Study Traces Ammonia Pollution in Africa
As global agriculture scales up over the next century to meet the needs of growing populations, it is likely that atmospheric ammonia (NH3) emissions will rise, too. Ammonia is naturally emitted by soils and vegetation fires, but most of it is added to the atmosphere by humans through agricultural activities such as fertilizer use and livestock ranching. When present in excess amounts in an ecosystem, ammonia can make soils more acidic and hinder plant growth. As an air pollutant, it can provoke heart- and lung-related illness.
In a new NASA-led study, scientists examined changing atmospheric ammonia concentrations over Africa, where human populations are predicted to double by 2050. In many African countries, governments are promoting fertilizer use to increase food production. In addition, the burning of living or dead trees and plants (biomass burning) is common in Africa; by one estimate, as much as 70 percent of the planet’s burned area each year occurs on the vast continent. It is an important area for examining ammonia on both a regional and global scale.
Using satellite data from the European Space Agency’s Infrared Atmospheric Sounding Interferometer (IASI), a team led by Jonathan Hickman of Columbia University and NASA’s Goddard Institute for Space Studies identified increases and decreases in ammonia concentrations across Africa between 2008 and 2018. They also identified some likely causes of those changes.
The map above on the left depicts the change in ammonia concentration across Africa, with a focus on three key areas. The map on the right shows changes in burned area between 2008 and 2018. Both come from a study published November 16, 2021, in the journal Atmospheric Chemistry and Physics.
“We have shown here that we can use satellite data to observe trends and monitor emissions of ammonia in specific regions, linked to specific activities or environmental events,” said Enrico Dammers, a scientist at the Netherlands Organization for Applied Scientific Research and a co-author of the paper.
The researchers found “multiple distinct stories about how air quality changes in response to growing agricultural activity across Africa,” said Hickman, the principal investigator. For instance, in West Africa, the end of the dry season and the peak in biomass burning corresponded with increases in atmospheric ammonia concentrations over the study period. Previous studies had attributed rising ammonia levels here to fertilizer use. However, Hickman and Dammers found that NH3 pollution increased most when farmers were preparing their land by burning it, yet before they started adding fertilizer.
In the Lake Victoria region, the expansion of farming led to increased fertilizer use. The IASI data indicated that much of the growth in ammonia concentrations in this region could be linked to areas where farmers were applying more fertilizer on both new and existing agricultural land.
In South Sudan, a 30,000 square-kilometer wetland fed by the Nile River—called the Sudd—was the only region that showed a clear decrease in atmospheric NH3 over the study period. About half of the Sudd is permanently flooded, while the other half is a floodplain that may or may not flood depending on how wet the year is. The researchers found that in drier years, when a larger portion of the wetlands dried up, ammonia concentrations increased. As the soil dried, it naturally emitted ammonia. In wetter years, ammonia concentrations were lower.
As Africans expand agricultural production in coming years, many areas could see higher concentrations of atmospheric ammonia. Similar trends have already played out across the globe. “Satellite analyses can help start to bridge the monitoring gap, providing early analyses of how changes in agriculture and other sources of ammonia are affecting the atmosphere,” Hickman said.
“These results are important to keep in mind as the world experiences a growing population and huge challenges with food security,” Hickman added. “Understanding how human-made and natural ammonia emission sources are changing is important for ensuring policies and technologies that promote sustainable agricultural development.”
NASA Earth Observatory images by Joshua Stevens, using data courtesy of Hickman, J. E., et al. (2021). Story by Alison Gold, NASA Earth Science News Team, with Mike Carlowicz.