Tropospheric Ozone Assessment Report II: Ozone distribution and variability in the oceanic and polar regions

Oceanic and polar regions are critical for global atmospheric processes, but an evaluation of ozone distributions and trends in these regions has been missing. In this assessment, part of the TOAR-II project, we report lower atmosphere ozone concentrations over open oceans and polar regions: with data from ships, buoys, aircraft and ground sites from 1975 to 2023, this is the most comprehensive dataset yet. Ozone trends were determined using Linear Mixed-effects Modelling (LMM). In most regions, trends have been modest since the mid-1970s (<1 nmol mol-1 per decade), positive in the mid-latitudes and tropical regions (with medium-low certainty), negative in the North and Tropical Pacific (with low and high certainty, respectively), increasing in Antarctica and decreasing in the Arctic. Seasonal effects often influence the long-term trends in extra-tropical regions. Five global and one hemispheric models were used to aid the interpretation of the sparse observations. The seasonal cycles are reasonably well reproduced, although models disagree with each other and with the observations on the distribution of ozone, especially in the tropics and in the Arctic. Changes in methane, iodine, shipping emissions, nitrogen oxides emissions, and terrestrial and marine volatile organic compound are likely to be important drivers for ozone over the oceans, although specific attributions are uncertain owing to insufficient data. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.