Impact of Particle Generation Method on the Apparent Hygroscopicity of Insoluble Mineral Particles

Calcite (CaCO 3 ) mineral particles are commonly generated by atomization techniques to study their heterogeneous chemistry, hygroscopicity, and cloud nucleation properties. Here we investigate the significant artifact introduced in generating calcium mineral particles through the atomization of a saturated suspension of the powder in water, by measuring particle hygroscopicity via CCN activation curves. Particles produced from atomization displayed hygroscopicities as large as κ app > 0.1, 100 times more hygroscopic than that obtained for dry-generated calcite, κ app = 0.0011. The hygroscopicity of the wet-generated particles increased as a function of time the calcite powder spent in water, and with decreasing particle size. Wet-generated calcium oxalate was more hygroscopic through wet- (κ app = 0.34) versus dry-generation (κ app = 0.048). Atomized calcium sulfate particles, however, were only slightly more hygroscopic (κ app = 0.0045) than those generated dry (κ app = 0.0016). Single-particle analysis by ATOFMS and SEM/EDX, and bulk analysis of the calcite powders by ICP-MS and IC revealed no significant soluble contaminants. The atomized particles were likely composed of components that dissolved from the powder and then re-precipitated, and appeared to contain little of the original mineral powder. The increased hygroscopicity of atomized calcite may have been caused by aqueous carbonate chemistry producing Ca(OH) 2 , Ca(HCO 3 ) 2 , and metastable hydrates with increased solubility. Surface water adsorption may have also played a role, in addition to uncharacterized soluble components produced by wet-generation, and the precipitation of amorphous phases including glassy states. This study suggests that using wet-generation methods to suspend mineral dust samples will not produce particles with the correct physicochemical properties in laboratory studies, a finding which has important implications for past and future laboratory studies focusing on understanding relationships between the hygroscopicity and chemistry of mineral dust particles.