Compact Open-Path Sensor for Fast Measurements of CO2 and H2O using Scanned-Wavelength Modulation Spectroscopy with 1f-Phase Method.

Xiang Li,Feng Yuan,Mai Hu,Ruifeng Kan,Lifang Shi,Chenguang Yang,Yabai He,Bin Chen

doi:10.3390/s20071910

Abstract

We report here the development of a compact, open-path CO2 and H2O sensor based on the newly introduced scanned-wavelength modulation spectroscopy with the first harmonic phase angle (scanned-WMS-θ1f) method for high-sensitivity, high temporal resolution, ground-based measurements. The considerable advantage of the sensor, compared with existing commercial ones, lies in its fast response of 500 Hz that makes this instrument ideal for resolving details of high-frequency turbulent motion in exceptionally dynamic coastal regions. The good agreement with a commercial nondispersive infrared analyzer supports the utility and accuracy of the sensor. Allan variance analysis shows that the concentration measurement sensitivities can reach 62 ppb CO2 in 0.06 s and 0.89 ppm H2O vapor in 0.26 s averaging time. Autonomous field operation for 15-day continuous measurements of greenhouse gases (CO2/H2O) was performed on a shore-based monitoring tower in Daya Bay, demonstrating the sensor’s long-term performance. The capability for high-quality fast turbulent atmospheric gas observations allow the potential for better characterization of oceanographic processes.

Highlights

Carbon dioxide (CO2 ) and water vapor (H2 O) are confirmed as two influential greenhouse gases (GHGs) existing in the biogeochemical system
The measurement of GHGs emissions and energy exchange in temporal resolution is paramount to illuminating complex processes, especially important in coastal zones, where exceptionally heterogeneous terrestrial inputs, elemental cycling due to upwelling events, and exchanges between open and coastal ocean changes
The performance of wavelength modulation spectroscopy (WMS) sensors is commonly evaluated with the detection limits by quantifying noise-equivalent absorbance (NEA)

Summary

Introduction

Carbon dioxide (CO2 ) and water vapor (H2 O) are confirmed as two influential greenhouse gases (GHGs) existing in the biogeochemical system. The measurement of GHGs emissions and energy exchange in temporal resolution is paramount to illuminating complex processes, especially important in coastal zones, where exceptionally heterogeneous terrestrial inputs, elemental cycling due to upwelling events (e.g., tides, currents, local land, or sea breeze), and exchanges between open and coastal ocean changes. Sensors 2020, 20, 1910 induce a high dynamic variability of heat, water vapor, and carbon dioxide [4,5,6]. In the last two decades, there has been substantial efforts to examine the global budgets of many trace gases and energy via various disciplines and modeling studies at coastal seas [7,8,9,10,11,12,13]. One of the remaining challenges is to interpret and upscale relatively sparse measurements to a regional or continental scale in a coastal environment [14]

Methods

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Conclusion