Field measurements of the acidity of cloudwater deposited on polypropylene surfaces and Sitka spruce shoots

Summary Enhancement of the acidity of water on conifer needles, caused by evaporation even during the presence of cloud, is investigated. Field measurements of the acidity of droplets on polypropylene surfaces, shaped like needle bearing conifer shoots, and on Sitka spruce shoots, showed that surface droplet acidity varied significantly over a few hours, due to many causes. The acidity of the droplets commonly reached 800 eq/l (pH = 3.1) and occasionally 1000 eq/l (pH = 3.0). Increases in H⁺ ion concentration due to evaporation of pure water from the droplets occurred particularly around the end of cloud events. It was concluded that variation of H⁺ concentration of water on conifer needles due to evaporation during the presence of cloud is likely to be less important than that due to the other reasons. In particular the increase in H⁺ concentration from the reduction in droplet volume, due to evaporation, after the end of a cloud event is likely to be the most significant.