GAW Report No. 205 - IGAC Project

CHAPTER 8 - KEY ISSUES AND OUTLOOKLead Author: David Parrish (1)Contributing Authors: Laura Gallardo (2) , Tong Zhu (3) , Megan L. Melamed (4) and Mark Lawrence (5)(1)NOAA ESRL Chemical Sciences Division, Tropospheric Chemistry Programme Lead. 325 Broadway R/CSD7,Boulder, CO 80305 USA(2)Departamento de Geofísica & Centro de Modelamiento Matemático, Universidad de Chile. Blanco Encalada2002, piso 4, Santiago, Chile(3)College of Environmental Sciences and Engineering, Peking University, Beijing, China(4)IGAC International Project Office, University of Washington/JISAO, Seattle, WA USA(5)Institute for Advanced Sustainability Studies, Potsdam, GermanyThe population growth, economic and industrial development, and rising standard of livingin the world’s megacities will bring not only new problems but also new opportunities. There aremanifold challenges that accompany megacity growth: providing food, shelter, transportation andother goods and services for an ever-increasing population. Surmounting these challenges bringsobvious problems that affect the health and welfare of the urban population as well as societal andecological environment in areas well beyond the urban centre. At the same time, megacities areoften looked upon as the economic engines of the world. As such, they also represent aconcentration of resources that provide opportunities to address these challenges more efficientlythan possible if the same population were dispersed in smaller cities and rural environments. Aspecial section entitled Cities in the 8 February 2008 issue of Science explored these issues[http://www.sciencemag.org/content/319/5864.toc]. The goal of this chapter is to highlight some ofthe key questions and issues that have been identified regarding the challenges and opportunitiesthat accompany the emergence and evolution of megacities. This chapter is not intended to becomprehensive or detailed. Each of the issues will require significant future analysis that will bepublished elsewhere.8.1 THE SCALING LAW OF AIR POLLUTION AND HEALTH EFFECTS OF URBANPOPULATIONIt is generally recognized that ambient concentrations of air pollutants are higher in largeurban areas than in smaller cities. However, given the difficulty of comparing measurements fromdifferent cities due to differences in instrument siting, meteorological conditions, and many others,there is little quantitative information regarding how urban pollutant concentrations depend onpopulation. Satellite measurements can now provide such information. Figure 1 shows thevertical column abundance of NO 2 measured by the SCIAMACHY satellite over ten western UScities [Kim et al., 2009]. Since the predominant source of NO 2 is emissions from anthropogenicactivities, which are predominately located at the Earth’s surface, NO 2 is largely confined to thenear surface layer of the atmosphere, and the vertical column abundance is proportional to theurban ambient concentrations. In addition to the apparent day-of-week cycle in the figure, it isclear that larger cities (Los Angeles, San Francisco) generally have higher NO 2 columns than lesspopulated cities. Figure 2 is a log-log plot of the relationship between the average weekday NO 2column as a function of the population of the urban area (population taken for 2005 from a UScensus bureau web site: http://www.census.gov/popest/metro/CBSA-est2009-annual.html).The linear relationship shown in Figure 2 suggests that the relationship between urban NO 2column and population can be approximately captured as a scaling law in the form of a power lawfunction of population, as has been found for many urban relationships [Bettencourt et al., 2007]:column NO 2= N " eq 8.1i.e., many diverse urban properties increase as the population size (N) raised to an exponent b,which is between 0 and 1. The line in Figure 2 indicates the linear least-squares fit to the points!285