Long range transport of smoke particles from Canadian forest fires to the Mediterranean basin during June 2013

The first Special Observation Period (SOP1) of the ChArMEx (Chemistry-Aerosol Mediterranean Experiment) campaign occurred from June 12 to July 5, 2013. We discuss measurements carried out in the central Mediterranean sector at the ENEA Station for Climatic Observation on the island of Lampedusa (35.5°N, 12.6°E, Italy), at the Global Atmospheric Watch (GAW) station on the island of Gozo (36.0°N, 14.3°E; Malta), at Capo Granitola station along the southern coast of Sicily (37.6°N, 12.6°E) and onboard the ATR42 French instrumented aircraft. Several aerosol layers at various altitudes were detected by lidar at Lampedusa, one of the ChArMEx supersites, throughout June 27 and in the early part of June 28. A first dense layer between 2 and 4 km altitude, associated with transport of desert dust from Africa, was present in the morning of June 27 and disappeared around noon. Around midday, several thin layers started to appear between 5 and 12 km, and down to 2 km afterwards. The aerosol optical depth (AOD) peaked at about 0.45 around noon. The occurrence of the layers above 2 km was associated with a progressive increase of the Ångström exponent (AE), revealing the growing amount of small sized particles. The AE exceeded 1.8 in the late afternoon. Radiosonde and microwave radiometer profiles show a very dry mid troposphere, with a moderate increase in relative humidity in correspondence with the aerosol layer at about 11 km. The low humidity suggests the intrusion of upper tropospheric airmasses in the mid troposphere. Multiwavelength lidar backscattering, depolarization, and color ratio profiles, as well as their morphological evolution, suggest that the layers between 2 and 12 km occurring in the afternoon are composed by fine aerosols and are not due to cloud particles. Backward trajectories show that the airmasses containing the fine particles originated from North America. They overpassed Canada in correspondence with intense wildfires occurring on June 22 and 23, as also revealed by MODIS, OMI, and CALIOP images and data. The circulation systems of those days was favorable to the long range eastward transport of these aerosol plumes towards Europe, down to the Mediterranean Sea. A further confirmation of the advection of Canadian airmasses over the Mediterranean central region during this period is provided by measurements made at Gozo. The Gozo AERONET Cimel data analysis on June 27 shows a single scattering albedo with spectral characteristics typical of biomass burning, with the contribution of fine particles to the total AOD growing progressively up to 92% in the early afternoon. At Capo Granitola the AOD peaked at about 0.8 in the late afternoon of June 27, with elevated values (> 2) of the AE. The ATR42 overflew the area on those days and additional observational evidence will be sought in its data.