GLOBALNI OKOLJSKI POJAVI
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1 GLOBALNI OKOLJSKI POJAVI Klasifikacija okoljskih pojavov glede na razširjenost (viri, posledice.) v okolju: Lokalni Lesni prah, formaldehid (IVERKA) Azbest (SALONIT) Regionalni Troposferski ozon Globalni Uèinek tople grede Tanjšanje ozonskega plašèa Kisli dež PCB v polarnih predelih Tanjšanje ozonskega plašèa PARADOKS Èe je smisel življenja ležanje na plaži.. Potem je nesmisel uporaba UV zašèitnih sprayev saj v ozraèje sprošèajo snovi, ki unièujejo ozonski plašè Ozonska luknja prviè opažena v 1980-ih nad Antarktiko (J. C. Farman et. al) LITERATURA M.J. Molina, F.S. Rowland, Nature 249(1974)810 F.S. Rowland, M.J. Molina, Chemical & Engineering News 15(1994)8 S. Solomon, Nature 347(1990)347 M. McFarlan, Environmental Science and Technology 23(1989)1203 J.S. Taylor, Journal of Chemical Education 67(1990)835 S. Madronich, F.R. de Gruijl, Nature 366(1993)23 R.S. Stolarski, Scientific American Jan. (1988)30 O.B. Toon, R.P. Turco, Scientific American Jun. (1991)68 F.S. Rowland, Annual Reviews of Physical Chemistry 42(1991)731 W.H. Brune, et al. Science 252(1991)1260 J.B. Kerr, C.T. McElroy, Science 262(1993)1032 M.A.K. Khalil, R.A. Rasmussen, Journal of Geophysical Research 98(1993)
2 Izražanje kolièine ozona Koncentracije Absolutna (molekule/ cm 3 ) Relativna (molski delež ppb, ppt) Debelina sloja Dobsonove enote (DU) 1DU = 0,01 mm sloj èistega ozona (pri gostoti, ki bi jo imel plin na zemeljski površini; P = 1 atm, T = 273 K) Opažanja Povpreèna kolièina ozona v atmosferi se giblje med 250 DU (ekvatorialni pas) in 450 DU (subpolarne regije) stratosferski vetrovi povzroèajo transport O 3 koncentracija se spreminja z letnimi èasi najvišje konc. ob koncu polarne zime najnižje konc. sredi polarne pomladi Opažanja II Kolièina ozona nad Antarktiko v obdobju upada Min. koncentracije so se zmanjšale za 50% Površina ozonske luknje (< 220 DU) se veèa presega površino severne Amerike Trend upadanja kažejo tudi povpreène koncentracije O 3 v nepolarnih podroèjih (60 N - 60 S) 2
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5 Pomen ozonskega plašèa Absorpcija svetlobe v molekulah plinov intenziteta svetlobe se pri prehodu skozi atmosfero znižuje ozon moèno absorbira v obmoèju med 220 in 320 nm (O 2 med 120 in 220 nm) Biološke posledice obèutljivost DNA je pri nižjih valovnih dolžinah veèja incidenca kožnega raka je sorazmerna z intenziteto UV svetlobe Osnove fotokemije E=hν ali E=hc/λ (h = 6, Js, c = 2, ms -1 ) èe v enem molu snovi vsaka molekula absorbira en foton, potem je absorbirana energija: E= / λ kjmol -1 (λ izrazimo v nm) energije fotonov v UV so podobnega reda velikosti kot reakcijske entalpije ( H ) za disociacijo molekul 5
6 Osnove fotokemije II za disociacijo kisika je H = 495 kj mol -1 kar ustreza valovni dolžini svetlobe 241 nm pri λ < 241 nm pride do fotokemijske reakcije (fotolize) O 2 + UV 2 O pri λ > 241 nm pride le do vzbujanja O 2 + UV O 2 * O 2 + Q (toplota) Nastanek in nekatalitièna razgradnja ozona O + O 2 O 3 + Q (1) koncentracija ozona je najveèja kjer doseže produkt intenzitete UV-C in koncentracije kisika najvišjo vrednost (18-25 km) - ozonski plašè natanèneje zapišemo reakcijo 1 kot O + O 2 + M O 3 + M + Q (2) vloga molekule M je v sprejemanju reakcijske toplote - temperaturna inverzija - mešanje je v stratosferi veliko poèasnejše kot v troposferi (gostota) Nastanek in nekatalitièna razgradnja ozona II O 3 + UV (λ < 320 nm) O 2 * + O* (3) O 3 + O O 2 (4) O + O O 2 (5) 6
7 Katalitièna razgradnja ozona X + O 3 XO+ O 2 XO + O X + O 2 O 3 + O 2 O 2 Mehanizem I Razgradnjo O 3 katalizirajo radikali NO, HO, Cl, F, Br, ki so tudi naravnega izvora. V stratosferi: N 2 O + O* 2NO Naravna razgradnja ozona NO + O 3 NO 2 + O 2 NO 2 + O NO + O 2 O 3 + O 2O 2 NO - najpomembnejši katalizator razgradnje O 3 v spodnji in srednji stratosferi, kjer je koncentracija O 3 najvišja. Nad 45 km je najpomembnejši katalizator OH radikal. (O* + CH 4 OH + CH 3 ) OH + O 3 HOO + O 2 HOO + O OH + O 2 O 3 + O 2O 2 Reakcijska hitrost in vpliv koncentracij -d[o 3 ]/dt = k [NO][O 3 ] Omejujoè dejavnik pri razgradnji O 3 je koncentracija O, ki omogoèa regeneracijo katalizatorja X. [O] je v nižji stratosferi nizka (malo UV, veliko O 2 ) zato poteka razgradnja ozona v tem podroèju po mehanizmu II 7
8 Mehanizem II X + O 3 XO+ O 2 X + O 3 X O + O 2 XO + X O X + X + O 2 2O 3 3 O 2 Primer: OH + O 3 HOO + O 2 HOO + O 3 OH + 2O 2 Cl in Br kot katalizatorja Njune spojine nastajajo tudi v naravnih procesih (oceani) CH 3 Cl + UV c Cl + CH 3 OH + CH 3 Cl Cl + produkti Cl + O 3 ClO + O 2 ClO + O Cl + O 2 O 3 + O 2O 2 Podobno za Br! Neaktivne oblike katalizatorjev Vsak atom Cl lahko razgradi veè molekul, a ga veèina obstaja v neaktivni obliki: ClO + NO 2 ClONO 2 (razpade pod UV) Cl + CH 4 HCl + CH 3 OH + HCl H 2 O + Cl (delež majhen) Podobno tudi Br a je reakcija z metanom poèasnejša!! 40 do 50 krat uèinkovitejši pri razgradnji O 3 a ga je manj Katalizatorji se iz stratosfere odstranjujejo preko difuzije neaktivnih oblik (HCl, HBr) v troposfero, kjer se spirajo v tla z dežjem. 8
9 Ozonska luknja Nastane kot posledica meteoroloških pogojev v spodnji stratosferi v èasu polarne zime nizke temperature znižanje tlaka rotacija Zemlje nastanek vorteksa ni izmenjave z bližnjimi zraènimi masami dolga obdobja nizkih T Polarni stratosferski oblaki PSC ji nastanejo s kondenzacijo vodnih par koncentracija vode je majhna zato so potrebne zelo nizke T (-80 C v èasu polarne zime, ko ni segrevanja stratosfere pri nastajanju O 3 ) Pri -80 C nastajajo drobni delci (verjetno mešanica tekoèine in kristalov!?), ki vsebujejo žveplove in dušikove kisline (Tip I) Nekaj stopinj pod -80 C nastajajo veèji delci ledu in verjetno dušikove kisline (Tip II) Reakcije na trdnih delcih ClONO 2 HCl Cl Cl HCl Kristal HCl HCl HCl Cl 2 UV Vodni sloj HOCl Cl OH UV ClONO 2 (g) + H 2 O(l) HOCl(aq) + HNO 3 (aq) HCl(g) H + (aq) + Cl - (aq) Cl - (aq) + HOCl (aq) Cl 2 (g) + OH - (aq) HCl(g) + ClONO 2 (g) Cl 2 (g) + HNO 3 (aq) 9
10 Tvorba radikalov in deaktivacija Halogeni, ki se v obdobju polarne noèi nakopièijo v stratosferi se pod vplivom UV svetlobe ob zaèetku pomladi pretvorijo v radikale Njihova deaktivacija ni možna, ker je veèina NO 2 vezana v kristalih kot HNO 3 Deaktivacija Cl je možna šele po razpadu PSC in se iz preostalih kristalov Tipa I sprosti HNO 3 HNO 3 + UV NO 2 + OH Denitrifikacija spodnje stratosfere Veèina kristlov Tipa II in veèjih kristalov Tipa I zaradi gravitacije izpade v troposfero in zmanjša koncentracijo NO 2 Antarktièna ozonska luknja zaradi denitrifikacije traja do dva tedna dlje, zmanjšanje koncentracije ozona pa je okoli 20% veèje Tanjšanje ozonskega plašèa v subpolarnih podroèjih Cl + O 3 ClO + O 2 OH + O 3 HOO + O 2 ClO + HOO HOCl + O 2 HOCl OH + Cl Denitrifikacija: NO 2 + O 3 NO 3 + O 2 NO 2 + NO 3 N 2 O 5 N 2 O 5 + H 2 O HNO 3 (H 2 SO 4 katalizator) 10
11 ClO (ppb) O3 (ppb) Zemljepisna širina Polarna noè Sonèna svetloba deaktivacija Koncentracija (ppb) PSC kemija Izguba O 3 èas 11
12 O 3 razkrajajoèe snovi ODS so organske spojine antropogenega izvora, ki vsebujejo halogene elemente in nimajo naravnega odtoka v troposferi - (ne razpadajo in niso topne v vodi) CFC - klorofloroogljiki HCFC - nadomestki za CFC hidroklorofloroogljiki Haloni (spojine broma ali joda) Druge ODS (organska topila CCl 4, CH 3 -CCl 3 ) 12
13 Kode za CFC in HCFC CFC-11, CFC-12, CFC-113 številu v kodi prištejemo 90. V vsoti nam posamezne cifre pomenijo število ogljikovih, vodikovih in florovih atomov v molekuli (po navedenem zaporedju!) Primer za CFC-11: = 101, sledi: 1 C, 0 H, 1F klorofloroogljik je CFCl 3 (derivat metana (1 C) z enim atomom flora (1 F) in ne vsebuje vodika (0 H)) Problem HCFC HCFC-22 je nadomestek za CFC-11 pretežno razpade že v troposferi zato je njegov potencial za razgradnjo O 3 le 5% potenciala, ki ga predstavlja CFC-11 razgradnja HCFC-22 pri kateri pride do sprošèanja Cl je hitrejša kot pri CFC-11 posledica je 15% veèji uèinek HCFC-22 v prvih 15 letih po izpustu in zanemarljiv dolgoroèni uèinek (hitreje razpada) najveèja zaskrbljenost velja ravno za naslednjih nekaj desetletij! HFC Hidrofloroogljiki so glavni dolgoroèni nadomestek za HCFC spojine (npr. CH 2 F- CF 3 - HFC-134a) življenjska doba v atmosferi je nekaj 10 let Pri razpadu zaradi OH nastane HF, ki pa je stabilna in ne daje F Problem je trifloroocetna kislina, ki nastane pri nekaterih reakcijah razgradnje HFC (in tudi HCFC) - možna akumulacija v mokrišèih in inhibicija rasti rastlin 13
14 Mednarodni sporazumi o proizvodnji CFC in drugih ODS Motrealski protokol 1987 London 1990, Kopenhagen 1992, Dunaj 1995, Montreal 1997 Prenehanje proizvodnje CFC razvite države do 1995 države v razvoju do 2010 Prenehanje proizvodnje HCFC razvite države do 2030 države v razvoju do 2040 poveèevanje proizvodnje prepovedano po 2015 Drugi ukrepi Proizvodnja halonov ustavljena 1994 obstojeèe zaloge se še uporabljajo (gasilni aparati) koncentracija teh snovi v atmosferi še narašèa Uporaba CH 3 Br (sterilizacija zemlje) bo prekinjena (èe bo zagotovljen ustrezen nadomestek - CH 3 J?) 2005 v razvitih državah (2001 v ZDA) 2015 v državah v razvoju (do 2002 omejili porabo na kolièine v 90-ih letih) Rezultati Koncentracija klora v troposferi dosegla maksimum 1994 Maksimalna koncentracija klora v stratosferi (4 ppb) dosežena konec 20. Stoletja Koncentracija Cl se bo poèasi zmanjševala dolgotrajnost difuzije do srednje in zgornje stratosfere kjer pride do fotodisociacije poèasnost reakcij preko katerih se Cl in Br izloèata iz stratosfere nadaljevanje izpustov Cl in Br 14
15 Predvidevanja Nadaljnje podvajanje koncentracije Cl v stratosferi bi vodilo do stalnega pojava ozonske luknje na Arktiki Antarktièna ozonska luknja ne bo veè opazna po polovici 21. Stoletja Vse napovedi, ki so rezultat intenzivnih raziskav v zadnjih 20 letih, so lahko le semikvantitativne! 15
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