The Sensitivity of PM₂₅Source-Receptor Relationships to Atmospheric Chemistry and Transport in a Three-Dimensional Air Quality Model

Abstract

Air quality model simulations constitute an effective approach to developing source-receptor relationships (socalled transfer coefficients in the risk analysis framework) because a significant fraction of particulate matter (particularly PM 2.5 ) is secondary (i.e., formed in the atmosphere) and, therefore, depends on the atmospheric chemistry of the airshed. In this study, we have used a comprehensive three-dimensional air quality model for PM 2.5 (SAQM-AERO) to compare three approaches to generating episodic transfer coefficients for several source regions in the Los Angeles Basin. First, transfer coefficients were developed by conducting PM 2.5 SAQM-AERO simulations with reduced emissions of one of four precursors (i.e., primary PM, sulfur dioxide (SO 2 ), oxides of nitrogen (NO x ), and volatile organic compounds) from each source region. Next, we calculated transfer coefficients using two other methods: (1) a simplified chemistry for PM 2.5 formation, and (2) simplifying assumptions on transport using information limited to basin-wide emission reductions. Transfer coefficients obtained with the simplified chemistry were similar to those obtained with the comprehensive model for VOC emission changes but differed for NO x and SO 2 emission changes. The differences were IMPLICATIONS Attainment of air quality standards for PM 2.5 will require developing quantitative relationships between PM 2.5 ambient levels and the emission sources of PM 2.5 and its precursors. We show that for cases where the secondary fraction of PM 2.5 is significant, the development of sourcereceptor relationships with a three-dimensional air quality model should use an explicit treatment of atmospheric chemistry. On the other hand, source-receptor relationships for the primary component of PM 2.5 require simulating individual source-receptor combinations that properly account for atmospheric transport between the source and the receptor.due to the parameterization of the rates of secondary PM formation in the simplified chemistry. In 90% of the cases, transfer coefficients estimated using only basin-wide information were within a factor of two of those obtained with the explicit source-receptor simulations conducted with the comprehensive model. The best agreement was obtained for VOC emission changes; poor agreement was obtained for primary PM 2.5 . INTRODUCTIONThe overall objective of this work is to investigate several alternative approaches to developing reliable sourcereceptor relationships that can be used in a risk analysis framework. Air quality model simulations constitute an effective approach to developing source-receptor relationships because a significant fraction of fine particulate matter (PM 2.5 ) is secondary (i.e., formed in the atmosphere) and, therefore, depends on the atmospheric chemistry of the airshed. These source-receptor relationships, which are called transfer coefficients (TCs) in the risk analysis framework, can be made specific to source regions and major primary pollutants.The air quality modeling ...

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