Editing Ozone depletion (section)

== Ozone hole and its causes ==
[[File:Uars ozone waves.jpg|thumb|right|upright=1.7|Ozone hole in North America during 1984 (abnormally warm, reducing ozone depletion) and 1997 (abnormally cold, resulting in increased seasonal depletion). Source: NASA<ref>{{cite web |last1=Nash |first1=Eric |last2=Newman |first2=Paul |url=http://earthobservatory.nasa.gov/IOTD/view.php?id=1771 |title=NASA Confirms Arctic Ozone Depletion Trigger |work=Image of the Day |publisher=[[NASA]] |date=September 19, 2001 |access-date=April 16, 2011}}</ref>]]

The Antarctic ozone hole is an area of the Antarctic stratosphere in which the recent ozone levels have dropped to as low as 33 percent of their pre-1975 values.<ref>{{cite web |title=Emissions of a banned ozone-depleting gas are back on the decline |url=https://research.noaa.gov/article/ArtMID/587/ArticleID/2713/Emissions-of-a-banned-ozone-depleting-gas-are-back-on-the-decline |website=NOAA Research News|date=11 February 2021 }}</ref> The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container. Within this [[polar vortex]], over 50 percent of the lower stratospheric ozone is destroyed during the Antarctic spring.<ref>{{cite web|last=Sparling |first=Brien |url=http://www.nas.nasa.gov/About/Education/Ozone/antarctic.html |title=Antarctic Ozone Hole |publisher=NASA Advanced Supercomputing Department |date=June 26, 2001 |url-status=unfit |archive-url=https://web.archive.org/web/20050312093001/http://www.nas.nasa.gov/About/Education/Ozone/antarctic.html |archive-date= March 12, 2005 }}</ref>

As explained above, the primary cause of ozone depletion is the presence of chlorine-containing source gases (primarily [[CFCs]] and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is substantially enhanced in the presence of [[polar stratospheric cloud]]s (PSCs).<ref>{{cite web |last=Parson |first=Robert |date= December 16, 1997 |url=http://www.faqs.org/faqs/ozone-depletion/antarctic |title=Antarctic ozone-depletion FAQ, section 7 |publisher=Faqs.org |access-date=April 16, 2011}}</ref>

These polar stratospheric clouds form during winter, in the extreme cold. Polar winters are dark, consisting of three months without solar radiation (sunlight). The lack of sunlight contributes to a decrease in temperature and the polar vortex traps and chills the air. Temperatures are around or below −80&nbsp;°C. These low temperatures form cloud particles. There are three types of PSC clouds—nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapid cooling water-ice (nacreous) clouds—provide surfaces for chemical reactions whose products will, in the spring lead to ozone destruction.<ref>{{cite journal |last1=Toon |first1=Owen B. |last2=Turco |first2=Richard P. |title=Polar Stratospheric Clouds and Ozone Depletion |date=June 1991 |journal=Scientific American |volume=264 |issue=6 |pages=68–74 |url=http://www.atmos.washington.edu/~davidc/ATMS211/articles_optional/Toon_Turco91_ozone.pdf |access-date=April 16, 2011 |bibcode=1991SciAm.264f..68T |doi=10.1038/scientificamerican0691-68 |archive-url=https://web.archive.org/web/20110225082537/http://www.atmos.washington.edu/~davidc/ATMS211/articles_optional/Toon_Turco91_ozone.pdf |archive-date=February 25, 2011 |url-status=dead }}</ref>

The [[photochemistry|photochemical]] processes involved are complex but well understood. The key observation is that, ordinarily, most of the chlorine in the stratosphere resides in "reservoir" compounds, primarily chlorine nitrate ({{chem|ClONO|2}}) as well as stable end products such as HCl. The formation of end products essentially removes Cl from the ozone depletion process. Reservoir compounds sequester Cl, which can later be made available via absorption of light at wavelengths shorter than 400&nbsp;nm.<ref>{{cite journal |last1=Sumi´nska-Ebersoldt |title=ClOOCl photolysis at high solar zenith angles: analysis of the RECONCILE self-match flight |date=July 2011 |journal=Atmos. Chem. Phys. |volume=12 |pages=1353–1365 |doi=10.5194/acp-12-1353-2012|bibcode = 2012ACP....12.1353S |first2=R. |last3=Wegner|first3=T. |last4=Grooß|first4=J.-U. |last5=Hösen|first5=E. |last6=Weigel|first6=R. |last7=Frey|first7=W. |last8=Griessbach|first8=S. |last9=Mitev|first9=V. |last10=Emde|first10=C. |last11=Volk|first11=C. M. |last12=Borrmann|first12=S. |last13=Rex|first13=M. |last14=Stroh|first14=F. |last15=von Hobe|first15=M. |issue=3 |last2=Lehmann|doi-access=free }}</ref> During the Antarctic winter and spring, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). Denitrification is the process by which the clouds remove {{chem|NO|2}} from the stratosphere by converting it to nitric acid in PSC particles, which then are lost by sedimentation. This prevents newly formed ClO from being converted back into {{chem|ClONO|2}}.

The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring. During winter, even though PSCs are at their most abundant, there is no light over the pole to drive chemical reactions. During the spring, however, sunlight returns and provides energy to drive photochemical reactions and melt the polar stratospheric clouds, releasing considerable ClO, which drives the hole mechanism. Further warming temperatures near the end of spring break up the vortex around mid-December. As warm, ozone and {{chem|NO|2}}-rich air flows in from lower latitudes, the PSCs are destroyed, the enhanced ozone depletion process shuts down, and the ozone hole closes.<ref>{{cite web |url=http://ozonewatch.gsfc.nasa.gov/facts/hole.html |title=Ozone Facts: What is the Ozone Hole? |work=Ozone Hole Watch |publisher=[[NASA]] |date=November 18, 2009 |access-date=April 16, 2011 |archive-date=November 20, 2010 |archive-url=https://web.archive.org/web/20101120062849/http://ozonewatch.gsfc.nasa.gov/facts/hole.html |url-status=dead }}</ref>

Most of the ozone that is destroyed is in the lower stratosphere, in contrast to the much smaller ozone depletion through homogeneous gas-phase reactions, which occurs primarily in the upper stratosphere.<ref>{{cite journal |last=Rowland |first=Frank Sherwood |author-link=Frank Sherwood Rowland |date=May 29, 2006 |title=Stratospheric ozone depletion |url=http://rstb.royalsocietypublishing.org/content/361/1469/769.full#disp-formula-5 |journal=Phil. Trans. R. Soc. B |volume=361 |issue=1469 |pages=769–790 |doi=10.1098/rstb.2005.1783 |pmc=1609402 |pmid=16627294 |quote=Free radical reactions for ozone removal: Reaction 4.1}}</ref>