  We demonstrate coherent dual frequency-comb spectroscopy for detecting
variations in greenhouse gases. High signal-to-noise spectra are acquired
spanning 5990 to 6260 cm^-1 (1600 to 1670 nm) covering ~700 absorption features
from CO2, CH4, H2O, HDO, and 13CO2, across a 2-km open-air path. The
transmission of each frequency comb tooth is resolved, leading to spectra with
<1 kHz frequency accuracy, no instrument lineshape, and a 0.0033-cm^-1 point
spacing. The fitted path-averaged concentrations and temperature yield dry-air
mole fractions. These are compared with a point sensor under well-mixed
conditions to evaluate current absorption models for real atmospheres. In
heterogeneous conditions, time-resolved data demonstrate tracking of strong
variations in mole fractions. A precision of <1 ppm for CO2 and <3 ppb for CH4
is achieved in 5 minutes in this initial demonstration. Future portable systems
could support regional emissions monitoring and validation of the spectral
databases critical to global satellite-based trace gas monitoring.
