Concentration and size distribution of water-extracted dimethylaminium and trimethylaminium in atmospheric particles during nine campaigns - Implications for sources, phase states and formation pathways

In this study, we determined the concentrations of water-extracted dimethylaminium (DMA(+)) and trimethylaminium (TMA(+)) in size-segregated atmospheric particles collected during three inland campaigns and one sea-beach campaign in Qingdao and live marine campaigns in marginal seas of China and the northwest Pacific Ocean. The averages of DMA(+) and TMA(+) in PM0.056-(10) (the sum of concentrations from 0.056 to 10 mu m) during each campaign ranged from 0.045 to 1.1 nmol m(-3) and from 0.029 to 0.53 nmol m(-3), respectively. The increased concentrations of DMA(+) and TMA(+) in PM0.056 - 10, particularly the 1-2 orders of magnitude increased ratios of DMA(+)/NH4+ and TMA+/NH4+, in the marine and sea-beach atmospheres indicated that the overwhelming majority was derived from marine sources. Size distributions of TMA(+) and DMA(+) were also investigated in terms of phase states and formation pathways, e.g., the dominant modes of particulate DMA(+) and TMA(+) in some samples were characterized by the mass median aerodynamic diameter at 0.1-0.2 pm against the dominant mode of NH4+ and SO42- at 0.7-0.9 mu m, while the ratios of DMA(+)/NH4+ and/or TMA(+)/NH4+ in <0.2 mu m particles increased by 3-10 times from the corresponding lower values in >0.2 mu m particles. This strongly implied that the particulate DMA(+) and TMA(+) at <0.2 mu m size range overwhelmingly existed in the liquid organic phase as unprotonated TMA and DMA, but those at the >0.2 mu m size range mainly existed in the aqueous (or solid) phase where the dominance of gas-aerosol equilibria would cause the ratios to be almost size-independent. The size-dependent phase states corresponded to their various formation pathways. (C) 2018 Elsevier B.V. All rights reserved.