An open-path dual-beam laser spectrometer for path-integrated urban NO2 sensing

Abstract

Fast and continuous in-situ monitoring of NO2 in urban air using optical sensing has been a topic of research recently. Due to possible interferences from other absorbers, spectroscopic measurements across a broad absorption band of NO2 are preferred over single-wavelength laser absorption measurements for specificity. While, in general, broadband spectrometers account for spectral interferences and exhibit high selectivity, laser spectrometers possess faster responses and higher sensitivity. In this study, we present an open-path and dual-beam laser absorption spectrometer (OP-DBLS) featured as sensitive, accurate and cost-effective urban NO2 monitor with fast and easy operation. This instrument consists of two lasers operating at wavelengths close to the NO2 absorption peak around 405 nm and the ratio of absorption at these wavelengths were used to remove cumulative influence of all broad baseline features such as aerosol interferences. The instrument was continuously operated for thirteen days at the middle ring road monitoring centre located in the northeast of Shanghai, P. R. China. The measurements were compared with concurrent measurements by a Chemiluminescence (CL) detector and the correlation of campaign-averaged hourly measurements depicting diurnal variations in NO2 concentrations was found to be ∼0.8. Furthermore, results from comparison against four other air quality monitors deployed elsewhere in the city showed excellent consistency.