On the impact of emissions from central European cities on tropospheric chemistry and climate

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 NOx and/or
NMVOC. The model was validated using surface measurements of key pollutants.
Selected air-quality measures were used as metrics describing the cities emission impact on regional air pollution. Due to urban emissions, significant ozone titration over occurs over cities while over rural areas futher from them, ozone production os modeled, mainly in terms of number of exceedances and accumulated exceedances over the threshold of 40 ppbv. Urban NO_x, SO₂ and PM2.5 emissions
also significantly contribute to concentrations in the cities themselves (up to 50-70% for NO_x and SO₂ , and up to 55% for PM2.5), 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.
The radiative impact of the perturbed air chemistry (gases/aerosols) due to urban emissions is dominated mainly by the aerosol direct/indirect effect and is characterized by decrease of temperatures up to -0.08 K at the surface but the decrease concerns the whole PBL. Decreases are modeled for the PBL height as well, while the impact on total precipitation and wind speed is statistically insignifcant. The impact of all (urban and non-urban) emissions is about up to -0.2, so considerable part of the radiative effects of all emissions is attributable to urban emissions. The partial climate impact of urban emissions induced ozone, sulfate and nitrate changes are presented as well.