An optical system for measuring in real-time the spatial distribution of aerosol particles, such as pesticides or the like, sprayed from a vehicle over an agricultural field. An optical sensor is mounted directly on the vehicle and sends an excitation light beam through the aerosol as it is being sprayed. The resulting scattered light is received and analyzed to deduce therefrom information on the spatial distribution of the particles. This information is relayed to a processing unit, which in turn provides spraying instructions, for example a warning signal, to the operator of the system. The spraying of the aerosol can therefore be controlled in real-time to avoid contamination of sensitive areas.