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

Smith, E. N., Baker, K. R., El-Sayed, M. M. H., Hennigan, C. J., Rosanka, S., and Carlton, A. G.

Abstract

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 (R² = 0.5 and R² = 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.