Scientific Arguments

CARIBIC uses a passenger jet airliner, which means that the altitude of our research flights (apart from ascending and descending) is about 10 to 12 km. This is the altitude at which modern jet aircraft function optimally.
Interestingly just at this altitude, we cruise in the UTLS region (Upper Troposphere-Lower Stratosphere region) for most of the aicraft movements. In the tropics we clearly fly in the free tropical troposphere well below the tropopause.

Didier Hauglustaine has clearly summarized the importance of the UTLS region in a brief article in the IGAC newsletter Issue number 26, June 2002. . . . The upper-troposphere / lower-stratosphere (UTLS) is still considered as a key region of the atmosphere as far as composition and climate interactions are concerned for the following reasons:

Radiative forcing by greenhouse gases such as water vapor and ozone remains especially sensitive in the UTLS to concentration changes, due to large temperature contrast with the surface.

The UTLS is the layer in which stratospheric and tropospheric air are mixed. The exchange between these atmospheric domains controls the influx of tracers into the stratosphere (including water vapor and long-lived greenhouse gases), and the O3 and NOx flux from the stratospheric reservoir down into the troposphere.

Chemistry in the lower stratosphere is very sensitive to temperature changes and, at high latitudes, to the presence of Polar Stratospheric Clouds (PSCs). Stratospheric ozone concentration and temperature changes are directly coupled providing an important climate-chemistry interaction. The UTLS also coincides with the height of transition between positive and negative trends as detected by ozone sondes.

Due to the influence of rapid convection within cloud structures and the large-scale vertical transport associated with convergence, the imprints of lower tropospheric events such as biomass burning and forest fires and several regional air pollution episodes are also imposed on the upper troposphere.

In this relatively dry region, species like peroxides and oxygenated hydrocarbons play an important role in generating HOx radicals. Heterogeneous chemical reactions on cirrus clouds may also affect the budget of ozone and other species at these altitudes.