How We Measure Wet Nitrogen and Sulfur

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33 seconds

An air quality scientist describes how we monitor the wet deposition of nitrogen and sulfur. Video courtesy of Mackenzie Reed/NPS


Nitrogen and sulfur air pollution is transported through the atmosphere in gaseous and particulate forms. When these compounds are deposited by rain, snow, or fog it is called wet deposition. Too much nitrogen or sulfur deposition can have harmful effects on park ecosystems.

We monitor wet deposition in parks by collecting precipitation and analyzing it for nitrogen and sulfur compounds including ammonium (NH4+), nitrate (NO3- ), and sulfate (SO42-).

Who is monitoring and where?

NPS staff and partners collect wet deposition in parks as part of the National Atmospheric Deposition Program (NADP) National Trends Network (NTN). This multi-agency cooperative network monitors precipitation chemistry at more than 250 sites across the United States and Canada. Many of these monitoring sites are inside parks or are close enough to be considered representative of park air quality conditions. Take a look at the Nitrogen & Sulfur tab on the NPS Air Quality Monitoring map to see where these sites are located.

When selecting deposition monitoring sites the NPS considers:

  • Data gaps
  • Air quality concerns
  • Regional representation

What equipment do we use and how does it work?

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35 seconds

Automated precipitation collection bucket system. Video courtesy of Mackenzie Reed/NPS


Precipitation is collected using an automated bucket system. When precipitation falls onto the sensor (shown above), a mechanical arm removes the lid, allowing precipitation to collect in the bucket. When the sensor dries, the automated system returns the lid to the bucket to minimize evaporation and sample contamination.

Every Tuesday, the site operator replaces the collection bucket with a new, clean bucket. The sample bucket is then taken to the field laboratory where it is weighed, and any precipitation collected is transferred to a shipping bottle. This bottle is sent to the NADP Central Analytical Laboratory in Madison, Wisconsin where the precipitation is analyzed to determine the concentrations of ammonium, nitrate and sulfate.

Person pointing to a rain gage
An air quality monitoring site operator explains the rain gauge system.
Photo courtesy of Mackenzie Reed

In addition to the bucket collection system, each site also uses an open-faced rain gauge to precisely monitor the total amount of rainfall at the site (shown to the right). The rain gage works by recording the weight of collected precipitation which can be later translated into inches of precipitation received. The wind shield surrounding the gauge prevents updrafts from carrying off snow before it can fall into the gauge. The rain gauge and automated bucket system share a common data logging application used to ensure that the automated bucket system is properly opening and closing when the rain gage is collecting precipitation.

How are the data used?

Wet deposition data are useful for:

  • Documenting levels of wet nitrogen and sulfur deposition in parks
  • Identifying wet deposition trends for ammonium, nitrate, and sulfate
  • Assessing the effectiveness of deposition reduction strategies
  • Informing emission reduction policies
  • Improving atmospheric modeling
  • Understanding the ecosystem effects of deposition

Explore park-specific wet deposition information on the Nitrogen and/or Sulfur portion of the NPS Air Quality Conditions & Trends website. Additional data summaries and deposition maps are available through the NTN website.

Last updated: March 24, 2022


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