19^th–20^th century semi-quantitative surface ozone along subtropical Europe to tropical Africa Atlantic coasts

Abstract. Tropospheric ozone (O₃) plays a key role in the climate system. Studying pre-industrial tropospheric O₃ implies two important challenges: i) the lack of observational records prior to the late 19^th century, which hampers understanding long-term climate trends, given O₃ crucial role, ii) and the uncertainties on their quantitative values in a non-polluted atmosphere across the planet. The ozonoscope was the first instrument used to measure ozone. It offers semi-quantitative estimates of surface O₃ when no other measurements were available. Despite their potential value, the digitisation, curation and publication of ozonoscope data remains largely unexplored. In this work, we initiate an effort to rescue surface O₃ ozonoscope records with a new data collection. We include data from 23 observatories covering Portugal and the African Atlantic regions, providing a latitudinal span from the extratropics in the northern hemisphere to the tropics in the southern hemisphere. This record represents the most extended ozonoscope data series to date, spanning 50 years of daily data and 58 years of monthly data, from 1855 to 1913.