RELAXED EDDY ACCUMULATOR FOR FLUX MEASUREMENT OF NANOMETER-SIZE PARTICLES

An investigation of the relaxed eddy accumulation (REA) technique to m easure the flux of ultrafine (similar to 1 nm in diameter) aerosol par ticles using unattached radon progeny as a tracer and the construction of a prototype system based on the REA principle is reported. The sys tem consisted of a sonic anemometer with a response frequency of 21 Hz , three screen/filter holders, a custom-built electronic circuit to co ntrol three electromagnetic inlet valves for sampling the up-, down- a nd neutral vertical winds, a high-capacity air blower and a portable P C. A 635-mire mesh screen/fiberglass filter combination was used in ea ch intake to provide a separate measure of the unattached-to-aerosol a nd attached-to-aerosol radon progeny. The 9 cm-diameter 635 mesh scree n, combined with an air flow rate of 230 L min(-1), resulted in 50% pe netration for 2.7 nm-diameter particles. Corrections for a system resp onse delay of 125 ms and the screen collection and alpha counting effi ciencies were incorporated into the flux calculation. The prototype RE A system was used during the summer/fall of 1996 at a semiarid site in central New Mexico. The sensitivity of the system was generally limit ed by the statistical counting error of the radioactivity collected on the screens. The technique was found most practical under conditions where both the ultrafine particle flux and radon concentration were hi gher than average. Comparing the measured fluxes for the unattached an d attached modes, under the assumption that the deposition velocity fo r particles in the attached mode was zero and averaging out the effect s of transient gradients in the radon and total aerosol concentrations , a deposition velocity for the unattached mode was deduced. Initial r esults for horizontal winds of 4 to 8 m s(-1) and an aerodynamic rough ness length of 30 cm under varied atmospheric stabilities at a 4 m sam pling height suggest corresponding deposition velocities for ultrafine particles in the range of 5 to magnitude 35 cm s(-1) These values are higher than predicted by some commonly used dry deposition models. (C ) 1998 American Association for Aerosol Research.