THE TEMPERATURE OF EVAPORATING SEA SPRAY DROPLETS

Evaporating sea spray droplets are often assumed to be at the temperature of a well-ventilated wet-bulb thermometer, T-wet. Although this assumption may be accurate enough in practice, it is incorrect on theoretical grounds. Spray droplets have curved surfaces, they contain dissolved salts, and they may be small enough that the air and water vapor surrounding them do not behave as continuous fluids. Each of these characteristics of aqueous solution droplets can potentially affect vapor exchange at a droplet's surface and, thus, its temperature; but the wet-bulb temperature accounts for none of these. This paper uses a full microphysical model to accurately predict the evaporating temperature, T-ev, of pure and saline droplets to investigate how close T-wet is to this temperature. In general, T-wet is within 0.2 degrees-0.3 degrees C of T-ev for droplets with salinities from 0 to 40 psu when the droplet radius is 10 mu m or greater. When the droplet radius is less than 10 mu m, however, T-wet can underestimate T-ev badly, especially for higher air temperatures. To provide accurate estimates of T-ev quickly, the paper describes an algorithm that predicts T-ev to within 0.3 degrees C of the temperature predicted by the full model for droplets with radii from 0.5 to 500 mu m when air temperatures are from -10 degrees to 30 degrees C, relative humidities are from 80% to 97.5%, and droplet salinities are from 0 to 40 psu.