Functional Group Reactivity in Brown Carbon Photolysis in Concentrated Solution from Methylglyoxal with Ammonium Sulfate

Abstract:

Atmospheric Brown carbon (BrC) is composed of a class of organic compounds that can absorb solar radiation present in the UV/VIS wavelength range. Research in this field has been focused on characterizing the formation and composition of BrC, however, the removal mechanisms of BrC are well understood. For example, previous work has been done with aqueous solutions of BrC using methylglyoxal with ammonium sulfate to produce imidazoles which are light-absorbing aerosol chromophores. These compounds were analyzed using high-resolution mass spectrometry. In this work, we probe BrC photolysis of a similar system to investigate how the known functional groups change during photolysis. This sample was analyzed using UV-Vis spectrometry to understand how the concentration and the solvent (water vs. acetonitrile) of the BrC solution affect the light absorptivity as a function of time. In addition, an Aerosol Mass Spectrometer and FTIR were used to further characterize the molecular functional groups that are modified during the aging process for concentrated BrC solutions from different time frames. For dilute solutions with UV/Vis, we observe that the BrC has a fast-aging time (a lifetime of roughly 8 minutes) with exponential decay of absorbance. For the concentrated samples, we observe a slower photolytic decay, likely as a result of decreased light flux due to the concentrated sample. These concentrated samples also show a shift in the functional groups with the potential formation of carbonyls as initial photolysis products.