SULFATE

1
SULFATE THE CLAW HYPOTHESIS
2
TOPICS FOR TODAY

• The Sulfur Cycle Sources of Sulfate
• The CLAW hypothesis
• How might sulfate concentrations be affected by
  climate change?

3
SULFATE THE DOMINANT AEROSOL COUNTER-ACTING GHG
WARMING
Hansen et al., 2005
4
SULFUR CYCLE
Most sulfur is tied up in sediments and soils.
There are large fluxes to the atmosphere, but
with short atmospheric lifetimes, the atmospheric
S burden is small. SO2 Anthropogenic (fossil
fuel combustion) source comparable to natural
sources (soils, sediments, volcanoes, DMS
oxidation) Sulfur is oxidized in the
atmosphere SO2 ---- H2SO4 S(IV)
S(VI) Sulfate is an important contributor to
acidity of precipitation. Sulfuric acid has a
low Pvap and thus partitions primarily to
aerosol/aqueous phase Strongly perturbed by
human activities!
5
GLOBAL SULFUR BUDGET Chin et al., 1996(flux
terms in Tg S yr-1)
SO42- t 3.9d
SO2 t 1.3d
cloud
42
OH
H2SO4(g)
8
18
4
OH
NO3
(CH3)2S
64
dep 6 dry 44 wet
DMS t 1.0d
10
dep 27 dry 20 wet
22
Phytoplankton
Volcanoes
Combustion Smelters
6
GLOBAL SULFUR EMISSION TO THE ATMOSPHERE
2001 estimates (Tg S yr-1) Industrial 57
Volcanoes 5 Ocean 15 Biomass burning
1
Chin et al. 2000
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FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS
SO42- (coal combustion) NO3- (fossil
fuel) NH4 (agriculture)
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TOPICS FOR TODAY

• The Sulfur Cycle Sources of Sulfate
• The CLAW hypothesis
• How might sulfate concentrations be affected by
  climate change?

9
THE CLAW HYPOTHESIS
Ncloud droplets (fixed LWP)
Scattering of solar radiation by droplets
Cloud Albedo
Cloud Nucleation
Loss of solar radiation to space -
CCN
NSS-sulfate
Oxidation
Solar irradiance below clouds
Tsurf of Earth
DMS(g)
Sea-to-air transport
Production of DMS by marine phytoplankton /- ?
DMS(aq)
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THE GAIA HYPOTHESIS
Gaia a complex entity involving the Earth's
biosphere, atmosphere, oceans, and soil the
totality constituting a feedback or cybernetic
system which seeks an optimal physical and
chemical environment for life on this planet.
James Lovelock
CO2
Control Surface Temperature?
DMS
weathering
conversion to chalk/granite
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DMS EMISSIONS
DMS emitted by planktonic algae (living), but
concentration in sea water not clearly connected
with productivity. Note phytoplankton contain
chlorophyll so often use satellite products of
chl-a as proxy ? But phytoplankton speciation
important!
In Future
12
ESTIMATING DMS EMISSIONS A CHALLENGE!
The results were compared to published fields of
geophysical and biological parameters. No
significant correlation was found between DMS and
these parameters, and no simple algorithm could
be found to create monthly fields of sea surface
DMS concentration based on these parameters.
Kettle et al., 1999
Broken stick regression Anderson et al.,
2000 DMS a log(CJQ) blog(CJQ) s log(CJQ) s
C chlorophyll J short-wave light Q nutrient
term
All data
Binned Data Fit
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ESTIMATES OF ANNUAL MEAN SURFACE DMS
DMS fluxes are not known to within a factor of 2
different regional success for different models
Belviso et al., 2004
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SULFATE SOURCE FROM DMS OXIDATION
18-27 of DMS is converted to sulfate IPCC, 2007
DMS
Sulfate
Sulfate from DMS
DJF
JJA
Gondwe et al., 2003
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THE IMPORTANT LINK NSS-SULFATE (FROM DMS) AS CCN
CLAW (1987) postulates Marine CCN vary from
30-200 cm-3 Sea salt particles are not important
CCN concentrations at cloud height are 1
cm-3 Sulfate is the only important water soluble
aerosol in marine air (100 cm-3)
Increasing N, leads to enhanced ?A
Average liquid water content of clouds (L) is
constant
r radius ? density of water N number
density of droplets
?N implies compensating ?r and thus an increase
in total surface area of droplets and increase in
cloud albedo
Proxy for LWP (or thickness of cloud)
Example N 30 would lead to a globally
averaged increase in solar albedo of 0.005 (and
associated cooling of 1.3K)
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IMPORTANCE OF CLAW?

• Special issue of Environmental Chemistry in 2007
  reviewed CLAW
• after 20 years of research the theory remains
  unproven
• primarily because too many complex processes
  involved
• theory was critical to start thinking of
  connections between biology, chemistry and cloud
  physics

• Challenges
• literature explorations mostly statistical
  CCN/sulfate correlations
• the process as a whole would occur over 1000s
  of km (a couple of days to oxidize DMS to
  SO2transport away from any phytoplankton bloom)
• on the whole, is the ocean light or temperature
  limited? Regional behaviour?
• how important is MBL nucleation vs. FT
  detrainment?
• how do sea salt and organic particles emitted
  from the ocean complicate CLAW?
• what about the role of increasing ocean
  stratification (reduction in nutrient
  availability and plankton growth)?
• the potential for enhanced convection to loft
  DMS higher into the FT
• the effect of ocean acidification on ocean
  productivity?
• floristic shifts due to warming oceans?

17
TOPICS FOR TODAY

• The Sulfur Cycle Sources of Sulfate
• The CLAW hypothesis
• How might sulfate concentrations be affected by
  climate change?

18
PREDICTED CHANGES IN (ANTHROPOGENIC) SO2
EMISSIONS
50 Drop
Globally SO2 emissions expected to decline,
largely due to reductions expected in NA, EU,
AS(?). Spread largely due to assumptions about
timing of emissions controls in Asia/India. DMS
source will become relatively more important.
IPCC 2007 (WG3)
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HOW WILL THE SULFUR CYCLE BE AFFECTED BY CLIMATE
CHANGE?
Transport? Lofting?
SO42-
SO2
cloud
OH
Oxidation processes? Lifetimes?
H2SO4(g)
OH
NO3
(CH3)2S
dep
DMS
dep
Phytoplankton
Volcanoes
Combustion Smelters
Species? Response? Ocean stratification? Flux out
of ocean?