Part I, Chemistry: David Stevenson

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Atmospheric modelling of the 1783-84 Laki eruption

• Part I, Chemistry David Stevenson
• Institute for Meteorology, University of
  Edinburgh, UK
• Thanks Colin Johnson, Dick Derwent, Bill Collins
  (UK Met Office)
• Part II, Climate Ellie Highwood

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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?
• Next talk
• Can it generate a climate impact?

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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 - 14 km
• Tropopause at 10 km
• Dry fog or blue haze recorded over Europe,
  Asia, Atlantic, Arctic, N. America
• This appears to have been a sulphuric acid
  aerosol layer in the troposphere and/or lower
  stratosphere

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Atmospheric model STOCHEM

• Global 3-D chemistry-transport model
• Meteorology Hadley Centre GCM
• 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

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STOCHEM framework
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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, albedo, 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

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Sulphur chemistry
O3(aq)
Only deposition rates determine the SO4 lifetime
Oxidation and deposition rates determine the SO2
lifetime
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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
• What was the vertical profile of emissions?

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1990 Anthropogenic SO2 emissions (annual total)
0.1
1
10
100
Tg(S)/yr/5x5
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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

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Zonal annual mean SO2
1990
1860
100
P (hPa)
300
1000
laki lo
laki hi
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Zonal annual mean SO4
1990
1860
laki lo
laki hi
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Atmospheric aerosol burden
Hi
Lo
Dashed lines Assume H2SO4.2H2O (35 more mass)
Global burden H2SO4 (Tg)
Background level
June 1783 Feb 1784
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Laki SO4 evolution
Upper Trop
Lower Strat
Surface
90N
lo
Eq
90S
May 1784
June 1783
SO4 / pptv
90N
hi
Eq
90S
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July SO2 (ppbv) Laki hi
Surface 0.5 km
550 hPa 5 km
0.1
0.2
0.5
2
10
20
50
1
5
100
0.1
0.2
0.5
2
10
20
50
1
5
100
350 hPa 8 km
200 hPa 12 km
0.1
0.2
0.5
2
10
20
50
1
5
100
0.1
0.2
0.5
2
10
20
50
1
5
100
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July SO4 (pptv) Laki hi
Surface 0.5 km
550 hPa 5 km
350 hPa 8 km
200 hPa 12 km
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Laki sulphur budget
Lo case
17 Tg(S) or 70 Tg (H2SO4.2H2O)
SO2 gas
Emissions 61 Tg(S)
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Laki sulphur budget
Hi case
22 Tg(S) or 89 Tg (H2SO4.2H2O)
SO2 gas
Emissions 61 Tg(S)
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Optical Depth July 1783 Mean
Lo max 0.24
Hi max 0.39
Assumes aerosol is H2SO4.2H2O 1 unit optical
depth 3 x 10-5 g cm-2 column aerosol Stothers
(1996) observed max td1 to 4 over Europe
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Impact on oxidants (H2O2)
surface
550 hPa
350 hPa
200 hPa
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SO2 lifetime
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Laki hi SO2 lifetime
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SO4 lifetime
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Conclusions

• Simulated a sulphate aerosol cloud across much of
  the NH during the 8-month eruption, in rough
  agreement with observations
• but modelled optical depths are maybe 3x too
  small?
• 60-70 of emitted SO2 is deposited before forming
  aerosol (previous studies assumed it all formed
  aerosol)
• Oxidant H2O2 is strongly depleted
• lengthens the SO2 lifetime
• more likely to be deposited as SO2
• Environmental impacts include poor SO2 air
  quality and SO2 deposition, as well as acid rain
• Now use the aerosol fields to calculate a climate
  impact

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IPCC(2001) value Lifetime (Volcanic Days
component)
Burden (Volcanic Tg(S) component) IPCC(2001)
value
Volcanic component IPCC(2001) value
Fluxes in Tg(S)/yr IPCC(2001) value
OH
6.3 12
0.29 (0.075) 0.46
SO4
SO2
0.81 (0.12) 0.77
1.0
4.9 5.3 (6.2)
1.8 1.1 (3.0)
H2O2
32
5.5
49 46
44
O3
17
0.35
30
6.2
41
9.2
7.1 9.5
0.75
0.3
9.0 9.3
12
1.4
71 76
0.56
DMS
MSA
4
1.4 2.2
Deposition
Deposition
15 24
Dry
Wet
Wet
Dry
1 1
Anthro- pogenic
Volcanic
Soil
Biomass burning
Oceanic