Heterogeneous uptake of ammonia and dimethylamine into sulfuric
and oxalic acid particles
Meike Sauerwein1 and Chak Keung Chan1,2,31Division 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
Received: 09 Nov 2016 – Accepted for review: 11 Nov 2016 – Discussion started: 17 Nov 2016
Abstract. Heterogeneous uptake is one of the major mechanisms governing the amounts of short-chain alkyl-amines and ammonia (NH3) 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 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 showed that the particulate dimethyl-aminium/ammonium molar ratios (DMAH/NH4) changed substantially during the uptake process, which was predominantly influenced by the extent of neutralization and the particle phase state. DMA uptake and NH3 uptake into concentrated H2SO4 droplets were initially similarly efficient, yielding DMAH/NH4 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 still acidic droplets. Once the droplets were neutralized, the stronger base DMA displaced some of the ammonium absorbed earlier, leading to DMAH/NH4 that were up to four times higher than the corresponding gas phase ratios. At 10 % RH, crystallization of partially neutralized sulfate particles prevented further DMA uptake, while NH3 uptake continued, and displaced DMAH+ after the solid particles were completely neutralized, forming almost pure ammonium sulfate. Displacement of DMAH+ by NH3 has also been observed in neutralized, solid oxalate particles. The results illustrate why in ambient liquid aerosols the DMAH/NH4 can be larger than DMA/NH3, despite of an excess of NH3 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 NH3 are of the same magnitude.
Sauerwein, M. and Chan, C. K.: Heterogeneous uptake of ammonia and dimethylamine into sulfuric
and oxalic acid particles, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-995, in review, 2016.