Also quoted here: Scientists Relate Urban Population to Air Pollution(Science Daily, Aug. 19, 2013)
Today we review a new look at the relationship between the size of cities and the level of air pollution world-wide, as estimated from satellite measurements of nitrogen dioxide, whose main source is vehicle emissions. Although there were regional differences, the pollution more than doubled when a city’s population increased from 1 to 10 million and even more (by a factor of 5) in China with its large number of new megacities. This result has major implications for urban planning of developing large cities, especially where vehicle emissions is a major contributer to that pollution and little is being done to limit them.
“Nitrogen dioxide (NO2) released primarily from combustion processes, such as traffic, is a short-lived atmospheric pollutant that serves as an air-quality indicator and is itself a health concern.”
“The correlation of surface NO2 with population is significant for the three countries and one continent examined here: United States (r = 0.71), Europe (r = 0.67), China (r = 0.69), and India (r = 0.59)”
“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” "Measurement of that relationship is potentially useful for developing future inventories and formulating air pollution control policies."
“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,”
"Despite large populations, Indian cities seem cleaner in terms of NO2 pollution than the study's other regions."