Nitrogen oxides in ambient air in industrial countries result mainly from emissions of nitric oxide (NO) from fossil fuel combustion. In the presence of ozone (O3), NO is rapidly converted into nitrogen dioxide (NO2). Further oxidation of NO2 leads to the formation of a range of compounds, the most important of which are: nitric acid (HNO3), peroxy acetyl nitrate (PAN) and nitrous acid (HNO2). The environmental effects of these compounds include eutrophication of natural ecosystems, acidification and photochemical air pollution. It is therefore necessary to understand the dry deposition processes for these compounds and use this understanding to provide estimates of dry deposition inputs to ecosystems across Europe. This review outlines current understanding of the exchange processes and methods used to estimate regional NO(y) deposition. Several methods have been used to measure dry deposition. Among these micrometerological methods provide the best approach for estimating fluxes in the field. However, few field measurements of the deposition velocity of NO2 to important ecosystems have been reported and the results have not always been conclusive. Measurement artefacts such as non-stationarity caused by local sources, monitors responding to other gases than NO2 and the influence of photochemical reactions have made field measurement very difficult. More recent field work however has provided strong indications that NO2 deposition to vegetation is controlled by stomatal opening. This implies that the deposition velocity shows a marked diurnal as well as an annual cycle with maximum values up to 1 cm s-1 during the day in summer. Few measurements of HNO2 exchange have been reported, but based on knowledge of its physical-chemical properties it is expected that HNO2 is taken up via stomata. Measurements of PAN also indicate uptake controlled by stomatal opening. Several measurements have shown that deposition of HNO3 is limited only by the rate of its aerodynamic transport to the surface leading to deposition velocities as large as 10 cm s-1 to forest. For NO the situation is more complex. Early laboratory studies using high concentrations showed stomatal uptake. However detailed field studies carried out more recently at ambient concentrations show that most ecosystems emit NO. To provide spatial patterns of nitrogen oxide deposition, information on the concentration in air as well as an estimate of the deposition velocity is needed. For the purpose of regional dry deposition estimates, the widely separated rural monitoring stations are adequate to provide broad-scale concentration fields for NO2. For the other gases, information is scarce and concentrations need to be a derived from calculations with long-range transport models. These coarse concentration fields may be combined with statistical information on land use within each grid cell. Deposition to each land-use category within each grid cell can then be calculated using a resistance layer model. An example of this approach is provided for the UK. This exercise showed that the input of NO2 varies between 1 kg N ha-1 year-1 for remote areas to 10 kg N ha-1 year-1 in more polluted areas, with NO2 concentrations in excess of 10 ppb.
