Transcript Global Sulphur Cycle - University of Edinburgh

Atmospheric Impact of
the 1783-1784 Laki
volcanic eruption
David Stevenson (University of Edinburgh)
Thanks to:
Ellie Highwood (Univ. Reading),
Colin Johnson, Bill Collins, Dick Derwent (Met Office)
Funding:
Talk Structure
•
•
Motivations: volcanoes and the atmosphere
Introduce:
•
•
•
Model experiments & results
Radiative forcing & climate impact
Conclusions / problems
– The Laki eruption
– Atmospheric chemistry model
– Tropospheric sulphur cycle
0530 15 June 1991
0630 15 June 1991
0730 15 June 1991
Solar
radiation
Aerosol reflects
Solar radiation – net cooling
SO2 oxidises to H2SO4 aerosol:
Residence time ~year (mid-stratosphere)
weeks-months (UT/LS)
days-weeks (troposphere)
Ash falls out
quickly (hours-days)
Pinatubo aerosol from the Space Shuttle
Aerosol layer ~20-25 km
Tropopause ~15 km
SO2 (gas) + OH → H2SO4 (aerosol)
Volcanoes and Climate
• Natural climate variability
IPCC
2001
• How much of this is due to volcanoes?
Volcanoes and Climate
• Test climate model sensitivity/response
Hansen et al. 1997 IPCC, 2001
GISS GCM
Volcanoes and Climate
• Focus to date on large explosive
eruptions, or those that leave a record in
the Greenland or Antarctic ice.
• What about large effusive eruptions?
‘Fires of the Earth –
The Laki Eruption 1783-1784’
Eyewitness account of the
eruption by the local vicar
(Rev. Jon Steingrimsson)
Recently reprinted by the
University of Iceland, and
translated into English
1/5 of Iceland’s population
died (10,000 people)
Much better than that cack by Tolkien
27 km long fissure
580 km2 of lava
1783-84 Laki eruption, Iceland
•
•
•
•
•
•
•
•
8 June 1783: 27 km long fissure opens
15 km3 of basalt erupted in 8 months
60 Tg(S) released
60% in first 6 weeks
Fire-fountaining up to ~800 - 1450 m
Eruption columns up to ~6 - 13 km
Tropopause at ~10 km
‘Dry fog’ or ‘blue haze’ recorded over Europe, Asia,
N. Atlantic, Arctic, N. America
• This appears to have been a sulphuric acid aerosol
layer in the troposphere and/or lower stratosphere
~200m Fire-fountaining at Etna, 2002
Photos from Tom Pfeiffer’s web-site: www.decadevolcano.net
Environmental impacts
•
•
•
•
20% of the Icelandic population die
Acid deposition destroys crops & grazing
Similar impacts across Europe
Cooling of NH (regionally extreme, e.g.
Alaska)
• Cooling for several years (Franklin, 1785)
• Famine
Questions
• Using our best estimate of the Laki
SO2 emissions, what is the modelled
impact on the global atmospheric
composition?
• Does it agree with observations?
• Can it generate a climate impact?
Atmospheric model:
STOCHEM
•
•
•
•
•
•
Global 3-D chemistry-transport model
Meteorology: HadAM3
GCM grid: 3.75° x 2.5° x 58 levels
CTM: 50,000 air parcels, 1 hour timestep
CTM output: 5° x 5° x 22 levels
Detailed tropospheric chemistry
• CH4-CO-NOx-hydrocarbons
• detailed oxidant chemistry
• sulphur chemistry
• Normally used for air quality/climate studies
• This version has high resolution tropopause
STOCHEM framework
Air parcel centres
Eulerian grid
from GCM
provides
meteorology
Interpolate met. data for each
air parcel
For each air parcel
• Advection step
• Interpolated winds, 4th order Runge-Kutta
• Plus small random walk component (=diffusion)
• Calculate emission and deposition fluxes
• Prescribe gridded emissions for NOx, CO, SO2, etc.
• Integrate chemistry
• Photochemistry (sunlight, clouds, etc.)
• Gas-phase chemistry (T, P, humidity, etc.)
• Aqueous-phase chemistry (cloud water, solubility, etc.)
• Mixing
• With surrounding parcels
• Convective mixing (using GCM convective clouds)
• Boundary layer mixing
Sulphur chemistry
Oxidants normally determined by background
photochemistry – but very high SO2 levels will deplete them
+OH
emissions
SO2
gas
+H2O2(aq)
SO4
aerosol
(in clouds)
Oxidation and
deposition rates
determine the
SO2 lifetime
+O3(aq)
dry(wet) deposition
Only
deposition rates
determine the
SO4 lifetime
wet(dry) deposition
Present-day tropospheric sulphur cycle
Burden
Tg(S)
Lifetime
Days
Fluxes in Tg(S)/yr
OH
0.29
6.3
0.81
SO2
SO4
1.1
30
H2O2
O3
32
5.3
17
9.2
7.1
9
71
Deposition
Dry Wet
49
1.4
12
DMS
4
MSA
Deposition
1
Biomass Anthro- Volcanic Soil
burning pogenic
15
Dry Wet
Oceanic
Laki sulphur emissions
• Analysis of the S-content of undegassed
magma suggests ~60 Tg(S) released by Laki
(Thordarson et al., 1996)
• ~1990 global annual anthropogenic input
• Compare to Pinatubo: ~20 Tg(S)
• What was the vertical profile of emissions?
1990 Anthropogenic SO2 emissions
(annual total)
Laki
value 61
Peak
value ~2
Total 72
0.1
1
10
Tg(S)/yr/5x5
100
Model experiments
• 1990 atmosphere
• Background ‘pre-industrial’ atmosphere
• Two Laki emissions cases
‘lo’: emissions evenly distributed 0-9 km
‘hi’: 75% emissions at 8-12 km, 25% at 0-3 km
• All runs had fixed (‘1996-97’) meteorology
No attempt made to simulate 1783 weather
•
•
•
•
Run for one year following start of eruption
Generate aerosol distributions
No feedback between aerosols  climate
Calculate radiative forcings and climate effects later
Zonal mean JJA SO2 & sulphate
Pre-industrial background
Tropopause
SO2
SO4
Laki
July SO2 (ppbv) Laki hi
Surface
0.5 km
0.1 0.2
0.5 1
2
5
10
20
50 100
550 hPa
5 km
0.1 0.2
0.5 1
2
5
10
20
350 hPa
8 km
0.1 0.2
0.5 1
2
5
10
20
50 100
50 100
200 hPa
12 km
0.1 0.2
0.5 1
2
5
10
20
50 100
July SO4 (pptv) Laki hi
Surface
0.5 km
50
100
200
500
1000 2000
5000
550 hPa
5 km
50
100
200
500
1000 2000
350 hPa
8 km
50
100
200
500
1000 2000
5000
5000
200 hPa
12 km
50
100
200
500
1000 2000
5000
Laki SO4 evolution
Upper Trop
Surface
90°N
Lower Strat
lo
Eq
90°S
May 1784
June 1783
10
90°N
Eq
90°S
20
50
100
200
500
1000 2000
5000
10
20
50
100
200
500
1000 2000
5000
10
20
50
100
200
500
1000 2000
5000
SO4 / pptv
hi
Laki sulphur budget
Hi case
+OH
Emissions
61 Tg(S)
SO2
gas
+H2O2(aq)
+O3(aq)
16%
16%
SO4
aerosol
4%
28%
37%
10%
90%
Dry
dep
Wet
dep
Dry
dep
Wet
dep
22 Tg(S)
or
89 Tg
(H2SO4.2H2O)
Impact on oxidants (JJA)
H2O2
OH
O3
Laki Sulphate budget (JJA)
LS
Production
Deposition
Increase
Transport to UT
tSO4 = 67 days
(no transport: >7 yrs)
UT
Transport from LS
Production
Deposition
Increase
Transport to LT
tSO4 = 10 days
(no transport: 32 days)
LT
Transport from UT
Production
Deposition
tSO4 = 5.3 days
Increase
-20
0
Tg(S)
20
Acid deposition to Greenland
Greenland Ice-core data
H2SO4 deposition rates mg(S)/m2/yr
Modelled
Observed
Background
5.0
2.9-8.6
Laki
63-65
18-107
Total atmospheric aerosol mass
Aerosol yield/peak loading
Total yield
Peak load
51-66
4.4-5.1
Clausen & Hammer(1988)
280
-
Zielinski (1995)
~40
-
Stothers (1996)
150
4.5
100-150
-
200
(~150?)
This work
Clausen et al (1997)
Thordarson and Self (2002)
Tg(H2SO4)
Radiative forcing & Climate impact
Ellie Highwood (Reading Univ)
• Aerosol fields inserted into Reading IGCM
• 3 experiments:
• 1. Hi/long decay (10 month e-fold)
• 2. Hi/short decay (3.6 month e-fold)
• 3. Lo
• Each has a 10 member ensemble of 3 yr runs
• Compare to control run with no forcing
• Only direct aerosol effect
Radiative forcings
Climate Impact
lo
hi
Climate impact
• Hi runs have NH cooling of –0.21K, in
good agreement with observations
(-0.14 to –0.27K)
• Lo runs show no significant cooling
• BUT: runs neglect indirect aerosol effects
• Hi runs also have cooling persisting for 3
years, due to feedbacks (ice/snow albedo)
Conclusions(1)
• 1st attempt at chemistry-climate modelling of the
Laki eruption
• Simulated a sulphate aerosol cloud across much
of the NH during the 8-month eruption
• Deposition to Greenland similar to ice-core
record
• 60-70% of emitted SO2 is deposited before
forming aerosol (previous studies assumed it all
formed aerosol)
• Mean lifetime ~week
• Atmospheric loading less than previous
estimates
Conclusions(2)
• Oxidants H2O2 & OH strongly depleted
– lengthens the SO2 lifetime
– more likely to be deposited as SO2
• Climate modelling suggests Hi scenario
gives ~0.2K cooling, and persists for >2
years; this matches observations
• But: many processes missing
• For more info: 2 papers in ACP:
www.copernicus.org/EGU/acp
• [email protected]
Problems(1)
• Volcanology:
– Emissions uncertain
• Magnitude
• Vertical profile
• Temporal distribution – episodic
– Plume processes, e.g. scavenging of SO2/SO4
by ash in the eruption column
Problems(2)
• Chemistry modelling
–
–
–
–
Not fully coupled
No aerosol microphysics
No coupling of aerosol to photolysis rates
Only 1 heterogeneous reaction (N2O5 loss)
Problems(3)
• Climate model
– Simplified radiation scheme – more bands
suggest forcing is smaller by factor 0.6
– Humidity assumptions – 80% RH increases
forcing by factor 2.6
– No aerosol indirect effects
– Climate sensitivity low compared to other
models: suggests duration of forcing may be
underestimated
2 Papers in Atmospheric Physics & Chemistry:
www.copernicus.org/EGU/acp/acp/3/487/acp-3-487.pdf
www.copernicus.org/EGU/acp/acp/3/1177/acp-3-1177.pdf