Expanding agriculture and increased fertiliser use in Africa are driving up ammonia air pollution, with serious consequences for human health, climate and the health of ecosystems, the National Aeronautics and Space Administration (Nasa) has said.

A new Nasa-led study, the first to document changing atmospheric ammonia (NH3) concentrations in Africa over an extended period, says agriculture is driving up nitrogen deposition, which could adversely affect ecosystems.

Ammonia is emitted naturally from soils and vegetation fires, but agricultural activities such as raising livestock and using fertiliser are also major sources, Nasa says. Ammonia emissions are bound to rise as agriculture scales up to meet the needs of growing populations.

Negative effects

Excess ammonia hinders plant growth and contributes to impairment of water bodies by excessive plant and algae growth (eutrophication) and soil acidification, resulting in a decline in crop productivity and biodiversity.

It also contributes to the production of inorganic aerosols, the primary constituents of fine particulate matter and a serious health hazard that can lead to heart and lung-related illness.

In the Lake Victoria region, the expansion of agricultural area led to increases in ammonia concentration over the study period, pointing to increased usage of fertiliser.

The researchers used satellite data from 2008 to 2018, identifying rises and falls in ammonia concentrations across the continent and the likely causes of those changes.

“We’re using (varying kinds) of satellite observations of the land surface and atmosphere to evaluate how NH3 concentrations have changed over Africa,” NASA said on November 16 in a report published in the journal Atmospheric Chemistry and Physics.

In West Africa, increases in ammonia concentration corresponded with the end of the dry season and a time when farmers were preparing their land by burning it.

Interestingly, ammonia concentration declined over the Sudd wetlands in South Sudan. About half of the Sudd is permanently flooded and the other half is a flood plain.

The researchers found that in drier years, ammonia concentrations over the Sudd increased as the soil dried, naturally emitting the gas. In wetter years, ammonia concentrations were lower.

Urea fertiliser alert

Burning of biomass, wetland shrinkage, environmental events and agricultural activity were shown to have led to the build-up in ammonia air pollution in Africa, with urea fertiliser and livestock excreta being the biggest culprits.

The report also shows that countries with the highest rates of increase in NH3 densities also had high rates of growth in carbon dioxide densities. Livestock numbers were also high.

“Understanding how human-made and natural ammonia emission sources are changing is important for ensuring policies and technologies that promote sustainable agricultural development,” said principal investigator Jonathan Hickman, a research scientist at Columbia University and NASA’s Goddard Institute for Space Studies in New York City.

“These results are important to keep in mind as the world experiences a growing population and huge challenges with food security,” he added.

By 2050, Africa’s population is predicted to be approach 2.5 billion –nearly double what it is today. African governments are promoting fertiliser use to increase food production while the burning of vegetation is common practice on the continent.

As Africa expands its agricultural processes, Hickman said, it will see higher concentrations of atmospheric ammonia. Similar trends have already played out across the globe.

“Fertiliser use in Africa is currently low but growing. Implementing practices that can limit NH3 losses from fertiliser as agriculture is intensified may help mitigate impacts on health and ecosystems,” said NASA. “These studies emphasise the need for agriculture to scale up sustainably.”

“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,” he said. He said they would continue observing ammonia concentrations across the continent into the future, to see how these trends change with time.

“These factors made Africa an important place to study ammonia emissions,” said principal investigator Jonathan Hickman, a research scientist at Columbia University and NASA’s Goddard Institute for Space Studies in New York City.

And adds, “Soil NH3 production is temperature-dependent, doubling with every 5∘C temperature increase, though the actual soil NH3 flux is determined in part by plant and soil physiological and physical factors.”

The 30,000 square kilometre wetland fed by the Nile River, called the Sudd, was the only region that showed a clear decrease in ammonia over the study period.

“We’ve shown here that we can use satellite data to observe trends and monitor emissions of ammonia in specific regions,” said Enrico Dammers, a scientist at the Netherlands Organization for Applied Scientific Research and co-author of the paper.

Although evidence of increasing NH3 over parts of Africa has been observed, NASA says mechanisms behind these trends are not well understood.

Hickman and his team used satellite data observed by the European Space Agency’s Infrared Atmospheric Sounding Interferometer (IASI).

“The change in mean NH3 intensity over the Sudd is strongly correlated with variation in wetland extent in the Sudd. In years when more area remained flooded during the dry season, NH3 intensities were lower,” said a NASA report.