Increasing our understanding of regional greenhouse gas transport, sources, and sinks requires accurate, precise, continuous measurements of small gas enhancements over long ranges. We demonstrate a coherent dual frequency-comb spectroscopy technique capable of achieving these goals. Spectra are acquired spanning 5990 to 6260  cm^−1 (1600–1670 nm) covering ∼700 absorption features from CO_2, CH_4, H_2O, HDO, and CO_213, across a 2 km path. The spectra have sub-1-kHz frequency accuracy, no instrument lineshape, and a 0.0033  cm^−1 point spacing. They are fit with different absorption models to yield dry-air mole fractions of greenhouse gases. These results are compared with a point sensor under well-mixed conditions to evaluate the accuracy of models critical to global satellite-based trace gas monitoring. Under heterogeneous conditions, time-resolved data demonstrate tracking of small variations in mole fractions, with a precision <1  ppm for CO_2 and <3  ppb for CH_4 in 5 min. Portable systems could enable regional monitoring.