Stratospheric Ozone in the Last Glacial Maximum

As the primary control on surface ultraviolet radiation, stratospheric ozone has a crucial impact on tropospheric chemistry. Using the Whole Atmosphere Community Climate Model (WACCM), a high-top model with interactive stratospheric chemistry, stratospheric ozone in the Last Glacial Maximum (LGM) is investigated. It is shown that due to the boundary conditions and altered atmospheric concentrations of greenhouse gases in LGM, the stratospheric temperature increases by up to 10 K, leading to faster ozone destruction. On other hand, the hydroxyl radical (OH) and nitrogen oxide (NOx) concentrations decrease by 25%, which acts to increase the ozone concentration. Because of these compensating effects, the upper stratospheric ozone in the LGM decreases only by 5-10%. In the lower stratosphere, changes in the stratospheric overturning circulation play a significant role in determining ozone abundances. Consistent with a weakening of the residual circulation, the lower stratospheric ozone increases by up to 10- 20% in the tropics and decreases in the extratropics by 5-10%. The total stratospheric column ozone increases by 2% in the tropics, owing to the cancelation of the changes of ozone in the upper and lower stratospheres, but decreases up to 8% at the extratropics. Such change is generally smaller than that caused by anthropogenic ozone-depleting substances in the present day. It is also found that in the LGM tropospheric ozone decreases by 30-55%, driven by decreased emissions of reactive nitrogen and carbon from the surface, and tropospheric mean OH increases by 16.4%. In addition to the implications for tropospheric chemistry, the ozone fields from this study are useful for the PMIP LGM simulations that often use prescribed ozone fields.