Self-Calibrated Multiharmonic CO2 Sensor Using VCSEL for Urban In Situ Measurement

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 situ
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.