Impact of the transport of aerosols from the free troposphere towards the boundary layer on the air quality in the Paris area

We propose a quantification of the downward transport of aerosols from the free troposphere (FT) to the planetary boundary layer (PBL). Aerosols are originally released at the surface as a consequence of anthropogenic activities, biomass burning, soil mobilization, etc. After being vertically transported into the FT, they are exposed to the long-range transport (LRT) and may subside to impact, in turn, surface air pollution in remote places. Using 5400 h of routine Lidar observations conducted at the SIRTA observatory in the Paris area (France), we identified 154 free tropospheric aerosol layers continuously monitored during their downward transport into the local PBL. One of these events - associated to a Saharan dust outbreak - is thoroughly documented in a case study. And a climatological analysis of surface PM10 levels recorded at air quality monitoring stations allows the impact of FT to PBL transport of aerosols to be quantified. This source is found to be significant for 15 out of the 16 stations, with average PM10 concentrations 2.14 mu g m(-3) (i.e. 12%) above climatological values after the injection of free tropospheric aerosols into the PBL. (c) 2007 Elsevier Ltd. All rights reserved.