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

Abstract. For the purpose of qualifying and quantifying the impact of urban emission from Central European cities on the present-day regional air quality, the regional climate model RegCM4.2 was coupled with the chemistry transport model CAMx, including two-way interactions. A series of simulations was carried out for the 2001–2010 period either with all urban emissions included (base case) or without considering urban emissions. Further, the sensitivity of ozone production to urban emissions was examined by performing reduction experiments with −20 % emission perturbation of NO_x and/or non-methane volatile organic compounds (NMVOC).

The modeling system's air quality related outputs were evaluated using AirBase, and EMEP surface measurements showed reasonable reproduction of the monthly variation for ozone (O₃), but the annual cycle of nitrogen dioxide (NO₂) and sulfur dioxide (SO₂) is more biased. In terms of hourly correlations, values achieved for ozone and NO₂ are 0.5–0.8 and 0.4–0.6, but SO₂ is poorly or not correlated at all with measurements (r around 0.2–0.5). The modeled fine particulates (PM_2.5) are usually underestimated, especially in winter, mainly due to underestimation of nitrates and carbonaceous aerosols.

European air quality measures were chosen as metrics describing the cities emission impact on regional air pollution. Due to urban emissions, significant ozone titration occurs over cities while over rural areas remote from cities, ozone production is modeled, mainly in terms of number of exceedances and accumulated exceedances over the threshold of 40 ppbv. Urban NO_x, SO₂ and PM_2.5 emissions also significantly contribute to concentrations in the cities themselves (up to 50–70 % for NO_x and SO₂, and up to 60 % for PM_2.5), but the contribution is large over rural areas as well (10–20 %). Although air pollution over cities is largely determined by the local urban emissions, considerable (often a few tens of %) fraction of the concentration is attributable to other sources from rural areas and minor cities. For the case of Prague (Czech Republic capital), it is further shown that the inter-urban interference between large cities does not play an important role which means that the impact on a chosen city of emissions from all other large cities is very small. At last, it is shown that to achieve significant ozone reduction over cities in central Europe, the emission control strategies have to focus on the reduction of NMVOC, as reducing NO_x (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 NO_x and/or NMVOC reduction ends up in decreased ozone pollution, mainly in terms of exceedances.