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Iron(III)-Catalyzed Chemistry in Biomass Burning Organic Aerosol
KATHERINE HOPSTOCK, Hind Al-Abadleh, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 35
Working Group: Aerosol Chemistry
Abstract
Biomass burning organic aerosol (BBOA) is one of the largest sources of organics in the atmosphere. Mineral dust and biomass burning smoke frequently occur in the same atmospheric environment. Typical biomass burning compounds, such as dihydroxybenzenes and their derivatives, are capable forming light-absorbing, insoluble polymeric particles upon reaction with soluble iron under conditions characteristic of aerosol liquid water. Despite mechanistic advances in iron-catalyzed chemistry with isolated BBOA precursors, it is not well understood how this chemistry translates to secondary brown carbon (BrC) formation from real BBOA. In this study, BBOA was generated through the pyrolysis of various types of biomass fuels, and the water-soluble fractions were reacted with iron chloride under dark, acidic conditions. We utilized spectrophotometry to monitor changes in the mass absorption coefficient (MAC) of the dissolved organics. MAC measurably increased at visible wavelengths after several hours of reaction. Soluble and insoluble reaction products were separated, and dissolved in aqueous and organic solvents, respectively. Analysis was conducted with ultra-performance liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer (UPLC-PDA-HRMS). Products absorbing at near-UV and visible wavelengths were tentatively assigned. In general, the insoluble products had higher molecular weights than the soluble products and starting BBOA compounds. The distribution of molecular formulas suggested that Fe(III) catalyzed oligomerization of phenols in solution. These results suggest that light-absorbing aerosol particles can be produced from Fe(III)-catalyzed reactions in aging BBOA plumes produced from smoldering combustion in the absence of any photochemistry. This chemistry has important implications for understanding the direct effect of BBOA/mixed dust aerosols on climate.