Evolution of Chemical and Optical Properties of Secondary Organic Aerosols Generated from Nighttime Oxidation of Unsaturated Heterocyclic Compounds
Studies have shown that the secondary organic aerosols (SOA), generated from the oxidation reactions of organic compounds, can produce light-absorbing species referred to as secondary brown carbon (BrC).1 BrC is a significant contributor to climate radiative forcing and atmospheric warming, however, chemical composition and optical properties of BrC aerosols remain poorly understood. Biomass burning (BB) has been recognized as the major source of primary and secondary BrC aerosols in the atmosphere.2–4 The nighttime chemistry in BB plumes can enhance the formation of BrC in the atmosphere.5,6 Heterocyclic compounds are emitted in large quantities from BB smoke and can act as possible precursors for BrC formation. In this work, we investigated the optical properties and chemical evolution of BrC formed during the nighttime NO3-initiated oxidation of furan, furfural, pyrrole, 1-methylpyrrole, and thiophene in laboratory chamber studies. Pyrrole oxidation led to formation of the most absorbing BrC, followed by furan- and furfural- derived SOA. Thiophene and 1-methylpyrrole oxidation produced only slightly absorbing SOA. Trends in signatures of different SOA functional groups are investigated to explain the observed differences in the measured optical properties: single scattering albedo (SSA), mass absorption coefficient (MAC), and refractive index (RI) of SOA.