Accepted in the journal Applied Optics
Experimental PHOCS spectra and retrieval fit. Shown above is an oxygen feature at 7815.67 cm-1 and a water feature at 7816.74 cm-1. Plotted above the spectrum are fit residuals.
Abstract: We describe the development of a near-infrared laser heterodyne radiometer (LHR): Precision Heterodyne Oxygen-Calibration 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 CO2 and H2O column retrievals. Vertical profiling is made feasible by interrogating the very low-noise absorption lines shapes collected at »0.0067 cm-1 resolution. PHOCS complements results from the Orbiting Carbon Observatory (OCO-2), Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS), and ground-based Fourier transform spectrometers. In this manuscript we describe the development of the instrument by Mesa Photonics and present the results of initial tests in the vicinity of Washington, D.C.