Broadband, single-channel cavity-enhanced absorption spectroscopy: towards a fast, modest-cost sensor for urban NO2

Nitrogen dioxide (NO2) is a priority air pollutant strongly associated with transport emissions, but its high spatial and temporal variability are challenging to measure. High quality instruments are costly and often unsuited to fast, portable measurements, while low-cost sensors are limited by measurement accuracy and are too slow for measurements on a second time scale. Here we present a strategy for a sensor based on cavity-enhanced absorption spectroscopy (CEAS) to achieve low ppb sensitivity to NO2 in tens of seconds. The approach uses a single, broadband wavelength channel (420–460 nm), a spectral region in which NO2 is the dominant absorber in urban settings. We describe two CEAS instruments that differ in cavity length and signal detection approach. One instrument had 43 cm cavity length, a photodiode detector and lock-in amplification, and achieved a precision of 3 ppb in 10 s. The second system had a 100 cm cavity and photomultiplier tube detector and achieved a precision of 1.1 ppb (in 10 s). Instrument accuracy was assessed against reference measurements in chamber experiments and one instrument was applied in an 11 week study of the effects of vehicle traffic on NO2 levels around a primary school in Cork city, Ireland. Despite moderate reflectivity cavity mirrors, the performance of these systems demonstrates that broadband, non-spectrally resolved CEAS is a fruitful approach to fast, modest cost urban measurements.