The Role of Vegetation on Urban Atmosphere of Three European Cities—Part 1: Evaluation of Vegetation Impact on Meteorological Conditions

Abstract

This study quantifies the vegetation impact on urban meteorology by means of the numerical model WRF (Weather Research and Forecasting model). The assessment was made for two months: July and January. These were considered as representative for the summer and winter seasons, for the reference year 2015 in three European cities: Bologna, Milano, and Madrid. Two simulations at 1 km resolution were conducted over the cities with and without the actual urban vegetation, called VEG and NOVEG, respectively, in the model input. Then, the impact of vegetation was evaluated as the difference between the two simulations (VEG-NOVEG) for temperature, relative humidity, and wind speed fields. In general, we found that, as can be expected, urban vegetation tends to cool the atmosphere, enhance the humidity, and reduce the wind speed. However, in some cases, areas with the opposite behaviour exist, so that no a priori results can be attributed to the presence of urban vegetation. Moreover, even when major impact is confined around grid cells where urban vegetation is present, changes in meteorological quantities can be observed elsewhere in the city’s area. The magnitude of urban vegetation impact is higher in summer than in winter and it depends on the city’s morphological peculiarities, such as urban texture and vegetation types and distribution: average July temperature variations due to the presence of urban vegetation reach peaks of −0.8 °C in Milano, −0.6 °C Madrid, and −0.4 °C in Bologna, while in January, the values range between −0.3 and −0.1 °C. An average heating effect of ca. +0.2 °C is found in some parts of Madrid in January. For relative humidity, we found increments of 2%–3% in July and 0.5%–0.8% in January, while a decrease in wind speed was found between 0.1 and 0.5 m/s, with the highest occurring in Madrid during July.

The Role of Vegetation on Urban Atmosphere of Three European Cities. Part 2: Evaluation of Vegetation Impact on Air Pollutant Concentrations and Depositions

“…For each city, the AMS used the same hourly gridded anthropogenic emissions as input for the VEG and NOVEG simulations, while the meteorological fields were different according to the presence or absence of vegetation [11]. Thus, all processes simulated by CTMs, such as transport and diffusion, gas-phase chemistry, aerosol chemistry and dynamics, and wet and dry deposition, are altered due to changed meteorological conditions, and some of them, such as gas-phase and aerosol chemistry, are also modified by BVOC emissions, that change because of vegetation and meteorology differences, as explained in the following Section 3.1.…”

“…The main components of the two AMS used to simulate air quality over Bologna and Milan (AMS-MINNI) and Madrid (WRF-CMAQ) are shown in Table 1. The details regarding the configuration of the meteorological model WRF can be found in D'Isidoro and Mircea et al (2023) [11], while here, the emphasis is on CTMs components responsible for gas-phase chemistry and aerosol processes.…”

“…Simulations Setup FARM simulations were carried out over three nested domains (Europe, Northern Italy, Bologna, and Milan cities), while CMAQ simulations were performed over four nested domains (Europe, Iberian Peninsula, central Spain, and Greater Madrid area). All these domains described in Table S1 were located inside the domains used by the WRF model to reconstruct the meteorological conditions shown in Figure S1 and Table S1 in D'Isidoro and Mircea et al (2023) [11].…”

“…This study investigates in a comprehensive way the simultaneous effects of vegetation on meteorology and air quality in three cities, Bologna, Milan, and Madrid, located in two countries, Italy and Spain, using two AMS that are described in detail in Section 2. It also includes, for the first time, a detailed description of urban vegetation at a single tree level (Section 2) using urban tree inventories provided by municipalities and of urban morphology [11]. The robustness of the results is ensured by the application of two stateof-the-art air quality modelling systems: AMS-MINNI [12][13][14][15] for Bologna and Milan and WRF-CMAQ [16][17][18] for Madrid.…”

“…Such realistic approach was developed and applied for the first time in the framework of the European Life project VEG-GAP (https://www.lifeveggap.eu/, accessed on 15 May 2023). The urban meteorological conditions' changes due to vegetation are extensively discussed in the companion article [11]. Here, their effects on air concentrations and dry depositions of O 3 , NO 2, and PM10 are shown (Section 3).…”