It is not clear which factors (i.e., PM mass concentration, number concentration, biological or chemical composition [7], sellectchem physical properties, mass burden, particle number, total area, or electrostatic characteristics [8]) have the most crucial influence on human health. Nevertheless, the population exposed to PM always demonstrates adverse health effects.Particles with aerodynamic diameters of between 10?3 and 100��m can occur everywhere in the ambient air. The number of particles with specific size present at a given site depends on many factors. These include the origin of PM at the discussed site [9], atmospheric processes (condensation, nucleation, and evaporation), chemical transformations, deposition, and removal with precipitation.
It should be mentioned that particles with a diameter smaller than 100nm, known as ultrafine particles, dominate the number concentrations but do make a small contribution to total aerosol particle mass [10, 11]. They represent excess health risks relative to fine (d < 2.5��m) or coarse particles (10��m < d < 2.5��m) of identical or similar chemical composition [12].It is increasingly recognised that ultrafine particles can have significant implications on public health in addition to mass concentrations of particulate matter [10�C12]. This is because ultrafine particles can easily be inhaled and deposited in the deeper regions of the respiratory tracts and have a higher surface area per unit volume than larger particles, thus increasing their capability to adsorb organic compounds, some of which are potentially carcinogenic [13].
Current legislation in Europe [14] requires mass concentration measurements of the PM10 and PM2.5 (ambient particles with aerodynamic diameter �� 10 and 2.5��m, resp.), whereas particle number concentration (including ultrafine particles) and size distribution are not routinely measured in monitoring networks [13, 15].A number of studies described number concentration of PM in cities and urban surroundings [16�C26]. It is clear that traffic is the most important source of ultrafine particles [17, 27, 28]. Emissions from gasoline- and diesel-fuelled vehicles alone can contribute to up to approximately 90% of the total particle number concentrations [29]. Kumar et al. [16] reports a summary of recently published studies on atmospheric nanoparticles in European cities.
This covers a total of about 45 sampling locations in 30 different cities within 15 European countries for quantifying levels of roadside and urban background particle Anacetrapib number concentrations (PNCs). Average PNCs at the reviewed roadside and urban background sites were found to be 3.82 �� 3.25 �� 104 and 1.63 �� 0.82 �� 104cm?3, respectively, giving a roadside to background PNC ratio of ~2.4.Biomass burning in local sources and nucleation processes significantly influence the particle number.