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

Abstract

Emissions of reactive oxidized nitrogen (NO and NO₂), 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 NO₂ (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 NO₂ deposition to terrestrial surfaces over large areas.

Increasing anthropogenic emissions of NO_x have led to an approximate doubling in surface O₃ concentrations since the last century. NO_x acts as a catalyst for the production of O₃ from volatile organic compounds (VOCs). Paradoxically, emission controls on motor vehicles might lead to increases in O₃ 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 stomatal uptake. Long-term flux measurements over moorland in Scotland show 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 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 O₃ 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 O₃ exposure (AOT₄₀) is used to derive a threshold O₃ 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.