A Clearer View of Hazy SkiesRemote Sensing Provides a Global Perspective on Pollution in the Atmosphere
When astronauts talk about viewing Earth from space, the conversation often turns to the planet’s mesmerizing beauty. They describe views of aquamarine coral reefs glimmering amidst the deep blue ocean; of armies of sand dunes marching across deserts; of clouds and lightning flashes dancing through the atmosphere.
For many, the view is deeply humbling. “For the first time in my life, I saw the horizon as a curved line. It was accentuated by a thin seam of dark blue light: the atmosphere,” said Ulf Merbold, a German astronaut who flew on Space Shuttle Columbia in 1983. “This was not the ‘ocean’ of air I had been told it was…I was terrified by its fragile appearance.”
For some astronauts, that thin blue line has appeared quite vulnerable. Many have noticed palls of haze lingering over parts of the world, the result of millions of tiny particles drifting in the atmosphere. Aerosol particles, which can be either liquid or solid, obstruct sunlight and cause distinct and vibrant features to blend into a hazy, featureless mélange of gray.
The particles that affect visibility have many sources, some of them natural. For instance, winds blow bits of dust and dried soil aloft; volcanoes occasionally belch thick plumes of ash; forest fires produce smoke; even vegetation and plankton can emit substances that contribute to haze.
But many of the particles are the result of human activity. Coal-burning power plants, smelters, and other industrial sources can produce sulfur dioxide gas, which reacts in the atmosphere to produce light-scattering sulfate particles. Combustion engines (mostly vehicles) release nitrogen oxide, which can form nitrate particles. Diesel vehicles, wood stoves, and cook stoves emit sooty black carbon. And while aerosol particles have the biggest impact on visibility, certain gases contribute to air pollution and harm human health. Ozone and sulfur dioxide, for instance, are mostly invisible to human eyes but noxious to the lungs.
From the time of the first weather satellites and human space flights in the 1960s, there were hints of air pollution. Some of the first arrived when Air Force scientists began to notice “anomalous gray shades” over the oceans as viewed by Defense Meteorological Satellite Program satellites. In 1965, Gemini VII astronauts took one of the first photographs of industrial air pollution from space. It wasn’t much to look at—a few faint smudges emanating from an otherwise grainy image of Galveston Bay, Texas.
Five decades later, the signs of air pollution are much clearer. Dozens of satellites now orbit Earth, and some of them collect information about polluting particles and gases. “In comparison to those first glimpses, the imagery and data we're getting back now are akin to looking at a modern high-definition, movie-sized color television rather than an old black-and-white set with bunny ears,” said Bryan Duncan, an atmospheric scientist at NASA’s Goddard Space Flight Center and deputy project scientist for Aura, an Earth-observing satellite.
Indeed, satellites are now sending back remarkable views of pollution that an astronaut with perfect vision could never see. Many modern satellites sense wavelengths beyond the visible portion of the spectrum, so they have mapped hot spots of gaseous pollutants over cities, power plants, oil and gas fields—even shipping lanes—that would otherwise be invisible.
The data offer reasons for both optimism and worry. On the one hand, the once heavily polluted skies over North America and Europe have cleared considerably in recent decades due to regulations, technology improvements, and economic changes. On the other hand, satellite sensors show the opposite happening in Asia, as populations increase and countries industrialize.
Serious challenges remain. As our ability to view air pollution on a global scale has improved, so has our understanding of its widespread health consequences. According to a 2014 report from the World Health Organization, about one in eight deaths is associated with exposure to air pollution, making it “the world’s largest single environmental health risk.”