Tracing acid rain with isotopes

In the study of acid rain, the isotopes of interest are the stable isotopes of sulphur and oxygen which comprise the sulphate molecule, one of the main acid constituents of our local rainfall. The principle behind such studies is that the ratio of the various isotopes of these two elements vary slightly, and that sulphate from a particular source will have characteristic values for these ratios. such ratios could then serve as 'fingerprints' that could be used to identify that source, even after the cloud of sulphate had travelled many hundreds of kilometers