Aerosol nucleation and its role for clouds and Earth's radiative forcing in the aerosol-climate model ECHAM5-HAM 1Max Planck Institute for Meteorology, Hamburg, Germany
2Atmospheric, Oceanic and Planetary Physics, University of Oxford, UK
3Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland
*now at: Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA
Received: 03 April 2010 – Published in Atmos. Chem. Phys. Discuss.: 10 May 2010 Abstract. Nucleation from the gas phase is an important source of aerosol
particles in the Earth's atmosphere, contributing to the number of cloud
condensation nuclei, which form cloud droplets. We have implemented in the aerosol-climate model ECHAM5-HAM a new scheme for neutral
and charged nucleation of sulfuric acid and water based on laboratory data, and
nucleation of an organic compound and sulfuric acid using a parametrization of
cluster activation based on field measurements. We give details of the implementation, compare results with observations, and
investigate the role of the individual aerosol nucleation mechanisms for clouds
and the Earth's radiative forcing. The results of our simulations are most consistent with observations when
neutral and charged nucleation of sulfuric acid proceed throughout the
troposphere and nucleation due to cluster activation is limited to the forested
boundary layer. The globally averaged annual mean contributions of the individual nucleation
processes to total absorbed solar short-wave radiation via the direct,
semi-direct, indirect cloud-albedo and cloud-lifetime effects in our
simulations are −1.15 W/m2 for charged H2SO4/H2O
nucleation, −0.235 W/m2 for cluster activation, and −0.05 W/m2
for neutral H2SO4/H2O nucleation. The overall effect of
nucleation is −2.55 W/m2, which exceeds the sum
of the individual terms due to feedbacks and interactions in the model.
Aerosol nucleation contributes over the oceans with −2.18 W/m2 to total
absorbed solar short-wave radiation, compared to −0.37 W/m2 over land.
We explain the higher effect of aerosol nucleation on Earth's radiative
forcing over the oceans with the larger area covered by ocean clouds, due to
the larger contrast in albedo between clouds and the ocean surface compared
to continents, and the larger susceptibility of pristine clouds owing to the
saturation of effects.
The large effect of charged nucleation in our simulations is not in
contradiction with small effects seen in local measurements: over southern Finland,
where cluster activation proceeds efficiently, we find that charged nucleation
of sulfuric acid and water contributes on average less than 10% to ultrafine
aerosol concentrations, in good agreement with observations.
Revised: 16 August 2010 – Accepted: 24 October 2010 – Published: 16 November 2010
Citation: Kazil, J., Stier, P., Zhang, K., Quaas, J., Kinne, S., O'Donnell, D., Rast, S., Esch, M., Ferrachat, S., Lohmann, U., and Feichter, J.: Aerosol nucleation and its role for clouds and Earth's radiative forcing in the aerosol-climate model ECHAM5-HAM, Atmos. Chem. Phys., 10, 10733-10752, doi:10.5194/acp-10-10733-2010, 2010.