Altitude Variability of Giant Sea-Salt Aerosol Size Distributions in the Layer Below Marine Stratocumulus Clouds over the SE Pacific during VOCALS

Giant sea-salt aerosol particles (a type of GCCN) form an important part of the aerosol size distribution in marine boundary layers.  Despite occurring in low concentrations, they may have a strong impact on the formation of warm rain in marine stratocumulus clouds.

During the 2008 VOCALS deployment of the NSF/NCAR C-130 in the SE Pacific, an external impaction system, the Giant Nuclei Impactor (GNI) was used to expose polycarbonate microscope slides in the free airstream outside the aircraft fuselage. This allowed for extensive analysis of GCCN size distributions of sea-salt particles with dry radius, 0.8 μm < r_d < 12 μm.

We examine the altitude variability of sea-salt GCCN (i) in the mixed layer, and (ii) in the decoupled cloud layer below upper-level marine stratocumulus clouds.  The sea-salt loading is often relatively well-mixed in the mixed layer, as also shown in prior studies (e.g. Lewis and Schwartz, 2004), but here we demonstrate a preferential reduction in GCCN mass loading in many de-coupled cloud-free layers; i.e. between the top of the mixed layer and the base of upper-level stratiform clouds.

GCCN may be lost as a result of cloud processing, either as precipitation sized drops or as smaller sized drops that sediment out of upper-level stratocumulus decks.  In both cases they change the subsequent ability of the clouds to develop precipitation through the GCCN mechanism.