Broadband cavity enhanced absorption spectroscopy for measuring atmospheric NO<sub>3</sub> radical

NO₃ radical is the most important oxidant in atmospheric chemistry at night, and it controls the oxidation and removal of various trace gas components in the atmosphere. The understanding of the chemical process of NO₃ radical is of great significance for studying the atmospheric pollution processes such as haze. The NO₃ radical has a low concentration and strong activity, so it is relatively difficult to measure accurately. We report here in this paper an instrument for unambiguously measuring NO₃ based on broadband cavity enhanced absorption spectroscopy (BBCEAS). To achieve the robust performance and system stability under diverse conditions, this BBCEAS instrument has been developed, with efficient sampling, and resistance against vibration and temperature change improved, and the BBCEAS instrument also has low-power consumption. The 660-nm-wavelemngth light-emitting diode (LED) is used as a light source of the BBCEAS system. The sampling gas path with low loss and suitable for domestic high-particle environment is designed. Through the LED light source test, the optimal working current and temperature can be obtained to achieve the acquisition of NO₃ absorption spectrum with high signal-to-noise ratio. Considering the fact that the water vapor absorption is an important interference factor for the measurement of NO₃ radical by BBCEAS, the daytime atmospheric measurement spectrum is used as a background spectrum, and participates in spectral fitting of NO₃ to reduce the effect of water vapor. The mirror reflectivity and effective cavity length are calibrated, and the Allan variance analysis is also carried out. The reflectance of the mirror can reach about 0.99993 at 662 nm (NO₃ absorption peak), and the corresponding theoretical effective optical path can reach more than 7 km, which can meet the measurement requirements of atmospheric NO₃ radicals. The detection limit (1σ) of 0.75 pptv for NO₃ is achieved with an acquisition time of 10 s and a total measurement error of about 16%. The atmospheric NO₃ radical observation is carried out in Hefei. During the observation period, the highest NO₃ concentration is 23.4 pptv, demonstrating the promising potential applications in in-situ, sensitive, accurate and fast simultaneous measurements of NO₃ in the future by using the developed broadband cavity enhanced absorption spectroscopy.