| Volatile organic compounds (VOCs) play very important roles in chemical processes occurred in the atmosphere. They are precursors of secondary pollutants like surface ozone and secondary organic aerosols, and are important factors depleting the stratospheric ozone-layer and influencing global radiation budget. Some toxic VOCs like benzene, may cause direct damage human health if they accumulate to relatively high levels in micro-environments, thus are of wide concerns. Urban streets are most important micro-environments for street levels of pollutants have great impacts on human health. Various studies both in China and in other countries reveal heavier pollution in urban streets; also the pollution by toxic organic compounds was not reflected by the normal air pollution index (API) reported to the public.This paper focused on exposure levels and sources of VOCs in urban streets of Guangzhou. Also street levels of PM10 and PM2.5 were investigated. The spatial and temporal variations of these pollutants have been studied. Unlike other studies in urban streets with air sampling in fixed sites, the present study conducted air sampling while we were walking along streets like pedestrians. This kind of "mobile" sampling would well represent pedestrians' exposure in urban streets. The results of the present study are as the following:1) Toxic VOCs detected in the air samples were dominated by monocycylic aromatic hydrocarbons (MAHs) and chlorinated hydrocarbons. Comparedto other public transportation modes in urban Guangzhou, pedestrians' exposure to VOCs and PM10 in average was much higher. Particularly pedestrians' exposure to MAHs was very high, e.g. benzene with an average of 44.7μg m-3 and a maximum of 120.9μg m-3, thus may pose potential health risks.2) This study suggested that reported API might not reflect people's risk due to air pollution in urban microenvironments like street canyons. For example, although API had good linear correlation with API, serious PM10 pollution in urban streets was not indicated by the reported API. Therefore, it is necessary to add some roadside monitoring stations in local air monitoring networks, and enhance the monitoring and control of some toxic air pollutants, like benzene, in urban air.3) Due to traffic and environmental conditions, people's exposure to VOCs were higher in old urban streets than in new urban streets, and in central urban streets than in sub-urban streets. Inter-comparisons among walk-only streets, part-time walk-only streets and traffic streets revealed that the levels of benzene and toluene were still very high in walk-only streets even if the traffic-related pollutants like ethylbenzene and xylenes were decreased, indicating additional sources other than vehicle emission in these walk-only streets.4) In urban Guangzhou streets, air pollutants showed similar temporal trends. They had the highest levels in spring and the lowest in summer; the higher levels of them were also found in weekend than in weekday, and in the afternoon than in the morning. As for diurnal variations of VOCs, their levels were relatively high in traffic rush hours; but PM2.5 and PM10 were only peaked in 20:00-24:00. Traffic flow and meteorological conditions played important roles in the temporal variations of air pollutants in streets.5) Based on factor analysis with 18 VOCs, BTEX ratios and their mutual correlations, traffic emission was the major source of air pollutants in urban Guangzhou streets. In addition, the solvent evaporation and plant emission were also the important sources for chloride hydrocarbons and monoterpenes, respectively.
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