Post Date: 13 July 2016

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

Abstract

Ammonia (NH₃) and short chain alkyl-amines are important alkaline gases to promote particle nucleation and growth in the atmosphere. Relative to NH₃, amines are often found to be enriched in condensed phase compared to the gas phase.

In this study the simultaneous uptake of NH₃ and dimethylamine (DMA) into sulfuric acid was measured for different amine-to-ammonia gas phase molar ratios at low and moderate relative humidity. Experimental and theoretical investigations of the water activities of five alkyl-amines and their mixtures with H₂SO₄ were further conducted to study the difference of hygroscopic characteristics of aminium and ammonium sulfates. The tendency of many amines to stick to various surfaces and to volatilize from their sulfate salt solutions posed major challenges on the practical implementation of the experiments and were thoroughly addressed in the experimental design.

DMA and NH₃ showed a similarly effective uptake into concentrated H₂SO₄ followed by kinetically favored uptake of NH₃ into still acidic droplets. After reaching neutral composition DMA partially displaced ammonium ions from liquid particles, leading to 2-4 times enhanced dimethylaminium concentrations in the condensed compared to the gas phase. At low relative humidity the presence of dimethylaminium inhibited the formation of ammonium bisulfate, requiring more NH₃ to partition into the condensed phase before crystallization occurred. This phase transition prevented further DMA uptake, while NH₃ partitioning continued and almost entirely displaced dimethylaminium forming ammonium sulfate.

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, but aminium sulfates were significantly more hygroscopic than ammonium sulfate. These findings evidently show the complexity of processes related to amine-ammonia co-absorption and indicate that the resulting aminium-to ammonium ratios can have a significant influence on the physicochemical properties of aerosols.