To understand the extent to which human activities are polluting Earth’s atmosphere and oceans, it’s important to distinguish human-made pollutants from compounds that occur naturally. A recent study co-authored by a Brown University professor does just that for ammonium, a compound that is produced by human activities like agriculture, as well as by natural processes that occur in the ocean. The research, based on two years of rainwater samples taken in Bermuda, suggests that ammonium deposited over the open ocean comes almost entirely from natural marine sources, not from anthropogenic sources.