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

Jiang, Huanhuan, Frie, Alexander L., Lavi, Avi, Chen, Jin Y., Zhang, Haofei, Bahreini, Roya, Lin, Ying-Hsuan
Environmental science & technology letters 2019 v.6 no.3 pp. 184-190
absorbance, aerosols, biomass, burning, carbon, furans, mass spectrometry, nitrates, oxidation, pyrroles, refractive index, smoke, thiophene, 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.