Modeling Secondary Organic Aerosol Production from Photosensitized Humic-like Substances (HULIS)

Humic-like substances (HULIS) are ubiquitous in atmospheric aerosols. Despite experimental evidence that HULIS can catalyze secondary organic aerosol (SOA) formation through photosensitizer chemistry, the potential contribution of this pathway to ambient SOA has not been quantified. In this study, GAMMA, a photochemical box model, was used to analyze the experimental data of Monge et al. ( Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 6840−6844) to quantify the kinetics of uptake of limonene by particles containing humic acid, a laboratory proxy for HULIS. The results indicate that limonene is taken up by irradiated particles containing humic acid efficiently, with a reactive uptake coefficient of 1.6 × 10–4. Consequently, simulations of limonene–HULIS photosensitizer chemistry under ambient conditions, simultaneously with other aqueous SOA formation processes, show that this pathway could contribute up to 65% of the total aqueous SOA at pH 4. The potential importance of this pathway warrants further laboratory studies and representation of this SOA source in atmospheric models.