Atmospheric modelling of the 1783-84 Laki eruption
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Transcript of Atmospheric modelling of the 1783-84 Laki eruption
Part I, Chemistry: David StevensonInstitute for Meteorology, University of Edinburgh, UKThanks: Colin Johnson, Dick Derwent, Bill Collins (UK Met Office)
Part II, Climate: Ellie Highwood
Atmospheric modelling of the 1783-84 Laki eruption
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?
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
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
STOCHEM framework
Eulerian gridfrom GCM provides
meteorology
Air parcel centres
Interpolate met. data for eachair 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, 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
Sulphur chemistry
SO2gas
emissions SO4aerosol
+OH
dry(wet) deposition wet(dry) deposition
+H2O2(aq)
(in clouds)+O3(aq)
Oxidants normally determined by background photochemistry – but very high SO2 levels will
affect them
Oxidation anddeposition rates
determine theSO2 lifetime
Onlydeposition rates
determine theSO4 lifetime
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?
1990 Anthropogenic SO2 emissions (annual total)
0.1 1 10 100Tg(S)/yr/5x5
Laki value 61
Total 72
Peakvalue ~2
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’) meteorologyNo 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 annual mean SO2
1990 1860
laki lo laki hi
100 pptv
5 ppbv
>10 ppbv
16 km
8 km
4 km
0 km
12 km
1000
300
100
P
(hP
a)
500 pptv
Zonal annual mean SO4
1990 1860
laki lo laki hi
500 pptv 100 pptv
300 pptv
1 ppbv
Atmospheric aerosol burden
LoHi
Dashed linesAssume
H2SO4.2H2O(35% more
mass)
BackgroundlevelGlo
bal b
urd
en
H2S
O4 (
Tg
) 8
6
4
2
0June 1783 Feb 1784
Laki SO4 evolutionSurface Upper Trop Lower Strat
lo
hi
90°N
Eq
90°S
90°N
Eq
90°S
June 1783
10 20 50 100 200 500 10002000 5000
SO4 / pptv10 20 50 100 200 500 10002000 5000 10 20 50 100 200 500 10002000 5000
May 1784
Surface0.5 km
550 hPa5 km
350 hPa8 km
200 hPa12 km
0.1 0.2 0.5 2 10 20 501 5 100
0.1 0.2 0.5 2 10 20 501 5 1000.1 0.2 0.5 2 10 20 501 5 100
0.1 0.2 0.5 2 10 20 501 5 100
July SO2 (ppbv) Laki hi
July SO4 (pptv) Laki hi
50 100 200 500 1000 2000
5000
50 100 200 500 1000 2000
500050 100 200 500 1000 2000
5000
50 100 200 500 1000 2000
5000
Surface0.5 km
550 hPa5 km
350 hPa8 km
200 hPa12 km
Laki sulphur budget
Emissions61 Tg(S)
SO2gas
+OH
SO4aerosol
14%
+O3(aq) 3%
Drydep
34%
Wetdep
38%
17 Tg(S)or
70 Tg (H2SO4.2H2O)
Drydep
12%
Wetdep
88%
11%
+H2O2(aq)
Lo case
Laki sulphur budget
Emissions61 Tg(S)
SO2gas
+OH
SO4aerosol
16%
+O3(aq) 4%
Drydep
28%
Wetdep
37%
22 Tg(S)or
89 Tg (H2SO4.2H2O)
Drydep
10%
Wetdep
90%
16%
+H2O2(aq)
Hi case
Assumes aerosol is H2SO4.2H2O1 unit optical depth = 3 x 10-5 g cm-2 column aerosolStothers (1996) observed max d~1 to 4 over Europe
Lo: max 0.24 Hi: max 0.39
Optical Depth – July 1783 Mean
Impact on oxidants (H2O2)
surface 550 hPa
350 hPa 200 hPa
SO2 lifetime
Laki hi SO2 lifetime
SO4 lifetime
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…
Dry Wet
OH
Anthro-pogenic
Volcanic Oceanic
Deposition
Wet
Deposition
SO4
0.81 (0.12)0.77
4.9
5.3 (6.2)
SO2
0.29 (0.075)0.46
1.8
1.1 (3.0)
Soil
DMS7176
Biomassburning
1524
12
7.19.5
4946
9.2
30
9.09.3
0.56
6.2
1.4
0.75
11
1.42.2
0.3
6.312 1.0
32
17
5.5
0.35
H2O2
O341
44
Dry
4 MSA
IPCC(2001) valueLifetime (VolcanicDays component)
Burden (Volcanic Tg(S) component)IPCC(2001) value
Volcanic componentIPCC(2001) value
Fluxes in Tg(S)/yrIPCC(2001) value