Evaluation of CMAQ-Predicted Gas Phase Water Soluble Organic Carbon

ELLIE SMITH, Kirk Baker, Christopher Hennigan, Marwa El-Sayed, Annmarie Carlton, University of California, Irvine

     Abstract Number: 262
     Working Group: Coast to Coast Campaigns on Aerosols, Clouds, Chemistry, and Air Quality

Abstract
Gas phase water soluble organic carbon (WSOCg) comprises many important trace species critical for Secondary Organic Aerosol (SOA) formation. The Community Multiscale Air Quality (CMAQ) model predicts aqueous pathways dominate SOA mass in the eastern U.S. In this work, we evaluate CMAQ’s predictive skill for accurate representation of WSOCg. We pair model predictions of WSOCg in space and time for 3 months of continuous mist chamber sampling in Baltimore, MD with retrospective CMAQ simulations for the eastern U.S. We sample CMAQ’s atmosphere with a virtual mist chamber, applying collection efficiencies as a function of Henry’s Law, and accounting for each individual species’ solubility and sampling bias. CMAQ predictions do not replicate the average diurnal pattern of measured WSOCg in any month and are generally biased low relative to measurements, especially in winter. However, parent volatile organic compounds (VOCs), NO2 and ozone predictions evaluate well compared to measurements at a nearby EPA Photochemical Assessment Monitoring Station (PAMS) site in a neighboring model grid cell, (r = 0.5; r = 0.7; and r = 0.8, respectively and all p values ≈ 0). CMAQ-predicted WSOCg is significantly (p ≈ 0) and positively correlated to model-predicted NOx (r = 0.9 in February/March; r = 0.5, in August) and exhibits similar morning and evening peaks. Measured WSOCg mass is significantly (p ≈ 0) correlated with temperature in the summer (r = 0.7), but not in winter. Modeled WSOCg concentrations are correlated with temperature in both seasons. These findings suggest that CMAQ represents VOC reactivity for ozone formation well, but not the processing that produces WSOCg, a dominant contributor to SOA in the region.