David Stevenson 1, Colin Johnson 2, Ellie Highwood 3, Bill Collins 2, & Dick Derwent 2 1 School of GeoSciences, University of Edinburgh 2 The Met Office 3 Department of Meteorology, University of Reading Production and removal of volcanic aerosol at tropopause levels: A model study of the Laki eruption

Talk Structure The Laki eruption Atmospheric sulphur chemistry Chemistry-climate model & experiments SO 2 & sulphate aerosol budgets Residence times & climate impact Conclusions & Problems 2 papers in Atmos. Chem. Phys. Discuss.

Laki eruption, Iceland 8 June 1783: 27 km long fissure opens 15 km 3 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 (~250 hPa) ‘Dry fog’ or haze recorded over Europe, Asia, N. Atlantic, Arctic, N. America This appears to have been a sulphuric acid aerosol layer in the troposphere and lower stratosphere

Photos from Tom Pfeiffer’s web-site: Fire-fountaining at Etna, 2002

Sulphur chemistry SO 2 gas emissions SO 4 aerosol +OH dry (wet) depositionwet (dry) deposition +H 2 O 2(aq) (in clouds) +O 3(aq) Oxidants normally determined by background photochemistry – but very high SO 2 levels will affect them Oxidation and deposition rates determine the SO 2 lifetime Only deposition rates determine the SO 4 lifetime

Chemistry-transport model: STOCHEM Global 3-D chemistry-transport model Meteorology: Hadley Centre GCM GCM grid: 3.75° x 2.5° x 58 levels CTM grid: 5° x 5° x 22 levels Detailed tropospheric chemistry CH 4 -CO-NO x -hydrocarbons detailed oxidant chemistry sulphur chemistry This version has high resolution tropopause NO aerosol microphysics or sedimentation

Model experiments Background ‘pre-industrial’ atmosphere Laki emissions: 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, and production/loss fluxes No feedback between aerosols  climate Just chemical and transport effects

Zonal mean JJA SO 2 & sulphate Laki SO 4 Pre-industrial background SO 2 Laki SO 2 emissions Tropopause

Impact on Oxidants  H 2 O 2 (%)  OH (%) (Zonal Mean NH JJA)

Zonal mean JJA Laki SO 2 sinks (ppbv/day) SO 2 + OH Aqueous phase oxidation Altitude / km Wet depositionDry deposition Lower Troposphere Upper Troposphere Lower Stratosphere

Laki SO 2 budget (JJA) Lower Stratosphere Upper Troposphere Lower Troposphere  SO2 = 25 days (no transport: 170 days)  SO2 = 12 days (no transport: 67 days)  SO2 = 5.3 days

Laki Sulphate budget (JJA) LS UT LT  SO4 = 67 days (no transport: >7 yrs)  SO4 = 10 days (no transport: 32 days)  SO4 = 5.3 days

Conclusions Production of volcanic aerosol depends on –Magnitude, duration, and altitude of SO 2 emissions –Oxidant availability and degree of depletion –Residence time of SO 2 Around the tropopause ~10-30 days at mid-high NH latitudes, mainly determined by transport timescales to lower levels, where oxidation and deposition are much more rapid Removal of aerosol depends on –Transport timescales to the troposphere where deposition processes operate. NH mid-high latitudes: ~2 months for LS, ~10 days for UT Bad assumptions sometimes made: –Aerosol production from SO 2 is 100% –Lifetime ~1 year in stratosphere (Pinatubo, tropical mid-stratosphere) E.g. Laki eruption, most emissions were around tropopause levels: –only ~30% converted to aerosol –mean aerosol lifetime 1-2 weeks More details in two 2003 ACPD papers