The role of nitrogen oxides in the long-range transport of photochemical oxidants

Abstract

Ozone episodes in the atmospheric boundary layer over Europe are superimposed on a background level which is slowly increasing. The change in the background concentration is probably controlled by changes in the emissions of nitrogen oxides (NO x ). It is likely that a further increase in the emissions of NO x in Europe and elsewhere in the Northern Hemisphere will contribute to a continued rise in free tropospheric ozone, while the occurrence of ozone episodes in the boundary layer may not change much. Nitrate derived from the emissions of NO x contributes significantly to acid deposition in Europe and elsewhere. Integrated over 1 year and over all of Europe, there is probably an almost linear relationship between a change in NO x emissions and the corresponding change in nitrate deposition. A reduction in the emissions of NO x in a photochemical episode in Europe could lead to only a slight decrease in the concentration of hydroxyl radical (OH) and ozone, while there might be a marked increase in hydrogen peroxide. The conversion of NO x to nitrate would become slightly slower, while the conversion of SO 2 to sulphate would remain fairly unchanged. The atmospheric lifetime of NO x and sulphur species could remain the same or increase slightly. In clean air, a downward trend in the emission of NO x (a few percent per year) is shown in model calculations to cause a reduction in the background concentration of ozone and OH, while H 2O 2 increases. This means that the gas-phase conversion of SO 2 and NO x , through the reaction with OH, to sulphate and nitrate will become less efficient. In addition, the dark reaction converting NO 2 to the NO 3 radical and further to nitrate through reaction with ozone, would become slower, while the wet-phase oxidation of SO 2 to sulphate would become more efficient. This means that reduced emissions of NO x would give rise to a more than proportional reduction in nitrate formation, while the efficiency of sulphate formation could increase. For the distribution of acid deposition this would imply an increase in the lifetime for sulphur compounds and a decrease in the lifetime for NO x compounds.