Concentration Patterns Sample Clauses

Concentration Patterns. There are certain variables the most important of which are season, location and time that can be predictive of the air pollutant’s levels. Close to roadsides and on weekdays PM10 and NO2 concentrations tend to be high as there is an increased volume of traffic [18]. In contrast, O3 levels tend to be higher at background sites [34] and the O3 ‘weekend effect’ term has been used often to describe the higher O3 levels at that time of the week. This increase can be attributed to higher VOC/NOx ratio [35], however there is variability that is dependent on the NO2/NO ratio [36]. In terms of season, NO2 and O3 have well documented opposing patterns that are directly related to their photochemistry. Specifically, in winter, NO2 levels present a zenith as days are shorter and there is lower solar energy and OH concentrations and heating related emissions are higher. In contrast, in summertime higher temperatures and sunlight lead to a peak of O3 levels [37]. Equally, these gaseous pollutants have opposite diurnal patterns; O3 slowly increases during sunrise, reaches a peak in the afternoon and levels persist to late afternoon [38]. In contrast, NO2 has two main concentration waves, a morning peak and an evening rush hour peak [38]. However, more diverse patterns may be observed during atypical meteorological conditions and pollution sources. PM10’s seasonal and diurnal pattern is similar to NO2’s levels, as it also presents higher concentrations during winter months due to lower ambient temperatures shifting the semi-volatile organic compounds from the gas phase into the particulate phase and due to higher emissions. In addition, during rush hour there is a peak due to increased primary emissions, though PM10 episodes usually occur in summer months and tend to be the result of long range transport upsurge and secondary particle formation due to increased photochemistry and stagnation [39].