Quantifying Stratospheric Ozone in the Upper Troposphere Using in Situ Measurements of HCl

A chemical ionization mass spectrometry (CIMS) technique has been developed for precise in situ measurements of hydrochloric acid (HCl). Aircraft measurements of HCl, ozone, and other gases were made in the summer of 2002 at subtropical latitudes in the upper troposphere and lower stratosphere. Significant abundances of HCl were found in many upper tropospheric air parcels as a result of stratosphere-to-troposphere transport. Minimum upper tropospheric HCl abundances are much lower than model mean estimates. Using the compact linear correlation of HCl with ozone in the lower stratosphere, the amount of stratospheric ozone in the upper troposphere can be uniquely distinguished from ozone that originated in the troposphere. This approach allows the processes affecting stratosphere to troposphere transport to be diagnosed in the atmosphere with greatly increased precision and accuracy.