Reducing secondary organic aerosol formation from gasoline vehicle exhaust
Monday, 2017/07/10 | 08:00:04
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Yunliang Zhao, Rawad Saleh, Georges Saliba, Albert A. Presto, Timothy D. Gordon, Greg T. Drozd, Allen H. Goldstein, Neil M. Donahue, and Allen L. Robinson ENVIROMENTAL SCIENCES
Figure 1: Emissions and SOA production data from photooxidation experiments with dilute gasoline vehicle exhaust for different vehicle classes: (A) NMOG emissions, (B) end-of-experiment SOA production, and (C) effective SOA yields. The boxes represent the 75th and 25th percentiles of the data from individual vehicle tests with the centerline being the median. The whiskers are the 90th and 10th percentiles. The SOA production from SULEV was comparable to that measured during dynamic blank experiments, indicated by the dashed line in B. As discussed in the text, the SOA yields for SULEV vehicles were not estimated due to the large uncertainty in SOA production. This figure combines data from 14 newly tested vehicles (1 pre-LEV, 3 LEV, 3 ULEV, and 7 SULEV vehicles) with previously published data for 11 additional vehicles (3 pre-LEV, 3 LEV, and 5 ULEV vehicles) from Gordon et al. (5). SignificanceSecondary organic aerosol (SOA) is a major component of atmospheric fine particles, which pose serious health risks and influence Earth’s climate. We combine laboratory measurements and computational modeling to investigate SOA formation from gasoline vehicle exhaust—an important source of air pollution in urban environments. We find a strong dependence of SOA formation from gasoline vehicle exhaust on oxides of nitrogen (NOx) concentrations. Our results suggest that changing atmospheric NOx levels over the next two decades will likely dramatically reduce the effectiveness of stricter new gasoline vehicle emissions standards to lower SOA concentrations in Los Angeles and other urban areas. AbstractOn-road gasoline vehicles are a major source of secondary organic aerosol (SOA) in urban areas. We investigated SOA formation by oxidizing dilute, ambient-level exhaust concentrations from a fleet of on-road gasoline vehicles in a smog chamber. We measured less SOA formation from newer vehicles meeting more stringent emissions standards. This suggests that the natural replacement of older vehicles with newer ones that meet more stringent emissions standards should reduce SOA levels in urban environments. However, SOA production depends on both precursor concentrations (emissions) and atmospheric chemistry (SOA yields). We found a strongly nonlinear relationship between SOA formation and the ratio of nonmethane organic gas to oxides of nitrogen (NOx) (NMOG:NOx), which affects the fate of peroxy radicals. For example, changing the NMOG:NOx 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. This highlights the importance of integrated emission control policies for NOx and organic gases.
See: http://www.pnas.org/content/114/27/6984.abstract.html?etoc PNAS July 3 2017; vol.144; no.27: 6984–6989 |
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