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