Dynamic-chemical coupling of the upper troposphere and lower stratosphere region

Abstract

The importance of the interaction between chemistry and dynamics in the upper troposphere and lower stratosphere for chemical species like ozone is investigated using two chemistry–climate models and a Lagrangian trajectory model. Air parcels from the upper troposphere, i.e. regions of lightning and aircraft emissions, are able to be transported into the lowermost stratosphere (LMS). Trajectory calculations suggest that the main transport pathway runs via the inter tropical convergence zone, across the tropical tropopause and then to higher latitudes, i.e. into the LMS. NO x from aircraft emissions at mid-latitudes are unlikely to perturb the LMS since they are washed-out while still in the troposphere. In contrast, NO x from tropical lightning has the chance to accumulate in the LMS. Because of the longer residence times of NO x in the LMS, compared to the upper troposphere, this excess NO x from lightning has the potential to form ozone in the LMS, which then is transported back to the troposphere at mid-latitudes. In the models, around 10% of the ozone concentration and 50% of the NO x concentration in the northern hemisphere LMS is produced by lightning NO x . At least 5% of the ozone concentration and 35% the NO x concentration at 150 hPa at mid-latitudes originates from tropical lightning in the climate–chemistry simulations.