Livestock operations can be a major source of ammonia emissions.

Melanie Blanding/Alamy Stock Photo

In advance, satellites reveal hidden hot spots of ammonia pollution

Ammonia is a significant ingredient in smog and a growing problem, but scientists have struggled to track emissions in detail from sources such as animal feedlots and fertilizer factories. Now, satellite data described in a new paper can help them pinpoint and measure ammonia hot spots, potentially providing an independent way of tracking progress toward national pollution targets.

“It’s a real historic moment,” says Mark Sutton, an environmental physicist with the Centre for Ecology & Hydrology in Edinburgh. “This paper presents a landmark, an incredible level of detail.”

Compared with soot, ozone, and other air pollutants, ammonia is a neglected stepchild, Sutton says, both in research and air quality regulations. Part of the difficulty is simply measuring it. The chemical typically stays in the atmosphere for less than a day, before it reacts with other molecules and turns into particulate matter. So it’s not easy to measure concentrations, even when studying it with ground-based instruments. In the Netherlands, for example, “We still can’t get a good picture of ammonia” despite a large number of monitoring stations, says Jan Willem Erisman, a nitrogen expert at the Vrije Universiteit Amsterdam. “This tells you the importance of satellite measurements.”

Remote sensing experts, however, were long skeptical of the chances of measuring ammonia with satellites, partly because of its low concentrations. “It’s one of the pollutants that we never thought we could measure from space,” says Cathy Clerbaux, a physicist with the French national research agency CNRS in Paris. Clerbaux oversees a satellite-based instrument called the infrared atmospheric sounding interferometer (IASI) that can easily measure the concentration of molecules such methane and ozone. But within a few years of the launch of the first IASI in 2007, Clerbaux and colleagues had teased out an initial map of major sources of ammonia, such as smoke from large fires. “It was very crude,” she recalls.

Now, they have refined the maps considerably, thanks to a clever way of processing 9 years of data, resolving emissions within square kilometers. With a near-global map of ammonia emissions, Martin van Damme of the Free University of Brussels set about identifying the sources of the most intense emissions. By checking images from Google Earth and other sources, Van Damme identified 83 places with agricultural sources, mostly large concentrations of cows, pigs, or chickens, where ammonia escaped from ponds or heaps of animal waste. Another 130 hot spots were factories that make fertilizer from ammonia, they report this week in Nature. The maps also include large areas where ammonia is released from manure or fertilizer applied to fields, such as the Indo-Gangetic Plain of India and Pakistan. (Vehicles emit ammonia from catalytic converters, contributing to poor air quality in cities, but those emissions don’t show up in the maps.)

The new data could help fill in gaps in the most comprehensive collection of air pollution records, called the Emissions Database for Global Atmospheric Research (EDGAR), which is run by the European Union’s Joint Research Centre in Ispra, Italy. The data in EDGAR are used in a variety of computer models for evaluating the environmental effects of ammonia and long-distance transport of pollution. Most of the ammonia hot spots were missing from EDGAR, which relies on countries to supply data. In addition, the hot spots that were present in the database tended to list much lower emissions than those found in the satellite data. Part of the reason for the discrepancies could be the difficulty of measuring ammonia from the ground, Clerbaux says. And some countries may not have regulations that require reporting of ammonia emissions, Sutton adds.

Satellites have limitations. IASI can’t see through clouds, for example, and it requires a temperature difference between the ground and atmosphere, which makes it hard to gather data from some places. Sutton says that, at the moment, the data are better for assessing relative changes rather than absolute amounts of ammonia. Still, the ability to compare overall emissions from countries will stimulate progress in reducing ammonia emissions, Sutton says.

The European Union, where ammonia is responsible for about 60% of particulate air pollution, has a target of reducing ammonia emissions by 6% by 2020, relative to 2005 levels. But ammonia levels are increasing, because of increased use of fertilizer and meat production. “The ambition is low, but the embarrassment of not being able to comply raises attention,” Sutton says. The capability to monitor progress by satellite “will shine a spotlight on ammonia and the need to take action.”