Gaseous ammonia and ammonium ions in the free troposphere

Abstract

Ammonia, the most important alkaline compound commonly found in the atmosphere, has a key role in aerosol chemistry. Gaseous ammonia is released at the Earth's surface primarily through the decomposition of organic material and, due to its extremely large solubility in water and reactivity with acid aerosol components, is efficiently removed by interaction with aqueous and acid aerosols. While the presence of ammonium ions (NH₄⁺) in aerosols and rain water is well established, there have been few studies of gaseous ammonia and its abundance is not well established, particularly in the free troposphere¹. This is probably due to the difficulty in developing sufficiently sensitive analytical methods, which are not affected by interference of ammonium-containing aerosols. Due to its large proton affinity or gas-phase basicity, ammonia vapour is expected to react also with atmospheric gaseous positive ions leading to gaseous NH₄⁺ ‘core ions’. Detection of the gaseous NH₄⁺ core ions, which have not yet been observed in the free atmosphere, may provide sensitive means of detecting atmospheric ammonia vapour^2,3,4. We report here on the first measurements of this ion in the free atmosphere at altitudes from 4,000 to 8,000 m. Ammonia vapour abundances inferred from these data are very low suggesting efficient heterogeneous ammonia removal and very low ammonia saturation pressures over aerosols.