Aerosol Radiative Forcing at Finokalia Sampling Station during the MINOS Experiment
Krzysztof Markowicz1, Piotr J. Flatau2,3, J. P. Crutzen3,4
and V. Ramanathan3
1Institute of Geophysics, University of Warsaw, Poland
2Naval Research Laboratory, Monterey, California
3Center for Atmospheric Science, Scripps Institution of Oceanography,
University of California, San Diego, USA
4Max-Planck-Institute for Chemistry, Mainz, Germany
We present observations of radiative forcing taken during the MINOS experiment
(July-August, 2001) at Finokalia Sampling Station (FSS) located on North-Eastern
shores of Crete, Greece. Clouds are scarce in summer at FSS making it almost an
ideal location for aerosol forcing studies. The main results are that (a) aerosols ,
mostly of anthropogenic origin, lead to a large reduction is the radiation absorbed
at the surface; the diurnal average reduction is as large as 15 to 30 W.m-2; b)
The reduction in surface solar radiation is about two to three times larger than
the reflected solar radiation at the top-of-the atmosphere, thus giving observational
proof for the large role of absorbing aerosols in the aerosol forcing; c) the forcing
efficiency ( the slope of the aerosol forcing with respect to the aerosol optical depth)
is similar to that observed for the heavily polluted flow from India during the INDOEX
observations; d) An important new finding is that. there are large differences between
forcing efficiencies for fire cases and more "typical" northerly outflow from Europe
(combined with the transport of dust from Sahara)
Total and diffuse broadband radiative (280-2800 nanometers) fluxes were measured using
the Kipp and Zonen pyranometers. The direct broadband flux was measured by pyrheliometer
mounted on sun tracker. Two Biospherical Instruments radiometers were used in shadowed
and non-shadowed mode to give information about the diffuse/direct radiation in the PAR
region (400-700nm). Aerosol optical thickness in the visible and near infrared, total
columnar water vapor and ozone were obtained using the Microtops and Analytical Spectral
Device instruments. Using simultaneous measurements of aerosol physical and radiative
properties we developed an aerosol model which, in conjunction with the radiative transfer
calculations, successfully explains the observed solar fluxes at the surface. The top of
atmosphere (TOA) fluxes determined form the radiative transfer model were validated using
satellite observations (CERES). The aerosol radiative forcing was obtained by two methods:
hybrid and differential. The mean value of aerosol forcing efficiency at the surface during
MINOS campaign is around -100Wm-2 . The mean value of AOT was 0.25 at 500nm, thus the
averaged total surface flux was decreased by -25Wm-2 owing to presence of aerosol. Based
on hybrid method we computed day to day variation of aerosol forcing efficiency. During
widespread fire episodes (7-11 August, 2001) in Turkey and Greece the surface forcing
efficiency increased to -140Wm-2 and was associated with low values of single scattering
albedo (less than 0.8) because of large absorption by soot particles. Thus biomass burning
had strong impact on the surface incoming solar radiation. We will also discuss strong
correlation between the observed AOT and the total water vapor content. This suggest that
the FSS aerosols were strongly hygroscopic and that the water vapor influences aerosol forcing.