Insights into secondary organic aerosol formation from the day- and nighttime oxidation of PAHs and furans in an oxidation flow reactor.

Secondary organic aerosols (SOA) formed by oxidation of typical precursors largely emitted by biomass burning, such as PAHs and furans, are still poorly characterized in terms of formation yields, physical and light absorption properties, particularly those generated at night following reaction with nitrate radicals (NO₃). In the present study, we evaluated and compared the formation yields, effective density (ρ_eff), absorption Ångström exponent (α), and mass absorption coefficient (MAC) of laboratory-generated SOA from three furan compounds (furan, 2-methylfuran, and 2,5-dimethylfuran) and four PAHs (naphthalene, acenaphthylene, fluorene, and phenanthrene). SOA were generated in an oxidation flow reactor from the reaction between hydroxyl radicals (OH; 0.1–20 equivalent aging days) or NO₃ radicals (0.05–6 equivalent aging nights of 14 h) with single furan or PAH. The ρ_eff , formation yields, α, and MAC of the generated SOA varied depending on the precursor and oxidant considered. The ρ_eff of SOA formed with OH and NO₃ tended to increase with particle size before reaching a "plateau". This was particularly evident for the nighttime chemistry experiments with NO₃ radicals (1.2 to 1.6 on average for particles > 100 nm). Such results highlighted potential differences in the chemical composition of the SOA, as well as probably in their morphology, according to the particle size. Three times lower SOA formation yields were obtained with NO₃ compared to OH. The yields of PAH SOA (18 to 76 %) were 5 to 6 times higher than those obtained for furans (3–12 %). While furan SOA showed low or negligible light absorption properties, PAH SOA was found to have a significant impact in the UV-Visible region, implying a significant contribution to atmospheric brown carbon (BrC). No increase in the MAC values was observed from OH to NO₃ oxidation processes, probably due to a low formation of nitrogen-containing chromophores through homogeneous gas phase oxidation processes with NO₃ only (without NO_x). Overall, the results obtained in this work demonstrated that PAHs are significant precursors of SOA emitted by biomass burning, through both, day- and nighttime processes, and have a substantial impact on the aerosol light absorption properties and so probably on climate. [ABSTRACT FROM AUTHOR]