Competitive Uptake of Dimethylamine and Trimethylamine against Ammonia on Acidic Particles in Marine Atmospheres.

Alkaline gases such as NH 3 and amines play important roles in neutralizing acidic particles in the atmosphere. Here, two common gaseous amines (dimethylamine (DMA) and trimethylamine (TMA)), NH 3 , and their corresponding ions in PM 2.5 were measured semicontinuously using an ambient ion monitor-ion chromatography (AIM-IC) system in marine air during a round-trip cruise of approximately 4000 km along the coastline of eastern China. The concentrations of particulate DMA, detected as DMAH + , varied from <4 to 100 ng m -3 and generally decreased with increasing atmospheric NH 3 concentrations. Combining observations with thermodynamic equilibrium calculations using the extended aerosol inorganics model (E-AIM) indicated that the competitive uptake of DMA against NH 3 on acidic aerosols generally followed thermodynamic equilibria and appeared to be sensitive to DMA/NH 3 molar ratios, resulting in molar ratios of DMAH + to DMA + DMAH + of 0.31 ± 0.16 (average ± standard deviation) at atmospheric NH 3 concentrations over 1.8 μg m -3 (with a corresponding DMA/NH 3 ratio of (1.8 ± 1.0) × 10 -3 ), 0.80 ± 0.15 at atmospheric NH 3 concentrations below 0.3 μg m -3 (with a corresponding DMA/NH 3 ratio of (1.3 ± 0.6) × 10 -2 ), and 0.56 ± 0.19 in the remaining cases. Particulate TMA concentrations, detected as TMAH + , ranged from <2 to 21 ng m -3 and decreased with increasing concentrations of atmospheric NH 3 . However, TMAH + was depleted concurrently with the formation of NH 4 NO 3 under low concentrations of atmospheric NH 3 , contradictory to the calculated increase in the equilibrated concentration of TMAH + by the E-AIM.