What happens to ammonia on leaf surfaces?

The exchange of ammonia between leaf surfaces and the atmosphere is bi-directional, and depends on the relative solution concentrations in or on the leaf, and concentrations in the atmosphere. The amount of ammonia (as ammonium ions) present at equilibrium in solution on leaf surfaces depends on temperature, and on the presence of other gases such as carbon dioxide and sulphur dioxide, which act as acids to neutralise the hydroxide ions formed when ammonia dissolves. Under ambient conditions, with low concentrations of ammonia and sulphur dioxide, equilibrium may not be achieved even over many hours, because of aerodynamic limitations in the transfer between the air and the surface. Unless chemical reactions occur to ‘fix’ ammonium on the surface, for example as involatile ammonium sulphate or organic nitrogen, any deposited ammonia will be returned to the atmosphere as surface water evaporates. Results from a simple model are presented to show the effects of different atmospheric components and temperature, and also of the rate of oxidation of dissolved sulphur dioxide, on the retention of ammonium on leaf surfaces.