Differences between eddy correlation fluxes for discrete and continuous scalars: applications to sea-spray aerosol fluxes

Almost all sea-spray source functions have been derived from indirect measurements with only a handful of recent studies having attempted direct eddy correlation measurements of the sea salt particle flux. The first direct eddy correlation measurements of fully size segregated aerosol fluxes over the open ocean was made during the UK SOLAS SEASAW cruise in the spring of 2007 on the RRS Discovery in the North Atlantic Ocean. The eddy correlation measurements of sea spray particles were made using the aerosol probe, CLASP, and a sonic anemometer alongside eddy correlation measurements of water vapor, CO₂, and Ozone.

Individual aerosol ogive functions were inspected to ensure that the cospectra were well behaved at the lowest frequencies included in the flux estimates. It was found that there is greater variability between individual aerosol flux estimates than the momentum flux, and substantially greater variability between the flux contributions from different frequencies within a given averaging period. The high degree of variability in the aerosol flux estimates can be ascribed to the inherently discrete nature of the surface source of particles – individual whitecaps or bubble plumes on the sea surface. The spatial separation of breaking wave crests may be of the order of 100 meters, this imposes a strong, irregular heterogeneity on the surface source, on scales similar to those of the dominant flux-carrying eddies, that does not exist for the flux of momentum or scalars such as heat, water vapor, or trace gases.

We will examine the effects of this variability on the aerosols fluxes along with the effects of relative humidity and particle deposition.