Effects of urban land expansion on the regional meteorology and air quality of Eastern China
Atmospheric Chemistry and Physics Discussions
Rapid urbanization throughout Eastern China is imposing an irreversible effect on local climate and air quality. In this paper, we examine the response of a range of meteorological and air quality indicators to urbanization. Our study uses the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem) to simulate the climate and air quality impacts of four hypothetical urbanization scenarios with fixed surface pollutant emissions during the month of July from 2008 to 2012. An
... 08 to 2012. An improved integrated process rate (IPR) analysis scheme is implemented in WRF/Chem to investigate the mechanisms behind the forcing–response relationship at the process level. For all years, as urban land area expands, concentrations of CO, elemental carbon (EC), and particulate matter with aerodynamic diameter less than 2.5 microns (PM<sub>2.5</sub>) tend to decrease near the surface (below ~ 500 m), but increase at higher altitudes (1–3 km), resulting in a reduced vertical concentration gradient. On the other hand, the O<sub>3</sub> burden averaged over all newly urbanized grid cells consistently increases from the surface to a height of about 4 km. Sensitivity tests show that the response of meteorology and pollutant concentrations to the spatial extent of urbanization are nearly linear near the surface, but nonlinear at higher altitudes. Over eastern China, each 10% increase in nearby urban land coverage (NULC) on average leads to a decrease of approximately 2% in surface concentrations for CO, EC, and PM<sub>2.5</sub>, while for O<sub>3</sub> an increase of about 1% is simulated. At 800 hPa, each 10% increase in the square of NULC enhances air pollution concentrations by 5–10%, depending on species. This indicates that as large tracts of new urban land emerge, the influence of urban expansion on meteorology and air pollution would be amplified. IPR results indicate that, for primary pollutants, the enhanced sink (source) caused by turbulent mixing and vertical advection in the lower (upper) atmosphere could be a key factor in changes to simulated vertical profiles. The evolution of secondary pollutants is further influenced by the upward relocation of precursors that impact gas phase chemistry for O<sub>3</sub> and aerosol processes for PM<sub>2.5</sub>. Our study indicates that dense urbanization has a moderate dilution effect on surface primary airborne contaminants, but may intensify severe haze and ozone pollution if local emissions are not well controlled.