Incoherent broadband cavity enhanced absorption spectroscopy for in situ measurements of NO2 with a blue light emitting diode

We report on the development of Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS) using a blue light emitting diode (LED) for the detection of NO2 in laboratory ambient air. Absorption of the oxygen collisional pair in the atmosphere was also detected in the same spectral range. The mirror reflectivity was determined using a standard gas sample mixture of NO2, and calibrated with the help of the absorption spectrum of the oxygen collisional pair in pure oxygen at atmospheric pressure. Optimization of the experimental parameters was investigated and is discussed in detail. For the first time in IBBCEAS involving broadband absorption spectra, averaging time for signal-to-noise ratio enhancement has been optimized using Allan variance plot. 18.1 ppbv NO2 in laboratory ambient air has been retrieved from the absorption spectra using differential fitting method over a 40 nm spectral region centered at 470 nm. A minimum detection sensitivity of about 2.2 ppbv (1 sigma) for NO2 at atmospheric pressure has been achieved using the optimal averaging time of 100 s by means of a high finesse optical cavity formed with two moderate reflectivity (similar to 99.55%) mirrors. No purging of the cavity mirrors by high purity He or N-2 gas streams was necessary to prevent contamination of the mirror faces for the in situ measurements.