Aqueous Phase Photo-oxidation of Brown Carbon Nitrophenols: Reaction Kinetics, Mechanism, and Evolution of Light Absorption

Abstract

Light absorbing organic aerosol particles, referred to as brown carbon, are geographically widespread and can have an important climate impact through the absorption of solar radiation. Recent studies, both in the laboratory and the field, have shown that brown carbon aerosols can be bleached of their color by direct photolysis and photo-oxidation reactions on the time scale of hours to days. However, the photo-oxidation of nitrophenol molecules, which are colored compounds often associated with biomass burning organic aerosol, show an enhancement in light absorption before the color is lost. This study investigates the mechanism of color enhancement and the fate of three nitrophenol compounds, specifically nitrocatechol, nitroguaiacol, and dinitrophenol, in the aqueous phase using online aerosol chemical ionization mass spectrometry (aerosol-CIMS). The second-order rate constants for the three nitrophenols with OH radicals in the aqueous phase at pH 7 (298 K), were determined to be 5 × 109 M–1 s–1, 5.2 ×...