Characterization of forest fire and Saharan desert dust aerosols over south-western Europe using a multi-wavelength Raman lidar and Sun-photometer

Abstract

In the morning of 21 July 2019, a dense forest fire smoke plume was detected over ´Evora (Portugal) in the lower troposphere. Around 13:00 UTC, a Sahara dust layer was detected by lidar between 3000 and 4000 m height above sea level. In this work results of these events using an EARLINET Raman lidar and AERONET Sunphotometer measurements are reported and discussed: the dense smoke layer, the mixing of smoke with dust and the lofted dust layer that crossed ´Evora. During the morning, in-situ hourly mean black carbon concentration reached a maximum of 6.28 ± 0.73 μgm􀀀 3. The AERONET fine mode aerosol optical depth at 500 nm reached 0.77 and the Ångstr¨om exponent at 440–870 nm varied between 1.45 and 1.82, indicating the presence of fine smoke particles in the atmospheric column. In the beginning of the afternoon, the lidar profiles indicated the presence of a smoke layer followed by two thin layers of pollution mixed with dust and a lofted dust layer. The smoke particles caused low particle linear depolarization ratios at 532 nm with mean value of 0.059 ± 0.002 in the smoke layer. In contrast, the lofted dust layer caused mean particle linear depolarization ratio at 532 nm of 0.26 ± 0.01. Mass concentrations were in the order of 22–29 μg m􀀀 3 in the smoke layer and 60–178 μg m􀀀 3 in the lofted dust layer. During the afternoon and at night, mixing dust layers were identified by using lidar data. In the afternoon, two aerosol layers exhibited mean particle linear depolarization ratios at 532 nm of 0.23 ± 0.01 and 0.25 ± 0.01 with the dust dominating the extinction and mass concentration in both aerosol layers. Mass concentrations of dust were in the order of 101–266 μg m􀀀 3 and 159–195 μg m􀀀 3. At night, the particle linear depolarization ratio presented low variability across the identified layer with mean value of 0.20 ± 0.01. The Ångstr¨om exponents were low, the mean lidar ratios were 50 ± 6 sr (355 nm) and 42 ± 4 sr (532 nm) and the aerosol optical depth was 0.32 (355 nm) and 0.30 (532 nm). Mass concentrations ranged in 10–28 μg m􀀀 3 for non-dust and in 77–186 μg m􀀀 3 for dust.