Measurements of pH versus drop size in natural rain

Results from numerical scavenging models 1–3 , which estimate the acidity of precipitation falling through a polluted atmosphere, have previously been compared with p H measurements of bulk rain water or of single drops of unknown size 4 . Here we describe results from an instrument which, for the first time, allows the variation of p H with drop size to be evaluated. Field measurements show a p H minimum for drops with radii near 0.5 mm, consistent with simple mass transfer considerations.