The regional impact of urban emissions on climate over central Europe: present and future emission perspectives

Abstract

Abstract. The regional climate model RegCM4.2 was coupled to the chemistry transport model CAMx, including twoway interactions, to evaluate the regional impact of urban emission from central European cities on climate for presentday (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) and future (2046)(2047)(2048)(2049)(2050)(2051)(2052)(2053)(2054)(2055) periods, and for the future one only emission changes are considered. Short-lived non-CO 2 emissions are considered and, for the future impact, only the emission changes are accounted for (the climate is kept "fixed"). The urban impact on climate is calculated with the annihilation approach in which two experiments are performed: one with all emissions included and one without urban emissions. The radiative impacts of non-CO 2 primary and secondary formed pollutants are considered, namely ozone (O 3 ), sulfates (PSO 4 ), nitrates (PNO 3 ), primary organic aerosol and primary elementary carbon (POA and PEC).The validation of the modelling system is limited to key climate parameters, near-surface temperature and precipitation. It shows that the model, in general, underestimates temperature and overestimates precipitation. We attribute this behaviour to an excess of cloudiness/water vapour present in the model atmosphere as a consequence of overpredicted evaporation from the surface.The impact on climate is characterised by statistically significant cooling of up to −0.02 and −0.04 K in winter (DJF) and summer (JJA), mainly over cities. We found that the main contributors to the cooling are the direct and indirect effects of the aerosols, while the ozone titration, calculated especially for DJF, plays rather a minor role. In accordance with the vertical extent of the urban-emission-induced aerosol perturbation, cooling dominates the first few model layers up to about 150 m in DJF and 1000 m in JJA. We found a clear diurnal cycle of the radiative impacts with maximum cooling just after noon (JJA) or later in afternoon (DJF). Furthermore, statistically significant decreases of surface radiation are modelled in accordance with the temperature decrease. The impact on the boundary layer height is small but statistically significant and decreases by 1 and 6 m in DJF and JJA respectively. We did not find any statistically significant impact on precipitation and wind speed. Regarding future emissions, the impacts are, in general, smaller as a consequence of smaller emissions, resulting in smaller urban-induced chemical perturbations.In overall, the study suggest that the non-CO 2 emissions play rather a minor role in modulating regional climate over central Europe. Much more important is the direct climate impact of urban surfaces via the urban canopy meteorological effects as we showed earlier.

Urban canopy meteorological forcing and its impact on ozone and PM<sub>2.5</sub>: role of vertical turbulent transport

“…The average urban canopy impact on temperature is very similar to the values presented in Huszar et al (2018a) who, for the two analyzed cities (Berlin and Prague), encountered increases up to 1.5-2 • C. The diurnal variation of the impact shows also large similarities in both the quantitative and qualitative sense, when the maximum impact occurs around late evening. The urban impact on temperature is consistent with previous observation (Gaffin et al, 2008) and model-based studies (Pichierri et al, 2012;Giannaros and Melas, 2012;Struzewska and Kaminski, 2012), while Sarrat et al (2006) simulated somewhat later timing of the maximum urban impact.…”

“…Figure 18 presents the JJA changes of O 3 due to the urbanization-induced K v enhancement for the three resolutions and six K v methods. Ozone is increased in all cases, ranging from 0.2 to 3 ppbv (about 5 %-10 %), as expected following Huszar et al (2018a). They showed that the main contributor to this increase is the reduced destruction due to the turbulence-enhanced vertical removal of NO x from the canopy layer and the increased turbulent flux from the RL during the night.…”

“…For example, Struzewska and Kaminski (2012) simulated similar reductions for central European cities, but Chinese cities in Zhu et al (2017) encountered comparable decreases too. Compared to the decreases in Huszar et al (2018a), reaching −1 ms −1 , our results suggest that, in the event of wind, higher resolutions bring stronger impact. Indeed, the 27 and the 9 km impacts are always less than the 3 km result for Prague.…”

“…Figure 28 presents the total-impact of the urban meteorological changes on the mean canopy concentrations of O 3 for each resolution and both seasons. Huszar et al (2018a) predicted that near-surface O 3 should increase due to the increased removal (turbulent dispersion) of NO x from urban areas and thus reduced titration, as well as due to enhanced turbulent transport from higher levels, although they pointed out that the overall effect is always a result of competitive impact of multiple influences. Indeed, in our results, ozone usu-ally increases too, but the magnitude of the increase changes substantially across resolutions, and it is different for the two cities.…”

“…Although higher temperatures favor ozone formation (Im et al, 2011), in urban areas, the situation can be different. Huszar et al (2018a) showed that due to higher temperatures alone, surface ozone in urban areas is reduced, while the main contribution is given by increased dry deposition velocities and increased flux of nitrogen oxides (NO x ) towards nitric acid (HNO 3 ). Sarrat et al (2006) concluded too that, especially during nighttime, UHI influences the NO+O 3 → NO 2 +O 2 reaction and ozone dry deposition reducing its concentrations.…”