Effect of urbanization on the urban meteorology and air pollution in Hangzhou

Urbanization has a substantial effect on urban meteorology. It can alter the atmospheric diffusion capability in urban areas and therefore affect pollutant concentrations. To study the effects of Hangzhou’s urban development in most recent decade on its urban meteorological characteristics and pollutant diffusion, 90 weather cases were simulated, covering 9 weather types, with the Nanjing University City Air Quality Prediction System and high-resolution surface-type data and urban construction data for 2000 and 2010. The results show that the most recent decade of urban development in Hangzhou substantially affected its urban meteorology. Specifically, the average urban wind speed decreased by 1.1 m s ^?1; the average intensity of the heat island increased by 0.5°C; and the average urban relative humidity decreased by 9.7%. Based on one case for each of the nine weather types, the impact of urbanization on air pollution diffusion was investigated, revealing that the changes in the meteorological environment decreased the urban atmosphere’s diffusion capability, and therefore increased urban pollutant concentrations. For instance, the urban nitrogen oxides concentration increased by 2.1 μg m ^?3 on average; the fine particulate matter (diameter of 2.5 μm or less; PM_2.5) pollution concentration increased by 2.3 μg m ^?3 on average; in highly urbanized areas, the PM_2.5 concentration increased by 30 μg m ^?3 and average visibility decreased by 0.2 km, with a maximum decrease of 1 km; the average number of daily hours of haze increased by 0.46 h; and the haze height lifted by 100–300 m. The “self-cleaning time” of pollutants increased by an average of 1.5 h.