Abstract. Urban areas are hot-spots of intense emissions and they influence air-quality not only locally but on regional or even global scales. The impact of urban emissions over different scales depends on the dilution and chemical transformation of the urban plumes which are governed by the local and regional scale meteorological conditions. These are influenced by the presence of urbanized land-surface via the so called urban canopy meteorological forcing (UCMF). In this study, we investigate for selected central European cities (Berlin, Budapest, Munich, Prague, Vienna and Warsaw), how the urban emission impact (UEI) is modulated by the UCMF for present day climate conditions (2015–2016) using three regional climate-chemistry models: the regional climate models RegCM and WRF-Chem (its meteorological part), the chemistry transport model CAMx coupled to either RegCM and WRF and the “chemical” component of WRF-Chem. The UCMF was calculated by replacing the urbanized surface by rural one while the UEI was estimated by removing all anthropogenic emissions from the selected cities. We analyzed the urban emissions induced changes of near surface concentrations of NO2, O3 and PM2.5. We found increases of NO2 and PM2.5 concentrations over cities by 4–6 ppbv, and 4–6 μgm−3, respectively meaning that about 40–60 % and 20–40 % of urban concentrations of NO2 and PM2.5 are caused by local emissions and the rest is the result of emissions from surrounding rural areas. We showed that if UCMF is included, the UEI of these pollutants is about 40–60 % smaller, or in other words, the urban emission impact is overestimated if urban canopy effects are not taken into account. In case of ozone, models due to UEI usually predict decreases around −2 to −4 ppbv (about 10–20 %), which is again smaller if UCMF is considered (by about 60 %). We further showed that the impact on extreme (95th percentile) air-pollution is much stronger, as well as the modulation of UEI is larger for such situations. Finally, we evaluated the contribution of the urbanization induced modifications of vertical eddy-diffusion to the modulation of UEI, and found that it alone is able to explain the modelled decrease of the urban emission impact if the effects of UCMF are considered. In summary, our results showed that the meteorological changes resulting from urbanization have to be included in regional model studies if they intend to quantify the regional fingerprint of urban emissions. Ignoring these meteorological changes can lead to strong overestimation of UEI.

Abstract. Urban areas are hot spots of intense emissions, and they influence air quality not only locally but on a regional or even global scale. The impact of urban emissions over different scales depends on the dilution and chemical transformation of the urban plumes which are governed by the local- and regional-scale meteorological conditions. These are influenced by the presence of urbanized land surface via the so-called urban canopy meteorological forcing (UCMF). In this study, we investigate for selected central European cities (Berlin, Budapest, Munich, Prague, Vienna and Warsaw) how the urban emission impact (UEI) is modulated by the UCMF for present-day climate conditions (2015–2016) using two regional climate models, the regional climate models RegCM and Weather Research and Forecasting model coupled with Chemistry (WRF-Chem; its meteorological part), and two chemistry transport models, Comprehensive Air Quality Model with Extensions (CAMx) coupled to either RegCM and WRF and the “chemical” component of WRF-Chem. The UCMF was calculated by replacing the urbanized surface by a rural one, while the UEI was estimated by removing all anthropogenic emissions from the selected cities. We analyzed the urban-emission-induced changes in near-surface concentrations of NO2, O3 and PM2.5. We found increases in NO2 and PM2.5 concentrations over cities by 4–6 ppbv and 4–6 µg m−3, respectively, meaning that about 40 %–60 % and 20 %–40 % of urban concentrations of NO2 and PM2.5 are caused by local emissions, and the rest is the result of emissions from the surrounding rural areas. We showed that if UCMF is included, the UEI of these pollutants is about 40 %–60 % smaller, or in other words, the urban emission impact is overestimated if urban canopy effects are not taken into account. In case of ozone, models due to UEI usually predict decreases of around −2 to −4 ppbv (about 10 %–20 %), which is again smaller if UCMF is considered (by about 60 %). We further showed that the impact on extreme (95th percentile) air pollution is much stronger, and the modulation of UEI is also larger for such situations. Finally, we evaluated the contribution of the urbanization-induced modifications of vertical eddy diffusion to the modulation of UEI and found that it alone is able to explain the modeled decrease in the urban emission impact if the effects of UCMF are considered. In summary, our results showed that the meteorological changes resulting from urbanization have to be included in regional model studies if they intend to quantify the regional footprint of urban emissions. Ignoring these meteorological changes can lead to the strong overestimation of UEI.

Abstract. Wind-blown dust (WBD) emitted by the Earth’s surface due to sandblasting can potentially have important effects on both climate and human health via interaction with solar and thermal radiation and reducing air-quality. Apart from the main dust "centers" around the world like deserts, dust can be emitted from partly vegetated middle and high latitude areas like Europe if certain conditions are suitable (strong winds, bare soil, reduced soil moisture, etc.). Using a wind-blown dust model (WBDUST) along with a chemical transport model (CAMx) coupled to a regional climate model (WRF), this study as one of the first ones provides a model based estimate of such emissions over Europe as well as the long-term impact of WBD emissions on the total PM concentrations for the 2007–2016 period. We estimated WBD emissions to about 0.5 and 1.5 Mg km−2 yr−1 in fine and coarse mode in average. Maximum emissions occur over Germany where the average seasonal fine and coarse mode emission flux can reach 0.2 and 0.5 g km−2 s−1, respectively. Large variability is seen in the daily averaged emissions with values up to 2 g km−2 s−1 for the coarse mode aerosol on selected days. The WBD emissions increased the modelled winter PM2.5 and PM10 concentrations by up to 10 and 20 μgm−3, respectively, especially over Germany, where the highest emissions occur. The impact on other seasons is lower. Much higher impacts are modelled however during selected days when occasionally the urban PM2.5 and PM10 concentrations are increased by more than 50 and 100 μgm−3. The comparison with measurements revealed that if WBD is considered, the summer biases are reduced however the winter PM is even more overestimated (so the bias increased). We identified strong overestimation of the modelled wind-speed (the maximum daily wind is almost 2 times higher in WRF than the measured ones) suggesting that WBD emissions are also overestimated hence the enhanced winter PM biases. Moreover, we investigated the secondary impacts of the crustal composition of fine WBD particles on secondary inorganic aerosol (SIA): sulphates (PSO4), nitrates (PNO3) and ammonium (PNH4). Due to perturbing the water pH value and thus the uptake of their gaseous precursors as well as due to increased aerosol surface serving as oxidation site, we modelled increased seasonal PSO4 and PNO3 concentrations by up to 0.1 μgm−3 and decreases for PNH4 (by up to -0.05 μgm−3), especially during winter. As the average daily impact, these numbers can however reach much larger values up to 1–2 μgm−3 for sulphates and nitrates while the decrease of ammonium due to WBD can reach -1 μgm−3 on selected days. The sensitivity test on the choice of the inorganic equilibrium model (ISORROPIA vs. EQSAM) showed that if EQSAM is used, the impact on SIA is slightly stronger (by a few 10 %) due to larger number of cations considered for water pH in EQSAM. Our results have to be considered as a first estimate of the long-term WBD emissions and the related effects on PM over Europe. More sensitivity studies involving the impact of the WBD model choice and the input data used to describe the land-surface need to be carried out in future to better constrain these emissions.

<p>Urban canopies impact the meteorological conditions in the planetary boundary layer (PBL) and above in many ways: apart from urban heat island effect, the urban breeze circulation can form. Further, the enhanced drag causes intensification of the turbulent diffusion leading to elevated PBL height and this drag, at the same time causes lower windspeeds. These changes together act as a 'meteorological forcing' for the chemical processes involing transport, diffusion and chemical transformation of urban pollutants in the urban canopy and over larger scales, therefor we use the term urban canopy meteorological forcing (UCMF). Using regional scale coupled chemistry-climate models over central Europe (involving models RegCM, CAMx and WRF-Chem),  we investigate here how the UCMF influences the urban emissions and their dispersion into regional scales. The analysis covers key pollutants as O<sub>3</sub>, NO<sub>2</sub> and PM2.5 and the 2015-2016 period. </p><p>While urban emissions contribute by about 60-80% to the total NO<sub>2</sub> and PM2.5 concentrations in cities, for ozone, they cause decrease in the urban cores and slight increase over sourrounding rural areas. More importantly, we found that if UCMF is considered, the impacts on all three pollutants are reduced, by about 20-30%. This is caused by the fact that vertical turbulence is greatly enhanced in urban areas leading to reduced fingerprint of the urban emissions (the case of NO<sub>2</sub> and PM2.5) while in case of O<sub>3</sub>, reduction of the NO<sub>2</sub> impact means smaller first-order titraltion therefor higher ozone concentrations - i.e. the ozone fingerprint of urban emissions is smaller. Our analysis showed that for evaluating the impact of urban emissions over regional scales, the meterological effects caused by the urban canopy have to be considered in modeling studies.</p>

Abstract. Urbanization or rural-urban transformation (RUT) represents one of the most important transformations of land-use. To account for the impact of such process on air-quality, multiple aspects of how this transformation impacts the air has to be accounted for. Here we present a numerical model (regional climate models RegCM and WRF coupled to chemistry transport model CAMx) based study for present day conditions (2015–2016) focusing on a range of central European cities and quantify the individual and combined impact of four potential contributors. Apart from the two most studied impacts, i.e. the urban emissions and the urban canopy meteorological forcing (UCMF, i.e. the impact of modified meteorological conditions) we focus also on two less studied contributors to RUT: the impact of modified dry-deposition due to transformed landuse and the impact of modified biogenic emissions due to urbanization induced vegetation modifications and changes in meteorological conditions affecting these emissions. To quantify each of these RUT components, we performed a series of simulations with CAMx driven with both RegCM and WRF were each effect was added to the simulations one-by-one while we focused on gas-phase key pollutants: nitrogen and sulfur dioxide (NO2 and SO2) and ozone (O3). The validation of the results using surface observations showed an acceptable match between the modelled and observed annual cycles of monthly pollutant concentrations for NO2 and O3 while some discrepancies in the shape of the annual cycle were identified for some of the cities for SO2 pointing to incorrect representation of the annual emission cycle in the emissions model used. We showed on an ensemble 19 European cities that the most important contributors to the impact of RUT are the urban emissions themselves, resulting in increases concentrations for nitrogen dioxide (by 5–7 ppbv on average) and sulfur dioxide (by about 0.5–1 ppbv) and decreases for ozone (by about -2 ppbv) and the urban canopy meteorological forcing resulting in decreases of primary pollutants (by about 2 ppbv for NO2 and 0.2 ppbv for SO2) and increases of those of ozone (by about 2 ppbv). These are the two major drivers of urban air pollution and our results showed that they have to be accounted for simultaneously as the impact of urban emissions without considering UCMF can lead to overestimation of the emission impact. Additionally, we quantified two weaker contributors: the effect of modified landuse on dry-deposition and the effect of modified biogenic emissions. Due to modified dry-deposition summer (winter) NO2 increases (decreases) by 0.05(0.02) ppbv while almost no average effect for SO2 in summer and a 0.04 ppbv decrease in winter is modelled. The impact on ozone is much stronger and reaches a 1.5 ppbv increase on average. Due to modified biogenic emissions, negligible effect on SO2 and winter NO2 is modelled, while for summer NO2, and increase by about 0.01 ppbv is calculated. For ozone, we found a much larger decreases between 0.5–1 ppbv. In summary, when analyzing the overall impact of urbanization on air-pollution for ozone, all four components has to be accounted for while for primary pollutants (i.e. NO2 and SO2), the two minor contributors can be neglected.