A method for determining emissions in an exhaust plume produced by a combustion engine of a vessel during cruise of the vessel, said emissions comprising the presence or concentration of at least one predetermined gas or the count and size of particles, the method comprising the steps of: - identifying the vessel, its position (A, B, C, D), bearing and speed; - determining meteorological conditions in an area cruised by the vessel; - providing an unmanned aerial vehicle comprising: - an electronic control system for controlling the vehicle's flight; - at least one sensor (23, 24, 25, 26) for determining emissions in the atmosphere surrounding the vehicle; - a data interface for passing signals to an external data collecting unit, said signals comprising at least one of: (a) an output signal of the at least one sensor (23, 24, 25, 26) and (b) data obtained from the output signal of the at least one sensor (23, 24, 25, 26); - determining a position and distribution of the exhaust plume on the basis of the position (A, B, C, D), bearing and speed of the vessel and further on the basis of said meteorological conditions; - controlling the UAV to: - fly through the exhaust plume; - determine said emissions in the exhaust plume by means of the at least one sensor (23, 24, 25, 26); and - transmit said signals to the external data collecting unit for further processing thereof, wherein the step of controlling the UAV comprises determining or adjusting a flight mission, including at least a flight trajectory for the UAV during flight, and wherein the step of determining or adjusting the flight trajectory is carried out on the basis of meteorological conditions, vessel position (A, B, C, D), bearing and speed, and sensor data provided by output signals of the at least one sensor (23, 24, 25, 26). - identifying the vessel, its position (A, B, C, D), bearing and speed; - determining meteorological conditions in an area cruised by the vessel; - providing an unmanned aerial vehicle comprising: - an electronic control system for controlling the vehicle's flight; - at least one sensor (23, 24, 25, 26) for determining emissions in the atmosphere surrounding the vehicle; - a data interface for passing signals to an external data collecting unit, said signals comprising at least one of: (a) an output signal of the at least one sensor (23, 24, 25, 26) and (b) data obtained from the output signal of the at least one sensor (23, 24, 25, 26); - an electronic control system for controlling the vehicle's flight; - at least one sensor (23, 24, 25, 26) for determining emissions in the atmosphere surrounding the vehicle; - a data interface for passing signals to an external data collecting unit, said signals comprising at least one of: (a) an output signal of the at least one sensor (23, 24, 25, 26) and (b) data obtained from the output signal of the at least one sensor (23, 24, 25, 26); - determining a position and distribution of the exhaust plume on the basis of the position (A, B, C, D), bearing and speed of the vessel and further on the basis of said meteorological conditions; - controlling the UAV to: - fly through the exhaust plume; - determine said emissions in the exhaust plume by means of the at least one sensor (23, 24, 25, 26); and - transmit said signals to the external data collecting unit for further processing thereof, - fly through the exhaust plume; - determine said emissions in the exhaust plume by means of the at least one sensor (23, 24, 25, 26); and - transmit said signals to the external data collecting unit for further processing thereof,