Pergamon J. Aerosol Sci., Vol. 26. Suppl 1, pp. $897-$898, 1995 Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00 ACTIVATION PROPERTIES OF MARINE AEROSOL IN THE NETHERLANDS A. Khlystov, G.P.A. Kos, H.M. ten Brink Netherlands Energy Research Foundation (ECN) P.O.Box 1, 1755 ZG Petten, The Netherlands A.Berner, C.Kruisz lnstitut fur Experimentalphysik der Universitat Wien, Vienna, Austria Keywords: ambient aerosol, activation, cloud chamber. A cloud chamber was built to study the effect of anthropogenic aerosol on cloud formation in the Netherlands. Emphasis was given to the investigation of cloud formation in marine air, since sensitivity studies show that the reflectivity of such clouds is most effectively influenced by the (extra) anthropogenic aerosol particles (Wigley, 1989; Charlson et al., 1992). For this reason the supersaturations in the study were as low as in typical marine stratus. The cloud chamber was described elsewhere (Khlystov et al., 1995). The very high flow rate enables the use of in-situ instruments like the Forward Scattering Spectrometer Probe (FSSP-100, Particle Measuring Systems Inc.) and high-flow cascade impactors for sampling and chemical characterization of the droplets (Bemer, 1989). The performance of the cloud chamber was tested with reference aerosol of varying number concentrations. It was found that particles above 0.07 Ixrn ("activation threashold") are activated which corresponds to the supersaturation of 0.15%. The effect of anthropogenic aerosols on cloud formation was determined from a comparison of the number of droplets formed in "clean" arctic marine air (usually coming from the NW direction) versus the number of droplets formed in marine air which had traveled over the UK ("polluted marine air"). The polluted air masses were further divided into a class of moderately and a class heavily polluted air, on the basis of the number of aerosol particles. It was desided that concentrations in the order of 102 cm-3 represent "clean" air, concentrations in the order of 103 cm-3 represent moderately polluted and concentrations in the order of 104 cm-3 represent heavily polluted air. Table 1. Average number of droplets versus number of cloud nuclei (particles larger than 0.07 jam) in different marine air masses; unit number per cm3 of air. The toatl number of aerosol particles is also given. Droplets Cloud nuclei Total aerosol Clean 6O 60 130 Moderately polluted 190 320 1010 Heavily polluted 380 4700 9900 $897