Relaxed eddy accumulation system for size-resolved aerosol particle flux measurements

A novel relaxed eddy accumulation ( REA) system for aerosol particle flux measurement has been developed and tested. The system consisted of a fast-response sonic anemometer, a flow system, and software for operating the valves and the concentration analysis system. The prototype was used during September - October 2001 at the SMEAR II station of the University of Helsinki. The REA system was operated with a varying threshold for valve switching determined by the running mean standard deviation of the vertical wind speed. Such a varying threshold made the flux proportionality coefficient beta independent of observation conditions. Using temperature as a tracer, beta was determined to be 0.392 +/- 0.002. The system was validated by comparing the carbon dioxide fluxes estimated by REA with the ones measured by the eddy covariance technique. The system was used subsequently for flux measurements of 50-nm aerosol particles and deposition velocity estimation. Observed deposition velocities over a pine forest during the autumn season were on the average 0.43 +/- 0.06 ( standard error) cm s(-1), which is higher than the earlier model estimates for forest canopies. Deposition velocity was dependent on the turbulence level and stability. To the authors' knowledge, no direct experimental data of deposition velocities on this size range is available in the literature.