Demonstration of a Simplified, Two-Wavelength Optical Approach to Measuring Nitrogen Dioxide in Cities

Nitrogen dioxide (NO2) is a major air pollutant in urban areas, and achieving good accuracy and sensitivity in low-cost measurements is desirable to monitor NO2 levels in settings with high spatio-temporal variability. This paper describes a ratiometric approach that uses the different absorption at two nearby wavelengths to quantify NO2. The response to NO2 and other potential interferences is calculated at 437.3 and 439.4 nm for a low-resolution (1.44 nm) system. Owing to its elevated concentration and strong absorption compared to other absorbing gases, NO2 dominates the ratio of light absorption at these wavelengths in urban settings. The approach is experimentally demonstrated in a simple measurement system comprising a blue LED, narrow bandpass filters and non-dispersive detectors. The approach was validated in atmospheric simulation chamber experiments over an 8 m pathlength and achieved a high level of agreement against a reference DOAS spectral analysis (R2 = 0.97). Mixing ratios of up to 12 ppm were measured with a standard deviation of 51 ppb, suggesting that low ppb-level sensitivity can be achieved in pathlengths of a few hundred metres. The spectral stability of the ratiometric method was demonstrated in the open atmosphere using a short open-path system with a pathlength of 45 m. The standard deviation of the ratio of intensities in the two channels was 0.2%, despite changes in the transmitted intensity of almost 90%. The ratiometric two-channel approach developed in this work can be used in both in situ and remote sensing configurations, and we suggest that it has potential for use in a range of settings, including for low-cost monitoring in low-income cities and towns and continuous emission monitoring.