Heterogeneous uptake is one of the major mechanisms governing the amounts of short-chain alkylamines and ammonia (NH3) 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 NH3 into sulfuric and oxalic acid particles at gaseous DMA?∕?NH3 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 dimethyl-aminium/ammonium molar ratios (DMAH?∕?NH4) 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 NH3 uptake into concentrated H2SO4 droplets were initially similarly efficient, yielding DMAH?∕?NH4 ratios that were similar to DMA?∕?NH3 ratios. As the co-uptake continued, the DMAH?∕?NH4 gradually dropped due to a preferential uptake of NH3 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?∕?NH4 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 NH3 uptake continued and displaced DMAH+, forming almost pure ammonium sulfate. Displacement of DMAH+ by NH3 has also been observed in neutralised, solid oxalate particles. The results can explain why DMAH?∕?NH4 ratios in ambient liquid aerosols can be larger than DMA?∕?NH3, despite an excess of NH3 in the gas phase. An uptake of DMA to aerosols consisting of crystalline ammonium salts, however, is unlikely, even at comparable DMA and NH3 gas phase concentrations.
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