GAW Report No. 205 - IGAC Project

CHAPTER 8 – KEY ISSUES AND OUTLOOKIt is clear that a single megacity in a large, densely populated region cannot effectivelyaddress its air quality issues in isolation; a region wide control strategy is required. The US facedthis issue in the 1980s when the Ozone Transport Assessment Group (OTAG) studied this issue.In that country regional transport is significant, but is dwarfed by its significance in Asia. China willhave to grapple with its daunting regional transport problem, which will become increasinglyimportant as that country continues to develop, but it may be much more difficult in the Indo-Gangetic Plain, which encompasses much of northern and eastern India, the most populous partsof Pakistan, parts of Nepal, and most of Bangladesh. Mobilizing the required internationalcoordination to develop effective air quality control policies will be difficult in a politicallychallenging region of the globe.8.4 AIR QUALITY CONTROL STRATEGIES: URBAN VERSUS DOWNWIND AREASVOC and NO X emitted to the atmosphere are oxidized through photochemical processesthat produce both ozone and SOA. The oxidation of anthropogenic emissions of these species isresponsible for the presence of elevated concentrations of ozone and SOA in urban areas. Overthe past decades, there has been a great deal of debate in the US regarding whether it is cheaperand more effective to control VOC or NO X emissions. This debate has been largely focused oncontrol of ozone rather than SOA [NRC, 1991]. A variety of analyses have argued that VOCcontrols are more effective and that NO X control may in fact be counter-productive. Thesearguments rely upon observationally based analysis, such as weekday-weekend differences[Blanchard et al., 2008 and references cited therein], EKMA type analysis [NRC, 1991] or evensimpler models [Stedman, 2004]. Unfortunately, photochemical ozone formation is a complexproblem involving emissions from urban cores, transport, distributed anthropogenic as well asbiogenic emissions of both VOC and NO X , etc. It is very likely that the relative effectiveness ofVOC and NO X controls is characteristic of each location, and varies with meteorology.It is also important to note that the debate described above has generally had a verylocalized focus – the air quality in the urban area for which emission controls are being considered– and impacts on downwind areas are not considered. If VOC emissions are controlled and NO Xemissions are not, ozone concentrations may well drop in the emission region, but the emitted NO Xmay be more efficiently transported to downwind regions, eventually forming a greater amount ofozone than in the absence of the VOC controls. This is an especially important consideration indensely populated regions such as Asia discussed in the preceding section.To reliably determine the most effective ozone or PM control strategy for a particular urbanarea or region requires air quality modelling incorporating accurate input from emission inventories,boundary condition characterization and meteorological fields as well as incorporating realisticdescriptions of chemical and physical processes [Song et al., 2010]. Arguably, such modelling tothe required accuracy is not yet possible for any urban area. Nevertheless, the detrimental healtheffects of air pollutants in the world’s megacities require that air quality improvement efforts mustbe implemented in lieu of such reliable information. Indeed, such efforts have been implementedand have resulted in great improvement in many cities (Figure 7). Fortunately, reduction ofemissions of any important pollutant brings beneficial effects from many perspectives, even if thereductions are not optimized for highest efficiency.8.5 CONTRIBUTION OF MEGACITIES TO REGIONAL AND GLOBAL CONCENTRATIONSStudying the impacts of megacities is challenging as they occur on different spatial scales,from the local to the global. Local and regional-scale measurements are crucial to understandingthe basic relationships between emissions, current ambient concentrations, and past trends overtime. There have been numerous observational studies of air pollution due to megacities on thelocal to global scale [Garland et al., 2008; Gurjar et al., 2008; Zhang et al., 2008]. MILAGRO was aparticularly large field programme that coordinated measurements to investigate the impact ofemissions from Mexico City on the air quality of the city and its surroundings [Molina et al., 2010,293