Laboratory Studies of Photolysis Aging of Brown Carbon Aerosols

DIEGO CALDERON-ARRIETA, Ana Morales, Taylor Estock, Anusha P.S. Hettiyadura, Chunlin Li, Yinon Rudich, Alexander Laskin, Purdue University

     Abstract Number: 537
     Working Group: Aerosol Chemistry

Abstract
Composition and optical properties of atmospheric brown carbon (BrC) aerosols constantly evolve as a combined result of direct photolysis and multi-phase chemical reactions. Diverse chemical compositions in BrC exhibit varying reaction kinetics against atmospheric transformations, leading to erratic results of BrC in their optics and associated climate effect. Direct photolysis commonly degrades chromophores relating to their molecular weight and phase state, and thus may decrease the corresponding mass absorption coefficient (MAC) values of BrC. However, the exact photolysis pathways in molecular levels are still unclear for most BrC, particularly the biomass pyrolysis BrC with wide-spanning MAC. To fill the gap, a series of BrC with varying absorption were collected from wood smoldering and underwent photolysis in the laboratory. We found that photolysis alters the distribution of BrC chromophores and modifies optical properties and gas-particle partitioning of airborne aerosol. Our study identifies specific groups of BrC chromophores that have varying degrees of susceptibility and resilience to photolysis aging. To examine these changes in chemical compositions, we employed a molecular characterization approach utilizing a hyphenated platform that combines high-performance liquid chromatography interfaced with a photodiode array detector and high-resolution mass spectrometer. Our data reveals BrC chromophores that are either photobleached or formed through photolysis-induced condensation reactions. Our presentation will highlight molecular identities of the BrC chromophores grouped by their optical properties and susceptibility to solar irradiation, as well as characteristic time scales for the photochemical aging. Our experiments provide insights into the photochemical stability of BrC and offer valuable information for atmospheric chemistry models.