Climatology of Aerosol Radiative Properties in the Free Troposphere

High altitude mountaintop observatories provide the opportunity to study aerosol properties in the free troposphere without the added expense and difficulty of making airborne measurements. Climatologies for free tropospheric aerosol radiative properties in cloud-free air, including light scattering, light absorption, light extinction, single scattering albedo, Ångström exponent, hemispheric back-scatter fraction and radiative forcing efficiency, from fourteen high altitude (2.2-5.1 km) measurement platforms are presented. These climatologies utilize data from twelve mountaintop observatories in the 20-50 N latitude band and two multi-year, in-situ aerosol vertical profiling programs. The amount of light absorption and scattering observed at these high altitude sites either peaks in the spring or it has a broad spring to summer enhancement. The seasonal variation of the aerosol single scattering albedo, back-scatter fraction and Ångström exponent changes from site to site but the timing can be related to aerosol sources known to impact the individual sites. The seasonal variation of in-situ aerosol light extinction from these high altitude measurements is in excellent agreement with extinction values derived from CALIPSO lidar measurements. There are systematic relationships among aerosol properties related to source types and atmospheric processing which could help constrain model parameterizations.