ACTIVATION PROPERTIES OF AMBIENT AEROSOL IN THE NETHERLANDS

A cloud chamber has been used to study the cloud activation of ambient aerosol in The Netherlands. The large dimensions and throughput of th e chamber allowed unperturbed collection of aerosol and droplets with cascade impactors and on-line measurements with cloud monitors (FSSP) inside the facility. The study provided maxima for the number of man-m ade aerosol acting as cloud nuclei in marine clouds in The Netherlands . Emphasis was given to the investigation of cloud formation in marine air, since sensitivity studies had shown that such clouds are most ef fectively influenced by the (extra) anthropogenic aerosol particles. F or this reason the supersaturations in the study were low (on average 0.12%), similar to those in actual marine stratus. The effect of the a nthropogenic aerosols on cloud formation was determined by comparing t he number of droplets formed in ''clean'' arctic marine air to the num ber of droplets formed in ''polluted'' marine air (air which had trave lled over the U.K.). Air masses with the total aerosol number concentr ation of the order of 100 cm(-3) were considered as ''clean'' marine a ir. Air masses with higher aerosol concentrations were divided into '' moderately'' and ''heavily'' polluted with total aerosol concentration s of the order of 1000 and 10,000 cm(-3), respectively. In the clean m arine air all potential cloud nuclei (particles lar er than the thresh old size of the smallest reference particles that were activated al gi ven supersaturation) were activated and the number of cloud droplets f ormed was on average 45 cm(-3). In the moderately polluted air 72% of potential cloud nuclei were activated and the average droplet number w as 190 cm(-3). The difference in the actual cloud droplet number and t he number of potential cloud nuclei could be explained by the presence of water-insoluble particles which do not activate. In the heavily po lluted air the average droplet concentration was around 320 cm(-3) whi ch is, on average, 24% of the number of potential cloud nuclei. Copyri ght (C) 1996 Elsevier Science Ltd