Effect of pH on the Chemical and Optical Properties of Brown Carbon Aerosol

It is recognized that tropospheric aerosols play an important role in climate by scattering and absorbing incoming solar radiation, leading to cooling at the Earth's surface and potentially warming in the atmosphere. For instance, aerosol formed from the reaction of di-carbonyl compounds, such as glyoxal, with ammonium salts have the potential to absorb light, commonly called Brown carbon (BrC) aerosol. Past studies on glyoxal-based BrC have been performed using aqueous solutions reacting ammonium salts or amines with glyoxal to form BrC analogs. However these studies usually do not explicitly control aerosol pH. Recent field observations suggest aerosol pH is often in the range pH= 0-2 and thus ammonium bisulfate shou ld dominate over ammonium sulfate. In the present study, BrC aerosol was formed by mixing solutions of glyoxal, ammonium salts, and ammonia to achieve various pH values while keeping the other species constant. After reaction for multiple days, the solutions were atomized and the composition was measured using Aerosol Mass Spectrometry (AMS) while the aerosol light extinction and absorption were measured using Cavity ring-down (CRD) and Photoacoustic spectroscopies (PAS) respectively. Visual inspection of the solutions revealed that relatively basic solutions (pH>5) changed to a dark yellow color over time. In contrast, the acidic solutions (pH<4) remained clear. In addition, aerosol from acidic solutions were composed of less organic mass compared to basic solutions as detected in the AMS, likely due to unreacted glyoxal partitioning out of the aerosol as it dried. Further, the AMS detected an increase in mass fragments believed to correlate to BrC molecules, such as imidazole, as the pH increased. Finally, an increasing trend in light absorption was detected in the PAS as the pH of the solutions increased. These results suggest that BrC formation is hindered under acidic conditions and therefore BrC may not be as warming as previously believed.