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For the purpose of qualifying and quantifying the impact of urban emission\udfrom Central European cities on the present-day regional air quality, the\udregional climate model RegCM4.2 was coupled with the chemistry transport\udmodel CAMx, including two-way interactions. A series of simulations was\udcarried out for the 2001–2010 period either with all urban emissions\udincluded (base case) or without considering urban emissions. Further, the\udsensitivity of ozone production to urban emissions was examined by performing\udreduction experiments with −20 % emission perturbation of NOx and/or non-methane volatile organic compounds (NMVOC).\ud \udThe modeling system's air quality related outputs were evaluated using\udAirBase, and EMEP surface measurements showed reasonable reproduction of the\udmonthly variation for ozone (O3), but the annual cycle of nitrogen\uddioxide (NO2) and sulfur dioxide (SO2) is more biased. In\udterms of hourly correlations, values achieved for ozone and NO2 are\ud0.5–0.8 and 0.4–0.6, but SO2 is poorly or not correlated at all\udwith measurements (r around 0.2–0.5). The modeled fine particulates\ud(PM2.5) are usually underestimated, especially in winter, mainly due to underestimation of nitrates and carbonaceous aerosols.\ud \udEuropean air quality measures were chosen as metrics describing the cities\udemission impact on regional air pollution. Due to urban emissions,\udsignificant ozone titration occurs over cities while over rural areas remote\udfrom cities, ozone production is modeled, mainly in terms of number of\udexceedances and accumulated exceedances over the threshold of\ud40 ppbv. Urban NOx, SO2 and PM2.5 emissions also significantly contribute to concentrations in the cities themselves (up to\ud50–70 % for NOx and SO2, and up to 60 % for PM2.5),\udbut the contribution is large over rural areas as well (10–20 %).\udAlthough air pollution over cities is largely determined by the local urban\udemissions, considerable (often a few tens of %) fraction of the\udconcentration is attributable to other sources from rural areas and minor\udcities. For the case of Prague (Czech Republic capital), it is further shown\udthat the inter-urban interference between large cities does not play an\udimportant role which means that the impact on a chosen city of emissions from\udall other large cities is very small. At last, it is shown that to achieve\udsignificant ozone reduction over cities in central Europe, the emission\udcontrol strategies have to focus on the reduction of NMVOC, as reducing\udNOx (due to suppressed titration) often leads to increased O3. The influence over rural areas is however always in favor of improved air quality, i.e. both NOx and/or NMVOC reduction ends up in decreased ozone pollution, mainly in terms of exceedances

On the long-term impact of emissions from central European cities on regional air quality