Nitrogen oxides in the troposphere – What have we learned from satellite measurements?

Abstract

Nitrogen oxides are key species in the troposphere where they are linked to ozone formation and acid rain. The sources of nitrogen oxides are anthropogenic to large extend, mainly through combustion of fossil fuels. Satellite observations of NO2 provide global measurements of nitrogen oxides since summer 1995, and these data have been applied for many studies on the emission sources and strengths, the chemistry and the transport of NOx. In this paper, an overview will be given on satellite measurements of NO2, some examples of typical applications and an outlook on future prospects. 1 NOx in the troposphere Nitrogen oxides play several important roles in the atmosphere. In the stratosphere, they act as catalysts in ozone destruction reducing ozone levels and at the same time form reservoir substances with the halogen oxides thereby reducing their ozone depletion potential. In the troposphere, photolysis of NO2 is the only known route for ozone formation. As ozone is also rapidly destroyed by reaction with NO, nitrogen oxides (NOx =N O +N O2 )a nd ozone are often in photochemical equilibrium in the troposphere. In the presence of HO2 or peroxy radicals (RO2), NO2 is reformed without destruction of an ozone molecule leading to accumulating of O3. Thereby, high levels of NOx together with volatile organic compounds and sufficient illumination lead to photochemical smog. Through reaction with ozone, NO2 forms the nitrate radical which is rapidly photolysed during daytime but at night takes over the role of OH as the most reactive radical in the troposphere. NO2 can also react with OH to form HNO3 which is easily taken up by droplets and aerosols. Nitrate aerosols are also formed via formation of N2O5 from the reaction of NO3 and NO2 and subsequent hydrolysis on surfaces and droplets. Through this pathway, NOx contributes to acidification of aerosols and cloud droplets which eventually impacts on ecosystems (acid rain). As emissions of SO2 have been dramatically reduced over the past decades, the relative importance of HNO3 has increased in most industrialised countries. Nitrogen oxides are also relevant for radiative forcing. Locally, high levels of NO2 can be reached and direct absorption of sunlight becomes significant (1). More indirectly, NOx levels determine the amount of ozone formed in the troposphere, and O3 is an important climate gas. At large concentrations, NOx also impacts on OH levels which in turn determines the life time of many greenhouse gases. A simplified overview on the chemistry of NOx in the troposphere is given in figure 1. The sources of NOx in the troposphere are both natural and anthropogenic. Microbial processes in soils, oxidation of biogenic NH3, wild fires and lightning are natural sources of NOx. Today, fossil fuel combustion is the largest source of NOx and together with the emissions from biomass burning, anthropogenic activities dominate the NOx budget. The uncertainty of