Hygroscopic properties of aminium sulfate aerosols

Abstract

Alkylaminium sulfates originate from the neutralisation reaction between short-chained amines and sulfuric acid and have been detected in atmospheric aerosol particles. Their physicochemical behaviour is less well characterised than their inorganic equivalent, ammonium sulfate, even though they play a role in atmospheric processes such as the nucleation and growth of new particles and cloud droplet formation. In this work, a comparative evaporation kinetics experimental technique using a cylindrical electrodynamic balance is applied to determine the hygroscopic properties of six short-chained alkylaminium sulfates, specifically mono-, di-, and tri-methylaminium sulfate and mono-, di-, and tri-ethyl aminium sulfate. This approach allows for the retrieval of a water-activity-dependent growth curve in less than 10ĝ€s, avoiding the uncertainties that can arise from the volatilisation of semi-volatile components. Measurements are made on particles >ĝ€5ĝ€μm in radius, avoiding the need to correct equilibrium measurements for droplet-surface curvature with assumed values of the droplet-surface tension. Variations in equilibrium solution droplet composition with varying water activity are reported over the range 0.5 to >ĝ€0.98, along with accurate parameterisations of solution density and refractive index. The uncertainties in water activities associated with the hygroscopicity measurements are typically ĝ€0.9 and ĝ€‰ĝ1/4ĝ€‰ĝ€±1ĝ€% below 0.9, with maximum uncertainties in diameter growth factors of ±0.7ĝ€%. Comparison with previously reported measurements show deviation across the entire water activity range.