A method for determining emissions in an exhaust plume (11) produced by a combustion engine of a vessel (10) during cruise of the vessel (10), said emissions including the presence or concentration of carbon dioxide (CO2) and/or sulphur dioxide (SO2) and/or the count and size of particles. The position and distribution of the exhaust plume (11) is determined or estimated on the basis of the position, bearing and speed of the vessel (10) and further on the basis of meteorological data, such as wind direction and speed. An unmanned aerial vehicle (UAV) (12), i.e. a so-called drone, is controlled to fly through the 10 plume (11) to make measurements of exhaust emissions of the vessel (10).