LOREN CARL MARZ - Board Certified Meteorologist (CCM #591)


The following are citations from studies recently completed involving diesel emissions and how they compare to emissions from other automotive technologies (emphasis added where appropriate):


"...The mass of particles emitted from diesel vehicles is regulated by EU legislation. Particles emitted from petrol vehicles are not regulated because the mass emitted is low. The number of particles emitted is not regulated for any type of vehicle, but may be an important indicator of the health impacts. Ultrafine particles are believed to penetrate deeper into the lungs and to stay there longer than larger particles. The mass of one particle with a diameter of 3 micrometer is equal to the mass of 1 million particles with a diameter of 0.03 micrometer. Most particles from gasoline engines fall below 0.1 micrometer. This is why they contribute little to the mass of particles collected during a conventional emissions test....

5.1 Diesel engines - For diesel engines the tests show that both the number of particles emitted and the size distribution appear to be relatively unaffected by load, excess air and driving style. The number of particles rises modestly with increasing fuel injection. For diesel vehicles the number of particles peak at around 60 nanometers (nm). However, the number of particles varies considerably between the different diesel vehicles. The worst emit more than three times as many particles as the best. Consequently there is in many cases room for improvement even without having to resort to particle filters.

The results show that the French particle filter very efficiently removes ultrafine particles from the exhaust. The number of ultrafine particles emitted is less than 20% of those from diesel vehicles without particulate filters. In many conditions the number of particles emitted from the Peugeot 607 HDi is lower than the numbers emitted from the cars with turbo SI engines.

5.2 NA and turbo SI-engines - At low specific power outputs vehicles with NA and turbo engines emit much fewer particles than vehicles with SI DI and diesel engines. The emissions are less than 1% of those from diesel engines. However, unlike diesel engines, the particle emissions from these engines show a large variation with load, air/fuel ratio and in some cases approach diesel levels of emission. There is a linear relationship between the amount of fuel injected and the number of particles produced per minute. Increasing the fuel injected by a factor of two produces a tenfold increase in the number of particles. Increasing the specific power output affects the number of particles in a similar way. At maximum torque the total number of ultrafine particles emitted from vehicles with SI turbo engines can be greater than those from diesel engines...

For NA and turbo SI vehicles the results show increasing numbers of particles emitted with decreasing lambda values. There is a large variation between car models depending on the extent to which they operate stoichiometrically. Where manufacturers have not taken responsibility for ensuring that Lambda=1 under high loads, these vehicles can be high emitters during fast accelerations. Some vehicles with SI engines approach diesel emission levels at relatively moderate specific torque.

The NA and turbo SI engines tend to produce smaller particles compared to those produced by vehicles with diesel engines. The smallest size category (i.e. 10 nm) represents the largest number of particles of all size categories. There may be even more particles of sizes below 10 nm though the equipment is unable to measure them.

As shown above, vehicles with NA and turbo SI-engines exhibit a large range of particle emissions. The number of particles emitted varies by a factor of 100 to 10,000 depending on load, torque, A/F ratio and other factors. Unlike diesel engines, particle emissions from NA and turbo SI vehicles are sensitive to driving behaviour.

5.3 Direct injection SI - Only three models in the test represented the SI DI vehicles. The results should thus be interpreted with some caution.

The particle emission from the SI DI engines appear to be relatively independent of the amount of fuel injected per minute as well as of torque and engine load. In most circumstances the number of ultrafine particles emitted approach those of the diesel vehicles. Unlike NA and turbo SI engines the SI DI engines produce relatively high numbers of particles at low specific torque. At low loads the numbers fall slightly below those of the diesel vehicles and at moderate and high loads they are close to those from vehicles with SI NA engines.

The vehicles with SI DI engines seem to behave like those with diesel engines when running lean and like those with conventional SI engines when running stoichiometric or rich. At 90 km/h one of the three engines runs lean while the others operates stoichiometrically (i.e. lambda = 1).

The size distributions are different from both those of conventional SI and diesel engines. When the SI DI engines run lean, the size distribution is similar to that of diesel engines, but at about half the numbers. When running stoichiometrically the size distribution is more similar to those of the NA and turbo engines but at numbers that are about one hundred times higher.

Experimental animal and limited human studies indicate that the smallest particles, that is those less than 0.1 microgram, cause more inflammation in the periphery of the lung than do larger particles (Expert Panel on Air Quality Standards, 1999). In this size range particle mass is unlikely to be important, as they are so small....

...The lowest number of particles are emitted from the diesel + filter vehicle, and are about one tenth of those emitted from the vehicles with the NA SI-engines. The filter was not regenerating during this test and it shows a removal efficiency of 99.99%....

...If fitting particle filters to diesel vehicles becomes widespread practice, petrol vehicles could become the major source of ultrafine particles...."

Source: Farnlund et al., "Emissions of Ultrafine Particles from Different Types of Light Duty Vehicles." Swedish National Road Administration

"...Particulate researchers in the United Kingdom, for example, have shown that PM from gasoline engines is normally distributed around 30 nanometers while diesel PM is normally distributed around 60 nanometers....

...gasoline particulates could pose an even greater health risk than diesel particulates since the greater number of our vehicles are gasoline fueled and the greater quantity of fuel consumed is gasoline...."


"...Number of particles & size is influenced by the driving pattern (new driving cycle?). The particulate filter is very effective (advantage for diesel in the future?)..."

Source: Environmental and Health Impact From Modern Cars, May 2002 and August 2002 (DEER Conference); Ecotraffic (Peter Ahlvic)

"...The order of mutagenic activity per microgram of extract in TA-98 without S-9 from the lowest to the highest was environmental sample tunnel < heavy-duty diesel < light-duty diesel < spark ignition...."

Source: A. L. Brooks, et al., "A comparison of genotoxicity of automotive exhaust particles from laboratory and environmental sources." Environmental Mutagenesis, Volume 6, Issue 5, March 1984, Pages 651 - 668

Mammalian cell genotoxicity:

"- Gasoline PM strongly active for DNA damage and for chromosomal damage
- Diesel PM strongly active for DNA damage and weakly active for chromosomal damage
- Gasoline SVOC strongly active for DNA damage and chromosomal damage
- Diesel SVOC inactive or weakly active for DNA damage or chromosomal damage"

Source: In Vitro Genotoxicity of Gasoline and Diesel Engine Vehicle Exhaust Particulate and Semi-Volatile Organic Compound Materials, L Liu, M Keane, M Cui, M Ensell, W Miller, K Kashon, T Ong, W Wallace; National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention; DEER Conference, August 2002,

Mammalian cell genotoxicity at 30 degrees F:

"- Gasoline PM strongly active for DNA damage and for chromosomal damage
- Diesel PM weakly active for DNA damage and for chromosomal damage
- Gasoline SVOC weakly active for DNA damage; strongly active for chromosomal damage
- Diesel SVOC inactive or weakly active for DNA damage or chromosomal damage"

Source: In Vitro Genotoxicity of Particulate and Semi-Volatile Organic Compound Exhaust Materials from a Set of Gasoline and a Set of Diesel Engine Vehicles Operated at 30 Degrees F, M Ensell, M Keane, T Ong, W Wallace; National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention; DEER Conference, August 2003,

"...cell growth seen...with diesel exhaust exposure [with diesel particulate filter]..."

Source: Comparison of Direct Exposure of Human Lung Cells to Modern Engine Exhaust Particles, John Storey, ORNL, 2003 DEER Conference;

"...Scientists and health professionals have long believed that engine emissions can be harmful to human health. Although much attention has been directed toward studying soot from diesel engines during the past 20 years, there have been few comparisons of hazards between diesel and gasoline emissions, particulate and vapor emissions, or emissions from normal-emitting and smoking vehicles. A recent study funded by the U.S. Department of Energy conducted at the Lovelace Respiratory Research Institute in Albuquerque, NM, and reported in the December 2002 issue of Toxicological Sciences, provides some significant new insights that help place the hazards of different vehicle emissions into a clearer context.

On the basis of preliminary findings suggesting that catalytically controlled gasoline emissions containing semivolatile organic compound (SVOC) vapors and particulate matter (PM) could be just as toxic as uncontrolled (engine-out) diesel emissions containing SVOC and PM, the Lovelace study evaluated combined SVOC and PM samples collected from several gasoline and diesel cars and pickup trucks with normal emissions, from single gasoline vehicles emitting visible white and black smoke, and from a diesel vehicle emitting black smoke. Reactions in lungs of rats were measured to indicate hazards for inflammation and tissue damage (responses that could aggravate asthma and may cause bronchitis), and mutagenicity in bacteria was measured as a crude indicator of cancer hazard. ..."

Source: DOE

"...Per unit of mass, the vapor-phase SVOC was ~5x more toxic than PM...
...Health effects were eliminated or reduced to non-significant levels [with use of DPF]...
...BOTTOM LINES - The problem is emissions - not diesel emissions..."


"...According to Dan Greenbaum, President of the independent Health Effects Institute, "If you look across all the public agencies that have reviewed scientific data, you see some consistency in them saying that diesel is either a probable human carcinogen or likely to be a human carcinogen, but that's one step below saying it's a known carcinogen." (1) In fact, although some studies have shown an association of diesel with lung cancer, those associations are considered weak in epidemiological terms because (2):

-Relative risks resulting from modeled exposures to diesel emissions are small.
-Many of the studies used dated information or inadequate data for source apportionment studies.
-Few of the studies were controlled for all possible confounders (interferences).
-To study measured actual exposures to diesel emissions concurrent with air monitoring, as did OHVT's Northern Front Range Air Quality Study (NFRAQS).

For example, the CARB and STAPPA conclusions are based on the flawed MATES II study conducted by the South Coast Air Quality Management District (SCAQMD) in Southern California. (3) The MATES II study arbitrarily assigned 70% of the cumulative risk from air toxics to diesel particulate matter (PM) emissions. To support this conclusion, SCAQMD used elemental carbon as a surrogate for diesel PM. Although the EPA has used a similar approach to estimate levels of diesel PM in its Hazardous Air Pollutant Exposure Model (HAPEM), the agency acknowledges that this approach raises issues of the use of a gas-phase surrogate species for a particulate compound and that CO and PM emissions from mobile sources are not always correlated. (4) Because SCAQMD did not directly measure diesel PM, it calculated diesel PM in the ambient air from assumptions based on data nearly 20 years old.

Even if these factors were reasonably accurate, SCAQMD left unaccounted 67% of the residual particulate carbon captured at air monitoring stations for PM2.5. This begs the question of what other sources contribute to the PM2.5 collected at the air monitoring stations. For example, the NFRAQS work indicated that light-duty gasoline vehicles contributed about 60% of the PM2.5 carbon measured at urban Denver sites and that these contributions were 2.5-3.0 times those from diesel ( This fraction closely matches the unaccounted residual particulate carbon from the SCAQMD PM2.5 air monitoring sites. The NFRAQS methodology is being extended to the Los Angeles area to verify the results found in Denver.

A more fundamental problem is that CARB's Scientific Review Panel also used the flawed MATES II study as the basis for its calculation that 410 out of a million Californians could get lung cancer from diesel exhaust PM. STAPPA compounded this error by simply taking the CARB estimates and extrapolating them across the country to predict 125,000 extra cases of cancer from diesel exhaust. (6) In turn, this calculation was used to support CARB's recommendation for the California Environmental Protection Agency to list diesel exhaust as a toxic air contaminant. CARB took this course despite being warned by the authors of a key railroad study that their work did not prove that diesel exhaust causes cancer. (7)..."

Source: "Plain Talk on Trucks", DOE

"...Results from the Northern Front Range Air Quality Study indicate that sources with emissions similar to light-duty gasoline vehicles contributed about 60% of PM2.5 carbon at urban Denver sites, and these contributions were 2.5 to 3 times the diesel exhaust contributions.... Are the risks from spark-ignition PM emissions really negligible relative to diesel PM?..."


"....Gasoline vehicles and diesel emissions account for 21% and 2% of the PM2.5 mass concentration, respectively...."

Source: Eugene Kim, Philip K. Hopke; "Source Apportionment of Fine Particles in Washington, DC, Utilizing Temperature-Resolved Carbon Fractions." Journal of the Air & Waste Management Association, Volume 54, July 2004, Pages 773-785


"...DPM [diesel particulate matter] accounted for an average 15% of ambient PM2.5, and gasoline PM accounted for an average of 52% of ambient PM2.5 in the 1994-95 Phoenix study...."

Source: EPA Hazard Assessment Document for Diesel Engine Exhaust (page 128), available at

"...Using emission factor measurements from on-road studies, it appears that gasoline vehicles comprise the majority of mobile source PM emissions in the US...."

Source: Alan W. Gertler, "Diesel vs. gasoline emissions: Does PM from diesel or gasoline vehicles dominate in the US?" Atmospheric Environment, Volume 39, Issue 13, April 2005, Pages 2349-2355 [Abstract available at]

The Health Effects Institute (HEI) has determined that a major study actually shows a decreasing risk of cancer with increasing length of exposure to diesel exhaust [although this reference is not used to suggest diesel exhaust is healthy or even innocuous; just to show that diesel PM carcinogenicity is not as unequivocal as is often implied]:

"...The Panel's analysis of the exposure-response association in the railroad worker data showed that the evidence for a positive association of lung cancer with cumulative exposure to diesel exhaust depends entirely on differences in risks among job categories. Train workers (with higher exposures) have higher risks compared with clerks (with low or no exposure). However, within all job categories, the relation of lung cancer risk to duration of employment is negative...."

Source: HEI, Diesel Emissions and Lung Cancer: Epidemiology and Quantitative Risk Assessment, p. 40 (June 1999);

"...Animal studies of post-1995 diesel technology do not show short term indicators of [cancer] risk..."


"...underground miners, many of whom experience the highest occupational DPM exposures, generally do not show elevations in lung cancer..."

Source: Hesterberg, et al, "A critical assessment of studies on the carcinogenic potential of diesel exhaust." Crit Rev Toxicol. 2006 Oct;36(9):727-76

"...In sum, the weight of evidence suggests most studies (70%) are indeterminate because limitations are too great [that Diesel Exhaust is carcinogenic]...."

Source: Gamble, "Lung cancer and diesel exhaust: A critical review of the occupational epidemiology literature." Critical Reviews in Toxicology, 2010; 40(3): 189-244

Update: The International Agency for Research on Cancer (IARC) has recently (June 2012) classified diesel exhaust as a "known carcinogen" (Group 1). However, that decision apparently was based on a large U.S. National Cancer Institute/National Institute for Occupational Safety and Health study of occupational exposure to diesel emissions in underground miners, which showed an increased risk of death from lung cancer in exposed workers ( The miners in this study were exposed to extremely high levels of diesel particulate matter (up to 1005 micrograms/m3 - from 1947- to 1997-diesel engine technology.

Update #2: The International Agency for Research on Cancer (IARC) has more recently (October 2013) classified all particulate matter as a "known carcinogen" (Group 1 - Thus, particulate matter from all other sources is now concidered a known carcinogen, not just diesel engine particulate matter.

And of course, Diesel Particulate Filters are very effective in removing Diesel PM:

"...Peugeot with SiC-Diesel particle filter (DPF) has much lower emissions of nanoparticles, than generally supposed until now. The usual SMPS measurements from the CVS dilution tunnel estimate the background concentrations in the CVS dilution air, which are about 40 times higher, than the concentrations of finest particles in the exhaust gas of the vehicle....

...Due to the air filter of the CVS air inlet there are about 2 times lower particle count concentrations in the CVS air than in the ambient air...

...the particle count-concentration in the exhaust gas is about 18 times lower, than in the CVS dilution air. Due to that both gases change the roles: the CVS air is the emission source and the exhaust gas acts as the dilution air...."


"...DPF "cleans" room air [US06 cycle]..."

Source: John Storey et al., ORNL, "Comparison of Direct Exposure of Human Lung Cells to Modern Engine Exhaust Particles." 2003 DEER Conference;

Source: Advanced Diesel Engines for the EU and US Automotive Markets, Prof. Dr.-Ing. Peter Hofbauer FEV Engine Technology, Inc.;

Source: From Mayer et al,"Nanoparticle-Emission of EURO 4 and EURO 5 HDV Compared to EURO 3 With and Without DPF."; SAE 2007-01-1112,

"...the report notes that there are on-road measurements indicating that a continuously-regenerating trap with a catalyst-coated filter (in addition to the catalyst before the filter) coupled with ultralow sulfur fuel and low sulfur lubricating oil can reduce PM emissions across all sizes of particle to levels virtually indistinguishable from ambient background levels. Furthermore, there are significant reductions in other pollutants with adverse health impacts, such as polycyclic aromatic hydrocarbons and the carbonaceous component of ultrafine particles resulting from the use of ultralow sulfur fuel with continuously regenerating traps. ..."

Source:, "Report on Ultra-fine Particles", page 7 (original report - Phil Johnson and Paul J. Miller, NESCAUM/NESCCAF, "Ultrafine Particles: Issues Surrounding Diesel Retrofit Technologies for Particulate Matter Control." February 5, 2007)

"...After the first 300 seconds of the NEDC cycle...particles measured from multiple particle counting systems are close to tunnel background.... (Page 28)
...the concentrations for the FTP are lower than for the NEDC, and the concentrations for the FTP were below baseline noise levels for the EEPS during the initial spikes of solid particles...." (Page 30)

Source: State Of California, California Environmental Protection Agency, Air Resources Board, "California's Informal Participation in the Particle Measurement Programme (PMP) Light Duty Inter-Laboratory Correlation Exercise (ILCE_LD) Final Research Report." October 2008,

"...~200 fold reduction in soot emissions during steady state operation [ULSD]...
...DPF effective over entire PM size range...
...~100 fold reduction in soot emissions during steady state operations...overall trap efficiency is >97% [high sulfur diesel fuel]..."

Source: Measuring motor vehicle PM emissions: Current issues and future, Matti Maricq

"...particle emissions coming out of the tailpipe are at or less than ambient..."


"...Independent engineers have told me that the air coming out of the exhaust pipe is cleaner than the air going into the engine's intake. They don't even have to vent the exhaust when working on the engine indoors...."


"...the emissions coming out of this bus are right now 1/60th of what's in the outdoor air that we're breathing right now..."

Source: Dr. Bruce Hill, Clean Air Task Force,, @3:32-3:58 mark

Gasoline engines aren't neccessarily "pristine" with respect to PM either:

"...According to the [EPA] study, the contribution of gasoline vehicles to the nationwide PM emission inventory is higher than previously estimated...."

Source: EPA,

"...Generally gasoline engines have somewhat lower concentrations in the upper end of the accumulation mode [where] most of the particle mass is found, thus they have lower mass emissions...
...Emissions of extremely small particles, D > 3 < 10 nm, are often more than an order of magnitude higher than emissions of particles for D > 10 nm...
...Simultaneous CPC measurements show that for most of these vehicles there are many particles below 10 nm...
...There does not seem to be a simple relationship between number and mass emissions...
...SI becomes more important as particles become smaller...
-Emissions of extremely small particles (Dp < 10 nm) at high road speed may be very high (1014 - 1016 part./kg fuel) even for nominal low emitters [gassers].
-Number emissions from two classes of SI engines may equal or exceed Diesel levels
- Normal emitters at high speed and load
- Worn engines with high oil consumption
-The Swiss EPA has proposed that Europe adopt a solid particle emission standard of 1011 part./km
- This corresponds to roughly 1012 part./kg fuel
- It is unlikely that our current on-road gasoline fleet meets this standard
- Will gasoline engines need exhaust filters?"

Source: Gasoline Vehicle Exhaust Particle Sampling Study, David Kittelson, et al, DEER 2003 Conference;, (paper)

"...Jack Peckham of the Global Refining and Fuels Report released information in a September 17 piece that sided with diesel in comparison to gasoline when dealing with emissions. According to a "real world vehicle testing report" by University of Minnesota renowned combustion particle scientists, "new data shows that PM number emissions from modern gasoline cars may equal or exceed diesel PM levels." It goes on to discuss gasoline PM emissions and that fact that gasoline engines may need a particulate filter much like that of a diesel. "Ironically, while diesels are sometimes demonized by air regulators," Peckham states, "diesel particulate filters (DPFs) nearly eliminate PM mass emissions and drastically reduce PM number emissions as well, as numerous studies now show." The University of Minnesota study, which "used a sophisticated, truck-mounted mobile emissions lab to chase a variety of vehicles around Minneapolis-St. Paul during the summer," showed that newer and older gasoline vehicles matched or exceeded diesel PM number emissions at high speed/load. (Gasoline Cars May Need Particulate Filters; PM Emissions No Longer Just A Diesel Issue; Global Refining & Fuels Report; September 17, 2003.)..."

Source: DTF Diesel in the News, September 2003 Wrap-up

"...since gasoline vehicles are now proven to emit substantial amounts of PM and "toxics," the current exclusive regulatory crackdown on diesel vehicle PM doesn't quite hit the clean-air bull's-eye, scientific studies indicate....

...researchers once again exploded the 'myth that gasoline doesn't put out much elemental carbon. Well, it does,' Kittelson said, 'especially under cold-start--there's a lot of elemental carbon....'"


"...It was also found that spark ignited engines emit numbers of small particles which are comparable to those from diesel engines...."


"...Diesel exhaust particles have been shown to display a multimodal size distribution (Kerminen et al. 1997) and are mainly carbonaceous agglomerates below 100 nm in diameter, whereas particles emitted by gasoline vehicles are also mainly carbonaceous agglomerates but considerably smaller, ranging from 10 to 80 nm (Morawska and Zhang 2002)...."


"...Gasoline vehicle exhaust is enriched in particulate PAH as compared with diesel exhaust...."

Source: DOE's Gasoline/Diesel PM Split Study; Eric M. Fujita, David E. Campbell, William P. Arnott, Barbara Zielinska and Judith C. Chow; Division of Atmospheric Sciences; Desert Research Institute, Reno, NV; Douglas R. Lawson, National Renewable Energy Laboratory, Golden, CO, DEER 2003;

"...SI vehicles, whether low or high emitters, emitted greater amounts of high-molecular-weight particulate PAHs (benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, and coronene) than did CI vehicles...."

Source: Fujita, et al, "Variations in Speciated Emissions from Spark-Ignition and Compression-Ignition Motor Vehicles in California's South Coast Air Basin." Journal of the Air and Waste Management Association, June 2007

"...Typically, gasoline UFPs [ultra-fine particles] contain a higher fraction of heavy polycyclic aromatic hydrocarbons (PAHs) than diesel exhaust (DE) which may have implications for the differential toxicity of these particles..."

Source: HEI Review Panel on Ultrafine Particles. 2013. "Understanding the Health Effects of Ambient Ultrafine Particles." HEI Perspectives 3. Health Effects Institute, Boston, MA, (Page 21)

"...Most of the particles added by the on-road fleet were below 50 nm in diameter....

...The higher the speed, the greater the particle concentration, and the smaller the particle size. This is a reasonable finding because at high vehicular speeds, particulate number emissions, especially from SI engines, increase with increased engine load, exhaust temperatures and exhaust flow...."

Source: D. B. Kittelson, W. F. Watts and J. P. Johnson, "Nanoparticle emissions on Minnesota highways." Atmospheric Environment, Volume 38, Issue 1, January 2004, Pages 9-19

"...Group average fuel-specific number emissions range from 3.9x1014 to 1.0x1017 particles/kg of fuel...

...The relative contribution of light-duty vehicles to particle number emissions increased as particle size decreased, for the smallest particles apportioned number emissions were 1.3x1016 and 7.1x1015 particles/kg of fuel for heavy-duty and light-duty vehicles, respectively. Comparison of on-road chase and apportionment results with chassis dynamometer tests in a certification type facility suggests that the latter may underestimate real-world number emissions....

...SI engines emit a higher proportion of smaller particles than do diesels..."

Source: D.B. Kittelson, W.F. Watts, J.P. Johnson, J.J. Schauer, and D.R. Lawson, "On-road and laboratory evaluation of combustion aerosols-Part 2: Summary of spark ignition engine results." Journal of Aerosol Science, Volume 37, Issue 8, August 2006, Pages 931-949,

"...Particle emissions are influenced by operational factors such as speed, engine load, or ambient temperature for which petrol cars show greater dependence. In some operational modes particle number emissions may reach the same range as those found in non-DPF diesel cars...."

Source: Combustion Particles and NOx from Diesel and Petrol Vehicles - A 2008 update including some biofuels and additional exhaust components, Ecotraffic

"...Modern DPF technology allows a very efficient removal of solid particles, and also of the nanoparticles...." (page 32)
"...Petrol engine particulates are composed of much smaller particles than those from diesel engines. In some reports, the petrol engine particles were found practically entirely in the nanoparticle range (<50 nm)...." (page 37)

Source: Particle and NOx Emissions From Automotive Diesel and Petrol Engines, Ecotraffic, November 2009

Some studies show that current gasoline direct injection (GDI) car engines do NOT meet the particle number (PN) emissions that new diesel cars are now required to meet in Europe...

Source: Kody Klindt, IAV Automotive Engineering Inc., "Reducing the particulate emission numbers in DI Gasoline Engines."

Source: Walter Piock, Guy Hoffmann, Axel Berndorfer, Patrick Salemi and Bernd Fusshoeller; "Strategies Towards Meeting Future Particulate Matter Emission Requirements in Homogeneous Gasoline Direct Injection Engines." SAE International, 2011-01-1212,

"...Filters are considered effective enough such that tailpipe emissions are less than ambient levels in laboratory..."

Source: Tim Johnson (Corning), Symposium on Particulate Matter, August 12-13, 2003

"...The cancer risk index is...higher for gasoline vehicles than for diesel vehicles...."

Source: John Fairbanks (DOE), Symposium on Particulate Matter, August 12-13, 2003

"...Trap-equipped diesels emit a lower particle number than even gasoline vehicles...."

Source: Axel Friedrich, Umweltbundesamt, Symposium on Particulate Matter, August 12-13, 2003

"...Current port fuel injected gasoline engines with catalytic converters are "clean", i.e., they emit very low masses of PM, but emit very many ultrafine particulates...."


"...DPF-filtered exhaust virtually eliminated the elemental (black) carbon and drastically reduced the organic carbon as well. DPF-filtered exhaust also drastically reduced CO, VOCs, and the speciated semi-volatile and heavy organics...."


"...New cars with diesel particle filters (DPF) can meet or beat new gasoline cars on particulate matter (PM) emissions, California Air Resources Board (CARB) deputy executive director Tom Cackette told the Society of Automotive Engineers (SAE) World Congress here....
...These PSA cars emit about 0.5 milligrams/mile of PM, versus about 2 mg/mile PM from advanced gasoline cars, he said...."


Particle numbers in particle size bins of two identical cars (Peugeot 307 - one with a 80 kW 1.6 liter gasoline engine (SI-P); the other with a 80 kW 2.0 liter HDi FAP diesel engine with CR injection and particulate filter (CI-CR/DPF)):

Particle size distribution for the Peugeot cars in US06

(Figure 40 on page 63)

The folowing graphic shows that not only is DPF highly effective in reducing the extremely small particles (~5 nm = 0.005 micrometers = "PM0.005"), the emissions of these extremely small particles from the gasoline vehicles are as high or higher than the non-DPF diesel vehicle (CI-UI/HP) even in the gentle NEDC test cycle:

(Figure 48 on page 69)

The folowing graphic shows that particle number emissions in gasoline engines approach and even exceed the particle number emissions in even uncontrolled diesel engines in the US06 test cycle (high speed/aggressive driving):

"...In US06, the particle number for the petrol cars increase substantially, i.e., approaching the level for the diesel car without a particle trap in one case and reaching an even higher levels in the other case (Figure 31). The levels for the diesel car with the particulate filter decreased with one order of magnitude compared to the results in NEDC. Thus the relative advantage of this car compared to the other cars was more pronounced in this driving cycle than in the previous case...." (Page 55)

Source: "Environmental and Health Impact From Modern Cars." Ecotraffic Report,

"...Smoky bars and casinos have up to 50 times more cancer-causing particles in the air than highways and city streets clogged with diesel trucks at rush hour, according to a study...

...Repace found an average level of respirable particles of 231 micrograms, or millionths of a gram, per cubic meter of air in the eight nightspots in Delaware. That is 15 times the 15-microgram Environmental Protection Agency limit for outdoor air, and 49 times the rush-hour average on Interstate 95 in Wilmington. It even tops the 199-microgram rush-hour level at the Baltimore Harbor Tunnel tollbooths...."

Source: Journal of Occupational and Environmental Medicine,

"...While it is undisputed that a number of such potentially hazardous substances are present in diesel exhaust, mere presence is not the relevant scientific issue; instead, the issue is "what is the dose?" For it is foundational in all toxicology that the dose makes the poison..."


"...the location impacted primarily by light-duty emissions is slightly more toxic than the sample impacted largely by heavy-duty emissions...."


"...The contribution from diesel exhaust varies throughout the year, but never exceeds 0.17 micrograms/m3...."


"...The annual DPM [diesel particulate matter] concentrations for Atlanta, Nashville, and Birmingham were 1.09, 0.55, and 0.47 micrograms/m3, respectively. ...The annual DPM contribution to the total PM2.5 for Atlanta, Nashville, and Birmingham were 3.7%, 2.5%, and 2.2%, respectively." [This is the contribution from all diesel sources - on- and off-road]

Source: L.A. Diaz-Robles, J.S. Fu, G.D. Reed and A.J. DeLucia, "Seasonal Distribution and Modeling of Diesel Particulate Matter in the Southeast US." Environment International, Volume 35, Issue 6, August 2009, Pages 956-964

For comparison, the current annual PM2.5 NAAQS limit is 12 micrograms/m3; the 24-hour limit is 35 micrograms/m3 (

"...Many articles in the popular press tend to incorrectly use the word "soot" as a synonym for fine particulate matter, when in fact soot - a black carbonaceous residue of incomplete combustion of fuel - is rarely the dominant component of ambient PM2.5 mass. By definition, soot would be identified as carbon (probably elemental but perhaps organic) in bulk analyses of PM2.5 samples...."


"Soot is produced by spark-ignition vehicles...and lots of other sources! (from the tailpipe of relatively new (7k mi) Toyota Prius at NREL)"

Source: (Slide #2)

"...The results demonstrate the high efficiency of diesel particle filters (DPFs) in curtailing nonvolatile particle emissions over the entire size range...."

Source: Martin Mohr, Anna-Maria Forss, Urs Lehmann, Laboratory for Internal Combustion Engines, Empa (Swiss Federal Laboratories for Materials Testing and Research), "Particle emissions from diesel passenger cars equipped with a particle trap in comparison to other technologies." Environ Sci Technol. 2006 Apr 1;40 (7):2375-83

"...DPF installed on each of these [diesel] vehicles was effective in trapping both nucleation and soot accumulation mode particles with filtration efficiencies above 99%...."

Source: De Filippo & Maricq, "Diesel Nucleation Mode Particles: Semivolatile or Solid?" Environmental Science & Technology, vol. 42, no. 21, pp.7957-7962, 2008

"...Mercedes-Benz Buses Meet EEV Emissions Standard Without Diesel Particulate Filter..."


"...Europe plans to establish an SPN standard for gasoline vehicles by September 1, 2014. It is possible that the standard will be set comparable to the standard for diesels [6X1011 /km], and thereby will force the use of GPF [particulate filters] on gasoline vehicles...."


"...The particle number emission from DPF-equipped vehicles are in the range 108-1010 /km and non DPF equipped vehicles are in the range 1013-1014 /km. This is in good agreement with previous studies of diesel vehicles..."

Source: Karlsson, "Measurement of Emissions from Four Diesel Fuelled Passenger Cars Meeting Euro 4 Emission Standards." AVL, 2005,

"...While direct injection spark-ignition vehicles exhibited high particle emission, only topped by the diesel vehicles without a particle filter, diesel vehicles with particle filters had lowest particulate emission...."

Source: Schreiber, et al., "Particle Characterization of Modern CNG, Gasoline and Diesel Passenger Cars." SAE Paper 2007-24-0123

"...DPFs can be >99% efficient by particle #...
...Efficiency is nearly independent of particle size..." [slide #9]

Source: Matti Maricq, "How Are Emissions of Nuclei Mode Particles Affected by New PM Control Technologies and Fuels?" HEI Annual Conference 2009,

"...Acceleration for the gasoline test fleet increased its [particle] emissions to a level that exceeded the [unfiltered] diesel test fleet emissions..."

Source: Appendix P, LEV III PM Technical Support Document, "Development Of Particulate Matter Mass Standards For Future Light-Duty Vehicles.", page P64

"...BC emission factors from gasoline vehicles are at least a factor of 2 higher than previous North American measurements, and a factor of 9 higher than currently used emission inventories in Canada..."

Source: John Liggio, et al, "Are Emissions of Black Carbon from Gasoline Vehicles Underestimated? Insights from Near and On-Road Measurements." Environmental Science & Technology, 2012, 46 (9), pp 4819-4828,

"...The [California] ARB projects that from 2008 to 2020, while emissions of all sources of PM are expected to increase by 3.2 percent, PM from all diesels will decline by 58 percent...."


"...Research has demonstrated that the level of PM from gasoline light-duty vehicles is more significant than previously thought..."

Source: EPA, Final Rule, "Control of Air Pollution from Motor Vehicles: Tier 3 Motor Vehicle Emission and Fuel Standards." (Page 23 of 1069)

"Study finds light-duty gasoline vehicles responsible for about half of [ambient] PM2.5 mass"

"...We improve upon earlier U.S. studies by reporting all three Pd-group PGEs (i.e., Rh, Pd, and Pt) emanating from an on-road fleet, nearly eliminating contributions from diesel-driven vehicles..."

Source: Ayse Bozlaker, et al. (2013) "Elemental Characterization of PM2.5 and PM10 Emitted from Light Duty Vehicles in the Washburn Tunnel of Houston, Texas: Release of Rhodium, Palladium, And Platinum." Environmental Science & Technology,

A significant amount of organic PM (secondary organic aerosols - SOA) is also produced indirectly by gasoline exhaust and evaporative VOC emissions from gasoline production/distribution/refueling:

"...PMC3. In Atlanta, the composition of the ultrafine particles less than 100 nm particles was dominated by carbon compounds. The major composition classes (expressed as percentage of particle mass) were: organic carbon (~74%), potassium (~8%), iron (~3%), calcium (~2%), nitrate (~2%), elemental carbon (~1.5%), and sodium (~1%) (Solomon et al., 2003b)....
...PMC17. In southeast Texas, high organic-carbon to elemental-carbon ratios suggest that much of the carbonaceous material in PM2.5 is not emitted directly, but is formed in the air through reactions involving both gaseous biogenic and anthropogenic VOC emissions...
...PMC19. Data from both the Atlanta and Houston Supersite Programs indicate that secondary formation of organic aerosols tended to be large compared to primary emissions..."

Source: Southern Oxdant Study,

"...Gasoline vehicles are predominant source of anthropogenic SOA precursors..."


"...Here we show that in the real urban atmosphere reactive anthropogenic VOCs (AVOCs) produce much larger amounts of SOA than these models predict, even shortly after sunrise.... third of global SOA could be from AVOCs...."

Source: Volkamer, et al., "Secondary organic aerosol formation from anthropogenic air pollution: Rapid and higher than expected." GEOPHYSICAL RESEARCH LETTERS, VOL. 33, 2006,

"...These new observations and a growing body of evidence in the recent literature show that the secondary formation of OM from anthropogenic VOCs emitted from urban sources can be an important source on regional scales...."

Source: J. A. de Gouw, et al., "Sources of particulate matter in the northeastern United States in summer: Direct emissions and secondary formation of organic matter in urban plumes." JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, 2008,

"...exhaust from gasoline vehicles contributes more to the production of secondary organic aerosols (SOA) than exhaust from diesel vehicles...
...Based on their study, the team estimated that within a day of processing, SOA from gasoline exhaust emissions may reach 4 Tg/yr, which is ~16% of recent global estimates of biogenic SOA....
...The contribution of diesel to SOA is almost negligible..."

Source: Bahreini, R., et al., "Gasoline emissions dominate over diesel in formation of secondary organic aerosol mass." Geophys. Res. Lett. (2012), (

"...gasoline exhaust readily forms secondary organic aerosol with a signature aerosol mass spectrum with similarities to the oxidized organic aerosol that commonly dominates the OA mass spectra in and downwind urban areas. This substantiates recent claims that gasoline SOA is a dominating source to SOA in and downwind large metropolitan areas (Bahreini et al. 2012)....

...the distribution of light aromatics was consistent with vaporized fuel and the apparent mass yield and aerosol mass spectra were both consistent with C6-C9 light aromatics being responsible for more than 90% of the SOA formation...."

Source: Nordin, et al., "Secondary Organic Aerosol Formation From Gasoline Passenger Vehicle Emissions Investigated in a Smog Chamber." Atmospheric Chemistry and Physics Discussion, 12, 31725-31765, 2012,

"...the mixture of organic vapors emitted by newer [gasoline] vehicles appears to be more efficient (higher yielding) in producing SOA than the emissions from older vehicles....

...These results for light-duty gasoline vehicles contrast with the results from a companion study of on-road heavy-duty diesel trucks; in that study late model (2007 and later) diesel trucks equipped with catalyzed diesel particulate filters emitted very little primary PM, and the photo-oxidized emissions produced negligible amounts of SOA...."

Source: Gordon, et al., "Secondary organic aerosol formation exceeds primary particulate matter emissions for light-duty gasoline vehicles." Atmospheric Chemistry and Physics. 09/2013; 13(9):23173-23216. (Absract)

"...scientists have been puzzled to learn that a subset of these particles, called secondary organic aerosols, has a greater total mass, and is thus more dangerous, than previously understood...."

Source: Veronique Perraud, et al., "Nonequilibrium atmospheric secondary organic aerosol formation and growth." Proceedings of the National Academy of Sciences, January 2012 (


"...The petrol cars had higher emissions of benzene, ethene, propene, and 1-3 butadiene than their diesel counterparts...
...Most harmful gaseous emiss. lower for diesel..."

Peugeot 307, 80 kW 1,6 liter gasoline (SI-P)
VW Golf, 77 kW 1,6 liter gasoline (SI-G)
Peugeot 307, 80 kW 2,0 HDi FAP diesel with CR injection and particulate filter (CI-CR/DPF)
VW Golf, 96 kW 1,9 TDI diesel, high-pressure pressure unit injectors (CI-UI/HP)"

Benzene emissions in NEDC

Emissions of alkenes in NEDC at +22C

PAH emissions in NEDC (Interpolated to +7C)

Source: Environmental and Health Impact From Modern Cars, May 2002 and August 2002 (DEER Conference); Ecotraffic (Peter Ahlvic)

"...It is interesting to note that the levels of several of the most toxic compounds emanating from gasoline engines are higher (for example, benzene, 1-3 butadeine, formaldehyde) than from diesels...."


"...Benzene accounts for one to two percent of the exhaust hydrocarbons, expressed as a percentage of total organic gases (TOG), in diesel engines....For gasoline-powered highway vehicles, the benzene fraction of generally higher than for diesel engines, about three to five percent...." (Page 2-73)

Source: EPA Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel Engines,

"...Dr Grosjean identified about 100 carbonyls in the Tuscarora Mountain and Caldecott Tunnels. Total carbonyl emission factors from diesel-powered trucks were found to be about 4 times those from gasoline powered cars when both were calculated on a distance-traveled basis. On a fuel-consumed basis, total carbonyl emission factors for diesel trucks were slightly less than for cars. Formaldehyde, acetaldehyde and acetone were the three major carbonyls present...."

Source: Airborne Carbonyls from Motor Vehicle Emissions in Two Highway Tunnels; Daniel Grosjean. (HEI Report)

"...At similar dilutions, Gasoline had more "stuff" than Diesel...

...the gasoline atmosphere has much higher volatile organics, including "air toxics"..."


"...Ambient levels of volatile organic MSATs [mobile source air toxics] in the SoCAB were mainly associated with gasoline vehicles...."

Source: Fujita and Zielinska, "Assessment of Exposures to Mobile Source Air Toxics". Air Toxics Workshop II, Houston, Texas June 12-13, 2007


"...Due to significantly lower HC emissions from diesel cars, the ozone forming potential...was generally significantly lower for this category of vehicle compared to petrol vehicles. ...Comparing the ozone forming potential averaged for the new petrol cars with the average for the new diesel cars, yields a difference of more than one order of magnitude...

...Note that the evaporative emissions from the vehicles were not included in the evaluation of the ozone forming potential, neither was this done for evaporative losses from fuel distribution and refuelling. As these emissions were higher for petrol cars, the ozone formation for these cars is underestimated....

...The ozone forming potential was considerably lower for diesel cars, i.e., roughly one order of magnitude lower than for petrol cars. This is mainly due to the low level of HC emissions for diesel cars compared to petrol cars. Should the evaporative emissions have been taken into account, the relative differences would have been even greater...."

HC emissions in NEDC

Source: Environmental and Health Impact From Modern Cars, May 2002 and August 2002 (DEER Conference); Ecotraffic (Peter Ahlvic)

"...However, diesel vehicles have a unique characteristic that distinguishes them from gasoline vehicles. While the emission rates of gasoline vehicles increase over time, diesel emissions remain the same throughout the vehicle's life...."

Source: U.S. Department of Energy, "Plain Talk on Trucks"

"...As expected, the CO level was significantly lower for diesel cars compared to their petrol counterparts. The ratio was almost one order of magnitude regardless of ambient temperature...."

Source: Environmental and Health Impact From Modern Cars, May 2002 and August 2002 (DEER Conference); Ecotraffic (Peter Ahlvic)

"...The diesel is distinguished by its low emissions of carbon monoxide and hydrocarbons. In addition, unlike gasoline engines, diesel engines do not emit evaporative volatile organic compounds (VOCs), which contribute to the production of ozone in the atmosphere. The advantage of the diesel also includes very low cold-start emissions.

In Europe and elsewhere, the diesel engine is relied upon to meet greenhouse gas emission reduction commitments and, particularly, to reduce carbon dioxide emissions. A comparison of vehicles equipped with a three-way catalyst gasoline engine versus a diesel engine meeting the same performance standards evidenced overall reduced greenhouse gas emissions of 45 percent for the diesel, with even better performance under real driving conditions. Diesel engines also can run on new bio-diesel fuels, making further greenhouse gas reductions possible.

Environmental benefits of the diesel also include lower emissions of greenhouse gases in the fuel production and distribution process, as well as smaller refueling and evaporation losses at the pump...."


"...There are other environmental advantages of diesels as well: near-zero evaporative hydrocarbon emissions due to the extremely low vapor pressure of diesel fuel, low cold-start emissions, and low in-use emissions deterioration...."


"...As tailpipe emissions are reduced due to improved exhaust emission control systems, the hydrocarbons produced by evaporation of the gasoline during distribution, vehicle refueling, and from the vehicle, become more and more significant. A recent European study found that 40% of man-made volatile organic compounds came from vehicles..."


"...Though the sites are well removed from large urban source regions, the observations show a clear anthropogenic influence on the hydrocarbon levels at these rural sites...."


This brings up another factor that's typically not considered when evaluating how "green" a specific vehicle is. These evaporative emissions from gasoline vehicles are in addition to the tailpipe emissions of the FTP 75 testing:

The FTP 75 emissions do not include evaporative emissions:

"...During the FTP, tailpipe exhaust is collected in three bags: the so-called cold bag for the first 5 cycles of driving, the stabilized bag for the next 13, and the hot bag for the 5 repeat cycles following the hot soak. For regulatory purposes, the measured mass emissions from each bag are substituted in a prescribed equation to determine the emission rate per unit of travel (in this case, grams per mile) of each regulated emission.

Evaporative emissions, including those resulting from leaks of liquid fuel, are measured separately using a variable-temperature SHED (sealed-housing-for-evaporative-determination) facility; i.e., an instrumented temperature-controlled room in which the test vehicle is housed...."

Source: Ozone-Forming Potential of Reformulated Gasoline, Commission on Geosciences, Environment and Resources; excerpt available at

Regulations permit a gasoline car to emit up to 0.05 g/mile of evaporative HCs ( So a gas vehicle could emit 0.01 g/mile tailpipe HC emissions plus 0.05 g/mile evaporative emissions (running loss) and still be classified "SULEV". Or a gas vehicle could be certified as LEV II/Tier 2 Bin 5 by achieving 0.09 g/mile tailpipe NMHC emissions plus 0.05 g/mile evaporative running loss which totals 0.14 g/mile NMHC emissions (i.e., barely meets Bin 10 - discontinued after the 2006 model year). That doesn't include "diurnal + heat soak" evaporative emissions while a vehicle is "sitting" (i.e., in addition to "running losses"). Diesel vehicles have trivial evaporative emissions ( [Page 28], [Page 5], [Page 19], in addition to the references previously cited), and are specifically exempted from these regulations. So gas vehicles have "hidden emissions" that diesels don't (with the exception of vehicles classified as "PZEV" [Partial Zero Emission Vehicle] and even those have higher evaporative losses during distribution and refueling).

A complete detailed analysis of "well-to-wheel" emissions of the 2014 ULEV/Tier 2 Bin 5 (TDI) and PZEV/T2B2 (gas) VW Passats is available here.

"...In cities, as much as 95 percent of all CO emissions may come from automobile exhaust..."


"...The primary source of CO from vehicles is the incomplete combustion of gasoline in engine cylinders. ...CO emissions by diesel vehicles are minimal, primarily because of the excess air used in the diesel combustion cycle...."

Source: The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska (2002), Board on Environmental Studies and Toxicology (BEST), Board on Atmospheric Sciences and Climate (BASC), Transportation Research Board (TRB), excerpt available at

"...gasoline vehicles emit the large majority of CO..."

Source: EPA Hazard Assessment Document for Diesel Engine Exhaust (Page 43), available at

"...Compared to spark-ignition engines, diesel engines typically produce very low NMOG emissions..."

Source: EPA, "Draft Regulatory Impact Analysis: Tier 3 Motor Vehicle Emission and Fuel Standards." (Section 1.4.2, Page 1-29 (49 of 532))

"...Gasoline automobiles are by far the largest contributors to the carbon monoxide and hydrocarbon inventories...

...Jon Van Gerpen, the agricultural and biological sciences department head at the University of Idaho, said untreated diesel vehicles are in the same category-they release comparable emissions-as gas-powered cars with fully functioning catalytic converters. Van Gerpen, an expert in the mechanics of compression ignition engines and their emissions, said that by putting an oxidation catalyst on a diesel engine, CO and HC emissions actually fall below measurement thresholds...."


"...The results suggest that the emissions of gasoline-powered vehicles still dominate the hydrocarbon distribution in northern mid-latitude urban areas..."

Source: Borbon, et al. (2013), "Emission ratios of anthropogenic VOC in northern mid-latitude megacities: observations vs. emission inventories in Los Angeles and Paris." Journal of Geophysical Research: Atmospheres, Volume 118, Issue 4. Pages 1585-2083,

"...However, recent OHVT-sponsored research has shown that ozone is present in higher concentrations during weekends than on weekdays in California and other urban U.S. locations, even though ozone precursors are present in lower concentrations on weekends. This is especially true of heavy-duty vehicles - they contribute about 50% less of ozone precursor emissions on weekends than they do on weekdays. The data are beginning to suggest that in urban locations, NMOCs are more important than NOx in determining ozone concentrations. These findings, if demonstrated to be correct, would suggest that reducing NOx emission reductions actually increase ozone concentrations in urban locations...."

Source: U.S. Department of Energy, "Plain Talk on Trucks", September 2001

"...NOx controls in Southern California (and other urban U.S. locations) are counterproductive for reducing ambient ozone, and they actually increase ambient ozone levels. Were it not for large concurrent HC emission reductions on weekends, weekend ozone would be even higher than it is, and the weekend/ weekday ozone difference would be even larger....


"...Gasoline exhaust and gasoline vapor account for ~80 percent of ambient NMHC in on-road samples and at regional air monitoring locations suggesting that gasoline emissions are responsible for the majority of ozone found in the SoCAB.... ozone disbenefit will result if NOx emissions are decreased at current levels of VOC until ambient NOx levels are decreased by roughly 90 percent to about 10-12 ppb...."

Source: Eric Fujita, et al, "Weekend/Weekday Ozone Study in the South Coast Air Basin."

"...modeling suggests that peak O3 will increase unless NOx mixing ratios are decreased by approximately 90% from current weekend levels...." ["IMPLICATIONS" on page 1]

Source: Fujita, E.M.; Stockwell, W.R.; Campbell, D.E.; Keislar, R.E.; Lawson, D.R., "Evolution of the Magnitude and Spatial Extent of the Weekend Ozone Effect in California's South Coast Air Basin, 1981-2000." J. Air & Waste Manage. Assoc. 2003, 53 (7), 802-815,

"...Lawson is openly sharing data that shows NOx emissions in the L.A. Basin on Sundays are approximately 50 percent less relative to the emissions of NOx on Wednesdays. Conversely, ozone formation in the L.A. Basin on Sundays is approximately 50 percent higher compared to Wednesdays...

...urban areas all across the country experience NOx reductions of 50 percent on Sundays relative to Wednesdays...


"...Some experts - most notably Lawson and Eric Fujita of the Desert Research Institute in Nevada, both former California air pollution officials - believe that regulators could keep ozone in check better by slowing the pace of nitrogen oxide reductions while doing more to cut volatile organic compounds....


"... Since the percent derived here is 35 percent, this implies that a UAM derived MIR factor for CO would then be 1.75 (i.e., 35/20) times 0.07 or 0.12 grams ozone per gram CO...."

Source: CARB (ICF)

"...CARB focuses on reducing NOx. But atmospheric scientist Gary Whitten of ICF notes that if the tradeoff of reducing NOx is to increase hydrocarbon [HC] and carbon monoxide [CO] emissions [which is exactly what is happening with regulations which favor gasoline engine emission profiles and disfavor diesel emission profiles, thus, intentionally or not, encourage the gasoline vehicle market while discouraging the diesel vehicle market], the environment would be poorly served. The reason, according to Whitten, is that a reduction in hydrocarbon and carbon monoxide emissions has a much greater beneficial impact on ozone formation than an equivalent reduction in NOx. Whitten concludes, "The effectiveness of THC for reducing ozone in these simulations must be as much as 8 times better than NOx reductions on an equal percentage of the mobile emissions basis."..."

Source: The Other Gasoline Crisis: Speeding Up the Shift From MTBE to Ethanol, David Morris and Jack Brondum, Institute for Local Self-Reliance, September 2000;

"...That has the EPA focusing on cutting nitrogen oxides, a method that scientists say reduces smog slowly and inefficiently...."


"...studies into the "weekend ozone effect" in California indicate that NOx reductions trigger increased ozone levels. Thus, future diesel NOx emission standards may be in fact counterproductive-meaning less clarity for clean air policy makers...."

Source: Dieselnet, "The Log", January 14, 2004;

"...Moreover, CO is a recognized precursor to urban ozone (or smog) that appears to be growing in importance relative to VOC. For example, the newest California LEV II regulations allow CO-to-VOC emissions ratios of up to 100-to-1 even though the ozone-forming potential of CO is only about 50-to-1 relative to exhaust VOC. As a consequence of such emissions ratios, the carbon monoxide from such vehicles can generate more urban ozone than their exhaust VOC emissions...."

Source: AIR QUALITY IMPACTS OF ETHANOL IN CALIFORNIA GASOLINE, By Dr. Gary Whitten, Systems Applications International, Inc.

"...emissions inventories of CO consistently show that the mobile contribution, especially in troublesome urban areas, is often as high as 90 percent....

...Carbon monoxide is a major ozone precursor. New studies...have shown that CO can be equivalent to 25 to 50 percent of the mobile-related contribution from volatile organic compounds (VOC)...."


"...Simplified chemistry:
UV radiation (hv), acting on atmospheric NO2 creates ozone (O3):
NO2 + hv ---> NO + O
O + O2 ---> O3

But ozone is also destroyed (titrated) if NO is still present:
O3 + NO ---> NO2 + O2
Why does O3 build up? If volatile organic compounds (VOCs) are also present, NO is consumed by other reactions and is not available to titrate the O3:
VOC + NO ---> NO2 + other products ..."


"...reducing NOx emissions causes the "ozone production efficiency" (OPE) of NOx to increase, offsetting the decrease in NOx emissions...

...OPE lower with higher NOx emissions..."

Source: from a presentation by Dr. James F. Meagher, NOAA's Air Quality Program Manager, at the "East Tennessee Ozone Study 2006 Science Workshop", May 2006, Ozone Formation - Is All NOx Created Equal?

"...The atomic oxygen produced by the photolysis of NO2 is very reactive and readily combines with O2 in the air to form O3. In the presence of NO, however, the O3 will immediately decompose, regenerating the nitrogen dioxide. This nitrogen dioxide photolytic cycle is summarized in the following three reactions.

NO2 + hv --> NO + O
O + O2 --> O3
O3 + NO --> NO2 + O2

Hence, while the presence of NO2 is required to form O3, the nitrogen dioxide photolytic cycle by itself does not generate net ozone, and cannot explain ozone accumulation....

...ROG [VOC] reductions (with constant NOx) always lead to a slowing of the ozone production process and lower peak ozone concentrations. NOx reductions (with constant ROG) can lead to a speeding up of the ozone process, and can increase or decrease peak ozone values depending on the ROG-to-NOx ratio.

Thus, whereas ROG control is never detrimental, NOx control can be detrimental, particularly in the central cores of urban areas...."

Source: Air Pollution Control, A Design Approach; Cooper, D. C.; Alley, F. C.; Third Edition, pages 595 and 600

"...Our analyses of weekday/weekend differences in ozone precursor emissions show that different emission reductions of ozone precursors than normally take place each weekend will be required before significant reductions in ambient ozone can be achieved in the Atlanta area. The principal limitation of our analysis is that current weekend emission reductions are not identical to those that might occur in the future as a result of emission control programs. Regional-scale changes in ozone levels potentially could result from combined urban and nonurban reductions of ozone precursor emissions. However, it is sobering that Atlanta sites exhibited so much difference between the weekend reductions of ozone precursors compared with much smaller changes in ozone levels. Any expectation of future declines of regional ozone concentrations due to precursor reductions in the metropolitan area should be further investigated...."


"...NOx concentration reductions on Sundays exceeded the Sunday reductions of CO and hydrocarbon concentrations in all cities that were studied... (Page 101)

...all areas exhibited ozone concentration increases for data averaged over the period 10 am through 5 pm, with median Sunday ozone levels ranging from 100 to 115 percent of the Wednesday concentrations... (Page 104)

...none of the metropolitan study areas exhibited significant reductions of weekend ozone levels, nor were significant ozone decreases observed downwind of the study areas on weekends...." (Page 118)


"...Much lower emissions of VOC, CO, and NOx on weekends, with larger NOx reductions than VOC and CO reductions
       - Up to 80% fewer trucks and buses and ~15% less light-duty traffic on roads in urban areas on weekends...
...Projected emission reductions from 2005- 2010 similar to today's weekend reductions of ozone precursor concentrations (future NOx emission reductions > VOC emission reductions)...
...In urban areas, NOx reductions increase ozone production; VOC (and CO) reductions decrease ozone production. For ozone production, these emission reductions offset each other....
...Downwind ozone levels do not appear to be sensitive to changes in NOx emissions (downwind ozone has traditionally been thought to be sensitive to changes in NOx emissions)..."

"...PM nitrate is reduced less than 3%...
...NOx reductions in urban areas currently do not reduce, and usually increase, ambient ozone...
...One state reviewer's comment: 'The over-arching conclusion that lower weekend concentrations of ozone precursors do not lead to lower ozone concentrations is impossible to dispute and has far reaching regulatory implications.'..."

Source: Weekday and Weekend Air Pollutant Levels in Ozone Problem Areas in the United States, Dr. Douglas R. Lawson, et al, NREL, August 23, 2005, DEER 2005;

"...CARB especially has been vigorously resisting the conclusions of independent researchers and offering other hypotheses to explain the weekend effect, including a change in the timing of emissions on weekends or carryover of pollution from increased driving on Friday and Saturday evenings.[10] Most of these explanations are not persuasive, or they merely obfuscate the debate. In fact, CARB's views have failed to pass the rigors of scientific peer review. The July 2003 issue of the Journal of the Air & Waste Management Association (JAWMA) devoted a special section to studies of the weekend effect, several of which are cited here. The journal's reviewers rejected CARB's submission.

EPA has similarly resisted the implications of the weekend effect. The technical documentation for EPA's proposed off-road diesel rule, released last May, approvingly cites the CARB paper that was later rejected by JAWMA.[11] Therein lies another irony: when EPA in 1999 promulgated a rule requiring a 90-percent reduction in NOx emissions from automobiles, the agency's own analysis concluded that the rule would increase ozone in many areas of the country.[12]..."


"...While this final rule will reduce ozone levels generally and provide significant ozone-related health benefits, this is not always the case at the local level. Due to the complex photochemistry of ozone production, NOx emissions lead to both the formation and destruction of ozone, depending on the relative quantities of NOx, VOC, and ozone catalysts such as the OH and HO2 radicals. In areas dominated by fresh emissions of NOx, ozone catalysts are removed via the production of nitric acid, which slows the ozone formation rate. Because NOx is generally depleted more rapidly than VOC, this effect is usually short-lived and the emitted NOx can lead to ozone formation later and further downwind. The terms "NOx disbenefits" or "ozone disbenefits" refer to the ozone increases that can result from NOx emission reductions in these localized areas....

...EPA's air quality modeling predicts NOx disbenefits in the areas identified by some studies as "VOC-limited" (e.g., Los Angeles)...." (Page 2-113)

Source: EPA, "Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel Engines." Available at

"...Under these conditions NOx reductions are highly effective in reducing ozone while VOC reductions have little effect. Such conditions are called "NOx-limited"....

...When NOx levels are relatively high and VOC levels are relatively low, NOx forms inorganic nitrates (i.e., particles) but relatively little ozone. Such conditions are called "VOC-limited." Under these conditions, VOC reductions are effective in reducing ozone, but NOx reductions can actually increase local ozone under certain circumstances...." (Page 2-41)

Source: EPA, "Regulatory Impact Analysis: Control of Emissions of Air Pollution from Locomotive Engines and Marine Compression Ignition Engines Less than 30 Liters Per Cylinder." See also "EPA Final Regulatory Impact Analysis: Control of Emissions of Air Pollution from Highway Heavy-Duty Engines.", "U.S. EPA Integrated Science Assessment for Oxides of Nitrogen - Health Criteria (First External Review Draft)." U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-07/093, "EPA Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel Engines.", Federal Register / Vol. 72, No. 63 / Tuesday, April 3, 2007

(Note EPA acknowledges that there are no "VOC disbenefits" in "NOx-limited" areas, but there are "NOx disbenefits" in "VOC-limited" areas)

"...EPA Rule Is Making Ozone Smog Worse Los Angeles NOx and VOC decline, respectively, about 20 percent and 15 percent on Saturdays relative to weekdays. Nevertheless, ozone rises about 20 percent. On Sundays, NOx and VOC decline even more--about 35 percent and 20 percent, respectively, relative to weekdays. Yet ozone levels climb higher still, to about 30 percent above weekday levels.

Los Angeles has one of the worst weekend effects in the nation, but the pattern is similar all over the country. In Atlanta, NOx and VOC decrease, respectively, 57 percent and 17 percent on Sundays relative to weekdays, but ozone levels don't change. In Cincinnati, NOx drops 40 percent on weekends, but with no change in ozone.

This is a problem because EPA and state regulators assume that reducing both VOC and NOx is necessary for attaining the federal eight-hour ozone standard, and they have built that assumption into NOx-reduction regulations that are costing Americans billions of dollars each year. weekend-effect research suggests reducing NOx is at best slowing the pace at which ozone declines, and is even making ozone worse in some cities....

...Most VOC comes from gasoline engines, mainly automobiles..."


The weekend effect has also been documented in Europe, even in suburban and rural locations:

"...Despite the known weekend decrease in emissions, average ozone exposures increased at weekends for rural, suburban and urban sites...."


"...The extremely important implication of this scientific finding (AN11) and findings from other studies is that because of the ozone-destruction effect of NOx under VOC-sensitive conditions and the widespread occurrence of such conditions (e.g., within intensely urbanized areas), air quality managers should view NOx control with caution...."

Source: Southern Oxidant Study (SOS),

"...The findings from this study suggest that timely attainment of O3 standards will require greater emphasis on reducing VOC emissions in major metropolitan areas....

...From a regulatory perspective, the O3 weekend effect is a significant issue even if peak O3 does not change on weekends despite weekend reductions in O3 precursor mixing ratios....

...Ozone isopleth diagrams, whether generated from simple box models or from three-dimensional grid model simulations, have consistently shown that reductions of VOC emissions do not change peak O3 levels if O3 formation is NOx-limited, whereas reductions of NOx emissions typically increase peak O3 levels if O3 formation is VOC-limited (until a transition to a NOx-limited condition has been achieved, after which further reductions of NOx begin to lower peak O3 levels)....

...the transition from NOx-saturated to NOx-limited O3 formation was estimated to occur downwind of the suburban-rural boundary at a distance corresponding to a mean air-mass travel time of approximately 30 min....

...Differences between downwind and upwind peak O3 levels showed no statistically significant variation among days of the week in any of the four study areas, despite large weekend reductions in mean concentrations of O3 precursors....

...We interpret the observed absence of differences between weekday and weekend peak O3, in combination with significantly lower ambient levels of NOx, as an indication that O3 formation in the urban study areas is VOC-limited. Our analyses of weekday/weekend differences in O3 precursors show that different emission reductions of O3 precursors than normally take place on weekends will be required to affect greater reductions in peak O3 in large metropolitan areas...."

Source: Blanchard, Tanenbaum, Lawson; "Differences between Weekday and Weekend Air Pollutant Levels in Atlanta; Baltimore; Chicago; Dallas-Fort Worth; Denver; Houston; New York; Phoenix; Washington, DC; and Surrounding Areas." Journal of the Air & Waste Management Association, Volume 58, December 2008, Pages 1598-1615, (abstract)

"...We recommend that the priorities for the control of VOC sources include motorcycles, gasoline vehicles, and solvent use because of their larger OFP [ozone formation potential] contributions...."

Source: Junyu Zheng, Min Shao, Wenwei Che, Lijun Zhang, Liuju Zhong, Yuanhang Zhang, and David Streets, "Speciated VOC Emission Inventory and Spatial Patterns of Ozone Formation Potential in the Pearl River Delta, China." Environ. Sci. Technol., Oct. 2009, 43 (22), pp 8580-8586, (abstract)

"...A dramatic decrease in primary pollutants emissions (83-98% in NO) causes an 8 ppbv increase in ozone at the urban core. Downwind (27 km), ozone decreases by only 5 ppbv. Nighttime O3 is shown to increase to 20 ppbv at the urban sites and 30 ppbv downwind...."

Source: Ilan Levy, "A national day with near zero emissions and its effect on primary and secondary pollutants." Atmospheric Environment, Volume 77, October 2013, Pages 202-212, (abstract)

Anyone still convinced that diesel NOx is the primary culprit in urban ozone (smog)?

Of course, as suggested in some references above, the weekend ozone effect shouldn't come as that much of a surprise, since the chemistry would predict that increases in ambient HC (VOC) concentrations relative to ambient NOx concentrations would result in increases in ambient O3 concentrations (ground-level ozone - GLO):

GLO production:

The ozone production pathway for CO would be:

CO + OH --> H + CO2   (oxidation by OH)
H + O2 + M --> HO2 + M   (formation of a peroxy radical)
HO2 + NO --> OH + NO2   (rxn with NO to form NO2; regeneration of OH radical)
NO2 --> NO + O   (photodissociation of NO2)
O + O2 + M --> O3 + M   (creation of O3)

For any generic hydrocarbon RH, the ozone production pathway would be:

RH + OH --> R + H2O   (oxidation by OH)
R + O2 + M --> RO2 + M   (formation of a peroxy radical)
RO2 + NO --> RO + NO2   (rxn with NO to form NO2)
NO2 --> NO + O   (photdissociation of NO2)
O + O2 + M --> O3 + M   (creation of O3)

Source: Michael E. Chang, School of Earth & Atmospheric Sciences, Georgia Institute of Technology (personal communications)

So hydrocarbons are responsible for converting NO into NO2 by producing peroxy radicals (RO2./HO2.). Peroxy radicals therefore compete with O3 in the NO into NO2 conversion process. Without the influence of HCs (or CO; CO behaves as a HC in the ozone production pathway), NOx is rather benign with respect to ozone formation, no matter how high the concentration of NOx becomes:

From Air Pollution, It's Origin and Control; Wark, Warner, Davis (Page 476):

[O3] = kI[NO2]/[NO] (I is the intensity of the sunlight.)

Empirical data show that kI = ~25 micrograms/m3 or about 0.015 ppm (15 ppb).

NOx emissions from diesel engines equipped with DPF is approximately 50% NO (Source: Karlsson, "Measurement of Emissions from Four Diesel Fuelled Passenger Cars Meeting Euro 4 Emission Standards." AVL, 2005,

Thus, if [NO2]/[NO] = 1, [O3] = (0.015)(1) = 0.015 ppm or 15 ppb, well below the NAAQS eight-hour standard of 75 ppb (near the natural background (pre-industrial) levels of 10 - 20 ppb).

And, as previously suggested, diesel emissions of HC and CO are very low while the HC and CO emissions from gasoline engines are relatively much higher. Thus, the typical emissions profile of diesel engines (near-zero HC and CO emissions, relatively higher emissions of NOx) could be expected to deplete GLO (i.e., "smog") in urban areas.

Diesels run on biodiesel have even lower HC and CO emissions:

Source: EPA

"... Specifically, dimethyl ether (DME) trucks developed and tested jointly by the National Traffic Safety and Environment Laboratory and Nissan Diesel Motor Co., have exceeded the upcoming 2010 US heavy-duty emissions regulations...."


So why the regulatory emphasis on NOx?

Comparison of latest passenger car emission regs with previous iteration


 Tier 1 (100K miles)Tier 2 (120K miles)Factor Decreased By
NOx1.25 g/mi (diesel)0.20 g/mi (Bin 8)*6.25*
NMHC0.31 g/mi0.125 g/mi (Bin 8)2.48
PM0.1 g/mi0.02 g/mi (Bin 8)5
CO4.2 g/mi4.2 g/mi0


 Tier 1 (100K miles)LEV II (120K miles)Factor Decreased By
NOx0.6 g/mi0.07 g/mi8.6
NMHC0.31 g/mi0.09 g/mi3.4
PM-0.01 g/mi-
CO4.2 g/mi4.2 g/mi0


* - Fleet average must meet Bin 5 limits for NOx (0.05 g/mile @ 50K miles; 0.07 g/mile @ 120K miles, which is equivalent to the LEV II limit), thus the mandated reduction factor for diesels is effectively 17.9.

With the referenced studies suggesting that decreasing NOx relatively more than HCs (or CO) may actually result in higher ground-level ozone levels in urban areas (where ozone is the most problematic and where much of the population is concentrated), the regulatory emphasis on NOx becomes questionable.

Even more perplexing than the emphasis on NOx reductions is the lack of regulatory concern for CO. Even though CO emissions have generally been on a downward trend over the past 20 years (although the latest data from EPA indicate that this trend may be reversing), some areas of the U.S. are still in non-attainment with the CO NAAQS:

"...Over 19 million people currently live in the 11 nonattainment areas for the CO NAAQS...."

Source: (Page 2-122)

"...As Table 1 shows, federal tailpipe emission standards were made substantially more stringent in 1981..."


[essentially NOTHING has been done regulatorily with CO since then down to Bin 5/LEV II.]

On the other hand, NO areas of the U.S. are in non-attainment with the NO2 NAAQS ( and the trend has generally been downward:


So not only are there areas that are in non-attainment with the CO NAAQS, but CO is an ozone precursor. Yet NOx is clearly targeted solely because it is an ozone precursor.

Notice also that the regulated levels of CO are nearly two orders of magnitude (i.e., nearly 100 times) higher than either NOx or HC. This regulatory logic does not seem appropriate, and appears to be a capitualtion to the typical emissions profiles of gasoline engines.

Fortunately, technology is becoming available that may permit light-duty diesels to meet the questionable NOx regulatory limits of Tier 2 and LEV II:

"...DDC developed a first-generation, integrated CIDI engine and emission control system for automobile and light-duty truck applications. This system, applied to a DCX Neon vehicle simulating a vehicle with a 2,250-lb inertia weight, includes CLEAN Combustion technology in conjunction with a first-generation emissions control system. It has achieved Federal Emission Standard Tier II Bin 3 NOx and PM emissions levels over the transient Federal Test Procedure 75 (FTP) cycle. Combined fuel economy (integrating FTP75 and highway fuel economy transient cycle tests) was measured at 63 miles per gallon. ..."

Source: DOE

"...Near Tier 2 Bin 9 Without Any Active NOx Aftertreatment [light truck]...
...Tier 2 Bin 8 Without Active Aftertreatment [passenger car]...
...Summary Tier 2 Bin 3 Emissions Demonstrated for Light Truck / SUV and Passenger Car Platform with Integrated Diesel and Aftertreatment (CSF and Urea SCR) System
Tier 2 Demonstrated for the Light Truck Platform over the US06 Cycle"

Source: Advanced Diesel Engine and Aftertreatment Technology Development for Tier 2 Emissions
R. Aneja, B. Bolton, B. Oladipo, Z. Pavlova-MacKinnon, A. Radwan
Detroit Diesel Corporation

"...Low Emissions - Potential to achieve levels of 2007/2010 Heavy-Duty Engine and Tier 2 emission standards without the use of NOx aftertreatment...."


"...under its Clean Automotive Technology R&D program, EPA is also evaluating unique diesel engine concepts with a goal of identifying a Clean Diesel Combustion concept that could simultaneously be extremely efficient, clean, and cost effective....EPA has now demonstrated lower engine-out nitrogen oxide emissions levels than anything reported in the literature, and this suggests the potential for diesel engine design, using innovative air, fuel, and combustion management and conventional particulate matter trap aftertreatment, that might be able to achieve Tier 2 Bin 5 NOx levels without the need for NOx aftertreatment...."


"...The diesel engine is one of the most promising technologies available today to reduce the environmental footprint of the transportation sector....

...This NOx emissions target is the ultimate level of the HD 2007 standard, and will be required for HD engines sold after 2010. From a light duty perspective, this is approximately equivalent to a diesel SUV or light pick-up truck emitting at the Tier 2-bin 5 level....

...We have run CDC engines at NOx levels that are 50% lower than this to demonstrate the ability to further reduce NOx levels, if there is ever a requirement...."


EPA's CDC Preliminary Results


"...Meta One shows that diesels can potentially meet the strictest emissions standards when combined with modern hybrid and after-treatment technologies....

...the Mercury Meta One concept shows that advanced technologies that we're developing promise the potential to deliver diesels that can be as clean as the cleanest gasoline engines..."


"...Applied in a Silverado full-size pickup truck, the reformer device helped reduce NOx emissions to 0.023 g/mi-well below the Tier 2 Bin 5 target of 0.05 g/mi...."


"...The Bosch 2,000-bar common-rail fueling system going into production in late 2007 has 4.5 times less [PM and NOx] emissions than the 1,600-bar system...."


"...In an effort to meet pollution regulations in the United States, Honda Motor Company is breaking out a plasma reactor to curb emissions in its diesel engines...."


"...These advanced diesel technologies will be integrated into a vehicle capable of meeting the lowest current worldwide exhaust emissions standard (SULEV, Tier 2 Bin 2) without compromising customer appeal, while maintaining significant fuel efficiency (therefore CO2) advantage over current US equivalent gasoline products...."


"...The research project aims to improve diesel engine combustion so that Tier 2 Bin 5 levels can be reached without aftertreatment systems.

Bin 2 levels will be met with the addition of advanced air-management systems, 2-stage turbocharging, advanced exhaust gas recirculation and closed-loop cylinder pressure-based engine controls, the company says...."


"...Nissan Motor is developing an advanced diesel powertrain that includes a new HC-NOx trap for emissions control that may be able to meet California's standard for super-ultra-low emission vehicles (SULEVs)...."


"...Nissan has developed clean diesel technology which meets the state of California's standard for super-ultra-low emission vehicles (SULEVs), as part of the company's ultimate goal of making engine exhaust to the atmospheric level...."


"...Ricardo President Dean Harlow announced that the company has achieved a breakthrough with its low-emissions diesel work and is demonstrating a Tier 2 Bin 2 (SULEV) solution (0.02 g/mi NOx) as well as an optimized Tier 2 Bin 5 solution that requires no NOx aftertreatment...."


Diesels currently aren't even a major source of criteria pollutants according to the lastest data available from EPA (2002 - annual emissions in thousands of tons), in spite of implications to the contrary...

 Total Diesel (r/nr)*Total Anthropogenic (r/nr)*Attributed to Diesel
CO2081 (1016/1065)112,049 (62K/24K)1.86%
NOx6681 (3395/3286)21,102 (7365/4086)31.7%
PM2.5305 (99/206)6803 (149/285)4.48%
VOCs456 (208/248)16,544 (4543/2688)2.76%

* r/nr = "road"/"non-road" mobile sources

Source: EPA

...and are expected to be an even smaller source in 2030 according to EPA projections...

 Total Diesel (r/nr)*Total Anthropogenic (r/nr)*Attributed to Diesel
CO658 (150/508)83,100 (37,549/36,907)0.9%
NOx2278 (261/2017)10,605 (1551/4493)21.5%
PM2.574 (10/64)2065 (67/247)3.6%
VOCs274 (139/248)14,338 (3126/2737)1.9%

* r/nr = "road"/"non-road" mobile sources

Source: U.S. Environmental Protection Agency, Assessment and Standards Division, Office of Transportation and Air Quality, "Regulatory Impact Analysis: Control of Emissions of Air Pollution from Locomotive Engines and Marine Compression Ignition Engines Less than 30 Liters Per Cylinder." Tables 3-96 - 3-100 (Pages 3-108 - 3-112), EPA420-R-08-001a, May 2008

"...Though sometimes perceived as a predominant source of air pollution, diesel engines actually contribute relatively small percentages of the overall emissions inventory, and the trends are generally declining. According to the latest EPA National Emissions Inventory, compared to all other sources, diesel engines as a whole (including on- and off-road, marine, and locomotive applications) account for 1.49% of PM (PM10) emissions, 1.85% of CO, 2.69% of volatile organic compounds (VOCs), 3.13% of sulfur dioxide (SO2), 4.38% of fine PM (PM2.5), and 32.61% of NOx..."

Source: Allen Schaeffer, Diesel Technology Forum

Less than 4.5% of the total PM2.5 burden was attributed to Diesels in the U.S. in 2001, and that may be an overestimate as suggested by the DOE "Plain Talk on Trucks" reference above. That's not the only reference:

"...In the recent Northern Front Range Air Quality Study (NFRAQS) [7] of the Denver, CO area, data obtained from actual vehicle emissions measurements were used to determine the proportion of PM2.5 emissions from various sources. Sophisticated analytical chemistry measurement techniques were used to differentiate PM2.5 emissions from various sources, for example, "smoking" gasoline engines, gasoline engine high emitters, well maintained gasoline engines, different diesel engines, and even cooking on barbecue grills. The data showed that 74 percent of the measured PM2.5 came from gasoline exhaust vs. 26 percent from diesel exhaust.

By comparison, the EPA Mobile 5 model was used to predict PM2.5 emissions inventory apportionment. The model predicted that 22 percent of PM2.5 emissions would come from gasoline exhaust vs. 78 percent from diesel exhaust. This contradicts the results of actual experimental measurements (see Figure 5) which suggests that the model may need some substantial revision to be considered to be reliable...."


"...Source apportionment studies are showing that more PM measurement data are needed to improve accuracy of mobile source inventory model predictions of emission contributions (gasoline cars vs. diesel trucks)....

...Health impacts research results indicate that emissions from high emitting gasoline vehicles even with catalytic converters are as toxic as or more so than emissions from diesel vehicles with no aftertreatment...."

Source: Environmental Science & Health Impacts Research, Dr. James J. Eberhardt, DEER 2004

"...The findings also show that most of the pollution does not come from smokestacks and tailpipes in the developed world. Instead, it comes from industrial emissions in south Asia and forest fires and other burning of vegetation across the planet...."


"...Based on recent emissions inventories, the majority of global BC emissions come from Asia, Latin America, and Africa...."


"... According to conventional wisdom, hydrogen-fueled cars are environmentally friendly because they emit only water vapor -- a naturally abundant atmospheric gas. But leakage of the hydrogen gas that can fuel such cars could cause problems for the upper atmosphere, new research shows...."


"...But diesel fuel is being made cleaner, said David Friedman, research director for clean vehicles for the Cambridge-based Union of Concerned Scientists, and could become 'a good option by the end of the decade.'..."


"...Diesel engines equipped with the latest emission controls emit lower levels of deadly fine particles than gasoline engines, according to various trade groups in the automobile industry.

'Diesel particle filters appear terrific at eliminating virtually all diesel particulate matter emissions,' Clean Air Watch President Frank O'Donnell said Nov. 27.

In the tests by the Association of Emissions Control by Catalyst, diesel engines with emission controls also produced about one-tenth of the carbon monoxide emitted by gasoline engines and less than half of the hydrocarbon emissions...."


Generically, diesel vehicles are not expected to have "well-to-wheel" emissions that are any higher than those of a generic gasoline-hybrid equivalent, and generally less than those of a conventional gasoline vehicle:

Source: Wang, "GREET Life-Cycle Analysis of Vehicle/Fuel Systems." Argonne National Laboratory, (Slides 21-26)

"...Electric cars lead to hidden environmental and health damages and are likely more harmful than gasoline cars and other transportation options...."


"...Electric vehicles and grid-dependent (plug-in) hybrid vehicles showed somewhat higher nonclimate damages than many other technologies for both 2005 and 2030. Operating these vehicles produces few or no emissions, but producing the electricity to power them currently relies heavily on fossil fuels; also, energy used in creating the battery and electric motor adds up to 20 percent to the manufacturing part of life-cycle damages....

...Fully implementing federal rules on diesel fuel emissions, which require vehicles beginning in the model year 2007 to use low-sulfur diesel, is expected to substantially decrease nonclimate damages from diesel..."

Source: National Academy of Sciences, "Hidden Costs of Energy: Unpriced Consequences of Energy Production and Use." Press Release, October 19, 2009,

"...Diesel, which has relatively high damages in 2005, has one of the lowest levels of damage in 2030. This is due to the substantial reductions in both PM and NOx emissions that a 2030 diesel vehicle is required to attain...." (Page 210 (of 506))

"...The grid-dependent electric options have somewhat higher damages and GWP than other technologies, even in our 2030 analysis, in large measure due to the continued conventional and GHG emissions from the existing and likely future grid at least as of 2030...." (Page 219 (of 506))

Source: National Academy of Sciences, "Hidden Cost of Energy: Unpriced Consequences of Energy Production and Use." (2009),

"...The results of a new, comprehensive modeling study characterizing light-duty electric drive vehicle (EDV) deployment in the US over 108 discrete scenarios do not demonstrate a clear and consistent trend toward lower system-wide emissions of CO2, SO2, and NOx as EDV deployment increases....

...the model deploys diesel and diesel hybrids in many scenarios, which may be a cost-effective way to reduce CO2 emissions given their higher efficiency compared to conventional gasoline vehicles...."

Source: Samaneh Babaee, Ajay S. Nagpure, and Joseph F. DeCarolis (2014), "How Much Do Electric Drive Vehicles Matter to Future U.S. Emissions?" Environmental Science & Technology,

"...Regulated emissions of NMHC, CO, and PM were 95, 98, and 89 percent below the EPA 2007 standard...

...Unregulated emissions that included single ring aromatics, PAH, nitroPAH, alkanes, alcohol and organic acids, hopanes/steranes, carbonyls, metals and elements, organic carbon, elemental carbon, and dioxins/furans were 79 to 99 percent lower than the emissions from a similar 2004 technology engine...." (Page 83)

Source: "Advanced Collaborative Emissions Study (ACES) Phase 1 Final Report." June 2009, Coordinating Research Council,

"...Regulated emissions of NOx, NMHC, CO, and PM were 61%, 100%, 97%, and 92% below the 2010 emission standards, respectively....

...Unregulated emissions that included single ring aromatics, PAH, oxyPAH, nitroPAH, alkanes, alcohol and organic acids, hopanes/steranes, carbonyls, metals and elements, organic carbon, elemental carbon, dioxins and furans were 50% to 99% percent lower than those emitted from 2007 technology engines tested in ACES Phase 1...." (Page 69 of 70)

Source: "Phase 2 of the Advanced Collaborative Emissions Study - Final Report." November 2013,

"...The first results of the most comprehensive study ever undertaken of the health effects of exposure to new technology diesel engines has found no evidence of gene-damaging effects in the animals studied, and only a few mild effects on the lungs, according to a report issued today by the Health Effects Institute (HEI)..."


"...Animals were generally healthy and gained weight as expected - Females exposed for 30 months, males for 28 months due to overall lower survival rates - No overt clinical signs of disease other than normal aging..." (Slide #19)

Source: ACES DOE Merit Review June 2013,

"...there is now sufficient evidence to conclude that health effects studies of pre-2007 DE likely have little relevance in assessing the potential health risks of NTDE exposures...."

Source: Hesterberg TW, Long CM, Sax SN, Lapin CA, McClellan RO, Bunn WB, Valberg PA. 2011. "Particulate matter in new technology diesel exhaust (NTDE) is quantitatively and qualitatively very different from that found in traditional diesel exhaust (TDE)." J Air Waste Manage Assoc 61:894-913,

"...A team of researchers in Germany has found that the mutagenicity of diesel engine exhaust (DEE) is eliminated in the gas phase by a Diesel Oxidation Catalyst (DOC)..."

Source: Gotz A. Westphal et al, "Mutagenicity of Diesel Engine Exhaust Is Eliminated in the Gas Phase by an Oxidation Catalyst but Only Slightly Reduced in the Particle Phase." Environmental Science & Technology (2012),

" is clear that using DPFs creates surplus emission reductions that translate directly into additional, quantifiable health benefits enjoyed by all Americans...."


"Study shows significant reductions in real-world heavy-duty diesel emissions from 2010 regulations"

"...Frey and Sandhu found that a truck in compliance with the older 1999 standards emitted 110 grams of NOx per gallon of fuel used, and 0.22 grams of PM per gallon of fuel used. A 2010 truck emitted 2 grams of NOx per gallon of fuel-a decrease of 98%. The PM emissions were 95% lower...."

Source: Sandhu, G.S., and H.C. Frey, "Real-World Measurement and Evaluation of Heavy Duty Truck Duty Cycles, Fuels, and Emission Control Technologies." Transportation Research Record, (in press),

"...According to co-author Dr. Tom Kirchstetter of LBNL, black carbon levels have decreased by about 90 percent over a 45-year period, beginning with the establishment of CARB in 1967, mostly as a result of state regulations for diesel engine emissions....

...The reductions occurred during a time when diesel fuel consumption increased by about a factor of five, attesting to the effectiveness of CARB regulations requiring cleaner fuels and vehicle technology...."


For analyses of other aspects of diesel engine technology, see:

Diesel Efficiency


Diesel Performance

(This page will be updated as new information becomes available)

Updated: 3/25/2014

For an objective comprehensive analysis of the environmental (specifically air quality) impact of gas vs. diesel, please see: ENVIRONMANTAL IMPACT ANALYSIS: GAS VS. DIESEL IN LIGHT-DUTY HIGHWAY APPLICATIONS IN THE U.S.