Why is it Important to Monitor Ammonia in the Atmosphere?
Atmospheric ammonia (NH3) gas is an important reactant in atmospheric chemistry, and affects the chemical deposition to ecosystems.
Ammonia Drives Atmospheric Chemical Reactions
Gaseous ammonia is the principle basic gas in the atmosphere, meaning that it forms a basic solution (pH >7.0) when dissolved in water. Ammonium (NH4+) fine particulate aerosol (less than 2.5 um in size, termed "PM2.5") is formed by the reaction of ammonia gas with sulfur, nitrogen, and other acidic species forming ammonium sulfate and ammonium nitrate particulate matter. These aerosols, along with carbon aerosol, constitute the major fraction of PM2.5 pollution in the atmosphere. Fine particulates contribute to visibility degradation at a number of sites, and has become a particular problem within Class 1 wilderness areas, including National Parks.
Particulate ammonium species act as cloud condensation nuclei, forming precipitation droplets, removing the ammonium from the atmosphere as wet deposition. Ammonium aerosol can also be removed from the atmosphere as dry deposition. Ammonia gas can also directly dissolve into cloud water.
Ammonia Impacts Ecosystems
Ammonia gas and ammonium ions are nutrients that stimulate plant and algal growth in nitrogen limited systems. Deposition of ammonium can promote crop growth, but it also can alter the structure and diversity of native plant communities. When an ammonium ion deposits to a soil surface, it can increase soil acidity through nitrification reactions, releasing hydrogen ions and converting ammonium to nitrate. Additionally, NH3 is thought to supply between 15-40% of nitrogen loading to estuaries, therefore NH3 plays a very important role in coastal eutrophication.
Results from the NADP indicate that ammonium concentrations in precipitation have increased in many parts of the U.S. This effect is particularly evident when viewed in the time series maps. During the same period, sulfate concentrations have decreased across the U.S., due to sulfur dioxide emissions limits implemented under the 1990 Clean Air Act Amendments. These increasing ammonium loadings with continued decreases in sulfate concentrations in the nation's rain both increase the overall precipitation pH. The combined effect of this is to reduce the incidence of acid rain in most regions of the U.S.
Sources of Ammonia in the Atmosphere
Domesticated animals and fertilizer represent the largest source of anthropogenic atmospheric ammonia emissions on a global basis, and ammonia emissions from agricultural operations are of growing concern.
According to the U.S. EPA, total U.S. ammonia emissions increased from 3.9 teragrams in 1990 to 4.5 teragrams in 2000, a rate increase of approximately 1.5%/year. These increased emissions are mostly due to increases in agricultural animal populations. In the continental U.S., the highest ammonia emissions in both 1990 and 2000 were in Iowa and Delaware, but the highest percentage state-level increase over this time period was in North Carolina (+66%). Spatial distribution of ammonia and total nitrogen wet deposition is very similar to distribution of ammonia emissions.
The Need for a National Monitoring Network?
Despite the importance of ammonia in atmospheric chemistry and its impacts on ecosystems, there have been no national studies or routine monitoring of ambient (that is, away from source) ammonia concentrations.
The Clean Air Status and Trends Network (CASTNET) has routinely measured atmospheric particulate ammonium concentrations, but not atmospheric ammonia. CASTNET is planning future gaseous monitoring by changing their monitoring method, but these changes are only in the planning stage.
A national ammonia gas monitoring network will improve our understanding of ammonia's role in the atmosphere and its impacts by:
- Determining the spatial distribution of ammonia concentrations,
- Determine the seasonality of these concentrations,
- Providing benchmarks to assist in meeting air quality goals,
- Providing information for scientific and policy needs.
A US-Canada Workshop on Ammonia (Chicago, Feb. 2007) came to a consensus result calling for a coordinated, national network of ammonia monitoring. Their consensus results were:
- A passive ammonia network is worthwhile to pursue, but it will be necessary to have widespread participation to be effective and achieve goals of the network;
- Participation of [The U.S. Department of] Agriculture is critical;
- A long-term network is preferable to a short-term one, even if other more quantitative measurements emerge;
- Radiello™ devices hold promise because of low detection limits and high reproducibility, but additional experience and measurements are needed;
- Weekly sampling is desirable, but under most ambient conditions only the Radiello devices have a low enough detection limit;
- NADP should be pursued as a coordinating body; and
- Sites with existing ammonia monitoring measurements should be highest priority for funding (fifteen sites were suggested as candidates for the first phase of deployments).
An NADP Initiative
A motion was made at the 2007 NADP Executive meeting in Annapolis, Maryland (June 12-13, 2007) to consider a special study with a goal of establishing an ammonia monitoring network. This network would measure atmospheric concentrations of ammonia gas using low-cost, passive samplers at a number of sites across North America.