[Detection of Atmospheric HONO and NO₂ by Incoherent Broadband Cavity Enhanced Spectroscopy]

We report on the development of an optical instrument based on LED-IBBCEAS for simultaneous measurements of nitrous acid (HONO) and nitrogen dioxide (NO₂) in ambient air. The light emitting from the LED centered at 365 nm was directly focused into the cavity formed with two high reflectivity mirrors, separated by a distance of 1. 76 m. The light output of the cavity was received with a portable spectrometer. The mirror reflectivity was calibrated by absorption spectra of NO₂ and O₂-O₂. In the spectral range of 353-376 nn, the maximum mirror reflectivity was found to be 0.999 17. Detection limits (1σ) of 0.6 ppbv for HONO and 1.8 ppbv for NO₂ were achieved with an acquisition time of 120 s. In order to test the accuracy of measured results by present setup, concentrations of NO₂ were recorded during continuous 56 hours and compared with data from a NOX analyzer equipped with a blue light converter. The least-square fit lines give gradients of 1. 09 and respective intercepts of 3.45, with a linear correction factor of 0.89. The concentrations of HONO and NO₂ in indoor air were monitored, the concentrations of HONO varied from near 0 to 5.3 ppbv in 24 hours, the averaged concentration was 1. 8 ppbv, and the concentrations of NO₂ varied from 5 to 51 ppbv at the same time, the averaged concentration was 21.9 ppbv.