We describe the features of a mid-infrared cavity ringdown spectrometer and report its capabilities for ultra-sensitive monitoring of atmospheric trace gases. A CO overtone sideband laser operating near λ=3 μm is utilized to excite a high-finesse cavity which provides an effective optical absorption pathlength of 3.6 km. The detection limit achieved is 100 volume parts per trillion for methane and 2 volume parts per billion for formaldehyde in atmospheric air (integration time: 20 s). Furthermore, a prototype for a mobile analyser based upon an all-solid-state infrared laser system was developed.