Nitrogen from Atmospheric Deposition

Nitrogen in the atmosphere has both natural (e.g. emissions from soil and vegetation, or natural wildfires) and human sources (e.g., transportation, agriculture, built infrastructure, and industrial emissions). Once these sources of nitrogen air emissions enter the atmosphere, some of it gets deposited back to land and water surfaces through deposition.

Deposition can be in two forms: wet deposition and dry deposition. Wet deposition is when nitrogen in the atmosphere is absorbed by precipitation (rain, fog and snow) and transported to the earth’s surface. Dry deposition is when nitrogen is deposited directly to the surface without the aid of precipitation.

An even smaller proportion of what is deposited eventually enters Puget Sound waters either via rivers and streams, or through direct deposition to marine surface areas.

Air Emissions of Nitrogen

Every three years, Ecology’s Air Quality Program releases an inventory of air emissions estimates from different sources, by county, and throughout Washington State. Most nitrogen air emissions are in the form of NH3 or NOx. NOx is a generic term for nitrogen oxides and includes both nitrogen oxide (NO) and nitrogen dioxide (NO2), but generally assumed to be >90% in the form of NO. For calculations presented here, we assumed all NOx emissions are in the form of nitrogen oxide (NO), which gives us an upper bound estimate since NO has a higher nitrogen content than NO2.

Based on the most recent 2011 inventory, transportation is the largest human source of nitrogen within Puget Sound counties, contributing to 78% of NOx-N emissions to the atmosphere. The top transportation sources include vehicle exhaust (gas and diesel) followed by commercial marine vessels. Agricultural activities (mainly fertilizer application and livestock waste) are the largest source of ammonia-nitrogen (NH3-N), contributing to 74% of NH3-N emissions. The top agricultural sources include livestock waste (both dairy and beef, plus others) followed by fertilizer application. Total atmospheric nitrogen emissions from Puget Sound counties from all sources combined add up to 85,300 metric tons/year – again, this assumes all NOx is in the form of NO.

Since NOx and NH3 are the main forms of nitrogen released into the atmosphere, the nitrogen contained in them both is a reasonable estimate of total atmospheric nitrogen emissions. When NOx-N and NH3-N emissions are added together, transportation is the largest nitrogen emissions source, followed by built infrastructure, agriculture, point sources and finally, natural sources. Nitrogen air emissions from all sources in Puget Sound counties totaled 77,400 metric tons/year, based on the 2011 inventory. Various chemical and physical processes affect how much of these emissions of nitrogen eventually get deposited to Puget Sound through either wet or dry deposition.

For example, nitrogen in rainfall can be taken up and stored by vegetation or seep into groundwater through different soil layers where bacteria can transform it to nitrogen gas and nitrate. Rainwater nitrogen can also runoff into streams and rivers where in-stream processes can assimilate it before these rivers enter Puget Sound. We don’t know how much of the nitrogen in rainfall that lands on Puget Sound watersheds eventually ends up in Puget Sound waters.

Wet and Dry Deposition

Wet deposition of nitrogen can be estimated using rainfall nitrogen concentrations. The National Atmospheric Deposition Program (NADP) monitors different constituents in rainfall throughout the U.S. Annual maps of precipitation-weighted mean concentrations (in mg/L) and deposition (in kg/ha) of nitrate (NO3-) and ammonia (NH4+) are available, in addition to downloadable data. There are four NADP monitoring stations located to the west of the Cascades within the Puget Sound basin where data have been collected since the 1980s or 1990s. Nitrogen concentration data are available at weekly, monthly, seasonal, and annual intervals.

Based on aggregating concentration data aggregated across all NADP stations within the Puget Sound Mohamedali et al. (2011a) estimated that the annual average wet deposition of inorganic nitrogen to Puget Sound was about 1.14 kilograms of nitrogen per hectare per year (kg N/ha-yr) between 1999 and 2008. Wet deposition from NADP stations located further away from urban influences (in the Olympics and near Mt. Rainier) were lower at 0.85 kg N/ha-yr, and wet deposition from stations closer to urban areas in the North Cascades were higher at 1.63 kg N/ha-yr over the same time period.

Dry deposition has been estimated by data collected by the EPA’s Clean Air Status and Trends Network (CASTNET). CASTNET monitoring is focused on long-term monitoring of air quality in rural areas, and currently has only one station operation in the Puget Sound region located within Mt. Rainier National Park, about 3.5 miles southwest of Mt. Rainier.

The figure below compares annual total nitrogen deposition, both wet and dry, near Mt. Rainier using both CASTNET data NADP data. The amount of wet deposition of nitrogen is much larger than dry deposition, with the latter contributing about 10% of total nitrogen deposition. Also interesting is the year-to-year variability in total nitrogen deposition rates. Between 1996 and 2012, deposition rates varied between 0.76 kg-N/ha (in 2009) and 2.51 kg-N/ha (in 1997).

Total Nitrogen Deposition (wet + dry) near Mt. Rainier between 1996 and 2012.
Source: http://www.epa.gov/castnet/javaweb/charts/MOR409_totn.png.

A recent "state of knowledge" report by the National Park Service, Thresholds for protecting PNW ecosystems from atmospheric deposition of nitrogen, summarized what we know (and don’t know) about atmospheric nitrogen loading to National Parks and US Forest Service lands within the Pacific Northwest.

Delivery to Puget Sound

These above aerial deposition rates are based on data collected near Mt. Rainier and away from Puget Sound’s urban centers. Actual deposition rates around Puget Sound are likely higher due to the influence of local air emissions, though we don’t know by how much. However, not all the nitrogen that is deposited out of the atmosphere ends up in Puget Sound.

Some of it is directly deposited to Puget Sound marine waters, and some of it is deposited to Puget Sound watersheds where various processes and pathways transform this nitrogen before delivery to Puget Sound via rivers and streams. For example, nitrogen can be taken up and stored by vegetation or seep into groundwater through different soil layers where bacteria can transform it to nitrogen gas. Nitrogen that enters rivers and streams can also be assimilated through in-stream processes before these rivers enter Puget Sound.

If we distribute the aerial deposition rates from 2011 across Puget Sound, an estimated 1,700 metric tons/year of atmospheric nitrogen is deposited directly to marine waters of Puget Sound, and an additional 3,700 metric tons/year is deposited to Puget Sound watersheds. As described earlier, a smaller but unknown proportion of this 3,700 metric tons/year of nitrogen ends up in Puget Sound since some is lost or transformed from watershed processes.