Whitecap aerosol productivity deduced from simulation tank measurements

Abstract

An estimate of dF 0 / dr , the sea spray aerosol production per increment droplet radius per unit time per unit area of the ocean, has recently been obtained from a term‐by‐term evaluation of the following expression: dF 0 / dr = dE /drτ −1 W ( U ). The differential whitecap aerosol productivity, dE / dr , the number of aerosol droplets per increment droplet radius produced during the decay of a unit area of whitecap, has been redetermined, for the size range 0.8 μm < r < 8 μ, from the results of recent experiments in the University College, Galway, whitecap simulation tank in which Particle Measuring Systems aerosol spectrometers were used. The characteristic decay time for whitecaps, τ, and the initial whitecap area were in the present study determined from the frame‐by‐frame analysis of cine‐film recordings of whitecaps generated in the simulation tank. The fraction of the sea surface covered by whitecaps W at wind speed U was taken from Monahan and O'Muircheartaigh (1980). The resulting expression, representing the local sea surface aerosol source function in units of m −2 s −1 μm −1 appropriate for inclusion in any model for the prediction of the vertical profile of aerosols within the marine boundary layer, is given by dF 0 / dr = 5.77 × 10 −6 U 3.41 (Δ dN / dr ), where the last term represents the change in aerosol concentration per increment droplet radius, expressed in m −3 μ −1 , that occurs in the air within the hood over the tank when an individual whitecap decays.