NASA Scientists Reveal How Air Pollution Relates to Major City Populations

How much air pollution are you sucking into your lungs every day? That's a good question. NASA scientists have taken a look at several major cities and have found out exactly how a city's pollution relates to the size of its population.

In order to examine exactly how population affects air pollution, the researchers employed satellite observations. This allowed them to directly measure air pollution's dependence on population in four of the planet's major air pollution regions: the United States, Europe, China and India. More specifically, the scientists focused on nitrogen dioxide, or NO2, which is a common pollutant from the burning of fossil fuels. The gas is also a precursor to the formation of near-ground ozone, which can cause respiratory problems and is an issue in many major cities.

After collecting the data from the satellite observations, the researchers then used an air quality computer model to derive the annual mean concentration of the gas near the ground, excluding hotspots such as power plants that could skew the urban relationship. They then overlaid pollution concentration with population density data to find out exactly how the two related to one another.

So what did they find? It turns out that the pollution-population relationship varies by region. For example, a city of 1 million people in Europe experiences six times higher nitrogen dioxide pollution than an equally populated city of 1 million people in India. This particular variation is a reflection of regional differences that include industrial development, per capita emissions and geography.

The researchers also found that the contribution to air pollution from surface-level NO2 in each region more than doubled when cities increased in population from 1 million to 10 million people. However, in China this increase was much larger--by about a factor of five.

The findings reveal exactly how pollution relates to population concentrations in various cities. While larger cities are typically more energy efficient with lower per-capita emissions, more people still translates to more pollution. Yet it also shows that there are vast regional differences depending on where the city is.

"Despite large populations, Indian cities seem cleaner in terms of NO2 pollution than the study's other regions," said Lok Lamsal, one of the researchers, in a news release.

The findings are important for understanding the connections between pollution and cities. In addition, it could help in the future development of pollution control policies.

The findings are published in the journal Environmental Science & Technology.