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Heterogeneous uptake is one of the major mechanisms governing theamounts of short-chain alkylamines and ammonia (NH3) in atmosphericparticles. Molar ratios of aminium to ammonium ions detected in ambientaerosols often exceed typical gas phase ratios. The present studyinvestigated the simultaneous uptake of dimethylamine (DMA) and NH3 intosulfuric and oxalic acid particles at gaseous DMA ∕ NH3 molar ratios of0.1 and 0.5 at 10, 50 and 70 % relative humidity (RH). Single-gasuptake 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 wasseverely influenced by the extent of neutralisation and the particle phasestate. In general, DMA uptake and NH3 uptake into concentratedH2SO4 droplets were initially similarly efficient, yieldingDMAH ∕ NH4 ratios that were similar to DMA ∕ NH3 ratios. As the co-uptakecontinued, the DMAH ∕ NH4 gradually dropped due to a preferential uptake ofNH3 into partially neutralised acidic droplets. At 50 % RH, once thesulfate droplets were neutralised, the stronger base DMA displaced some ofthe ammonium absorbed earlier, leading to DMAH ∕ NH4 ratios up to fourtimes higher than the corresponding gas phase ratios. However, at 10 % RH,crystallisation of partially neutralised sulfate particles prevented furtherDMA uptake, while NH3 uptake continued and displaced DMAH+, formingalmost pure ammonium sulfate. Displacement of DMAH+ by NH3 has alsobeen observed in neutralised, solid oxalate particles. The results canexplain why DMAH ∕ NH4 ratios in ambient liquid aerosols can be largerthan 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

Heterogeneous uptake of ammonia and dimethylamine into sulfuric and oxalic acid particles