According to a paper published by CARB staff in 2015 (Propper et al., "Ambient and Emission Trends of Toxic Air Contaminants in California." Environ. Sci. Technol. 2015, 49, 11329-11339, https://pubs.acs.org/doi/10.1021/acs.est.5b02766), ambient diesel particulate matter (DPM) concentration in the Southern California Air Basis (SoCAB) was <0.6 µg/m3 in 2012 (Figure 2). According to EPA, the average PM2.5 concentration in 2012 was 15.6 µg/m3 @SoCAB. That would result in DPM accounting for 3.8% of particulate matter at most. The trajectory is down from there and continues to decrease in relative importance (EPA estimates that DPM will contribute ~0.2 µg/m3 in the Western U.S. by 2025). Even in 1998, i.e., long before DPF was in widespread use in diesel vehicles, the "Northern Front Range Air Quality Study" showed that light-duty gasoline vehicles contributed ~60% of PM2.5 carbon at U.S. urban sites tested, and that was 2.5 to 3 times the contribution of diesel exhaust (Page 10-3 of full report). According to a European (UK) report (https://uk-air.defra.gov.uk/assets/documents/reports/cat09/1907101151_20190709_Non_Exhaust_Emissions_typeset_Final.pdf), non-exhaust emissions are a much larger component of vehicular PM10 and PM2.5 emissions than the exhaust. According to Figure 2 and Figure 3 on page 25, exhaust PM emissions will continue to become a smaller and smaller portion of traffic-generated PM10 and PM2.5. So even in Europe it doesn't appear DPM is major component of ambient PM2.5. I agree that there are likely differences in the toxicity of particles. However, GDI petrol cars emit relatively higher levels of PM emissions, and those emissions are ENRICHED in high-molecular-weight PAHs relative to diesel PM. Some of those PAHs are carcinogenic. See https://enrg.io/gasolne-engines-emit-particulates-diesels/ for example. Where's the outrage about those PM emissions?

"...Here, we investigate the mechanisms by which diesel exhaust, a major component of air pollution, induces neurotoxicity...." Except diesel exhaust PM is a very MINOR component of ambient PM2.5. According to CARB's state emission inventory for 2020, diesel exhaust (on- AND off-road) contributed 21.5 tons/day of PM2.5 in California out of a total of 788.3 tons/day from all sources. 21.5/788.3 = 2.7%. What about the other 97.3% of PM2.5 from non-diesel sources? Is PM2.5 from other sources innocuous? Researchers need to start looking for a new villain for a pollution source other than diesel. Some modern diesels have been shown to actually remove particles from ambient air, even relatively clean air in some scenarios (https://www.auto-motor-und-sport.de/tech-zukunft/dieselabgase-partikelmessungen-im-realbetrieb/).

According to a companion study (https://www.emissionsanalytics.com/news/2020/1/8/burning-issue-tyres-and-air-quality), most of the tire PM mass is >PM2.5, and even >PM10. However, there are still relatively high PN emissions in the nano-particle size range.

According to the linked T&E report - "...Combining the per km impact of regeneration on regulatory PN emissions with regulatory PN results on non-regenerating tests did not result in an exceedance of the 6 x10^11/km PN limit. This is also the case when the same calculation is performed for particles larger than 10 and 4nm...." (Page 28) In fact, both vehicles tested easily met the 6X10^11/km standard (<2X10^11/km), even accounting for the more frequent active regenerations claimed in the report and using the smaller particle sizes. There's nothing in the regulation stating that the 6X10^11/km standard is a "not-to-exceed" one.

Why are "diesel engine fumes" being blamed as the source of BC? Another study conducted between 2011 and 2013, i.e., before widespread use of DPF in Europe, showed that the primary source of BC in London is residential wood smoke (Bohnenstengel et al. “Meteorology, Air Quality, and Health in London.” Bulletin of the American Meteorological Society, May 2015, 779-804). The latest diesel vehicles have DPFs that are so effective that they're actually filtering not only dirty urban air, but even relatively clean air (https://www.auto-motor-und-sport.de/tech-zukunft/werkstatt/dieselabgase-partikelmessungen-im-realbetrieb/). BC emissions from these vehicles would clearly be negative!

It should be noted that sufficient concentration of particles is essential for precipitation to occur. Particles act as cloud condensation nuclei. Recent research has found that precipitation efficiency peaks at particle concentrations well above natural background levels.

In addition to the disbenefits of a regulatory strategy that reduces NOx emissions relatively more than VOC/HC emissions, as seen in this study and ozone weekend effect studies, reducing ambient NOx levels relatively more than ambient VOC levels results in more secondary organic aerosol (SOA) formation, based on another recent study - https://www.greencarcongress.com/2017/06/20170622-robinson.html. It's becoming increasingly clear that the regulatory focus on NOx emissions almost exclusively couldn't be more misguided.

This study is obsolete because modern diesels with SCR generally have LOWER NO2 emissions than unfiltered diesels in spite of much higher NO2:NOx ratios from DPF: https://ww3.arb.ca.gov/research/seminars/kirchstetter/kirchstetter.htm#CARBscience

Focusing exclusively on NOx emissions is an incomplete way of assessing exhaust emissions impact on air quality. Lower NOx emissions at the expense of higher emissions of other criteria pollutants is potentially disbeneficial.

A couple of comments about sources of air pollution. Diesel engines are usually the default villain as the source of air pollution. However: - Modern diesel vehicles (i.e., w/DPF) have far fewer PM emission than petrol, both in terms of mass and number - https://www.fleetnews.co.uk/news/environment/2018/06/22/new-diesels-found-to-be-71-cleaner-than-petrol-models - A source apportionment study in London found that PM emissions from residential wood burning was a significantly larger source of ambient PM than diesel vehicles - Bohnenstengel et al., “Meteorology, Air Quality, and Health in London,” Bulletin of the American Meteorological Society, May 2015. - A study published this month concludes that coal-fired electricity generation units are the largest source of ambient ultrafine particles - https://www.sciencedaily.com/releases/2019/03/190312075916.htm

What is the definition of "green car"? Has one been formally established?

Agree with Peter. If you look closely at the recent report from the European Environmental Agency ("Electric vehicles from life cycle and circular economy perspectives; TERM 2018: Transport and Environment Reporting Mechanism (TERM) report," EEA Report No 13/2018 - https://www.greencarcongress.com/2018/11/201081126-eea.html), the results are at best mixed. The GHG emissions and (maybe) local emissions are generally lower for BEV, but human toxicity and ecotoxicity are MUCH higher, mainly from the production phase for the battery. Diesel ICEV generically is not only much lower than BEV in these parameters, it's lower than gasoline ICEV ( see Figures 3.4, 4.2, 6.2, and 6.3).

For what it's worth, Fulcrum claims that they can produce jet/diesel fuel for "less than $1.00/gallon" - http://fulcrum-bioenergy.com/benefits/low-cost-producer/ The cost of ULSD pre-tax is about $2.20/gallon per EIA.

@E-P, The harvesting of forest biomass in lieu of, or in addition to, prescribed burning was just a thought. It's certainly possible that the cost of collecting the biomass may be too high. I'm not clear about your position on biofuels however. You seem to be adamantly opposed to large-scale biofuel implementation here, yet you seem to support it in a comment you made earlier this year: Only replacing 30% of the petroleum used in the USA is setting the sights far too low. A billion dry tons of biomass can make at least 800 million tons of methanol (probably more), which comes to over 260 billion gallons. Total US gasoline consumption, which is about 45% of all US petroleum consumed, is only about 140 billion GPY. Between PHEVs and methanol you could replace 100% of motor fuel and 70% or more of total petroleum.... https://www.greencarcongress.com/2018/01/20180124-biomass.html Can you clarify your position in this regard?

When you have so much money tied up in the assertion that they are correct, the burden of proof should be on those who benefit, not on the skeptics. The same can be said of any technologies or fuels that are outside the "business-as-usual" scenario of primarily continued fossil fuel use for transportation. We likely don't even have full understanding of future ramifications of fossil fuels as they increasingly come from harder-to-process feedstocks like tar sands, for example. I was directly involved with federal and state forestry services' "prescribed burn" programs (controlled burning), mostly in the "smoke management" realm. When I was involved, over 20 million areas of biomass were intentionally burned each year for wildfire management purposes. That of course results in considerable emissions of GHG, PM, NOx, and many other pollutants, completely uncontrolled. That doesn't include the tens of millions of acres burned each year from unintentional wildfires. I always though harvesting at least some of that biomass for fuel production should at least be considered. It may be far too expensive, or have other obstacles, but I personally think it should be looked at. The fuel loadings of forests need to be further reduced, IMHO.

I completely agree. But by the same token, the assertion that most biofuels have negligible benefits is also based on assumptions which could be way off, isn't it? Argonne National Laboratory has an extensive list of publications for data used in GREET (https://greet.es.anl.gov/list.php). Since GREET is updated yearly, the model incorporates data from the very latest research available.

That may be true, but the GREET model is supposed to take all of those emission sources into account, at least as well as they're currently understood. GREET has dozens of tabs within the model that address ancillary emissions from each feeedstock source. It even includes emissions generated from production of agriculture machinery.

What other data do you have regarding GHG emissions from biofuels E-P? According to the latest version of Argonne National Laboratory's GREET model (https://www.greencarcongress.com/2018/10/20181012-greet.html), FT diesel from biomass has 88% lower WTW GHG emissions than fossil diesel (39 g/mile (FTD) vs. 320 g/mile ULSD)). That isn't really negligible, is it?

I disagree that most biofuels have negligible net benefits. The average carbon intensity of the approved renewable diesel fuels in California's LCFS is 30 g CO2e/MJ, which is over 70% reduction in WTW CO2 emissions. https://www.greencarcongress.com/2018/11/20181115-rd.html

I have too, but they're all older-generation diesel vehicles which do not have DPF after-treatment technology. Why is diesel singled out from the gas version of this vehicles (a 3.6 liter gas engine will also be available according to the article)? Consider the following: "...Emissions Analytics has revealed that the latest Euro 6d-Temp diesel models emit, on average, 71% less particulates than petrol equivalents...." Source: https://www.fleetnews.co.uk/news/environment/2018/06/22/new-diesels-found-to-be-71-cleaner-than-petrol-models "...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..." Source: http://nouvelles.umontreal.ca/en/article/2017/07/13/diesel-is-now-better-than-gas/ "...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....." Source: https://www.empa.ch/web/s604/soot-particles-from-gdi

Diesels in the U.S are neither polluting nor dirty. Every light-duty diesel vehicle line certified in the U.S. is subject to months of extensive additional emissions testing beyond the official certification test procedures, and has since the NOV was issued to VW in September 2015. The additional testing is conducted to ensure the vehicles' certified emissions are representative in all "driving cycles and conditions that may reasonably be expected to be encountered in normal operation and use." Even in Europe, diesels certified to Euro 6d-temp ('type-approval"), all 270 of them, meet the regulatory limit for NOx emissions: https://www.acea.be/uploads/press_releases_files/RDE-compliant_diesels_November_2018.pdf

I believe EPA estimates that ~3 billion gallons of waste cooking oil are currently produced by restaurants per year in the U.S. That's still a rather limited supply for renewable diesel though.

Large wind turbines can promote surface warming by disrupting surface-based nocturnal inversions which typically form over land surfaces (absent significant synoptic feature). Instead the lower levels of the atmosphere remain mixed and surface temperatures don't cool as much at night.

But again, from an air quality perspective, lower NOx emissions at the expense of higher HC, CO, PM/PN, and NH3 emissions is potentially counter-productive. Comparing the CARB certified emissions of the ROUSH "near-zero" propane engine and a competitor diesel engine (Detroit Diesel DD8)... ROUSH - 0.04 g/hp-hr (NMHC); 0.01 g/hp-hr (NOx); 5.0 g/hp-hr (CO); 0.002 g/hp-hr (PM) DD8 - 0.000 g/hp-hr (NMHC); 0.05 g/hp-hr (NOx); 0.4 g/hp-hr (CO); 0.000 g/hp-hr (PM). https://www.arb.ca.gov/msprog/onroad/cert/mdehdehdv/2018/roush_hdoe_a3440086_6d8_0d02_lpg.pdf (ROUSH) https://www.arb.ca.gov/msprog/onroad/cert/mdehdehdv/2018/detroitdiesel_mhdd_a2900163_5d1-7d7_0d20-0d01.pdf (Detroit Diesel) The ROUSH engine also has 18% higher GHG emissions than the DD8 Diesel per cert.

From an air quality perspective, lower NOx emissions at the expense of higher HC, CO, PM/PN, and NH3 emissions is a dubious proposal, and even potentially counter-productive.