Gas-Phase Water-Soluble Organic Carbon: CMAQ Model Evaluation in Baltimore County

Prediction of gas-phase water-soluble organic carbon (WSOC_g), a precursor for secondary organic aerosol formed through processing in atmospheric waters (aqSOA), has not yet been evaluated in models. We pair the WSOC_g predictions from the U.S. EPA's Community Multiscale Air Quality (CMAQ) model with continuous mist chamber measurements during February-March 2015 and August 2016 in Baltimore County, MD. We simulate mist chamber collection of WSOC_g from CMAQ's atmosphere with application of compound-specific collection efficiencies as a function of Henry's law. CMAQ predictions of WSOC_g mass concentrations are highest in August, while measurements are highest during February-March. CMAQ does not replicate the average diurnal pattern of the measured WSOC_g in any month. The CMAQ prediction of directly emitted VOCs that oxidize to form WSOC_g is more reasonable, and the model skill for nitrogen dioxide (NO₂) and ozone (O₃) is relatively excellent in comparison (R ² = 0.5 and R ² = 0.6, respectively; p ≈ 0). These findings suggest that representation of organic gases and their chemistry in this CMAQ simulation is sufficient to accurately predict the criteria pollutants NO₂ and O₃, but not necessarily the chemical transformations that produce WSOC_g, an important precursor for aqSOA.