Application of the CACM and MPMPO modules using the CMAQ model for the eastern United States

Abstract

The Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics (MPMPO) have been updated with a detailed treatment of the oxidation mechanisms and secondary organic aerosol (SOA) formation potentials of α‐pinene, β‐pinene, and d‐limonene. The updated CACM and MPMPO modules have been incorporated into the Community Multiscale Air Quality (CMAQ) model. The revised CMAQ model was used to simulate air quality over the eastern United States, with a particular focus on New England (NE) for the period 3–4 August 2004, which was part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign. On 3 August, 24‐hour‐average organic aerosol (OA) and PM2.5 concentrations were approximately 7.0 μg m−3 and 13.0 μg m−3, respectively, at Thompson Farm (TF), a rural site in southeastern New Hampshire. The model results (e.g., ozone (O3), PM2.5, and individual PM2.5 chemical components) were compared against various observational data sets (e.g., AIRMAP, IMPROVE, SEARCH, and AIRNOW), as well as CMAQ model predictions using the CB‐IV gas‐phase mechanism and the SORGAM SOA module. Both CMAQ model simulations with CACM/MPMPO and with CB‐IV/SORGAM predicted O3, PM2.5, sulfate, and ammonium reasonably well but underestimated elemental and organic carbon aerosol. SOA predictions from CACM/MPMPO and from CB‐IV/SORGAM were very close for the sites where OA concentrations were available on 4 August, though sensitivity of SOA predictions at TF to domain‐wide NOx emissions and temperature variations differed significantly. Additionally, on the basis of the predicted chemical composition of OA from CMAQ with CACM/MPMPO, 24‐hour averages of the ratio of the organic mass to organic carbon were determined to be in the range of 1.1 to 1.9, depending on the relative abundance of SOA and primary organics.