Ammonia in the atmosphere: a review on emission sources, atmospheric chemistry and deposition on terrestrial bodies

Gaseous ammonia (NH3) is the most abundant alkaline gas in the atmosphere. In addition, it is a major component of total reactive nitrogen. The largest source of NH3 emissions is agriculture, including animal husbandry and NH3-based fertilizer applications. Other sources of NH3 include industrial processes, vehicular emissions and volatilization from soils and oceans. Recent studies have indicated that NH3 emissions have been increasing over the last few decades on a global scale. This is a concern because NH3 plays a significant role in the formation of atmospheric particulate matter, visibility degradation and atmospheric deposition of nitrogen to sensitive ecosystems. Thus, the increase in NH3 emissions negatively influences environmental and public health as well as climate change. For these reasons, it is important to have a clear understanding of the sources, deposition and atmospheric behaviour of NH3. Over the last two decades, a number of research papers have addressed pertinent issues related to NH3 emissions into the atmosphere at global, regional and local scales. This review article integrates the knowledge available on atmospheric NH3 from the literature in a systematic manner, describes the environmental implications of unabated NH3 emissions and provides a scientific basis for developing effective control strategies for NH3.

Ammonia exchange between the atmosphere and the surface waters at two locations in the Chesapeake Bay

Excess phytoplankton production, which contributes to hypoxic conditions, is nitrogen limited in the Chesapeake Bay during the summer months. Therefore, understanding the flux of ammonia by direct deposition to the biologically active surface layer is critical to understanding the nutrient dynamics of the bay. This paper presents the results of a 2-yr study measuring gaseous ammonia (NH3) and aerosol ammonium (NH4+) in Baltimore and Solomons, MD, from which direct atmospheric loading of total ammonia (Nt = NH3 + NH4+) to the Chesapeake Bay is estimated. Mean atmospheric concentrations of total ammonia for Baltimore and Solomons were 2.7 +/- 1.7 and 1.0 +/- 0.8 microg of N m(-3), respectively. Monte Carlo estimates of gross dry deposition ranged from <100 to 4900 microg of N m(-2) d(-1). However, based upon water quality parameters, Monte Carlo estimates of gross volatilization of NH3 were calculated to range from <100 to 7700 microg of N m(-2) d(-1). The resulting net air-sea exchange flux varied seasonally from a net deposition into the water during the winter to a net volatilizing into the atmosphere during the summer. A total of 60% of the paired air-water samples had flux estimates that were not significantly different than equilibrium at the 90% confidence interval. The gross deposition, gross volatilization, and net air-sea fluxes were greater and more variable in Baltimore relative to the rural site. Atmospheric ammonia concentrations decrease during the winter at the rural site. However, the net exchange is still into the water due to an exponential decrease in [NH3]eq with temperature. These results indicate that the nitrogen-limited Chesapeake Bay can act as a source of ammonia to the local atmosphere.

Experiments and simulations of ion-enhanced interfacial chemistry on aqueous NaCl aerosols

A combination of experimental, molecular dynamics, and kinetics modeling studies is applied to a system of concentrated aqueous sodium chloride particles suspended in air at room temperature with ozone, irradiated at 254 nanometers to generate hydroxyl radicals. Measurements of the observed gaseous molecular chlorine product are explainable only if reactions at the air-water interface are dominant. Molecular dynamics simulations show the availability of substantial amounts of chloride ions for reaction at the interface, and quantum chemical calculations predict that in the gas phase chloride ions will strongly attract hydroxl radicals. Model extrapolation to the marine boundary layer yields daytime chlorine atom concentrations that are in good agreement with estimates based on field measurements of the decay of selected organics over the Southern Ocean and the North Atlantic. Thus, ion-enhanced interactions with gases at aqueous interfaces may play a more generalized and important role in the chemistry of concentrated inorganic salt solutions than was previously recognized.