Long-term ambient air pollution levels in four Chinese cities: inter-city and intra-city concentration gradients for epidemiological studies

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Abstract
The field data collection of an air pollution epidemiologic study was carried out from 1993 to 1996 in four Chinese cities of Lanzhou, Chongqing, Wuhan, and Guangzhou. In each city, an urban district and a suburban district were selected. Ambient concentrations of total suspended particles (TSP), size-fractionated particulate matter including PM2.5, PM2.5–10, and PM10, sulfur dioxide (SO2), and oxides of nitrogen (NOx) were measured in these districts. The results indicate the presence of wide inter-city and intra-city gradients in long-term ambient levels of these measured pollutants. Across the eight districts, the 1993–1996 4-year means of TSP, SO2, and NOx ranged from 198 to 659 μg/m3, from 14.6 to 331 μg/m3, and from 31.5 to 239 μg/m3, respectively, and the 1995–1996 2-year means of PM2.5, PM2.5–10, and PM10 ranged from 51.5 to 142 μg/m3, from 29.2 to 107 μg/m3, and from 80.7 to 232 μg/m3, respectively. These pollution ranges substantially extended the upper end of the pollution ranges of previous air pollution epidemiologic studies conducted in North America and Europe. In each district, significant correlations among the measured pollutants were observed for daily concentrations. However, the gradient patterns in long-term means of different pollutants were different across the eight districts. (e.g., PM2.5–10 and TSP were highest in the Lanzhou urban district, PM2.5 and PM10 were highest in the Guangzhou urban district, SO2 was highest in the Chongqing urban district, and NOx was highest in the Guangzhou urban district). In general, seasonal variations were present in the ambient concentrations with high levels often occurring in winter months. The eight districts may be classified into four district clusters based on integrated levels of all measured pollutants. These features of the ambient air pollution have important implications for epidemiological studies and may provide unique opportunities to study exposure–effects relationships in the four Chinese cities.