AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Photosensitized SOA Production by Humic Acid in Aqueous Aerosols
WILLIAM TSUI, V. Faye McNeill, Columbia University
Abstract Number: 142 Working Group: There Must be Something in the Water: Cloud, Fog and Aerosol Aqueous Chemistry for Aerosol Production
Abstract Photosensitized reactions involving humic acid, a proxy for water-soluble humic-like organic compounds (HULIS), have been experimentally observed to contribute to secondary organic aerosol (SOA) growth (Monge et al., 2012). However, the extent of photosensitized reactions in ambient aerosols remains poorly understood and unaccounted for in atmospheric models. Here we use GAMMA 4.0, a photochemical box model that couples gas-phase and aqueous-phase aerosol chemistry, to analyze photosensitized SOA formation by humic acid in laboratory and ambient settings. We find that this pathway may be a significant contributor to aqueous aerosol SOA (aaSOA) formation, particularly under less acidic conditions where other aaSOA formation mechanisms are less efficient.
Reactive uptake coefficients were determined for reactions of gas-phase limonene and isoprene with photoactivated humic acid-containing particles based on the experimental data of Monge et al. (2012) using GAMMA. Reactions of these VOCs with photoactivated humic acid are efficient. This plus the fact that HULIS is relatively abundant in ambient aerosols results in a reactive uptake coefficient nearly 500 times greater than that for limonene with photoactivated imidazole carboxaldehyde (IC)-containing particles. For simulations using GAMMA under ambient conditions, we find that the contribution of SOA formation from this pathway is much greater than for the IC pathway, contributing up to 58% by mass of aaSOA growth. The contribution of this pathway to overall SOA production will be explored using a regional model.