Development of a Quantum Cascade Laser-Based Sensor for Environmental HONO Monitoring in the Mid-Infrared at 8 μm

The development of a fast in situ optical sensor based on a continuous-wave quantum cascade laser operating at 8 μm for the measurement of environmental gaseous nitrous acid (HONO) is reported. A strong absorption line at 1252.2109 cm ^–1 is found to be free of interferences and used for the first time for ambient HONO detection. Loss of HONO on the wall of the gas sample absorption cell is evaluated. The resulting 1-s detection limit for HONO is 110 pptv with a folded path length of 172 m. Continuous monitoring of HONO and NO ₂ mixing ratios variations along with the basic meteorological parameters was performed during a field campaign of several days. Typical photochemical reactions of HONO have been observed on sunny and snowy days. Atmospheric HONO mixing ratios observed in this study indicate that direct emission from traffic exhausts and local heterogeneous reactions is potentially important sources of HONO.