Smallest ozone hole in decades: how it happened, why it matters

Smallest ozone hole in decades: how it happened, why it matters

The annual ozone hole reached its peak extent of 16. 4 million sq km on September 8, then shrank to less than 10 million sq km for the remainder of September and October, satellite measurements show.

During September and October, the ozone hole over the Antarctic has been the smallest observed since 1982, NASA and US National Oceanic and Atmospheric Administration (NOAA) scientists have reported. The annual ozone hole reached its peak extent of 16. 4 million sq km on September 8, then shrank to less than 10 million sq km for the remainder of September and October, satellite measurements show. NASA has described it as great news for the Southern Hemisphere.

What is ozone and where is it in the atmosphere?

Ozone (O3) is a highly reactive gas composed of three oxygen atoms. It is both a natural and a man-made product that occurs in the Earth's upper atmosphere (the stratosphere) and lower atmosphere (the troposphere).  Depending on where it is in the atmosphere, ozone affects life on Earth in either good or bad ways.

Stratospheric ozone is formed naturally through the interaction of solar ultraviolet (UV) radiation with molecular oxygen (O2).  The "ozone layer," approximately 6 through 30 miles above the Earth's surface, reduces the amount of harmful UV radiation reaching the Earth's surface.

Tropospheric or ground-level ozone – what we breathe – is formed primarily from photochemical reactions between two major classes of air pollutants, volatile organic compounds (VOC) and nitrogen oxides (NOx).  These reactions have traditionally been viewed as depending upon the presence of heat and sunlight, resulting in higher ambient ozone concentrations in summer months.  Within the last decade, however, high ozone concentrations have also been observed under specific circumstances in cold months, where a few high elevation areas in the Western U.S. with high levels of local VOC and NOx emissions have formed ozone when snow is on the ground and temperatures are near or below freezing.  Ozone contributes to what we typically experience as "smog" or haze, which still occurs most frequently in the summertime, but can occur throughout the year in some southern and mountain regions.

Although some stratospheric ozone is transported into the troposphere, and some VOC and NOx occur naturally, the majority of ground-level ozone is the result of reactions of man-made VOC and NOx. Significant sources of VOC are chemical plants, gasoline pumps, oil-based paints, autobody shops, and print shops. Nitrogen oxides result primarily from high temperature combustion. Significant sources are power plants, industrial furnaces and boilers, and motor vehicles.

WHY IT’S SMALL THIS YEAR: There have been abnormal weather patterns in the atmosphere over Antarctica. In warmer temperatures like this year, fewer polar stratospheric clouds form and they don’t persist as long, limiting the ozone-depletion process. While it is good news, NASA has cautioned it is important to recognise that what we are seeing this year is not a sign that atmospheric ozone is suddenly on a fast track to recovery.