In-situ competition of amines with ammonia to be neutralized in ambient air

Acid-base neutralization is one type of most reactions in ambient air. Amines are reported to vie with ammonia (NH_3gas) for acid gas to generate new particles even with several orders smaller concentrations. Due to lack of direct observations, in-situ competition of amines with NH_3gas in ambient air to be neutralized remains poorly understood. Hourly concentrations of two popular gaseous amines, i.e., dimethylamine (DMA_gas) and trimethylamine (TMA_gas), and NH_3gas and their particulate partners in marine atmospheres were measured by AIM-IC in this study. The result reveals that the competition of DMA_gas with NH_3gas to be neutralized reasonably matches with the differences on their effective Henry's Law constants and concentrations. Under NH_3gas <0.3 µg m^-3, DMA_gas can be neutralized to be particulate dimethylaminium (DMAH⁺) by approximately 90%. The reverse is generally true under NH_3gas >1.8 μg m^-3. As expected, DMA_gas can always be neutralized largely when NH_3gas to be insufficient to neutralize abundant acids. The particulate TMA (TMAH⁺) is detectable or comparable to TMA_gas only under conditions of no freshly formed NH₄NO₃. The preexisting TMAH⁺ is surprised to deplete largely concurrently with strong formation of NH₄NO₃ under conditions of insufficient NH_3gas, contradictory to the common knowledge, i.e., abundant acids favor more alkaline gas to be neutralized like NH_3gas and DMA_gas. We thereby deliver a hypothesis including gas-aerosol equilibriums related to organic and water aerosol phases to interpret the unique finding. The preexisting TMAH⁺extracted into the water phase is likely to be reacted with other organics by polymerization.