4 Conclusions
Using composition data from multiple campaigns spanning a variety of environments, we calculated statistics of the OXL:SO42- mass ratio (median: 0.0217; R = 0.76; N = 2948), with 95% confidence interval bounds indicating a relative uncertainty range of ~±30% about the median (0.0154 – 0.0296). Ground-based size-resolved measurements show overall agreement with the proposed range, specifically within the submicrometer mode, suggesting our results are robust within the mixed layer for PM1. Results from remote marine measurements from AToM over the Pacific and near-surface Atlantic Oceans also corroborate the bootstrapped OXL:SO42- range. As analyzed environments span continental and coastal North America; west, east, and central Pacific Ocean; and west and central Atlantic Ocean, the confidence interval of the ratio is proposed to be robust to a wide range of factors that can impact the formation and removal of both OXL and SO42-. Furthermore, remarkable similarity betwen our 95% confidence interval (0.0154 – 0.0296) and the ratio of yields between SO42- and aqueous SOA (~0.008 – 0.033) (Ervens et al., 2011) supports the hypothesis of a generalizable range of OXL:SO42-.
One exception to the hypothesized OXL:SO42- range was the occurrence of gas-phase OXL and/or its precursors partitioning onto dust aloft during CAMP2Ex. Additionally, BB emissions as a source of both OXL and SO42- may produce a strong correlation and greatly elevate their ratio. Thus, we caution against interpretating a strong OXL and SO42- correlation as a standalone signature of aqueous processing when coarse particle types (e.g., dust) and/or BB emissions are present.
Given its relative uncertainty range (~±30% about the median) when taken across multiple environments, the 95% confidence interval of the OXL:SO42- ratio could be used to assess the relative extent of aqueous processing by comparing inferred OXL concentrations between air masses, with the implicit assumption that sampled SO42- mainly originates from aqueous processing, which is expected to be particularly true but not limited to near clouds. We emphasize that the OXL:SO42- ratio applies specifically to aqueous-processed aerosol (including via clouds or wet aerosol particles) and that an estimation of total SOA from this ratio requires additional information about the ratio of OXL and SOA. Furthermore, gas-phase oxidation is an important source of uncertainty in the ratio as it can be the dominant SO42-pathway at times and may partly explain OXL:SO42- variability between campaigns.
Examining the OXL:SO42- ratio in other parts of the world and seasons would be beneficial to further gauge its variability as well as to identify other potentially confounding factors. Future analysis employing the multi-seasonal ACTIVATE campaign will provide a valuable dataset for investigations of this nature.