Multi-Component and Multi-Point Trace Gas Sensing in Wavelength Modulation Spectroscopy Based on Wavelength Stabilization

Multi-component and multi-point trace gas sensing in the wavelength modulation spectroscopy is demonstrated based on the frequency-division multiplexing and time-division multiplexing technology.A reference photodetector is connected in series with a reference gas cell with the constant concentration to measure the second-harmonics peak of the components for wavelength stabilization in real time.The central wavelengths of the distributed feedback lasers are locked to the target gas absorption centers by the reference second-harmonics signal using a digital proportional-integral-derivative controller.The distributed feedback lasers with different wavelengths and modulation frequencies are injected into the gas cell to achieve multi-components gas measurement by the frequency-division multiplexing technology.In addition,multi-point trace gas sensing is achieved by the time-division multiplexing technology using a photoswitch and a relay unit.We use this scheme to detect methane(CH4)at 1650.9 nm and water vapor(H2O)at 1368.597 nm as a proof of principle with the gas cell path length of 10 cm.The minimum detection limits achieved for H2O and CH4 are 1.13 ppm and 11.85 ppm respectively,with three-point gas cell measurement;thus 10.5-fold and 10.1-fold improvements are achieved in comparison with the traditional wavelength modulation spectroscopy.Meanwhile,their excellent R-square values reach 0.9983 and 0.99564 for the concentration ranges of 500 ppm to 2000 ppm and 800 ppm to 2700 ppm,respectively.

MEASUREMENT OF CARBON MONOXIDE COLUMN CONTENT AND OTHER ATMOSPHERIC TRACE GASES FROM INFRARED SPECTRA

The measurement of column content of carbon monoxide,methane,and other atmospheric traces gases as well as aerosols has been conducted during the fall of 1992 and 1997,using an infrared spectrometer of moderate resolution in the cooperation of the Laboratory for Middle Atmosphere and Global Environment Observation(LAGEO),Institute of Atmospheric Physics, Chinese Academy of Sciences,with the Institute of Atmospheric Physics,Russian Academy of Sciences.The variation characteristics of atmospheric CO column content in Beijing area are presented.The analysis of the measurement shows that the CO column concentration of 0.10 mg cm^(-2)can be considered as the background over Beijing area.and the concentration as high as 0.75 mg cm^(-2)was observed under some unfavorable meteorological situations.

Effects of pressure and noise on the stability of photoacoustic signals of trace gas components

In essence,photoacoustic spectroscopy(PAS)technology is based on the thermal effect of gas infrared absorption and the acoustic theory of photoacoustic(PA)cell.PAS technology has a good application effect on environmental monitoring in agriculture.In this study,carbon monoxide and sulfur dioxide were used as examples to explain the potential application of PAS technology and analyze the influence mechanism of pressure and noise on the PA signal.The relationship between PA signal amplitude and the concentration of gas was determined by calibration.The pressure and noise characteristics were experimentally studied,and the relationship between the PA signal and pressure&noise was obtained.The theoretical analysis and experimental results not only provided a basis for further correction of the influence of pressure,noise and other factors on PA signal but also provided technical support for improving the field application of trace gas non-resonance PA detection device for environmental monitoring in agriculture.

Wide dynamic detection range of methane gas based on enhanced cavity absorption spectroscopy

Integrated cavity output spectroscopy(ICOS) is an effective technique in trace gase detection.The strong absorption due to the long optical path of this method makes it challenging in the application scenes that have large gas concentration fluctuation,especially when the gas concentration is high.In this paper,we demonstrate an extension of the dynamic range of ICOS by using a detuned laser combined with an off-axis integrating cavity.With this,we improve the upper limit of the dynamic detection range from 0.1%(1000 ppm) to 20% of the gas concentration.This method provides a way of using ICOS in the applications with unpredictable gas concentrations such as gas leak detection,ocean acidification,carbon sequestration,etc.

The Vertical Transport of Air Pollutants by Convective Clouds．Part Ⅲ：Transport Features of Different Cloud Systems

The vertical transport features of gaseous pollutants, with a negative exponent profile of concentration, by different types of convective cloud systems are numerically investigated by using a two-dimensional, reactlonless convective cloud transport model. The results show that an isolated, weak storm is able to pump pollutant gas out PBL and transport it to the mid-troposphere, whereas a deep. intense thunderstorm can very efficiently transport air pollutants up to the mid and upper troposphere and laterally spread with anvil, forming an extensive concentration surge layer at aliitnde of ten odd kilometers altitude. Each type of convective transport results in concentration reduction in PHL. In a wind shear environment the transport efficiency of deep thunderstorm significantly increases and the pollutants enter into clouds on the downshear side at low level and spread downwind in anvil layer. On the other hand, for a cumulus cloud with plenty of liquid water. the gas dissolution effect is increased, and the irreversible aqueous reactions, in extreme, may significantly weaken the vertical transports of pollutant gases even with solubility coefficients no more than 103 M atm-1.

Measurements of NO_2 mixing ratios with topographic target light scattering-differential optical absorption spectroscopy system and comparisons to point monitoring technique

A topographic target light scattering-differential optical absorption spectroscopy （＇IbTaL-DOA~） system is de- veloped for measuring average concentrations along a known optical path and studying surface-near distributions of atmospheric trace gases. The telescope of the ToTaL-DOAS system points to targets which are located at known dis- tances from the measurement device and illuminated by sunlight. Average concentrations with high spatial resolution can be retrieved by receiving sunlight reflected from the targets, A filed measurement of NO2 concentration is performed with the ToTaL-DOAS system in Shijiazhuang in the autumn of 2011. The measurement data are compared with con- centrations measured by the point monitoring technique at the same site. The results show that the ToTaL-DOAS system is sensitive to the variation of NO2 concentrations along the optical path.

Trace Ammonia Detection Based on Near-Infrared Fiber-Optic Cantilever-Enhanced Photoacoustic Spectroscopy

A trace ammonia(NH3)detection system based on the near-infrared fiber-optic cantilever-enhanced photoacoustic spectroscopy(CEPAS)is proposed.A fiber-optic extrinsic Fabry-Perot interferometer(EFPI)based cantilever microphone has been designed to detect the photoacoustic pressure signal.The microphone has many advantages,such as small size and high sensitivity.A near-infrared tunable erbium-doped fiber laser(EDFL)amplified by an erbium-doped fiber amplifier(EDFA)is used as a photoacoustic excitation light source.To improve the sensitivity,the photoacoustic signal is enhanced by a photoacoustic cell with a resonant frequency of 1624 Hz.When the wavelength modulation spectroscopy(WMS)technique is applied,the weak photoacoustic signal is detected by the second-harmonic detection technique.Trace NH3 measurement experiments demonstrate that the designed fiber-optic CEPAS system has a linear response to concentrations in the range of 0 ppm‒20 ppm at the wavelength of 1522.448 nm.Moreover,the detection limit is estimated to be 3.2 ppb for a lock-in integration time of 30 s.