VCSEL-Based Atmospheric Trace Gas Sensor Using First Harmonic Detection

Abstract

This paper presents a measurement system based on 1 the first harmonic in tunable diode laser absorption spectroscopy 2 using a vertical-cavity surface-emitting laser to measure the 3 atmospheric CO 2 and H 2 O concentrations. The developed system 4 separates the residual amplitude modulation signal from the 5 harmonics and then eliminates it. A digital signal processing 6 is developed to autonomously infer the wavelength and light 7 intensities of the laser. The gas concentrations are determined 8 without extra calibration. The long-term measurements are taken 9 to validate the precision and accuracy of the system. Based 10 on the Allan variance analysis, the broad wavelength scanning 11 enhances the measurement precision, and the first-harmonic 12 detection can achieve about two times as high precision as the 13 traditional second-harmonic detection. The field measurements 14 implemented in early spring of 2018 in Munich were compared 15 with the commercial nondispersive infrared (NDIR) sensor. 16 The outcomes have revealed that our system has high accuracy 17 for the gas concentration measurements and high consistency 18 with the NDIR sensor. The diurnal variations of CO 2 concen-19 tration have demonstrated that CO 2 concentration in urban 20 areas is affected by the biosphere and meteorological conditions 21 and the daily anthropogenic activities. Furthermore, the air 22 trajectory analysis based on the HYSPLIT model has found that 23 the CO 2 emission sources primarily come from the southeast 24 of Munich. The developed system described in this paper has a 25 great potential for in situ trace gas concentration measurements, 26 the analysis on the polluted gas distributions, and the verification 27 of the pollutant transport model in urban areas. 28 Index Terms-Trace gas concentration, tunable diode laser 29 absorption spectroscopy (TDLAS), wavelength modulation spec-30 troscopy (WMS), first harmonic, vertical-cavity surface-emitting 31 laser (VCSEL). 32 I. INTRODUCTION 33 G REENHOUSE gases (GHG) have a significant negative 34 impact on the climate change. Research has indicated 35 that carbon dioxide (CO 2), one of the dominant GHG, has 36

Assessment of Urban CO2 Measurement and Source Attribution in Munich Based on TDLAS-WMS and Trajectory Analysis

“…In situations with an uneven trajectory distribution, PSCF proves somewhat superior in predicting the potential emission sources compared to CWT.Atmosphere 2020, 11, 58 2 of 15 atmospheric CO 2 , such as remote sensing via satellite [5][6][7], ground-based column measurement [8][9][10][11], and in-situ measurement [12][13][14][15][16]. In-situ techniques have the advantages that they can measure the atmospheric components at a high spatial and temporal resolution under various meteorological conditions, and during day and night, as opposed to other techniques.The most common in-situ gas sensors are based on NDIR (nondispersive infrared) [17][18][19][20] and TDLAS (tunable diode laser absorption spectroscopy) [21][22][23][24][25] technologies. However, most commercial gas sensors require periodic calibrations, which would take a lot of effort and possibly interrupt measurements [26][27][28][29].…”

“…A precise and self-calibrated CO 2 /H 2 O sensor based on TDLAS-WMS technique has been developed by our group. Multi-harmonic detection is utilized in the sensor to improve the measurement precision [24,25].Locating potential CO 2 emission sources around the measurement site, a further aim of our study requires not only temporal variability analysis but transport modeling as well. A common method with which to analyze long-term measurements relies on simulated backward trajectories; for example, those calculated with the HySPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model [35][36][37].…”

“…The most popular statistical methods for evaluating this precisely are trajectory clustering, PSCF (potential source contribution function), and CWT (concentration weighted trajectory) methods. PSCF infers the probability that an area contributes to high pollutant concentration at the receptor site, while CWT uses a computed weighted concentration to identify the strength of emission sources arriving at the site [38,39].Our group previously developed a self-calibrated TDLAS-WMS gas sensor [24,25]. In this paper, we utilized the developed sensor to measure CO 2 concentration in Munich, Germany for an entire year, from February 2018 to January 2019.…”

“…The most common in-situ gas sensors are based on NDIR (nondispersive infrared) [17][18][19][20] and TDLAS (tunable diode laser absorption spectroscopy) [21][22][23][24][25] technologies. However, most commercial gas sensors require periodic calibrations, which would take a lot of effort and possibly interrupt measurements [26][27][28][29].…”

“…A precise and self-calibrated CO 2 /H 2 O sensor based on TDLAS-WMS technique has been developed by our group. Multi-harmonic detection is utilized in the sensor to improve the measurement precision [24,25].…”