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 (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 dimethylaminium/ 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.