Measurement of suspended particulate matter in the ambient atmosphere

The aim of the work described in this report was to develop a new static aerosol sampler for the collection of suspended paniculate matter in the ambient atmosphere, suitable for use as a Reference Instrument in a standard Reference Method to be applied within the framework of the European air pollution monitoring programmes. It was agreed at the outset that the performance of the new instrument should be consistent with the recommendations of the International Standards Organisation (ISO), first aspirating the inhalable fraction and subsequently providing the thoracic subtraction.At the beginning, it became clear that the quantitative definitions of inhalability as contained within existing conventions do not apply over the full range of windspeeds appropriate to the ambient atmosphere. Under a separate contract, a new set of wind tunnel experiments was carried out to provide the missing information, using a full-scale tailor’s mannequin. The results led to a proposal to modify the inspirability conventions which was recently adopted by the ISO and will appear in the revised set of recommendations. This development is important in relation to the present proposed Reference Instrument, and the new criterion was used in the present work as the working criterion for the new sampler.The flowrate of the original prototype instrument developed under the first contract was 30 1/min. New experiments were carried out to characterise the aspiration efficiency of that original instrument over the full range of particle sizes (up to 100 �m) and windspeeds (up to 10 m/s) expected to be encountered in practice. It was shown that performance was not satisfactory over these ranges of conditions so the decision was made to discard that earlier instrument However, a 70 1/min sampling head that was developed under the separate CEC contract had been shown to exhibit desirable aspiration characteristics, closely matching the inhalability criterion over the full range of conditions prescribed. Since in any case a higher flowrate was desirable, it was decided to use this sampling head as a starting point for the Reference Instrument which was the objective of the present project. Basically this head is of the ‘rotating-head’ type, consisting of a 25 mm circular orifice in a tapered conical head which is rotated steadily through 360 � about a vertical axis (so that there is no preferred orientation with respect to the wind).From this starting point, two versions were developed, the first for the inhalable fraction alone and the second for the thoracic subtraction consistent with the ISO recommendations. For the latter, a size selector made of porous plastic foam media was found, after making the appropriate choice of foam grade and overall dimensions, to provide the desired penetration characteristics. The development work itself also involved finding solutions to a number of problems, including not only achieving the desired inhalability-matching entry characteristics (for both head versions) but also delivering the aspirated aerosol uniformly to the foam size-selector with minimal losses inside the head itself. Inside each head version, the collecting filter was contained within a frame which facilitated handling before and after actual sampling. A quick-release ‘plug-in’ mechanism was developed for connecting the head to the rest of the sampler system, thus enabling rapid and simple interchange between the head options, depending on which fraction it was required to sample.One or other of these two sampling heads, the drive mechanism, pump, and flow metering and control are all incorporated into a portable weatherproof cabinet. The search for a commercially-available flow control system turned out to be unsuccessful within the budgetry constraints that were imposed (i.e. in the overall cost of the resultant instrument). Therefore a simple system was developed inhouse, incorporating feedback control between a pressure sensor on the exhaust flow and a flow control valve, on the pump inlet. “”