We report on the development effort of a nanosecond-pulsed seeded optical parametric generator (OPG) for remote trace gas measurements. The seeded OPG output light is single frequency with high spectral purity and is widely tunable both at 1600nm and 3300nm with an optical-optical conversion efficiency of ~40%. We demonstrated simultaneous tuning over the methane (CH 4 ) absorption line at idler wavelength, 3270.4nm, and carbon dioxide (CO 2 ) absorption line at signal wavelength, 1578.2nm. In this paper, we will also discuss open-path atmospheric measurements with this newly developed laser source.