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

Abstract

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 five marine campaigns in marginal seas of China and the northwest Pacific Ocean. The averages of DMA⁺ and TMA⁺ in PM_0.056-₁₀ (the sum of concentrations from 0.056 to 10μm) during each campaign ranged from 0.045 to 1.1nmolm^-3 and from 0.029 to 0.53nmolm^-3, respectively. The increased concentrations of DMA⁺ and TMA⁺ in PM_0.056-₁₀, 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 0.1-0.2μm against the dominant mode of NH₄⁺ and SO₄^2- at 0.7-0.9μm, while the ratios of DMA⁺/NH₄⁺ and/or TMA⁺/NH₄⁺ in <0.2μm particles increased by 3-10 times from the corresponding lower values in >0.2μm particles. This strongly implied that the particulate DMA⁺ and TMA⁺ at <0.2μm size range overwhelmingly existed in the liquid organic phase as unprotonated TMA and DMA, but those at the >0.2μm 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.

Keywords: Aminium salts; Aqueous phase; Liquid organic phase; Marine sources; Size distribution.