The atmospheric budget of oxidized nitrogen and its role in ozone formation and deposition

Emissions of reactive oxidized nitrogen (NO and NO 2 ), collectively known as NO x , from human activities are c . 21 Tg N annually, or 70% of global total emissions. They occur predominantly in industrialized regions, largely from fossil fuel combustion, but also from increased use of N fertilizers. Soil emissions of NO not only make an important contribution to global totals, but also play a part in regulating the dry deposition of NO and NO 2 (NO x ) to plant canopies. Soil microbial production of NO leads to a soil ‘compensation point’ for NO deposition or emission, which depends on soil temperature, N and water status. In warm conditions, the net emission of NO x from plant canopies contributes to the photochemical formation of ozone. Moreover, the effect of NO x emissions from soil is to reduce net rates of NO 2 deposition to terrestrial surfaces over large areas. Increasing anthropogenic emissions of NO x have led to an approximate doubling in surface O 3 concentrations since the last century. NO x acts as a catalyst for the production of O 3 from volatile organic compounds (VOCs). Paradoxically, emission controls on motor vehicles might lead to increases in O 3 concentrations in urban areas. Removal of NO and NO 2 by dry deposition is regulated to some extent by soil production of NO; the major sink for NO 2 is stomatal uptake. Long‐term flux measurements over moorland in Scotland show very small deposition rates for NO 2 at night and before mid‐day of 1–4 ng NO 2 ‐N m −2 s −1 , and similar emission rates during afternoon. The bi‐directional flux gives 24‐h average deposition velocities of only 1–2 mm s −1 , and implies a long life‐time for NO x due to removal by dry deposition. Rates of removal of O 3 at the ground are also influenced by stomatal uptake, but significant non‐stomatal uptake occurs at night and in winter. Measurements above moorland showed 40% of total annual flux was stomatal, with 60% non‐stomatal, giving nocturnal and winter deposition velocities of 2–3 mm s −1 and daytime summer values of 10 mm s −1 . The stomatal uptake is responsible for adverse effects on vegetation. The critical level for O 3 exposure (AOT 40 ) is used to derive a threshold O 3 stomatal flux for wheat of 0·5 μg m −2 s −1 . Use of modelled stomatal fluxes rather than exposure might give more reliable estimates of yield loss; preliminary calculations suggest that the relative grain yield reduction (%) can be estimated as 38 times the stomatal ozone flux (g m −2 ) above the threshold, summed over the growing season.