Evolution of Light Absorption of Brown Carbon Aerosol following Ozonolysis

Tropospheric aerosols play an important role in Earths radiative balance by scattering and absorbing incoming solar radiation. Recently, there has been an increased focus on the fraction of organic aerosol capable of absorbing light, commonly referred to as brown carbon (BrC). It has been suggested that one pathway for BrC formation is via the reaction of di-carbonyl compounds, such as glyoxal, with amines or ammonium salts to form imidazole-based structures capable of absorbing light. Despite the known light absorption by BrC, there is still significant uncertainty in the atmospheric importance of BrC. Studies suggest that different forms of atmospheric aging can alter the ability of BrC to absorb incoming solar radiation. Most studies of the brown carbon bleaching have focused on photolysis or oxidation by OH radicals. Here we report the first measurements of brown carbon bleaching by ozonolysis. BrC aerosols are passed into a flow tube and exposed to ozone for the equivalent of hours of ozone exposure in the atmosphere. Following the flow tube, the optical properties of the aerosol are characterized with Photoacoustic and Cavity Ring down spectroscopies while chemical composition is monitored with Aerosol Mass Spectrometry. Along with observing compositional changes, we have been able to monitor the evolution of the aerosols complex refractive indexes over time as the aerosol becomes aged with ozone. In addition, kinetic information such as rate constants and uptake coefficients were obtained for the ozonolysis reaction, which can assist in modeling in simulating the radiative impact and lifetime of BrC aerosol in the atmosphere.