Heterogeneous uptake of ammonia and dimethylamine into sulfuric and oxalic acid particles
Heterogeneous uptake is one of the major mechanisms governing the amounts of short-chain alkylamines and ammonia (NH<sub>3</sub>) in atmospheric particles. Molar ratios of aminium to ammonium ions detected in ambient aerosols often exceed typical gas phase ratios. The present study inves...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/6323/2017/acp-17-6323-2017.pdf |
| Summary: | Heterogeneous uptake is one of the major mechanisms governing the
amounts of short-chain alkylamines and ammonia (NH<sub>3</sub>) 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<sub>3</sub> into
sulfuric and oxalic acid particles at gaseous DMA ∕ NH<sub>3</sub> 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 ∕ NH<sub>4</sub>) 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<sub>3</sub> uptake into concentrated
H<sub>2</sub>SO<sub>4</sub> droplets were initially similarly efficient, yielding
DMAH ∕ NH<sub>4</sub> ratios that were similar to DMA ∕ NH<sub>3</sub> ratios. As the co-uptake
continued, the DMAH ∕ NH<sub>4</sub> gradually dropped due to a preferential uptake of
NH<sub>3</sub> 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<sub>4</sub> 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<sub>3</sub> uptake continued and displaced DMAH<sup>+</sup>, forming
almost pure ammonium sulfate. Displacement of DMAH<sup>+</sup> by NH<sub>3</sub> has also
been observed in neutralised, solid oxalate particles. The results can
explain why DMAH ∕ NH<sub>4</sub> ratios in ambient liquid aerosols can be larger
than DMA ∕ NH<sub>3</sub>, despite an excess of NH<sub>3</sub> in the gas phase.
An uptake of DMA to aerosols consisting of crystalline ammonium salts, however,
is unlikely, even at comparable DMA and NH<sub>3</sub> gas phase concentrations. |
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| ISSN: | 1680-7316 1680-7324 |