Trends in Lower Stratospheric Ozone from 1998–Present Based on Balloon-Borne In Situ Measurements

A recent study of lower stratospheric (LS) 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 LS 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 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 since 1998 over all three sounding sites. However, upon realization that the fixed bottom limits of LS columns for the middle latitudes (13 km/147 hPa) include up to 2.5 km of the upper troposphere during summer months, we implemented LS column limits relative to the tropopause to determine genuine trends for LS ozone. The use of tropopause-relative partial column limits also reduces the impacts of decadal-scale increases in tropopause heights that may contribute to the decreasing trends in LS ozone when the partial columns are defined by fixed altitude/pressure limits.