Stratospheric Polar Processes and Ozone

Contact Michelle Santee

One of the overarching goals of the Aura mission is to track the stability of the stratospheric ozone layer. At issue is whether global stratospheric ozone will recover as anticipated as the abundances of ozone-depleting substances decline in response to international regulations.

The primary agent responsible for the formation of the ozone hole that forms over Antarctica every austral spring is anthropogenic chlorine. Stratospheric chlorine loading is presently near its peak but waning; assuming compliance with existing protocols, it should return to pre-1980 levels by about 2050.

Although detection and attribution of small changes in chlorine-catalyzed ozone loss are challenging problems, some abatement of lower stratospheric polar ozone depletion may become apparent during the Aura timeframe. There are, however, important linkages between climate change and ozone depletion that could delay recovery of the ozone layer. Changes in stratospheric temperature, humidity, and circulation patterns brought about by climate change could exacerbate polar ozone destruction processes. These issues are of particular concern in the Arctic, where wintertime temperatures often hover near the thresholds at which the processes leading to severe chlorine-catalyzed ozone destruction are triggered.

Aura MLS measures vertical profiles of many of the key species involved in polar processing and ozone loss in the lower stratosphere. In addition to temperature and ozone itself, MLS is providing the first simultaneous, collocated daily global measurements of both ClO, the primary form of reactive (ozone-destroying) chlorine in the stratosphere, and HCl, the main stratospheric chlorine reservoir (relatively inactive) species. MLS also measures two other minor chlorine species, HOCl and (in version 3) CH3Cl (the only significant natural source of organic chlorine).

In addition, MLS measures H2O and HNO3, the main components of the polar stratospheric clouds that form in the very low temperatures in the winter polar regions in both hemispheres; these cloud particles provide surfaces on which the heterogeneous chemical reactions that convert reservoir chlorine to reactive forms can take place, thus priming the atmosphere for severe ozone destruction.

N2O and CO are "tracers" of stratospheric air motions; MLS measurements of these species provide critical information needed to disentangle the effects of transport and mixing from those of chemical loss on the observed ozone distributions. Other MLS measurements that are arguably relevant for lower stratospheric polar processes and ozone loss include volcanic SO2 (for helping diagnose the influence of major volcanic eruptions on the ozone layer), and HCN, CH3CN, and cloud ice water content in the upper troposphere (for helping assess the impact of pollutants and other species lofted from below that may affect ozone chemistry in the stratosphere).