EVOLUTION OF CHLORINE AND NITROGEN SPECIES IN THE LOWER STRATOSPHERE DURING ANTARCTIC SPRING - USE OF TRACERS TO DETERMINE CHEMICAL-CHANGE

Observations of O-3, HCl, NO, and NO2 from the Halogen Occultation Exp eriment (HALOE) provide a means to investigate chemical change in the lower stratosphere over Antarctica during the first 23 days of October 1992. Two long-lived species also observed by HALOE, HF and CH4, are used as tracers to identify a series of air parcels having similar Cl- y and NOy abundances. The set of parcels chosen using tracer analysis show uniformly low O-3 mixing ratios, less than 1 ppm on the 480 K sur face (about 18-20 km). HCl mixing ratios for those parcels with less t han 1 ppm of ozone nearly tripled during the time period, and NO + NO2 abundances rose sharply, by a factor of 6 dr 7. These trends in HCl, NO, and NO2 agree qualitatively with model calculations which show tha t the formation of HCl proceeds quickly when O-3 levels fall so low th at (1) the rate of the reaction Cl + O-3 --> ClO + O-2 slows and (2) t he rate of the reaction NO + ClO --> NO2 + Cl becomes faster than the rate of the competing reaction NO + O-3 --> NO2 + O-2. Under these con ditions, Cl increases at the expense of ClO, and HCl is formed via the reaction Cl + CH4 --> HCl + CH3. Stratospheric chlorine is thus shift ed from reactive species to the long-lived, reservoir molecule HCl. Th e repartitioning of the active chlorine family in favor of HCl halts t he processes that destroy ozone and makes available active nitrogen in the form of NO and NO2. The investigation confirms earlier results an d validates tracer analysis as a reliable method to probe chemical cha nge in the stratosphere.