Transcript Biogenic_Cloud_Sources

Biogenic Cloud Sources
Spring 2012, Lecture 9
1
Alternative Cloud Formation Method
• In the previous lecture, we saw various ways
in which clouds could form
• There is another way clouds form, and it may
be part of a cybernetic system
2
Sulfur Cycle
• Sulfur is carried to the ocean as dissolved
sulfate ion
• Sulfur must be returned from the ocean, if the
land is not to be depleted of sulfur
• Traditionally, it has been stated that hydrogen
sulfide gas is emitted from the ocean
• Hydrogen sulfide is familiar to many people as
the smell of a rotten egg
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Hydrogen Sulfide
• Hydrogen sulfide stinks and is quite poisonous
• If the oceans were emitting large quantities of this
gas, we would smell it and travel on the oceans might
be hazardous
• It is quickly oxidized to sulfur dioxide in the presence
of oxygen, and is unstable in the air
• It is quite soluble, and should be washed out of the
atmosphere quickly
• It probably would not have time to be carried over the
land
4
James
Lovelock
• James Lovelock, geophysiologist and creator of the
GAIA hypothesis, concludes that hydrogen sulfide
cannot be the agent for the return of sulfur to the land
• Instead, Lovelock believes that dimethyl sulfide
((CH3)2S), emitted by many marine organisms, might
be the carrier gas
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Dimethyl Sulfide
• Dimethyl sulfide, unlike hydrogen sulfide, has a
pleasant odor when dilute, and that the smell of the
sea, or of fresh caught marine fish, is partly that of
dimethyl sulfide
• Dimethyl sulfide (DMS) is a naturally produced
biogenic gas essential for the Earth's biogeochemical
cycles
• Certain species of phytoplankton, microscopic algae
in the upper ocean, synthesize the molecule
dimethylsulfoniopropionate (DMSP) which is the
precursor to DMS
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DMSP Decomposition
Note the positive charge on the sulfur, and the
negative charge on the proprionic acid part of the
DMSP molecule
•
DMS Molecule
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Dimethyl Sulfide Decomposition
• Dimethyl sulfide decomposes slowly to yield sulfate
and methane sulfonate
• Both compounds can undergo further oxidation to
yield sulfuric acid (H2SO4) and methane sulfonic acid
(CH3SO3H)
• Rainfall can remove these compounds and deposit
them on the ground, but this cycle takes considerably
longer than the breakdown of hydrogen sulfide, thus
allowing sulfur to return to the land
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Lovelock and DMS
• Lovelock's own work has contributed one clue about
dimethyl sulfide
• Lovelock traveled on the research vessel Shackleton
on a voyage from Wales to Montevideo, Uruguay
• Using a gas chromatograph he measured
dimethylsulfide, carbon disulfide, and halocarbon
gases
• He arranged for colleagues to continue this work after
he left the ship
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Marine Deserts
• Later, M. O. Andreae made extensive measurements
of dimethyl sulfide over the oceans
• These measurements confirmed the ability of
dimethyl sulfide to be the major carrier of sulfur from
ocean to land
• The vast ‟desert” areas, characterized by low
biological productivity, are particularly large
generators of dimethyl sulfide, and they cover about
40% of the earth's surface
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Satellite Data
• The base of the ocean food web consists of
abundant and productive phytoplankton
• Phytoplankton are tiny ocean plants that
convert carbon dioxide to organic carbon, and
whose green chlorophyll is visible from space
• Satellites can detect this chlorophyll
• Satellite data can be used to estimate ocean
productivity, and thus detect marine deserts
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Where are Marine Deserts?
• Satellite image showing areas of high biologic
productivity in the ocean (green) and low
productivity (blue) – these are marine deserts
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Biological Productivity in Marine Deserts
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Lack of biological productivity is due to lack of nutrients
These areas are not devoid of life
One example is the Sargasso Sea
Such areas are known to have floating strands of seaweed
The seaweed strands correspond to the sparse sagebrush,
creosote bush, and other plants of the terrestrial deserts
• In support of Lovelock’s contention that DMS is the
important carrier gas for sulfur, Yoch (2002) provides a
table comparing emission amounts of several
organosulfur compounds (next slide)
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Global Emissions of Organosulfur Compounds
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Magnitude of DMS Emissions
• The magnitude of the marine DMS emissions is
remarkable, considering that over half of the DMSP
released is demethylated and that a significant
fraction of the DMS is oxidized by bacteria in the
water column before it can be released to the
atmosphere
• Generation of dimethyl sulfide by marine algae may
thus be important for land organisms, because sulfur
is scarce on land, but seems fortunate and not related
to climate control
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Benefit to Phytoplankton
• Why do phytoplankton generate
dimethylsulfoniopropionate (DMSP)?
• In what way does it benefit them?
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Battling Salt
• Most marine organisms fight a constant battle
with salt
• Any desiccation of a marine organism that
raises the NaCl concentration is potentially
fatal
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Function of DMSP
• Marine phytoplankton generate a compound
called dimethylsulfoniopropionate (DMSP).
• This compound is known as a betaine, because
it is similar to the compound
trimethylammonio acetate, or betaine
• Although the exact function of DMSP is
unclear, it has possible roles in osmoprotection
and cryoprotection
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Betaine
• Betaine was first isolated from beets
• Beets are known to have a high salt tolerance.
Betaines may convey this salt tolerance,
possibly by complexing the salt ions
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Nitrogen or Sulfur?
• Other, yet unknown, functions for
dimethylsulfoniopropionate may exist
• It seems reasonable that terrestrial plants
would use a nitrogen betaine, because nitrogen
is plentiful on land, and that marine organisms
would use a sulfur betaine because of the
abundance of sulfur in the sea
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Benefit of Sulfur Transfer to Land?
• Algae that generate dimethylsulfoniopropionate,
whatever its role, are a source of dimethyl sulfide
• Algae decompose after dying or being eaten
• The resulting products include acrylic acid ion and
dimethyl sulfide
• Lovelock argues those marine algae, working in their
own best interest, have contributed a mechanism for the
transfer of sulfur to land
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Decomposition of Rocks into Soil
• Sulfur on land would enhance the growth of
land plants, which in turn speed the
decomposition of rocks into soil
• Such decomposition would enhance the flow
of nutrients into the ocean, to the benefit of
marine organisms
• Thus a symbiotic relationship (one beneficial
to both partners) develops at a distance, and
without conscious thought
22
Marine Deserts and DMS
• The generation of dimethyl sulfide may be
purposeful rather than fortuitous, but it still
does not appear related to the climate
• The major generation area for dimethyl
sulfide, the marine deserts, may provide a clue
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Oxidation of DMS
• Meteorologists Robert Charlson and Stephan
Warren argue that the rapid oxidation of
dimethyl sulfide could provide small droplets
of sulfuric acid over the ocean
• Sulfuric acid is powerfully hygroscopic and
could yield the necessary condensation nuclei
to produce clouds
• Methane sulfonic acid can serve the same
purpose
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Development of Condensation Nuclei
• Areas far removed from the continents have few
condensation nuclei
• Condensation nuclei are often derived from airborne
dust generated on the continents
o Thus, these areas are expected to generate few clouds
o If increased dimethyl sulfide production were to generate
large scale cloud cover over what is generally a dark blue
ocean, the albedo would change
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Albedo Effect
• Andreae, Charlson, Lovelock, and Warren
have calculated the probable effect on the
climate
• They find that the increased albedo is
comparable in magnitude but opposite in sign
to the greenhouse effect
• Could this be part of a climate regulatory
system?
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Slow vs. Rapid Oxidation
• It was previously argued that the oxidation of
dimethyl sulfide was slow, thus allowing sulfur
to reach the interior of the continents
• Yet the argument for increased cloud cover
requires the rapid oxidation of dimethyl sulfide
• How is this possible?
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Kinetic Barrier
• We have previously
seen that the rate of
reaction is a kinetic
process, involving
an energy barrier
• The easiest way to
speed up a slow
reaction is to
introduce a catalyst
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What is the Catalyst?
• It is known that iodine compounds catalyze the
oxidation of dimethyl sulfide in marine
environments
• If we can find a source of iodine compounds,
we might have a regulatory mechanism
• Lovelock suggests that another betaine,
methyliodonio propionate, might exist in large
algae such as brown seaweed, Laminaria
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Laminaria digitata
• Large, flexible plants with a rubbery texture - Size can range
from less than one meter to ten meters or more for L.
saccharina or L. longicruris
• Color ranges from light brown to dark brown
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• Source of methyl iodide
Iodide Catalysis
• Methyl iodide released by sea plants is an
important part of the iodide cycle, transferring
iodine to the land
• Methyl iodide produced by the plants could
also serve to form iodine compounds, capable
of oxidizing dimethyl sulfide to sulfuric acid
• This could trigger cloud formation over the
ocean
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Benefits of Marine Clouds - 1
• Cloud formation can trigger an increase in
wind velocity
• The mixing of surface waters increases
• This brings nutrients from waters below the
photosynthetic layer upward, where they can
be utilized by the marine algae
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Benefits of Marine Clouds - 2
• In addition the clouds will probably result in
increased atmospheric precipitation
• This may wash aerosol dust out of the air into
the ocean
• In areas where sufficient terrestrial aerosol
dust exists, this would provide an additional
source of nutrients, particularly scarce
nitrogen, to the marine algae
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Potential Future Benefit
• Clouds also filter radiation reaching the ocean
surface
• In particular, they may absorb harmful UV
radiation
• If the ozone hole problem becomes worse, this
could become a more important benefit in the
future
34
Antarctic Glacial Evidence
• Some French glaciologists have recently reported the
existence of sulfuric and methane sulfonic acid in ice
cores from Antarctica
• Robert Delmas and colleagues and C. Saigne and M.
Legrand have studied ice cores dating from the
present to 30,000 years before present
• They found a strong inverse correlation between
global temperature and the deposition of these acids
• During the last ice age, two to five times more of
these acids were deposited than during warmer times
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Acid Sources
• Sulfuric acid has several natural sources
• Methane sulfonic acid, on the other hand, has
only one known source, the atmospheric
oxidation of dimethyl sulfide
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Other Benefits
• Thus it appears that increased marine
production of dimethyl sulfide might have
contributed to a cooling of the atmosphere
• Increased marine photosynthesis would also
help lower carbon dioxide levels, reducing
radiative forcing
• Increased marine precipitation would also
dissolve more carbon dioxide, carrying it into
the ocean
37
Marine Desert Expansion
• We saw before that satellites can delineate
marine deserts
• They can also be used to monitor changes
• It has been found that the low-productivity
area of the Western Pacific has been increasing
in area
38
Climate Model Predictions
• Climate models predict that expansion of marine
deserts due to climate change will be slow, taking
centuries to see
• Using six climate models for the period between the
beginning of the Industrial Revolution and 2050, it
was predicted that total growth of low-productivity
areas in the Northern and Southern Hemispheres
would range from 0.7 percent to 8.1 percent,
depending on various parameters
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SeaWIFS Satellite Data
• Ocean biologist Jeffrey Polovina and colleagues Evan
Howell and Mélanie Abécassis, working with NASA
data from the NASA Sea-Viewing Wide Field-ofView Sensor (SeaWiFS) processed, archived, and
distributed by the Ocean Biology Processing Group at
NASA’s Goddard Space Flight Center, made some
interesting discoveries
• The team also used sea surface temperature data from
the Advanced Very High Resolution Radiometer
(AVHRR)
40
Observational Data
• Polovina said, “We were very surprised. We looked at
the North Atlantic, the South Atlantic, the Indian
Ocean—we saw the same trends all over the globe.
Over nearly the past decade, regions with low surface
chlorophyll were expanding into nearby ocean basins.
The total area lost was quite enormous.”
• “The expansion of low-productivity waters matched
up with significant increases in sea surface
temperature.”
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Observation vs. Computer Model
• Polovina said, “We’ve measured more than
even the high range in only nine years.”
• Step-by-step desertification has led to a 1 to 4
percent loss in productive waters per year
Abécassis said, “The actual rate of expansion
was much bigger than the models predicted.”
42
Future Implications
• “This is a unique biological signal that we’re seeing.
What this means is that the ability of the oceans to
support life has decreased,” he said. “The density of
mahi-mahi, shark, tuna, et cetera, will be less.”
• Abécassis agreed, saying, “The expansion of poor
productivity areas is a major concern—with
implications for ocean food webs and potentially a lot
of changes for fisheries.”
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Study Duration -1
• The study covered nine years
• Although nine years is a reasonably long time
series, scientists prefer to base their research
on longer time series of thirty or more years to
ensure that natural cycles, such as El Niño and
its opposite, La Niña, characterized by lower
than normal sea surface temperatures, have
been taken into account.
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Study Duration - 2
• “During the period of our study there have
been several La Niña events, so the large-scale
climate has been unique,” Polovina said. “It’s
possible that the trend will reverse in the
coming decade. Or, it could be that the trend
holds and the losses really are worse than what
models are indicating they should be.”
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Pacific Ocean Data
Chlorophyll during the nine-year study period declined in the area of
the North Pacific shown above in December from 1998 to 2006. Lowproductivity areas —regions with 0.07 milligrams (0.000002 ounces)
or less of chlorophyll per cubic meter (35 cubic feet)—expanded by
more than 500,000 square kilometers per year between 1998 and 2006
in the North Pacific. (Courtesy J. Polovina)
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