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

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 × 109 M–1 s–1, and 3.7 × 109 M–1 s–1 for nitrocatechol, nitroguaiacol, and dinitrophenol, respectively. For a representative aqueous OH concentration, these rate constants correspond to an aqueous lifetime with respect to OH on the order of hours. While the nitrophenol molecules react rapidly with OH, the initial products, which are functionalized by additional electron-donating OH groups, likely lead to the observed absorption increase in the visible range. Further photo-oxidation fragments the aromatic structure to produce smaller, highly oxygenated molecules which no longer absorb strongly at visible wavelengths. These products include furoic acid, glyoxylic acid, malonic acid, oxalic acid, and isocyanic acid. All three nitrophenols investigated formed similar products during photo-oxidation, suggesting that these results could be generalized to this larger class of compounds.