Abstract
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.
Highlights
Continuing in the methylaminium sulfates series, a slight decrease in GFr is observed in the high water activity region for DMAS and TMAS, whereas at lower water activities, the hygroscopic properties of these compounds converge to (NH4)2SO4 within the uncertainties of the measurements as the amount of water in the particles decreases
The trends that have been observed for TEAS when compared to the other five aminium sulfate systems seem to be completely plausible, and this may indicate that the assumed initial TEA concentration is reasonable
The role of short-chained alkylamines in the formation of new particles has been investigated in recent literature and found to be significant
Summary
Quantifying the response of aerosol particles to variations in relative humidity (RH) in the atmosphere is important for predicting the impact of aerosols on climate through both direct and indirect radiative forcings (Albrecht, 1989; Kanakidou et al, 2005; Kolb et al, 2010), for understanding the mechanisms of heterogeneous chemistry (Dennis-Smither et al, 2014; Lee et al, 2012), and the partitioning of products in the condensed phase (Dusek et al, 2006; Topping et al, 2013a), and for inferring the deposition and loss mechanism of aerosols on surfaces and on inhalation into the respiratory tract (Broday and Georgopoulos, 2001; Haddrell et al, 2015). Despite the volatility of short-chained alkylamines, these compounds can undergo gas-to-particle partitioning due to a variety of chemical processes (Ge et al, 2011a): direct solubilisation, oxidation reactions that lead to the formation of secondary organic aerosols, acid–base reactions similar to those of ammonia (Seinfeld and Pandis, 2006) with both inorganic (sulfuric, nitric and hydrochloric acids) and organic acids (Lavi et al, 2015; Liu et al, 2012), and displacement reactions of ammonium cations with aminium cations (Bzdek et al, 2010; Qiu and Zhang, 2013).
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