Precision heterodyne oxygen-corrected spectrometry: vertical profiling of water and carbon dioxide in the troposphere and lower stratosphere

We describe the development of a near-infrared laser heterodyne radiometer: the precision heterodyne oxygen-corrected spectrometer (PHOCS). The prototype instrument is equipped with two heterodyne receivers for oxygen and water (measured near 1278 nanometers) and carbon dioxide (near 1572 nanometers) concentration profiles, respectively. The latter may be substituted by a heterodyne receiver module equipped with a laser to monitor atmospheric methane near 1651 nanometers. Oxygen measurements are intended to provide dry gas corrections and—more importantly—determine accurate temperature and pressure profiles that, in turn, improve the precision of the ${CO}_2$ and $H_{2}O$ column retrievals. Vertical profiling is made feasible by interrogating the very low-noise absorption lines shapes collected at $\approx 0.0067{cm}^{-1}$ resolution. PHOCS complements the results from the Orbiting Carbon Observatory (OCO-2), Active Sensing of ${CO}_2$ Emissions over Nights, Days, and Seasons (ASCENDS), and ground-based Fourier transform spectrometers. In this paper, we describe the development of the instrument by Mesa Photonics and present the results of initial tests in the vicinity of Washington, DC.