Impact of Environmental Conditions on Secondary Organic Aerosol Production from Photosensitized Humic Acid

Recent studies have shown the potential of the photosensitizer chemistry of humic acid, as a proxy for humic-like substances in atmospheric aerosols, to contribute to secondary organic aerosol mass. The mechanism requires particle-phase humic acid to absorb solar radiation and become photoexcited, then directly or indirectly oxidize a volatile organic compound (VOC), resulting in a lower volatility product in the particle phase. We performed experiments in a photochemical chamber, with aerosol-phase humic acid as the photosensitizer and limonene as the VOC. In the presence of 26 ppb limonene and under atmospherically relevant UV-visible irradiation levels, there is no significant change in particle diameter. Calculations show that SOA production via this pathway is highly sensitive to VOC precursor concentrations. Under the assumption that HULIS is equally or less reactive than the humic acid used in these experiments, the results suggest that the photosensitizer chemistry of HULIS in ambient atmospheric aerosols is unlikely to be a significant source of secondary organic aerosol mass.