Evaluation of aerosol pollution using a three device system.

The characterization of air pollution)in a particular area is connected with the acquisition of a series of statistically significant data. The actual case referred to is a sampling campaign developed in a station in an urban area (Rome), throughout a period of 12 months. For controlling the behaviour of aerosol pollution according to a most complete characterization the three following apparatuses have been employed: (a) continuous impact sampler (Hirst Spore Trap) (b) continuous filtration sampler (Aisi Sampler) (c) differential 4-stage sampler (Mammarella). Through comparison of data resulting from the three series of sampling it has been possible to analyze: (i) the behaviour of pollution in its complexity (ii) the rate of pollution due to coarse aerosol particles (iii) the median diameters of different fractions of aerosols. INTRODUCTION There are several methods for sampling and classifying air pollutants; in addition the types of samplers, when used to determine both aerosol or gaseous (vapour) pollution, may differ. With regard to aerosol sampling procedures, in particular, the best known and most frequently applied principles are the following: settling, impaction (both on liquids and solids), filtration and the application of thermal or electrical forces. Settling procedures are principally employed for characterizing coarse aerosols; in fact the more diameters of particles decrease, for a given mass value, the more the settling speed decreases. The collection efficiency for small particles is therefore more and more influenced by meteorological parameters (namely, temperature and wind). Consequently such a type of procedure may be considered useful only for aerosol particle sizes of over 30—40 microns. Higher efficiencies may be obtained however by exposing deposit gauges for long periods of time, utilizing in this case the washing action of rain. Impaction procedures, both on liquids and solids, exhibit a wide range of efficiencies, which depend principally on the suction speed and the diameter of suction openings. However, in contrast to the settling technique, an impaction sampling implies a volumetric measure of the inspired flow and, as a consequence, one may correlate the pollution levels with the flow volume assayed. 749