Evolution of the vertical profile and flux of large sea-salt particles in a coastal zone

Abstract

In the vicinity of the North Carolina Outer Banks we observed both steady onshore flow conditions and a continental air mass transition into a marine boundary layer. Using the CIRPAS Twin Otter aircraft, we measuredc hangesin the columnb urden of sea salt as the air mass was advected out to sea. We also measured the flux of whitecap-generatesde a-saltp articlesi n neutrallys tablea tmospherea t wind speedso f 4, 8, and1 2r n s- •. Productioonf saltp articleass s malla s0 .27/•mi n diametewr aso bserved. Furthermore,w e measureds alt particle size distributionsa t variousw ind speedsd uring alongs horew ind and near steadys tate conditionsU. sing thesem easurementsa s a frame of reference,w e discussth e very large differencesi n the reported size and flux of sea salt presentedi n the literature. The disagreemenitn reported salt fluxesi s larger for smallersizedp articles( almosta n order of magnitude)a nd is most likely due to assumptionms ade when the fluxesw ere computed,e speciallyt he particle dry depositionv elocitya nd air mass history.H owever,f or giant salt particlesw ith short atmosphericli fetimes (>-10/•m in diameter),t here is generala greementb etweenf luxesa nd size distributionsm easuredi n this studya nd previouso nes.R eported salt particle size distributionsin the literature also vary considerablyu nder similar steadyw ind and stability conditions.F rom these and our results it is clear that no more than half of the variance in salt particle concentration can be explainedb y wind speeda lone, suggestingth at the idea of "steadys tate" in the marine boundary layer rarely exists at midlatitudes.