Topics in lower stratospheric ozone

Abstract :

In this presentation I will cover 2 topics regarding lower stratospheric ozone.

1) Recent work suggests that during the period of the summertime North American Monsoon (NAM), ozone depletion could occur as a result of catalytic ozone destruction associated with the cold and wet conditions caused by overshooting convection. Aura Microwave Limb Sounder (MLS) water vapor measurements do show that the NAM region is wetter than other parts of the globe in regards to both the mean and extremes. However, definitive evidence of ozone depletion occurring in that region has not been presented. In this study, we examine coincident measurements of water vapor, ozone, and tropospheric tracers from aircraft data taken during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) aircraft campaign looking specifically for ozone depletion in regions identified as impacted by overshooting convection. Although we do find evidence of lower ozone values in air impacted by convective overshoots, using tropospheric tracers we attribute those observations to input of tropospheric air rather than catalytic ozone destruction. Additionally, we explore the consequences of these lower ozone values on surface UV and conclude that there is minimal impact on the UV index.

2) A recent study of lower stratospheric ozone measured by satellite-based sensors reported that downward trends in the tropics and middle latitudes have continued since 1998. Here, we present post-1998 trends in lower stratospheric ozone deduced from >20-year records of balloon-borne, in situ measurements of ozone at three sounding sites: Boulder, Colorado (39.95°N); Hilo, Hawaii (19.72°N); and Lauder, New Zealand (45.04°S). Lower stratospheric (LS) ozone columns are calculated from simultaneous measurements of ozone, temperature and pressure at high vertical resolution (5-10 m). Using the same fixed altitude/pressure limits for LS columns as the recent study, we too find statistically significant downward trends in LS ozone from 1998-2017 over all three sounding sites. However, these altitude/pressure limits actually include tropopause air, which may be changing in time. We explore this impact on the trend calculation using ozone sonde data from Boulder, Co.

Acknowledgements: This work was done in conjunction with Eric Ray, Sean Davis and Dale Hurst, all affiliated with the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado.