Fly ash are combustion particles produced in coal-fired power plants and carried by flue gases into the air. Despite continuous efforts to contain it, fly ash has been observed in aerosol plumes at a global scale. In fact, because of its morphology and size distribution, fly ash particles show long transport and relatively high atmospheric lifetimes. Fly ash particles are rich in metal oxides that can interact with atmospheric trace gases, induce cloud formation, and leach metals. Yet, relatively little is known about the impact of the atmospheric processing of fly ash in the environment. Here we report recent efforts to increase our understanding of the photochemistry of trace gas nitric acid on metal oxides, studies on weathering of fly ash, and effects on cloud formation and biogeochemical cycles. Chemisorbed nitric acid photochemistry on components of atmospheric particles contributes to the renoxificaton of the troposphere as the particle surface is acidified. Simulations of the low pH deliquescent layer on fly ash were carried out to investigate the iron mobility from well-characterized fly ash samples from three distinctive locations: U.S., India, and Europe. Proton-promoted iron leaching from fly ash showed significant variability between source regions. The combustion process of fly ash also proved to be significant, as well-combusted fly ash samples leached mostly Fe<sup>3+</sup> rather than bioavailable Fe<sup>2+</sup>. Overall, iron leach showed no clear correlation with particle properties other than its water affinity, which was investigated through experiments of adsorption/desorption of water. The resulting water uptake isotherms suggest that a higher water affinity leads to multilayer formation, with monolayers forming above 25% relative humidity. The adsorption/desorption isotherms show that water tends to remain adsorbed onto the surface of fly ash samples that contain a higher fraction of particles in the sub-micron scale. This apparent selective hysteresis suggests a capillary adhesion effect in addition to some limited porosity on carbonaceous particles. Overall, our studies indicate that fly ash can have a direct impact in cloud properties and iron deposition.

Atmospheric fate of fly ash: From heterogeneous photochemistry of nitric acid to particle weathering

254th National Meeting (2017)

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