10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Secondary Organic Aerosol Formation from Urban Sources: Current Understanding and New Results from WINTER in the NE US
JOSE-LUIS JIMENEZ, Jason Schroder, Patrick Hayes, Joost de Gouw, Viral Shah, Lyatt Jaegle, Pedro Campuzano-Jost, Douglas Day, Benjamin A. Nault, University of Colorado-Boulder
Abstract Number: 1018 Working Group: Air Quality in Megacities: from Sources to Control
Abstract Secondary organic aerosols (SOA) are particle-phase species formed from chemical reactions of gaseous organic compounds. SOA formed from urban sources (i.e. from emissions within urban areas) is a substantial contributor to particle concentrations in polluted regions, with important impacts on human health, visibility, and climate. In this presentation we will summarize recent progress on the understanding of SOA formation from urban sources, and to point towards the areas that require further research. We will also present results from the WINTER 2015 field study in the NE US, which was one of the first studies to characterize urban SOA with an advanced aircraft payload in winter.
The amount of SOA formed from urban sources can be quantitatively characterized and compared across different locations using techniques that normalize for the effects of changing emissions, dispersion, and photochemical age, such as the ratio of excess OA to excess CO (above their respective background levels), or the regression slope of SOA to Ox (O3 + NO2). Results from many developed world megacities (e.g. Los Angeles, Paris, Tokyo, Mexico City, NE US) are quite similar, highlighting the global importance of SOA from urban sources. A timescale of 1 day or less for formation of urban SOA is consistently observed. Results for smaller urban areas can be ambiguous if the signal-to-background ratios are small. Three-dimensional models should be evaluated using these same techniques, as if their results were experimental data. Merely comparing measured to modeled SOA concentrations can lead to very erroneous conclusions due to differences in emissions, dispersion, and photochemical aging (e.g. Woody et al., ACP, 2016).
SOA formation in WINTER was consistent with summer studies, once differences in OH are accounted for. SOA formed from anthropogenic volatile organic compounds (VOCs) emitted in urban areas is important, while SOA formed from biogenic VOCs emitted within the urban area is typically a very small contributor. However, biogenic SOA advected into urban areas from sources outside of it can be an important and even dominant contributor to urban SOA concentrations. Pre-2007 models of urban SOA are low by about an order-of-magnitude. This huge urban SOA underestimation still affects recent versions of widely-used models, e.g. CMAQ v.5.0.2 and the non-volatile POA version of GEOS-Chem v10. Vapor wall losses in Teflon chamber SOA experiments affected older SOA models, and contributed to this underprediction. The most commonly used Volatility Basis Set (VBS)-type models produce too much SOA and do so much too slowly, compared with observations. There is strong evidence that urban VOCs and semivolatile and intermediate volatility species (S/IVOCs) are both important as SOA precursors. Updated models still predict SOA formation that is too slow compared to observations, suggesting the importance of faster chemical pathways such as autoxidation.
Motor vehicles are significant, but probably no longer dominant contributors to urban SOA. There are indications that SOA from cooking emissions may also be important, which would help explain the substantial non-fossil fraction of the SOA from urban precursors. Solvents, personal care products, and other volatile chemical products (VCPs) are also likely important (McDonald et al., Science, 2018). Both cooking and solvents/PCPs will likely become more important in the future as vehicle emissions continue to be aggressively reduced, and more research on SOA formation from both sources is urgently needed. SOA from urban sources is poorly characterized for megacities in the developing world.