GAW Report No. 205 - IGAC Project

CHAPTER 8 – KEY ISSUES AND OUTLOOKmegacities on global tropospheric NO X concentrations. Under the low-emission future scenarioS2, the influence of megacities is generally reduced, and under the high-emission future scenarioS4, although the local influence of megacities is increased, the geographical extent of the influencebecomes smaller. In terms of ozone response, the individual model grid cells that containmegacities respond to the megacity emissions differently depending on their latitude. Tropicalmegacity grid cells generally show increased ozone year-round, while northern extratropicalmegacities generally show reduced ozone year-round. Lawrence et al. [2007] offer anotherperspective, examining the “regional pollution potential” of emissions from megacities. That studyfound that the long-range transport characteristics of generic tracers are strongly affected by themegacity location and can be grouped into various regions. Eurasian megacities tend to bothretain the most pollution in the region immediately surrounding the city, and also to export the mostpollution to the boundary layer more than 1000 km away from the city, while tropical cities,especially from Southeast Asia, tend to primarily export the most pollution to the uppertroposphere. Vertical transport, especially by deep cumulus convection, is particularly importantfor determining the differences between the regions.8.6 URBAN HEAT ISLAND IN MEGACITIESUrban structures change the surface of land and sky view factor, and therefore the surfacebalance of energy, such as the short and long wave radiation, and the sensible and latent heatfluxes. This change together with the anthropogenic heat sources are the main causes of highertemperatures in urban centres than in suburban and rural areas, and hence the formation of urbanheat island (UHI).Urban heat island effect can be described with urban heat island intensity (UHII). UHII isdefined as the spatially-averaged temperature difference between an urban area and itssurrounding rural area [Kim and Baik, 2005]. UHII can be quantified by calculating air or surfacetemperature differences between an urban area and nearby rural area simultaneously with similargeographic features; satellite imagery has also been used to obtain surface-temperature basedUHII over a study area under clear skies. [Memon et al., 2009].High UHII have been reported in studies of megacities, such as 10 °C in Beijing [Hung et al.,2005], 8 °C in Tokyo [Saitoh et al., 1995] and Paris [Lemonsu and Masson, 2002], and 5 °C in NewYork [Gedzelman et al. 2003]. There is also a trend of temperature increase in large cities. Forexample, in Osaka the UHII increased from approximately 2.4 °C in 1901 to almost 3 °C after 1981,while the UHIIs of Seoul, Tokyo, and Taipei, have increased by 1 °C to 2 °C [Kataoka et al., 2009].The higher temperature in urban centres of megacities is associated with higher health risk.It is found that the anthropogenic heat plays an important role for UHII and the boundary layerdevelopment, and also has a significant impact on local circulation, such as land sea circulation[Lin et al., 2008]. The formation of an UHI has been shown to have a diurnal variation. For example,over Paris the UHII is stronger at night than during the day thus impacting the structure of theatmospheric boundary layer, which has an important impact on primary and secondary regional airpollutants such as ozone and nitrogen oxide [Sarrata, et al., 2006].With elevated temperatures, UHI has significant impacts on outdoor air quality, especiallyon the concentration of photochemical oxidants. Narumi et al., [2009] used an atmosphericdispersion model to reproduce the observed air pollution conditions for the typical summer day.The results showed that a 1°C increase in temperature leads to an increase of 11% in themaximum photochemical oxidant concentration. A temperature increase of up to 3°C will increasethe concentration of photochemical oxidants by 19 ppb. The concentration of photochemicaloxidants was 30 ppb higher in the afternoon due to the effect of biogenic VOCs, indicating theirstrong impact on photochemical oxidant concentrations under UHI condition.Few studies have discussed the association between UHI and air quality under synopticpatterns. Lai and Cheng [2009] analyzed this association in the Taichung metropolis region. The295