Multiphase Ozone Oxidation of Catechol and Its Products after OH- and Light-Driven Processing

Phenolic compounds are some of the most abundant emissions of biomass burning during wildfires. Catechol, the most abundant isomer of benzenediol in biomass burning emissions, undergoes oxidation in the aqueous phase of cloud droplets to form secondary organic aerosol (SOA), including products that absorb light at visible wavelengths, called brown carbon (BrC) chromophores. After cloud evaporation, the remaining submicron SOA particles are susceptible to further oxidant- and light-driven processing. Here, we investigate the multiphase ozone oxidation of the reaction mixture from the aqueous OH-initiated oxidation of catechol, i.e., simulated OH-driven processing in clouds, using a coated-wall flow-tube apparatus. Reactive uptake of ozone was determined for thin films from the OH-driven processing of catechol with and without further irradiation of the thin films, i.e., simulated light-driven processing after cloud evaporation, at low and moderate relative humidity (RH). The experimental time series were reproduced using kinetic multilayer modeling, which, along with qualitative microscopy experiments, provided insights into the diffusivity of these materials. After OH-driven processing, the thin films exhibited uptake coefficients of 2 × 10^-6 and 9 × 10^-6 at 0 and 50% RH, respectively, and 4 h of exposure to 130 ppb of ozone. After OH- and light-driven processing, the uptake coefficients were lower, 2 × 10^-7 at 0% RH and 4 × 10^-6 at 50% RH, for the same ozone exposure. Consequently, the reaction mixture of catechol was plasticized upon uptake of water vapor but vitrified under UV irradiation. Kinetic multilayer modeling shows that slower ozone diffusion at low RH and after light-driven processing can lead to an increase in the atmospheric lifetime of reactive species from less than 1 h to more than a day.