Pollution of Lakes and Rivers Chapter 15: Ozone depletion, acid rain, and climatic warming: the problems of multiple stressors Copyright © 2008 by DBS.

Pollution of Lakes and Rivers

Chapter 15:Ozone depletion, acid rain, and climatic

warming: the problems of multiple stressors

Copyright © 2008 by DBS

Contents

• 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 DepletionThe Challenges of Multiple Stressors

• ‘several environmental problems culminating into a larger, potentially more serious calamity’

• 3 human-related alterations of the atmosphere:

– Climate change

– Acid deposition

– Ozone depletion

• Negative impacts are significantly amplified when they occur simultaneously – multiple stressors

Cartoon

Allotropes of Oxygen

Allotropes of Oxygen

Chapman TheoryAbove stratosphere oxygen absorbs UV-C and exists as O atoms

O2 + hν → O + O ΔH = 495 kJ/mol (<241 nm) (1)

Oxygen atom could react with oxygen molecule to form O3

O + O2 + M → O3 ΔH = -100 kJ/mol (2)

O3 formed could react with O atoms or absorb solar radiation

O3 + hν → O2 + O (<320 nm) (3)

O + O3 → 2O2 ΔH = -390 kJ/mol (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

+ O3

- O3

Chapman TheoryPhotostationary 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 DepletionStratospheric Ozone Depletion, Ultraviolet Radiation, and Cancer

• 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

Melanoma

Phytoplankton

UV-B damage

O2 + O3 as Natural UV Filter

absorbs

transmits

Absorption of UV-C and UV-B by O3

What is a Dobson Unit?

• 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?

• Occurs at the beginning of Southern Hemisphere spring (August-October)

• The average concentration of O3 in the atmosphere is about 300 Dobson Units

Not a “hole” but a region of depleted O3 over the Antarctic

Ozone is ‘thinning’ out

Any area where O3 < 220 DU is part of the O3 hole

Mid-lattitudes

• Arosa, Switzerland

• Continuous record back to 1931

• Slow, steady decline, of about 3% per decade during the past twenty years

• Enhanced by volcanic eruptions (Mt. Pinatubo)

Kerr, 2002

SummaryNatural change or related to human activities?

The Big Surprise of 1985

• 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

Min O3 at Antarctic in Spring (Sep-Nov)

Chemical explanation?

Physical explanation?

Ozone Destruction

• In september destruction occurs ~2 % per day

• Most O3 is wiped out at 15-20 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 DepletionRefrigerators, Air Conditioners, Aerosol Cans, and the Ozone Layer

• 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 DepletionRefrigerators, 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

O3 + O → O2 + O2

Ozone DepletionRefrigerators, Air Conditioners, Aerosol Cans, and the Ozone Layer

• 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)

Sch

indl

er e

t al

. (1

996)

As DOC concentrations increase, the depth of UV-B penetration decreases

Ozone DepletionRefrigerators, 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 DepletionRefrigerators, 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 DepletionReconstructing Past UV Penetration in Lakes

• 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 DepletionPaleo-optics: Tracking Long-term Changes in the Penetration of UV Radiation

• Boreal forest lakes have high DOC, biota relatively immune from damage

• Diatom transfer functions used to reconstruct past DOC

• 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…

Ozone DepletionPaleo-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

Pie

nitz

and

Vin

cent

, 20

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Ozone DepletionBiological Responses to Enhanced UV-B Exposure:

Evidence from Fossil Pigments

• Biological response may be directly related to UV

• 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

• Alpine study – pigments increasec. 1850 – 1900

• Suggests higher UV penetration

• Tree ring data suggests cool weather and droughts dec. DOC

Ozone DepletionBiological Responses…

• Fossil pigments increased in post acidification sediments of Lake 302S (Exp. Lakes Area)

• 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.

Leav

itt e

t al

(19

99)

Ozone DepletionMultiple stressors: New Combinations of Old Problems

• A

Ozone DepletionSummary

• 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

• 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

• 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

• 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.