LIMITS ON HETEROGENEOUS PROCESSING IN THE ANTARCTIC SPRING VORTEX FROM A COMPARISON OF MEASURED AND MODELED CHLORINE

Forty-day photochemical model runs are compared with ground-based stra tospheric ClO observations taken during the austral spring of 1993. Ou r purpose is to explore the range of required heterogeneous processing within which we can reproduce the duration and degree of chlorine act ivation within the Antarctic spring vortex. Heterogeneous processing o n nitric acid trihydrate (NAT) polar stratospheric clouds (PSCs) or su percooled ternary solution (STS) -type particles is shown to be necess ary to maintain chlorine in active forms during ozone hole formation i n September, even for small HNO3 amounts, or chlorine deactivates soon er than observed. The lower limits for the surface areas required are quite small, however. Thus the record ozone losses observed during Sep tember of 1993 may be attributed to catalytic loss due to chlorine mai ntained in active forms by heterogeneous processing despite the sparse particle loading of the Antarctic lower stratosphere at that time. Th e ozone loss rates predicted by the model during the formation of the springtime Antarctic ozone hole indeed agree quite well with observati ons. The one-dimensional model is also able to reproduce both the obse rved timing and rate for subsequent deactivation of chlorine. Renitrif ication from PSC evaporation is not required for this deactivation, as HCl reformation is very rapid at low ozone values.