Miniaturized Near Infrared Heterodyne Spectroradiometer for Monitoring CO2, CH4 and CO in the Earth Atmosphere

Abstract:

The paper describes the concept of a compact, lightweight heterodyne NIR spectro-radiometer suitable for atmospheric sounding with solar occultations, and the first measurement of CO₂ and CH₄ absorption near 1.60mm and 1.65 mm with spectral resolution l/dl ~ 5*10⁷. Highly stabilized DFB laser was used as local oscillator, while single model quartz fiber Y-coupler served as a diplexer. Radiation mixed in the single mode fiber was detected by quadratic detector using p-i-n diode within the bandpass of ~10 MHz. Wavelength coverage of spectral measurement was provided by sweeping local oscillator frequency in the range 1,1 см^-1. With the exposure time of 10 min, the absorption spectrum of the atmosphere over Moscow has been recorded with S/N ~ 300.

We retrieved methane vertical profile using Tikhonov method of smooth functional, which takes into account a priori information about first guess profile. The reference to model methane profile means that the regularization procedure always selects a priorivalues unless the measurements contradict this assumption.The retrieved methane profile demonstrates higher abundances in the lower scale height compared to the assumed model profile, well expected in the megalopolis center. The retrievals sensitivity is limited by 10 ppb, with the exception of the lower part of the profile where the tendency to lower values is revealed. Thus the methane abundance variations may be evaluated with relative accuracy better than 1%, which fits the requirements of greenhouse gas monitoring.

The retrievals sensitivity of CO2 is about 1-2 ppm. CO2 observations was also used to estimate stratoshere wind by doppler shift of absorption line.

Due to higher spectral resolution, lower sensitivity to atmospheric temperatures and other external factors, compared to heterodyne measurements in the thermal IR spectral range, the described technique provides accuracy comparable with much more complicated high resolution measurements now used in TCCON stations. Relative simplicity of the proposed scheme opens a perspective to employ it for high resolution spectroscopy in various applications. In particular, it may allow solar occultation observations of CO₂, CО, CH₄, H₂S, C₂H₄ and other gases from spacecraft, airborne or ground-based platforms.