Evaluating the effects of contact time and leaching solution on measured solubilities of aerosol trace metals

Aerosol trace metals have large impacts on oceanic biogeochemistry, atmospheric chemistry and human health. Several leaching protocols have been used to extract dissolved aerosol trace metals, leading to large uncertainties in their measured solubilities. However, the impacts of leaching protocols on measured solubilities are not well understood. This work compared solubilities of eight metals determined using two leaching solutions (ultrapure water versus 5 mmol/L acetate buffer at pH of 4.3) for ambient aerosol particles, and examined the effects of contact time (2 versus 8 h) on measured solubilities. In ultrapure water, overall increase in contact time from 2 to 8 h led to increase in metal solubilities, and the effects of contact time on measured solubilities exhibited considerable variabilities for different metals; on the other hand, the effects of contact time on measured solubilities were rather limited in acetate buffer. Compared to contact time (2 versus 8 h), leaching solutions (ultrapure water versus acetate buffer) were found to have greater impacts on measured solubilities: for the eight metals examined, median values of metal solubilities in acetate buffer were 1.7–15.3 times larger than those in ultrapure water when the contact time was 2 h. Furthermore, the effects of leaching solutions on measured solubilities exhibited substantial variabilities for different metals, and appeared to be larger for metals with lower solubilities. For a given metal, the effects of contact time and leaching solutions on measured solubilities also displayed substantial variations among different samples, implying that there were no apparent relationships between metal solubilities determined using different leaching protocols.