Chromophores and chemical compositions of brown carbon aerosol before and after photooxidation of combustion emissions

Abstract. Brown carbon (BrC) aerosols affect earth's climate and originate mainly from biomass burning. However, chromophores and chemical composition of BrC remain difficult predict, especially considering BrC from different sources and aging e.g. by photooxidation. To address this gap, we studied emissions of burning beech wood, straw, plastics, and cow dung in an oxidative flow reactor allowing to conduct photooxidation aging of fresh emissions by using mass spectrometry and excitation emission spectroscopy with a parallel factor analysis. After photooxidation, volatile organic compounds and heavier molecules were oxidized and formed more oxidized products. Phenolic-like substances (PLS) and less oxygenated humic-like substances (LO-HULIS) dominated the fluorescence of primary organic aerosol with 88 ± 7 %. After photooxidation, the PLS chromophore significantly decreased from 45 ± 8 % to 10 ± 6 % and humic-like substance increased from 55 ± 8 % to 90 ± 6 %, especially highly-oxygenated humic-like substance (HO-HULIS). The HULIS chromophores were unsaturated and contained high fractions with 5 %–10 % of nitrogen containing molecules. In contrast, the PLS chromophores had low oxidation states and contained lower fraction (2 %) of nitrogen containing molecules. After photooxidation, oxidation of PLS chromophores and volatile organic compounds in presence of NOx were converted into HULIS with the higher fraction of nitrogen containing and unsaturation chromophores. This study extends the current understanding of formation and photochemcial aging of brown carbon chromophorese e.g. PLS, HO-HULIS, and LO-HULIS from open fires including their molecular composition. This will facilitate modelling of brown carbon aerosol e.g. in trasport models.