| As part of the Mediterranean Intensive Oxidant Study (MINOS) performed
during August 2001 in the Eastern Mediterranean Region, intensive
measurements of chemical and radiative properties of atmospheric aerosols
were performed at two remote sites on Crete Island, located in the marine
boundary layer (MBL), and in the lower free troposphere (FT), respectively.
Gravimetric particulate mass, as well as chemically-derived masses of water
soluble ions, organic and elemental carbon, and tracer elements for dust
aerosols were measured for fine (<1.2 µm) and coarse (>1.2 µm)
particles at the two sampling sites. Although strongly bound water, mainly
associated with inorganic species, could have slightly altered our results
(10% of the reconstructed mass), chemical mass closure was achieved most
of the time for the fine and coarse size fractions and at both sites. Our
conversion factor of 2.1 for organic carbon (OC) to particulate organic
matter (POM) is at the upper end of those reported in the literature, but
fits with the aged smoke particles collected during the campaign. The
results indicate that this conversion factor changed during the campaign
along with the BC/TC ratio.
The particulate mass (PM) concentration for fine aerosols at the MBL and FT
sites averaged 17.4±4.7 µg/m3 and 11.2±3.2 µg/m3, respectively, and is among the highest reported in the literature
for remote sites; more than 90% of this PM was composed equally of
ammonium sulfate and carbonaceous aerosols. Comparison between the MBL and
FT sites showed a slight vertical gradient for PM that was not observed for
dust aerosols, which averaged 10.5±4.8 and 11.7±5.0 µg/m3 for the MBL and FT sites, respectively.
The results were used to reconstruct the ambient light scattering
coefficient (σsp) that was measured at ambient Relative
Humidity (RH) for fine particles at the MBL site. Reconstruction of σsp was achieved using ratios of wet to dry scattering, f(RH), that
depend on RH for ammonium sulfate, but are kept equal to 1 for POM. This
results in a low water adsorption for our organic-rich carbonaceous
aerosols, although these aged biomass smoke aerosols are supposed to be
highly oxidized. Mass scattering efficiencies of the main aerosol components
were obtained by multivariate regression analysis, and were 2.66 and 4.19 m2/g
(at the 95% confidence level) for dry ammonium sulfate and POM,
respectively. The calculations indicate that one third of the reconstructed
σsp was due to water uptake by ammonium sulfate aerosols,
demonstrating their major role in the radiative aerosol properties in the
eastern Mediterranean.
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