Variability in Aromatic Aerosol Yields under Very Low NO Conditions at Different HO₂/RO₂ Regimes

Current chemical transport models generally use a constant secondary organic aerosol (SOA) yield to represent SOA formation from aromatic compounds under low NO conditions. However, a wide range of SOA yields (10 to 42%) from m-xylene under low NO conditions is observed in this study. The chamber HO₂/RO₂ ratio is identified as a key factor explaining SOA yield variability: higher SOA yields are observed for runs with a higher HO₂/RO₂ ratio. The RO₂ + RO₂ pathway, which can be increasingly significant under low NO and HO₂/RO₂ conditions, shows a lower SOA-forming potential compared to the RO₂ + HO₂ pathway. While the traditional low-NO chamber experiments are commonly used to represent the RO₂ + HO₂ pathway, this study finds that the impacts of the RO₂ + RO₂ pathway cannot be ignored under certain conditions. We provide guidance on how to best control for these two pathways in conducting chamber experiments to best obtain SOA yield curves and quantify the contributions from each pathway. On the global scale, the chemical transport model GEOS-Chem is used to identify regions characterized by lower surface HO₂/RO₂ ratios, suggesting that the RO₂ + RO₂ pathway is more likely to prove significant to overall SOA yields in those regions. Current models generally do not consider the RO₂ + RO₂ impacts on aromatic SOA formation, but preliminary sensitivity tests with updated SOA yield parameters based on such a pathway suggest that without this consideration, some types of SOA may be overestimated in regions with lower HO₂/RO₂ ratios.