Characteristics of dimethylaminium and trimethylaminium in atmospheric particles ranging from supermicron to nanometer sizes over eutrophic marginal seas of China and oligotrophic open oceans

In this study, we characterized dimethylaminium (DMA⁺) and trimethylaminium (TMA⁺) in size-segregated atmospheric particles during three cruise campaigns in the marginal seas of China and one cruise campaign mainly in the northwest Pacific Ocean (NWPO). An 14-stage nano-MOUDI sampler was utilized for sampling atmospheric particles ranging from 18μm to 0.010μm. Among the four cruise campaigns, the highest concentrations of DMA⁺ and TMA⁺ in PM₁₀ were observed over the South Yellow Sea (SYS) in August 2015, i.e., 0.76±0.12nmolm^-3 for DMA⁺ (average value±standard deviation) and 0.93±0.13nmolm^-3 for TMA⁺. The lowest values were observed over the NWPO in April 2015, i.e., 0.28±0.16nmolm^-3 for DMA⁺ and 0.22±0.12nmolm^-3 for TMA⁺. In general, size distributions of the two ions exhibited a bi-modal pattern, i.e., one mode at 0.01-0.1μm and the other at 0.1-1.8μm. The two ions' mode at 0.01-0.1μm was firstly observed. The mode was largely enhanced in samples collected over the SYS in August 2015, leading to high mole ratios of (DMA⁺+TMA⁺)/NH₄⁺ in PM_0.1 (0.4±0.8, median value±standard deviation) and the ions' concentrations in PM_0.1 accounting for ~10% and ~40% of their corresponding concentrations in PM₁₀. This implied that (DMA⁺+TMA⁺) likely played an important role in neutralizing acidic species in the smaller particles. Using SO₄^2-, NO₃^- and NH₄⁺ as references, we confirm that the elevated concentrations of DMA⁺ and TMA⁺ in the 0.01-0.1μm size range were probably real signals rather than sampling artifacts.