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Season-height trends

Information on the vertical distribution of ozone is obtained from ozone soundings. The vertical profile time series have been homogenised and are now of the highest possible quality for further research. The 51 years time series of Uccle ozone sounding data is visualized in the figure below. The vertical coordinate is the altitude relative to the tropopause (tropopause location = 0), which marks the border between the troposphere (= temperature globally decreasing with increasing height; almost all weather phenomena take place in the troposphere) and the stratosphere, in which the temperature rises again with increasing height due to the absorption of solar UV light by the ozone molecules. As ozone is formed by different processes in the troposphere and the stratosphere, this distinction allows for a better study of the ozone concentrations at those different atmospheric layers. 

With an average tropopause location around 11 km at Uccle, it should be obvious from the figure that the maximum ozone concentrations (blue-violet colours) occur between altitudes of 20 to 25 km. 


Here it is: the 51 years time series of the vertical ozone profiles (ozone partial pressures in mPa) measured at Uccle by ozonesondes. The altitude level is expressed in km relative to the tropopause height.

To have a better idea of the time variability of the atmospheric ozone concentrations, we calculated the mean ozone profile for the different subsequent decades from the dataset of ozone profiles shown here above. These 5 different mean ozone profiles are presented by different colours in the graph here below, with the altitude relative to the tropopause height again as vertical coordinate.

From this graph, it can be noted that the first decade of observations (1969-1978, in black) had the lowest mean tropospheric ozone amounts (negative values of the vertical coordinate), while the highest stratospheric ozone amounts were measured during this decade. The minimum mean stratospheric ozone concentrations were measured in Uccle during the 1989-1998 decade (green coloured graph), a decade also characterised with a high amount of (harmful) tropospheric ozone concentrations. During this decade, the amounts of chlorine and bromine substances (like chlorofluorocarbons or CFCs and halons) in the stratosphere reached peak  values (with its maximum in 1997), and these substances deplete very efficiently the ozone molecules in the stratosphere.

Since the decreasing concentrations of ozone depleting substances in the stratosphere thanks to the Montreal Protocol emission regulations, the ozone layer is recovering (see blue and green curves, corresponding to the periods 1999-2008 and 2009-2019 respectively), but the tropospheric ozone concentrations also remain high. We can also clearly see that the stratospheric ozone amounts measured at the beginning of our time series are still not reached during the most recent years. Full ozone layer recovery is predicted to take place around 2050, and also depends on climate change, which interacts with stratospheric ozone amounts.


Mean vertical ozone profiles calculated for the subsequent 5 decades in which ozone sounding data are available at Uccle.

We now calculate the relative trends in the ozone concentrations by a least square linear regression on the ozone partial pressures at the different altitude levels. For the entire 1969-2019 time period, presented in black in the figure below, the ozone concentrations have been decreasing throughout the stratosphere between -1 to -2 %/decade, while increasing in the troposphere at a rate close to +2%/decade. However, it should be noted that these trends in the stratosphere are the result of a strong decreasing ozone amounts in the period 1969-1996 (around -5%/decade, in red), followed by increasing ozone amounts  (around +3%/decade, in green) around the ozone layer maximum in the period 1997-2019. The choice of splitting the time period in the year 1997 stems from the peak concentrations of ozone depleting substances in the stratosphere (the same trend turning point is chosen in the total ozone trends), but also coincides with the change of ozonesonde sensor type at Uccle.   


Vertical ozone concentration trends in % per decade, for different time periods. The error bars denote the 2σ standard errors of the linear regression slope determination and can be considered as a rough estimate of the trend uncertainty.

Finally, the linear trends are obtained on the monthly mean ozone partial pressures, and we show these trends for the different months separately in the figure below, for our entire time period of 51 years of measurements. The most significant decrease in the ozone concentrations around the ozone maximum (between 10 to 15 km in altitudes relative to the tropopause) took place in spring and early summer (March to June, dark red coloured), when the concentrations at the ozone maximum are high (highest from February to April). At altitudes above the ozone maximum, the ozone amounts also decreased significantly in November-December.

Another interesting finding from this figure is that the less significant increases in tropospheric ozone concentrations (light blue colours) arise during the summer months, when tropospheric ozone production is highest due to photochemical reactions involving sunlight and ozone precursor gases from anthropogenic fossil fuel and biofuel combustion, crop burning, chemical solvents, etc.


Season-height cross-section of ozone trends in percent per year at Uccle for the period January 1969 to December 2019. Areas where the trend is statistically significant at the 95 % level are coloured darker (red for negative and blue for positive trends). The altitude level is in units (km) relative to the tropopause height.