An LP-DOAS instrument with a laser driven light source for open-path measurement of atmospheric NO2 in Shanghai

Abstract

Shanghai is currently faced with the complex atmospheric pollution problem such as ozone (O3) and PM2.5. O3 is responsible for oxidization of atmospheric constituents that in turn produces secondary pollutants. Reducing atmospheric oxidation is the underpinning strategy for the prevention and control of pollution in this city. According to the observation data from the online automatic monitoring station of Shanghai, the city's O3 concentration has been rising steadily. In 2013, there are 100 days with ozone as the main pollutants, accounted for 31.9%. By Leighton relationship (K6[O3][NO]=K4[NO2] = 1), concentration of O3 is closely related to that of NO2. So, continuous monitoring of atmospheric NO2 has important significance. An instrument was developed for measuring atmospheric NO2 concentration using a powerful light source in conjunction with the long-path differential optical absorption spectroscopy (LP-DOAS) technique. This system installed at the central section of Shanghai used a laser driven light source (LDLS) and a sensitive TE cooled spectrometer. The LDLS showed better stability, high optical efficiency when compared to traditional arc lamp and light emitting diodes. The light sent into the atmosphere was returned by a retro reflecting mirror placed 1.3km away, and the light returned was sent to a high sensitive TE cooled spectrometer. The spectral range of measurement was 415–455nm and the spectra collected were averaged over 10 min, prior to analyzing for NO2 concentration. Continuous observations were carried out for two weeks and the NO2 concentration retrieved ranged from (20–600) ug/m3 with an estimated detection limit of 6.77 ug/m3. The measurement results indicate that it is feasible to use this instrument for detecting the NO2 with high spatial and temporal sensitivity.