Atmospheric Processing of Anthropogenic Combustion Particles: Effects of Acid Media and Solar Flux on the Iron Mobility from Fly Ash

Atmospheric combustion particles, such as fly ash emitted from coal-fired power plants, are a potential source of atmospheric iron, with significant implications in climate and global biogeochemical cycles. While the iron content and speciation of fly ash depend closely on the source region and combustion process, few studies have been carried out comparing the atmospheric processing of fly ash produced from coal-fired power plants in different regions. In this study, we present an investigation of iron dissolution in acidic aqueous solutions: HNO3 and HCl at pH 1 and pH 2 under daytime and nighttime conditions for three fly ash samples from three different sources: United States (USFA), Central Europe (EUFA), and India fly ash (INFA). Iron mobility and speciation depend on the source region and the combustion efficiency that generates fly ash. In HCl suspensions, proton-promoted mechanisms lead to larger fractions of aqueous-phase iron leached from fly ash particles, with poorly combusted samples providing significant fractions of Fe2+. In HNO3 suspensions, a surface-mediated redox reaction suppresses the mobility of Fe2+, leading to the formation of nitrites. In the presence of solar radiation, previously unrecognized pathways of atmospheric processing enhance the formation Fe2+ and nitrous acid from combustion particles. The information provided herein could be significant to increase our understanding of the effects of combustion particles in the atmospheric chemical balance regarding iron and nitrites.