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

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 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, 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 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

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) Laki value 61 Total 72 Peak value ~2 0.1 1 10 100 Tg(S)/yr/5x5

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 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

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

Atmospheric aerosol burden 8 6 4 2 Hi Lo Dashed lines Assume H2SO4.2H2O (35% more mass) Global burden H2SO4 (Tg) Background level June 1783 Feb 1784

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

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

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

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%

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%

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

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…

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