This project will examine the influence of particle size on atmospheric reactions of iron and, in turn, the influence of particle size on iron solubility. In particular, how particle size affects iron solubility during atmospheric processing by two different mechanisms will be investigated. First, the effect of particle size on iron solubility during photoreduction of iron in cloud waters will be determined. Photochemical reduction of aqueous iron has been well studied in authentic and simulated cloud waters, however, particle size has never been considered as a variable but may be important because only a restricted aerosol size fraction can form cloud condensation nuclei (CCN). Previous laboratory studies focusing on particles too big or too small to act as CCN may under- or overestimate the soluble iron resulting from photochemistry in real atmospheric systems. Second, the influence of particle size on iron solubility during reaction with gaseous sulfur dioxide will be investigated. Exposure of iron phases mixed with sodium chloride will be studied at two relative humidities (20% and 80%) representative of dry continental and wet marine environments, respectively. This will allow determination of whether gas-particle or liquid-particle reactions are more important during iron-sulfur interactions. Following exposure to sulfur dioxide, the iron isotope composition of the dissolved iron fraction will be determined and used to elucidate mechanisms.

This research will be a valuable contribution to understanding iron solubility in continental and marine aerosols. The results will be relevant to the broader question of how atmospheric iron affects ocean productivity and, hence, its relation to the global carbon cycle. Iron is a limiting nutrient in many parts of the open ocean and substantial effort has been put forth to determine the availability of iron in atmospheric particulate matter to marine life. This study will also advance the training of the next generation of atmospheric scientists. The project will fund one Ph.D. student at Arizona State University (ASU), with this research being the core of his/her thesis work. In addition, one undergraduate student each year from Northern Arizona University (NAU) will work with the principal investigators and the Ph.D. student at ASU. The Department of Chemistry at NAU is a non-Ph.D. granting department. Therefore, collaboration with ASU researchers will provide undergraduate students at NAU training in research approaches and techniques that are not normally possible at NAU. Furthermore, NAU is among the national leaders in granting degrees to Native American students. These unique demographics will be drawn upon to recruit environmentally-focused Native American undergraduates through the Institute for Tribal Environmental Professionals at NAU.