Variability in Aromatic Aerosol Yields under Very Low NO x Conditions at Different HO 2 /RO 2 Regimes.

Current chemical transport models generally use a constant secondary organic aerosol (SOA) yield to represent SOA formation from aromatic compounds under low NO x conditions. However, a wide range of SOA yields (10 to 42%) from m -xylene under low NO x conditions is observed in this study. The chamber HO 2 /RO 2 ratio is identified as a key factor explaining SOA yield variability: higher SOA yields are observed for runs with a higher HO 2 /RO 2 ratio. The RO 2 + RO 2 pathway, which can be increasingly significant under low NO x and HO 2 /RO 2 conditions, shows a lower SOA-forming potential compared to the RO 2 + HO 2 pathway. While the traditional low-NO x chamber experiments are commonly used to represent the RO 2 + HO 2 pathway, this study finds that the impacts of the RO 2 + RO 2 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 HO 2 /RO 2 ratios, suggesting that the RO 2 + RO 2 pathway is more likely to prove significant to overall SOA yields in those regions. Current models generally do not consider the RO 2 + RO 2 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 HO 2 /RO 2 ratios.