Scientists at NASA have used satellite observations to measure air pollution's dependence on population in four of the planet's major air pollution regions: the United States, Europe, China and India.
They found 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, according to the research led by Lok Lamsal, of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Lamsal said that energy usage patterns and per capita emissions differ greatly between India and Europe and destpite large populations, Indian cities seem cleaner in terms of NO2 pollution than the study's other regions.
The variation is a reflection of regional differences such as industrial development, per capita emissions and geography.
Previously, researchers have measured the relationship between population and several urban characteristics, like infrastructure, employment and innovation.
Lamsal said that they showed that the relationship is also applicable to pollution and measurement of that relationship is potentially useful for developing future inventories and formulating air pollution control policies.
The researchers focused on nitrogen dioxide, or NO2, a common pollutant from the burning of fossil fuels. The gas is a precursor to the formation of near-ground ozone, which can cause respiratory problems and is a problem in many major metropolitan areas.
NO2 is also unhealthy to breathe in high concentrations. One feature of the gas, however, is that it's a good proxy for urban air quality.
Lamsal and colleagues studied data collected by the Ozone Monitoring Instrument on NASA's Aura satellite, which measures NO2 throughout the atmosphere in the afternoon around the world.
Next they used an air quality computer model to derive from the satellite data the annual mean concentration of the gas near the ground in some of the Northern Hemisphere's major polluting regions, excluding hotspots such as power plants that could skew the urban relationship. By overlaying pollution concentration with population density data, the researchers could examine the relationship.
Results across the different regions showed divergent NO2 surface concentrations in urban areas of 1 million people: 0.98 parts per billion (U.S.), 1.33 ppb (Europe), 0.68 ppb (China) and 0.23 ppb (India). The same regions saw various degrees of pollution increases in cities with population of 10 million people: 2.55 ppb (U.S.), 3.86 ppb (Europe), 3.13 ppb (China) and 0.53 ppb (India).
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, although in China the increase was much larger, by about a factor of five.
Even though larger cities are typically more energy efficient with lower per-capita emissions, more people still translates to more pollution. But the study reveals some noteworthy regional differences.
The study has been published in Environmental Science and Technology.
The researchers said that further investigation is needed in order to clarify the causes behind the regional differences.