1. Chemistry and Transport in the Lower Stratosphere Wuhu Feng 1, Martyn Chipperfield 1, Howard Roscoe 2 1. Institute for Atmospheric Science, School of the Environment, University of Leeds, U.K 2. British Antarctic Survey, Madingley Road, Cambridge, U.K. fengwh@env.leeds.ac.uk 3. Antarctic Ozone Loss 2. SLIMCAT 3D CTM 3D off-line chemical transport model.  -  vertical coordinate. Detailed stratospheric chemical scheme. Model simulation from 1989, then seasonal runs for the selected winter spring. (resolution 2.8 o x 2.8 o x 24 levels ) 1. Introduction Chemical transport models (CTMs) are powerful tools to study the processes controlling the observed polar ozone depletion in the lower stratosphere (LS). Here we show how the SLIMCAT 3D CTM has been improved and now produces good simulations both in the Antarctic and Arctic regions for recent years. Some sensitivity experiments (i.e. initialization, model horizontal resolution, chemical processes and radiation scheme) are also shown to investigate their effect on the calculation of chemistry and transport in the LS in the Arctic polar winter/spring. 4 Arctic Ozone Loss Fig. 1. Daily minimum TOMS total O 3 between 50 o S and 90 o S compared with SLIMCAT output from May 2 to Nov. 30. Fig. 2. Comparison of O 3 sonde data at 450K at Neumayer (71 o S, 352 o E) for 2000 and 2002 with SLIMCAT. Fig. 3. Variation of Cl y species averaged southward of 60 o S at 450K for 2000 and 2002. SLIMCAT successfully reproduces the evolution of observed O 3 both in the Antarctic and Arctic: Recent improvements to boundary conditions, model resolution, chemical and radiation processes in the model lead to better tracer transport and polar ozone loss; Two separated, strong mixing regions are consistent with split O 3 hole in SH in 2002. Less late chlorine activation and strong descent in this winter. Very early chlorine activation occurred in 2002/03 Arctic winter. Fig. 5. Evolution of the log-normalised equivalent length (EL) as a function of equivalent latitude on 450K. Fig. 4. Diagnosed chemical O 3 loss rate (ppbv/day) due to two main catalytic cycles: ClO+ClO (left) and ClO+BrO (right) Fig 11. Polar ClO x and HCl at 493K for recent 10 Arctic winters Fig. 6. Log-normalised EL mapped onto 380 K and 521 for Sep. 26 in 2000 and 2002. Fig. 7. Comparison of O 3 sonde observations (+ marks) at Ny-Alesund (79 o N,12 o E) with SLIMCAT for selected Arctic winters: 1999/2000 (left), 2002/03 (middle) and 2003/04 (right). Also shown are some sensitivity tests. Fig. 8. Comparison between aircraft data (ER-2 and M55 flights) and SLIMCAT. Fig. 9. Comparison of MKIV data with SLIMCAT runs with/without heterogeneous reaction. Fig 10. Minimum T northward of 50 o N at 456K and 575K Fig. 12. Polar O 3 loss (65 o -90 o N) for 10 years. Acknowledgements. Emily Schuckbrugh for equivalent length code; Gert König-Langlo and P. von der Gathen for O 3 sonde data; B. Sen, G. Toon, J. F. Blavier for MKIV data; C.R. Webster, C.M. Volk, A. Ulanovsky, F. Ravegnani, J. Host, E.C. Richard for ER2 and M55 aircraft data; NASA for TOMS data and BADC for ECMWF analyses. This work was supported by U.K. NERC, EU TOPOZ III and QUILT projects. References Feng W, et al, JAS, (in press) Feng W, et al, ACP, (submitted).