Transport of aerosols over the French Riviera – link between ground-based lidar and spaceborne observations

For the first time, a 355 nm backscatter N2-Raman lidar has been deployed on the western part of the French Riviera to investigate the vertical aerosol structure in the troposphere. This lidar system, based at the AERONET site of Toulon–La Garde, performed continuous measurements from 24 June to 17 July 2014, within the framework of the multidisciplinary program Mediterranean Integrated Studies at the Regional and Local Scales (MISTRALS). By coupling these observations with those of the spaceborne instruments Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP), Spinning Enhanced Visible and InfraRed Imager (SEVIRI), and Moderate Resolution Imaging Spectroradiometers (MODIS), the spatial extents of the aerosol structures are investigated. The origins of the aerosol plumes are determined using back trajectories computed by the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT). This synergy allowed us to highlight plumes of particulate pollutants moving in the low and medium free troposphere (up to ∼5 km above the mean sea level) towards the French Riviera. This pollution originates from the Spanish coast, more particularly from Costa Blanca (including Murcia) and Costa Brava–Costa Daurada (including Barcelona). It is mainly due to traffic, but also to petrochemical activities in these two regions. Desert aerosol plumes were also sampled by the lidar. The sources of desert aerosols have been identified as the Grand Erg Occidental and Grand Erg Oriental. During desert dust events, we highlight significant differences in the optical characteristics in terms of the backscatter-to-extinction ratio (BER, inverse of the lidar ratio) between the planetary boundary layer, with 0.024 sr−1 (∼42 sr), and the free troposphere, with 0.031 sr−1 (∼32 sr). These differences are greatly reduced in the case of pollution aerosol plume transport in the free troposphere (i.e., 0.021 and 0.025 sr−1). Transported pollution aerosols appear to have similar BER to what is emitted locally. Moreover, using the correlation matrix between lidar aerosol extinction profiles as a function of altitude, we find that during transport events in the low free troposphere, aerosols may be transferred into the planetary boundary layer. We also note that the relative humidity, which is generally higher in the planetary boundary layer (>80 %), is found to have no significant effect on the BER.