When astronaut William Anders flew on the first manned mission to orbit the moon in 1968, he photographed the surreal view of Earth rising above the lunar horizon. "We came all this way to explore the moon, and the most important thing is that we discovered the Earth," Anders famously said. Back on the planet though, problems were brewing in the atmosphere.

Exhaust from cars and pollutants like sulfur from power plants masked the sky. In large cities, air pollution caused some people to experience burning in their lungs and eyes. Acid rain contaminated fresh water sources and damaged plant life. Earth's atmosphere was experiencing chemical chaos, but scientists didn't necessarily know the extent or have a detailed explanation. Nevertheless, the U.S. government introduced the 1970 Clean Air Act to reduce some of the pollutants and chemical chaos.

Since then, decades of satellite-based observations have helped researchers understand the chemical processes in the atmosphere that affect human lives in the short-term and long-term.

The atmosphere begins at the ground level where people live and breathe. Air pollution can be seen as a smoky haze lingering in the sky and blocking the sun's light. As early as the 1940s, Los Angeles would shut down for days as large masses of toxic smog filled the sky and burned people's throats and eyes. At the time, the cause and long-term effects of the noxious clouds were not well understood.

Today, scientists know that this Los Angeles pollution was different from the sulfur-laden smog of coal towns. This modern smog contained ozone - a pollutant that is not emitted from tailpipes or smoke stacks but is formed through chemical reactions in the sunlit atmosphere. While ground-level ozone is not presently measured by any space-based instrument, the Aura satellite can measure nitrogen dioxide, which is a key ingredient to the formation of ozone.

Narrowing Down Nitrogen Oxides\
Pollutants flood into the atmosphere through human activities, such as agricultural fires, power plant emissions and exhaust from the tailpipes of cars. One major pollutant is nitrogen dioxide - a brownish gas that can envelop cities. Nitrogen dioxide can lead to respiratory problems, but is also an ingredient of ground-level ozone, which can burn the lungs when inhaled.

The Ozone Monitoring Instrument (OMI) on the Aura satellite began monitoring levels of nitrogen dioxide worldwide shortly after its launch. OMI data show that nitrogen dioxide levels in the United States have decreased at a rate of 4 percent per year from 2005 to 2010 - a time period when stricter government policies on power plant and vehicle emissions came into effect. As a result, ground-level ozone concentrations also decreased. OMI data also showed a 2.5 percent decrease of nitrogen dioxide per year during the same time period in Europe, which had enacted similar legislation.

While air quality in the United States has improved, the issue still persists nationwide. Since bright sunlight is needed to produce unhealthy levels of ozone, ozone pollution is largely a summertime issue. As recent as 2012, about 142 million people in America- 47 percent of the population- lived in counties with pollution levels above National Ambient Air Quality Standards, according to the Environmental Protection Agency. The highest levels of ozone tend to occur on hot, sunny, windless days.

Air pollution also remains an issue worldwide. The World Health Organization reported that air pollution still caused one in eight deaths worldwide in 2012. Outside of the United States and Europe, OMI showed an increase in nitrogen dioxide levels. Data from 2005 to 2010 showed China's nitrogen dioxide levels increased at about 6 percent and South East Asia increased levels at 2 percent per year. Globally, nitrogen dioxide levels increased a little over half a percent per year during that time period.

"We knew we could measure nitrogen dioxide levels, but I don't think anyone thought we would be able to actually get quantitative emissions estimates," said Anne Douglass, Aura project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "That's important because in North America and Europe, you have good estimates of how much of a pollutant is going into the atmosphere, but in most of the world, you don't."

Sifting Through Sulfur Dioxide
People also influence the production of sulfur dioxide, an invisible gas pollutant primarily emitted into the atmosphere by coal-burning power plants and melting and extracting metals from ore via smelting. Sulfur dioxide combines with other chemicals in clouds to produce acid rain or becomes sulfate aerosols that affect health and climate. Over the past decade, OMI identified large sulfur dioxide concentrations around power plants and volcanoes.

In 2005, the U.S. Environmental Protection Agency passed the Clear Air Interstate Rule that focused on reducing power plant pollution even more, especially pollutants drifting from one state to another. The rule requires 27 eastern states and Washington, D.C., to reduce air pollutants, namely sulfur dioxide and nitrogen dioxide.

Many U.S. power plants installed devices that limit the release of sulfur dioxide in the atmosphere. OMI data showed a 40 percent decline in sulfur dioxide concentrations over the largest power plants from the 2005-2007 period to the 2008-2010 period. OMI also observed large decreases in nitrogen dioxide around power plants during that time period. However, similar to nitrogen dioxide, many other countries have not reduced sulfur dioxide emissions. Airing Out Aerosols

Nitrogen and sulfur dioxide concentrations are important targets for reducing air pollution, but many other air pollutants also affect air quality and human health.

Small particles in the atmosphere called aerosols or particulate matter can penetrate deep into a person's lungs, cause allergic reactions and infections and increase the risk of heart and lung disease. Particulate matter is emitted from various sources, such as smoke from burning wood, which increases particulate matter pollution in the winter.

Aerosols are formed from gases like sulfur dioxide and ammonia and can be carried large distances depending on the prevailing wind. Aura's Total Emission Spectrometer (TES) instrument measures ammonia, which comes from animal waste, fertilizers and soil.

"The Tropospheric Emissions Spectrometer measures a lot of constituents that you can't see like ammonia and agricultural emissions," Douglass said. "We're beginning to get ammonia estimates that we did not have before."

Aura's decade of work has set the stage for future air quality monitoring instruments. The European Space Agency will be launching the follow-up TROPOspheric Monitoring Instrument, or TROPOMI, that will continue Aura's OMI measurements with better ground resolution and precision and measure methane and carbon monoxide levels.

NASA also plans to launch the Tropospheric Emissions: Monitoring of Pollution, or TEMPO, instrument that will observe ozone, nitrogen dioxide, sulfur dioxide, formaldehyde and aerosols with higher ground resolution and frequency over the United States, Canada and Mexico.

"Pollution is a global issue because it can travel long distances in the wind," said Douglass. "By using satellites, we can develop a valuable global inventory of pollutants and understand how air quality may be changing."