Incoherent broadband cavity enhanced absorption spectroscopy for measurements of HONO and NO2 with a LED optical source

An application of incoherent broadband cavity enhanced absorption spectroscopy with a near-ultraviolet LED (peak 372 nm and FWHM is 13 nm) to simultaneously detecting HONO and NO2 is described. The light emitted from the LED is collimated and then coupled into an 70 cm long high finesse cavity formed with two high reflectivity mirrors. The spectra are respectively recorded when the cell is filled with He and then N2, and the mirror reflectivity is determined from the change in transmitted intensity due to the difference in Rayleigh scattering cross-section between He and N2. The maximum of mirror reflectivity is 0.99962 at 390 nm in a spectral region of 360-390 nm, and corresponding maximum of light path length is about 1.71 km when NO2/HONO mixture is measured. The concentrations of HONO and NO2 are obtained using least-squares fit. Detection sensitivity (1) of 0.6 ppbv for HONO and 1.9 ppbv for NO2 are achieved using an acquisition time of 1000 s. The experimental results demonstrate the possible application of this technology to in situ monitoring the trace gases in the atmosphere.