Importance of stratospheric ozone

In the troposphere, the layer of the atmosphere closest to Earth, ozone is typically present only in low concentrations. Its concentrations are greatest in the second layer of Earth’s atmosphere, the stratosphere, and peak at around 25 kilometres altitude. At this level of the atmosphere, ozone is beneficial because it absorbs damaging UV sunlight, reducing the levels experienced at Earth’s surface.

Ozone-depleting substances and the ozone hole

Ozone-depleting substances are synthetic gases that destroy stratospheric ozone. These harmful substances are mostly used as refrigerants in air conditioners and refrigerators, as propellants in aerosol cans, in fire extinguishers, and as solvents. Most ozone-depleting substances are also greenhouse gases.

In the 1980s, scientists were concerned that stratospheric ozone was at risk from ozone-depleting substances, with associated increases in UV sunlight reaching Earth’s surface. Increases in UV sunlight due to depleted ozone were observed globally during the 1980s and 1990s, particularly at high latitudes (McKenzie et al, 2011).

The thickness of the ozone layer in a column of air is measured in Dobson units (DU). One DU represents the amount of ozone molecules needed to produce a 0.01-millimetre layer of pure ozone at Earth’s surface. The ozone ‘hole’ is an area where the ozone layer is less than 220 DU (NASA, 2017a). Each spring, the hole forms over Antarctica. It is caused mostly by ozone-depleting substances.

In winter, ozone-depleting substances are trapped within the polar vortex, an ever-present clockwise flow of air around Antarctica that strengthens over winter. Polar stratospheric clouds form within the polar vortex. Ozone-depleting substances react with these clouds, producing reservoirs of chlorine and bromine. In spring, UV sunlight reacts with these chemicals in a process that quickly breaks down the ozone layer, leading to the springtime hole over Antarctica.

Since the 1987 Montreal Protocol under the Vienna Convention (see box 8), the global production of ozone-depleting substances has decreased by 98 percent (data from 1986 to 2015). The ozone hole has started to shrink in response. In 2016, the mean maximum size of the ozone hole was 20.9 million square kilometres, a 21 percent decrease from its largest mean maximum size in 2006 (26.6 million square kilometres). It is possible that the ozone hole will cease to form by the middle of this century, and ozone levels will return to their normal levels, more than 60 years since the world took action to reduce emissions of the harmful substances (Solomon et al, 2016). This is an excellent example of the benefits of international policy on environmental outcomes.

New Zealand’s use of ozone-depleting substances

New Zealand does not manufacture ozone-depleting substances and has phased out their importation and domestic use as required by the Montreal Protocol. We do, however, use some ozone-depleting substances in essential cases. For example, we use hydrochlorofluorocarbons for health and safety, such as in inhalers and fire extinguishers. We use methyl bromide for quarantine, to prevent the introduction of harmful pest organisms into our environment, and for pre-shipment purposes required by our trading partners. There is currently no internationally accepted alternative to methyl bromide. Common goods that are fumigated include log and timber products for export, imported fruits and vegetables, and contaminated shipping containers (Minister for the Environment, 2015).

New Zealand’s ozone concentrations, UV levels, and global warming

The ozone layer over New Zealand thins during summer, providing less protection from UV sunlight at a time when Earth is closest to the sun. Long-term average daily ozone concentrations vary during the year by about 29 percent, with minimum concentrations in March and maximum concentrations in October.

The ozone hole does not have a large effect on ozone concentrations over New Zealand, and therefore on our UV levels. In early spring when the ozone hole forms over Antarctica, ozone amounts are naturally relatively high in New Zealand. However, when the ozone hole breaks up in late spring, it can send ‘plumes’ of ozone-depleted air over New Zealand. This briefly decreases column ozone levels by about 5 percent, about the same amount as daily variation (Ajtić et al, 2004).

Studies are beginning to emerge about how UV levels and ozone concentrations over New Zealand might be affected by changes in winds and cloud patterns associated with global warming and its interactions with ozone depletion (UNEP, 2017). These complex interactions are likely to result in both risks and benefits of exposure to UV sunlight for the environment and society (Williamson et al, 2014).