Response of lower stratospheric HCl/Cl-y to volcanic aerosol: Observationsfrom aircraft, balloon, space shuttle, and satellite instruments

Vertical profiles of HCl/Cl-y measured by space shuttle (Atmospheric Trace Molecule Spectroscopy (ATMOS)) and balloon (MkIV) instruments from 1985 thr ough 1994 show that the increased volcanic aerosol from Mount Pinatubo had little effect on chlorine partitioning above 21 km. Below 21 km, however, l arge changes in HCl/l(y) were observed. Aircraft (Aircraft Laser Infrared A bsorption Spectrometer), balloon (MkIV, Far Infrared Spectrometer 2), space shuttle (ATMOS), and satellite (Halogen Occultation Experiment) measuremen ts below 21 km reveal an increase in HCl/Cl-y between early 1993 and late 1 997 of 31 +/- 9%. This growth rate is 2 to 3 times larger than model estima tes of the effect of the change in aerosol surface area and volume over thi s time period. For 1993 the five independent direct measurements of HCl/Cl- y produce a mean value of 0.57 +/- 0.05, whereas by 1997 the mean value was 0.75 +/- 0.07, in excellent agreement with the pre-Pinatubo mean of 0.74 /- 0.11. Measurements made in 1992 at the peak of the post-Pinatubo midlati tude aerosol loading demonstrate large variability, which is qualitatively consistent with model predictions. This variability appears to be attributa ble to the temperature sensitivity of the Cl-y partitioning when aerosol ab undances are high.