TRACING THE SOURCES OF THE CHEMICAL COMPOSITION OF PRECIPITATION BY CLUSTER ANALYSIS J. SLANINA, J. H. BAARD, W. L. ZIJP The Netherlands Energy Research Foundation (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands and W.A.H. ASMAN Institute for Meteorology and Oceanography (IMOU), 5 Princetonplein, 3584 CC Utrecht, The Netherlands (Received April 2, 1982; revised July 21, 1982) Abstract. Cluster analysiswas applied to a precipitation chemistrydata base obtained by samplingby means of 8 wet-onlysamplers on one location in the northwestern part of The Netherlands for sampling periods of 2 and 3 days. Two main groups were found, correspondingto either continental or maritime origin, which coincided with prior meteorological analysis. Each main group consisted of two subgroups. Polluted maritime air seems to be an important source of H +, presumably due to the low NH3 concentration in maritime air and hence little neutralization of strong acids. 1. Introduction Tracking the source regions for important components in precipitation has been a challenge for many scientists involved in precipitation chemistry. To explain the concentrations of components in rain-water, different approaches have been used: - Correlation of nearby (100 km) precursor emission with the concentrations of reaction product in precipitation (e.g. SO2-emission with SO42--concentration in precipitation) (Vermeulen, 1978). With this approach it is assumed that the chemical composition of precipitation is largely due to nearby sources. This is generally not the case, because pollutants are usually transported far away from the emission sources before they get a chance to be scavenged by precipitation (OECD, 1979). Another reason is that some pollutants are very slowly removed in the precursor- form and can only be removed efficiently after conversion to other products. As this conversion process takes some time, this also means that those components are not likely to be found near the emission sources. - Sampling during short periods (e.g. 1 day) combined with trajectory analysis and knowledge about the location and the strength of the emissions (Asman et al., 1981). The chemical composition of precipitation is not only influenced by emissions, but also by meteorological factors like rainfall rate or total amount of precipitation. This makes it more difficult to trace the origin of the pollutants found in precipitation. An approach to this problem was not to use absolute concentrations, but to use the ratio of concentrations of couples of components and then to try to divide precipitation periods into groups (Asman et al., 1981). In this way a division of rain periods could be made, Water, Air, and Soil Pollution 20 (1983) 41-45. 0049-6979/83/0201-0041500.75. Copyright © 1983 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.