Incoherent broadband cavity enhanced absorption spectroscopy for measurements of atmospheric HONO

Abstract

We report the development of an incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) based on an ultraviolet light emitting diode (UV-LED), and the IBBCEAS instrument is used for simultaneously measuring of the atmospheric HONO and NO2. The cavity-enhanced method is characterized by high sensitivity and spatial resolution. The incoherent broadband light is focused into a high-finesse optical cavity, two highly reflecting mirrors form the ends of the cavity, and the light is then trapped between the two highly reflecting mirrors, resulting in long photon residence time and long optical path length. The effects of the Rayleigh scattering of the gases in the cavity and stability of the UV-LED light source were discussed in this paper. The reflectivity of the highly reflecting mirror was calibrated by the difference of Rayleigh scattering of He and N2, and the optimum averaging time of the IBBCEAS instrument was confirmed to be 320 s by the Allan variance analysis. Detection limits (1) of 0.22 ppb for HONO and 0.45 ppb for NO2 were achieved with an optimum acquisition time of 320 s. In order to test the accuracy of measured results by the IBBCEAS instrument, concentrations of HONO and NO2 were recorded during about continuous three days by the IBBCEAS instrument and compared with the results obtained by a different optical absorption spectroscopy (DOAS) instrument. The results of HONO show a linear correction factor (R2) of 0.917, in a slope of 0.897 with an offset of 0.13 ppb; NO2 concentration measured by the IBBCEAS instrument accords well with the result obtained by the DOAS instrument, with a linear correlation of R2 = 0.937, in a slope of 0.914 with an offset of-0.17 ppb.