A compact and low power detector has been developed for the in situ measurement of atmospheric NO2 using broadband cavity enhanced absorption spectroscopy. Absorption by the O2–O2 collisional pair was used to determine the cavity mirror reflectivity, thus enabling the retrieval of absolute absorber concentrations by differential spectral fitting techniques. Quantitative amounts of ambient NO2 (between 3 and 32 parts per billion) were retrieved from spectra recorded in the presence of ambient aerosol with statistical uncertainties approaching 100 ppt for a 60 s averaging period. The instrument's response was compared to that of a commercial chemiluminescence detector and was found to agree to within 6%
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