Relaxed Eddy Accumulator for Flux Measurement of Nanometer-Size Particles

ABSTRACT

An investigation of the relaxed eddy accumulation (REA) technique to measure the flux of ultrafine (∼1 nm in diameter) aerosol particles using unattached radon progeny as a tracer and the construction of a prototype system based on the REA principle is reported. The system consisted of a sonic anemometer with a response frequency of 21 Hz, three screen/filter holders, a custom-built electronic circuit to control three electromagnetic inlet valves for sampling the up-, down- and neutral vertical winds, a high-capacity air blower and a portable PC. A 635-wire mesh screen/fiberglass filter combination was used in each intake to provide a separate measure of the unattached-to-aerosol and attached-to-aerosol radon progeny. The 9 cm-diameter 635 mesh screen, combined with an air flow rate of 230 L min⁻¹, resulted in 50% penetration for 2.7 nm-diameter particles. Corrections for a system response delay of 125 ms and the screen collection and alpha counting efficiencies were incorporated into the flux calculation. The prototype REA system was used during the summer/fall of 1996 at a semiarid site in central New Mexico. The sensitivity of the system was generally limited 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 higher than average. Comparing the measured fluxes for the unattached and attached modes, under the assumption that the deposition velocity for particles in the attached mode was zero and averaging out the effects of transient gradients in the radon and total aerosol concentrations, a deposition velocity for the unattached mode was deduced. Initial results for horizontal winds of 4 to 8 m s⁻¹ and an aerodynamic roughness length of 30 cm under varied atmospheric stabilities at a 4 m sampling 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.