Ammonia emission abatement does not fully control reduced forms of nitrogen deposition

Human activities and population growth have increased the natural burden of reactive nitrogen (N) in the environment. Excessive N deposition on Earth's surface leads to adverse feedbacks on ecosystems and humans. Similar to that of air pollution, emission control is recognized as an efficient means to control acid deposition. Control of nitrogen oxides (NO_x = NO + NO₂) emissions has led to reduction in deposition of oxidized nitrogen (NO_y, the sum of all oxidized nitrogen species, except nitrous oxide [N₂O]). Reduced forms of nitrogen (NH_x = ammonia [NH₃] + ammonium [NH₄⁺]) deposition have, otherwise, increased, offsetting the benefit of reduction in NO_y deposition. Stringent control of NH₃ emissions is being considered. In this study, we assess the response of N deposition to N emission control on continental regions. We show that significant reduction of NH_x deposition is unlikely to be achieved at the early stages of implementing NH₃ emission abatement. Per-unit NH₃ emission abatement is shown to result in only 60-80% reduction in NH_x deposition, which is significantly lower than the demonstrated 80-120% benefit of controlling NO_x emissions on NO_y deposition. This 60-80% effectiveness of NH_x deposition reduction per unit NH₃ emission abatement reflects, in part, the effects of simultaneous reductions in NO_x and SO₂ emissions.