Visibility Conditions: In Summer
2001 the MINOS (= Mediterranean INtensive Oxidant Study) field
campaign took place near Finokalia in a rural region in the East
of Crete. The measurement site was located 150 m above sea level.
Hazy.jpg shows the view in northern direction. The scenery below
the site directly at sea level is presented in Clear.jpg. The
horizon can be clearly noticed as a sharp line and atmospheric
opacity does not occur at all. However at the elevation of the
measuring site (150 m above sea level) the horizon is nearly totally
invisible. A significant haziness dominates the view. At an elevation
of 3 m a horizontal visibility of approximately 6 km would be
possible from the geometrical point of view. At the measuring
site the visibility could be even 40 km. An island is located
12 km off shore. Since this island is not visible, one can conclude
that the actual visibility is below 12 km at the measurement site.
Aerosol Measurements: In the beginning of August the total particle concentration was 4000 to 6000 particles per cm3 for particles with a diameter larger than 3 nm. These are extremely high values for a rural region. Therefore it can be assumed that the aerosol particles cause the observed haziness. Size distributions of particles larger than 0.1 µm were provided by optical particle counters as presented in dNlogD.jpg.
Unfortunately, no information about the size distribution in
the range between 3 nm and 100 nm is available. The condensation
particle counter UCPC only measures the number density of the
particles, but not the particle sizes.
Calculation of Visibility: Using
extinction cross-sections, Mie theory and the visibility equation
by Koschmieder one can conclude that most particles between 3
and 100 nm have a size of approximately 50 nm. This is the case,
because after taking into account the extinction due to the particles
above 100 nm (which can be calculated from the measured size distribution)
other particle sizes would result in a reduction of visibility
that is too low compared to the observed range of visibility of
Chemical Analysis: the major part
of these aerosol particles is sulphate. The meteorological analysis
of the air mass trajectories show that at this time the air came
from the industrial areas in the North of the Black Sea. Sulphur
dioxide was emitted there and photochemically transformed on their
way to Crete that means oxidized to sulphuric acid. Because of
the low vapour pressure H2SO4 condenses into small droplets. The
fact that the measured particles are so small indicates that the
nucleation occurred not very long ago.
Conclusion: Although one can see a distinct horizon directly from the coast, the air in the East of Crete is blurred by an aerosol stemming from anthropogenic gaseous emissions. Thereby the gaseous emissions were advected from an industrial region located far away. These small aerosol particles limit the visibility at an elevation of 150 m above the sea to less than 12 km, although the visibility could reach 40 km according to geometrical considerations. With sufficient visibility the horizon should be also visible as a sharp line between ocean and atmosphere.
Clear.jpg and Hazy.jpg: S. Borrmann, Crete, MINOS Field Campaign,