The TAPIOWCA project (for long-Term Assessment, Proxies and Indicators of Ozone and Water vapour changes affecting Climate and Air quality) aims to tackle several technical challenges and, then, to reassess the present-day mean distribution, climate variability and long-term trend in water vapour and ozone across the troposphere and stratosphere.

In particular, we propose

1. to improve the agreement between tropospheric ozone Climate Data Records (CDRs):
   1. by homogenising IASI measurements, and,
   2. by applying a harmonisation scheme that corrects for differences in the vertical perception of all major CDRs;
2. to characterise remaining differences between tropospheric ozone CDRs;
3. to investigate, using multiple linear regression (MLR) techniques, whether changes in ozone precursor concentrations or in exchanges between stratosphere and troposphere can explain tropospheric ozone trends;
4. to better capture interannual variability through an optimisation of the time lag of the response of ozone and water vapour to natural processes;
5. to analyse all available ozone and water vapour CDRs resolved in three dimensions, so that the full spatial structure of climate parameters can be revealed, hereby linking (a) the global and regional perspectives, and, (b) patterns in the troposphere and stratosphere;
6. to adopt a systematic and comprehensive approach across the project, in order to reduce uncertainty and to maximise robustness of the project’s assessments.

Ultimately, TAPIOWCA strives to find (partial) answers to several open science questions and to contribute substantially to environmental assessments of climate change, the ozone layer, and air quality. We therefore align the project’s methodology, output and timeline to the needs of ongoing and planned initiatives by the climate monitoring and trend assessment communities, in particular, TOAR-II, CEOS AC-VC/WGCV, SPARC LOTUS, WMO/UNEP, and IPCC.