Daily Cycles in Urban Aerosls Observed in Florence (Italy) by means of an Automatic 532-1064 nm Lidar

An unattended Lidar operating at 532-1064 nm was used for the continuous monitoring of the Planetary Boundary Layer (PBL) of Florence (Italy). This was the first time that such a well-established remote-sensing technique has been used in Italy as a monitoring tool for the long-term study of urban aerosols: time-height-backscatter plots were used for the interpretation of the PBL dynamics and the vertical distribution of the aerosols during the summer period, while the aerosol backscatter and its wavelength-dependency were used to estimate the mass concentration and the median size of the aerosols 40 m above the ground. The marked wavelength- dependency shown by traffic-related aerosols permitted a distinction between fresh urban and background/rural aerosols. It is the first time that such a well known remote-sensing technique is used for the long-term monitoring of urban aerosols properties. In summer, the daily cycles of aerosol mass concentration and the backscatter Ångström coefficient showed well-defined and very similar daily cycles, with a marked peak in the morning (of just three hours duration) and shallow minima in the afternoon. The evidenced morning mass peak was produced by small particles (0.05-0.1 um estimated mode diameter), while there was a prevalence of larger particles of rural origin in the afternoon. The observed cycles were the result of the coupling between the traffic cycle and the daily surface-wind cycle: fresh combustion aerosols produced in town were advected above the Lidar in the morning by the persisting night-time breeze, while a vertical dilution of urban aerosols occurred in the afternoon due to the development of the mixed layer. The reversal of the breeze in the late morning, induced by the local topography, was also responsible for the low aerosol concentration observed in the afternoon: rural air containing a low concentration of relatively large aerosols was advected above the Lidar, and dilution of urban aerosol through turbulent mixing occurred. By assuming an exponential decay of aerosol concentration with height, the Lidar- derived scaling height showed a minimun (100-200 m) in the afternoon,in coincidence with the advection of rural air from the suburbs. The mass concentration variations were found to be highly correlated with traffic at night and in the early morning, when the local breezewind brought the urban plume above the Lidar site. The high temporal resolution of the LIDAR made it possible to follow the aerosol mass variations with a 5 min resolution, fast enough to follow the rapid aerosol peak evolution in the morning: the peak mass concentration was 5-10 times larger than during the rest of the day, suggesting the inadequacy of the daily-averaged aerosol mass measurements at present carried out by local authorities. The mass variations were superimposed over a slowly-varying background mass concentration of unknown origin. An increase in aerosol size and a decrease in mass concentration were observed in the afternoon, because of the advection of rural air coupled with vertical mixing. The same rapidity is not achieved by the local aerosol monitoring samplers.