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.