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CHM210H1 (2)
Lecture 4

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Lecture 4 Reading – Pages 37-64 Ozone (O ) 3 a gas that is present in small concentrations throughout the atmosphere Dobson units (DU) are used to measure the total amt of atmospheric ozone lying over a point on Earth - 1 DU is equivalent to a 0.01mm (0.001cm) thickness of pure ozone at the density if would possess if it were brought to ground-level (1atm) pressure and 0°C temperature Stratospheric winds transport ozone from tropical regions (where most of it is produced) toward polar regions - The closer to the Equator you live, the less the total amount of ozone that protects you from UV light Antarctic ozone hole – discovered by Dr. Joe C. Farman in the British Atlantic Survey - Had been recording ozone data over this region since 1957; noticed that the total amounts of ozone each October had been gradually decreasing with each year, esp. during mid- September to mid-October (precipitous declines beginning in the late 1970s) - The period of time from September to November correspond to the spring season at the South Pole - Uncertainty as to whether hemical mechanism involving air pollutants was suspected to contribute to the ozone hole o Chlorine – produced mainly from gases released into the air in large quantities as a consequence of their use (e.g. in air conditioners) o Scientists predicted that chlorine would destroy ozone, but only to a small extent and only after several decades had elapsed We can preduct that the hole will continue to reappear each spring until about the middle of this century, and that a corresponding hole may appear above the Arctic region Losses of ozone during the 1980s and early 1990s were greater the higher the latitude in the northern and southern hemispheres - 1996-2005 – this trend of ozone loss was reversed; the gains in the northern hemisphere approximately cancelled the earlier losses - 1997-2008 – ozone recovery (of 1%) had begun, but only in the upper atmosphere, compared to losses of 14% there over the 2 previous decades Stratospheric ozone over Antarctica is reduced by about 50% for several months each year, due mainly to the action of chlorine - Ozone hole occurs from September to November, springtime in the Antarctic; has been appearing since about 1979 Ozone hole occurs as a result of special polar winter weather conditions in the lower stratosphere (where ozone concentrations are highest), that temporarily convert all the chlorine stored in inactive forms Cl and ClONO int2 active forms Cl and ClO - As a result, the high concentration of active chlorine causes a large (but temporary) annual depletion of ozone - Conversion of inactive to active chlorine occurs at the surface of particles formed by a solution of water, sulfuric acid, nitric acid o Nitric acid (HNO )3formed by combination of hydroxyl radical (OH) with nitrogen dioxide gas (NO ) 2 In most parts of the world, the stratosphere is cloudless – condensation of water vapour into liquid droplets or solid crystals that would form clouds doesn’t normally occur in the stratosphere, since it has a very small concentration of water - In the lower stratosphere, temperature drops to -80°C over the South Pole in the winter (24- hour darkness) that condensation does occur o The usual mechanism that warms the stratosphere (release of heat by O + O 2 reaction) is absent b/c of the lack of production of atomic oxygen from O and O 2 3 when there is total darkness - Since the polar stratosphere becomes so cold during total darkness, the air pressure drops b/c it’s proportional to the Kelvin temperature (PV = nRT) o Vortex – whirling mass of air in which winds can get up to 300km per hour; formed when the air pressure decreases in the polar stratosphere o Since matter can’t penetrate the vortex, the air inside is isolated, and remains very cold for a long time Polar stratospheric clouds (PSCs) – formed from the particles produced by condensation of the gases within the vortex - The first particles to form are small, and contain water, sulfuric acid, and nitric acids Chemical rxs that lead ultimately to ozone destruction occur in a thin aqueous layer on the surface of the PSC ice crystals - Upon contact, ClONO (g) +2H O(aq) 2 HO+l(aq) + HN- (aq) 3 - In the aqueous layer, HCl(g)  H (aq) + Cl (aq) - Rx of 2 new forms of dissolved chlorine – Cl (aq) + HOCl(aq) Cl (g) + OH (2q) - o Molecular chlorine (Cl )2escapes to the surrounding air - Net rx – HCl(g) + ClONO (aq) 2 Cl (g) + 2NO (aq) 3 o H and OH ions reform water During the dark winter months, molecular chlorine accumulates within the vortex in the lower stratosphere, becoming the predominant chlorine-containing gas - Once a little sunlight reappeaars in the very early Antarctic spring, or the air mass moves to the edge of the vortex where there is some sunlgiht, the chlorine molecules are decomposed by the light into atomic chlorine (Cl) – Cl + sunl2ght  2Cl - Similarly, gaseous HOCl molecules released from the surface of the crystals undergo photochemical decomposition – HOCl + sunlight  OH + Cl o Massive destruction of ozone by the Cl produced in these reactions then ensues via catalytic reactions Stratospheric temperatures above the Antarctic remain below -80°C even in the early spring, the crystals persist for months; any Cl that is converted back to HCl by the rx with methane is subsequently reconverted to Cl on t2e crystals, then back to Cl by sunlight Inactivation of chlorine monoxide (ClO) by conversion to ClONO doesn’t oc2ur – all the NO 2 necessary for this rx is temporarily bound as nitric acid in the crystals - The larger crystals move downward under the influence of gravity into the upper troposphere, removing NO from t2e lower stratosphere over the South Pole - This denitrification process in the lower stratosphere extends the life of the Antarctic ozone hole and increases ozone depletion Chlorine only returns to its inactive forms when the PSCs and the vortex have vanished - Liberation of HNO from the remaining crystals into the gas phase results in conversion to 3 NO b2 sunlight – HNO + UV 3NO + OH 2 Air containing normal amts of NO mixes2with polar air once the vortex breaks down in late spring; nitrogen dioxide combines with chlorine monoxide to form chlorine nitrate, which is catalytically inactive - Catalytic destruction cycles cease operation and ozone concentration builds back up toward its normal level a few weeks after the PSCs have disappeared and the vortex has ceased - Before the ozone levels build back up in the spring, some ozone-poor air can move away from the Antarctic and mix with surrounding air, lowering stratospheric ozone concentrations in adjoining geographic regions (Australia, New Zealand, southern South America) Lower stratosphere – region where PSCs form and chlorine is activated; concentration of free oxygen atoms is small - Few atoms are produced there due to the lack of UV-C light that is req’d to dissociate O 2 - Any atomic oxygen atoms produced in this way collide with abundant O molecules 2o form ozone, O 3 Most ozone destruction in the ozone hole occurs via Mechanism II; X and X’ being atomic chlorine; overall reaction Cl + O 3ClO + O (Step21) - Figure 2.4, page 44 – if step 1 is the process by which ozone destruction occurs, the O and 3 ClO display opposing trends - At sufficient distances from the South Pole (90°S), the concentration of ozone is relatively high, and that of ClO is low, since chlorine is mainly in its inactive forms - However, as one travels closer to the Pole and enters the vortex region, the concentration of ClO suddnly becomes high, and that of O falls 3ff sharply at the same time – most of the chlorine has been activated and most of the ozone has consequently been destroyed 2 ClO radicals (produced in 2 separate Step 1 events) combine temporarily to form a nonradical dimer, dichloroperoxide (ClOOCl, or Cl O ) 2 22lO  Cl-O-O-Cl (Step 2a) - Rate of the rx becomes high, which is important to ozone loss – chlorine monoxide concentration has risen steeply due to the activation of the chlorine Once the sunlight has become considerably more intense in the Antarctic spring, ClOOCl absorbs UV light and splits off one Cl atom; the resulting ClOO radical is unstable, and decomposes in about a day, releasing the other Cl atom - ClOCl + UV light  ClOO + Cl (Step 2b) - ClOO  O + C2 (Step 2c) - Net result – conversion of 2 ClO molecules to atomic chlorine via the intermediacy of the transient dimer ClOOCl o Step 2 overall – 2ClO  [ClOOCl] –(light) 2Cl + O 2 o By these processes, ClO returns to Cl, the ozone-destroying form Add overall Step 2 reaction to two times Step 1 (factor of 2 req’d to produce 2 intermediate ClO species needed in reaction 2a so none remains in the overall equation) – the overall reaction is 2O  3O 3 2 - Complete catalytic ozone destruction cycle exists in the lower stratosphere under special weather conditions – when a vortex is present; cycle also requires very cold temperatures o Under warmer conditions, ClOOCl is unstable and reverts back to 2 ClO molecules before it can undergo photolysis About ¾ of the ozone destruction in the Antarctic ozone hole occurs by the above mechanism, in which chlorine is the only catalyst - 2a is the rate-determining step (combination of 2 ClO molecules), o Rate law is second order in [ClO], so it proceeds at a substantial rate, and the destruction of ozone is only significant when [ClO] is high Lower stratosphere above Antarctica – ozone destruction rate is about 2% per day in September, due to combined effects of various catalytic reaction sequence - By early October, the ozone is almost completely gone between 15-20km altitude Special vortex conditions in the lower stratosphere above the Antarctic in winter cause denitrification, and led to conversion of inactive chlorine into Cl a2d HOCl, which produce Cl when sunlight appears In 2010, significant ozone depletion didn’t start to occur until August, possibly when sunshine first hit the chlorine - Maximum depletion/hole size occurred in late September/early October – temperatures start rising more, and were sufficient to start melting crystals by mid-October - Hole collapses about a month later How to measure the extent of ozone depletion? - Surface area covered by low ozone – grew rapidly and approximately linearly during the 1980s; the size of the hole in maximum depletion years (1998, 2006) has been somewhat larger than in that period) o Overall – no overall increase or decrease since the early 1990s - Minimum amount of overhead ozone – sharply decreased in springtime from 1978 to late 1980s; replaced by a slower decline o Since then, minimum ozone has remained fairly constant since the mid-1990s o Exception – 2002 - Average length of time that ozone depletion occurs – increased in recent years o Some reduction is seen in midwinter, summer, and spring; some persistence of depletion from year to year - Vertical region over which almost total depletion occurs (12-22km) – hasn’t increased since the mid-1990s An ozone hole above the Arctic didn’t start to form at the same time as in the Antarctic - Partial springtime ozone depletion has occurred several times since the mid-1990s; less severe than in Antarctica - Stratospheric temperature over the Arctic doesn’t fall as low nor for as long, and air circulation to surrounding areas is less limited o Flow of tropospheric air over mid-latitude mountain ranges in the northern hemisphere creates waves of air that can mix with polar air and warm the Arctic stratosphere Since the air doesn’t get as cold, polar stratospheric clouds form less frequently over the Arctic, and don’t last as long as over the Antarctic - Only small crystals were formed; not large enough to fall out of the stratosphere and denitrify it - During extended polar night, chlorine nitrate and hydrogen chloride react on the surface of the small particles to produce molecular chlorine, which dissociates to atomic chlorine, then reacts with an ozone molecule to become chlorine monoxide Before the mid-1990s, the vortex above the Arctic broke up before late winter, so NO - 2 containing air mixed with vortex air before much sunlight returned to the polar region in spring - Stratospheric air temperature usually rose above -80°C by early March – nitric acid was converted back to gaseous nitrogen dioxide before the Cl O mec2an2sm could occur - Increases in NO fr2m both these sources – activated chlorine was transformed back to ClONO be2ore it could destroy much ozone Both depletion of ozone and increase in CO levels themselves cool the stratosphere, leading to 2 even more depletion if cooling occurs in the spring, extending the period in which PSCs remain Amount of UV-B from sunlight reaching ground level increases by a factor of 3-6 in the Antarctic during the early spring b/c of the appearance of the ozone hole Biologically, the most dangerous UV doses under hole conditions occur in late spring (November-December), when the Sun is higher in the sky than in the earlier months; low overhead ozone values still prevail - Southern Argentina has shown abnormally high UV levels – ozone-depleted stratospheric air from the Antarctic travelled over the area o Stalling of the hole’s edge over Tierra d
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