Basic Science of Ammonia

Atmospheric ammonia is very important in the chemistry of the atmosphere, and as a chemical component addition to ecosystems.

Ammonia is a basic gas which reacts with many different anions (SO₄^- and NO₃^- are particularly important) that form aerosol solids in the atmosphere. Ammonium aerosol is one of the largest chemical constituents of this fine particulate matter, which are solid/liquid aerosols with diameters less than 2.5 micrometers in size (so called PM_2.5). Ammonium is typically found in PM_2.5 as either ammonium sulfate or ammonium nitrate.

For example, when ammonia and nitric acid are present in the gas phase at concentrations above the saturation point, the two species will react to form ammonium nitrate aerosol (NH₄NO₃).

NH₃ (gas) + HNO₃ (gas) <> NH₄NO₃ (particulate)

Warmer temperatures drive this equilibrium towards ammonia gas and away from ammonium nitrate . The same is true as ammonia gas (and nitric acid) is consumed in other reactions.

In likewise fashion, ammonia gas will react with sulfuric acid (and other forms) to produce ammonium sulfate. As more sulfur dioxide is oxidized and sulfate ions are available, ammonium sulfate particulate is formed. Ammonium sulfate is preferentially formed over ammonium nitrate. With less sulfate available, more ammonium nitrate is available.

Ammonia gas and ammonium particulate are also important contributors to ecosystems. This ammonium mass contributes to nitrogen deposition (both reduced and oxidized nitrogen) which is important in stimulating plant growth in nitrogen-limited systems , and can alter the structure and diversity of native plant communities. Ammonium associated with sulfate and nitrate contributes to acidic deposition (i.e., precipitation pH<5.0), and can lead to adverse impacts in acid-sensitive ecosystems. Also, particulate ammonium adsorbs and scatters light in the atmosphere, resulting in distinctly lower visibilities as in clean regions such as our National Park system.

Data from the NADP indicate that atmospheric ammonia concentrations are increasing. In contrast, sulfate concentrations are generally decreasing due to reduced SO₂ emissions due to reductions imposed by the 1990 Clean Air Act Amendments.