Simulations from eleven coupled chemistry‐climate models (CCMs) employing nearly identical forcings have been used to project the evolution of stratospheric ozone throughout the 21st century. The model‐to‐model agreement in projected temperature trends is good, and all CCMs predict continued, global mean cooling of the stratosphere over the next 5 decades, increasing from around 0.25 K/decade at 50 hPa to around 1 K/decade at 1 hPa under the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario. In general, the simulated ozone evolution is mainly determined by decreases in halogen concentrations and continued cooling of the global stratosphere due to increases in greenhouse gases (GHGs). Column ozone is projected to increase as stratospheric halogen concentrations return to 1980s levels. Because of ozone increases in the middle and upper stratosphere due to GHG‐induced cooling, total ozone averaged over midlatitudes, outside the polar regions, and globally, is projected to increase to 1980 values between 2035 and 2050 and before lower‐stratospheric halogen amounts decrease to 1980 values. In the polar regions the CCMs simulate small temperature trends in the first and second half of the 21st century in midwinter. Differences in stratospheric inorganic chlorine (Cly) among the CCMs are key to diagnosing the intermodel differences in simulated ozone recovery, in particular in the Antarctic. It is found that there are substantial quantitative differences in the simulated Cly, with the October mean Antarctic Cly peak value varying from less than 2 ppb to over 3.5 ppb in the CCMs, and the date at which the Cly returns to 1980 values varying from before 2030 to after 2050. There is a similar variation in the timing of recovery of Antarctic springtime column ozone back to 1980 values. As most models underestimate peak Cly near 2000, ozone recovery in the Antarctic could occur even later, between 2060 and 2070. In the Arctic the column ozone increase in spring does not follow halogen decreases as closely as in the Antarctic, reaching 1980 values before Arctic halogen amounts decrease to 1980 values and before the Antarctic. None of the CCMs predict future large decreases in the Arctic column ozone. By 2100, total column ozone is projected to be substantially above 1980 values in all regions except in the tropics.

Hg 0 + I 2 → HgI 2 Absolute N 2 , 1 atm 296 ± 1 < (1.27 ± 0.58) × 10 -19 Raofie et al. 2 M06-2X/aug-cc-pVTZ-PP High pressure limit 3.94 × 10 -14 T 1.06 e -159080/RT Auzmendi-Murua et al. 3 Hg 0 + I → HgI RRKM/B3LYP N 2 , 1 atm 180-400 4.0 × 10 -13 (T/298) -2.38 Goodsite et al. 4 Hg 0 + Br 2 → HgBr 2 Absolute Air, N 2 , 1 atm 298 ± 1 < (9 ± 2) × 10 -17 Ariya et al. 5 Absolute Air, 1 atm ∼298 No reaction detected Sumner et al. 6 Absolute Air, 1 atm 296 (6.0 ± 0.5) × 10 -17 Liu et al. 7 CCSD(T)/AVTZ 1 atm 298-2000 1.62 -9 e -110800/RT Wilcox and Okano 8 M06-2X/aug-cc-pVTZ-PP High pressure limit 4.70 × 10 -14 T 1.06 e -169190/RT Auzmendi-Murua et al. 3 Hg 0 + BrO → HgBrO Relative N 2 , 1 atm 298 10 -15 < k < 10 -13 Raofie and Ariya 9 Hg 0 + Br → HgBr Ab initio N/A, 1 atm 1.01 × 10 -12 e 1738/RT Khalizov et al. 10 RRKM/B3LYP N 2 , 1 atm 200-300 3.7 × 10 -13 (T/298) -2.76 Goodsite et al. 4 ; Goodsite et al. 11 Absolute N 2 , 0.26-0.79 atm 243-293 (1.46 ± 0.36) × 10 -32 [cm 6 molec -2 s -1 ] Donohoue et al. 12 (T/298) (-1.86±1.49) CCSD(T) Ar, 1 atm 260 1.2 × 10 -12 Shepler et al. 13 Relative Air, N 2 , 1 atm 298 ± 1 (3.2 ± 0.9) × 10 -12 Ariya et al. 5 Absolute CF 3 Br, 0.26 atm 397 ~3 × 10 -16 molec -1 s -1 Greig, G. et al. 14 CCSD(T)/AVTZ 1 atm 298-2000 6.64 × 10 -14 (T/298) -0.859 Wilcox and Okano HgBr + Br → HgBr 2 Absolute CF 3 Br, 0.26 atm 397 ~7 × 10 -14 Greig, G. et al. 14 RRKM/B3LYP N 2 , 1 atm 180-400 2.5 × 10 -10 (T/298) -0.57 Goodsite et al. 4 CCSD(T)/AVTZ 1 atm 298-2000 3.32 × 10 -12 (T/298) -9.18 Wilcox and Okano CCSD(T)/aVTZ 1 atm 298 6.33 × 10 -11 Dibble et al. 15 ; Wang et al.

Episodes of high bromine levels and surface ozone depletion in the springtime Arctic are simulated by an online air-quality model, GEM-AQ, with gas-phase and heterogeneous reactions of inorganic bromine species and a simple scheme of air-snowpack chemical interactions implemented for this study. Snowpack on sea ice is assumed to be the only source of bromine to the atmosphere and to be capable of converting relatively stable bromine species to photolabile Br₂ via air-snowpack interactions. A set of sensitivity model runs are performed for April 2001 at a horizontal resolution of approximately 100 km&times;100 km in the Arctic, to provide insights into the effects of temperature and the age (first-year, FY, versus multi-year, MY) of sea ice on the release of reactive bromine to the atmosphere. The model simulations capture much of the temporal variations in surface ozone mixing ratios as observed at stations in the high Arctic and the synoptic-scale evolution of areas with enhanced BrO column amount ("BrO clouds") as estimated from satellite observations. The simulated "BrO clouds" are in modestly better agreement with the satellite measurements when the FY sea ice is assumed to be more efficient at releasing reactive bromine to the atmosphere than on the MY sea ice. Surface ozone data from coastal stations used in this study are not sufficient to evaluate unambiguously the difference between the FY sea ice and the MY sea ice as a source of bromine. The results strongly suggest that reactive bromine is released ubiquitously from the snow on the sea ice during the Arctic spring while the timing and location of the bromine release are largely controlled by meteorological factors. It appears that a rapid advection and an enhanced turbulent diffusion associated with strong boundary-layer winds drive transport and dispersion of ozone to the near-surface air over the sea ice, increasing the oxidation rate of bromide (Br^&minus;) in the surface snow. Also, if indeed the surface snowpack does supply most of the reactive bromine in the Arctic boundary layer, it appears to be capable of releasing reactive bromine at temperatures as high as −10 °C, particularly on the sea ice in the central and eastern Arctic Ocean. Dynamically-induced BrO column variability in the lowermost stratosphere appears to interfere with the use of satellite BrO column measurements for interpreting BrO variability in the lower troposphere but probably not to the extent of totally obscuring "BrO clouds" that originate from the surface snow/ice source of bromine in the high Arctic. A budget analysis of the simulated air-surface exchange of bromine compounds suggests that a "bromine explosion" occurs in the interstitial air of the snowpack and/or is accelerated by heterogeneous reactions on the surface of wind-blown snow in ambient air, both of which are not represented explicitly in our simple model but could have been approximated by a parameter adjustment for the yield of Br₂...

An eight-member ensemble of ECHAM5-HAMMOZ simulations for a boreal summer season is analysed to study the transport of aerosols in the upper troposphere and lower stratosphere (UTLS) during the Asian summer monsoon (ASM). The simulations show persistent maxima in black carbon, organic carbon, sulfate, and mineral dust aerosols within the anticyclone in the UTLS throughout the ASM (period from July to September), when convective activity over the Indian subcontinent is highest, indicating that boundary layer aerosol pollution is the source of this UTLS aerosol layer. The simulations identify deep convection and the associated heat-driven circulation over the southern flanks of the Himalayas as the dominant transport pathway of aerosols and water vapour into the tropical tropopause layer (TTL). Comparison of model simulations with and without aerosols indicates that anthropogenic aerosols are central to the formation of this transport pathway. Aerosols act to increase cloud ice, water vapour, and temperature in the model UTLS. Evidence of ASM transport of aerosols into the stratosphere is also found, in agreement with aerosol extinction measurements from the Halogen Occultation Experiment (HALOE) and Stratospheric Aerosol and Gas Experiment (SAGE) II. As suggested by the observations, aerosols are transported into the Southern Hemisphere around the tropical tropopause by large-scale mixing processes. Aerosol-induced circulation changes also include a weakening of the main branch of the Hadley circulation and a reduction of monsoon precipitation over India