Abstract:
While the stratospheric ozone protects the biosphere against ultraviolet (UV) radiation,
tropospheric ozone acts like a greenhouse gas and an indicator of anthropogenic pollution. In this paper,
we combined ground-based and satellite ozone observations over Irene site (25.90◦ S, 28.22◦ E), one of
the most ancient ozone-observing stations in the southern tropics. The dataset is made of daily total
columns and weekly profiles of ozone collected over 20 years, from 1998 to 2017. In order to fill in some
missing data and split the total column of ozone into a tropospheric and a stratospheric column, we used
satellite observations from TOMS (Total Ozone Mapping Spectrometer), OMI (Ozone Monitoring
Instrument), and MLS (Microwave Limb Sounder) experiments. The tropospheric column is derived
by integrating ozone profiles from an ozonesonde experiment, while the stratospheric column is
obtained by subtracting the tropospheric column from the total column (recorded by the Dobson
spectrometer), and by assuming that the mesospheric contribution is negligible. Each of the obtained
ozone time series was then analyzed by applying the method of wavelet transform, which permitted
the determination of the main forcings that contribute to each ozone time series. We then applied
the multivariate Trend-Run model and the Mann–Kendall test for trend analysis. Despite the
different analytical approaches, the obtained results are broadly similar and consistent. They showed
a decrease in the stratospheric column (−0.56% and −1.7% per decade, respectively, for Trend-Run
and Mann–Kendall) and an increase in the tropospheric column (+2.37% and +3.6%, per decade,
respectively, for Trend-Run and Mann–Kendall). Moreover, the results presented here indicated that
the slowing down of the total ozone decline is somewhat due to the contribution of the tropospheric
ozone concentration.
