Hygroscopic Properties of Aminium Sulphate Aerosols

Alkylaminium sulphates originate from the neutralisation reaction between short-chained amines and sulphuric acid and have been detected in atmospheric aerosol particles. Their physicochemical behaviour is less well characterised than their inorganic equivalent, ammonium sulphate, 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 sulphates, specifically mono-, di- and tri-methylaminium sulphate and mono-, di- and tri-ethyl aminium sulphate. This approach allows the retrieval of a water activity dependent growth curve in less than 10 s, avoiding the uncertainties that can arise from the volatilisation of semi-volatile components. Measurements are made on particles >5 m 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 >0.98, along with accurate parameterisations of solution density and refractive index. The uncertainties in water activities associated with the hygroscopicity measurements are typically <±0.2% at water activities >0.9 and ∼±1% below 0.9, with maximum uncertainties in diameter growth factors of ±0.7%. Comparison with previously reported measurements show deviation across the whole water activity range.