A method for identifying sulfuric acid in single cloud and fog droplets

We describe herein a novel method for identifying sulfuric acid in single droplets of mist, fog or cloud. The method is based on a microchemical reaction displayed by the droplets upon a shiny cleavage plane [ ] of Iceland spar – the rhombohedral variety of calcite. Thus, single droplets containing sulfuric acid respond by a typical microchemical reaction wherein they create prismatic, hemi-bipyramidal or lenticular twinned gypsum crystals on the calcite surface. For acid identification, droplets were collected on the four stages of a cascade impactor. The method was tested both in the laboratory and in the field in cloudy and foggy conditions. The sulfur mass in single droplets varied between 10−12 and 10−13 g and the pH ranged from 2.0 to 4.0.