Interpretation of eddy-correlation measurements of particulate deposition and aerosol flux

Abstract

Eddy correlation measurement of aerosol particle fluxes has recently become feasible with the advent of fast response particle measurement devices such as multichannel spectrometers based on optical scattering, nephelometers and electrical charge devices. Preliminary work in this area has identified several problems in data interpretation. Those investigated in this paper are: (1) particle equilibrium with ambient relative humidity, (2) errors caused by humidity fluctuations, (3) errors caused by horizontal wind fluctuations and (4) statistical validity of the data. Consideration of evaporation response time indicates that particles with radius less than roughly 1.0 μm are continually in equilibrium, but that when the environment is nearly saturated, equilibrium is a strong function of relative humidity. Errors in the vertical particle flux caused by humidity fluctuations are shown to be proportional to the saturation ratio flux, w′ S′. Over the ocean, this can lead to an apparent upward particle flux near the surface which when characterized by a dry deposition velocity, v d , is on the order of c 1 2 d u ∗ . An analysis of the statistical properties of particle-covariance averages indicates that the minimum uncertainty in dry deposition velocity is of the order of σ w / trN where σ 2 w is the vertical variance and N the total number of particles used to obtain v d .