TDL based spectrometer for simultaneously monitoring local and column greenhouse gases

Abstract:

In this paper we present a concept of TDL based spectrometer for local and column greenhouse gases monitoring in particular for CO₂, CH₄, N₂O and O₂. The optical scheme of spectrometer’s single gas monitoring channel is shown on the figure 1 for CO₂ as an example.

For each gas monitoring laser radiation is divided into four channels.One of the channels is used for monitoring CO₂ abundance in the local atmosphere. This channel is presented by off-axis integrated cavity output spectroscopy (OA ICOS). Mirrors with reflection coefficient R~99.99% are used in order to achieve effective optical path equal to 1500 meters for high precision CO₂ monitoring. Measurements will be conducted at a pressure of 100 milibar and a compact diaphragm pump will be used. Reference gas spectra will be used for diode laser frequency stabilization. These spectra will be recorded in specified channel, which is also based on OA ICOS technique. Mirrors reflection coefficients are equal to ~99% in this case. A frequency scale calibration will be provided by Fabry-Perot (FP) ethanol in third channel. The same FP-ethanol will be used for each gas measurements.

The last channel is presented by heterodyne spectroscopy and will be used for average column and vertical profiling measurements of greenhouse gases. The intermediate frequency bandwidth is 10 MHz. The spectral coverage is provided by laser wavelength sweep with 100 kHz frequency. Signal registration will be carried out by high-speed electronics without principal nonlinear elements in analog signal path.

Present concept incorporates channels for local and column monitoring of CO₂ (1.605µ), CH₄ (1.65µ), O₂ (1.27µ), N₂O (2.25µ) and can be easily extended to H₂O (1.39µ), CO (2.33µ), etc. measurements.