Correlation between cloud condensation nuclei concentration and aerosol optical thickness in remote and polluted regions M. O. Andreae Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
Abstract. A large number of published and unpublished measurements of cloud
condensation nuclei (CCN) concentrations and aerosol optical thickness (AOT)
measurements have been analyzed. AOT measurements were obtained mostly from
the AERONET network, and selected to be collocated as closely as possible to
the CCN investigations. In remote marine regions, CCN0.4 (CCN at a
supersaturation of 0.4%) are around 110 cm−3 and the mean
AOT500 (AOT at 500 nm) is 0.057. Over remote continental areas, CCN are
almost twice as abundant, while the mean AOT500 is ca. 0.075. (Sites
dominated by desert dust plumes were excluded from this analysis.) Some, or
maybe even most of this difference must be because even remote continental
sites are in closer proximity to pollution sources than remote marine sites.
This suggests that the difference between marine and continental levels must
have been smaller before the advent of anthropogenic pollution.
Over polluted marine and continental regions, the CCN concentrations are
about one order of magnitude higher than over their remote counterparts,
while AOT is about five times higher over polluted than over clean regions.
The average CCN concentrations from all studies show a remarkable
correlation to the corresponding AOT values, which can be expressed as a
power law. This can be very useful for the parameterization of CCN
concentrations in modeling studies, as it provides an easily measured proxy
for this variable, which is difficult to measure directly. It also implies
that, at least at large scales, the radiative and microphysical effects of
aerosols on cloud physics are correlated and not free to vary fully
independently. While the observed strong empirical correlation is
remarkable, it must still be noted that there is about a factor-of-four
range of CCN concentrations at a given AOT, and that there remains
considerable room for improvement in remote sensing techniques for CCN
Citation: Andreae, M. O.: Correlation between cloud condensation nuclei concentration and aerosol optical thickness in remote and polluted regions, Atmos. Chem. Phys., 9, 543-556, doi:10.5194/acp-9-543-2009, 2009.