Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 6323-6339, 2017
https://doi.org/10.5194/acp-17-6323-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
23 May 2017
Heterogeneous uptake of ammonia and dimethylamine into sulfuric and oxalic acid particles
Meike Sauerwein1 and Chak Keung Chan1,2,3 1Division of Environment, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
2Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water, Bay, Kowloon, Hong Kong
3School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong
Abstract. 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.

Citation: Sauerwein, M. and Chan, C. K.: Heterogeneous uptake of ammonia and dimethylamine into sulfuric and oxalic acid particles, Atmos. Chem. Phys., 17, 6323-6339, https://doi.org/10.5194/acp-17-6323-2017, 2017.
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Short summary
Heterogeneous uptake is one of the mechanisms influencing the amounts of alkylamines and ammonia in atmospheric particles. The present study investigates the simultaneous uptake of dimethylamine and ammonia at different gas molar ratios into sulfuric and oxalic acid particles at low and high relative humidity. Results showed that the particulate dimethylaminium/ammonium molar ratios changed substantially during the uptake process, depending on the extent of neutralisation and phase state.
Heterogeneous uptake is one of the mechanisms influencing the amounts of alkylamines and ammonia...
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