Microscopy and chemistry of particles collected on TEOM filters: Swansea, south Wales, 1998-1999

Tapered element oscillating microbalances (TEOMs) are used in the UK Automa tic Monitoring Network for the continuous measurement of ambient airborne p articles. Used TEOM filters from Swansea, Cardiff and Pembroke were examine d under high-resolution field emission scanning electron microscopy (FESEM) . Clusters of calcium sulphate crystals, gypsum (CaSO4 . 2H(2)O) and anhydr ite (CaSO4) were abundant on spring and summer filters, and not present on autumn and winter filters. From textural considerations, the sulphates must have crystallised on the filter surfaces, either by dissolution and recrys tallisation of CaSO4 collected as particles, or by direct precipitation fro m saline water collected on the filters; in much the same way as the format ion of 'desert roses' by the evaporation of saline pore waters in desert sa nds. The proposed mechanism for the formation of these crystals has two imp ortant implications. Firstly, if the filters are episodically saturated wit h water, then on occasion the recorded masses will consist of both particle s plus water, causing errors in the results of continuous monitoring; an im portant consideration for epidemiological studies based on TEOM data. Secon dly, past toxicological experiments undertaken on TEOM-derived 'PM10' may h ave investigated material containing a significant component of in situ for med crystals, rather than the original PM10. (C) 2001 Elsevier Science Ltd. All rights reserved.