The development of Open path cavity-enhanced absorption spectroscopy for detecting ambient nitrate radicals

Nocturnal oxidation driven by nitrate radicals is an important process in atmospheric chemistry, regulating the fate of volatile organic compounds and nitrogen oxide, and affecting the particulate pollution levels. While detecting NO₃ is challenging due to its extremely low concentration. Currently, techniques such as Cavity Ring-Down Spectroscopy (CRDS) and Cavity-enhanced absorption Spectroscopy (CEAS) are widely used in NO₃ measurement but suffer from the sampling loss due to its high reactivity. Here, we try to develop an open CEAS system to detect the ambient NO3, which eliminates the sampling loss. However, this method has its technical challenges, including the interferences of water absorptions during the NO3 absorption window near 662 nm and the effects of particle extinction and relative humidity and temperature during the field observation. We applied a small cavity cage (~40 cm high reflectivity mirror distance) during the hardware design, which features great stability. In addition, we calculated the real-time water vapor cross-section by measuring the ambient temperature and relative humidity to retrieve the water vapor concentration with high accuracy. And we proposed an I0-database method to eliminate the effects of particle extinction and variations in environmental meteorological conditions. Finally, we will present the instrumental performance in the laboratory tests and field applications.