Aerosol nucleation over oceans and the role of galactic cosmic rays

[摘要] We investigate formation of sulfate aerosol in the marine troposphere fromneutral and charged nucleation of H₂SO₄ and H₂O. A box modelof neutral and charged aerosol processes is run on a grid covering the oceans.Input data are taken from a model of galactic cosmic rays in the atmosphere, andfrom global chemistry and transport models.We find a weak aerosol production over the tropical oceans in the lower andmiddle troposphere, and a stronger production at higher latitudes, most notablydownwind of industrial regions. The strongest aerosol production howeveroccurs in the upper troposphere over areas with frequent convective activity, inparticular in the tropics.This finding supports the proposition by which non-sea salt marine boundarylayer aerosol in tropical regions does not form in situ, but nucleates in theupper troposphere from convectively lifted and cloud processed boundary layerair rich in aerosol precursor gases, from where it descends in subsiding airmasses compensating convection.Convection of boundary layer air also appears to drive the formation ofcondensation nuclei in the tropical upper troposphere which maintains thestratospheric aerosol layer in the absence of volcanic activity.Neutral nucleation contributes only marginally to aerosol production in oursimulations. This highlights the importance of other mechanisms, includingcharged binary and ternary, and neutral ternary nucleation for aerosolformation.Our analysis indicates that the variation of ionization by galactic cosmic raysover the decadal solar cycle does not entail a response in aerosol productionand cloud cover via the second indirect aerosol effect that would explainobserved variations in global cloud cover.We estimate that the variation in radiative forcing resulting from a responseof clouds to the change in galactic cosmic ray ionization and subsequentaerosol production over the decadal solar cycle is smaller than the concurrentvariation of total solar irradiance.

[学科分类] 大气科学