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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 (NO_x) 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.

As part of the WINTER (Wintertime Investigation of Transport, Emissions, and Reactivity) campaign, a Particle‐into‐Liquid Sampler with a fraction collector was flown aboard the National Center for Atmospheric Research C‐130 aircraft. Two‐minute integrated liquid samples containing dissolved fine particulate matter (PM₁) species were collected and analyzed off‐line for the smoke marker levoglucosan using high‐performance anion‐exchange chromatography‐pulsed amperometric detection to compare levoglucosan with aerosol mass spectrometer (AMS) biomass burning markers and investigate the contribution from residential burning during the study. Levoglucosan was correlated with AMS organic aerosol (R² = 0.49) and with carbon monoxide (CO;R² = 0.51) for all flights. Levoglucosan was not correlated with the inorganic smoke marker water‐soluble potassium but was correlated with the AMS markers ∆C₂H₄O₂⁺(high resolution,R² = 0.60) and ∆m/z60 (unit mass resolution,R² = 0.61). However, at low levoglucosan, AMS markers deviated potentially due to interferences from other sources or differences with the species captured by the AMS markers. Analysis of levoglucosan changes relative to carbon monoxide as plumes advected from source regions showed no systematic levoglucosan loss for plumes up to 20 hr old. Based on literature residential burning source ratios and measured levoglucosan, contributions of organic carbon (OC) due to residential burning were estimated. The contribution ranged from ~30 to 100% of the OC, with significant variability depending on the source ratio used; however, the results show that biomass burning was a significant PM₁OC source across the entire sampling region. A GEOS‐Chem model simulation predicted significantly less smoke contribution.