Using data collected by three of NASA's high-tech aerosol monitoring satellites, Israeli scientists have tracked pollution trends for 189 megacities, including New York City, Tokyo and Mumbai.
Called MODIS-Terra, MODIS-Aqua, and MISR, the satellites were launched by NASA from 2000 through 2002. Their combined data constitute an accurate survey of aerosol concentrations a few hundred metres above the Earth.
Fifty-eight of these megacities have populations exceeding five million. The thickest layers of global smog-caused by traffic, industry, and natural minerals, among other factors, are found over the world's megacities, the American Journal of Climate Change reported.
On-the-ground monitoring stations do not always provide the most accurate picture as monitoring stations depend heavily on local positioning, and some cities put stations in urban centres while others build on the edge of a city.
Pinhas Alpert, professor of geophysics and planetary science at the Tel Aviv University (TAU) and head of the Porter School of Environmental Studies, with graduate student Olga Shvainshteinand and Pavel Kishcha, is relying on satellites for a comprehensive view of atmospheric pollutants.
Their method is the first to provide standardised global testing of pollution levels, which Alpert believes will also hold countries accountable for their emissions and encourage more environmentally friendly practices, according to a TAU statement.
Alpert likens the use of three satellites to the traditional Jewish idea of the three-judge panel.
"In the Jewish tradition, individual judges don't decide cases. There must be a minimum of three. You need a majority opinion," he said.
"By merging the data from three imperfect sensors, their flaws are mostly counterbalanced. In cases where the three sensors show differing signs of pollution levels, more research is required."
Europe and Northeast and Central North America are seeing the largest decreases in aerosol concentrations overall.
Among the cleanest cities were Houston, with a 31 percent decrease over the time period; Curitiba, Brazil, with a 26 percent decrease; and Stockholm, Sweden, with a 23 percent decrease.