Transcript Document

SULFATE & THE CLAW HYPOTHESIS
TOPICS FOR TODAY
1.
The Sulfur Cycle & Sources of Sulfate
2.
The CLAW hypothesis
3.
How might sulfate concentrations be affected by climate
change?
SULFATE: THE DOMINANT AEROSOL COUNTERACTING GHG WARMING
[Hansen et al., 2005]
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:
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!
SO2 ---- > H2SO4
S(+IV)
S(+VI)
GLOBAL SULFUR BUDGET [Chin et al., 1996]
(flux terms in Tg S yr-1)
cloud
42
SO2
4
NO3
18
t = 3.9d
OH
t = 1.3d 8
SO42-
H2SO4(g)
OH
(CH3)2S
DMS
t = 1.0d
10
64
dep
27 dry
20 wet
22
Phytoplankton
Volcanoes
Combustion
Smelters
dep
6 dry
44 wet
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]
FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS
Thermodynamic rules:
H 2O
H 2 SO4 ( g ) 
 SO42  2 H 
H 2O
NH 3 ( g ) 
 NH 4  OH 
Sulfate always forms an aqueous aerosol
Ammonia dissolves in the sulfate aerosol
totally or until titration of acidity, whichever
happens first
H 2O
HNO3 ( g ) 
 NO3  H 
Nitrate is taken up by aerosol if (and only if)
excess NH3 is available after sulfate
titration
HNO3 and excess NH3
NH 3 ( g )  HNO3 (Highest
g)
NH
4 NO3 (aerosol ) can also form a solid aerosol
concentrations
if RH is low
in industrial Midwest
(coal-fired power
SO42- (coal combustion)
NO3-plants)
(fossil fuel)
NH4+ (agriculture)
TOPICS FOR TODAY
1.
The Sulfur Cycle & Sources of Sulfate
2.
The CLAW hypothesis
3.
How might sulfate concentrations be affected by climate
change?
Ncloud droplets
(fixed LWP)
Cloud Nucleation
+
CCN
Scattering of solar
radiation by droplets
+
THE CLAW
HYPOTHESIS
Cloud Albedo
Loss of solar
radiation to space
-
NSS-sulfate
Oxidation
+
Tsurf of
Earth
Solar
irradiance
below clouds
DMS(g)
Sea-to-air transport
+
DMS(aq)
Production of DMS by marine
phytoplankton
+/- ?
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
DMS
weathering
…conversion to
chalk/granite
Control
Surface
Temperature?
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!
Flux (F)=Akc
In Future:
More ice- Increase
free ocean? winds?
A=ocean surface area
k=transfer velocity
c = gradient across air/sea interface
More DMS
in ocean?
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) < s
DMS = a + b[log(CJQ) – s]
log(CJQ) > s
All data
C = chlorophyll
J = short-wave light
Q = nutrient term
Binned Data + Fit
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]
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]
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
4 3
L =  r N
3
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
Example:
N +30% would lead to a globally averaged
increase in solar albedo of +0.005 (and associated
cooling of 1.3K)
Proxy for LWP (or thickness of cloud)
Average liquid water content of clouds (L) is constant:
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 1000’s of km (a couple of days to oxidize
DMS to SO2…transport 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?
TOPICS FOR TODAY
1.
The Sulfur Cycle & Sources of Sulfate
2.
The CLAW hypothesis
3.
How might sulfate concentrations be affected by
climate change?
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)]
HOW WILL THE SULFUR CYCLE BE AFFECTED BY
CLIMATE CHANGE?
Transport? Lofting?
cloud
SO42SO2
Oxidation processes?
Lifetimes?
NO3
OH
H2SO4(g)
OH
(CH3)2S
DMS
Phytoplankton
Species? Response?
Ocean stratification?
Flux out of ocean?
dep
Volcanoes
Combustion
Smelters
dep