Car-Borne Measurements of Atmospheric NO2 by a Compact Broadband Cavity Enhanced Absorption Spectrometer

We report car-borne measurements of atmospheric NO2 close to the ground by incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS). A compact IBBCEAS spectrometer with a blue light emitting diode (LED) having a central wavelength of 458 nm, a full width at half maximum of 25 nm, and a 50-cm-long cavity was developed for mobile measurements. The NO2 detection limit of the spectrometer was calculated as 1.9 parts per billion by volume (ppbv) for a 30 s acquisition time by stabilizing the LED emitting spectrum, optimizing the NO2 reference cross sections, and by calibrating the reflectivity of the cavity mirrors. The accuracy of the spectrometer was verified by measuring NO2 samples with various mixing ratios between 1–200 ppbv, which were produced by a gas dilution system in the laboratory. Three distinct journeys in Nanjing and the surrounding areas were selected as observation routes. The atmospheric NO2 close to the ground was measured by the spectrometer from August 4 to 7, 2013. The mixing ratios of NO2 ranged from 3 to 144 ppbv. These results were compared with the column density of NO2 measured by a passive differential optical absorption spectroscopy (DOAS) instrument on the same car. Hence, we demonstrate the feasibility of using the spectrometer for car-borne measurements of atmospheric NO2.