Microphysical Modelling of Chlorine Activation and Ozone Depletion

 

Research Staff: Katja Drdla

The effect of a range of assumptions about polar stratospheric clouds (PSCs) on ozone depletion during the 1999-2000 Arctic winter were assessed during 2001 using a coupled microphysical/photochemical model. Simulations spanned a large range of denitrification levels, with up to 80% vortex-average denitrification; individual trajectories had even larger denitrification values, and up to 40% dehydration. In addition, the composition of the PSCs was varied (ternary solutions, nitric acid trihydrate, nitric acid dihydrate, or ice) to explore sensitivity to heterogeneous reaction rates.

The presence of PSCs in the lowermost stratosphere during the month of February was found to be critical in causing severe ozone depletion below 500 K. Heterogeneous reactions on these PSCs were able to reactivate the ClONO2 produced as sunlight returned to the vortex. Only to 30% to 40% vortex-average ozone loss, depending on denitrification level, would have occurred without this chlorine reactivation; with chlorine reactivation, an additional 21% to 32% ozone loss is possible. During February (unlike earlier in the winter) the extent of chlorine reactivation and severity of ozone loss were sensitive to the heterogeneous reaction rates; varying the heterogeneous reactivity altered ozone loss by 11%. The heterogeneous chemistry primarily occurred from 0 to 4 K below the nitric acid trihydrate condensation point; however, many uncertainties influence heterogeneous reaction rates at these temperatures.

The chlorine reactivation during February also prevented denitrification from enhancing ozone loss until March: 70% vortex-average denitrification only enhanced ozone depletion by 3% on March 10th. The break-up of the vortex at this time probably limited the extent of ozone depletion during the 1999-2000 winter. If the vortex had remained stable until April 15th, further ozone loss could have been caused by the observed levels of denitrification. On April 15th, 16% ozone loss (out of a total 68% ozone loss) could be caused by 70% denitrification. Ozone loss intensifies non-linearly with enhanced denitrification: in individual air parcels with 90% denitrification, more than 40% ozone loss in mid-April can be attributed to denitrification alone.

 

Collaborator: M. R. Schoeberl, GSFC

Point of Contact: Katja Drdla, 650/604-5663, katja@aerosol.arc.nasa.gov