The technique of differential optical absorption spectroscopy (DOAS) is applied to monitor concentrations of nitrogen dioxide (NO 2 ) and aerosol, the major pollutants in the urban atmosphere. Two DOAS paths with optical distances of a few kilometers have been set up in the Chiba city area. The spectra of visible radiation emitted from xenon flashlights installed on tall constructions are analyzed in comparison with the laboratory spectrum of NO 2 . The intensity stability of the light sources enables the retrieval of aerosol extinction along the paths. It is found that the temporal behavior of both pollutants is similar to those observed at nearby ground sampling stations, though the effect of difference in the observation height often appears. Wind Doppler lidar measurement has also been undertaken from an observation site near an industrial complex along the Tokyo Bay, suggesting the influence of wind direction on the pollutant concentrations observed in the region.
Differential optical absorption spectroscopy (DOAS) is a useful technique for measuring nitrogen dioxide (NO 2) and aerosol, the most important species in urban environmental pollution. This paper reports on the results of our dual path DOAS measurements recently conducted in Chiba City, Japan, using xenon flashlights equipped on tall constructions as aviation obstruction lights. Because of the proximity of the southern DOAS path to an industrial area, it is found that the level of air pollution generally increases with the dominance of westerly winds, from the plausible source area to the observation light path. This situation is consistent with the result of wind lidar measurement covering a sector of ±28˚ with the observation range of approximately 2.8 km. In spite of the fact that the two DOAS paths, having path lengths of 5.5 and 3.5 km each, are located in separated regions of Chiba City, the observed temporal behavior was similar for both nitrogen dioxide and aerosol, though the southern path tends to exhibit slightly higher pollution levels than the northern counterpart. Additionally it is confirmed that size information of aerosol particles can be derived from the DOAS data through the analysis of the wavelength dependence of the aerosol optical thickness, which shows fairly good correlation with the mass ratio between PM2.5 and suspended particulate matter (SPM) obtained from the in-situ sampling station measurement. Thus, the DOAS approach can also be utilized for obtaining information on PM2.5 that is considered to be more harmful to human health than SPM. H. Saito et al.
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