Design, development, and application of novel all-solid-state spectroscopic gas sensors is reported. These compact instruments employ mid-infrared difference-frequency generation pumped by room-temperature semiconductor lasers, and are capable of real-time selective measurement of trace gases in ambient air with better than 1 ppb precision (part per billion, by mole fraction). Their features and performance in the laboratory and field environment are examined and compared to that of traditional spectroscopic sensors.