Aquatic Geochemistry 1: 355-374, 1996. 355 © 1996 Kluwer Academic Publishers. Printed in the Netherlands. Factors Controlling the Solubility of Aerosol Trace Metals in the Atmosphere and on Mixing into Seawater LUCINDA J. SPOKES and TIM D. JICKELLS School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. (Received: 9 May 1995; in final form: 1 December 1995) Abstract. Previous work has shown that the type and pH history of an aerosol governs trace metal solubility in rainwater. This study concentrates on the crustal elements AI, Fe and Mn and identifies additional processes which affect dissolution not only in the atmosphere but also on mixing into seawater. Aerosol dissolution experiments (at aerosol concentrations of about 30 mg 1-1) show manganese exhibiting high solubility at the low pH values typical of clouds (54 -4- 2.5% at pH 2, with results expressed in mole percent units) with 85% of this increase occurring within 6 hours of acidification. The percentage dissolution decreases to 50% at pH values representative of rainwater (pH 5.5) and to 26 4- 4% at pH 8, typical of seawater. No such dramatic solution phase removal occurs at pH 8 in the presence of inorganic anions (to a final solubility of 44 4- 2%). Thus the extent of manganese dissolution depends strongly on whether aerosols are cycled through acidic environments and on subsequent inorganic complexation once rainwater mixes into sea. Aluminium shows highest dissolution (7.1 + 0.6%) at low pH with 78% of this increase occurring within 6 hours of acidification. Rapid solution phase removal occurs on increasing the pH to that representative of rainwater (to 0.9 4- 0.4% with 87% of this decrease occurring within 15 min). As a consequence of acid cycling and aluminium's amphoteric nature, solubility is enhanced at seawater pH (2.3 -4-0.3%) over that in rain. Iron shows a strong pH-solubility relationship with highest solubility at low pH (4.7 4- 0.2%), 70% of this value being reached within 6 hours of acidification, and decreasing rapidly to 0.17% as pH is raised to 8. Addition of inorganic anions at pH 8 to simulate mixing into seawater causes a further decrease in solubility, perhaps due to anion induced colloid destabilisation. Photochemical reduction also effects solubility under low pH conditions with Fe(II) comprising 1% of the total iron in the Saharan Aerosol used and 8.4% in an Urban material at a pH of .~ 2. This element Shows rapid solution phase removal with increasing particulate load which is tentatively rationalised in terms of a simple Kd approach. Key words: aerosol dissolution, atmosphere, rainwater, seawater, trace metals, speciation, pH cycling, photochemistry, particulate load. 1. Introduction The atmosphere is now recognised as a major source of trace metals to the oceans (Duce et al., 1991). Metals enter the marine environment from the atmosphere through either wet or dry deposition and their fate, in terms of biogeochemical reactivity, depends on their solubility both prior to deposition, in the case of precip- itation, as well as after mixing into seawater. There has been considerable debate as to these solubility estimates (see Jickells (1995) for review) although progress