*2.3. Selection of Spectral Absorption Lines*

To achieve reliable measurements of trace gas concentrations, a spectral simulation is performed to determine whether the selected lines have sufficient strengths for measurements and are well isolated from the absorptions by other gas species without any serious interference. H2O and CO2 have several strong absorption bands in the infrared spectral range between 1.0 μm and 2.5 μm, as shown in Figure 1 [30]. For example, the line strengths of CO2 near 2.0 μm wavelength region are much stronger than in 1.6 μm region. So in this study, we used a ~2.0 μm diode laser for more sensitive detections. Figure 1c,d show the simulation of spectral absorption around 4991 cm−<sup>1</sup> and 7181 cm<sup>−</sup>1, based on the HITRAN2016 database [31] for 2% H2O and 400 ppmv CO2 in air under nominal conditions (*P* = 1 atm, *T* = 296 K, path length *L* = 15 cm for H2O or 20 m for CO2, respectively). The results indicate that the target lines for H2O and CO2 detection are appropriate with minimum spectral inference. So the diode lasers of 1.392 μm wavelength (NEL, NLK1E5GAAA) and 2.004 μm wavelength (NEL, KELD1G5BAAA) are used in this work.

**Figure 1.** (**a**,**b**) Spectral line strengths; (**c**,**d**) simulation of spectral absorption with 2% H2O and 400 ppmv CO2 in air at a temperature of 296 K and pressure of 1 atm, in the near infrared wavelength regions (**c**) 1.392 μm, path length 15 cm, and (**d**) 2.004 μm, path length 20 m, respectively.

#### *2.4. Experimental Setup*

A schematic of the laser gas analyzer and experimental setup developed for flux measurement is shown in Figure 2, comprised of three units: a laser gas analyzer, an ultrasonic anemometer for three-dimensional wind speeds, power supply, and data acquisition. Among them, our developed laser gas analyzer consists of a miniaturized TDLAS

measuring system and two gas absorption cells. The two diode lasers are driven by current controllers and temperature controllers with precision setting voltages, generated by digital-to-analog converters (DAC, Analog Devices AD5682, 14 bits) in combination with a microcontroller (ST, STM32H743VIT6). The wavelengths of diode lasers are ramped at a rate of 2 kHz via their operation currents. The fiber-coupled laser output is collimated and focused to either a single-pass absorption cell (15 cm optical path length) for H2O measurements, or a multi-pass absorption cell (Herriott style, 50 passes, total 20 m optical path length, 51-mm dimeter mirrors with 99.99% highly-reflective dielectric coatings around 2.0 μm) for CO2 measurements. The laser radiation is detected by a wavelength-extended (up-to 2.6 μm) InGaAs photodiode (GPD Optoelectronics), and then amplified (Analog Devices AD8065) and recorded by an analog-to-digital converter (ADC, 16 bits, on the STM32H743 microcontroller). The final results of gas concentrations and wind speeds are sent to a laptop computer by an Ethernet port and saved to a memory card. A GPS receiver is used to provide time information for synchronizing the data between the ultrasonic anemometer and the laser gas analyzer.

**Figure 2.** Schematic of the laser gas analyzer and experimental setup for flux measurements. (**a**) the three major components of the system; (**b**–**d**) details of the miniaturized TDLAS system.
