Increases in fertiliser use and livestock waste  have resulted in India having  the world’s highest atmospheric ammonia concentrations. Photo: / Peter Casier  CC BY NC ND

Ammonia increasing over agricultural areas

The first global, long-term satellite study of airborne ammonia gas has revealed increasing levels of the pollutant over four of the world’s most productive agricultural regions.

Using satellite data, new research has discovered steadily increasing ammonia concentrations from 2002 to 2016 over agricultural areas in the United States, Europe, China and India. The study “Increased atmospheric ammonia over the world’s major agricultural areas detected from space” was published in March in the journal Geophysical Research Letters.

Excess ammonia is harmful to plants and reduces air and water quality. In the troposphere – the lowest part of the atmosphere – ammonia reacts with nitric and sulphuric acids to form tiny particles (PM2.5) that contribute to aerosol pollution that is damaging to human health. Ammonia can also fall back to Earth as wet or dry deposition on vegetation, soils and surface waters, causing eutrophication.

The increases in ammonia are broadly tied to emissions from crop fertilisers and livestock waste, changes in atmospheric chemistry and warming soils that retain less ammonia.

Each major agricultural region highlighted in the study experienced a slightly different combination of factors that correlate with increased ammonia in the air from 2002 to 2016.

The United States, for example, has not experienced a dramatic increase in fertiliser use or major changes in fertiliser application practices. But legislation to reduce acid rain in the 1990s resulted in reductions in emissions of sulphur dioxide (SO2) and nitrogen oxides (NOx) that most likely had the unintended effect of increasing levels of 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 a plausible explanation for the increase in ammonia over the same time frame.

Europe experienced the least dramatic increase in atmospheric ammonia of the four major agricultural areas highlighted by the study. The researchers suggest this is due in part to successful limits on ammonia-rich fertilisers and improved practices for treating animal waste. As in the United States, significant reductions in emissions of SO2 and NOx and thus in atmospheric acids that would normally remove ammonia from the atmosphere help to explain the increased ammonia levels.

“The decrease in acid rain is a good thing. Aerosol loading has plummeted – a substantial benefit to us all,” said Russell Dickerson, a professor at the University of Maryland and co-author of the study.
China has greatly expanded agricultural activities since 2002, widening its use of ammonia-containing fertilisers and increasing ammonia emissions from animal waste. The increase in ammonia has spiked aerosol loading in China, and is a major contributor to the thick haze seen in Beijing.

In India, a broad increase in fertiliser use coupled with large contributions from livestock waste have resulted in the world’s highest concentrations of atmospheric ammonia. But the researchers note that ammonia concentrations have not increased as quickly as over other regions, most likely due to increased emissions of SO2 and NOx and, consequently, some increased scrubbing of ammonia from the atmosphere.

In all regions, the researchers attributed some of the increase in atmospheric ammonia to climate change, reflected in warmer air and soil temperatures. Ammonia vaporises more readily from warmer soil, so as the soils in each region have warmed year by year, their contributions to atmospheric ammonia have also increased since 2002.

The authors hope that a better understanding of atmospheric ammonia will help policy makers craft approaches that better balance the high demand for 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 fertiliser, which makes it harder to maintain clean air and water. Wise agricultural practices and reduced greenhouse gas emissions can help avoid adverse effects.”

Christer Ågren

Sources: Joint press release by the American Geophysical Union and the University of Maryland; Science Daily, 16 March 2017.


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