NASA - National Aeronautics and Space Administration

We present a new cavity ring-down spectroscopy system which was developed for variable-temperature absorption measurements (220–290  K) of atmospheric gases. This laser spectrometer was developed in the framework of the NASA Orbiting Carbon Observatory-2 project to improve our understanding of line shape parameters for carbon dioxide and oxygen. The apparatus consists of a monolithic, fixed-mirror ring-down cavity within a temperature-regulated enclosure, which is interrogated by a tunable, single-frequency diode laser. We experimentally characterize and model the dependence of the spectrum detuning axis at each setpoint temperature, and show that absolute frequencies are stable to within 200 kHz over several hours, corresponding to temperature stabilities better than 1  mK. We measure the R16e (30013-0001) 12C16O2 transition and carry out multi-spectrum analyses using two line profiles incorporating speed-dependent (quadratic approximation) and Dicke narrowing (hard collision assumption) effects. The resulting broadening coefficient and temperature exponent are in excellent agreement (0.05% level) with previous high-resolution Fourier-transform spectroscopy measurements, and the speed dependent broadening parameter is within 3% of the theoretical value. ground-platforms require increasingly accurate line-by-line spectroscopic parameters over the 220–300 K temperature range. For example, the Orbiting Carbon Observatory-2 (OCO-2) NASA satellite mission [1] has a target relative uncertainty of 0.3% in the retrieval of the column-integrated carbon dioxide ­ (CO2) molar fraction. NASA has similar measurement goals for ­CO2 which are required for the TCCON Project and ASCENDS mission, as well as in the European Space Agency (ESA) MERLIN mission which detects methane (­CH4) in the Earth’s atmosphere. All of these measurements require accurate, predictive forward models for the relevant spectrally resolved absorption coefficient over an atmospheric column that traverses the lower troposphere through the upper stratosphere. These spectroscopic models need to be validated over a wide range of pressure, temperature and composition consistent with the conditions of interest.

The cavity-ring down spectroscopy (CRDS) laserabsorption technique was introduced by O’Keefe and Deacon in 1988 using pulsed lasers [2] and is now routinely implemented using myriad custom and commercial spectrometers. The essential aspect of CRDS is to measure the decay rate of a circulating light beam that leaks out of a high-finesse optical resonator (i.e. ring-down cavity) containing a light-absorbing sample. CRDS is now usually implemented with single-frequency, continuous-wave (CW) lasers [3, 4]. These types of lasers have bandwidths that are much less than the cavity mode spacing, thus allowing for single-mode excitation and yielding decay signals that can be modeled by a single exponential function. For this case, the circulating probe beam encompasses a narrow range of optical frequencies centered around one cavity mode, and the sample absorption coefficient equals the change in the decay rate divided by the speed of light. This situation leads to high spectral