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A number of recent studies have shown that iron dissolution in Fe-containing dust aerosol can be linked to source material (mineral or anthropogenic), mineralogy, and iron speciation. All of these factors need to be incorporated into atmospheric chemistry models if these models are to accurately predict the impact of Fe-containing dusts into open ocean waters. In this report, we combine dissolution measurements along with spectroscopy and microscopy to focus on nanoscale size effects in the dissolution of Fe-containing minerals in low-pH environments and the importance of acid type, including HN[O.sub.3], [H.sub.2]S[O.sub.4], and HCl, on dissolution. All of these acids are present in the atmosphere, and dust particles have been shown to be associated With nitrate, sulfate, and/or chloride. These measurements are done under light and dark conditions so as to simulate and distinguish between daytime and nighttime atmospheric chemical processing. Both size (nano-versus micron-sized particles) and anion (nitrate, sulfate, and chloride) are found to play significant roles in the dissolution of [alpha]-FeOOH under both light and dark conditions. The current study highlights these important, yet unconsidered, factors in the atmospheric processing of iron-containing mineral dust aerosol. mineral dust aerosol | iron-containing dust | nanoscale iron oxides | acid dissolution | photoinduced dissolution doi/ 10.1073/pnas.0910809107