Sources of Oxygenated Organic Molecules and Their Impacts on Organic Aerosol in China

Oxygenated organic molecules (OOMs), the oxidation products of organic precursors with low volatility and a high oxygen atom number, are an important driver of new particle formation (NPF) and secondary organic aerosols (SOA) formation. The sources and formation processes of OOMs are highly complicated, especially in populous regions (e.g., China) with diverse emissions of anthropogenic and biogenic precursors. However, current models fail to capture OOM formation from different precursors due to the absence of important reaction pathways (e.g., autoxidation, dimerization) and the oversimplified treatment of the oxidation of semi-volatile and intermediate-volatility precursors (I/SVOCs). In this work, we develop a Precursor-resolved Integrated two-dimensional Volatility Basis Set (I2D-VBS) model framework, which simulates the multi-generational ageing of organic emissions in the full volatility range on a precursor level and explicitly tracks irregular radical reactions, including autoxidation, dimerization, and RO isomerization followed by accelerated autoxidation. The parameterizations within the Precursor-resolved I2D-VBS are optimized by simulating chamber and flow-tube experiments measuring OOMs and SOA formed from individual precursors. We then incorporate the Precursor-resolved I2D-VBS in the CMAQ chemical transport model and simulate OOM formation in China. The CMAQ/I2D-VBS model successfully reproduced OOM concentrations at various sites across China, achieving accuracy within ±40% for total OOM concentrations and within a factor of 2 for volatility-binned OOM concentrations. The model results reveal that over 60% of total OOM concentrations are from I/SVOC in China; nevertheless, for extremely low-volatility OOMs (logC*<=-5) that are important for initial particle growth, ~60% of them are from anthropogenic VOCs in the North China Plain and ~80% of them are from biogenic VOCs in Southeast China. Multi-generational OH oxidation is the reaction pathway contributing most to OOM formation (>70%), followed by autoxidation (~20%). Overall, OOMs contribute around half of SOA concentrations in China, highlighting the critically important role of OOMs in SOA formation.