Continuous wave cavity ring-down spectroscopy measurement of NO₂ mixing ratios in ambient air

The performance of a continuous wave diode laser based cavity ring-down (CRD) spectrometer is demonstrated for retrieval of NO2 mixing ratios in ambient air. Measurements are made at wavelengths around 410 nm, a region free from other spectroscopic interferences, and the instrument is shown to be capable of quantitative and precise NO2 mixing ratio determinations without any requirement for calibration. The minimum concentration of NO2 that can be measured by the spectrometer is estimated to be 0.1 ppbv (parts per billion by volume) in 50 s at atmospheric pressure. Using dynamic dilution of a standard NO2 sample, the performance of the spectrometer has been validated for NO2 measurements over the range of 0.2 to 80 ppbv. NO2 measurements performed directly on laboratory air are compared with data obtained with a calibrated chemiluminescence instrument and shown to be in excellent agreement.