Abstract
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.
Highlights
Carbon dioxide (CO2 ) is the most important anthropogenic greenhouse gas (GHG) in the atmosphere
The diurnal CO2 cycle is: the CO2 concentration decreases in the daytime and reaches a minimum in the afternoon; afterwards, it increases at night and reaches a maximum in the morning, becoming more and more noticeable from February to July
The precision and accuracy of the sensor were verified with Allan Variance analysis and compared with a commercial
Summary
Carbon dioxide (CO2 ) is the most important anthropogenic greenhouse gas (GHG) in the atmosphere. In Germany, continuous observations have been performed to measure atmospheric CO2 for decades [2,3,4] These sites are always located in rural areas or high mountains, exhibiting lesser impacts from local pollution, and cannot detect the urban emissions directly. TDLAS is often combined with WMS (wavelength modulation spectroscopy), a technique that utilizes a faster but smaller amplitude sinusoidal signal together with a slower and larger ramp signal to modulate the laser wavelength It uses a lock-in amplifier (LIA) to decompose the absorption signal into its harmonic components. A precise and self-calibrated CO2 /H2 O sensor based on TDLAS-WMS technique has been developed by our group. Note that the CO2 recorded for the month of January in this article was measured in 2019, and the rest of the results were performed considering the data from 2018
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