Transport processes and ozone budgets in the upper troposphere- a synthesis of EOS MLS measurements and chemistry transport model studies

Ozone in the lowest 10-15km of the atmosphere (the troposphere) is the third most important greenhouse gas; increases in ozone since the pre-industrial era are thought to have contributed the equivalent of about one quarter of the CO2 greenhouse gas warming over the same period. It is in the upper part of the troposphere that ozone is most effective as a greenhouse gas. It is important that we understand the processes that control present-day ozone in this region in order to be able to make predictions of how the concentrations of this greenhouse gas will change in the future, and the consequences for future climate. The 'budget' of ozone in the upper troposphere is controlled by both chemical and transport processes. In the tropics, nitrogen oxides generated by lightning discharges are particularly important for ozone formation in the upper troposphere. Other emission sources have their origin at the Earth's surface. These emissions, or ozone formed 'in-situ' from these emissions, are transported in convective updraughts, or by prevailing weather systems, to the upper troposphere. Another source of ozone in the upper troposphere comes from import from the stratosphere, which is the atmospheric layer above the troposphere. We will apply complex computer models that simulate physical and chemical processes in the Earth's atmosphere to understand how ozone is formed and transported in the troposphere. These models rely on observations to judge their ability to simulate all these relevant processes. However, in the upper troposphere, especially in the sub-tropics and tropics, observations are scarce. Satellite measurements can help to solve this problem, but most instruments (called nadir-viewing instruments) can only make measurements that represent a much larger portion of the troposphere than just its upper region. In July 2004, the Aura satellite was launched with the Earth Observing System (EOS) Microwave Limb Sounder (MLS) on board. MLS is one of the first satellite instruments able to make accurate measurements of ozone in the upper troposphere with relatively high vertical resolution (a few km). The aim of this proposal is the combined use of MLS measurements and model simulations, to investigate the relative importance of both different sources of emissions, as well as different transport processes, in determining the seasonal and regional variation of ozone in the upper troposphere. We will then investigate how lightning and transport may change in the future, due to regional shifts in convection and rainfall as the climate warms, and the implications for ozone in the tropical upper troposphere.