We report the application of quantum cascade (QC) lasers to measurement of atmospheric trace gases in both closed path and open path configurations. The QC laser, a recently available commercial device, is Peltier cooled and pulsed, with emission near 965 cm-1. We use direct absorption with a rapid sweep integration and spectral fits to derive absolute concentrations from tabulated line parameters without calibration. In the closed path configuration, with absorption in a long pathlength multipass cell (210 m, 50 Torr), we examined laser line widths and sensitivity limits. We measured ammonia with a precision of 0.05 nmole/mole (0.05 ppbv) RMS at 1 Hz, limited by detector noise. The laser linewith was 0.007 cm-1 HWHM, based on measurements of ethylene absorption line shapes with a current pulse width of ~14 ns. In the open path configuration, we measured ammonia in the exhaust of automobiles driving through a probe beam. Atmospheric pressure line broadening and turbulence limit the sensitivity, giving a column density noise level at 20 Hz of 1.4 ppm-m. We observed ammonia column densities up to 40 ppm-m in the exhaust plumes. In future systems we will include a CO2 channel, allowing normalization to fuel use rate.