Natural NOx sources - Importance, representations in models, and contribution to ozone

Kurzfassung

Nitrogen oxides (NOx) are an important prerequisite for the formation of ozone, which is toxic to humans and a greenhouse gas. Accordingly, emissions of NOx influence the air quality and the radiation budget of the atmosphere. Therefore, many studies investigate the influence of anthropogenic emissions on the budget of ozone and NOx. However, for such studies not only the anthropogenic emissions, but also the natural emissions must be well known. Examples of such natural sources are the emissions of NOx from soil (soil NOx) as well as NOx emissions due to lightning (lightning NOx). The mechanisms of both processes are relatively well understood, but the global emissions are only poorly constrained. Current estimates of lightning NOx emissions, for example, are in the order of 2-8 Tg (N) a-1 (Schuman & Huntrieser 2007), while total global emissions from the aviation sector are below 1 Tg (N) a-1. The implementation of these emissions in (regional) chemistry climate models is a challenging task. Even though the mechanisms are relatively well known, the processes must be heavily parametrised. For example, parametrisations of soil NOx emissions depend mainly on soil temperature, soil moisture and fertiliser usage, while emissions of lightning NOx are strongly coupled to convection parametrisations. We will give a short overview on the mechanisms producing emissions of soil NOx and lightning NOx and discuss the global amounts of these emissions. Further we will shortly explain the problems of calculating these emissions in models like COSMO-CLM/MESSy. Exemplarily we will present results of COSMO-CLM/MESSy which highlight the importance of soil NOx emissions for the ozone and NOx budget in Europe. The results of this simulation show, that over Europe the contribution of emissions of soil NOx as well as other biogenic emissions contribute on average up to 20 % to the tropospheric ozone column.