Fifty years of balloon-borne ozone profile measurements at Uccle, Belgium: a short history, the scientific relevance, and the achievements in understanding the vertical ozone distribution
Authors: Van Malderen Roeland , De Muer Dirk , De Backer Hugo , Poyraz Deniz , Verstraeten Willem W. , De Bock Veerle , Delcloo Andy W. , Mangold Alexander , Laffineur Quentin , Allaart Marc , Fierens Frans , Thouret Valérie 
Published: August 2021

Abstract. Starting in 1969 and comprising three launches a week, the
Uccle (Brussels, Belgium) ozonesonde dataset is one of longest and densest
in the world. Moreover, as the only major change was the switch from
Brewer-Mast (BM) to electrochemical concentration cell (ECC) ozonesonde
types in 1997 (when the emissions of ozone-depleting substances peaked), the Uccle time series is very homogenous. In this paper, we briefly describe the efforts that were undertaken during the first 3 decades of the 50 years
of ozonesonde observations to guarantee the homogeneity between ascent and
descent profiles, under changing environmental conditions (e.g. SO2),
and between the different ozonesonde types. This paper focuses on the 50-year-long Uccle ozonesonde dataset and aims to demonstrate its past,
present, and future relevance to ozone research in two application areas: (i) the assessment of the temporal evolution of ozone from the surface to the
(middle) stratosphere, and (ii) as the backbone for validation and stability
analysis of both stratospheric and tropospheric satellite ozone
retrievals. Using the Long-term Ozone Trends and Uncertainties in the
Stratosphere (LOTUS) multiple linear regression model (SPARC/IO3C/GAW,
2019), we found that the stratospheric ozone concentrations at Uccle
have declined at a significant rate of around 2 % per decade since 1969, which is also rather
consistent over the different stratospheric levels. This overall decrease
can mainly be assigned to the 1969–1996 period with a rather consistent
rate of decrease of around −4 % per decade. Since 2000, a recovery of between +1 % per decade and +3 % per decade of the stratospheric ozone levels above Uccle has been
observed, although it is not significant and is not seen for the upper stratospheric
levels measured by ozonesondes. Throughout the entire free troposphere, a
very consistent increase in the ozone concentrations of 2 % per decade to 3 % per decade has been measured since both 1969 and 1995, with the trend since 1995 being in
almost perfect agreement with the trends derived from the In-service
Aircraft for a Global Observing System (IAGOS) ascent/descent profiles at
Frankfurt. As the number of tropopause folding events in the Uccle time
series has increased significantly over time, increased
stratosphere-to-troposphere transport of recovering stratospheric ozone
might partly explain these increasing tropospheric ozone concentrations,
despite the levelling-off of (tropospheric) ozone precursor emissions and
notwithstanding the continued increase in mean surface ozone concentrations.
Furthermore, we illustrate the crucial role of ozonesonde measurements for
the validation of satellite ozone profile retrievals. With the operational
validation of the Global Ozone Monitoring Experiment-2 (GOME-2), we show how the
Uccle dataset can be used to evaluate the performance of a degradation
correction for the MetOp-A/GOME-2 UV (ultraviolet) sensors. In another example, we
illustrate that the Microwave Limb Sounder (MLS) overpass ozone profiles in
the stratosphere agree within ±5 % with the Uccle ozone profiles
between 10 and 70 hPa. Another instrument on the same Aura satellite
platform, the Tropospheric Emission Spectrometer (TES), is generally positively
biased with respect to the Uccle ozonesondes in the troposphere by up to
∼ 10 ppbv, corresponding to relative differences of up to
∼ 15 %. Using the Uccle ozonesonde time series as a
reference, we also demonstrate that the temporal stability of those last two satellite retrievals is excellent.