V.A. Kapitanov

Design and characteristics of a differential Helmholtz resonant photoacoustic cell for infrared gas detection

The Helmholtz resonant photoacoustic (PA) cell is a very convenient design of PA system for air pollution monitoring based on infrared molecular absorption. A simple differential Helmholtz resonator designed for flow measurements is presented in this work. The investigation of the PA systems characteristics based on this design includes experimental study of the responsitivity both of the separate photoacoustic cell and the whole photoacoustic system applied to trace gases detection. The experimental observations are compared to the theoretical predictions. A simple arrangement to enhance the photoacoustic signal of the whole system by a factor of 2 is presented.

Optimisation of photoacoustic resonant cells with commercial microphones for diode laser gas detection.

The theoretical and experimental study of the differential Helmholtz resonant (DHR) cell sensitivity under variation of the total gas pressure is made for various commercial microphones. Near-infrared lasers (room-temperature diode lasers) were used to measure the response of DHR cell versus pressure of the absorbing gas and frequency of the laser radiation modulation. Several molecular absorbers like H2O, CH4, mixed with molecular buffer gases were used to investigate the behavior of the photoacoustic (PA) signal characteristics with a DHR cell. The experimental data are compared with the results of computer simulation. The minimal detectable concentrations of gases were determined for the DHR cell for each commercial microphone.

Influence of the overlapping of the atmospheric gas absorption spectra on the retrieval of the total methane content in the atmosphere from the transmission in the 1.6 to 1.7 μm spectral region

The influence of the spectral lines of atmospheric gases on the retrieval of the methane concentration from the measurement of the solar radiation spectrum in the spectral region of 1.6–1.7 μm is studied. The modeling of the atmospheric transmission function for different vertical profiles of the methane and ethylene concentration in this range has shown the necessity of taking into account the ethylene spectral absorption lines in radiation calculations in this range in addition to the CH4, H2O, and CO2 usually taken into account.

Numerical modeling of SF6 photoacoustic gas analyzer in the atmosphere with frequency modulation of thermal radiation

A simple design of a photoacoustic gas analyzer with a thermal source and an interference filter is suggested. The photoacoustic gas analyzer operation was modeled in the LabView visual programming environment while measuring the sulfur hexafluoride (SF6) concentration in atmospheric air. The optimal parameters of its elements were determined, i.e., a photoacoustic cell, an interference filter, an infrared thermal source, and microphones. The influence of the main absorbing components of the air on the PA signal amplitude was considered. It is shown that the use of low-cost commercial elements of the gas analyzer allows detection of atmospheric SF6 to a threshold level of about 0.06 ppm.

Design and modeling of a photoacoustic gas analyzer with a thermal source for carbon isotope ratio analysis

A diagram is suggested and numerical simulation is carried out of a photoacoustic gas analyzer with a thermal source and first order interference filter for carbon isotope ratio analysis. An optimal spectral range for gas analyzer operation is selected. In the wavelength range selected, the spectral parameters of the atmosphere are analyzed, as well as of some gases, which, being present in samples under study, could affect the measurement results. The gas analyzer suggested allows us to measure the carbon isotope ratio 13C/12C in CO2 samples with a minimal error of about 0.5‰, and could be useful for detection of such gases as SO2, CO, and NH3, absorption lines of which fall in the selected spectral range.

Laser gas-analyzers for studying kinetics of gas-exchange between vegetation biosystems and atmosphere

Specifications and examples of application of laser gas-analyzers of near- and mid-infrared ranges for diagnostics of greenhouse gases (CO2, C2H4, and CH4) emission by vegetation biosystems both under standard conditions and under impacts of natural and anthropogenic stresses are considered in the report. The gas-analyzer complex includes: 1) gas-analyzer with a multipass absorption cell based on the frequency-tunable diode laser (1600-1650 nm) for measuring sub-background methane concentrations (≤ 20 ppbV); 2) gas-analyzer based on the wave-guide CO2-laser equipped with the intracavity acoustic detector capable of fast frequency scanning in the 9 and 10 μm generation bands and having the ethylene-sensitivity better than 1 ppbV. The technology of measurements of gas emission by plant stripped leaves and coniferous needles is described and the measurements of CO2 and CH4 emissions are presented.

High resolution diode laser spectroscopy of H2O spectra broadened by nitrogen and noble gases

The absorption spectra of pure H2O with mixtures of broadening gases N2, Ar, Xe, He, Ar and air have been measured in 1.39 mμ spectral region by high resolution spectrometer based on diode laser (DFB NEL, Japan). For the processing of pure water spectra and it’s mixtures with a different broadening gases in a wide pressure range we used a multispectrum fitting procedure developed at IAO. The program is based on a relatively simple Rautian-Sobel’man line profile and linear pressure dependence of the line profile parameters. H2O measured spectra bulk processing results in the retrieving of such line parameters: zero-pressure line center positions, intensities, self-broadening and self-shift coefficients of pure water, broadening and shift coefficients for other gases which are describes the experiment with the minimum residuals in a wide pressure range.

Photoacoustic spectroscopy of the expired air at a human respiratory pathology

There are about 600 volatile compounds in air expired by man, including molecules-biomarkers of endogenous (produced in organism) origin, the specificity of generation and excretion mechanisms of which is sufficient for studies of both normal and pathologic processes. The goal of this work is analysis of the human-expired air by means of the intracavity laser photoacoustic sensor intended for detection of air gas admixtures having the absorption bands in the wavelength spectral range 9.2 - 10.8 μm. The sensor principle of operation is based on the photoacoustic effect caused by gas absorption of the CO2 laser radiation. Measurements were conducted with a test group consisting of 100 persons and a group of patients (60 persons) affected by different bronchopulnionary pathologies. The state of health of each group members was determined from questioning data. The spectral absorption dependences were normalized to referent points and then investigated in respect to a qualitative coincidence. The typical results are presented below.

Spatial distribution of methane over Lake Baikal surface (Invited Paper)

Anomolies in the atmospheric methane distribution over Lake Baikal have been studied with a diode gas analyzer having the threshold sensitivity of 0.04 ppm. Local areas, in which the methane concentration two to three times exceeded the background value equal to 2.00 ± 0.16 ppm, were detected.