Simplified Diagrammatic Representation of Ozone and Nitrogen Dioxide Formation. In the troposphere O3 formation is dependent on NOx, sunlight and VOCs. In the presence of UV light O3 undergoes photolysis and hydroxyl radicals can form hydroperoxyl radicals which then can react with O3 to form NO2. Also, NO2 can form O3 and the latter can react with NO to regenerate NO2. The formation process of the third pollutant investigated in this thesis is rather different; this is because unlike the aforementioned gaseous pollutants, PM10 is not a single molecular entity but consists of a range of chemical species which in turn are derived from different sources. Therefore, PM composition and size is dependent on primary emissions and subsequent atmospheric processing. PM can originate from natural and anthropogenic sources, primary particles are directly emitted as solids or liquids and examples of sources include burning of fuels, the wear and tear of tyres and brakes (anthropogenic sources) as well as sea salt and Saharan dust (natural sources) [18, 19]. Secondary particles are formed in the atmosphere by chemical reactions of NOx, VOCs and SO2 [18] that lead to new PM formation via condensation and nucleation reactions [19]. However, the formation of secondary PM requires a longer time frame and therefore within street canyons (e.g. Marylebone road due to building structure) direct particle emissions comprise the larger contribution of roadside’s increments [12]. PM’s secondary formation process is very nicely depicted by Xxxxxx [19] in Figure 2. Figure 2: ‘Atmospheric Cycling of Aerosols’ [19] A widely accepted characterization of PM is based on particle size. Particles with a 50% cut-off aerodynamic diameter of 10μm are referred as PM10. These particles are also known as thoracic because the inhaled mass fraction penetrates beyond the larynx to the air ways [20]. PM10 can be subdivided into coarse particles (PM2.5– PM10, diameter 2.5–10μm); fine particles (PM2.5, diameter <2.5μm); and ultrafine particles (UFPs, diameter <0.1μm) [21]. Each fraction is thought to represent different formation mechanisms, typical urban PM can be conceptually represented by having a trimodal distribution which reflects the size and composition of the particle [22]. The coarse mode particles (diameter >1 μm) are produced by mechanical and resuspension processes, this size is associated with reduced atmospheric visibility and due to their larger size they are subject to gravitational influence. Coarse particles have been shown to consist almost completely of non...
