We report the development of a laser gas analyzer that measures gas concentrations at a data rate of 100 Hz. This fast data rate helps eddy covariance calculations for gas fluxes in turbulent high wind speed environments. The laser gas analyzer is based on derivative laser absorption spectroscopy and set for measurements of water vapor (H₂O, at wavelength ~1392 nm) and carbon dioxide (CO₂, at ~2004 nm). This instrument, in combination with an ultrasonic anemometer, has been tested experimentally in both marine and terrestrial environments. First, we compared the accuracy of results between the laser gas analyzer and a high-quality commercial instrument with a max data rate of 20 Hz. We then analyzed and compared the correlation of H₂O flux results at data rates of 100 Hz and 20 Hz in both high and low wind speeds to verify the contribution of high frequency components. The measurement results show that the contribution of 100 Hz data rate to flux calculations is about 11% compared to that measured with 20 Hz data rate, in an environment with wind speed of ~10 m/s. Therefore, it shows that the laser gas analyzer with high detection frequency is more suitable for measurements in high wind speed environments.

We developed a cavity ringdown spectrometer by utilizing a step-scanning and dithering method for matching laser wavelengths to optical resonances of an optical cavity. Our approach is capable of working with two and more lasers for quasi-simultaneous measurements of multiple gas species. The developed system was tested with two lasers operating around 1654 nm and 1658 nm for spectral detections of ¹²CH₄ and its isotope ¹³CH₄ in air, respectively. The ringdown time of the empty cavity was about 340 µs. The achieved high detection sensitivity of a noise-equivalent absorption coefficient was 2.8 × 10⁻¹¹ cm⁻¹ Hz^−1/2 or 1 × 10⁻¹¹ cm⁻¹ by averaging for 30 s. The uncertainty of the high precision determination of ${\mathsf{\delta }}^{13}{\mathrm{CH}}_4$ in air is about 1.3‰. Such a system will be useful for future applications such as environmental monitoring.

To achieve multi-gas measurements of quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors under a frequency-division multiplexing mode with a narrow modulation frequency interval, we report a frequency-domain detection method. A CH₄ absorption line at 1653.72 nm and a CO₂ absorption line at 2004.02 nm were investigated in this experiment. A modulation frequency interval of as narrow as 0.6 Hz for CH₄ and CO₂ detection was achieved. Frequency-domain 2f signals were obtained with a resolution of 0.125 Hz using a real-time frequency analyzer. With the multiple linear regressions of the frequency-domain 2f signals of various gas mixtures, small deviations within 2.5% and good linear relationships for gas detection were observed under the frequency-division multiplexing mode. Detection limits of 0.6 ppm for CH₄ and 2.9 ppm for CO₂ were simultaneously obtained. With the 0.6-Hz interval, the amplitudes of QEPAS signals will increase substantially since the modulation frequencies are closer to the resonant frequency of a QTF. Furthermore, the frequency-domain detection method with a narrow interval can realize precise gas measurements of more species with more lasers operating under the frequency-division multiplexing mode. Additionally, this method, with a narrow interval of modulation frequencies, can also realize frequency-division multiplexing detection for QEPAS sensors under low pressure despite the ultra-narrow bandwidth of the QTF.

A heterodyne radiometer (LHR) based on the balanced-detection method has been developed to evaluate system performance under both balanced-detection and single-channel-detection modes. Experimental results demonstrate that the balanced-detection LHR exhibits superior suppression of laser-induced noise compared to the traditional single-channel-detection LHR. The developed system achieved a noise equivalent power (NEP) of 2.53 × 10 ⁻¹⁵ W/√Hz, which is only 1.7 times of the theoretical quantum limit. To further assess system performance, measurements of atmospheric carbon dioxide (CO ₂ ) column abundance were conducted. The measurement precision of the balanced-detection LHR was determined to be approximately 0.6%, which is 5.67 times higher than that of the single-channel-detection LHR.