Articles | Volume 17, issue 6
Research article
31 Mar 2017
Research article |  | 31 Mar 2017

Hygroscopic properties of aminium sulfate aerosols

Grazia Rovelli, Rachael E. H. Miles, Jonathan P. Reid, and Simon L. Clegg

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 10 s, avoiding the uncertainties that can arise from the volatilisation of semi-volatile components. Measurements are made on particles > 5 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 > 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 entire water activity range.

Short summary
The response of airborne particles to relative humidity (RH) in the atmosphere, referred to as their hygroscopicity, is a key property that determines their water content. The hygroscopicity depends on the chemical species that make up the particle. Here, we investigate the growth in particle size and composition that occurs with increasing RH for particles containing a series of amine compounds and sulfuric acid, typical atmospheric components, providing a comprehensive and accurate data set.
Final-revised paper