the regions’ megacities, <strong>and</strong> a variety of <strong>in</strong>fluences need to beconsidered when form<strong>in</strong>g air quality policy to try <strong>and</strong> addressthese issues.• A London case study found that isoprene (a proxy for biogenicVOCs) still plays a relatively small role <strong>in</strong> overall VOC reactivity,<strong>and</strong> thereby ozone formation. The exception to this isperiods of high temperature <strong>and</strong> sunlight, which <strong>in</strong>creases biogenicemissions significantly (e.g., summer 2003 heat wave).While significant decreases <strong>in</strong> anthropogenic VOCs were observedover the past decade, likely ow<strong>in</strong>g to effective regulationof emissions, isoprene has not seen a rise <strong>in</strong> importance relativeto the anthropogenic VOC fraction (von Schneidemesseret al., 2010). Furthermore, a significant fraction of the isoprene<strong>in</strong> urban areas likely orig<strong>in</strong>ates from anthropogenic sources. Toreliably assess the <strong>in</strong>fluence of biogenic emissions <strong>in</strong> a regionit may be necessary to <strong>in</strong>clude the measurement of monoterpenespecies <strong>in</strong> rout<strong>in</strong>e monitor<strong>in</strong>g, as isoprene, which is frequentlyused as a proxy for biogenic emissions, may not besufficient. These results may have implications for urban airquality policies <strong>and</strong> monitor<strong>in</strong>g practices, as well as emission<strong>in</strong>ventories used <strong>in</strong> modell<strong>in</strong>g. (Summarized from (von Schneidemesseret al., under review)).• A modell<strong>in</strong>g study of ozone formation <strong>in</strong> the Pearl RiverDelta region of Ch<strong>in</strong>a found that ozone formation is likelyVOC-limited <strong>in</strong> the urban, <strong>in</strong>l<strong>and</strong> areas <strong>and</strong> NOx-limited <strong>in</strong>the more rural areas with a predom<strong>in</strong>ance of aged emissions(Wang et al., 2010). The Pearl River Delta region of Ch<strong>in</strong>ahas significant photochemical pollution episodes. Emissionsof nitrogen oxides <strong>in</strong> the PRD area are dom<strong>in</strong>ated by mobilesources (47%) <strong>and</strong> power generation (39%); the three largestsources of VOCs are mobile sources (38%), evaporationlosses of solvents <strong>and</strong> petroleum (24%) <strong>and</strong> biogenic sources(23%), with the largest concentration of emissions over the urbanareas <strong>in</strong> the region. The ozone response to reductions ofanthropogenic VOCs <strong>and</strong> NOx, separately <strong>and</strong> simultaneouslyrevealed significant spatial differences <strong>in</strong> VOC- <strong>and</strong> NOx-limitedconditions. In the urban areas, less efficient ozone productionresulted from <strong>in</strong>tense NOx emissions which reacted with<strong>and</strong> thereby suppressed ozone concentrations. As emissionswere transported out of the urban areas <strong>and</strong> concentrations ofNOx were lower, ozone production efficiency <strong>in</strong>creased. Thesevary<strong>in</strong>g regimes of ozone production efficiency over the PRDregion are shown <strong>in</strong> Figure 3. Additionally, synoptic weatherconditions <strong>in</strong> the PRD significantly <strong>in</strong>fluence the formation <strong>and</strong>distribution of ozone. (Summarized from (Wang et al., 2010)).The different regimes <strong>and</strong> their effect on ozone production efficiencyshould be considered when form<strong>in</strong>g <strong>and</strong> implement<strong>in</strong>gpolicy for emission reductions to achieve the largest benefits.• High resolution of emissions is important on a local/megacity scale for underst<strong>and</strong><strong>in</strong>g pollutant production <strong>and</strong> exposure,but less important for climate studies that aim at underst<strong>and</strong><strong>in</strong>gchanges <strong>in</strong> pollutant production over a regionalto global scale. Urban- to global-scale models are used tosimulate <strong>and</strong> improve underst<strong>and</strong><strong>in</strong>g of ozone production <strong>and</strong>distribution. However, urban scale models are typically used toassess ozone <strong>in</strong> terms of air quality, while global scale modelsfocus on ozone <strong>in</strong> terms of climate change <strong>and</strong> its role asa greenhouse gas. As the emissions from megacities lead<strong>in</strong>gto ozone production are substantial, <strong>and</strong> spatial distributionof emissions are not even, this study aimed at underst<strong>and</strong><strong>in</strong>g<strong>and</strong> quantify<strong>in</strong>g the <strong>in</strong>accuracies that arise when ozone productionis modelled on a coarse scale (e.g. <strong>in</strong> a global chemistry-transportmodel). This was accomplished by apply<strong>in</strong>g aregional model us<strong>in</strong>g f<strong>in</strong>er <strong>and</strong> coarser resolution of emissionsover large urban areas. The ma<strong>in</strong> difference result<strong>in</strong>g from thevary<strong>in</strong>g resolution of the emissions was that f<strong>in</strong>er resolutionwas able to capture areas of high NOx <strong>in</strong> pollution plumeswhich caused suppressed ozone levels, whereas <strong>in</strong> the coarseresolution few grid boxes reached such high concentrations ofNOx. This resulted <strong>in</strong> a small enhancement of ozone production<strong>in</strong> the coarser grid models <strong>in</strong> most of the cases that werestudies. Overall these differences were small <strong>and</strong> the coarserresolution was deemed sufficient for climate studies, whereasthe high resolution would be more important for local air pollutionstudies. (Summarized from (Hodnebrog et al., 2011)).An established approach to focus on urban scale processes <strong>in</strong>regional models is two-way nest<strong>in</strong>g. Two-way nest<strong>in</strong>g <strong>in</strong>volvessimulat<strong>in</strong>g several sub-doma<strong>in</strong>s <strong>and</strong> their <strong>in</strong>teractions, simultaneously,however this technique is computationally expensive.In CityZen, the impact of megacities on the regional scale wasaddressed by <strong>in</strong>novative chemistry-transport modell<strong>in</strong>g us<strong>in</strong>gscale bridg<strong>in</strong>g approaches: grid stretch<strong>in</strong>g (Siour et al., underreview) <strong>and</strong> upwards nudg<strong>in</strong>g (Maurizi et al., 2011). Gridstretch<strong>in</strong>g <strong>in</strong>volves a s<strong>in</strong>gle grid with variable horizontal resolutionthat permits focus<strong>in</strong>g on selected areas, while upwardsnudg<strong>in</strong>g uses data from high resolution model runs <strong>and</strong> forcesthis data <strong>in</strong>to low resolution areas to improve concentrationfields. By allow<strong>in</strong>g a feedback of the <strong>in</strong>ner (high resolution) gridarea to the larger, coarser grid scale, these strategies offer analternative to two-way nest<strong>in</strong>g. Both approaches confirmedthat an improved representation of megacities was crucial tocapture air quality <strong>in</strong> their vic<strong>in</strong>ity although the added valuewas limited compared to the classical meso-scale s<strong>in</strong>gle-nestedapproach regard<strong>in</strong>g air pollution export at the larger (regional)scale.Figure 3. (Wang et al., 2010) <strong>Regional</strong> distribution of ozone productionefficiency over the Pearl River Delta region based on the overall chemicalproduction of ozone <strong>and</strong> NOz over the surface layers dur<strong>in</strong>g daytime on16 October 2004. Less efficient ozone production (1-5) is found <strong>in</strong> theurban areas with <strong>in</strong>tensive NOx emissions, whereas high ozone productionefficiency (>11) is found <strong>in</strong> upw<strong>in</strong>d <strong>and</strong> rural areas with less NOx sourceemissions.• Under climate conditions expected for 2030, model simulationspredict <strong>in</strong>creased ozone concentrations, however, projectedemission reductions will more than compensate for this<strong>and</strong> therefore most areas have predicted ozone reductions(Figure 4). Until 2030, accord<strong>in</strong>g to model results, the predict-THE EGGS 7 back to contents