Using a new satellite-based method, NASA scientists have located 39 unreported and major human-made sources of toxic sulphur dioxide emissions.

The unreported emission sources, found in the analysis of satellite data from 2005 to 2014, are clusters of coal-burning power plants, smelters, oil and gas operations found notably in the Middle East, but also in Mexico and parts of Russia.

In addition, reported emissions from known sources in these regions were—in some cases—two to three times lower than satellite-based estimates.

Altogether, the unreported and underreported sources account for about 12 percent of all human-made emissions of sulphur dioxide - a discrepancy that can have a large impact on regional air quality, said lead author of the study Chris McLinden, atmospheric scientist with Environment and Climate Change Canada in Toronto.

Sulphur dioxide (SO2) is a known health hazard and contributor to acid rain.

Current, sulphur dioxide monitoring activities include the use of emission inventories that are derived from ground-based measurements and factors, such as fuel usage.

“We now have an independent measurement of these emission sources that does not rely on what was known or thought known,” McLinden noted.

“When you look at a satellite picture of sulphur dioxide, you end up with it appearing as hotspots - bull’s-eyes, in effect—which makes the estimates of emissions easier,” he said.

The findings were detailed in the journal Nature Geoscience.

“Quantifying the sulphur dioxide bull’s-eyes is a two-step process that would not have been possible without two innovations in working with the satellite data,” co-author Nickolay Krotkov, atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, pointed out.

First was an improvement in the computer processing that transforms raw satellite observations from the Dutch-Finnish Ozone Monitoring Instrument aboard NASA’s Aura spacecraft into precise estimates of sulphur dioxide concentrations.

Being able to detect smaller concentrations led to the second innovation.

The researchers used a new computer programme to more precisely detect sulphur dioxide that had been dispersed and diluted by winds.

They then used accurate estimates of wind strength and direction derived from a satellite data-driven model to trace the pollutant back to the location of the source, and also to estimate how much sulphur dioxide was emitted from the smoke stack.