Editing Ozone layer (section)

==Depletion==
{{Main|Ozone depletion}}
[[File:Future ozone layer concentrations.gif|thumb|300px|NASA projections of stratospheric ozone concentrations if [[chlorofluorocarbon]]s had not been banned]]

The ozone layer can be depleted by [[free radical]] [[catalysis|catalysts]], including [[nitric oxide]] (NO), [[nitrous oxide]] (N<sub>2</sub>O), [[hydroxyl]] (OH), atomic [[chlorine]] (Cl), and atomic [[bromine]] (Br). While there are natural sources for all of these [[chemical species|species]], the concentrations of chlorine and bromine increased markedly in recent decades because of the release of large quantities of man-made [[organohalogen]] compounds, especially [[chlorofluorocarbon]]s (CFCs) and [[bromofluorocarbons]].<ref>{{cite book |chapter-url=http://www.eia.doe.gov/oiaf/1605/archive/gg97rpt/chap5.html |chapter=Halocarbons and Other Gases |publisher=Energy Information Administration |title=Emissions of Greenhouse Gases in the United States 1996 |date=1997 |access-date=June 24, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080629032506/http://www.eia.doe.gov/oiaf/1605/archive/gg97rpt/chap5.html |archive-date=June 29, 2008}}</ref> Atmospheric components are not sorted out by weight in the [[homosphere]] because of wind-driven mixing that extends to an altitude of about 90 km, well above the ozone layer. So despite being heavier than diatomic nitrogen and oxygen, these highly stable compounds rise into the [[stratosphere]], where Cl and Br [[radical (chemistry)|radicals]] are liberated by the action of ultraviolet light. Each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. By 2009, nitrous oxide was the largest ozone-depleting substance (ODS) emitted through human activities.<ref>{{cite web |title=NOAA Study Shows Nitrous Oxide Now Top Ozone-Depleting Emission |publisher=NOAA |date=August 27, 2009 |url=http://www.noaanews.noaa.gov/stories2009/20090827_ozone.html |access-date=November 8, 2011}}</ref>

The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet radiation reaches the Earth's surface at a higher intensity. Ozone levels have dropped by a worldwide average of about 4 percent since the late 1970s. For approximately 5 percent of the Earth's surface, around the north and south poles, much larger seasonal declines have been seen, and are described as "ozone holes". "Ozone holes" are actually patches in the ozone layer in which the ozone is thinner. The thinnest parts of the ozone are at the [[polar regions of Earth|polar points of Earth's axis]].<ref>{{cite web |title=ozone layer {{!}} National Geographic Society |url=https://education.nationalgeographic.org/resource/ozone-layer |access-date=May 30, 2022 |website=education.nationalgeographic.org}}</ref> The discovery of the annual depletion of ozone above the Antarctic was first announced by [[Joe Farman]], [[Brian G. Gardiner (meteorologist)|Brian Gardiner]], and [[Jonathan Shanklin]], in a paper which appeared in ''[[Nature (journal)|Nature]]'' on May 16, 1985.

Regulation attempts have included but not have been limited to the [[Clean Air Act (United States)|Clean Air Act]] implemented by the [[United States Environmental Protection Agency]]. The Clean Air Act introduced the requirement of [https://www.epa.gov/criteria-air-pollutants/naaqs-table National Ambient Air Quality Standards (NAAQS)] with ozone pollutions being one of six criteria pollutants. This regulation has proven to be effective since counties, cities, and tribal regions must abide by these standards and the EPA also provides assistance for each region to regulate contaminants.<ref>{{cite web |last=US EPA |first=OAR |title=Ozone Implementation Regulatory Actions |date=December 14, 2016 |language=en |website=epa.gov |url=https://www.epa.gov/ground-level-ozone-pollution/ozone-implementation-regulatory-actions |access-date=May 30, 2022}}</ref> Effective presentation of information has also proven to be important in order to educate the general population of the existence and regulation of ozone depletion and contaminants. A scientific paper was written by Sheldon Ungar in which the author explores and studies how information about the depletion of the ozone, [[climate change]], and various related topics. The ozone case was communicated to lay persons "with easy-to-understand bridging metaphors derived from the popular culture" and related to "immediate risks with everyday relevance".<ref>{{cite journal |last=Ungar |first=Sheldon |title=Knowledge, ignorance and the popular culture: climate change versus the ozone hole |date=July 2000 |journal=Public Understanding of Science |language=en |volume=9 |issue=3 |pages=297–312 |issn=0963-6625 |doi=10.1088/0963-6625/9/3/306 |s2cid=7089937 |url=https://journals.sagepub.com/doi/10.1088/0963-6625/9/3/306|url-access=subscription }}</ref> The specific metaphors used in the discussion (ozone shield, ozone hole) proved quite useful and, compared to global climate change, the ozone case was much more seen as a "hot issue" and imminent risk. Lay people were cautious about a depletion of the ozone layer and the risks of skin cancer.

[[Satellite]]s burning up upon re-entry into Earth's atmosphere produce [[aluminum oxide]] (Al<sub>2</sub>O<sub>3</sub>) [[nanoparticle]]s that endure in the atmosphere for decades.<ref name=GeophysResearchLtrs_20240611/> Estimates for 2022 alone were ~17 metric tons (~30{{nbsp}}kg of nanoparticles per ~250{{nbsp}}kg satellite).<ref name=GeophysResearchLtrs_20240611/> Increasing populations of [[satellite constellation]]s can eventually lead to significant ozone depletion.<ref name=GeophysResearchLtrs_20240611>{{cite journal |last1=Ferreira |first1=Jose P. |last2=Huang |first2=Ziyu |last3=Nomura |first3=Ken-ichi |last4=Wang |first4=Joseph |title=Potential Ozone Depletion From Satellite Demise During Atmospheric Reentry in the Era of Mega-Constellations |journal=Geophysical Research Letters |date=11 June 2024 |volume=51 |issue=11 |doi=10.1029/2024GL109280 |doi-access=free|bibcode=2024GeoRL..5109280F }}</ref>

"Bad" ozone{{Clarification needed|date=January 2025}} can cause adverse health risks respiratory effects (difficulty breathing) and is proven to be an aggravator of respiratory illnesses such as [[asthma]], [[chronic obstructive pulmonary disease|COPD]], and [[emphysema]].<ref>{{cite journal |last1=Zhang |first1=Junfeng (Jim) |last2=Wei |first2=Yongjie |last3=Fang |first3=Zhangfu |title=Ozone Pollution: A Major Health Hazard Worldwide |date=2019 |journal=Frontiers in Immunology |volume=10 |page=2518 |issn=1664-3224 |pmid=31736954 |doi=10.3389/fimmu.2019.02518 |doi-access=free |pmc=6834528}}</ref> That is why many countries have set in place regulations to improve "good" ozone{{Clarification needed|date=January 2025}} and prevent the increase of "bad" ozone in urban or residential areas. In terms of ozone protection (the preservation of "good" ozone) the [[European Union]] has strict guidelines on what products are allowed to be bought, distributed, or used in specific areas.<ref>{{cite web |title=Ozone Regulation |language=en |website=ec.europa.eu |url=https://ec.europa.eu/clima/eu-action/protection-ozone-layer/ozone-regulation_en |access-date=May 30, 2022}}</ref> With effective regulation, the ozone is expected to heal over time.<ref>{{cite web |last=US EPA |first=OAR |title=International Treaties and Cooperation about the Protection of the Stratospheric Ozone Layer |date=July 15, 2015 |language=en |website=epa.gov |url=https://www.epa.gov/ozone-layer-protection/international-treaties-and-cooperation-about-protection-stratospheric-ozone |access-date=May 30, 2022}}</ref>
[[File:TOMS Global Ozone 65N-65S.png|thumb|Levels of atmospheric ozone measured by satellite show clear seasonal variations and appear to verify their decline over time.]]

{{Main|Ozone depletion and climate change}}

In 1978, the United States, Canada, and [[Norway]] enacted bans on [[chlorofluorocarbon|CFC]]-containing [[aerosol spray]]s that damage the ozone layer but the European Community rejected a similar proposal. In the U.S., chlorofluorocarbons continued to be used in other applications, such as refrigeration and industrial cleaning, until after the discovery of the Antarctic ozone hole in 1985. After negotiation of an international treaty (the [[Montreal Protocol]]), CFC production was capped at 1986 levels with commitments to long-term reductions.<ref>{{cite journal |title=The Evolution of Policy Responses to Stratospheric Ozone Depletion |journal=Natural Resources Journal |date=1989 |last=Morrisette |first=Peter M. |volume=29 |pages=793–820 |url=http://www.ciesin.org/docs/003-006/003-006.html |access-date=April 20, 2010}}</ref> This allowed for a ten-year phase-in for developing countries<ref>An Interview with Lee Thomas, EPA's 6th Administrator. [http://www.epaalumni.org/history/video/interview.cfm?id=28 Video], [https://www.epaalumni.org/userdata/pdf/60740780F5ACB3D5.pdf#page=1 Transcript] (see p15). April 19, 2012.</ref> (identified in Article 5 of the protocol). Since then, the treaty was amended to ban CFC production after 1995 in developed countries, and later in developing countries.<ref>{{cite web |title=Amendments to the Montreal Protocol |publisher=EPA |date=August 19, 2010 |url=http://www.epa.gov/ozone/intpol/history.html |access-date=March 28, 2011}}</ref> All of the world's 197 countries have signed the treaty. Beginning January 1, 1996, only recycled or stockpiled CFCs were available for use in developed countries like the US. The production phaseout was possible because of efforts to ensure that there would be substitute chemicals and technologies for all ODS uses.<ref>{{cite web |title=Brief Questions and Answers on Ozone Depletion |publisher=EPA |date=June 28, 2006 |url=http://www.epa.gov/ozone/science/q_a.html |access-date=November 8, 2011}}</ref>

On August 2, 2003, scientists announced that the global depletion of the ozone layer might be slowing because of the international regulation of ozone-depleting substances. In a study organized by the [[American Geophysical Union]], three satellites and three ground stations confirmed that the upper-atmosphere ozone-depletion rate slowed significantly over the previous decade. Some breakdown was expected to continue because of ODSs used by nations which have not banned them, and because of gases already in the stratosphere. Some ODSs, including CFCs, have very long atmospheric lifetimes ranging from 50 to over 100 years. It has been estimated that the ozone layer will recover to 1980 levels near the middle of the 21st century.<ref name="wmo2010">{{cite book |title=Scientific Assessment of Ozone Depletion: 2010 |date=2011 |publisher=WMO |chapter=Stratospheric Ozone and Surface Ultraviolet Radiation |chapter-url=http://acdb-ext.gsfc.nasa.gov/Documents/O3_Assessments/Docs/WMO_2010/Chapter_2.pdf |access-date=March 14, 2015}}</ref> A gradual trend toward "healing" was reported in 2016.<ref name=healing>{{cite journal |title=Emergence of healing in the Antarctic ozone layer |vauthors=Solomon, Susan etal |journal=Science |volume=353 |issue=6296 |pages=269–74 |date=June 30, 2016 |bibcode=2016Sci...353..269S |pmid=27365314 |doi=10.1126/science.aae0061 |doi-access=free |hdl=1721.1/107197 |hdl-access=free}}</ref>

Compounds containing [[carbon–hydrogen bond|C–H bonds]] (such as [[hydrochlorofluorocarbon]]s, or HCFCs) have been designed to replace CFCs in certain applications. These replacement compounds are more reactive and less likely to survive long enough in the atmosphere to reach the stratosphere where they could affect the ozone layer. While being less damaging than CFCs, HCFCs can have a negative impact on the ozone layer, so they are also being phased out.<ref>{{cite web |title=Ozone Depletion Glossary |publisher=EPA |url=http://www.epa.gov/ozone/defns.html#hcfc |access-date=September 3, 2008}}</ref> These in turn are being replaced by [[hydrofluorocarbon]]s (HFCs) and other compounds that do not destroy stratospheric ozone at all.

The residual effects of CFCs accumulating within the atmosphere lead to a concentration gradient between the atmosphere and the ocean. This organohalogen compound dissolves into the ocean's surface waters and acts as a [[chlorofluorocarbon|time-dependent tracer]]. This tracer helps scientists study ocean circulation by tracing biological, physical, and chemical pathways.<ref>{{cite journal |title=Observations of CFCs and SF6 as Ocean Tracers |last=Fine |first=Rana A. |date=2011 |journal=Annual Review of Marine Science |volume=3 |pages=173–95 |bibcode=2011ARMS....3..173F |pmid=21329203 |doi=10.1146/annurev.marine.010908.163933 |url=http://yyy.rsmas.miami.edu/groups/cfc/pubs/Fine_AnnRevMarineSci3_2011.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150210212306/http://yyy.rsmas.miami.edu/groups/cfc/pubs/Fine_AnnRevMarineSci3_2011.pdf |archive-date=February 10, 2015}}</ref>