Atmospheric chemical processing dictates aerosol aluminum solubility: insights from field measurement at two locations in Northern China

Deposition of mineral dust aerosol into open oceans impacts marine biogeochemistry, and the deposition flux can be constrained using dissolved aluminum (Al) in surface seawater as a tracer. However, aerosol Al solubility, a critical parameter used in this method, remains highly uncertain. We investigated seasonal variations of Al solubility for supermicron and submicron particles at two locations in Northern China. Aerosol Al solubility was very low at Xi'an (0.11 %–9.1 %), showed no apparent variation with seasons or relative humidity, and was not correlated with sulfate or nitrate; in contrast, it was much higher at Qingdao (0.06 %–23.4 %), exhibited distinct seasonal variability, and increased with relative humidity and the abundance of sulfate and nitrate. All these features observed can be explained by the effects of atmospheric chemical processing. Mineral dust transported to Xi'an, an inland city in Northwest China, was still not obviously aged and thus chemical processing had little effect on Al solubility; after arriving at Qingdao, a coastal city in the Northwest Pacific, mineral dust was substantially aged by chemical processing, leading to significant enhancement in Al solubility. Our work further reveals that aerosol liquid water and acidity play vital roles in the dissolution of aerosol Al by chemical processing. We suggest that chemical aging can lead to spatiotemporal variation of aerosol Al solubility, and this should be considered when using dissolved Al in surface seawater to constrain oceanic dust deposition.