Atmospheric reduced nitrogen: Sources, transformations, effects, and management

ABSTRACT

Human activities have increased atmospheric emissions and deposition of oxidized and reduced forms of nitrogen, but emission control programs have largely focused on oxidized nitrogen. As a result, in many regions of the world emissions of oxidized nitrogen are decreasing while emissions of reduced nitrogen are increasing. Emissions of reduced nitrogen largely originate from livestock waste and fertilizer application, with contributions from transportation sources in urban areas. Observations suggest a discrepancy between trends in emissions and deposition of reduced nitrogen in the U.S., likely due to an underestimate in emissions. In the atmosphere, ammonia reacts with oxides of sulfur and nitrogen to form fine particulate matter that impairs health and visibility and affects climate forcings. Recent reductions in emissions of sulfur and nitrogen oxides have limited partitioning with ammonia, decreasing long-range transport. Continuing research is needed to improve understanding of how shifting emissions alter formation of secondary particulates and patterns of transport and deposition of reactive nitrogen. Satellite remote sensing has potential for monitoring atmospheric concentrations and emissions of ammonia, but there remains a need to maintain and strengthen ground-based measurements and continue development of chemical transport models. Elevated nitrogen deposition has decreased plant and soil microbial biodiversity and altered the biogeochemical function of terrestrial, freshwater, and coastal ecosystems. Further study is needed on differential effects of oxidized versus reduced nitrogen and pathways and timescales of ecosystem recovery from elevated nitrogen deposition. Decreases in deposition of reduced nitrogen could alleviate exceedances of critical loads for terrestrial and freshwater indicators in many U.S. areas. The U.S. Environmental Protection Agency should consider using critical loads as a basis for setting standards to protect public welfare and ecosystems. The U.S. and other countries might look to European experience for approaches to control emissions of reduced nitrogen from agricultural and transportation sectors.

Implications: In this Critical Review we synthesize research on effects, air emissions, environmental transformations, and management of reduced forms of nitrogen. Emissions of reduced nitrogen affect human health, the structure and function of ecosystems, and climatic forcings. While emissions of oxidized forms of nitrogen are regulated in the U.S., controls on reduced forms are largely absent. Decreases in emissions of sulfur and nitrogen oxides coupled with increases in ammonia are shifting the gas-particle partitioning of ammonia and decreasing long-range atmospheric transport of reduced nitrogen. Effort is needed to understand, monitor, and manage emissions of reduced nitrogen in a changing environment.

Acknowledgment

This paper was commissioned by the Critical Review Committee of the Air & Waste Management Association. The authors appreciate the guidance, comments, and support of the oversight committee, particularly Susan Wierman, Bret Schichtel, and Lisa Bucher and other members. The authors also appreciate the work of Heather Flaherty, Fatemeh Rezaei, Kimberley Driscoll, McKenzie Brannon, and Connor Olson in the development of visuals for the manuscript and the presentation. The authors appreciate the thoughtful discussions with Chris Clark, Christy Goodale, Bob Howarth, Hans Paerl, and Justin Coughlin. This paper is a contribution of the Hubbard Brook Ecosystem Study.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/10962247.2024.2342765

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Charles Driscoll

Charles T. Driscoll is Distinguished and University Professor of Civil and Environmental Engineering at Syracuse University, Syracuse, NY. Driscoll’s scholarly work addresses the effects of disturbance on forest, freshwater and marine ecosystems, including air pollution (mercury, sulfur, and nitrogen deposition), land-use, and climate change.

Jana B. Milford

Jana B. Milford is Emerita Professor in the Department of Mechanical Engineering and the Environmental Engineering Program at the University of Colorado Boulder. Her research and teaching interests focus on air quality modeling and data analysis, thermal sciences, environmental impacts of energy systems, and environmental law and policy.

Daven K. Henze

Daven K. Henze is Professor and the S.P. Chip and Lori Johnson Faculty Fellow in the Department of Mechanical Engineering at the University of Colorado Boulder. His research interests include aerosols, air quality, climatology, and atmospheric chemistry.

Michael D. Bell

Michael D. Bell is an ecologist with the National Park Service - Air Resources Division who studies how air pollution harms ecosystems. He develops management tools, such as Critical Loads, to guide the protection and recovery of ecosystems from air pollution.