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On-road vehicle emissions now account for a minor fraction of organic aerosol in Los Angeles
Citation:
Schulze, B., K. Barsanti, H. Pye, P. Wennberg, J. Seinfeld, C. Kenseth, E. Pennington, P. Van Rooy, K. Seltzer, J. Crounse, B. Barletta, S. Meinardi, AND D. Blake. On-road vehicle emissions now account for a minor fraction of organic aerosol in Los Angeles. AAAR Fall Meeting, Albuquerque, New Mexico, October 18 - 22, 2021.
Impact/Purpose:
Regulatory policies aimed at controlling emissions from on-road vehicles have substantially reduced aerosol mass loadings in Los Angeles over the last seventy years. Despite continuing investments in vehicle emissions controls, however, the rate of decline in organic aerosol (OA) mass loadings has slowed in recent years. No individual emission source is predicted to be responsible for a majority of ambient OA. Simulations of future emission scenarios suggest that only minor reductions to the urban OA burden will occur over the next decade, and demonstrate that effectively reducing urban OA will require increased focus on emissions from non-road and area sources.
Description:
Regulatory policies aimed at controlling emissions from on-road vehicles have substantially reduced aerosol mass loadings in Los Angeles over the last seventy years. Despite continuing investments in vehicle emissions controls, however, the rate of decline in organic aerosol (OA) mass loadings has slowed in recent years. Although the increasing relevance of non-traditional sources of urban volatile organic compounds (VOCs) such as volatile chemical products (VCPs) has been advanced as a potential explanation for this observation, the relative importance of individual emission sources to the current urban OA budget remains unconstrained. Here, we compare high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measurements of aerosol mass loading and composition in Pasadena, CA from meteorologically similar periods in 2010 and 2020 to the results of a detailed 0-D box model of OA formation to gain insight into urban OA sources. Positive matrix factorization (PMF) is applied to the OA mass spectral dataset to distinguish mass loadings of urban and regional background OA. The model incorporates recently published inventories of OA precursor emissions in LA County in 2010 and 2020 as well as state-of-the-science parameterizations of emission volatility (e.g., I/SVOCs) and secondary OA (SOA) formation using a volatility basis set framework. Model results reproduce measured OA mass loadings within measurement uncertainty for both years, and suggest that on-road vehicle emissions in 2020 accounted for less than 20% of urban OA. No individual emission source is predicted to be responsible for a majority of ambient OA. Rather, VCPs, cooking-related sources, non-road combustion engines, and asphalt all contribute significantly to urban OA loadings. Simulations of future emission scenarios suggest that only minor reductions to the urban OA burden will occur over the next decade, and demonstrate that effectively reducing urban OA will require increased focus on emissions from non-road and area sources.
