Behavior of Water-to-Air Trace Gas Exchange for Ammonia and Amines at Low Wind Speeds Using Chemical Ionization Mass Spectrometry

Abstract

Ammonia (NH3) and alkylamines play critical roles in atmospheric aerosol chemistry, influencing cloud formation, Earth’s radiative balance, and air quality. Despite significant contributions from water-based sources, associated emissions of ammonia and alkylamines remain poorly characterized, especially in freshwater environments. As such, this lack of understanding complicates efforts to accurately predict ammonia and amine fluxes from aquatic sources. For example, the widely used two-film resistance model often leads to substantial misestimation of these fluxes under low-wind conditions. Compounding this uncertainty, Henry’s Law, which is commonly applied to these compounds, relies on published Henry’s Law constants that can differ by more than 2 orders of magnitude, further resulting in inaccurate modeling of the water-to-air transfer of these highly polar gases. In this study, we experimentally investigate the real-time trace gas exchange (i.e., flux) of ammonia and alkylamines in model aqueous solutions (10–9 to 10–5 M) across the air–water interface using a custom flux measurement system (FMS) coupled with a hydronium chemical ionization mass spectrometer (CIMS). Carrier gas velocities ranged from 10–4 to 10–3 m s–1 to examine how low wind speeds influence emission rates. Observed gas concentrations within the FMS headspace were compared to theoretical concentrations predicted by the two-film resistance model and Henry’s Law. Our findings provide direct experimental evidence of ammonia and alkylamine gas exchange from aqueous systems in low wind conditions, helping to constrain their emission behavior and refine our understanding of aquatic contributions to the atmosphere.