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 (k(375)), the single scattering albedo at 375 nm (SSA(375)), 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 (k(375) = 0.015 0.003, SSA = 0.86 0.01, < MAC >(290-700 nm) = 3400 +/- 700 cm(2) g(-1)) is also more absorbing than furan SOA (< MAC >(290-700 nm) = 1100 +/- 200 cm(2) g(-1)) and thiophene SOA (k(375) = 0.003 0.002, SSA(375) = 0.98 0.01, (MAC)(290-700) (nm) = 3000 +/- 500 cm(2) g(-1)). Compared to other SOA systems, MACs reported in this study are higher than those from biogenic precursors and similar to high-NOx 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.