The sensitivity of PM2.5 source-receptor relationships to atmospheric chemistry and transport in a three-dimensional air quality model

Air quality model simulations constitute an effective approach to developin g source-receptor relationships (so-called transfer coefficients in the ris k analysis framework) because a significant fraction of particulate matter (particularly PM2.5) is secondary (i.e., formed in the atmosphere) and, the refore, depends on the atmospheric chemistry of the airshed. In this study, we have used a comprehensive three-dimensional air quality model for PM2.5 (SAQM-AERO) to compare three approaches to generating episodic transfer co efficients for several source regions in the Los Angeles Basin. First, tran sfer coefficients were developed by conducting PM2.5 SAQM-AERO simulations with reduced emissions of one of four precursors (i.e., primary PM, sulfur dioxide (SO2), oxides of nitrogen (NOx), and volatile organic compounds) fr om each source region. Next, we calculated transfer coefficients using two other methods: (1) a simplified chemistry for PM2.5 formation, and (2) simp lifying assumptions on transport using information limited to basin-wide em ission reductions. Transfer coefficients obtained with the simplified chemi stry were similar to those obtained with the comprehensive model for VOC em ission changes but differed for NOx and SO2 emission changes. The differenc es were due to the parameterization of the rates of secondary PM formation in the simplified chemistry. In 90% of the cases, transfer coefficients est imated using only basin-wide information were within a factor of two of tho se obtained with the explicit source-receptor simulations conducted with th e comprehensive model. The best agreement was obtained for VOC emission cha nges; poor agreement was obtained for primary PM2.5.