Greenhouse gases have a significant impact on general global climate change. A self-calibrated multiharmonic measurement system based on tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation spectroscopy, using the vertical-cavity surface-emitting laser, is developed and applied to measure the CO 2 and H 2 O concentrations in the urban areas. The precision of the developed system is evaluated using the Allan variance method, and it is found that the multiharmonic can enhance the precision of the system and the first-to-third harmonics are sufficient to achieve the lowest Allan deviation. To validate its accuracy, comparison experiments with the commercial nondispersive infrared (NDIR) instrument (Li-Cor 840A) are performed and the results demonstrate the high accuracy of the TDLAS system and its consistency with the NDIR instrument. Measurements were carried out in autumn and winter of 2017 in the city center of Munich. The results indicate that the in si t u measurement of the CO 2 concentration in the downtown area correlates with both natural and anthropogenic activities. Due to its high precision and accuracy, the self-calibrated multiharmonic measurement system has a great potential to further study the emission and distribution of CO 2 in the urban areas.
Anthropogenic carbon dioxide (CO2) emissions mainly come from cities and their surrounding areas. Thus, continuous measuring of CO2 in urban areas is of great significance to studying human CO2 emissions. We developed a compact, precise, and self-calibrated in-situ CO2/H2O sensor based on TDLAS (tunable diode laser absorption spectroscopy), WMS (wavelength modulation spectroscopy), and VCSEL (vertical-cavity surface-emitting laser). Multi-harmonic detection is utilized to improve the precision of both measurements to 0.02 ppm for CO2 and 1.0 ppm for H2O. Using the developed sensor, we measured CO2 concentrations continuously in the city center of Munich, Germany, from February 2018 to January 2019. Urban CO2 concentrations are strongly affected by several factors, including vegetation photosynthesis and respiration (VPR), planetary boundary layer (PBL) height, and anthropogenic activities. In order to further understand the anthropogenic contribution in terms of CO2 sources, the HySPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model was applied to calculate six-hour backward trajectories. We analyzed the winter CO2 with the trajectory clustering, PSCF (potential source contribution function), and CWT (concentration weighted trajectory) methods, and found that local emissions have a great impact on urban CO2 concentration, with main emission sources in the north and southeast directions of the measurement site. In situations with an uneven trajectory distribution, PSCF proves somewhat superior in predicting the potential emission sources compared to CWT.
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
In this work, the recent lattice Boltzmann model with self-tuning equation of state (EOS) [R. Huang et al., J. Comput. Phys. 392, 227 (2019)] is improved in three aspects to simulate the thermal flows beyond the Boussinesq and ideal-gas approximations. First, an improved scheme is proposed to eliminate the additional cubic terms of velocity, which can significantly improve the numerical accuracy. Second, a local scheme is proposed to calculate the density gradient instead of the conventional finite-difference scheme. Third, a scaling factor is introduced into the lattice sound speed, which can be adjusted to effectively enhance numerical stability. The thermal Couette flow of a nonattracting rigid-sphere fluid, which is described by the Carnahan-Starling EOS, is first simulated, and the better performance of the present improvements on the numerical accuracy and stability is demonstrated. As a further application, the turbulent Rayleigh-Bénard convection in a supercritical fluid slightly above its critical point, which is described by the van der Waals EOS, is successfully simulated by the present lattice Boltzmann model. The piston effect of the supercritical fluid is successfully captured, which induces a fast and homogeneous increase of the temperature in the bulk region, and the time evolution from the initiation of heating to the final turbulent state is analyzed in detail and divided into five stages.
A method is proposed that employs the ratios of the 2nd and 4th harmonics at the line center to measure line width under high absorption. The measured line width is then applied in the ''calibration-free 2f/1f'' formula to detect gas pressure and concentration. To verify this method, the transitions of CO 2 at 6,982.0678 cm -1 and H 2 O at 6,979.2995 cm -1 are selected to measure the line width, gas pressure, and concentration in laboratory and field environments, respectively. Experimental results are very consistent with the expectations.
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