We have designed and built a wavelength-tunable optical source for standoff detection of gaseous chemicals by differential absorption spectrometry in the long-wave infrared. It is based on a nanosecond 2 mu m single-frequency optical parametric oscillator, whose idler wave is amplified in large aperture Rb:PPKTP crystals. The signal and idler waves are mixed in ZnGeP2 crystals to produce single-frequency tunable radiation in the 7.5-10.5 mu n range. The source was integrated into a direct detection lidar to measure sarin and sulfur mustard inside a dosed chamber, in an integrated path configuration with a noncooperative target.