Aerosol nucleation over oceans and the role of galactic cosmic rays J. Kazil1,2, E. R. Lovejoy2, M. C. Barth3, and K. O'Brien4 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO,USA 2Atmospheric Chemical Processes Group, Atmospheric Chemistry Division, NOAA ESRL, Boulder, CO, USA 3MMM/ACD, National Center for Atmospheric Research, Boulder, CO, USA 4Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA
Abstract. We investigate formation of sulfate aerosol in the marine troposphere from
neutral and charged nucleation of H2SO4 and H2O. A box model
of 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, and
from global chemistry and transport models.
We find a weak aerosol production over the tropical oceans in the lower and
middle troposphere, and a stronger production at higher latitudes, most notably
downwind of industrial regions. The strongest aerosol production however
occurs in the upper troposphere over areas with frequent convective activity, in
particular in the tropics.
This finding supports the proposition by which non-sea salt marine boundary
layer aerosol in tropical regions does not form in situ, but nucleates in the
upper troposphere from convectively lifted and cloud processed boundary layer
air rich in aerosol precursor gases, from where it descends in subsiding air
masses compensating convection.
Convection of boundary layer air also appears to drive the formation of
condensation nuclei in the tropical upper troposphere which maintains the
stratospheric aerosol layer in the absence of volcanic activity.
Neutral nucleation contributes only marginally to aerosol production in our
simulations. This highlights the importance of other mechanisms, including
charged binary and ternary, and neutral ternary nucleation for aerosol
Our analysis indicates that the variation of ionization by galactic cosmic rays
over the decadal solar cycle does not entail a response in aerosol production
and cloud cover via the second indirect aerosol effect that would explain
observed variations in global cloud cover.
We estimate that the variation in radiative forcing resulting from a response
of clouds to the change in galactic cosmic ray ionization and subsequent
aerosol production over the decadal solar cycle is smaller than the concurrent
variation of total solar irradiance.
Citation: Kazil, J., Lovejoy, E. R., Barth, M. C., and O'Brien, K.: Aerosol nucleation over oceans and the role of galactic cosmic rays, Atmos. Chem. Phys., 6, 4905-4924, doi:10.5194/acp-6-4905-2006, 2006.