Chemistry of iron in marine aerosols

Abstract

Abstract. Remote marine aerosols collected at four Pacific island stations (Midway, Oahu, Enewetak, and Fanning) and during four Pacific cruises were used in determining the chemical form of iron in marine atmospheric aerosols. Aerosols collected from an urban area, Xian, near the Chinese loess plateau were also studied using the same procedures. Marine and urban Chinese aerosol samples and Chinese loess samples were leached by acidic deionized water over a pH range of 2.0–5.6 to determine the extent of dissolution of iron in atmospheric waters. The results indicated that (1) more than 90% of the atmospheric iron in remote marine aerosols over the North Pacific is released in 2 N acidic solutions, and in the mean ~50% of the atmospheric iron in remote marine aerosols was in the form of Fe(II); and (2) the solubility of iron in the marine aerosols was 5–17 times greater than that of iron in Chinese loess at the same pH. This may be largely due to the photoreduction of Fe(III) to Fe(II) during its long‐range transport. At a total iron concentration of less than 20 nmol/kg, ∼55% of the atmospheric iron dissolves in solutions with a pH of 3.8 and 25–30% dissolves at a pH of 4.8. We postulate that (1) the mixing of mineral dust with sulfate/sulfuric acid aerosols as a result of coalescence processes within clouds might be a mechanism to increase the solubility of atmospheric iron in cloud water or rainwater; and (2) photochemical reduction of Fe(III) to Fe(II) may be an important mechanism increasing the solubility of iron, which would be rapidly released to rainwater, fog, or cloud droplets when the aerosol particles contact them during long‐range transport over the open ocean. The total atmospheric iron input was estimated to be ∼15 ‐ 25 × 1012 g/yr for the entire North Pacific Ocean.