(8e) Exploring Atmospheric Organic Aerosol Source Apportionment In the United States for An Entire Simulation Year

The contribution of specific sources, both anthropogenic and biogenic, to atmospheric pollution varies widely throughout space and time. The relationship between these sources and ambient receptors is complicated by variability in the sources themselves, meteorological conditions, and chemistry that occurs during the pollutants’ atmospheric lifetime. These relationships must be well-characterized for relevant pollutant species in order to inform policy decisions that seek to improve air quality with the most effective emissions reduction strategies. The Particulate Matter Source Apportionment Technology (PSAT) algorithm [Wagstrom et al., 2008] was developed and applied to a regional-scale, three-dimensional chemical transport model (PMCAMx) to address this need from several perspectives for inorganic constituents of atmospheric particulate matter (PM) (e.g. sulfate, ammonium, nitrate, elemental carbon, etc).

For this study, we focus on organic aerosol (OA), a major contributor to the PM mass composition. Understanding the OA system is more complicated than understanding the inorganic aerosol constituents (ammonium, sulfate, nitrate, chloride, etc) due to the large number of distinct organic species present, widely varying structure and properties, and their complex oxidation pathways. Previously, we have modeled OA formation and continued atmospheric oxidation with the regional-scale CTM, PMCAMx-2008 [Murphy and Pandis, 2009]. Here we combine PSAT and PMCAMx-2008 to simulate organic aerosol formation with source resolution for the US throughout an entire year, 2008.

A notable conceptual improvement in PMCAMx-2008 is the treatment of POA evaporation upon dilution in the atmosphere. In previous work, a uniform representative volatility distribution was applied to OA mass from all sources. In this work, we expand that analysis to treat source-specific volatility distributions from diesel, gasoline, and biomass burning constituents to quantify the effect of that assumption on the dominance of particular sources to the ambient OA loading. The continued gas-phase oxidation of organic vapors can change their volatility over time and this has an effect on the long-range transport and properties of these species. We investigate this relationship by distinguishing between local, short range, mid range and long range pollutants at several receptor sites in the US. We finally assess the average chronological age of OA mass throughout the US domain while taking into account the continued oxidation and semivolatile partitioning of the aerosol and vapors. The analysis from this study confirms the highly regional, dispersed nature of organic aerosol pollution and suggests that local emissions reduction efforts alone may be insufficient for effective mitigation of this pollutant.

Wagstrom, K. M., et al. (2008), Development and application of a computationally efficient particulate matter apportionment algorithm in a three-dimensional chemical transport model, Atmos. Environ., 42, 5650– 5659, doi:10.1016/j.atmosenv.2008.03.012.

Murphy, B. N., and S. N. Pandis (2009), Simulating the formation of semivolatile primary and secondary organic aerosol in a regional chemical transport model, Environ. Sci. Technol., 43, 4722–4728.