Impact of sea breeze on vertical structure of aerosol optical properties in Dunkerque, France

During July 2008, we used an elastic backscattering LIDAR to monitor the aerosol vertical distribution at the coastal area of Dunkerque, France. Here we report the sea breeze event which was observed with more highlighted effect of aerosol on 25 July. By combining LIDAR measurements with Sun photometer-retrieved aerosol optical thickness, we estimated an average LIDAR ratio of 33 sr (± 14 sr) for the estimation of aerosol extinction profiles during the sea breeze. The LIDAR derived aerosol extinction in the first 200 m is clearly affected by the sea breeze and increases by more than 100% at the time of sea breeze arrival. A sharp convective boundary layer height decrease is observed in the LIDAR data due to the formation of the thermal internal boundary layer in the lowest part of the sea-to-land flow. PM2.5 concentration increases due to the thermal internal boundary layer formation and reaches its maximum between 1 and 2h after the front overpass. Except during the front overpass, the PM2.5 is well correlated to the inverse of the mixing height detected by the LIDAR.

Research highlights
► LIDAR aerosol extinction profiles reveals the drastic increase in PM10 and PM2.5 due to the sea breeze onset.
► The sea breeze reduces the mixed layer depth and consequently increases PM2.5.
► Before and after the sea breeze, PM2.5 diurnal variability is explained by the vertical development of the mixing layer.