Development of a lightweight broadband cavity-enhanced spectrometer system for simultaneous monitoring of HONO and NO₂

Abstract. Nitrous acid (HONO) is an important source of hydroxyl radicals in the troposphere, but its measurement remains challenging because of its low atmospheric abundance and rapid temporal variability. In this work, we developed a compact broadband cavity-enhanced absorption spectrometer (BBCEAS) for simultaneous measurements of HONO and NO₂ in the near-UV spectral region. The instrument uses a high-reflectivity optical cavity, with a measured mirror reflectivity of 0.99986 at 368 nm, and a spectral acquisition time of 0.15 s. Laboratory characterization demonstrated stable and sensitive measurements of both species. The instrument achieved 1σ precisions of 193 pptv for HONO and 584 pptv for NO₂ at 1 s time resolution, improving to 30 pptv and 76 pptv, respectively, at a 60 s averaging time. Intercomparison measurements showed excellent agreement with reference analyzers, with nearly unit regression slopes for both HONO and NO₂. Controlled humidity experiments showed no systematic dependence of the retrieved concentrations on water vapour over the investigated range, indicating robust performance under varying humidity conditions. In addition, a 24 h ambient deployment in Syracuse, New York, USA, demonstrated the practical applicability of the instrument for continuous measurements of atmospheric HONO and NO₂. The instrument provided high-temporal-resolution measurements of HONO and NO₂ and enabled continuous measurements of HONO at 10s of pptv levels. The combination of high sensitivity, rapid acquisition, compact design, and stable performance provides potential for continuous monitoring across diverse environments and for integration with mobile and airborne platforms.