GAW Report No. 205 - IGAC Project

CHAPTER 6 - EUROPE6.1.3 Pollution levelsThe air pollution in the European MPCs which results from the emissions discussed in theprevious section, as part of the broader topic of general urban air quality, has a very long history ofslowly increasing recognition and research. In particular, London has a very important historical rolein this respect, with the “London Smog” events dating back at least to the 17 th century, culminatingin the weeklong tragedy in December, 1952, which caused about 12,000 deaths. Many other largecities have been renowned for their poor air quality, as well as for the substantial clean-up effortsover more recent years, e.g., the reduction of acid rain throughout Europe in the latter part of the20 th century.6.1.4 Outflow characteristics and effects on regional ozone-related atmospheric chemistryTo our knowledge, only one published study thus far [Butler and Lawrence, 2009] hasexamined the impact of megacities worldwide on regional and global ozone-related atmosphericchemistry, and none have done so yet for aerosol and climate impacts. A few further studies haveexamined this for specific regions [e.g., for Asia in Guttikunda et al., 2005], but none to ourknowledge specifically for the European MPCs. Some studies on the impacts of individual cities willbe discussed in the following sections.Butler and Lawrence [2009] used a zeroth-order approach, the so-called “annihilationscenario”, to examine the megacity effects by removing their emissions from the correspondinggridcells in a global emissions inventory (at 1 o horizontal resolution) before interpolating to theglobal model grid, and comparing these results with a simulation including the normal totalemissions. The results show that the overall impact of megacities on the major ozone precursorgases NO X and CO are of the order of 5-10%, corresponding to the relative contribution ofmegacities to the global total emissions of the precursor gases, while the impact on global O 3 ismuch smaller, around 1%. The change in July mean surface ozone is shown for four scenarios inFigure 3. The exact dependence of the regional chemistry on the emissions varies as a function ofgeographical location, and corresponds particularly strongly with the latitudes of the MPCs. In theindividual grid cells containing megacities, the response for European megacities is similar to mostother extratropical megacities, with a reduction in ozone year-round, and often an increase in ozonein the downwind grid cells, particularly in summertime; in tropical megacity grid cells, on the otherhand, ozone generally increases year-round. The influence is found to change for various futurescenarios. Under a future scenario with a maximum feasible reduction of emissions, the influence ofmegacities is generally reduced, while under a high-emission future scenario, although the localinfluence of megacities is increased, the geographical extent of the influence becomes smaller. Onenote worth making about these results is that the tendency for global emissions datasets tounderestimate the emissions compared to city-specific datasets, as discussed above, also meansthat the impacts of megacities are probably somewhat underestimated in these simulations.Finally, a special characteristic of the European MPCs has been pointed out and quantifiedby Lawrence et al. [2007], who examined results of simulations with generic, gas-phase tracers withthree different representative lifetimes (1, 10 and 100 days) emitted from 36 MPCs distributedglobally. Using metrics to rank different outflow characteristics (“regional pollution potentials”) of theMPCs, it was found that the MPCs in this region tend to be ranked highest amongst all globalregions in terms of the tendency for pollutants to both accumulate locally in the surface layer of theregion immediately surrounding each MPC, as well as to be transported extensive distances (e.g.larger than 1000 km) downwind, while still remaining in the boundary layer. Conversely, theemissions from European MPCs are least effectively transported into the upper tropospherecompared to other world regions. Two major open issues for follow-up studies are currently beinginvestigated: the same kind of simulations with aerosol tracers (including sedimentation,scavenging and deposition) are being performed for comparison to the gas-phase tracer results [D.Kunkel et al., 2011], and comparable regional model simulations are being set up for the Europeanregion to determine the impact of using a considerably higher resolution in a non-hydrostatic modelon the pollutant dispersion characteristics [I. Coll et al., LISA, pers. comm.].196