City pollution from cars smogging your view? Have a look at the coastal pollution in the map shown above. It’s not an atmospheric pollutant that you can see with your naked eye, but its presence can lead to cardiovascular and respiratory problems in humans.

What is it? Nitrogen dioxide (NO2), which in elevated amounts lead to unhealthy levels of nitrogen oxides (NOx), fine particles, and ozone in the air we breath.

Just what is causing this red alert pattern along the coasts? That’s the rub, it’s a combination of city pollution, off-shore drilling, and ship traffic.

Ships and airplanes can both leave whispy cloud tracks called “contrails” in the air that are easily seen from space.

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But the specific amount of NO2 that ships release near the coasts gets lost among the other emissions of nitrogen oxides from other pollution sources.

Shipping is estimated to contribute 15 to 30 percent to the global NOx pollution. To test this, several satellites are monitoring atmospheric NO2 levels. This map shows the ocean and sea-based contribution of NO2 into the atmosphere from 2005 to 2012 as measured by a Dutch and Finnish-built Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite.

Take a look at the difference between the hazy NO2 level drifting between southern Africa and Australia, the result of westerly winds blowing soot from fires in Africa across the Indian Ocean, and the distinct red line along the transoceanic shipping route between Sri Lanka and Singapore.

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Other areas of the ocean such as in the Pacific and the Atlantic have just as much shipping traffic, but the routes do not leave such a precise NO2 trail for the satellite to detect, because in those waters ships are often forced to skirt around storms.

Earth and climate scientists work with maps like this one to help better evaluate the human contribution to atmospheric pollution.