Impact of Ozonolysis on Brown Carbon Light Absorption

Abstract:

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. While some aerosols are purely scattering, there has been an increased focus on organic aerosol capable of absorbing light, commonly referred to as brown carbon (BrC). One suggested pathway for formation of brown carbon is via the reaction of di-carbonyl compounds, such as glyoxal, with amines or ammonia to form imidazole-based structures capable of absorbing light. Despite the known light absorption by brown carbon, there is still significant uncertainty in the atmospheric importance of brown carbon. Studies suggest that different atmospheric oxidants can alter the ability of BrC to absorb incoming solar radiation, leading to less absorption, however most previous studies have focused on oxidation with OH radical chemistry. As the impact of ozone on BrC is poorly constrained, it is difficult to fully characterize the climatic impact of carbonyl-amine based BrC. Here we report the first measurements of how ozonolysis affects the composition and optical properties of glyoxal-based BrC and discuss the potential importance of ozonolysis on the radiative impact of BrC aerosol. BrC aerosols are passed into a flow tube and subject to ozone exposure at levels equivalent to hours of ozone exposure in the atmosphere. Aerosol Mass Spectrometry is used to measure changes in the aerosol composition, while a combination of Photoacoustic and Cavity Ring Down spectroscopies characterizes the changes in the optical properties of the brown carbon aerosol. We summarize our findings and discuss implications for the climatic effect of aged brown carbon aerosols.