Alkylaminium sulfates are frequently detected in ambient aerosols, and are believed to be important in the nucleation of new particles in the atmosphere, despite the comparatively low gas phase concentrations of amines. In this study, water activities and osmotic coefficients have been measured, using a chilled mirror dew point technique, of aqueous mixtures of sulfuric acid and the following alkylaminium sulfates: methylaminium, ethylaminium, dimethylaminium, diethylaminium, and trimethylaminium sulfate. The samples were prepared by mixing solutions of the five corresponding amines and aqueous sulfuric acid and determining the exact aminium to sulfate molar ratios by ion chromatography. The results were correlated using an extended Zdanovskii-StokesRobinson equation to enable concentration/water activity relationships to be calculated over the entire composition range from pure aqueous sulfuric acid to pure aqueous aminium sulfate. Water activities and osmotic coefficients for aminium:sulfate ratios of 1:1 (the bisulfate salts) and lower showed great similarity with ammonium bisulfate, but osmotic coefficients for the 2:1 ratio (the aminium sulfates) were significantly larger (and water activities lower) than for ammonium sulfate. These results differ from those obtained in Clegg et al.'s (2013) study. The relative values of the osmotic coefficients, in concentrated solutions, suggest that the numbers of methyl or ethyl groups in the aminium ion may have a stronger lowering effect on water activity than the alkyl chain length.

Alkyl aminium sulfates (AASs) can affect the physicochemical properties of atmospheric aerosols such as hygroscopicity. Previous laboratory experiments have shown that the water content in AAS bulk solutions is higher than in aqueous ammonium sulfate solution in the range of 60-95% relative humidity (RH). Furthermore, amine was found to evaporate from the solution during the preparation of AASs from the parent amine and sulfuric acid solutions. Here we report the hygroscopicities of deposited particles of four AASs at different aminium-to-sulfate molar ratios (A/Ss) in the range of <3-90% RH using air-flow cells coupled with in situ micro-Raman spectroscopy. Normalized integrated areas of O-H stretching peaks in the Raman spectra were converted to water-to-solute molar ratios (WSRs) at various RH values. Evaporation of amine was also observed in most cases and the exact A/Ss of sample particles or solutions were determined by ion chromatography. Mono-methylaminium sulfate (MMAS) and mono-ethylaminium sulfate (MEAS) particles were stable at A/S = 2.0, but di-methylaminium sulfate (DMAS) and tri-methylaminium sulfate (TMAS) suffered from DMA and TMA evaporation and eventually equilibrated to the A/S of 1.5 and 1.0, respectively. At these stable compositions MMAS and MEAS exhibited phase transitions in the super-saturation region, while DMAS and TMAS showed a continuous and reversible water uptake. Besides, an approach to estimate the hygroscopicities of DMAS and TMAS particles at an initial A/S larger than that of the stable compositions was presented. In the range of 60-95% RH, the WSRs of all the studied AAS particles were consistent with a previous study based on experimental values and the extended Zdanovskii-Stokes-Robinson equation. In general, all the studied AASs were more hygroscopic than their corresponding ammonium counterparts within the studied RH range and evaporation of amine needs to be corrected in studying unstable AAS particles.

Heterogeneous uptake is one of the major mechanisms governing the amounts of short-chain alkylamines and ammonia (NH 3 ) in atmospheric particles. Molar ratios of aminium to ammonium ions detected in ambient aerosols often exceed typical gas phase ratios. The present study investigated the simultaneous uptake of dimethylamine (DMA) and NH 3 into sulfuric and oxalic acid particles at gaseous DMA / NH 3 molar ratios of 0.1 and 0.5 at 10, 50 and 70 % relative humidity (RH). Single-gas uptake and co-uptake were conducted under identical conditions and compared. Results show that the particulate dimethylaminium/ammonium molar ratios (DMAH / NH 4 ) changed substantially during the uptake process, which was severely influenced by the extent of neutralisation and the particle phase state. In general, DMA uptake and NH 3 uptake into concentrated H 2 SO 4 droplets were initially similarly efficient, yielding DMAH / NH 4 ratios that were similar to DMA / NH 3 ratios. As the co-uptake continued, the DMAH / NH 4 gradually dropped due to a preferential uptake of NH 3 into partially neutralised acidic droplets. At 50 % RH, once the sulfate droplets were neutralised, the stronger base DMA displaced some of the ammonium absorbed earlier, leading to DMAH / NH 4 ratios up to four times higher than the corresponding gas phase ratios. However, at 10 % RH, crystallisation of partially neutralised sulfate particles prevented further DMA uptake, while NH 3 uptake continued and displaced DMAH + , forming almost pure ammonium sulfate. Displacement of DMAH + by NH 3 has also been observed in neutralised, solid oxalate particles. The results can explain why DMAH / NH 4 ratios in ambient liq-uid aerosols can be larger than DMA / NH 3 , despite an excess of NH 3 in the gas phase. An uptake of DMA to aerosols consisting of crystalline ammonium salts, however, is unlikely, even at comparable DMA and NH 3 gas phase concentrations.

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&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Heterogeneous uptake is one of the major mechanisms governing the amounts of short-chain alkyl-amines and ammonia (NH&lt;sub&gt;3&lt;/sub&gt;) gases resident in atmospheric particles. Molar ratios of aminium to ammonium ions detected in ambient aerosols often exceed typical gas phase ratios. The present study investigated the simultaneous uptake of dimethylamine (DMA) and NH&lt;sub&gt;3&lt;/sub&gt; into sulfuric and oxalic acid particles at gaseous DMA/NH&lt;sub&gt;3&lt;/sub&gt; molar ratios of 0.1 and 0.5 at 10&amp;#8201;%, 50&amp;#8201;%, and 70&amp;#8201;% relative humidity (RH). Single gas uptake and co-uptake were conducted under identical conditions and compared. Results showed that the particulate dimethyl-aminium/ammonium molar ratios (DMAH/NH&lt;sub&gt;4&lt;/sub&gt;) changed substantially during the uptake process, which was predominantly influenced by the extent of neutralization and the particle phase state. DMA uptake and NH&lt;sub&gt;3&lt;/sub&gt; uptake into concentrated H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; droplets were initially similarly efficient, yielding DMAH/NH&lt;sub&gt;4&lt;/sub&gt; that were similar to DMA/NH&lt;sub&gt;3&lt;/sub&gt; ratios. As the co-uptake continued the DMAH/NH&lt;sub&gt;4&lt;/sub&gt; gradually dropped due to a preferential uptake of NH&lt;sub&gt;3&lt;/sub&gt; into still acidic droplets. Once the droplets were neutralized, the stronger base DMA displaced some of the ammonium absorbed earlier, leading to DMAH/NH&lt;sub&gt;4&lt;/sub&gt; that were up to four times higher than the corresponding gas phase ratios. At 10&amp;#8201;%&amp;#8201;RH, crystallization of partially neutralized sulfate particles prevented further DMA uptake, while NH&lt;sub&gt;3&lt;/sub&gt; uptake continued, and displaced DMAH&lt;sup&gt;+&lt;/sup&gt; after the solid particles were completely neutralized, forming almost pure ammonium sulfate. Displacement of DMAH&lt;sup&gt;+&lt;/sup&gt; by NH&lt;sub&gt;3&lt;/sub&gt; has also been observed in neutralized, solid oxalate particles. The results illustrate why in ambient liquid aerosols the DMAH/NH&lt;sub&gt;4&lt;/sub&gt; can be larger than DMA/NH&lt;sub&gt;3&lt;/sub&gt;, despite of an excess of NH&lt;sub&gt;3&lt;/sub&gt; in the gas phase; the uptake of DMA to aerosols consisting of crystalline ammonium salts, however is unlikely, even if the gas concentrations of DMA and NH&lt;sub&gt;3&lt;/sub&gt; are of the same magnitude.&lt;/p&gt;

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