PII: 80021-8502(96)00296-0 J. AerosolSci.. Vol. 27. Suppl. I. pp. 5447-5448. 1996 Copyright «:l 1996 Elsevier Science Ltd Printed in Oreat Britain. All rights reserved 0021·8502/96 $15.00 + 0.00 SIZE DISTRIBUTIONS OF IONIC SPECIES IN AMBIENT AEROSOLS IN SE ENGLAND Meng-Chen Chung and I. Colbeck Institute for Environmental Research, Department of Biological and Chemical Sciences, University of Essex, Colchester C04 3SQ, U.K. KEYWORDS: Size distribution, cascade impactor. METHOD Aerosol size distributions of ionic species have been studied in the semi-rural area, east of Colchester since February 1995. Sampling was performed with the MaylRE cascade impactor, which contains seven impaction stages and a backup filter. Coarse particles (>2.5 urn) were collected on stages 1 to 4, and fine particles «2.5 urn) on stages 5 to 7 and the backup filter. Samples were analyzed by ion chromatography for anions(Cr, N03- & SOl) and cations (NH/, Na+, K+, Mg 2 • & Ca 2+). RESULTS Sulphate was found to be the most abundant particulate in this area. The variation of the mean monthly concentration of sol and N03' were found to be in the range of 3.4-8.7 J.lglm 3 and 1.2-3.2 ug/m', respectively. High levels of acidic species were observed in the winter season. Ammonium particles showed good correlation with both nitrate and sulphate. It may therefore be concluded that ammonia appears to be the main species for the neutralization of acidic aerosols ( Kitto and Harrison, 1992). Acidic particles dominated the fine mode, with the average ratio: 84% of anions to cations (neq.zm'), Sulphate was found to be the most abundant ion of the fine particles, whereas sodium and chloride particles were the most abundant of the coarse particles (Figure 1). Little chloride was found on the backup filter stage «0.5 um). Marine air contributes the high concentration of chloride in Colchester. Elevated levels of acidic pollutants were observed on several occassions, for example the period of 2nd-3rd February 1995. Here mass concentrations of N0 3 " and sol were 2 to 3 times higher than the average levels (Figure 2). The cause of this high level of acidic aerosol was deduced by the backward air mass trajectory method, and it mainly came from the industrial area of North England. S447