Ammonia, anhydrous

Phototransformation in air

Description of key information

Ammonia does not absorb at wavelengths >290 nm and, therefore, is not expected to be susceptible to direct photolysis by sunlight. Ammonia may be photolytically dissociated at wavelengths of less than 222nm, resulting in the production of amino and ammonia radical species. The gas phase reaction of ammonia with photochemically produced hydroxyl radicals is thought to contribute about 10% to the overall atmospheric removal process.

In the atmosphere, ammonia can be removed by rain or snow washout. Reactions with acidic substances, such as H2SO4, HCl, HNO3, or N oxides (all produced in high concentrations from anthropogenic activities) produce ammonium aerosols, which can undergo dry or wet deposition. The gas phase reaction of ammonia with photochemically produced hydroxyl radicals is thought to contribute about 10% to the overall atmospheric removal process. The best estimate of the half-life of atmospheric ammonia is a few days (ATSDR; Toxicological Profile for Ammonia. Atlanta, GA: Agency for Toxic Substances and Disease Registry, US Public Health Service (2004)).

Additional information

Ammonia reacts with ozone, hydroxyl radical, and atomic oxygen. Oxidation by ozone is a first order reaction with respect to the concentration of ammonia and is catalyzed by hydroxide ions over the pH range 7-9. Ammonia and ozone react to produce ammonium nitrate aerosols.  Photolytic degradation and reaction with photolytically produced hydroxyl radicals (·OH) in the troposphere are major pathways for the removal of atmospheric ammonia.  Some of the ammonium in the atmosphere is oxidized to oxides of the nitrogen and nitrate ion, which represents a significant contribution to the total acidity of rainfall.  Various ammonium complexes may also be formed by the heterogeneous reaction of atmospheric ammonia with nitric oxide-soot surfaces in the atmosphere. There are two primary photochemical reactions that destroy ammonia in the atmosphere. Ammonia may be photolytically dissociated at wavelengths of less than 222nm, resulting in the production of amino and ammonia radical species. The second reaction is a thermal anhydrous reaction between ammonia and sulphur dioxide resulting in the formation of ammonium sulphate aerosols.

 

The kinetic behavior of OH radicals in the presence of a large excess of NH3 was studied by pulsed photolysis. OH radicals were produced by vacuum-uv photolysis of H2O and were monitored by resonance fluorescence.  The rate constant for the reaction of OH with NH3was determined to be 1.5(±0.4) x 10-13cm3molecules-1s-1at 25°C.

The photodegradation of ammonia was investigated using a flash photolysis system. The rate constant for the photodegradation of ammonia under the conditions of this study is reported to be 1.47 (±0.7) x 10-13cm³ molecule-1s-1at 25°C.