Atmospheric Monitoring Using Infrared Heterodyne Radiometry

Abstract

Atmospheric constituents have unique vibrational-rotational signature lines within the infrared spectrum and the signature intensities and line shapes vary with the constituent concentration and the atmospheric density. The recent development of stable, single frequency, single-mode laser local oscillators and nearly quantum-noise-limited heterodyne receivers have permitted the development of infrared heterodyne radiometers (IHR's) which provide good sensitivity and excellent specificity for the remote examination of individual atmospheric constituent signature lines. A 9 to 11 μm IHR employing a CO₂ laser local oscillator has been developed and can be used to resolve the spectral signature of atmospheric constituents such as SO₂, O₃ C₂H₄, and NH₃. The IHR has a bandwidth of 100 MHz (33 x 10⁻³ cm⁻¹) and a minimum detectable power spectral density of 5.4 x 10⁻²⁴ W/Hz for a 1-second integration time. For atmospheric monitoring applications the IHR telescope collects the thermal energy radiating from the earth at: (1) a clear spectral window, and (2) a spectral region in which the signature lines of the constituent gases at various layers of the atmosphere will be energized by the upwelling thermal radiation. When the vertical temperature distribution of the atmosphere is known, the concentration of the atmospheric constituent gas can be determined as a function of altitude from the radiance data collected at the IHR using an iterative mathematical technique.