Production velocity of sea spray droplets

[1] The sea spray generation function dF/dr₀ predicts the rate at which droplets of initial radius r₀ are produced at the sea surface. Because this function is not readily measurable in the marine environment, however, it is often inferred from measurements of the near-surface droplet concentration, C(r₀), through an assumed velocity scale, the effective spray production velocity. This paper proceeds in reverse, though: It uses a reliable estimate of dF/dr₀ and 13 sets of measurements of C(r₀) over the ocean to calculate the implied effective production velocity, V_eff, for droplets with initial radii r₀ from 5 to 300 μm. It then compares these V_eff values with four candidate expressions for this production velocity: the dry-deposition velocity, V_Dh; the mean wind speed at the significant wave amplitude (A_1/3), the standard deviation in vertical droplet velocity, σ_wd; and laboratory measurements of the ejection velocity of jet droplets, V_ej. The velocity scales and V_ej agree best with the implied V_eff values for 20 ≤ r₀ ≤ 300 μm. The deposition velocity, V_Dh, which is the velocity most commonly used in this application, agrees worst with the V_eff values. For droplets with r₀ less than about 20 μm, the analysis also rejects the main hypothesis: that dF/dr₀ and C(r₀) can be related through a velocity scale. These smaller droplets simply have residence times that are too long for spray concentrations to be in local equilibrium with the spray production rate.