NASA scientists find evidence that directly links urban population to air pollution.

You are bound to be subjected to more air pollution if you live in a big city compared to people living in smaller adjacent areas.  Though it is no secret that population is a major contributing factor to air population, until NASA's recent study, there has been no proper evidence that shows to what extent population contributes to air pollution.

In a new study, scientists from NASA directly measured air pollution's dependence on population in four of the planet's major air pollution regions: the United States, Europe, China and India. Researchers found that the link between population and pollution varies from region to region. For example, researchers found that people living in a city of 1 million people in Europe experience six times higher nitrogen dioxide pollution than an equally populated city of 1 million people in India. Researchers have previously measured the relationship between population and several urban characteristics, such as infrastructure, employment and innovation.

 "We show that the relationship is also applicable to pollution," Lok Lamsal, of NASA's Goddard Space Flight Center in Greenbelt, Md said. "Measurement of that relationship is potentially useful for developing future inventories and formulating air pollution control policies."

For this study, researchers concentrated on nitrogen dioxide, a common air pollutant that is dangerous if inhaled in large quantities. They studied data collected by the Ozone Monitoring Instrument on NASA's Aura satellite, which measures NO2 in the atmosphere in the afternoon around the world. Using an air quality model, researchers then derived the annual mean concentration of the gas near the ground in some of Northern Hemisphere's major polluting regions.

The findings revealed different NO2 surface concentrations in urban areas of 1 million people like 0.98 parts per billion in the U.S., 1.33 ppb in Europe, 0.68 ppb in China and 0.23 ppb in India.

"Energy usage patterns and per capita emissions differ greatly between India and Europe," Lamsal concluded. "Despite large populations, Indian cities seem cleaner in terms of NO2 pollution than the study's other regions." 

The researchers said that further studies are required to determine the reasons behind regional differences.

An earlier study by researchers from the University of North Carolina found that over two million deaths occur annually as a direct consequence of man-made air pollution.