Sulfur (S) and nitrogen (N) compounds are essential nutrients for life. In excess, however, S and N can be toxic, even life threatening, and in national parks excess N and S can upset the delicate balances in natural ecosystems. N and S species can cause rain and snowfall to be acidic, and particulate sulfate (SO42-) and nitrate (NO3-) can cause significant visibility impairment and adversely affect human health.