Study finds power law scaling relationship between urban population and NO2 pollution
25 August 2013
A study by NASA scientists and their colleagues at Dalhousie University (Canada) and NOAA based on satellite data has found a significant correlation of surface nitrogen dioxide (NO2) with population in the three countries and one continent examined: United States (r = 0.71), Europe (r = 0.67), China (r = 0.69), and India (r = 0.59). These regions contain two-thirds of total global anthropogenic NOx emissions. NO2 is 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.
Urban NO2 pollution, they found, like other urban properties, is a power law scaling function of the population size: NO2 concentration increases proportional to population raised to an exponent. (Even though larger cities are typically more energy efficient with lower per-capita emissions, more people still translates to more pollution.) The value of the exponent varies by region from 0.36 for India to 0.66 for China, reflecting regional differences in industrial development and per capita emissions. The study was published in the ACS journal Environmental Science & Technology.
The NO2 scaling laws investigated here have profound implications for air-quality management. For example, in a Chinese city of 10 million people, the average outdoor ambient NO2 concentrations are 100.66 = 4.6 times higher than in an average city of 1 million. This greater concentration occurs even though cities with higher populations have higher energy efficiency and, therefore, lower per capita NO2 emissions. For example, in China, the total NOx emissions from an urban area are a factor of 100.63 = 4.3 times lower from one larger average city of 10 million people compared to 10 smaller average cities with 1 million people each. The other regions that we examined exhibited smaller increases in NO2 pollution with population, reflecting a combination of emission controls and per capita fuel use.—Lamsal et al.
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, Md. The variation is a reflection of regional differences such as industrial development, per capita emissions and geography.
The researchers used tropospheric NO2 columns retrieved from the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite, which measures NO2 throughout the atmosphere in the afternoon around the world. They then used local scaling factors from a three-dimensional chemistry-transport model (GEOS-Chem) to relate the OMI NO2 columns to ground-level concentrations. The OMI-derived surface NO2 data are significantly correlated (r = 0.69) with in situ surface measurements.
They selected 239 US, 757 European, 244 Chinese, and 265 Indian cities with population greater than 50,000; they excluded urban domains containing large power plants, identified as CO2 annual emissions >18 Tg for the year 2007, that could skew the results.
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.
Energy usage patterns and per capita emissions differ greatly between India and Europe. Despite large populations, Indian cities seem cleaner in terms of NO2 pollution than the study’s other regions.—Lok Lamsal, lead author
The researchers say that further investigation is needed in order to clarify the causes behind the regional differences.
L. N. Lamsal, R. V. Martin, D. D. Parrish, and N. A. Krotkov (2013) Scaling Relationship for NO2 Pollution and Urban Population Size: A Satellite Perspective. Environmental Science & Technology 47 (14), 7855-7861 doi: 10.1021/es400744g
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