Transcript Environmental Chemistry

Environmental Chemistry
Chapter 2:
The Ozone Holes
Part 1 - Pollution of the Stratosphere
Part 2 - The Ozone Hole
Copyright © 2012 by DBS
The ozone hole provides a classic case of the workings of
science at its best: the unexpected discovery of an important
effect, proposals of theories to explain it, quick mounting of a
logistically difficult experimental field program to test the
theories, and a blending of laboratory data, field observations,
and computer models to achieve understanding, in this case
within only two years
Graedel and Crutzen, 1993
2.1 Dobson Units for Overhead Ozone
•
1 DU is the number of molecules of O3 required to create a layer of O3
0.01 mm (0.001 cm) thick at 0 °C and 1 atm
•
Over the Earth’s surface, the O3 layer’s average thickness is about 300
DU (3 mm layer) if brought to sea level
O3 “Hole”
[O3] ~100 DU
Determination of O3 Concentration
•
What is the total mass of ozone
corresponding to 350 DU?
V(O3) = 4/3π [(r+d)3 -r3]
[(r+d)3 - r3] = [r3 + d3 + 3rd2 + 3r2d - r3] ~ 3r2d
…since r>>d
350 DU = 3.5 mm
or 3.5x10-3 m
r = 6400 km
or 6.4x106 m
V(O3)
= 4/3 π [3r2d] = 4 π r2d
= 4 x 3.14 x (6.4 x 106)2 x 3.5 x 10-3
= 1.8 x 1012 m3 or 1.8 x 1015 L
n = PV/RT = 1.0 atm x 1.8 x 1015 L / 0.082 L atm mol-1K-1 x 273 K
= 8 x 1013 moles
= 8 x 1013 mols x 48 g mol-1 = 4 x 1015 g
1 DU at 1 atm…
d = thickness of O3 (1 DU=10-5 m), r = 6.4 x 106 m
From Ideal gas law, PV = nRT = NkBT
N = number of molecules
kB=R/NA Boltzmann’s const, kB=1.381 x 10-23 JK-1
From before:
V = 4 πr2d
NO3
= [(Pstp x V) /(kB.Tstp)]
= [(Pstp x 4 πr 2 d) /(kB.Tstp)]
Dividing both sides by 4 πr 2
NO3/4πr 2 = [(Pstp x d)/(kB.Tstp)] = 2.69 x 1020 molecules m-2
If all ozone in the atmosphere were spread out around the earth in a homogeneous
spherical shell at standard temperature, Tstp ( 273.16 K) and standard pressure, Pstp
(101325 Pa), one DU is equivalent to a horizontal density (number of ozone molecules
per unit area) of 2.69 x1020 molecules per square meter
2.2 History of the Annual Hole Above
Antarctica
• O3 recorded since
1957 by Farman at
Halley Bay (British
Antarctic Station)
• Area of USA =
10 x 106 km2
Sept-Oct period is Antarctic Spring
The Big Surprise of 1985
•
Farman et al. revealed a dramatic
and unpredicted decline in
stratospheric O3 in a surprising
location
– Antarctica
– Shocked the world
– Showed dramatic decline in
springtime O3 starting in 1970’s
30% by 1985
70% by 2000
Isolated local concern or global problem?
Min O3 at Antarctic in Spring
(Sep-Nov)
Chemical explanation?
Physical explanation?
TOMS
Indirect measurements
Measures O3 by
mapping UV light
emitted by the Sun to
that scattered from the
Earth's atmosphere
back to the satellite
O3 is inferred from
Earth’s albedo
Shows strong spatial variability
Low around equator, high in mid-latitude (why)
Very low at Antarctic (especially in September/October)
http://jwocky.gsfc.nasa.gov
What is the Ozone Hole?
•
Occurs at the beginning of
Southern Hemisphere spring
(August-October)
•
The average concentration of
O3 in the atmosphere is
about 300 Dobson Units
Any area where
O3 < 220 DU is part of
the O3 hole
Ozone is ‘thinning’ out
Not a “hole” but a region of
depleted O3 over the Antarctic
2.3 Ozone in Temperate Areas
• Ozone depletion seen world-wide
• Losses during 80’s and 90’s were greater at higher latitudes
(close to poles)
• Trend was reversed 1996 - 2005
2.4 The Activation of Catalytically Inactive
Chlorine
• The ozone hole occurs due to special polar winter weather
conditions in the lower stratosphere, where ozone
concentrations usually are highest, that temporarily convert all
the chlorine that is stored in the catalytically inactive forms HCl
and ClONO2, into active forms •Cl and •ClO
2.4 The Activation of Catalytically Inactive
Chlorine
•
Conversion of inactive Cl to active •Cl forms on particles formed by a
solution of water, sulfuric acid and nitric acid
•
Most parts of the world stratosphere is cloudless
•
Temperature in lower stratosphere over South Pole drops to -80 ºC in
Antarctic winter, results in ice crystal formation
•
Total darkness prevents Chapman mechanism
•
Also pressure drop (PV=nRT) in combination with Coriolis force
produces an insolated vortex with speeds in excess of 180 mph
2.4 The Activation of Catalytically Inactive
Chlorine
•
Particles produced by condensation of gases within the vortex form
Polar Stratospheric Clouds (PSCs)
•
Chemical reactions that lead to O3 loss occur in an aqueous layer at
the surface of PSCs
•
Exposure of sunlight in the early Antarctic spring (our Fall) initiates
destruction of O3
Activation of Cl On Ice Particles
(Polar Stratospheric Clouds)
•
Cl resides in stable "reservoir" compounds, HCl and chlorine nitrate (ClONO2)
ClONO2(g) + H2O(aq) → HOCl(aq) + HNO3(aq)
HCl(aq) → H+(aq) + Cl-(aq)
•
Reaction of the Cl- with HOCl produces molecular Cl2 gas
Cl-(aq) + HOCl → Cl2(g) + OH-
Net:
HCl + ClONO2 → Cl2 + HNO3
Cl2 + hν → 2 •Cl
Activation of Cl On Ice Particles
(Polar Stratospheric Clouds)
•
Massive destruction of ozone by atomic chlorine then ensues by catalytic reactions
•
Any •Cl converted to HCl by reaction with CH4 is reconverted by PSCs and sunlight
to •Cl
•
Inactivation of •ClO by conversion to ClONO2 does not occur since all NO2 is bound
as HNO3 in the PSCs
•
Only when PSCs and vortex have vanished does Cl return to inactive forms
•
Air containing NO2 mixes with vortex in spring to form catalytically inactive ClONO2
•
Ozone levels return to normal
2.5 Reactions that Create the Ozone Hole
•
Lower stratosphere – where PSCs form and •Cl is activated, [O] is
small due to low amount of UV-C
•
O3 destruction based on O3 + O pathway not important here (Mech. I)
•
Most ozone loss in the ozone hole is via Mech. II
2.5 Reactions that Create the Ozone Hole
•
Mechanism II
•
With both X and X’ being atomic •Cl
2.5 Reactions that Create the Ozone Hole
•
Mechanism II:
Step 1: Cl• + O3 → ClO• + O2
•
Confirmation that O3 loss occurs by this reaction is shown below
Anticorrelation of
• ClO with O3
2.5 Reactions that Create the Ozone Hole
•
Mechanism II:
Step 2a:
•
2ClO• → Cl-O-O-Cl
Dichloroperoxide formation rate is high due to inc. •Cl
Step 2b: ClOOCl + hv → ClOO + •Cl
Step 2c: ClOO → O2 + •Cl
Net:
2ClO• → [ClOOCl] → 2Cl• + O2
Conversion of 2 chlorine reservoir species to chlorine radical
2.5 Reactions that Create the Ozone Hole
•
Mechanism II:
•
Adding step 2 to 2 x step 1 we obtain:
2 x step 1
step 2
•
Thus a complete catalytic ozone destruction cycle exists in the lower
stratosphere under these special (cold/vortex) conditions
The New Catalyic Cycle
Reactions Responsible for the Hole
Step 1
and 2
represent
Mech II
Step 1: •Cl + O3 → ClO• + O2
Step 2: 2ClO• → Cl-O-O-Cl
Step 2b: Cl-O-O-Cl → •Cl + ClOO
Step 2c: ClOO → •Cl + O2
Occurs when [O]
(needed for Mech I)
is low
Step 2 net: 2ClO• → ClOOCl + hν → 2 •Cl + O2
Net: 2O3 → 3O2
controls season
One molecule of chlorine can degrade over 100,000 molecules of ozone before it is removed from the
stratosphere or becomes part of an inactive compound
These inactive compounds, for example ClONO2, are collectively called 'reservoirs'. They hold chlorine
in an inactive form but can release an active chlorine when struck by sunlight
Nearly 75% of the ozone depletion in the antartica occurs by this mechanism (Cl. As a catalyst)
Why are ClO Concentrations So High?
During Polar winter
Special vortex conditions
+
Low temperature
+
Denitrification of ClONO2
@ice crystal
Cl2 + HNO3
sunlight
Stratospheric ‘containment vessel’ over S. pole
•Cl
2.5 Reactions that Create the Ozone Hole
•
~ 75 % of ozone destruction in the hole occurs by mechanism II with Cl
as the only catalyst
•
Slow step is 2a combination of 2 ClO molecules
•
Rate = k[ClO]2
•
Double ClO concentration, rate x4
2.5 Reactions that Create the Ozone Hole
•
Ozone loss above Antarctica ~ 2 % per day
•
By early October almost all ozone is lost 15 – 20 km
2.5 Reactions that Create the Ozone Hole
•
Seasonal evolution
and decline of
Antarctic ozone
hole
2.6 The Size of the Antarctic Ozone Hole
•
Measured according to:
– Surface area of low ozone
– Minimum overhead ozone (see 2002)
– Length of time O3 depletion occurs
– Vertical region over which O3 depletion occurs
2.6 The Size of the Antarctic Ozone Hole
2.7 Stratospheric Ozone Destruction of
the Arctic
•
•
Did not start to form until mid 1990s
Less severe than Antarctica due to higher temperatures and
meteorology
Summary
2.8 Increases in UV at Ground Level
•
Increases in UV-B have been measured in spring at mid-latitude
regions
•
6-14 % increase
2.9 CFC Decomposition Increases
Stratospheric Chlorine
•
Increase in stratospheric chlorine primarily due to use and release of
chlorofluorocarbons (CFCs)
•
Nontoxic, nonflammable, nonreactive (at Earth’s surface!), and have
useful condensation properties (used as coolants)
•
CFCs have no tropospheric sink, so all molecules eventually reach the
stratosphere
•
CFCs are heavier than air, why do they rise?
•
Photochemically decomposed by UV-C
•
Atmospheric lifetimes are long
2.10 Other Chlorine-Containing OzoneDepleting Substances
•
Carbon tetrachloride (CCl4)
–
–
–
–
•
No tropospheric sink
Ozone-Depleting Substance (ODS)
Used as solvent and in manufacture of CFCs
Long atmospheric lifetime (26 yrs)
Methyl Chloroform (CH3CCl3)
–
–
–
Used in metal cleaning
Approx. half removed by reaction
with OH
Atmospheric Lifetime (5 yrs)
2.12 CFC Replacements
•
CFCs and CCl4 have no tropospheric sinks (not soluble in
water/rain), not decomposed by UV-A or visible light
•
HCFCs contain H atoms bonded to C atom. Removed in the
troposphere by hydroxyl radicals (H-abstraction)
•
•
CHF2Cl (HCFC-22) the current replacement for refrigerator coolants
Long term ozone destroying potential is small
•
Reliance on HCFCs would lead to build up of Cl
•
Products free of Cl are ultimate replacement
2.12 CFC Replacements
•
Hydrofluorocarbons, HFCs, are the compound of choice in USA
e.g. CH2F-CF3 (HFC-134a), CH2F/CHF2CF mix
•
No chlorine atoms!
•
Rest of world uses cyclopentane or isobutane
2.13 Halons
•
Halons, used in fire extinguishers e.g. CF3Br, CF2BrCl (hydrogen free)
•
No tropospheric sinks
•
Photochemically decomposed to Cl, Br, F atoms
•
Bromine is significant ozone problem
2.14 Can Stratospheric Fluorine Destroy
Ozone?
•
F and HF formed by decomposition of CF, HCFCs, HFCs, and halons
•
Reaction with methane and other H-containing gases is rapid and
produces stable HF
•
Why no F cycle?
OH + HF
•
endothermic
Atomic F is ‘deactivated’ before it can destroy ozone
2.15 International Agreements that
Restrict ODSs
•
•
•
•
‘Precautionary Principle’
Use of CFCs in most aerosols banned in 1970s in USA
Montreal Protocol (1987) signed by most countries to phase out CFCs
Based on Rowland and Molina’s work
2.15 International Agreements that
Restrict ODSs
•
•
•
•
CFC production in developed countries ended in 1995
Developing countries had until 2010
CFC-12 has longer atmospheric lifetime than CFC-11
CCl4 slight decline due to lack of sinks
CFC-12 > lifetime than CFC-11, no sinks for CCl4 or CFC-113
Use of HCFCs on the rise, temporary substitute for CFCs
2.15 International Agreements that
Restrict ODSs
•
Observations in 2000 indicated chlorine content of the stratosphere has
peaked
–
–
–
–
Slowness in the decline of stratospheric chlorine due to:
Long travel time to rise to middle stratosphere
Slowness of removal
Continued inputs
•
Recent projections predict the Antarctic hole area will decrease around
2023, fully recover by 2070
•
Without International agreements to protect the atmosphere predictions
indicate large increase in skin cancers around the world
Antarctic Hole Size and Minimum O3
• NASA FACTS
http://ozonewatch.gsfc.nasa.
gov/meteorology/index.html
Cf. Fig. 1-2, 1-3
Chapter 2 Homework
P2-1: A minor route for ozone destruction involves Mechanism II with bromine as X’ and chlorine as X (or vice-versa).
The ClO and BrO free radical molecules produced in these processes then collide with each other and rearrange
their atoms eventually yield O2 and atomic chlorine and bromine. Write out the mechanism for this process, and
add up the steps to determine the overall reaction.
Box 2-1 problem 1: Deduce the overall reaction equation for the reaction sequence given in Box 2-1.
P2-6: The free radical CF3O is produced during the decomposition of HFC-134a. Show the sequence of reactions by
which it could destroy ozone acting as an X catalyst in a manner reminiscent of OH.
P49 Activity: Using the information to be found at www.ozonewatch.gsfc.nasa.gov and other websites, compare the
history of the most recent Antarctic hole to the time evolution of the 2010 hole in Fig. 2-6. Did the maximum
depletion, maximum area, and minimum temperate exceed 2010 values and did they occur at about the same time
as they did in 2010? Photocopy or download Figure 2-1 and manually add data for more recent years to the two
bar graphs. Are there signs yet from your data that the hole is becoming smaller in area or depletion is lessening?
(4 points)