Effects of urban land expansion on the regional meteorology and air quality of eastern China

Atmos. Chem. Phys., 15, 8597–8614, 2015 www.atmos-chem-phys.net/15/8597/2015/ doi:10.5194/acp-15-8597-2015 © Author(s) 2015. CC Attribution 3.0 License. Effects of urban land expansion on the regional meteorology and air quality of eastern China W. Tao1 , J. Liu1 , G. A. Ban-Weiss2 , D. A. Hauglustaine3 , L. Zhang4 , Q. Zhang5 , Y. Cheng6 , Y. Yu7 , and S. Tao1 1 Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China 2 Sonny Astani Department of Civil and Environmental Engineering, University of Southern California, CA, USA 3 Laboratoire des Sciences du Climat et de l’Environnement, UMR8212, CEA-CNRS-UVSQ, Gif-sur-Yvette, France 4 Laboratory for Climate and Ocean-Atmosphere Sciences, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China 5 Center for Earth System Science, Tsinghua University, Beijing 100084, China 6 Chinese Academy of Meteorological Sciences, Beijing, China 7 Nanjing Municipal Environmental Monitoring Centre, Nanjing, Jiangsu 210013, China Correspondence to: J. Liu () Received: 27 January 2015 – Published in Atmos. Chem. Phys. Discuss.: 8 April 2015 Revised: 6 July 2015 – Accepted: 10 July 2015 – Published: 3 August 2015 Abstract. 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 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 (PM2.5 ) 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 other hand, the O3 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 responses of 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 on average leads to a decrease of approximately 2 % in surface concentrations for CO, EC, and PM2.5 , while for O3 an increase of about 1 % is simulated. At 800 hPa, pollutants’ concentrations tend to increase even more rapidly with an increase in nearby urban land coverage. This indicates that as large tracts of new urban land emerge, the influence of urban expansion on meteorology and air pollution would be significantly amplified. IPR analysis reveals the contribution of individual atmospheric processes to pollutants’ concentration changes. It 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 influenced by the upward relocation of precursors that impact gas-phase chemistry for O3 and aerosol processes for PM2.5 . 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. 1 Introduction Urbanization refers to the growth of urban populations and the vast expansion of urban areas. According to the 2011 revision of the United Nations (UN) World Urbanization Published by Copernicus Publications on behalf of the European Geosciences Union. 8598 W. Tao et al.: Effects of urban land expansion on the regional meteorology and air quality Prospects, the global proportion of the population living in urban areas is likely to increase to 68 % (about 6.2 billion) by 2050, and the urban population in less developed regions will almost double from 2.7 billion in 2011 to 5.1 billion in 2050 (Heilig, 2012). The environmental side-effects of urbanization, such as inadvertent climate modification (Changnon, 1992) and air quality degradation (Mage et al., 1996), remain an important research topic with societal relevance. The radiative, thermal, hydrologic, and aerodynamic properties of urban land surfaces are distinct from those of natural surfaces (e.g., forests, grassland), resulting in unique exchange processes of energy, moisture, and momentum with the ambient atmosphere and thus distinct climatic conditions in urban areas (Oke, 1987). The features of urban climate (e.g., urban heat island (UHI), wind profiles in the urban canopy layer) have been extensively observed, modeled and comprehensively reviewed (e.g., Arnfield, 2003; Kanda, 2007; Souch and Grimmond, 2006). The urban climate is characterized by multiple scales (Britter and Hanna, 2003; Fisher et al., 2006; Oke, 2006) e.g., flows in the roughness sublayer at micro-scale are not subject to Monin–Obukhov similarity relationships, whereas upper flows in the inertial layer are in equilibrium with the underlying surface, and can be described by mesoscale dynamics. Another feature of urban climatology is, heterogeneity, namely the high nonuniformities of roughness elements (e.g., impervious road, green belt) in urban areas make it rather complicated to generalize the urban flow details from one landscape to another (Fernando et al., 2001). Factors such as anthropogenic heat (Fan and Sailor, 2005), chemistry–climate feedbacks (Rosenfeld, 2000), and topography could alter the characteristics of urban climatic conditions, and the intensity of background wind speed or land-sea breezes could impact the structure of the urban boundary layer (Fisher et al., 2006; Rotach et al., 2002) and the ventilation conditions as well (Ryu et al., 2013; Yoshikado and Tsuchida, 1996). Up to now, a number of urban canopy schemes have been developed (e.g., Coceal and Belcher, 2004; Di Sabatino et al., 2008; Harman et al., 2004; Luhar et al., 2014; Solazzo et al., 2010; Trusilova et al., 2013; Wang et al., 2011). Among them, four schemes with different complexities have been implemented in the mesoscale meteorological model (e.g., WRF) to account for the effects of urban areas on urban climate, namely bulk (BULK; Liu et al., 2006), a singlelayer urban canopy model (SLUCM; Kusaka and Kimura, 2004), building effect parameterization (BEP; Martilli et al., 2002), and a building energy model (coupled to BEP, denoted as BEP + BEM; Salamanca et al., 2010). The BULK schem (...truncated)