Probing the pH dependence of brown carbon formation: Insights from laboratory studies on aerosol particles and bulk phase solutions

Light-absorbing organic aerosol (brown carbon, BrC) can have a significant impact on the radiative balance of the Earth’s atmosphere. However there are still substantial uncertainties regarding the formation, composition, and radiative properties of BrC. In this study, we conducted laboratory experiments to investigate the pH dependence of BrC formation in both aerosol particles and bulk phase solutions. Using glyoxal, ammonia, and ammonium salts, we generated precursor solutions under varying bulk pH conditions ranging from 0.69 to 8.43. Drying the solutions either in the bulk or aerosol phase resulted in BrC formation. The resulting organic material was analyzed to determine its chemical composition and optical properties. Under the set of conditions investigated here, neutral to basic conditions of relevance to cloud water favored BrC formation for both aerosols and bulk solutions. In contrast, BrC products were formed under acidic conditions only in the aerosol phase. Due to rapid equilibration with the gas phase and evaporative losses of water, the aerosols probed here likely had extremely low pH values, well below the bulk pH of 0.69. By achieving such acidic conditions in the aerosol phase, new acid-catalyzed pathways are possible to form BrC. These findings indicate brown carbon formation is favored at both high and very low pH, and further point to the importance of using aerosol samples in studies of pH dependent chemistry of relevance to the atmosphere. Copyright © 2023 American Association for Aerosol Research