THE ATMOSPHERIC BUDGET OF OXIDIZED NITROGEN AND ITS ROLE IN OZONE FORMATION AND DEPOSITION

Emissions of reactive oxidized nitrogen (NO and NO2), collectively kno wn as NOx, from human activities are c. 21 Tg N annually, or 70 % of g lobal total emissions. They occur predominantly in industrialized regi ons, largely from fossil fuel combustion, but also from increased use of N fertilizers. Soil emissions of NO not only make an important cont ribution to global totals, but also play a part in regulating the dry deposition of NO and NO2 (NOx) to plant canopies. Soil microbial produ ction of NO leads to a soil 'compensation point' for NO deposition or emission, which depends on soil temperature, N and water status. In wa rm conditions, the net emission of NOx from plant canopies contributes to the photochemical formation of ozone. Moreover, the effect of NOx emissions from soil is to reduce net rates of NO2 deposition to terres trial surfaces over large areas. Increasing anthropogenic emissions of NOx have led to an approximate doubling in surface O-3 concentrations since the last century. NOx acts as a catalyst for the production of O-3 from volatile organic compounds (VOCs). Paradoxically, emission co ntrols on motor vehicles might lead to increases in O-3 concentrations in urban areas. Removal of NO and NO2 by dry deposition is regulated to some extent by soil production of NO; the major sink for NO2 is sto matal uptake. Long-term flux measurements over moorland in Scotland sh ow very small deposition rates for NO2 at night and before mid-day of 1-4 ng NO2-N m(-2) s(-1), and similar emission rates during afternoon. The bi-directional flux gives 24-h average deposition velocities of o nly 1-2 mm s(-1), and implies a long life-time for NOx due to removal by dry deposition. Rates of removal of O-3 at the ground are also infl uenced by stomatal uptake, but significant non-stomatal uptake occurs at night and in winter. Measurements above moorland showed 40 % of tot al annual flux was stomatal, with 60% non-stomatal, giving nocturnal a nd winter deposition velocities of 2-3 mm s(-1) and daytime summer val ues of 10 mm s(-1). The stomatal uptake is responsible for adverse eff ects on vegetation. The critical level for O-3 exposure (AOT(40)) is u sed to derive a threshold O-3 stomatal flux for wheat of 0.5 mu g m(-2 ) s(-1). Use of modelled stomatal fluxes rather than exposure might gi ve more reliable estimates of yield loss; preliminary calculations sug gest that the relative grain yield reduction (%) can be estimated as 3 8 times the stomatal ozone flux (g m(-2)) above the threshold, summed over the growing season.