Photolytic aging of organic aerosol from pyrolyzed urban materials

Abstract

Emissions from large-scale fires significantly contribute to the atmospheric burden of primary organic aerosol (OA). The frequency of fires occurring at the wildland–urban interface (WUI) is increasing, with biomass and a wide range of human-engineered materials serving as fuels. The chemical composition and optical properties of OA from WUI fires are poorly characterized, and this work seeks to understand how direct photolytic aging alters the light absorbing properties of particulate matter generated from WUI fires. Ten flammable urban materials were selected to represent structural and furnishing components commonly combusted in urban fires: electrical 12 AWG wire (white PVC coating), ceiling tile, synthetic fabric, electrical 23 AWG wire (purple PVC coating), lumber, drywall, fiberboard, vinyl flooring, plywood, and carpet. Each material was pyrolyzed at 600 °C under N₂ in a tube furnace and resulting smoke particles were collected on Teflon filters or deposited on fused silica optical windows. OA samples were aged by exposing them to simulated solar radiation (near-UV radiation, 280–400 nm) directly on the collection substrates. Mass absorption coefficients (MAC) and Absorption Angström Exponents (AAE) of the unaged and aged OA were calculated from spectrophotometry measurements taken after extracting OA in suitable organic solvents. We observed variable trends after short-term exposure to UV radiation: six out of ten types of urban OA exhibited photoenhancement, two exhibited photobleaching, and the remaining two experienced negligible change after 2 h of photolytic aging. The average AAE value for unaged OA was 8.8 ± 1.3 and decreased to 7.3 ± 0.9 after aging, reflecting increased absorption in the visible range of the spectrum after the UV exposure, and an evolution from very weakly to weakly absorbing brown carbon. Long-term UV exposure results indicate that most photochemical change occurs within the first few hours of irradiation. These results suggest that WUI fires efficiently produce brown carbon, which becomes increasingly light-absorbing in sunlight.