Historical evidence for a dramatic increase in the nitrate component of acid rain

The concentrations of nitrate and ammonium ion in rainfall have an important effect on precipitation pH. We have collected sets of rainfall analyses from non-urban sites in North America and western Europe, which start from last century, and we show here that there is a marked increase in the annual deposit of nitrate ion compared with relatively stable levels of ammonium ion deposit. The increase apparent in the data from the US parallels the increases in nitrogen oxide emissions from combustion processes and is large enough for the nitrate ion to contribute almost as much to the acidity of rainfall as the sulphate ion. © 1982 Nature Publishing Group.