CONTRAST: Convective Transport of Active Species in the Tropics

Summary

The Convective Transport of Active Species in the Tropics (CONTRAST)  experiment is designed to measure the chemistry and transport of reactive chemical species into  the tropical Tropopause Transition Layer (TTL) over the Western Pacific warm pool area.  During the boreal winter season, tropospheric air masses are preferentially transported into the  lower stratosphere in this region. Thus, the sources, chemistry, and transport of trace gases and  their degradation products in the region can substantially impact the chemistry of the lower  stratosphere. Considerable attention is being given to the role of tropical convection on the  delivery of reactive gases to the TTL. For example, uncertainties in the abundance of reactive  halogen species and the fate of short lived organic halogen compounds, particularly  bromocarbons, in the tropical upper troposphere leads to significant uncertainties in the  photochemistry of stratospheric ozone, especially in the lowermost stratosphere. The  CONTRAST mission will provide new measurements in this data sparse region to determine  the influence of convection on the reactive chemistry of the tropical upper troposphere, to  evaluate the role of different transport pathways in shaping the chemical composition, and to  define the chemical budgets for air entering the stratosphere in this region.    The research proposed here is to measure a wide range of organic trace gases collected from the  HAIS Advanced Whole Air Sampler (AWAS) on the NCAR GV aircraft. Up to 60 samples will  be collected on each flight in either autonomous or on-command sampling mode. The trace  gases to be targeted include a range of organic molecules, including a both short-lived and longlived halocarbon compounds, as well as non-methane hydrocarbons, organic nitrates, and  selected sulfur species. The gases targeted for this study will provide information to diagnose  halogen transformations and budget of the TTL, to estimate relative age of air in the UT/LS, and  to distinguish air mass sources in the region. Trace organic gas data analysis will be  coordinated with similar measurements from the TOGA and other instruments on the GV and  with two collaborating research missions (ATTREX and CAST). The combined data will  provide a unique characterization of tropical atmospheric chemical composition from the  surface through the TTL and into the lower tropical stratosphere to 20 km altitude.