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.