Long-term variation of PM_2.5levels and composition at rural, urban, and roadside sites in Hong Kong: Increasing impact of regional air pollution

Long-term study of air pollution plays a decisive role in formulating and refining pollution control strategies. In this study, two 12-month measurements of PM2.5mass and speciation were conducted in 00/01 and 04/05 to determine long-term trend and spatial variations of PM2.5mass and chemical composition in Hong Kong. This study covered three sites with different land-use characteristics, namely roadside, urban, and rural environments. The highest annual average PM2.5concentration was observed at the roadside site (58.0±2.0 μg m-3(average±2σ) in 00/01 and 53.0±2.7 μg m-3in 04/05), followed by the urban site (33.9±2.5 μg m-3in 00/01 and 39.0±2.0 μg m-3in 04/05), and the rural site (23.7±1.9 μg m-3in 00/01 and 28.4±2.4 μg m-3in 04/05). The lowest PM2.5level measured at the rural site was still higher than the United States' annual average National Ambient Air Quality Standard of 15 μg m-3. As expected, seasonal variations of PM2.5mass concentration at the three sites were similar: higher in autumn/winter and lower in summer. Comparing PM2.5data in 04/05 with those collected in 00/01, a reduction in PM2.5mass concentration at the roadside (8.7%) but an increase at the urban (15%) and rural (20%) sites were observed. The reduction of PM2.5at the roadside was attributed to the decrease of carbonaceous aerosols (organic carbon and elemental carbon) (>30%), indicating the effective control of motor vehicle emissions over the period. On the other hand, the sulfate concentration at the three sites was consistent regardless of different land-use characteristics in both studies. The lack of spatial variation of sulfate concentrations in PM2.5implied its origin of regional contribution. Moreover, over 36% growth in sulfate concentration was found from 00/01 to 04/05, suggesting a significant increase in regional sulfate pollution over the years. More quantitative techniques such as receptor models and chemical transport models are required to assess the temporal variations of source contributions to ambient PM2.5mass and chemical speciation in Hong Kong.