Multiphase laboratory studies of wildfire smoke plume aging under wet and dry conditions

Prof. David O. De Haan,
Department of Chemistry & Biochemistry, University of San Diego

"Field studies have shown that when wildfire smoke plumes age under dry conditions in the atmosphere, approximately half of the total brown carbon (BrC) absorption tends to fade with time due to photobleaching. Cloud-processed smoke plumes, however, do not fade noticeably, but the chemical or physical transformations behind this effect are unknown. To address this gap, we burned four types of biomass samples under simulated smoldering conditions (400 C) and fractionated the smoke for a series of chamber experiments. In one set of experiments, whole smoke, the VOC fraction, and denuded smoke where oxidized by OH, ozone, or NO3 under wet and dry conditions in a steel cloud chamber. In a second set of experiments, smoke POA, denuded POA, and water-soluble gas components of smoke were collected and re-aerosolized at constant rates into an oxidation flow reactor (OFR) for integrated OH exposure of 2 – 9 d of atmospheric oxidation. Observations of aerosol bounce indicated that aerosol particles had viscosities of > 1 Pa s even before photooxidation. No bleaching at 405 nm and little chemical reactivity was observed when smoke aerosol was exposed to the highest levels of OH radicals under dry conditions in the OFR. These observations suggest that diffusion is limited in smoke OA and that OH reactivity is limited to smoke aerosol surfaces. Ozone exposure, on the other hand, oxidized smoke OA on a 12-min timescale in the steel chamber., especially under dry conditions. "