1. 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

2. 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?

3. 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 ~ m
Eruption columns up to ~ 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

4. 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

5. STOCHEM framework Air parcel centres Eulerian grid from GCM provides
meteorology
Interpolate met. data for each
air parcel

6. 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

7. Sulphur chemistry SO2 SO4
Oxidants normally determined by background photochemistry – but very high SO2 levels will affect them
SO2
gas
emissions
SO4
aerosol
+OH
+H2O2(aq)
(in clouds)
+O3(aq)
Oxidation and
deposition rates
determine the
SO2 lifetime
dry(wet) deposition
wet(dry) deposition
Only
deposition rates
determine the
SO4 lifetime

8. 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?

9. 1990 Anthropogenic SO2 emissions (annual total)
Laki
value 61
Total 72
Peak
value ~2
0.1 1 10
100
Tg(S)/yr/5x5

10. 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 (‘ ’) 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

11. Zonal annual mean SO2 1990 1860 laki lo laki hi 100 pptv 100 500 pptv
16 km
8 km
4 km
0 km
12 km
P (hPa)
300
1000
laki lo
laki hi
5 ppbv
>10 ppbv

12. Zonal annual mean SO4 1990 1860 laki lo laki hi 500 pptv 100 pptv
1 ppbv

13. Atmospheric aerosol burden8 6 4 2 Hi Lo
Dashed lines
Assume
H2SO4.2H2O
(35% more
mass)
Global burden H2SO4 (Tg)
Background
level
June Feb 1784

14. Laki SO4 evolution Surface Upper Trop Lower Strat lo hi 90°N Eq 90°S
June 1783
May 1784 10 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
90°N hi Eq
90°S

15. July SO2 (ppbv) Laki hi Surface 0.5 km 550 hPa 5 km 350 hPa 8 km
0.1
0.2
0.5 1 2 5 10 20 50
100
0.1
0.2
0.5 1 2 5 10 20 50
100
350 hPa
8 km
200 hPa
12 km
0.1
0.2
0.5 1 2 5 10 20 50
100
0.1
0.2
0.5 1 2 5 10 20 50
100

16. July SO4 (pptv) Laki hi Surface 0.5 km 550 hPa 5 km 350 hPa 8 km
100
200
500
1000
2000
5000 50
100
200
500
1000
2000
5000
350 hPa
8 km
200 hPa
12 km 50
100
200
500
1000
2000
5000 50
100
200
500
1000
2000
5000

17. Laki sulphur budget SO2 SO4 Lo case 17 Tg(S) or 70 Tg (H2SO4.2H2O) gas
aerosol
14%
Emissions
61 Tg(S)
11%
+H2O2(aq)
+O3(aq) 3%
Dry
dep
34%
Wet
dep
38%
Dry
dep
12%
Wet
dep
88%

18. Laki sulphur budget SO2 SO4 Hi case 22 Tg(S) or 89 Tg (H2SO4.2H2O) gas
aerosol
16%
Emissions
61 Tg(S)
16%
+H2O2(aq)
+O3(aq) 4%
Dry
dep
28%
Wet
dep
37%
Dry
dep
10%
Wet
dep
90%

19. 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 td~1 to 4 over Europe

20. Impact on oxidants (H2O2)
surface
550 hPa
350 hPa
200 hPa

21. SO2 lifetime

22. Laki hi SO2 lifetime

23. SO4 lifetime

24. 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…

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