Brown Carbon Formation from Nighttime Chemistry of Unsaturated Heterocyclic Volatile Organic Compounds

Nighttime atmospheric processing enhances the formation of brown carbon aerosol (BrC) in biomass burning plumes. Heterocyclic compounds, a group of volatile organic compounds (VOCs) abundant in biomass burning smoke, are possible BrC sources. Here, we investigated the nitrate radical (NO₃)-initiated oxidation of three unsaturated heterocyclic compounds (pyrrole, furan, and thiophene) as a source of BrC. The imaginary component of the refractive index at 375 nm (k₃₇₅), the single scattering albedo at 375 nm (SSA₃₇₅), and average mass absorption coefficients (MAC_{290-700 nm}) of the resulting secondary organic aerosol (SOA) are reported. Compared to furan and thiophene, NO₃ oxidation of pyrrole has the highest SOA yield. Pyrrole SOA (k₃₇₅ = 0.015 ± 0.003, SSA = 0.86 ± 0.01, MAC_{290-700 nm} = 3400 ± 700 cm² g^-1) is also more absorbing than furan SOA (MAC_{290-700 nm} = 1100 ± 200 cm² g^-1) and thiophene SOA (k₃₇₅ = 0.003 ± 0.002, SSA₃₇₅ = 0.98 ± 0.01, MAC_{290-700 nm} = 3000 ± 500 cm² g^-1). Compared to other SOA systems, MACs reported in this study are higher than those from biogenic precursors and similar to high-NO_x anthropogenic SOA. Characterization of SOA molecular composition using high-resolution mass spectrometric measurements revealed unsaturated heterocyclic nitro products or organonitrates as possible chromophores in BrC from all three precursors. These findings reveal nighttime oxidation of fire-sourced heterocyclic compounds, particularly pyrrole, as a plausible source of BrC.