Modelling Human Exposure to Air Pollutants in an Urban Area

http://www.revistadechimie.ro 61 Road traffic is one of the main sources of atmospheric pollution in urban areas, where the contribution of traffic flows, congestions and high population density typically increases the exposure of the settled population to pollutants [1-3]. One of the most important family of traffic-related pollutants is represented by nitrogen oxides (NOx), whose main contributors to their emissions in Europe are the mobile sources (38.4%), followed by the energy production sector (21.1%), the commercial, institutional and household sectors (14.8%), energy use in industry (13.4%) and other minor sectors (12.3%) [4]. Unlike nitrogen monoxide (NO), nitrogen dioxide (NO2) produces adverse effects on human health, such as the development of bronchitis, pneumonia, asthma and the pulmonary growth reduction in chronically exposed adults and children [5, 6]. To preserve the human health, the World Health Organization (WHO) established a limit value of 40μg m-3 for the annual mean concentration of NO2. An hourly threshold of 200 μg m-3 was also set in order to prevent the effects of acute exposure, such as medium airways inflammations and amplification of allergic reactions [5]. Since emitted NO tends to be rapidly converted to NO2, also within the O3 chemical reactions forced by solar radiation [5], NO2 is commonly found near the emission sources. Thus, NO2 can be considered a typical indicator of air pollution from road traffic in urban areas. Critical situations for European urban areas (such as intense road traffic and particular meteorological conditions due to the complex urban structures) make the compliance of the imposed limits a difficult task. The compactness of urban fabrics, the presence of streets with highly developed buildings in the vertical dimension and the lack of open spaces favour the stagnation of pollutants and increase the population exposure. The structuring elements of the urban fabric are the so-called street canyons, intended as narrow roads laterally delimited by two continuous rows of buildings. At this spatial scale, the street canyon represents the basic geometric unit of the urban fabric and defines the vertical dimension of the urban canopy layer, which is the atmospheric layer included between the soil and the roofs [7], after which the urban boundary layer starts to develop. The canyon length (L), its width (W) and the height (H) of the buildings are relevant parameters for the dispersion of pollutants [8].