Daily cycles in urban aerosols observed in Florence (Italy) by means of an automatic 532–1064 nm LIDAR

An unattended light detection and ranging (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, 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. In summer, the aerosol mass concentration and the backscatter Ångström coefficient showed well-defined and very similar daily cycles with a marked peak in the morning and shallow minima in the afternoon. The morning mass peak was produced by small particles (0.05–0.1 μm mode diameter), while there was a prevalence of larger particles in the afternoon. The observed cycles were the result of a coupling between the traffic cycle and the daily surface-wind cycle: fresh urban aerosols produced in town were advected above the LIDAR in the morning by the persisting nighttime breeze. The reversal of the breeze in the late morning was responsible for the low-aerosol concentration observed in the afternoon, when rural air containing a low concentration of relatively large aerosols reached 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 minimum (100–200 m) in the afternoon, in coincidence with the advection of air from the suburbs. The mass concentration variations were found to be strictly correlated with traffic at night and in the early morning, when the local breeze 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 carried out at present by the local authorities.