The number of landfills in Sweden is not exactly known, but the number of closed landfills containing organic waste is estimated to be between 4000 and 8000. Some of these are close to large and growing cities and are of interest to exploitation. Urban air quality affects people's health and well-being, real-time measurement and modelling of gases in the air is important for planning future cities. In this paper, we present a cost-effective method for monitoring methane and CO2 based on gas sensors mounted on an unmanned aircraft (UAV, drone). A drone-based wireless system is developed with MOS sensors (MQ2, MQ4 and TGS2611), Arduino-nano, XBee wireless communications modules and the interface to a base station's computer is written in Python. The system was tested in real field measurements with good results.