f!) Pergamon PII: SOO21-8S02(96)OOO79-1 I. AerDsoi Sci., Vol. 27, Suppl. I, pp. SIl-SI4, 1996 Copyright <C 1996 Eloevier Science LId Printed in Greal Britain. All rights reserved 0021-8502/96 + 0.00 PARTICLE SIZE DISTRIBUTIONS OF ATMOSPHERIC MERCURY IN URBAN AND RURAL AREAS N. PIRRONE·#, G. J. KEELER# and I. ALLEGRINI· # Air Quality Laboratory, Department of Environmental and Industrial Health. The University of Michigan, Ann Arbor, MI 48109·2029, USA" Institute for Air Pollution Research, National Research Council, 00016 Monterotondo ST., Rome, Italy KEYWORDS Atmospheric Mercury; Urban Area; Rural Area; Sampling; Particle Size Distribution Natural and anthropogenic high-temperature sources such as forest fires, active volcanoes, coal- and oil-fired power plants, non-ferrous metal production from primary and secondary smelters, solid waste and sewage sludge incinerators and a miscellaneous category (i.e., mercury production, other coal uses, cement kilns) release gas-phase and particle-phase Hg to the atmosphere (Pirrone et al.. 1996) with major fractions of its particulate mass often in the size range below 1 urn (Keeler et al., 1995; Pirrone et al., 1995). These particle fractions are especially important as they have long residence times in the atmosphere and the knowledge of their chemical and physical characteristics with changing meteorological conditions is important in atmospheric transport and deposition models, for relating mesoscale variations in airborne Hg concentrations with regional and global circulation patterns, and in assessing long-term ecological and health impacts on different environmental ecosystems as well as on biota (Pirrone et al., 1995; Pirrone esal., 1995-a). This paper presents particle size distributions of atmospheric Hg observed in recent years in several urban and rural areas of the United States. A five-stage microorifice cascade impactor (MOl) was used to collect size-fractionated mercury. The MOl was chosen because of its moderately high flow rate, 30 L min-I and relatively low pressure drop (Keeler et al., 1995). Regulation of the pressure drop is important as vaporization of particle associated water inside the impactor can result in distortion of the size distribution. S1,S2,S3,S4,S5 (pgl m3) RH (%) 100 M 1 2 4 5 7 B 9 10 14 16 17 March 1994 Figure I. Ambient concentrations of particulate Hg in Detroit measured with a 5-stage MOl with a particle cut-offdiameter of 5.0 urn for S 1,2.5 urn for S2, 1.0 urn for S3, 0.5 urn for S4 and 0.18 urn for SS. S13