Uptake of ozone and particulate matter by urban green in the city of Rome

The availability of green areas is a basic element for the quality of life in urban environments. Indicators such as “amount of public green spaces per inhabitant”, “public parks” and “recreation areas” are often included among those used to evaluate the sustainability of the urban environment. Beside its well known aesthetic and recreational benefits, urban green, and particularly urban trees, could generate a series of important ecosystem services. For example, urban trees modify the microclimate by humidifying the surrounding atmosphere through leaf transpiration, limiting the urban heat island effect and providing shade and wind shelter that, in turn, can reduce building energy requirements. The reduction of atmospheric carbon dioxide and other “greenhouse” gases, as well as the enhanced deposition sink for gaseous and particulate pollution, are other important functions of the so called “urban forests”. For this reason, modeling plant-atmosphere interactions in urban environment has being raising an increasing interest in recent years, with the purpose to assess the effects of vegetation management on urban air quality. Here we present the results of a modeling exercise, implemented in the frame of the HE.R.E.P.L.U.S. (HEalth Risk from Enviromental Pollution Levels in Urban Systems) Project, funded under EU FP7. Our aim was to provide an estimate of O3 and PM10 removed by evergreen and deciduous broadleaved trees in the Municipality of Rome (ab. 128530 ha), to quantify the role of urban woody vegetation in a Mediterranean megacity. Ozone fluxes were simulated for each day of the reference year, and referred to unitary area of soil surface, based on O3 air concentration and plant stomatal conductance for each vegetation type. The latter was simulated through a semi-empirical process based model (MOCA-Flux) originally developed to estimate the net primary productivity of plant community, and based on the “big leaf” assumption. The model has been parameterized by using physiological and structural data collected, during different field campaigns, on the main woody species in the Rome metropolitan area. The simulation was run for the years 2003-2004, characterized by different climatic conditions that affected plant functional performance. In the year 2003, the values of total ozone yearly removed by evergreen broadleaves, deciduous broadleaves and conifers in Rome municipality were 103.9, 144.4 and 62.7 ton, respectively. In the year 2004, instead, the three functional groups removed 77.4, 151.7, 77.8 ton of O3, respectively. Instead, the maximum values of total PM10 removed by evergreen broadleaves, deciduous broadleaves and conifers were 418.3, 254.1 and 264.5 ton in 2003, and 415.2, 560.7 and 278.1 in 2004, respectively. These results are coherent with what reported in literature, showing that vegetation could contribute in improving air quality in urban areas. Moreover, they highlight the complementary role of the main tree functional groups in the air pollution removal in two years with very different climatic conditions: the extremely hot and dry year 2003, and the year 2004, which is more representative of the average long-term climatic pattern of the city of Rome.