The impact of horizontal transport on the chemical composition in the tropopause region: lightning NO x and streamers

Abstract

The chemical composition of the region of the extra-tropical tropopause is influenced by chemistry and transport processes of either troposphere and stratosphere. On the basis of simulations with the coupled chemistry–climate model ECHAM4.L39(DLR)/CHEM two specific chemistry–transport interactions are investigated aiming to determine their importance for the ozone budget in the tropopause region. First, the transport and chemistry of nitrogen oxides from tropical lightning are analyzed. Second, the impact of low ozone air masses transported from the tropics into higher latitudes by wave breaking events (so-called streamers) is investigated. The first mechanism leads to an increase of ozone in the order of 5–10%. This additional ozone is not only formed in the tropical upper troposphere and then transported into the lower-most stratosphere (LMS), but it is also produced in the LMS, where the production is small but the residence time is long. The second mechanism is only important in the area 100–30 hPa, where ozone is not solely chemically controlled. In the lowest stratosphere at around 30°N and 30°S ozone decreases by up to 80%. In the tropopause region ozone decreases by 30% in summer and 50% in winter. The analysis clearly shows that the interactions of transport and chemistry in the tropopause region are highly complex and a simulation of that area requests a reasonable representation of the troposphere and the stratosphere.