Atmospheric nitrogen deposition: revisiting the question of the invisible organic fraction

Nitrogen-containing organic matter in rain and aerosol makes up a substantial proportion – 25–35% – of the total nitrogen in atmospheric deposition. Despite this quantitative significance, it is “invisible” in current policy; and existing global air quality and nitrogen deposition monitoring programmes routinely measure nitrate and ammonium but not the organic component. Although recent research, especially in Asia and Latin America, is helping to build up a global picture of organic nitrogen deposition, data sets are sparse and the chemical composition of the organic matter is relatively poorly characterised. As a result, its role in ecosystems and Earth system processes remains unclear. Organic nitrogen should now be included in initiatives dealing with nitrogen deposition and its management.