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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.