THE EVOLUTION OF CLO AND NO ALONG AIR PARCEL TRAJECTORIES

Back trajectory analysis of Arctic and Antarctic aircraft data reveals that high ClO concentrations are associated with predicted polar stra tospheric cloud (PSCs) encounters. The CIO concentrations within the A rctic and Antarctic polar vortices vary widely but appear to be invers ely related to parcel solar exposure since the last PSC interaction. T hese results imply that production of NO(x) from HNO3 photolysis and r eaction with OH is the mechanism for the loss of chlorine radicals thr ough the reformation of chlorine nitrate. Highly denitrified air-parce ls show no change in ClO with solar exposure. The recovery process is quantitatively duplicated using a model of chemistry along trajectorie s. Although PSC processing is the primary mechanism for producing elev ated ClO amounts, back trajectories apparently unperturbed by PSC's al so show slightly elevated ClO levels in 1992 compared to Arctic 1989 a nd Antarctic 1987 measurements presumably due to the presence of Pinat ubo aerosol.