Fate of US and Canadian combustion nitrogen emissions

Abstract

While approximately 7.5 × 10¹² grams (7.5 Tg) of combustion nitrogen are emitted yearly in the United States and Canada^1–3, only 1.5–2.3 Tg are deposited as acid rain over that same region^4–6. Past nitrogen budget estimates^4,10 as well as recent observations¹¹ suggest that dry deposition and export play major roles. Here we report how the yearly accumulated deposition and transport of combustion nitrogen is simulated for the first time by a global transport model with realistic meteorology. The model predicts that dry deposition over the United States and Canada accounts for at least 2.1 and most probably 3.5 Tg of the emissions. The remainder is exported, principally over the North Atlantic. But at most 0.2 Tg, less than 3% of the estimated European emissions⁴, is predicted to reach Europe from North America. Furthermore, the model predicts that, while less than 40% of the nitrogen deposited as acid rain in the northeastern United States and eastern Canada comes from emissions in that region, almost all of the dry-deposited nitrogen and over half of the total acid nitrogen deposition comes from there.