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Molecular Characterization of Atmospheric Brown Carbon Chromophores Formed in Irradiated Proxies of Aqueous Secondary Organic Aerosol
MARIA MISOVICH, Anusha P.S. Hettiyadura, Wenqing Jiang, Qi Zhang, Alexander Laskin, Purdue University
Abstract Number: 233
Working Group: Aerosol Chemistry
Abstract
Guaiacyl acetone (GA), a prominent product found in biomass burning emissions, partitions rapidly into atmospheric aqueous phase systems such as fogs. In the aqueous phase, GA undergoes rapid oxidation reactions to form aqueous secondary organic aerosol (aqSOA). The photooxidation of GA in aqueous solution was examined in this study as a proxy of aqSOA formation related to atmospheric ageing of biomass burning emissions. An aqueous solution containing GA was oxidized by the triplet excited state of an aromatic carbonyl (3C*), produced from dimethoxybenzaldehyde in an irradiation experiment. Samples of the resulting reaction mixtures were collected at different reaction times, and chemical components of the aqSOA mixtures were analyzed using reversed phase liquid chromatography (LC) interfaced with a photodiode array detector (PDA) and a high resolution Orbitrap mass analyzer (HRMS) operated with an electrospray ionization (ESI) source. Recorded UV-Vis and HRMS spectra of the LC separated aqSOA components showed that less polar products, eluting at late retention times (>50 min), dominate light absorbing brown carbon (BrC) properties of aged aqSOA formed at longer irradiation times, while chromophores composed of relatively small polar species, eluting at <30 min, dominate BrC absorption at shorter irradiation times. Normalized top down UV-visible absorbance plots indicate that total BrC absorbance initially increases with irradiation, but decreases as irradiation continues. Overall, LC-PDA-HRMS chromatograms indicate three classes of products with strong light-absorbing characteristics. Elemental formulas were assigned to each class of products using HRMS data. The first class of products includes the monomeric products of GA functionalized with different groups. The second class includes various GA dimers and other oxygenated compounds with two aromatic rings. The third class of products contains nonpolar aromatic compounds formed as a result of non-pyrogenic charring processes.