Heterogenous reactions on atmospheric carbonaceous particles

Abstract

Elevated particle concentrations in the atmosphere have received significant attention due to their multiple effects from urban to global scales. The sources and formation mechanisms of these air pollutants, however, are poorly understood, especially atmospheric reactions involving oxidation and condensation of gas phase hydrocarbons, catalyzed by atmospheric particles. This work examines these particle catalyzed, gas to particle transformation processes, through detailed photochemical modeling of air quality episodes in Texas. Two heterogeneous reaction pathways were incorporated into a widely used photochemical model. The pathways that were examined were (1) Heterogeneous formation of sulfuric acid on carbonaceous particles and (2) Acid catalyzed condensation reactions of low molecular weight aldehydes. The impact of these pathways on air quality in Texas was examined for a period during which wildfires generated significant amounts of carbonaceous atmospheric aerosol. This episode was chosen for analysis because the rates of the heterogeneous reactions were expected to be significant during this period, leading to observable signals in ambient data. Simulations, together with ambient data, indicated that wood smoke mediated sulfate formation reactions, not accounted for in most current photochemical models, may have led to 5-10 µg/m3 of sulfate formation. In photochemical simulations, wood-smoke mediated sulfate formation was modeled by calculating the rate of impingement of SO2 molecules on the wood smoke particles, and then assuming that a fraction of the impingements resulted in reaction. For reaction probabilities on the order of 0.01, the model predicted magnitudes, spatial distributions and temporal distributions of sulfate concentrations consistent with observations. Simulations, together with ambient data, indicated that acid aerosol mediated organic aerosol formation reactions, not accounted for in most current photochemical models, may have led to 1-5 µg/m3 of organic aerosol. In photochemical simulations, acid mediated organic aerosol formation was modeled by calculating the rate of impingement of aldehyde molecules on acidic particles, and then assuming that a fraction of the impingements resulted in reaction. For reaction probabilities on the order of 0.0005, the model predicted magnitudes, spatial distributions and temporal distributions of organic aerosol consistent with observations. Ambient data and model results in this work demonstrated that the heterogeneous reactions on carbonaceous particles are significant. The methodology developed in this work, most importantly, can be applied to other heterogeneous reactions to be incorporated into a photochemical model.