Atmospheric Nitrogen

 

This graphic illustrates how a proportion of nitrogen air emissions get deposited to Puget Sound. The atmosphere has nitrogen that comes from natural and human nitrogen emissions. Natural sources include emissions from soil and vegetation or natural wildfires. Human sources include transportation (vehicle exhaust, aircraft emissions, and commercial vessels and emissions from trains), agriculture (agricultural burning, fertilizer application, and livestock waste), and built infrastructure (e.g. emissions from wastewater treatment facilities, combustion, and residential burning).

Once these sources of nitrogen air emissions enter the atmosphere, some of it gets deposited back to land and water surfaces through atmospheric deposition. A small proportion of what gets deposited onto watersheds eventually makes it into Puget Sound indirectly via rivers and streams.

Nitrogen air emissions

Emissions from all counties within Puget Sound add up to 77,400 metric tons/year. 

Transportation is the largest source of nitrogen emissions. The top transportation sources include vehicle exhaust (gas and diesel) followed by commercial marine vessels. The next largest sources include built infrastructure, agriculture (mainly livestock waste and fertilizer application), point sources, and finally, natural sources.

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 is generally assumed to be more than 90% in the form of NO.

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.

Atmospheric Deposition

This map shows the magnitude of atmospheric deposition of total nitrogen in 2016 in and around the Puget Sound region. These deposition estimates come from AIRPACT, a computerized system for predicting air quality in the Pacific Northwest developed by scientists at Washington State University in collaboration with other agencies in the Pacific Northwest. The model uses available emissions information, combined with the chemistry and physics of air pollutants and meteorological conditions, to forecast air quality.

The map shows that deposition is higher in northern Whatcom County near the Canada border and along the eastern shoreline of Puget Sound. Depending on prevailing winds, emissions from one area of Puget Sound can influence deposition in other areas of Puget Sound.

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.