Pergamon Atmospheric Environment Vol. 30, No. 1, pp. 145-153, 1996 Copyright © 1995 ElsevierScience Ltd Printed in Great Britain. All rights reserved 1352-2310/96 $9~50 + 0.00 1352-2310(95)00164-6 PARTICLE SIZE EFFECT FOR METAL POLLUTION ANALYSIS OF ATMOSPHERICALLY DEPOSITED DUST M. A. AL-RAJHI,* S. M. AL-SHAYEB,* M. R. D. SEAWARD* and H. G. M. EDWARDSt * Department of Environmental Science, and tChemistry and Chemical Technology, University of Bradford, Bradford BD7 1DP, U.K. (First received 16 September 1994 and in final form 30 March 1995) Abstract--The metallic compositions of 231 atmospherically deposited dust samples obtained from widely-dil~.~ring environments in Riyadh city, Saudi Arabia, have been investigated in relation to the particle size distributions. Sample data are presented which show that particle size classification is very important when analysing dust samples for atmospheric metal pollution studies. By cross-correlation and comparison, it was found that the best way to express the results of the metal concentration trend was as an average of particle ratios. Correlations between the six metals studied, namely Pb, Cr, Ni, Cu, Zn and Li, were found for every particle size (eight categories) and reveal that the metal concentrations increased as the particle size decreased. On the basis of this work, it is strongly recommended that future international standards for metal pollutants in atmospherically deposited dusts should be based on particle size fractions. Key word index: Atmospheric dust pollution, metal deposition, dust particle size, heavy metals, trace elements, arid environment, Saudi Arabia. 1. INTRODUCTION In urban and industrial environments, street dust is a common material which can represent an important source of toxic metals, especially to young children. Dust composition and accumulation can vary with several factors, including weather, traffic density, in- dustrial activity and the proximity of mobile soil. Heavy metal pollution in street dust has been pre- viously studied; Ho (1990) studied the effect of Pb reduction in petrol on the lead content of kerb-side dust in Hong Kong. Ward (1990) concluded that lead concentrations in the environs of the London orbital motorway (M25) vt recently opened sections had rap- idly increased from background measurements taken before the arrival of heavy motor vehicle activity. In attempts to identify metal pollutant sources, other workers have fractionated dust samples accord- ing to the particle sizes. This procedure enables inves- tigators to undertake more accurate comparisons be- tween different locations. Fcrgusson and Ryan (1984) studied the elemental composition of street dust from large and small urban areas related to city type, source and particle size. They found that the concen- trations of most elements increase with a decrease in the size of the dust particles. Beckwith et al. (1985) investigated the relationship between metal levels and particle size distribution across a residential road over a six-month period. Their results showed a gradual increase in metal sediment concentrations towards the 145 centre of the road and a higher affinity of heavy metals with the smaller particle sizes. Hopke (1980) investi- gated one approach to the identification of sources that contribute potentially hazardous elements to roadway dusts, based on the assumption that particles derived from a given source exhibit distinguishable size, density, and ferromagnetic characteristics and that they also exhibit distinguishable interrelation- ships between their constituent trace elements. Linton (1980) has described an approach to environmental lead tracing, which combines two experimental methods for assigning distinguishing characteristics. The first involves fractionating each dust sample into subsamples according to the particle size. The second method involves scanning electron microscopic exam- ination and energy dispersive X-ray analysis of indi- vidual particles in each subsample. The study area chosen for the present work, Riyadh city, suffers from heavy atmospheric dustfall, the average estimated as 55.7tkm-2month -1 (El- Shobokshy, 1985). This is attributed to the geographi- cal situation of Riyadh, which is surrounded by vast desert areas, and to ongoing major constructional activities in the city. In the same study, El-Shobokshy reported the average deposition of heavy metals over the city to be 11.3 t km - 2 month - ~. The present study aims to investigate the relationship between the atmo- spherically deposited dust particle size and metal con- tent pollution based on data derived from Riyadh city. The spatial distribution of heavy metal pollutants in