Secondary Brown Carbon Formation From Photooxidation of Furans From Biomass Burning.

Furans are a major class of volatile organic compounds emitted from biomass burning. Their high reactivity with atmospheric oxidants leads to the formation of secondary organic aerosol (SOA), including secondary brown carbon (BrC) that can affect global climate via interactions with solar radiation. Here, we investigate the optical properties and chemical composition of SOA generated via photooxidation of furfural, 2‐methylfuran, and 3‐methylfuran under dry (RH < 5%) and humid (RH ∼ 50%) conditions in the presence of nitrogen oxides (NOx) and ammonium sulfate seed aerosol. Dry furfural oxidation has the greatest BrC formation, including reduced nitrogen‐containing organic compounds (NOCs) in SOA, which are dominated by amines and amides formed from reactions between carbonyls and ammonia/ammonium. Based on the products detected, we propose novel formation pathways of NOCs in furfural photooxidation, which can contribute to BrC via accretion reactions during the photochemical aging of biomass burning plumes. Plain Language Summary: Biomass burning is a substantial source of both gas‐ and particle‐phase carbon to the atmosphere, including light‐absorbing aerosol known as brown carbon. Brown carbon can be emitted directly from combustion activities and also formed through the oxidation of gas‐phase carbon, known as secondary brown carbon. In this study, we investigate secondary brown aerosol formation from furans (chemicals with a five‐membered aromatic ring with four carbon atoms and one oxygen atom), one of the major classes of gaseous compounds emitted from biomass burning. We find that furfural is an important subset of furans that generates substantial brown carbon during oxidation, with corresponding formation of nitrogen‐containing organic compounds. Key Points: Photooxidation of furans in the presence of NOx and ammonium sulfate aerosol leads to secondary brown carbon formationOptical properties of secondary brown carbon from furfural photooxidation are comparable to ambient biomass burning observationsReduced nitrogen species are associated with brown carbon formation from furfural photooxidation and are enhanced under dry conditions [ABSTRACT FROM AUTHOR]