Title: Simulating Aqueous Secondary Organic Aerosol Formation and Cloudwater Chemistry in Gas-Aerosol Model for Mechanism Analysis
Description: Aerosols are known to have a large, uncertain effect on air quality and climate. Chemical processing of organic material in aqueous aerosols is known to form secondary organic aerosols (SOA), which make up a significant portion of particulate mass in the atmosphere. However, lack of clarity surrounding the importance of each source of SOA to total aerosol mass contributes to the uncertainties in their environmental impact. Disagreements between chemical models and field measurements suggest that some processes are misrepresented or are missing in current models. This work considers three pathways of SOA formation using Gas-Aerosol Model for Mechanism Analysis (GAMMA), a photochemical box model developed by the McNeill group featuring coupled gas phase and detailed aqueous phase aerosol chemistry. Imidazole-2-carboxaldehyde (IC), a light-absorbing organic species, has been observed to contribute to SOA formation as a photosensitizer. Currently, the extent of photosensitized reactions in ambient aerosols remains poorly constrained.

Reactive uptake coefficients were determined from experimental studies of IC-containing aerosols and scaled for ambient simulations in GAMMA. Results of remote ambient simulations show that IC is unlikely to be a significant source of SOA largely due to its lack of abundance in atmospheric aerosols. Humic-like substances (HULIS) have also been experimentally shown to catalyze SOA formation through photosensitizer chemistry. We use GAMMA to quantify the uptake kinetics of limonene in these photosensitizer experiments. Ambient GAMMA simulations of this SOA formation pathway show that limonene-HULIS photosensitizer chemistry can contribute up to 65% of total aqueous SOA at pH 4. Further laboratory studies are recommended for this SOA source to assess the need for its inclusion in aerosol models. Chemical processing of organic material in cloudwater is another known source of SOA. We use GAMMA to consider the impact of the coupled effect of processing in both aqueous aerosol and cloudwater on isoprene epoxydiol (IEPOX) SOA.

Simulations show that cloudwater at pH 3 – 4 can also be a potentially significant source of IEPOX SOA, largely due to higher water content in cloudwater than in aerosols. Thus, cloud processing may be a significant contributor to IEPOX SOA formation and could account for differences between predicted SOA mass and ambient measurements where mass transfer limitations in aerosol particles can be expected. This work concludes with recommendations for future work in GAMMA. Parameterization of glyoxal reactive uptake could allow for more accurate predictions of glyoxal oxidation product distributions. The inclusion of online thermodynamic calculations of inorganic species in GAMMA can better constrain several multiphase chemical processes, such as the highly pH-dependent uptake of IEPOX and sulfate formation. Updated detailed mechanisms of transition metal ion chemistry would also improve predictions of sulfate formation.