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

Nitrate radical is an important nocturnal oxidant in the atmosphere, regulates the fate of volatile organic compounds and nitrogen oxide, and affects the air quality. While the concentration of NO₃ in the ambient is low to several to tens of parts per trillion by volume in general on the surface. During the past decades, people attempted to detect ambient NO₃ by developing several techniques including Differential optical absorption spectroscopy (DOAS), Laser-induced fluorescence (LIF), Cavity Ring-Down Spectroscopy (CRDS), and Cavity-enhanced absorption Spectroscopy (CEAS). The latter two are widely used in NO₃ measurement but suffer from the sampling loss change due to its high reactivity. Here we try to develop an open CEAS system to measure NO₃ in the ambient air. This method is free of sampling loss but has its technical challenges, that as the interferences of water absorptions during the NO₃ absorption window near 662 nm. Different from previous studies, we applied a small cavity cage (~40 cm high reflectivity mirror distance) during the hardware design, which features good stability. In addition, we used a sensor to measure ambient temperature and relative humidity, which helped us to calculate the real-time water vapor cross-section to retrieve the water vapor concentration with high accuracy. At last, we will report the instrumental performance in the laboratory tests and field applications.