Investigations of ground-based clouds at the Mt Brocken

Results of the 1996 field season (April-November) of the cloud chemistry program starting in 1992 at the Mt. Brocken are presented. The mountain site (1142 m a.s.l.) is located in the Hart Mountains/Germany. For a large part of the database (about 40%), both cloud water and drizzle and/or precipitation were present at the same time. A separation into two groups was done: non-precipitating clouds and mixed category of events. More than 2000 one hour samples were collected and analysed for major ions. The mean cloud water pH for the frostfree period in 1996 was 3.9. Histograms giving the distribution of pH values show that the pH values of samples from non-precipitating clouds ranges from 2.5 up to values > 8 and that the distribution is bimodale. For the 1996 field season period as a whole, pH values of cloud water were lower than those of mixed samples (cloud water, drizzle, precipitation) at the Brocken site. This was related to higher concentrations of all ions in cloud water. Cloud base measurements using a ceilometer confirmed the correlation between liquid water content and height above cloud base and consequently the influence of sampling height relative to the cloud base on the total ionic content of cloud water.