Ammonia (NH3) is a compound composed of one nitrogen (N) and three hydrogen (H) molecules. It is usually found as a gas and is known for its pungent odor.
Ammonia is a gas. It is colorless, but has a very distinctive odor. You may have smelled it in common household cleaning products, fertilizers, or at a livestock operation.
When in gaseous form, ammonia has a short atmospheric lifetime of only a few hours (2) and usually deposits near (distance varies based on climatic conditions) its source via wet or dry deposition. Since ammonia gas is highly soluble in water, it readily dissolves in atmospheric water vapor in the lower atmosphere and is deposited back to the Earth’s surface via wet deposition (rain). In dry climates or seasons, dry deposition to surrounding plant and soil surfaces is the primary fate of ammonia gas (2). Studies have shown that the majority of gaseous ammonia will deposit within 700 m of livestock parimeters (29), but trace ammounts of deposition have been seen up to 10 km away.
When ambient ammonia gas reaches the soil surface, it is usually absorbed into the soil or reacts with water in the soil and is converted into its ionic form, ammonium (NH4+). If not taken up by plants or converted to nitrate via microbes, disassociation of ammonium back to ammonia and eventually leads to reduction of soil pH. Additionally, the surplus nitrogen in the soil can either be taken up by plants, stored in the soil, returned to the atmosphere via denitrification, or removed from the soil in runoff to surface waters or leaching to groundwater (3). This runoff of nitrogen from soils, as well as the direct deposition of ammonia to surface waters, contributes to eutrophication of waterways and changes in aquatic ecosystems (4, 5).
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When gaseous ammonia, a base, combines with acidic gas species (primarily nitric and sulfuric acid) in the atmosphere (contributed mainly from nitrous and sulfuric oxides (NOx and SOx, respectively) from industrial and vehicle combustion processes) it forms a solid called an ammonium salt, also known as secondary particulate matter, fine particulate matter, or PM2.5 (PM2.5 is particulate matter that is 2.5 microns in diameter or smaller). To give you an idea of how small that is, a human hair is around 60 microns in diameter.
Due to their small diameter and increased atmospheric lifetime of approximately 15 days, these fine particulates are able to travel further distances than gas phase ammonia before being dry or wet deposited to the ground surface. Given the right climatic and atmospheric conditions, these small particles can travel from agricultural areas, to pristine mountain ecosystems or urban locations where they can mix with other pollutants and accumulate in the atmosphere, contributing to smog and human respiratory health issues.
Rocky Mountain National Park is a prominent example of the impact of nitrogen deposition on fragile mountain ecosystems (6, 7). Recent research has shown that the transport of ammonia gas and ammonium particles (contributed from various sources) has contributed significant amounts of nitrogen to the Park's soils, changing plant species and native ecosystem balance (6, 7). It is currently unknown how much of this nitrogen is contributed from livestock systems, but research is underway to improve estimates.
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