Impact of Residual Water Vapor on the Simultaneous Measurements of Trace CH4 and N2O in Air with Cavity Ring-Down Spectroscopy

Qianhe Wei,Ping Yang,Jing Wang,Bincheng Li,Binxing Zhao

doi:10.3390/atmos12020221

Abstract

Methane (CH4) and nitrous oxide (N2O) are among the most important atmospheric greenhouse gases. A gas sensor based on a tunable 7.6 μm continuous-wave external-cavity mode-hop-free (EC-MHF) quantum cascade laser (from 1290 to 1350 cm−1) cavity ring-down spectroscopy (CRDS) technique was developed for the simultaneous detection of CH4 and N2O in ambient air with water vapor (H2O) mostly removed via molecular sieve drying to minimize the impact of H2O on the simultaneous measurements. Still, due to the broad and strong absorption spectrum of H2O in the entire mid-infrared (mid-IR) spectral range, residual H2O in the dried ambient air due to incomplete drying and leakage, if not properly accounted for, could cause a significant influence on the measurement accuracy of the simultaneous CH4 and N2O detection. In this paper, the impact of residual H2O on the simultaneous CH4 and N2O measurements were analyzed by comparing the CH4 and N2O concentrations determined from the measured spectrum in the spectral range from 1311 to 1312.1 cm−1 via simultaneous CH4 and N2O measurements and that determined from the measured spectrum in the spectral range from 1311 to 1313 cm−1 via simultaneous CH4, N2O, and H2O measurements. The measured dependence of CH4 and N2O concentration errors on the simultaneously determined H2O concentration indicated that the residual H2O caused an under-estimation of CH4 concentration and over-estimation of N2O concentration. The H2O induced CH4 and N2O concentration errors were approximately linearly proportional to the residual H2O concentration. For the measurement of air flowing at 3 L per min, the residual H2O concentration was stabilized to approximately 14 ppmv, and the corresponding H2O induced errors were −1.3 ppbv for CH4 and 3.7 ppbv for N2O, respectively.

Highlights

Global warming is considered to be the cause of climate change, and it has brought great consequences to human society [1]
There are three factors that should be considered for wavelength selection: (1) Absorption lines of CH4, N2O, and H2O should all appear in the selected spectral range; (2) the intensity of absorption lines must be moderate for measurements of either strong absorptions of species present in trace amounts or measurements of weak absorptions of abundant species; (3) the absorption lines should be relatively isolated from each other and from other CH4, N2O, and H2O lines to minimize the influence of neighboring lines and line-mixing effects
The influence of residual H2O on the simultaneous measurement of CH4 and N2O trace greenhouse gases in ambient air with tunable mid-IR cavity ring down spectroscopy (CRDS) coupled with EC-quantum cascade lasers (QCL) in the spectral range from 1311 to 1313 cm−1 has been investigated by treating the residual H2O either as a background gas or as a target gas, whose concentration is being determined simultaneously with the CH4 and N2O concentrations

Summary

Introduction

Global warming is considered to be the cause of climate change, and it has brought great consequences to human society [1]. Shao et al have used a single DFB diode laser emitting at 2.33 μm combined with the TDLAS technique for continuously detecting atmospheric CO and CH4 at a level of 0.73 and 36 ppbv, respectively [10] Among these various techniques, CRDS is a direct absorption technique with a significantly improved sensitivity than the conventional direct absorption spectroscopy due to its long effective absorption path length and insensitivity to intensity fluctuations of the light source [11,12,13], which has been widely used to detect trace gases in real time with ultrahigh sensitivity and relatively low system complexity in recent years [14]. Tang et al developed a CRDS setup for the simultaneous CH4 and N2O detection of ambient air in the spectral range between 1290 and 1350 cm−1 with the ppt-level sensitivity and ppb-level accuracy by drying the water vapor (H2O) inside the sample cell down to sub-ppm level so the influence of H2O on the simultaneous CH4 and N2O measurements becomes negligible [20]

Methods

Results

Conclusion