New Insights from Reexamination of In Situ Measurements of ClO in the UTLS from Aircraft and Balloons

It is widely recognized that in situ measurements of ClO from the NASA ER-2 aircraft in the late 1980s demonstrated the causal link between emissions of man-made chlorine compounds and ozone destruction in the polar stratosphere (Antarctic and Arctic). Less well known, although of potential interest today, are subsequent observations of ClO in the lowermost stratosphere from other aircraft and balloons. As if often the case with field observations, some important results are overlooked while more pressing topics are addressed, which was the case for more than a decade as the polar ozone hole phenomenon dominated stratospheric halogen studies. This presentation will focus on a series of observations of ClO (and, in some cases, BrO) using a suite of instruments that employed a common calibration procedure and that were deployed on multiple aircraft (the NASA WB-57 and NASA DC-8) and more than a dozen balloons launched from four locations (Laramie, Wyoming; Lauder, New Zealand; Kiruna, Sweden, and McMurdo Station, Antarctica). In particular, I will examine important details of halogen chemistry that were not addressed in early publications, including the range of abundances of ClO observed in the lowermost stratosphere in the tropics and mid-latitudes, evaluate limitations of the methods used in those measurements, and address the implications for our understanding of ozone balance in the lowermost stratosphere. I will recommend new observations that can be made with existing and improved instruments to resolve outstanding questions in atmospheric halogen chemistry in the upper troposphere and lower stratosphere, particularly in the tropics.