Deliquescence induces eddy covariance and estimable dry deposition errors

The effects of deliquescent processes on eddy covariance estimates of dry deposition are considered, both theoretically and using some representative data. Turbulent and latent heat fluxes near the Earthʹs surface imply a vertical “saturation ratio flux” (Fairall, C.W., Atmospheric Environment, 18 (1984) 1329). Deliquescence, instantaneous relative to turbulent time-scales, responds to saturation ratio (or relative humidity) fluctuations and induces covariance between vertical winds and size-resolved number concentrations of hygroscopic aerosols. The induced covariance represents an error in the estimation of surface exchange by direct application of the eddy covariance technique. Under deliquescent conditions (high relative humidity), resulting errors in dry deposition estimates are shown to be often as large as typical deposition velocities reported for small particles, depending on the shape of the number distribution and the magnitudes of heat and vapor fluxes in the boundary layer.