Evaluating simulations of organic aerosol volatility and degree of oxygenation in eastern China

Abstract. Volatility and oxygen-to-carbon (O/C) molar ratios are critical properties of organic aerosols (OA), influencing their viscosity, hygroscopicity, and light absorption thereby resulting in impacts on air quality and climate. While atmospheric models often track these properties to simulate OA evolution, their performance remains insufficiently evaluated. This study assessed OA volatility and O/C simulations by comparing CMAQ model outputs using official AERO7i and community-contributed two-dimensional volatility basis set (2D-VBS) schemes, against two field measurements in eastern China. Apart from baseline modelling, two additional simulations using AERO7i incrementally incorporated low-volatility/semi-volatile/intermediate-volatility organic compound (L/S/IVOC) emissions and enhanced anthropogenic secondary organic aerosol (SOA) yields. An optimized 2D-VBS simulation further constrained O/C ratios of primary organic aerosol (POA) emissions using observations. The results showed that OA mass concentrations were underestimated by 24 % in 2D-VBS and 27 %–34 % with updated AERO7i, likely due to underrepresented vehicular POA emissions and nighttime SOA formation. All simulations captured the substantial contribution of low-volatility products (C*<0.1 µg m−3) but failed to reproduce the detailed volatility distributions within this range. Simulated O/C ratios were biased low in aged air masses (notably with 2D-VBS) and slightly overestimated in areas with more local emissions using updated AERO7i. Misrepresentations of OA volatility significantly led to biases in viscosity predictions, while the hygroscopicity parameter played a more important role. These findings highlight the need to better constrain OA volatility and O/C in models to improve projections of OA air quality and climate impacts.