2019年化学周 - Depletion of Ozone Layer

In 1969 Dutch chemist Paul Crutzen published a paper that described the major nitrogen oxide catalytic cycle affecting ozone levels. Crutzen demonstrated that nitrogen oxides can react with free oxygen atoms, thus slowing the creation of ozone (O₃), and can also decompose ozone into nitrogen dioxide (NO₂) and oxygen gas (O₂). Some scientists and environmentalists in the 1970s used Crutzen’s research to assist their argument against the creation of a fleet of American supersonic transports (SSTs). They feared that the potential emission of nitrogen oxides and water vapour from these aircraft would damage the ozone layer. (SSTs were designed to fly at altitudes coincident with the ozone layer, some 15 to 35 km [9 to 22 miles] above Earth’s surface.) In reality, the American SST program was canceled, and only a small number of French-British Concordes and Soviet Tu-144s went into service, so that the effects of SSTs on the ozone layer were found to be negligible for the number of aircraft in operation.

In 1974, however, American chemists Mario Molina and F. Sherwood Rowland of the University of California at Irvine recognized that human-produced chlorofluorocarbons (CFCs)—molecules containing only carbon, fluorine, and chlorine atoms—could be a major source of chlorine in the stratosphere. They also noted that chlorine could destroy extensive amounts of ozone after it was liberated from CFCs by UV radiation. Free chlorine atoms and chlorine-containing gases, such as chlorine monoxide (ClO), could then break ozone molecules apart by stripping away one of the three oxygen atoms. Later research revealed that bromine and certain bromine-containing compounds, such as bromine monoxide (BrO), were even more effective at destroying ozone than were chlorine and its reactive compounds. Subsequent laboratory measurements, atmospheric measurements, and atmospheric-modeling studies soon substantiated the importance of their findings. Crutzen, Molina, and Rowland received the Nobel Prize for Chemistry in 1995 for their efforts.

Human activities have had a significant effect on the global concentration and distribution of stratospheric ozone since before the 1980s. In addition, scientists have noted that large annual decreases in average ozone concentrations began to occur by at least 1980. Measurements from satellites, aircraft, ground-based sensors, and other instruments indicate that total integrated column levels of ozone (that is, the number of ozone molecules occurring per square metre in sampled columns of air) decreased globally by roughly 5 percent between 1970 and the mid-1990s, with little change afterward. The largest decreases in ozone took place in the high latitudes (toward the poles), and the smallest decreases occurred in the lower latitudes (the tropics). In addition, atmospheric measurements show that the depletion of the ozone layer increased the amount of UV radiation reaching Earth’s surface.

This global decrease in stratospheric ozone is well correlated with rising levels of chlorine and bromine in the stratosphere from the manufacture and release of CFCs and other halocarbons. Halocarbons are produced by industry for a variety of uses, such as refrigerants (in refrigerators, air conditioners, and large chillers), propellants for aerosol cans, blowing agents for making plastic foams, firefighting agents, and solvents for dry cleaning and degreasing. Atmospheric measurements have clearly corroborated theoretical studies showing that chlorine and bromine released from halocarbons in the stratosphere react with and destroy ozone.

Over the past 30 years humans have made progress in stopping damage to the ozone layer by curbing the use of certain chemicals. But more remains to be done to protect and restore the atmospheric shield that sits in the stratosphere about 9 to 18 miles (15 to 30 kilometers) above the Earth's surface.

Atmospheric ozone absorbs ultraviolet (UV) radiation from the sun, particularly harmful UVB-type rays. Exposure to UVB radiation is linked with increased risk of skin cancer and cataracts, as well as damage to plants and marine ecosystems. Atmospheric ozone is sometimes labeled as the "good" ozone, because of its protective role, and shouldn't be confused with stratropospheric, or ground-level, "bad" ozone, a key component of air pollution that is linked with respiratory disease.

Ozone (O₃) is a highly reactive gas whose molecules are comprised of three oxygen atoms. Its concentration in the atmosphere naturally fluctuates depending on seasons and latitudes, but it generally was stable when global measurements began in 1957. Groundbreaking research in the 1970s and 1980s revealed signs of trouble.

In 1974, Mario Molina and Sherwood Rowland, two chemists at the University of California, Irvine, published an article in Nature detailing threats to the ozone layer from chlorofluorocarbon (CFC) gases. At the time, CFCs were commonly used in aerosol sprays and as coolants in many refrigerators. As they reach the stratosphere, the sun's UV rays break CFCs down into substances that include chlorine.

The groundbreaking research—for which they were awarded the 1995 Nobel Prize in chemistry—concluded that the atmosphere had a “finite capacity for absorbing chlorine” atom in the stratosphere.

One atom of chlorine can destroy more than 100,000 ozone molecules, according to the U.S. Environmental Protection Agency, eradicating ozone much more quickly than it can be replaced.

Molina and Rowland’s work received striking validation in 1985, when a team of English scientists found a hole in the ozone layer over Antarctica that was later linked to CFCs. The "hole" is actually an area of the stratosphere with extremely low concentrations of ozone that reoccurs every year at the beginning of the Southern Hemisphere spring (August to October). Spring brings sunlight, which releases chlorine into the stratospheric clouds.

Recognition of the harmful effects of CFCs and other ozone-depleting substances led to the Montreal Protocol on Substances That Deplete the Ozone Layer in 1987, a landmark agreement to phase out those substances that has been ratified by all 197 UN member countries. Without the pact, the U.S. would have seen an additional 280 million cases of skin cancer, 1.5 million skin cancer deaths, and 45 million cataracts—and the world would be at least 25 percent hotter.

More than 30 years after the Montreal Protocol, NASA scientists documented the first direct proof that Antarctic ozone is recovering because of the CFC phase-down: Ozone depletion in the region has declined 20 percent since 2005. And at the end of 2018, the United Nations confirmed in a scientific assessment that the ozone layer is recovering, projecting that it would heal completely in the (non-polar) Northern Hemisphere by the 2030s, followed by the Southern Hemisphere in the 2050s and polar region by 2060.

Monitoring of the ozone layer continues, and it’s finding that the recovery may not be as straightforward as hoped. A study in early 2018 found that ozone in the lower stratosphere unexpectedly and inexplicably has dropped since 1998, while another pointed to possible ongoing violations of the Montreal pact.

The world is not yet in the clear when it comes to harmful gases from coolants. Some Hydrochlorofluorocarbons (HCFCs), transitional substitutes that are less damaging but still harmful to ozone are still in use. Developing countries need funding from the Montreal Protocol's Multilateral Fund to eliminate the most widely used of these, the refrigerant R-22. The next generation of coolants, hydrofluorocarbons (HFCs) do not deplete ozone, but they are powerful greenhouse gases that trap heat, contributing to climate change.

Though HFCs represent a small fraction of emissions compared with carbon dioxide and other greenhouse gases, their planet-warming effect prompted an addition to the Montreal Protocol, the Kigali Amendment, in 2016. The amendment, which came into force in January 2019, aims to slash the use of HFCs by more than 80 percent over the next three decades. In the meantime, companies and scientists are working on climate-friendly alternatives, including new coolants and technologies that reduce or eliminate dependence on chemicals.

The recognition of the dangers presented by chlorine and bromine to the ozone layer spawned an international effort to restrict the production and the use of CFCs and other halocarbons. The 1987 Montreal Protocol on Substances That Deplete the Ozone Layer began the phase out of CFCs in 1993 and sought to achieve a 50 percent reduction in global consumption from 1986 levels by 1998. A series of amendments to the Montreal Protocol in the following years was designed to strengthen the controls on CFCs and other halocarbons. By 2005 the consumption of ozone-depleting chemicals controlled by the agreement had fallen by 90–95 percent in the countries that were parties to the protocol.

During the early 2000s, scientists expected that stratospheric ozone levels would continue to rise slowly over subsequent decades. Indeed, some scientists contended that, as levels of reactive chlorine and bromine declined in the stratosphere, the worst of ozone depletion would pass. Factoring in variations in air temperatures (which contribute to the size of ozone holes), scientists expected that continued reductions in chlorine loading would result in smaller ozone holes above Antarctica (which since 1992 have spanned more than 20.7 million square km [8 million square miles]) after 2040. The expected increases in ozone would be gradual primarily because of the long residence times of CFCs and other halocarbons in the atmosphere. Total ozone levels, as well as the distribution of ozone in the troposphere and stratosphere, would also depend on other changes in atmospheric composition—for example, changes in levels of carbon dioxide (which affects temperatures in both the troposphere and the stratosphere), methane (which affects the levels of reactive hydrogen oxides in the troposphere and stratosphere that can react with ozone), and nitrous oxide (which affects levels of nitrogen oxides in the stratosphere that can react with ozone).

Scientists in 2014 observed a small increase in stratospheric ozone—the first, they thought, in more than 20 years which they attributed to worldwide compliance with international treaties regarding the phase out of ozone-depleting chemicals and to upper stratospheric cooling because of increased carbon dioxide. Upon more thorough study, however, scientists in 2016 announced that stratospheric ozone concentrations had actually been increasing in the upper stratosphere since 2000 while the size of the Antarctic ozone hole had been decreasing. Overall zone concentrations away from the poles have continued to fall since 1998; however, a 2018 study showed that declines in the lower stratosphere outpaced gains in the upper stratosphere.

Since ozone is a greenhouse gas, the breakdown and anticipated recovery of the ozone layer affects Earth’s climate. Scientific analyses show that the decrease in stratospheric ozone observed since the 1970s has produced a cooling effect—or, more accurately, that it has counteracted a small part of the warming that has resulted from rising concentrations of carbon dioxide and other greenhouse gases during this period. As the ozone layer slowly recovers in the coming decades, this cooling effect is expected to recede.