THE FRACTIONATION OF ATMOSPHERIC COARSE AEROSOL BY A TUNNEL SAMPLER EMPLOYING SINGLE STAGE IMPACTORS

A wind tunnel sampler employing single-stage impactors has been used for the collection and size fractionation of ambient aerosols. Nine impactor strips, ranging in width from 1 to 25mm, each with a different cut-off diameter, together with a May impactor and a filer pack probe were installed in parallel within the tunnel. A fan at the tunnel outlet maintained a constant air flow within the tunnel and iso-axial sampling was achieved by a wind vane mounted on the tunnel. The size fractionation characteristics of the impaction strips were determined experimentally. At a flow rate of 8ms-1 the 50% cut-off diameter of the strips ranged from 7.8 to 38.8µm. The total mass of atmospheric sulphate and nitrate determined by a May cascade impactor was similar to that collected by a filter pack. The ambient size distributions measured by the tunnel sampling and May cascade impactor displayed a number of common features. It was found that, on average, 90% of the sulphate and 70% of the nitrate mass lies below 8 µm. Although well defined and reliable in terms of aerodynamic diameter the efficiency curves did not match the sharp cut-off behaviour of conventional cascade impactors. However, samples collected on impaction strips offer the advantages of easy handling, excellent means of particle examination by optical or electron microscopy and quantitative chemical speciation. The main disadvantage arises from overloading which limits the sampling time and consequently the aerosol mass collected.