Using in situ measurements of HCl to identify stratospheric air in the upper troposphere

The transport of ozone from the lower stratosphere (LS) to the upper troposphere (UT) is an important term in the budget of upper tropospheric ozone. Stratospheric tracers are often used to quantify the presence of stratospheric ozone in the upper troposphere. We present new in situ measurements of HCl in the UT/LS made using the NOAA Chemical Ionization Mass Spectrometer (CIMS) onboard the WB-57F high-altitude aircraft. The measurements were made at subtropical and mid latitudes in July 2002 as part of the NASA CRYSTAL-FACE mission. HCl is a unique tracer of stratospheric air in the troposphere because it is produced in the stratosphere, is long lived except in the winter vortices, and has no significant abundance or source in the upper troposphere. The vertical profiles of HCl will be contrasted to other routinely measured UT/LS exchange tracers (e.g., NO_y, H_2O, CO_2, CO and condensation nuclei). A positive linear correlation of HCl and O_3 was observed in the LS on three flights. In addition, occasions were found for which HCl was non-zero at the tropopause and in some regions of the UT, indicating mixing of stratospheric air into the UT. We propose that the combination of the linear HCl-O_3 correlation in the LS and CIMS HCl measurements (with detection limits < 20 pptv) is of value in estimating or bounding the amount of stratospheric ozone present in UT air parcels.