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Monday, May 1, 2017
Legislation to reduce acid rain in early 1990s most likely had unintended effect of increasing gaseous ammonia in the atmosphere.
Mar 17, 2017
The first global, long-term satellite study of airborne ammonia gas has revealed "hot spots" of the pollutant over four of the world's most productive agricultural regions, according to the University of Maryland.
Using data from the National Aeronautics & Space Administration's (NASA) Atmospheric Infrared Sounder (AIRS) satellite instrument, the University of Maryland-led research team discovered steadily increasing ammonia concentrations from 2002 to 2016 over agricultural centers in the U.S., Europe, China and India. Increased atmospheric ammonia is linked to poor air and water quality.
The study results, published March 16 in the journal Geophysical Research Letters, also describe the probable causes for increased airborne ammonia in each region. Although the specifics vary between areas, the increases in ammonia are broadly tied to crop fertilizers, livestock animal waste, changes to atmospheric chemistry and warming soils that retain less ammonia.
The results could help illuminate strategies to control pollution from ammonia and ammonia byproducts near agricultural areas, the researchers said.
"Our study reports the first global, long-term trends of atmospheric ammonia from space," said Juying Warner, associate research scientist in atmospheric and oceanic science at the University of Maryland. "Measuring ammonia from the ground is difficult, but the satellite-based method we have developed allows us to track ammonia efficiently and accurately. We hope that our results will help guide better management of ammonia emissions."
Gaseous ammonia is a natural part of Earth's nitrogen cycle, but excess ammonia is harmful to plants and reduces air and water quality. In the troposphere — the lowest, densest part of the atmosphere, where all weather takes place and where people live — ammonia gas reacts with nitric and sulfuric acids to form nitrate-containing particles that contribute to aerosol pollution that is damaging to human health, the researchers explained. Ammonia gas can also fall back to Earth and enter lakes, streams and oceans, where it contributes to harmful algal blooms and "dead zones" with dangerously low oxygen levels.
"Little ammonia comes from tailpipes or smokestacks; it's mainly agricultural — from fertilizer and animal husbandry," said Russell Dickerson,University of Maryland professor of atmospheric and oceanic science. "It has a profound effect on air and water quality — and ecosystems. Here in Maryland, ammonia from the atmosphere contributes as much as a quarter of the nitrogen pollution in the Chesapeake Bay, causing eutrophication and leading to dead zones that make life very difficult for oysters, blue crabs and other wildlife."
Each major agricultural region highlighted in the study experienced a slightly different combination of factors that correlated with increased ammonia in the air from 2002 to 2016.
The U.S., for example, has not experienced a dramatic increase in fertilizer use or major changes in fertilizer application practices, but Warner, Dickerson and their colleagues found that successful legislation to reduce acid rain in the early 1990s most likely had the unintended effect of increasing gaseous ammonia.
The acids that cause acid rain also scrub ammonia gas from the atmosphere, so the sharp decrease in these acids in the atmosphere is the most plausible explanation for the increase in ammonia over the same time frame, they said.
Europe experienced the least dramatic increase in atmospheric ammonia of the four major agricultural areas highlighted in the study. The researchers suggest that this is due, in part, to successful limits on ammonia-rich fertilizers and improved practices for treating animal waste. Much like in the U.S., a major potential cause for increased ammonia traces back to reductions in atmospheric acids that would normally remove ammonia from the atmosphere.
"The decrease in acid rain is a good thing. Aerosol loading has plummeted — a substantial benefit to us all — but it has also increased gaseous ammonia loading, which we can see from space," Dickerson said.
In China, a complex interaction of factors is tied to the increase in atmospheric ammonia. The study authors suggested that efforts to limit sulfur dioxide — a key precursor of sulfuric acid, one of the acids that scrubs ammonia from the atmosphere — could be partially responsible. However, China has also greatly expanded agricultural activities since 2002, widening its use of ammonia-containing fertilizers and increasing ammonia emissions from animal waste. Warming of agricultural soils due, at least in part, to global climate change could also contribute.
"The increase in ammonia has spiked aerosol loading in China. This is a major contributor to the thick haze seen in Beijing during the winter, for example," Warner said. "Also, meat is becoming a more popular component of the Chinese diet. As people shift from a vegetarian to a meat-based diet, ammonia emissions will continue to go up."
In India, a broad increase in fertilizer use, coupled with large contributions from livestock waste, resulted in the world's highest concentrations of atmospheric ammonia. The researchers noted, though, that ammonia concentrations there have not increased nearly as quickly as in other regions.
The researchers suggested that this is most likely due to increased emissions of acid rain precursors and, consequently, some increased scrubbing of ammonia from the atmosphere. This leads to increased levels of haze — a dangerous trend confirmed by other NASA satellite instruments, Dickerson said.
In all regions, the researchers attributed some of the increase in atmospheric ammonia to climate change, which is reflected in warmer air and soil temperatures. Ammonia vaporizes more readily from warmer soil, so as the soils in each region have warmed year by year since 2002, their contributions to atmospheric ammonia have also increased.
The study also ascribed some fluctuations in ammonia to wildfires, but these events are sporadic and unpredictable. As such, the researchers excluded wildfires in their current analysis.
"This analysis has provided the first evidence for some processes we suspected were happening in the atmosphere for some time," Warner said. "We would like to incorporate data from other sources, such as the Joint Polar Satellite System, in future studies to build a clearer picture."
The researchers hope that a better understanding of atmospheric ammonia will help policy-makers craft approaches that better balance the high demands on agriculture with the need for environmental protection.
"As the world's population grows, so does the demand for food — especially meat," Dickerson said. "This means farmers and ranchers need more fertilizer, which makes it harder to maintain clean air and water. Wise agricultural practices and reduced greenhouse gas emissions can help avoid adverse effects."