the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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 primarily 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.
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- RC1: 'Comment on egusphere-2025-2879', Anonymous Referee #1, 21 Aug 2025 reply
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Overview
This study evaluates the performance of CMAQ model with SAPRC07-aero7 mechanism in simulating organic aerosol (OA) mass concentrations, volatility distributions, and O/C ratios with two field measurements in eastern China. The authors conducted several sensitivity simulations, including adding emissions of IVOC and updating SOA formation mechanisms. The simulations were well designed but still difficult to capture the properties of measured OA, like the mass concentrations, volatility distributions, and O/C ratios. These limitations affected predictions of OA physicochemical properties such as glass transition temperature (Tg), viscosity, and hygroscopicity. The findings highlight the need for better constraints to improve model accuracy in simulating air quality and OA properties.
The manuscript is well-structured, and its conclusions are insightful, offering valuable guidance for future research. It is recommended for publication with some revisions.
Major comments:
The authors carefully explored potential reasons for the underestimation of SOA at two sites in China. However, underpredicted POA emissions (including POC and PNCOM) could significantly affect SOA partitioning and contribute to the observed biases. To evaluate this, I recommend an additional sensitivity simulation in which POA emissions are increased to match observational levels—particularly at the GZ site—to examine whether SOA predictions improve as a result. Furthermore, comparing the diurnal variations of observed and estimated emissions may help identify missing sources and better constrain emission uncertainties. While this would be a sensitivity test, inaccuracies in emission inventories are a well-known issue affecting model performance. In reality, the underprediction of SOA is likely due to a combination of underestimated POA emissions and missing or incomplete SOA formation pathways.
Minor comments: