Download Pollution of Lakes and Rivers Chapter 15

Pollution of Lakes and Rivers
Chapter 15:
Ozone depletion, acid rain, and climatic
warming: the problems of multiple
stressors
Copyright © 2008 by DBS
Contents
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The challenges of multiple stressors
Stratospheric ozone depletion, ultraviolet radiation, and cancer
Refrigerators, air conditioners, aerosol cans, and the ozone layer
Reconstructing past UV penetration in lakes
Paleo-optics: tracking long-term changes in the penetration of ultraviolet radiation in lakes
Biological responses to enhanced UV-B exposure: evidence from fossil pigments
Multiple stressors: new combinations of old problems
Ozone Depletion
The Challenges of Multiple Stressors
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‘several environmental problems culminating into a larger, potentially more
serious calamity’
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3 human-related alterations of the atmosphere:
– Climate change
– Acid deposition
– Ozone depletion
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Negative impacts are significantly amplified when they occur
simultaneously – multiple stressors
Cartoon
Allotropes of Oxygen
Allotropes of Oxygen
Chapman Theory
Above stratosphere oxygen absorbs UV-C and exists as O atoms
O2 + hν → O + O
ΔH = 495 kJ/mol (<241 nm)
(1)
+ O3
Oxygen atom could react with oxygen molecule to form O3
O + O2 + M → O 3
ΔH = -100 kJ/mol
(2)
O3 formed could react with O atoms or absorb solar radiation
O3 + hν → O2 + O
O + O3 → 2O2
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(<320 nm)
ΔH = -390 kJ/mol
(3)
- O3
(4)
A third molecule ‘M’ (N2 or H2O) facilitates as a heat energy carrier
(is not required when there is more than one molecule produced)
Enthalpies show a great deal of heat is generated
Because of the temperature inversion vertical mixing of air is slow in the
stratosphere
Chapman Theory
Photostationary state is
established
O3 + hν ↔ O2 + O
Net effect: one form of ‘odd
oxygen’ is converted into
another (O3 to O)
• At night back reaction
dominates
• At day forward reaction
dominates (hν)
JO3
Ozone Depletion
Stratospheric Ozone Depletion, Ultraviolet Radiation, and Cancer
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O3 present in trace amounts
Natural
Uneven distribution
Max at 25 km
Causes inversion
Earth’s ‘sun-screen’
O2 and O3 filter certain UV
wavelengths
Solar Flux
Phytoplankton
UV-B damage
Melanoma
O2 + O3 as Natural UV Filter
absorbs
transmits
Absorption of UV-C and UV-B by O3
What is a Dobson Unit?
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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
A column of air with an O3 concentration of 1 DU would contain about
2.69 x 1016 O3 molecules cm-2 at the base of the column
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
What is the Ozone Hole?
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Occurs at the beginning of
Southern Hemisphere spring
(August-October)
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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
Mid-lattitudes
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Arosa, Switzerland
Continuous record back
to 1931
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Slow, steady decline, of
about 3% per decade
during the past twenty
years
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Enhanced by volcanic
eruptions
(Mt. Pinatubo)
Kerr, 2002
Natural change or
related to human
activities?
Summary
The Big Surprise of 1985
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Farman et al. revealed a dramatic
and unpredicted decline in
stratospheric O3 in a surprising
location
– Antarctica
– Showed dramatic decline in
springtime O3 starting in 1970’s
30% by 1985
70% by 2000
Chemical explanation?
Min O3 at Antarctic in Spring
(Sep-Nov)
Physical explanation?
Ozone Destruction
• In september destruction
occurs ~2 % per day
• Most O3 is wiped out at 1520 km altitude
Could not be explained by
natural cycles since under low
light conditions [O] is too low
(UV-C required to produce O
does not penetrate past 20 km)
Ozone Depletion
Refrigerators, Air Conditioners, Aerosol Cans, and the Ozone Layer
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Known since 1970’s that human produced chlorine and bromine gases
destroy ozone
CFC’s inert in troposphere but sink for O3 in stratosphere
Uses: air conditioning, refrigerators, aerosol cans, plastic foams etc.
CFCl3 + hν → CFCl2 + •Cl
Ozone Depletion
Refrigerators, Air Conditioners, Aerosol Cans, and the Ozone Layer
Catalyst X (chlorine or bromine
atom) cleaves an oxygen (O) atom
from an ozone (O3) molecule,
forming OX and O2
Oxygen atom (O) can then remove
the oxygen atom attached to the
catalyst, releasing the catalyst
•X + O3 → •XO + O2
•XO + O → •X + O2
O 3 + O → O 2 + O2
Ozone Depletion
Refrigerators, Air Conditioners, Aerosol Cans, and the Ozone Layer
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Increased UV-B penetration should be cause for concern
Humic and fulvic fractions of DOC absorbs strongly (Vincent and Roy, 1993)
and act as natural sun-screen in lakes
With less DOC UV-B penetration is greater (Vincent and Pienitz, 1996)
As DOC concentrations
increase, the depth of UV-B
penetration decreases
Schindler et al. (1996)
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Ozone Depletion
Refrigerators, Air Conditioners, Aerosol Cans, and the Ozone Layer
Triple whammy
• Situation is made worse due to combined effects of our multiple stressors
– acid precipitation (Yan et al, 1996) and
– warming (Schindler et al, 1996)
• 3 stressors act in concert to increase UV-B exposure of aquatic organisms
Ozone Depletion
Refrigerators, Air Conditioners, Aerosol Cans, and the Ozone Layer
The top panel shows in their “natural
states”, with DOC being delivered to
the lake from the catchment
Acidity - The middle panel shows
how DOC in the lake is reduced with
acidic precipitation:
(i) increases photodegradation of
DOC, (ii) precipitates DOC from
water column – clearer lakes
Warming - The lower panel shows
how the supply of DOC to the lake is
further diminished under drought
conditions, as less DOC is delivered
to the lake
Ozone Depletion
Reconstructing Past UV Penetration in Lakes
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No direct information, rely on indirect methods:
1. Infer past lakewater DOC (Pienitz et al, 1999), reconstruct past underwater
radiation climate (paleo-optics) and relate this to vegetation change
(Pienitz and Vincent, 2000) or acidic deposition (Dixit et al, 2001)
2. Track past UV-B penetration by studying biotic response – pigment
changes in blue-green algae (Leavitt et al, 1997)
Ozone Depletion
Paleo-optics: Tracking Long-term Changes in the Penetration of UV Radiation
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Boreal forest lakes have high DOC, biota relatively immune from damage
Diatom transfer functions used to reconstruct past DOC
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Data has been gathered for Arctic tree line regions where it is used to
reconstruct position of the coniferous tree line (Pienitz et al, 1997)
Canadian Sub-Arctic, Queens Lake…
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Ozone Depletion
Pienitz and Vincent, 2000
Paleo-optics: Tracking Long-term Changes in the Penetration of UV Radiation
Using a diatom DOC transfer function researchers inferred a
warming period between 5000 – 3000 yrs BP
Ozone Depletion
Biological Responses to Enhanced UV-B Exposure:
Evidence from Fossil Pigments
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Biological response may be directly
related to UV
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Leavitt et al (1997,1999) has shown
use of fossil pigments to track UV
(i) alpine lakes subjected to droughts
(ii) artificially acidified lake in Ontario
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Alpine study – pigments increase
c. 1850 – 1900
Suggests higher UV penetration
Tree ring data suggests cool
weather and droughts dec. DOC
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Biological Responses…
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Fossil pigments increased in post acidification
sediments of Lake 302S (Exp. Lakes Area)
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Lake was artifically acidified in 1980
Lead to decline in lakewater DOC
Concentrations of UV-absorbing fossil pigment in the sediments of Lake
302S, NW Ontario. This lake was artificially acidified in 1980, resulting in a
decrease in DOC and a corresponding increase in UV-B penetration.
Leavitt et al (1999)
Ozone Depletion
Ozone Depletion
Multiple stressors: New Combinations of Old Problems
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Ozone Depletion
Summary
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Ozone depletion, acid rain and climate change are often discussed in
isolation
Cumulative effects may result in more severe damage
Decreases in the DOC due to acidification and rising temperatures may
have severely affected a lakes ability to filter out harmful UV-B radiation
Paleolimnological methods used to track changes in lake DOC include
diatom transfer functions and fossil pigments
As new problems are constantly being identified, dealing with multiple
stressors is possibly the greatest challenge facing environmental scientists
for the foreseeable future.
References
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Battarbee (2000)
Blokker et al (2005)
Dixit et al (2001)
Gorham (1996)
Hengeveld (1991)
Hodgson et al (2005)
Leavitt, P.R., Vinebrooke, R., Donald, D.B., Smol, J.P. and Schindler, D.W. (1997) Past
ultraviolet radiation environments revealed using fossil pigments in lakes. Nature, Vol. 388, pp.
457-459.
Leavitt et al (1999)
Leavitt et al, (2003)
Laurion et al (1997)
References
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Macdonald et al (1993)
Pienitz (1997 a and b)
Pienitz et al (1999)
Pienitz, R. and Vincent , W. (2000) Effects of climate change relative to ozone depletion on UV
exposure in subarctic lakes. Nature, Vol. 404, pp. 484-487.
Schindler, D., Curtis, P.J., parker, B. and Stainton, M. (1996) Consequences of climate warming
and lake acidification for UV-B penetration in North American boreal lakes. Nature, Vol. 379, pp.
705-708.
Schindler (1998)
Schindler (2001)
References
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Smol, J.P., Walker, I.R. and Leavitt, P.R. (1991) paleolimnology and hindcasting climatic trends.
Verhandlungen der Internationalen Vereinigung von Limnologen, Vol. 24, pp. 1240-1246.
Smol , J.P. and Cumming , B.F. (2000) Tracking long-term changes in climate using algal
indicators in lake sediments. Journal of Phycology, Vol. 36, pp. 986-1011.
Verleyen et al (2005)
Vincent , W.F. and Roy, S. (1993) Solar ultraviolet-B radiation and aquatic primary production:
damage, protection and recovery. Environmental Reviews, Vol. 1, pp. 1-12.
Vincent , W. and Pienitz , R. (1996) Sensitivity of high-latitude freshwater ecosystems to global
change: temperature and solar radiation. Geoscience Canada, Vol. 23, pp. 231-236.
Yan, N.D., Keller, W., Scully, N., Lean, D. and Dillon, P. (1996) Increased UV-B penetration in a
lake owing to drought-induced acidification. Nature, Vol. 381, pp. 141-143.