Effects of Urban Land Forcing on Local and Downwind Air Quality, a Case Study for East Asia

Abstract:

Urban land surfaces are distinct from natural surfaces as their unique radiative, thermal, hydrologic and aerodynamic properties. In this study, we have investigated the response of a range of meteorological and air quality indicators to urban land expansion based on the Weather Research and Forecasting model coupled with chemistry (WRF/Chem). Specifically, we simulate the climate and air quality impacts of four hypothetical urbanization scenarios during the month of July from 2008 to 2012 over eastern China, a region experiencing the fastest urbanization. We find that as urban land expanses, though emissions are held constant, concentrations of CO, elemental carbon (EC), and PM_2.5 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 contrary, the O₃ burden averaged over all newly urbanized grid cells consistently increases from the surface to a height of about 4 km. The responses of pollutant concentrations to the spatial extent of urbanization are linear near the surface, but nonlinear (or intensified) at higher altitudes. The perturbations in boundary layer height, 2-m temperature and 2-m relative humidity also increase linearly with the spatial extent of urban land expansion (R² >0.96). Our work indicates that as large tracts of new urban land emerge, the influence of urban expansion on meteorology and air pollution would be significantly amplified. An improved integrated process rate (IPR) analysis scheme is implemented in WRF/Chem to investigate the non-negligible and unique role of urban land forcing in impacting the advection, turbulent mixing, and dry/wet removal of pollutants. IPR indicates 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 largely influenced by the upward relocation of precursors that impacts gas-phase chemistry for O₃ and aerosol processes for PM_2.5. Our preliminary results indicate that this “elevation effects” of urban land expansion could foster the trans-Pacific transport of airborne pollutants, and it is necessary we further quantify and analyze.