Since the 1980s, air pollution has increased worldwide, but it has increased at a much faster pace in regions close to the equator. Researchers from the University of North Carolina at Chapel Hill, the University of Colorado, Boulder and their colleagues have now shown that this changing global emissions map is creating more total tropospheric ozone worldwide compared to the amount of pollution being emitted, signaling an effect that could be difficult to reign in without strategic policy planning.
In the study, published in Nature Geoscience, the team used a global chemical transport model to simulate changes in tropospheric ozone concentrations from 1980 to 2010, and to separate the influences of changes in the spatial distribution of global anthropogenic emissions of short-lived pollutants, the magnitude of these emissions, and the global atmospheric methane concentration. They found that the increase in ozone burden due to the spatial distribution change slightly exceeds the combined influences of the increased emission magnitude and global methane.
Based on their findings, they suggested that emission increases in Southeast, East and South Asia may be most important for the ozone change. The spatial distribution of emissions dominates global tropospheric ozone, suggesting that the future ozone burden will be determined mainly by emissions from low latitudes, they concluded.
Emissions are growing in places where there is a much greater effect on the formation of ozone. A ton of emissions in a region close to the equator, where there is a lot of sunlight and intense heat, produces more ozone than a ton of emissions in a region farther from it.—J. Jason West, corresponding author
The work provides strategic insight on where in the world to reduce emissions of pollutants that form ozone. When present in the lower atmosphere (troposphere), ozone 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 proximity to the equator.
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.
The findings were surprising. We thought that location was going to be important, but we didn’t 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.—J. Jason West
The findings point to several strategies for reducing ground-level ozone across the world, such as decreasing emissions of ozone precursors in regions close to the equator, particularly those with the fastest growth of emissions.
A more challenging scenario is that even if there is a net reduction in global emissions, ozone levels may not decrease if emissions continue to shift toward the equator. But continuing aircraft and satellite observations of ozone across the tropics can monitor the situation and model forecasts can guide decision making for controlling global ozone pollution.—Owen R. Cooper, co-author
Yuqiang Zhang, Owen R. Cooper, Audrey Gaudel, Anne M. Thompson, Philippe Nédélec, Shin-Ya Ogino & J. Jason West (2016) “Tropospheric ozone change from 1980 to 2010 dominated by equatorward redistribution of emissions” Nature Geoscience doi: 10.1038/ngeo2827