In Situ and Loss-Free Detection of Nitrate Radical via Open-Path Cavity-Enhanced Absorption Spectroscopy: Indoor Air Application

Nitrate radical (NO₃) is an important oxidant in the atmosphere. Current optical cavity technology is the predominant method for measuring the NO₃ radicals. Yet, it suffers from high and variable sampling losses, especially at high humidity and particulate matter loading conditions, which bring large measurement uncertainties. Here, we introduce an open-path cavity-enhanced absorption spectroscopy (OP-CEAS) method for measuring ambient NO₃, which is characterized by negligible sampling loss and demonstrates enhanced measurement performance over the closed-path CEAS. An algorithm combining a dynamic reference library and real-time H₂O absorption cross-section calculator is designed to eliminate the interference of intensified water vapor absorption and baseline drift introduced by the fluctuations in temperature and humidity on NO₃ spectrum analysis. Our algorithm allows for robust NO₃ measurements of OP-CEAS in conditions of elevated humidity and outperforms the closed-path CEAS system. The device was successfully applied in a typical indoor environment and captured high levels of NO₃, which provides a pathway for the exploration of indoor oxidant chemistry. Our results indicate that this OP-CEAS approach possesses a notable capacity for continuous and loss-free measurements for NO₃, and it offers a dependable approach for indoor air-quality studies.