Pollution emitted in tropical countries, like India, is driving the increase in global ozone - one of the primary causes of air pollution-related respiratory problems and heart disease, scientists say.
Since the 1980s, air pollution has increased worldwide, but has increased at a much faster pace in tropical regions.
Research from the University of North Carolina at Chapel Hill in the US shows that this changing global emissions map is creating more total ozone worldwide compared to the amount of pollution being emitted, signalling an effect that could be difficult to reign in without strategic policy planning.
"Emissions are growing in places where there is a much greater effect on the formation of ozone," said Jason West, from UNC-Chapel Hill.
"A tonne of emissions in a region close to the equator, where there is a lot of sunlight and intense heat, produces more ozone than a tonne of emissions in a region farther from it," said West.
The study provides a much-needed path forward on where in the world to strategically reduce emissions of pollutants that form ozone, which - when present in the lower atmosphere - is one of the primary causes of air pollution-related respiratory problems and heart disease.
West said that China's emissions increased more than India's and Southeast Asia's from 1980 to 2010, but Southeast Asia and India, despite their lower growth in emissions during this period, appear to have contributed more to the total global ozone increase due to their relative proximity to the equator.
The reason is that ozone, a greenhouse gas and toxic air pollutant, is not emitted but forms when ultraviolet light hits nitrogen oxides.
When these pollutants interact with more intense sunlight and higher temperatures, the interplay speeds up the chemical reactions that form ozone.
Higher temperatures near the equator also increase the vertical motion of air, transporting ozone-forming chemicals higher in the troposphere, where they can live longer and form more ozone.
"We thought that location was going to be important, but we did not suspect it would be the most important factor contributing to total ozone levels worldwide. Our findings suggest that where the world emits is more important than how much it emits," said West.
Researchers used a computer model to simulate the total amount of ozone in the troposphere, atmosphere's part where ozone is harmful to humans, between 1980 and 2010.
They used a unique European data set of ozone observations from commercial aircraft to confirm the strong increases in ozone above Asia.
Then they superimposed a map of how much pollution the world was emitting in 1980 onto where the world was emitting it in 2010, and vice versa, in addition to another scenario of the growth of methane gas, to determine what is driving the world's increase in ozone production.
The study appears in the journal Nature Geoscience.
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