Daily modulation of PAHs in the atmosphere

On the long period the presence of the pollutant sources together with the prevailing weather conditions heavily influences the level of atmospheric pollution affecting a certain area. At daily scale, atmospheric stabilization and mixing processes in the boundary layer regulate the load of pollutants through a mechanism of dilution or concentration and this effect can overtake the sources input to the pollution. Moreover wide differences according to the daytime can be observed especially in the summer with regard to the atmospheric reactivity. Several investigations have been carried out concerning the day/night modulation of particle bound atmospheric PAHs through 12-hours sampling, but shorter time ranges were rarely investigated. In this study the daily PAH trends was observed through 6-h sampling intervals. Samples were collected in winter and summer. Several in field campaigns were carried out with sampling starts fixed at different times in order to better observe the PAH modulation. Similarly to that of other air contaminants, also the PAH daily modulation is affected by vertical mixing processes developing in the atmosphere. Comparing the night-time samples to the day-time ones, both in the cold and the warm seasons, the crucial impact of atmospheric stability is confirmed by the high PAH concentrations recorded during the night when traffic source is almost absent, and conversely by the relatively low values observed in the day time, i.e. just when the effect of the emission sources is more relevant, but the atmospheric turbulence increases. Besides, the ratio between the concentrations of benzo(e)pyrene and benzo(a)pyrene was used in the perspective of assessing the relative contributions of the emission sources and physical processes on the PAHs concentrations. This ratio is widely used to evaluate the atmospheric reactivity and looks as suitable for investigating the degradation rate ("ageing") underwent by atmospheric particulates. The BeP/BaP ratio always grows during the hours of the day characterized by high heating, which suggests that the air vertical mixing neutralizes the impact of intense fresh emission. On the contrary, during the hours of air stability the pollutants are accumulated and trapped near the ground and the BeP/BaP ratio decreases. Due to meteorological conditions typical of the period, the BeP/BaP ratios are lower in the winter than in summer, however it is important to underline that atmosphere presents oxidative properties even in the cold season; this is shown by the day/night BeP/BaP differences recorded.