Heterogeneous uptake of ammonia and alkyl-amines by sulfuric acid particles

Abstract

Ammonia (NH₃) and short-chain alkyl-amines (R₃N) are alkaline gases that play a crucial role in the formation and growth of secondary aerosols. Heterogeneous uptake into atmospheric particles is the dominant removal mechanism of gaseous NH₃ and R₃N. To date, no experimental data are available on the simultaneous uptake of both gases. While properties of ammonium sulfates formed during these atmospheric processes have been well explored, aminium salts often lack a complete characterization, especially in terms of their hygroscopic parameters. In this study, the simultaneous uptake of NH₃ and dimethylamine (dimethylammonium, DMA) into sulfuric acid (H₂SO₄) was measured for different gas molar ratios and relative humidities. Subsequently, water activities of five R₃N and their mixtures with H₂SO₄ were studied, in order to investigate the difference of hygroscopic characteristics of the formed aminium and ammonium sulfates. The tendency of R₃N to stick to various surfaces and to volatilize from their sulfate salt solutions posed major challenges to the practical implementation of the experiments and were thoroughly addressed in the experimental design. The uptake of DMA and NH₃ was strongly dependent on the particle phase state and extent of neutralization. Uptake into concentrated H₂SO₄ droplets was initially similarly effective, followed by a favorable uptake of NH₃ into still acidic droplets. At low relative humidity, the phase transition of partially neutralized particles prevented further DMA uptake, while NH₃ partitioning continued to neutralize the particle. Eventually, NH₃ displaced dimethyl-aminium from the solid neutralized particles to form ammonium sulfate crystals. Yet, the presence of aminium ions suppressed crystallization, requiring more NH₃ to partition into the particle before phase change occurred. At higher relative humidity the particle remained as droplets and DMA and NH₃ did not compete until the particle was fully neutralized, where DMA (as the stronger base) started to displace part of the ammonium ions from the droplet. Investigations of the hygroscopicity of aminium sulfates revealed that water activities of aminium-to-sulfate ratios of 1:1 (the bisulfate salts) and lower, showed great similarity with ammonium bisulfate. Aminium sulfates, however, were significantly more hygroscopic than ammonium sulfate. Thus, the lower the extent of neutralization of the sulfate, the more similar are the hygroscopic parameters of aminium and ammonium sulfates. The extent of neutralization of H₂SO₄ and the particle phase state (solid or liquid) determine the uptake (and displacement) of R₃N relative to NH₃. The resulting amount of aminium sulfate relative to ammonium sulfate in the particle, plus the achieved neutralization ratio in turn significantly influence the hygroscopic properties of the particle and its phase state.

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology