Aqueous phase photo-chemical processing of selected brown carbon species from biomass burning and vehicle exhaust

Photochemical cloud processing of biomass burning plume has been suggested as a potential source of secondary organic aerosol (SOA) recently. Nitrophenols and the analogs are a class of readily water-soluble light-absorbing compounds that are often found in biomass burning (BB) brown carbon (BrC). A number of studies found that they could undergo aqueous-phase photo-oxidation to form SOA, and often a change of light absorption was observed alongside the aqueous phase SOA (aqSOA) production. Here we plan to conduct both direct photolysis and OH oxidation experiments of four selected BrC compounds from BB and vehicle exhaust, i.e. 4-nitrocatechol (4NC), 4-methyl-5- nitrocatechol (4M5NC), 3-methyl-6-nitrocatechol (3M6NC), and 5-nitrosalicylic acid (5NSA). We will investigate the aqueous phase photochemical processing of these compounds with particular focus on the photo-enhancement/photo-bleaching properties of the formed aqSOA, characterization of aqSOA products, and the possible reaction pathways and mechanisms. Various analytical techniques will be applied, including UV- visible spectrometer to monitor the evolution of light absorptive properties with increasing reaction time, high performance liquid chromatography electron spray ionization tandem mass spectrometry to identify and quantify the formed aqSOA products, and nuclear magnetic resonance and Fourier-transform infrared spectroscopy to obtain complementary chemical structure information of both reactants and products. Results from this study will expand our knowledge on the possible precursors and pathways of atmospheric aqSOA formation, the photo-degradation of BrC species and how that influences their light absorptive properties.