Atmospheric impact of the 1783–1784 Laki eruption: Part I Chemistry modelling

[摘要] Resultsfrom the first chemistry-transport model study of the impact of the 1783–1784Laki fissure eruption (Iceland: 64°N, 17°W) upon atmospheric compositionare presented. The eruption released an estimated 61 Tg(S) as SO₂into the troposphere and lower stratosphere. The model has a highresolution tropopause region, and detailed sulphur chemistry. Thesimulated SO₂ plume spreads over much of the NorthernHemisphere, polewards of ~40°N. About 70% of the SO₂ gas isdirectly deposited to the surface before it can be oxidised to sulphuricacid aerosol. The main SO₂ oxidants, OH and H₂O₂,are depleted by up to 40% zonally, and the lifetime of SO₂consequently increases. Zonally averaged tropospheric SO₂concentrations over the first three months of the eruption exceed 20 ppbv,and sulphuric acid aerosol reaches ~2 ppbv. These compare to modelledpre-industrial/present-day values of 0.1/0.5 ppbv SO₂ and0.1/1.0 ppbv sulphate. A total sulphuric acid aerosol yield of 17–22Tg(S) is produced. The mean aerosol lifetime is 6–10 days, and the peakaerosol loading of the atmosphere is 1.4–1.7 Tg(S) (equivalent to 5.9–7.1Tg of hydrated sulphuric acid aerosol). These compare to modelledpre-industrial/present-day sulphate burdens of 0.28/0.81 Tg(S), andlifetimes of 6/5 days, respectively. Due to the relatively shortatmospheric residence times of both SO₂ and sulphate, theaerosol loading approximately mirrors the temporal evolution of emissionsassociated with the eruption. The model produces a reason-able simulationof the acid deposition found in Greenland ice cores. These results appearto be relatively insensitive to the vertical profile of emissions assumed,although if more of the emissions reached higher levels (>12 km), thiswould give longer lifetimes and larger aerosol yields. Introducing theemissions in episodes generates similar results to using monthly meanemissions, because the atmospheric lifetimes are similar to the reposeperiods between episodes. Most previous estimates of the global aerosolloading associated with Laki did not use atmospheric models; this studysuggests that these earlier estimates have been generally too large inmagnitude, and too long-lived. Environmental effects following the Lakieruption may have been dominated by the widespread deposition of SO₂gas rather than sulphuric acid aerosol.

[学科分类] 大气科学