Abstract: To address the problem of continuous and accurate measurement of atmospheric NO2 at night, a nighttime atmospheric NO2 detection system based on target reflection light is developed. The system mainly consists of a transmitting unit and a signal receiving unit, with a 3.5 W high-power semiconductor laser with a central wavelength of 445 nm as its light source, and according to the laser differential absorption characteristics of NO2 in the range of 440-450 nm, the laser detection wavelengths for NO2 are determined to be λon = 444.8 nm and λoff = 446.7 nm. Firstly, the sensitivity of wavelength and intensity of the semiconductor laser to its temperature and current was studied, and the quantitative relationship between the wavelength and intensity of laser and the temperature and current of laser was determined. Then basd on the quantitative relationship, the laser wavelength can be adjusted by controlling the temperature stability of the semiconductor laser while changing the current, and the influence of laser wavelength adjustment on laser intensity changes during the detection process can be eliminated. Finally, based on the constructed system, NO2 sample gas experiments were conducted, and the fitting values of NO2 differential absorption cross section were determined. After the determination of system parameters, field experiments were carried out at Anhui University and Science Island (the campus of Hefei Institutes of Physical Science, Chinese Academy of Sciences) respectively. NO2 nighttime concentration near the ground at two observation points was successfully obtained, and the measurement results were compared with the data from National Environmental Monitoring Station and the data from the long-path differential optical absorption spectroscopy (LP-DOAS) at Science Island respectively. It was found that the observation data of the developed system showed good consistency with the data of National Environmental Monitoring Station and the data of LP-DOAS, with coefficients of determination of 0.902 and 0.891, respectively, verifying the accuracy and reliability of the system. Furthermore, NO2 vertical distribution detection experiments were conducted, and NO2 concentration values at different heights were successfully obtained, proving the feasibility of the developed system in NO2 vertical distribution detection.

Key words: semiconductor laser, NO2 measurement, differential absorption, vertical column concentration