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Effects of Ammonium Sulfate on the Photodegradation of Toluene-Derived Brown Carbon Secondary Organic Aerosol
ALEXANDRA KLODT, Rachel O'Brien, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 73
Working Group: Aerosol Chemistry
Abstract
Secondary organic aerosol (SOA) derived from aromatic precursors contributes to the pool of atmospheric brown carbon by absorbing solar radiation in the near-UV and visible region of the solar spectrum. Previous studies have observed changes in the absorption spectrum of brown carbon and some other atmospherically relevant compounds with the addition of ionic species. However, there is a lack of consensus on the effect of ionic strength on chromophoric compounds and aerosols. Some studies have shown increased photodegradation rates with increasing ionic strength, while others observed increased absorbance but reduced rates of photobleaching. In this work, toluene SOA produced by photooxidation in the presence of NOx was used to study the effect of ionic strength on photodegradation rates of brown carbon SOA. The SOA was dissolved in aqueous solution at pH 4 and pH 1 and aged with photolysis in the presence or absence of 1 M ammonium sulfate, and UV-Vis spectroscopy was used to monitor the changes in absorption over time in both conditions. In a parallel set of experiments, SOA was photolyzed directly on a filter, and the distribution of functional groups was tracked using FTIR spectroscopy. Electrospray ionization high-resolution mass spectrometry with liquid chromatography separation (LC-ESI-PDA-HRMS) was finally used to observe changes in SOA composition from photolysis in both sets of experiments, allowing analysis of changes in chromophore abundance between the conditions. Preliminary results in the aqueous phase show increased initial absorbance but decreased photobleaching with ammonium sulfate, while the filter experiments suggest that photolysis causes the loss of nitroaromatic compounds as SOA degrades. Results from this study will provide a more complete understanding of the processes influencing the aging of brown carbon aerosol in the atmosphere.