The chemical boundary condition for the stratosphere is controlled by convective coupling from the boundary layer and free troposphere and this implies considerable variability in the flux of short lived species crossing the 390 K θ surface in the tropics. Convective injection of material from the boundary layer extends to the tropopause in the tropics such that short-lived compounds such as bromine, iodine, formaldehyde, along with a spectrum of species carried by aerosols, are episodically and irreversibly injected into the stratosphere in the tropics and through tropopause folds into the middleworld in the subtropics. The conundrum of increasing water vapor in the stratosphere (∼ 10%/decade at 40°N) and decreasing tropopause temperature is resolved by changes in the chemical composition of the TTL in the vicinity of convective outflow extending up to the 380–390 K θ level.
A key shortcoming in our knowledge of the coupling between climate change and chemical changes centers on the mechanisms that establish the spatial and temporal pattern of the chemical composition of the TTL and how that chemical composition is superimposed on the horizontal and vertical transport structure defining the dynamic meteorology of the tropics. Because of the vastly different response to temperature, oxidation, heterogeneous chemical processing, photolysis, hydrolysis, etc., at the molecular level of tracers such as:
- water vapor and its isotopes
- source species
- reactive intermediates such as formaldehyde, PAN, and HCl
- aerosol composition, etc.
the proper complement of simultaneously observed species provide a powerful diagnostic tool defining both the chemical structure of the TTL and the transport processes that couple the upper troposphere to the lower stratosphere. At the same time, it is these coupled chemical-transport processes that hold the key to changes in Earth’s climate in the face of forcing by CO2, CH4, N2O, CFCs, etc., in combination with convective ventilation of the boundary layer that contains highly variable amounts of nitrate, sulfate, organic, heavy metal, soot, etc. from industrial and biomass sources. It is the union of these changes and the power of their coupling that places considerable demand on the choice of species to be observed, the flight trajectories to be selected and the seasonal coverage to be achieved. This area of research is also the one that demands the greatest cooperation among the various communities—modeling, satellite, stratosphere, troposphere, and laboratory.