Transported Smoke from Africa over the Southeast Atlantic Ocean from CALIOP Version 4 Data: Signature of an Increase in Size of Smoke Particles

The spaceborne lidar CALIOP has been providing high quality profile information on aerosols and clouds since June 2006. Recently significant improvements in the calibration of the lidar have been achieved in both the 532 nm and 1064 nm channels with the release of the version 4.1 (V4) level 1 data products. The V4 level 2 algorithms, which were subsequently optimized to take full advantage of the improved calibrations, are expected to lead to a better representation of optical properties of the various tropospheric aerosol species. In particular, there was a significant anomaly in earlier data versions on the classification of aerosol layers over the Southeast Atlantic Ocean, where many smoke layers transported from the biomass burning areas of Southern Africa were misclassified as marine layers. We show that many more layers in V4 are now classified as smoke layers as expected. Further, we show evidence of these smoke particles increasing in size, as demonstrated by an increase in their particulate color ratios, as they are transported over the Southeast Atlantic Ocean from the source regions over Southern Africa. We will present seasonal and inter annual variation of this effect using the 12 years of CALIOP data. The depolarization measurements from CALIOP as well as from the CATS instrument aboard International Space Station will also be presented. We hypothesize that this inferred size increase is possibly due to hygroscopic swelling of the smoke particles which is reflected in the higher relative humidity in the middle troposphere for profiles with smoke. Mie scattering calculations indicate that the increase in particulate color ratio of smoke particles from land (~0.7 at 3 km) to ocean (~0.9 at 3 km) is consistent with a mean radius change of ~0.03 μm, which is similar to the increase in fine mean radius (0.02 µm) observed between the AERONET stations in Mongu and Ascension Island.