Aerosol Optical Properties in the Boundary Layer over the Remote Southern Ocean

Krista Laursen, Darrel Baumgardner, and Bruce Morley

In November and December, 1995, the NCAR C-130, as part of the Aerosol Characterization Experiment (ACE-1) carried a complement of particle measurement sensors to the Southern Hemisphere and made measurements in the boundary layer and free troposphere over the ocean to the south and west of Tasmania. One of the questions being addressed by the experiment was how aerosols and humidity interact to modify the radiative balance of the atmosphere. The complement of aerosol and radiation sensors on board the aircraft provide the means to measure some aerosol optical properties directly and to combine the measurements and derive other properties indirectly. A 0.536 and 1.06 um wavelength backscatter lidar was operated in both upward and downward pointing directions. From these measurements we have a direct measure of the aerosol backscatter component. Comparisons of the directly measured backscatter component with backcsatter calculated from in situ aerosol size distributions in vertical profiles provides a means of deriving particle refractive index and to evaluate how backscatter changes with humidity and particle size. The optical depth is derived using an empirical relationship between backscatter and extinction that is found from in situ measurements. Finally, the vertical profiles of extinction derived from the lidar is used in a model to predict the amount of downwelling radiation that should reach the sea surface and then compared with measurements of this radiation made with a hemispheric radiometer.

The observations show a great deal of structure in the optical depth of the boundary layer with a clear humidity dependency as particles deliquesce and enlarge as they take on water. The predicted short wave radiation is in very good agreement with that measured with the radiometer.