Increasing importance of deposition of reduced nitrogen in the United States

Abstract

Rapid development of agriculture and fossil fuel combustion greatly increased US reactive nitrogen emissions to the atmosphere in the second half of the 20th century, resulting in excess nitrogen deposition to natural ecosystems. Recent efforts to lower nitrogen oxides emissions have substantially decreased nitrate wet deposition. Levels of wet ammonium deposition, by contrast, have increased in many regions. Together these changes have altered the balance between oxidized and reduced nitrogen deposition. Across most of the United States, wet deposition has transitioned from being nitrate-dominated in the 1980s to ammonium-dominated in recent years. Ammonia has historically not been routinely measured because there are no specific regulatory requirements for its measurement. Recent expansion in ammonia observations, however, along with ongoing measurements of nitric acid and fine particle ammonium and nitrate, permit new insight into the balance of oxidized and reduced nitrogen in the total (wet + dry) US nitrogen deposition budget. Observations from 37 sites reveal that reduced nitrogen contributes, on average, ∼65% of the total inorganic nitrogen deposition budget. Dry deposition of ammonia plays an especially key role in nitrogen deposition, contributing from 19% to 65% in different regions. Future progress toward reducing US nitrogen deposition will be increasingly difficult without a reduction in ammonia emissions.

Keywords: agriculture; ammonia; dry deposition; nitrogen oxides; wet deposition.

Fig. 1.

Comparisons of the 3-y average NH₄⁺ percentage of wet inorganic nitrogen deposition across the United States in 1990–1992 (Left) and 2010–2012 (Right). To help visualize spatial patterns, isopleths were produced by interpolating NH₄⁺ mole percentages at individual monitoring sites using a cubic inverse-distance weighting of sites within 500 km of each observation station. The black dots on the map represent locations of sites with 3-y data available for each time period. The NH₄⁺ percentage on a molar basis [(NH₄⁺%) = (NH₄⁺)/(NO₃⁻ + NH₄⁺) × 100%] is noted at each site.

Fig. 2.

Absolute percentage change of NH₄⁺ and NO₃⁻ in wet deposition across the country. C_10–12 is the average NH₄⁺ or NO₃⁻ flux (kg N/ha per year) in each state between 2010 and 2012 and C_90–92 is the average NH₄⁺ or NO₃⁻ flux (kg N/ha per year) between 1990 and 1992. Only sites in Fig. 1 with both 1990–1992 and 2010–2012 data available are used to calculate the average flux for each state.

Fig. 3.

Spatial and temporal trends in dry inorganic N deposition at 37 locations across the United States. Included are deposition of gaseous nitric acid and ammonia and PM_2.5 ammonium and nitrate. Fractional reduced N contributions are represented by circle color. The total deposition from these four species is indicated by circle size. The bar charts depict monthly average contributions of individual dry reduced and oxidized N deposition pathways for eight selected regions. The average total dry inorganic N deposition fluxes in different regions are shown by the number in each figure.

Fig. S1.

Pie charts of seasonal N deposition species pathways (Upper) and total monthly measured precipitation (Lower) in Florida area (FL11 and FL19).

Fig. 4.

Ratio of annual NH₃ dry deposition rates estimated using the MLM vs. bidirectional approaches. Regions are indicated at top of graph. *Due to a lack of meteorological data, the bidirectional flux model is not parameterized appropriately for site KY98. **Due to vegetation type, the bidirectional flux model is not parameterized appropriately for site IL11.

Fig. S2.

Seasonal ratios of MLM vs. bidirectional NH₃ dry deposition estimates. *Due to lack of meteorological data, bidirectional flux model is not parameterized appropriately for the KY98 site. **Due to feature of surface plants, bidirectional flux model is not parameterized appropriately for the IL11 site.

Fig. 5.

Spatial trends in total reactive inorganic N deposition across the United States from July 2011 to June 2013. Fractional reduced N contributions to total N deposition (dry + wet) at the 37 sites are represented by circle color. The total inorganic nitrogen deposition is indicated by circle size. The pie charts show average fractional contributions of individual reduced and oxidized N deposition pathways for the eight regions, with each pie area proportional to the average total inorganic nitrogen deposition (also listed under each pie).