Summertime re-circulations of air pollutants over the north-eastern Iberian coast observed from systematic EARLINET lidar measurements in Barcelona

Regular aerosol backscatter measurements using an elastic-backscatter lidar were performed from May 2000 to December 2002 in Barcelona (Spain) in the frame of EU EARLINET Project (European Aerosol Research Lidar Network). Vertical profiles retrieved in a regular schedule confirmed the presence of multiple aerosol layers of regional origin above the mixing layer during numerous measurement days. Analysis of the meteorological situation of the corresponding days showed common synoptic regimes, which are typical in summertime around the region. As documented by earlier studies, under strong insolation and weak synoptic forcing, sea breezes and mountain-induced winds develop to create re-circulations of pollutants along the eastern Iberian coast. Layers are formed when aerosols are injected from the mountains into the return flow at various heights and distances from the coast. Aerosol layers can be found above the mixing layer up to 4000 m with variable thickness typically ranged between 100 and 1000 m. The mixing height mainly oscillates between 400 and 800 m asl in periods of maximum insolation. Four selected episodes are analysed combining lidar profiles, radiosoundings and synoptic meteorology information. One of them includes regional re-circulation aerosols at low levels and upper Saharan dust layers. Maximum backscatter coefficients ranged from 1×10−6 to 2×10−6 (m sr)−1 at the wavelength of 1064 nm. Assuming a lidar ratio of 30 sr (60 sr for Saharan dust), aerosol optical depths above the mixing layer ranged from 0.016 to 0.073. Detailed mesoscale analysis of one episode is performed by means of high resolution modelling with the PSU/NCAR Mesoscale Model 5 (MM5).