Atmospheric organic particulate matter (PM) affects climate, air quality and health; however most atmospheric models, including the USEPA's Community Multi-scale Air Quality (CMAQ) model do not accurately predict ambient concentrations. CMAQ underpredicts organic PM summertime concentrations at the surface and aloft, and fails to accurately capture the variability observed in measurements. The discrepancies between observations and CMAQ predictions of organic PM likely arise from poorly characterized secondary organic aerosol (SOA) formation processes.

Improvements in the ability to forecast ambient organic PM, especially the important SOA contribution, are critical to developing effective air quality management plans and protecting public health. The CMAQ aerosol module has recently been updated to include newly identified SOA precursors, aerosol species, and formation processes (e.g., cloud processing). An evaluation of model results compared to ambient concentration measurements and the relative importance of various SOA pathways will be presented.