Cities cover a very small part (<1%) of the world’s surface area, but account for around 70% of global energy-related CO2 emissions. Moreover, half of the world’s population lives in cities, a share that will probably reach 70% by 2050. In this context, accurately measuring a city’s emissions is a major challenge for developing coherent reduction plans. An international consortium of around 40 scientists is deploying a range of instruments for 2 weeks (under a meteorological balloon, on board research aircraft and on the ground) to measure emissions from the city of Reims and its surroundings. An adventure to follow on the dedicated website.