During a solar eclipse, sunlight incident on the Earth is reduced due to the (partial) shadow of the Moon. Atmospheric trace gas concentrations which are influenced by the amount of available sunlight, such as nitrogen dioxide (NO2), may be affected due to the disrupted photolysis processes. Large-scale observations of the increased NO2 concentrations caused by the solar eclipse would improve our understanding of the sensitivity of NO2 in the atmosphere to short-term variations in sunlight. Spaceborne measurements can provide valuable information about the large-scale spatial distribution of NO2, which is provided daily by the TROPOMI instrument aboard the Sentinel-5 Precursor satellite by measuring and retrieving locally reflected sunlight. However, the TROPOMI NO2 retrieval is unable to derive reliable concentrations during a solar eclipse, as solar eclipses are not taken into account in its retrieval algorithm. In this research, we have adjusted the NO2 retrieval of TROPOMI such that it can handle solar eclipses and study the large-scale response of NO2 during two solar eclipses over Europe in 2021 and 2022. We found a large-scale increase of NO2 in the adjusted measurements, which linearly correlated with the degree of obscuration. We compared the measured NO2 increase with the values from the atmospheric chemistry model TM5 including an applied eclipse implementation and we found a close agreement in most areas that are not highly polluted. Our measurements and model predict a NO2 increase of 60%±12% and 70%±7% for an obscuration fraction of 1, respectively. More advanced chemistry modelling work is needed to explain the measurements in highly populated areas. We conclude that our results demonstrate that the TROPOMI algorithm is capable of correctly measuring NO2 after an adjustment of the NO2 retrieval. We have shown that it is possible to adjust an atmospheric trace gas retrieval for the influence of a solar eclipse. Moreover, we are the first to provide evidence for an increase in NO2 during a solar eclipse using space-based measurement techniques and to quantify this increase on a large scale with the same instrument. Our measurements can be used to test atmospheric chemistry models, possibly improving their sensitivity to solar eclipses but also artificial shadows on the Earth induced by sunlight-intercepting geoengineering approaches.

To answer the question of habitability of other planets, it is crucial to find liquid water. As a planet’s surface might be difficult to characterise through observations, the observation of cloud composition and coverage could possibly reveal the presence of large bodies of surface water. Climate code SPEEDY is used to investigate relations between cloud patterns on rocky exoplanets with oceans for various planet parameters, such as obliquity and incident stellar flux, and the observable signals of such exoplanets are computed. SPEEDY was chosen for the modelling of rocky exoplanets, because of its speed, since our aim is to run simulations for various planet parameters, and its flexibility, since it allows the adaptation of planet properties such as the presence and distribution of continents. The planet’s rotational period is found to have the most obvious influence on the cloud pattern: with increasing rotational speed, bands of clouds form, parallel to the equator, with the number of bands increasing with the rotational speed. The total flux and polarisation of starlight that is reflected by the planets with cloud bands as functions of the wavelength and the planetary phase angle are computed. Also the influence of the integration time of the observations on the reflected light signals is studied. Our main recommendation for further research is to broaden the applicability of SPEEDY for exoplanet research by first gaining more insight into the parametrisations and then by adapting them where necessary to allow wider parameter settings.