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-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.