Monitoring of environmental nitrogen dioxide concentration using visible acousto-optic differential optical absorption spectroscopy

Atmospheric concentration measurement of nitrogen dioxide (NO₂) pollutant was demonstrated by a new type of differential optical absorption spectroscopy (DOAS) using a spectral scanning device of a visible acousto-optic tunable filter. The measurement requires a stable artificial light source such as a xenon lamp, and the light beam is directed into the environment where the concentration of NO₂ is to be monitored. The retrieval of NO ₂ concentration is then achieved by analyzing the residual light using the DOAS signal processing. In this paper, we present results obtained from this new DOAS system during continuous measurement of atmospheric NO ₂ concentration in the campus of the City University of Hong Kong. Another DOAS system, using a miniature CCD grating spectrometer, was established as a control experiment. The CCD spectrometer acts as a traditional DOAS system for performance evaluation. Instead of using the NO₂ absorption cross section for concentration retrieval, differential absorption area provides an alternative quantity for concentration retrieval. The monitoring results from both of the DOAS systems are compared with the pollutant concentrations reported in a nearby pollutant monitoring station, operated by the Hong Kong Environmental Protection Department.