Long-range transport and mixing of aerosol sources during the 2013 North American biomass burning episode: analysis of multiple lidar observations in the western Mediterranean basin

摘要

Long-range transport of biomass burning (BB) aerosols between North Americaand the Mediterranean region took place in June 2013. A large number ofground-based and airborne lidar measurements were deployed in the westernMediterranean during the Chemistry-AeRosol Mediterranean EXperiment (ChArMEx)intensive observation period. A detailed analysis of the potential NorthAmerican aerosol sources is conducted including the assessment of theirtransport to Europe using forward simulations of the FLEXPART Lagrangianparticle dispersion model initialized using satellite observations by MODISand CALIOP. The three-dimensional structure of the aerosol distribution inthe ChArMEx domain observed by the ground-based lidars (Minorca, Barcelonaand Lampedusa), a Falcon-20 aircraft flight and three CALIOP tracks, agreesvery well with the model simulation of the three major sources considered inthis work: Canadian and Colorado fires, a dust storm from western US and thecontribution of Saharan dust streamers advected from the North Atlantic tradewind region into the westerlies region. Four aerosol types were identifiedusing the optical properties of the observed aerosol layers (aerosoldepolarization ratio, lidar ratio) and the transport model analysis of thecontribution of each aerosol source: (i) pure BB layer, (ii) weakly dusty BB,(iii) significant mixture of BB and dust transported from the trade windregion, and (iv) the outflow of Saharan dust by the subtropical jet and not mixedwith BB aerosol. The contribution of the Canadian fires is the major aerosolsource during this episode while mixing of dust and BB is only significant atan altitude above 5 km. The mixing corresponds to a 20–30 % dustcontribution in the total aerosol backscatter. The comparison with the MODISaerosol optical depth horizontal distribution during this episode over the westernMediterranean Sea shows that the Canadian fire contributions were as large asthe direct northward dust outflow from Sahara.