It appears that the California regulatory agencies are using diesels as a scapegoat for their air quality problems. They've ARBITRARILY classified diesel particulate matter (DPM) as a "toxic air contaminant" and a carcinogen while giving all other sources of combustion PM a free pass. Even EPA hasn't classified DPM as carcinogenic saying that evidence of it's carcinogenicity is weak to moderate and that even a zero risk couldn't be ruled out based on a plethora of studies (Hazard Assessment Document for Diesel Engine Exhaust).
Gasoline PM hasn't undergone this level of scrutiny. However, gasoline PM has been shown to have higher levels of high molecular weight polynuclear aromatic hydrocarbons (PAH), many of which are carcinogens (e. g. , benzo[ghi]perylene). Gasoline PM has also been shown to be more toxic than DPM. CNG bus PM is more than 7 times more mutagenic than DPM. Yet these sources are NOT listed as toxic air contaminants/carcinogens.
Furthermore, gasoline exhaust and gasoline vapors are responsible for formation of secondary PM (secondary organic aerosols - SOA - PM produce by gaseous precursors in the atmosphere). Consider this:
Per Southern California Air Quality Management District's (SCAQMD) own data, ambient PM2. 5 = 21. 6 micrograms/cubic meter (ug/m3) in 2002-2003 in LA, 35% of that was organic carbon (OC), ~5% was EC (
http://arb.ca.gov/pm/pmmeasures/pmch05/southcoast05.doc; figure O-4); at least 50% of the OC is SOA in SoCAB (John H. Seinfeld, Prasad Pai, and David Allen, “AEROSOL-FORMING POTENTIAL OF ATMOSPHERIC ORGANIC COMPOUNDS”.
http://www.narsto.org/files/files/AssessAerosol.pdf; Shaocai Yu, Robin L. Dennis, Prakash V. Bhave and Brian K. Eder, “Primary and secondary organic aerosols over the United States: estimates on the basis of observed organic carbon (OC) and elemental carbon (EC), and air quality modeled primary OC/EC ratios”.
Atmospheric Environment, Volume 38, Issue 31, October 2004, Pages 5257-5268; average ambient DPM in LA is 1. 5 ug/m3; (
http://www.gvrd.bc.ca/air/pdfs/DieselParticulateMatterStudy.pdf, page 4):
21. 6 ug/m3 X 0. 35 (35% OC) = 7. 56 ug/m3 OC
7. 56 ug/m3 X 0. 5 (~50% of OC is SOA in SoCAB) = 3. 78 ug/m3 SOA
~76% of ambient NMHC in SoCAB is from gasoline exhaust and gasoline vapors (
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2002/session5/2002_deer_fujita.pdf, Steven G. Brown, Anna Frankel and Hilary R. Hafner; “Source apportionment of VOCs in the Los Angeles area using positive matrix factorization”.
Atmospheric Environment, Volume 41, Issue 2, January 2007, Pages 227-237).
3. 78 ug/m3 X 0. 755 (75. 5%) = 2. 85 ug/m3
Therefore, it is likely that just the SOA from gasoline NMHC accounts for more ambient PM2. 5 mass than DPM in SoCAB (almost twice as much); PLUS, a majority of primary PM emissions from gasoline vehicles is OC.
SOA just from evaporative emissions from gasoline storage/distribution:
3. 78 ug/m3 X 0. 325 (~33% of NMHC are evaporative emissions per Brown, et al. ) = 1. 23 ug/m3; i. e. , nearly as much as the calculated ambient DPM (1. 5 ug/m3).
I think that gasoline vehicles are primarily responsible for the air quality problems in Southern California (smog, ambient PM), but cracking down on those wouldn't be politically popular, would it?
