The photo above was taken by the satellite showing the atmosphere above southwestern Asia. Cold winter weather and burgeoning industrial economies have made for difficult breathing in Asia and the Middle East. News reports from Tehran, Beijing, and other cities have described hazy skies with very low visibility; restrictions on driving, factory operations, and outdoor activity; and hospitals full of people with lung ailments.
The map above shows the concentration of nitrogen dioxide in the atmosphere. Shades of orange reflect the relative abundance of the polluting gas. Nitrogen dioxide is a key emission from the burning of fossil fuels by cars, trucks, power plants, and factories; the combustion of fuel also produces sulfur dioxides and aerosol particles. When the weather is hot and sunlight strongest, nitrogen dioxide emissions usually lead to the creation of ground-level ozone. In the winter, the gas is less likely to breed ozone, but it does linger for a long time and contribute to fine particle pollution. Year-round, it is a good proxy for the presence of air pollution.
About a quarter-century ago, scientists and policymakers unveiled what the United Nations calls “the most successful treaty in UN history.” On September 16, 1987, the first 24 nations signed on to the Montreal Protocol on Substances that Deplete the Ozone Layer; 173 more have signed on in the years since. The international agreement likely saved the world from an environmental crisis, while setting an example for how to develop and implement environmental policy.
Prompted by scientific observations from the laboratory, the ground, aircraft, and satellites, the Montreal Protocol first reduced and then banned the chlorine- and bromine-based chemicals (particularly chlorofluorocarbons, or CFCs) that destroy atmospheric ozone. The destruction of the ozone layer allows more of the Sun’s ultraviolet radiation to reach the surface of the planet, increasing the risk of sunburns, skin cancer, and eye damage. The most prominent and infamous sign of depletion is the annual “ozone hole” that forms around the South Pole.
In 1979—when scientists were just coming to understand that atmospheric ozone could be depleted—the area of ozone depletion over Antarctica grew to 1.1 million square kilometers, with a minimum ozone concentration of 194 Dobson Units. In 1987, as the Montreal Protocol was being signed, the area of the hole reached 22.4 million square kilometers and ozone concentrations dropped to 109 DU. By 2006, the worst year for ozone depletion to date, the numbers were 29.6 million square kilometers and just 84 DU. By 2011, the most recent year with a complete data set, the hole stretched 26 million square kilometers and dropped to 95 DU. According to NASA scientist Pawan Bhartia, “The Antarctic hole is stabilizing and may be slowly recovering. Our focus now is to make sure that it is healing as expected.” The amount of ozone-depleting substances (ODS) in the atmosphere has stopped rising in recent years, and may actually be decreasing.
About a decade later, the effects of these federal and state efforts have left the air far cleaner than it was before. Total nitrogen oxide emissions have fallen by 47 percent in the United States since the late 1990s. Emissions from electric power generation and highway vehicles, the two largest sources, have decreased by 68 percent and 43 percent respectively, according to EPA statistics. The declines are even large enough to be seen from space. The measurements by NASA are depicted in the map above. The larger the yellow circle, the larger the decrease in polluting gases.
CITES:
Jesse Allen and Michael Carlowicz., using data provided courtesy of the Aura OMI science team.
Robert Simmon, using imagery from the Ozone Hole Watch. Caption by Mike Carlowicz.
Robert Simmon, using data from Duncan et al., 2013. Caption by Adam Voiland.
