Automotive Sector Emissions
Loren Carl Marz
Certified Consulting Meteorologist #591
ADVANTAGES OF DIESEL TECHNOLOGY IN LIGHT-DUTY AUTOMOTIVE APPLICATIONS
The following are citations from studies recently completed involving diesel emissions and how they compare to emissions from other mobile sources (emphasis added where appropriate):
PARTICULATE MATTER (PM) EMISSIONS
"...Emissions Analytics has revealed that the latest Euro 6d-Temp diesel models emit, on average, 71% less particulates than petrol equivalents...."
"...gasoline cars emitted on average 10 times more carbonaceous PM at 22 degrees C and 62 times more at -7 degrees C compared to diesel cars..."
"...Swiss researchers have concluded that some GDI engines emit just as many soot particles as unfiltered diesel cars did in the past...
...The results were sobering: every tested gasoline car emitted ten to 100 times more fine soot particles than the diesel Peugeot. Under the microscope, the particles from the gasoline engines were similar in size to the soot particles that had given diesel a bad name: primary particles measuring ten to 20 nanometers in size, which congregate into particle agglomerates measuring 80 to 100 nanometers before leaving the exhaust...."
"...The average PN emitted during 180 seconds by GDI and PFI vehicles were 3.09E+13 and 2.12E+13 particles respectively. Comparing to 2017 Euro 6 New European Driving Cycle (NEDC) limit on cumulative particles emitted over the entire test cycle, most PFI and GDI vehicles exceeded this limit in 6-12 seconds after a cold-cold start. In addition, EPA Tier 3 particle mass requirements are exceeded for tested GDI vehicles due to their characteristically high concentration of accumulation mode particles. In comparison, diesel vehicles with DPFs were the cleanest, with particle concentrations close to background levels....
...The average total PN emissions from these two Diesel vehicles was 6E+11 particles, nearly 20 times lower than the PFI and GDI vehicles (1E+13 particles)....
...PN was found to exceed the total PN limit for Euro 6 test standards (w/ NEDC drive cycle) for all the tested vehicles except for one PFI and the two Diesel w/DPF vehicles. Most GDI and PFI vehicles emitted E+13 particles, with the lowest PFI at 2.2E+12 and the highest GDI at 1.2E+14. In comparison, Diesel w/DPF vehicles emitted only 6E+11 particles...."
Source: Badshah, H., Kittelson, D., and Northrop, W., "Particle Emissions from Light-Duty Vehicles during Cold-Cold Start," SAE Int. J. Engines 9(3):2016, doi:10.4271/2016-01-0997, http://papers.sae.org/2016-01-0997/ (Abstract)
"...Particle concentration from DPF equipped diesel engines were found to be the lowest, while GDI and 8-cylinder PFI engines had the highest particle emissions....
...A PN [particle number] emissions index concept was developed to rank post 2010 model-year vehicles tested relative to a 4-cylinder diesel with DPF, used as the best available technology for low solid particle emissions. GDI engines had the highest PN indices, with up to a factor of 8000 higher for particles larger than 25 nm (Dp > 25 nm) and up to 900 times higher for particles smaller than 25 nm (DP < 25 nm)...."
Source: Badshah, H. and Khalek, I., "Solid Particle Emissions from Vehicle Exhaust during Engine Start-Up," SAE Int. J. Engines 8(4):2015, doi:10.4271/2015-01-1077, http://papers.sae.org/2015-01-1077/ (Abstract)
"...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." http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OAR-2011-0135-5096
"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, http://www.greencarcongress.com/2013/12/20131224-bolzaker.html
"...In general, PM emissions are on the order of 0.01mg/km +/- 0.005mg/km (+/- 1 sigma), thereby nearly 100% (99.89%) below the US-EPA Tier2-Bin5 standard...
...even during DPF regeneration events PM emissions levels remain ~27% below the regulatory standard..."
Source: Thompson et al., "In-Use Emissions Testing of Light-Duty Diesel Vehicles in the United States." International Council on Clean Transportation Report, May 15, 2014, http://www.theicct.org/sites/default/files/publications/WVU_LDDV_in-use_ICCT_Report_Final_may2014.pdf, Page 83
"...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." http://www.arb.ca.gov/regact/2012/leviiighg2012/levappp.pdf, page P64
"...Over the FTP-75 phase 1 (cold-start) drive cycle, the BC mass emissions from the two GDI vehicles at 0 oF (-18 oC) varied from 57 to 143 mg/mi...For the two PFI vehicles, the BC mass emissions over the FTP-75 phase 1 drive cycle at 0 oF varied from 111 to 162 mg/mi..."
Source: Tak W. Chan et al. (2014) "Black Carbon Emissions in Gasoline Exhaust and a Reduction Alternative with a Gasoline Particulate Filter." Environmental Science & Technology 48 (10), 6027-6034, http://pubs.acs.org/doi/abs/10.1021/es501791b (Abstract)
"...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, http://pubs.acs.org/doi/abs/10.1021/es2033845?mi=vb6ex5&af=R&pageSize=20&searchText=nanoparticle (Abstract)
"...Although GDI engines emit lower levels of CO2, they emit more black carbon -the climate-warming particle commonly known as soot- and toxic volatile organic compounds, such as benzene and toluene..."
"...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...."
"...Due to the overlapping filtration mechanisms, both large as well as small particles can be held back reliably, thus achieving a filtering efficiency of nearly 100% across the entire spectrum of sizes...
...Since the diffusion speed increases with decreasing particle size, smaller particles are actually separated the most effectively...." [Page 8 of 18]
Source: Fiebig et al., "Particulate emissions from diesel engines: correlation between engine technology and emissions." Journal of Occupational Medicine and Toxicology, 2014, 9:6, http://www.occup-med.com/content/pdf/1745-6673-9-6.pdf
"...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
"...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 [6X10E+11 /km], and thereby will force the use of GPF [particulate filters] on gasoline vehicles...."
Source: PRELIMINARY DISCUSSION PAPER - PROPOSED AMENDMENTS TO CALIFORNIA'S LOW-EMISSION VEHICLE REGULATIONS - PARTICULATE MATTER MASS, ULTRAFINE SOLID PARTICLE NUMBER, AND BLACK CARBON EMISSIONS, CARB, http://www.arb.ca.gov/msprog/levprog/leviii/meetings/051810/pm_disc_paper-v6.pdf (page 10)
"...The particle number emission from DPF-equipped vehicles are in the range 10E+8 - 10E+10 /km and non DPF equipped vehicles are in the range 10E+13 - 10E+14 /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
"...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 (https://www.epa.gov/criteria-air-pollutants/naaqs-table).
"...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 PM
2.5 mass. By definition, soot would be identified as carbon (probably elemental but perhaps organic) in bulk analyses of PM2.5 samples...."
"...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: http://www.aecc.be/en/content/pdf/AECC%20Newsletter%20March-April%202007.pdf, "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, http://www.arb.ca.gov/research/veh-emissions/pmp-ld/CARB_Golden_Vehicle_PMP_Report_Final-05JAN09.pdf
"...Multiple internal and external testing programs and studies all show that properly functioning PM filters effectively eliminate PM emissions from diesel truck exhaust...."
Source: State Of California, California Environmental Protection Agency, Air Resources Board, "Evaluation of Particulate Matter Filters in On-Road Heavy-Duty Diesel Vehicle Applications." http://www.arb.ca.gov/msprog/onrdiesel/documents/DPFEval.pdf, May 2015, Page 10
"...~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..."
"...With particulate filters, what comes out of the tailpipe (exhaust gas) typically contains fewer particles than the air entering the vehicle's engine -- the particulate filter literally acts as a cleaner...."
"...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, http://www.youtube.com/watch?v=Wm3i6YQOKKc, @3:32-3:58 mark
"...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
"...we conclude with a high degree of reliability that it is misleading to claim that people's exposure to diesel engines of road motor vehicles is the cause of increased risk of lung cancer. Eighty three per cent of particulate matters emissions in European Union countries (EEA, 2012a) and 97 per cent in the United States of America (EPA 2013) and Canada is generated by other economic sectors, mainly the commercial, institutional and household sector. Therefore, the claim that emissions from diesel engine exhausts from road transport are the main cause of lung cancer in humans needs to be seriously challenged...."
"...Gasoline vehicle exhaust is enriched in particulate PAH as compared with diesel exhaust...."
Source: 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;"DOE's Gasoline/Diesel PM Split Study." http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2003/session9/2003_deer_fujita.pdf (Slide #15)
"...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, http://pubs.healtheffects.org/getfile.php?u=893 (Page 21)
"...The World Health Organization (WHO) considers even the tiniest dose of benzo(a)pyrene harmful. The EU settled on an air limit of one nanogram per cubic meter. Levels in exhaust emissions were found to be as much as 1,700 times above this limit. Or to put it another way, one cubic meter of exhaust gas transforms up to 1,700 cubic meters of clean air into a mixture deemed carcinogenic according to the EU standard.
Once again, the diesel vehicle with particle filter fared much better: in the test, the Peugeot emitted only 45 nanograms of carcinogenic substances - 6 times less than the best one of the analyzed gasoline cars...."
"...Even with the GPF, the PAH concentrations were several times higher in the GDI vehicles than in diesel...."
Source: 21st ETH Conference on Combustion Generated Nanoparticles, https://www.dieselnet.com/news/2017/07eth.php (Empa, M.Munoz)
"...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 3.9x10E+14 to 1.0x10E+17 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 1.3x10E+16 and 7.1x10E+15 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
"...particle pollution from UK wood burning is now estimated to be more than double diesel exhaust..."
"...New low-emission [wood-burning] stoves cause much higher emissions (above 500,000 part/cm3) than new trucks with particulate filters (below 1,000 part/cm3)..."
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, http://www.ncsu.edu/sos/v.html
"...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....
...one 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, http://cires.colorado.edu/~jjose/Papers/2006GL026899_published.pdf
"...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, http://zardoz.nilu.no/~andreas/publications/137.pdf
"...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), http://www.agu.org/news/press/pr_archives/2012/2012-14.shtml (http://www.greencarcongress.com/2012/03/bahreini-20120303.html
"...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, http://www.atmos-chem-phys-discuss.net/12/31725/2012/acpd-12-31725-2012-print.pdf
"...the emissions from some vehicles generating as much as 6 times the amount of SOA as primary particulate matter after three hours of oxidation inside the chamber at typical atmospheric oxidant levels....
...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. http://www.researchgate.net/publication/260830202_Secondary_organic_aerosol_formation_exceeds_primary_particulate_matter_emissions_for_light-duty_gasoline_vehicles (Absract)
"...Speciated emissions data indicate that secondary organic aerosol produced from gasoline vehicle exhaust may exceed primary PM emissions...."
Source: May et al., "Gas- and particle-phase primary emissions from in-use, on-road gasoline and diesel vehicles." Atmospheric Environment, Volume 88, May 2014, Pages 247-260, http://www.sciencedirect.com/science/article/pii/S1352231014000715 (Abstract)
"...Our results clearly demonstrate that IVOCs from gasoline engines are an important class of SOA precursors..."
Source: Yunliang Zhao et al. (2016), "Intermediate Volatility Organic Compound Emissions from On-Road Gasoline Vehicles and Small Off-Road Gasoline Engines" Environmental Science & Technology, http://pubs.acs.org/doi/abs/10.1021/acs.est.5b06247 (Abstract)
"...Secondary organic aerosol (SOA) formation in the environment is often overlooked when discussing ambient particle loadings...while road transport is the principal source of primary particles, emission levels of ambient aerosol precursor compounds-such as NH3, SO2, NOx and NMVOC-that are important contributors to total ambient particle loadings far exceed those of primary particles...
...diesel exhaust is quite passive, producing very little secondary organic aerosols..."
Source: 18th ETH Conference on Combustion Generated Nanoparticles, June 2014, http://www.dieselnet.com/news/2014/07eth.php
"...catalyzed aftertreatment, especially DOC + DPF, is extremely effective at eliminating SOA from diesel combustion emissions. Gordon et al. found nearly zero SOA production from emissions from two different DOC + DPF-equipped HDDV operated over an urban driving cycle. In addition, SOA from the emissions of a single LDDV were dramatically reduced from 461 to 19 mg kg-fuel-1 after a DOC was installed..."
Source: Gentner et al. "Review of Urban Secondary Organic Aerosol Formation from Gasoline and Diesel Motor Vehicle Emissions." Environ. Sci. Technol., 2017, 51 (3), pp 1074-1093, http://nature.berkeley.edu/ahg/pubs/Gentner_etal_EST_2017.pdf
"...changing the NMOG:NOx [ratio] from 4 to 10 ppbC/ppbNOx increased the SOA yield from dilute gasoline vehicle exhaust by a factor of 8. We investigated the implications of this relationship for the Los Angeles area. Although organic gas emissions from gasoline vehicles in Los Angeles are expected to fall by almost 80% over the next two decades, we predict no reduction in SOA production from these emissions due to the effects of rising NMOG:NOx on SOA yields...."
Source: Yunliang Zhao et al. "Reducing secondary organic aerosol formation from gasoline vehicle exhaust," Proceedings of the National Academy of Sciences, May 2017, http://www.pnas.org/content/early/2017/06/13/1620911114.abstract (Abstract)
"...emissions from new gasoline cars (both EU and US) produce up to 6.5 times more SOA than POA after 5-10 hours of atmospheric aging. New diesel cars produced no detectable SOA, contrasting with old diesels, for which SOA production is about equal to emitted POA...."
Source: Platt et al. "Gasoline cars produce more carbonaceous particulate matter than modern filter-equipped diesel cars." Scientific Reports 7, Article number: 4926 (2017), https://www.nature.com/articles/s41598-017-03714-9
"...Gasoline sources were predicted to contribute about 13 times more OA than diesel sources; this difference was driven by differences in SOA production...."
"...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 (http://health.heraldtribune.com/2012/02/20/new-studies-of-air-pollution-reveal-a-hidden-danger-to-hearts-and-lungs/)