The Workshop on Environmental Education at the School Level in the Field of Air Quality, held on September 8-9 on the Greek island of Thira, provided an opportunity to measure aerosol optical thickness in a unique environment. Thira is the largest of the five-island Santorini group, located in the Aegean Sea at the southern boundary of the Cyclades islands (latitude 35.25šN, longitude 25.20šE). Santorini has been formed and re-formed by volcanic activity over tens of thousands of years. Today, the volcano on Nea Kemeni, in the center of the island group, is still considered active, but it produces only occasional small releases of gases through fumaroles.

Two circumstances dominate the aerosol environment at Santorini. First, marine aerosols from the Aegean Sea make the sky viewed at low elevation angles above the horizon look hazy. As a result, the nearby islands look less clear than one might expect based on the brilliant sunlight and clear skies that persisted throughout the conference. Second, it is not uncommon for dust clouds from northern Africa to blow north across the Mediterranean. Figure 1 shows such an occurrence, as seen through the eyes of the spacecraft-based Total Ozone Mapping Spectrometer (TOMS) on August 31, 2000. The aerosol index shown in Figure 1 is related to, but not the same thing as, aerosol optical thickness. Note the large "blob" of aerosols just to the west of Greece on this date.

Figure 1. TOMS aerosol index for August 31, 2000.

Santorini is famous for its beautiful sunsets, which hundreds of people gather to watch from the towns perched atop the cliffs along the western edge of Thira. Although the sunset shown in Figure 2 is not from August 31, 2000, the brilliant orange-red color and the obscured sea-atmosphere boundary are still probably due to the effects of both marine aerosols and African dust. (Sunsets during the workshop were not colored this brilliantly and TOMS images revealed no significant amounts of dust during the time of the workshop.)

Figure 2. Sunset viewed from the Island of Thira, Santorini, Greece (used by permission from webmaster, www.santorini.net, 2000).

Figure 3 shows aerosol optical thickness (AOT) based on sun photometer measurements made from various locations on Thira during the workshop. (None of the measurements were made near the coast at sea level.) For comparison, there is also a set of measurements from Drexel University in Philadelphia, Pennsylvania, USA, made on the morning of September 6, just before I left for the workshop. (Drexel is at an elevation of about 20 meters.) This was a clear day in Philadelphia, as a cold front had just passed through the area. The sky above Santorini also appeared very clear and blue, with haze from marine aerosols revealed visually only by looking out over the water at nearby islands, as mentioned above. Although there is no reason why measurements from these two very different locations should be simply related, it is nonetheless an interesting comparison for clear-sky aerosol optical thickness. It may be that the higher values from Santorini can be attributed to the effects of marine aerosols.

Figure 3. Aerosol optical thickness data from Santorini, with data taken at Drexel University just prior to departing for Santorini.