Secondary Brown Carbon Aerosol Resists Bleaching by Ozone under Acidic Conditions.

Abstract

Light-absorbing organic aerosol (brown carbon, BrC) can affect Earth's radiative balance. However, owing to uncertainties in BrC sources, composition, and lifetime, the radiative impact of BrC is poorly constrained. In particular, the effects of heterogeneous oxidation and the influence of aerosol pH on the lifetime and light absorption properties of BrC are not well established. In a series of laboratory experiments, we characterize the changes in the chemical composition and optical properties of BrC aerosol upon heterogeneous oxidation by ozone (O₃). BrC analogs were generated by reacting glyoxal with ammonium sulfate in bulk solutions. The resulting solutions were pH adjusted before being atomized and oxidized in a flow reactor, with online measurements of the aerosol optical and chemical properties to monitor changes from oxidation. For the conditions investigated here, we find that ozonolysis diminishes the ability of BrC material to absorb light, presumably due to the degradation of the BrC chromophores. While the BrC has a lifetime of 1-2 h due to ozonolysis, it effectively stops bleaching after <6 h of atmospheric processing, leaving behind an ozone (O₃) resistant fraction of BrC. We observed a pH dependence on oxidation and bleaching with acidic BrC bleaching more slowly and remaining more absorbing than more basic samples. Given that submicron atmospheric aerosols are typically acidic and rapidly undergo partial bleaching, we suggest that the complex refractive index (RI; m) of secondary glyoxal-ammonium BrC should be modeled using data from the recalcitrant fraction of acidic aerosols. This study reports aerosols generated from a pH = 1.51 solution having a RI of m = 1.48 + 1.2 × 10^-3 i and m = 1.53 + 2.9 × 10^-4 i at 405 and 532 nm, respectively after aging with O₃. A comprehensive treatment of BrC lifetime will require this process to be considered in conjunction with other bleaching mechanisms such as photolysis and reactions with OH.