Intergovernmental GR-RO research Programme

Air Quality assessment and study of transport processes along the axis Athens-Aegean sea (Greece) and Bucharest (Romania) using lidar techniques


Saharan dust outbreak over the Balkans as observed by synergy of active and passive sensors. A case study of long-range transport of aerosols between Greece and Romania in September 2012

paper abstract - submitted to ATMOSPHERIC CHEMISTRY AND PHYSICS Journal  

A Saharan dust outbreak over both Greece and Romania was captured during the 1st AQua-GRO campaign, starting on the night of September 23 in Athens, hitting Oxylithos and Timisoara on September 24 early morning, and reaching Bucharest and Iasi before noon (Fig. 1).

Backscatter and depolarization profiles computed from Bucharest lidar data for Sep. 24 are shown in Fig. 2. The strong depolarizing dust layer arriving at 2.5 - 3.5 km altitude is slowly going down and is mixed with local aerosol after sunset.

Dust particles arrived first at Greek stations and Timisoara, then at Bucharest and at Iasi stations, in this order, which is consistent with lidar measurements.

The mixing of Saharan dust with continental polluted aerosols results in a modification of both intensive (AE, SSA) and extensive (AOD) optical properties. Note that the daily mean Angstrom exponent (AE) decreases when dust particles arrive above the station, while the Single Scattering Albedo (SSA) increases. This is due to the fact that mineral dust is a rather large (small AE), low absorbing (high SSA) particle. It will contribute to the mixed aerosol by decreasing the mean AE and increasing the mean SSA of the initial small and absorbing continental polluted aerosols. In terms of the AOD, which is an extensive parameter and depends on the particles' concentration, dust particles will be added to the existing continental polluted aerosols. As consequence, although mineral dust has strong scattering and low absorbing properties, the AOD increases.

A significant variation of the optical and microphysical properties of the free troposphere aerosols was identified on 23-24 September both in Romania and Greece.  Both lidars identified the arrival of the mineral dust at each station, as well as the altitude of the layers. The origin of the air masses was assessed by using HYSPLIT backtrajectories. Integrated column parameters from sunphotometer showed different values at the 5 stations, with a clear gradient from South to North, even when the Saharan dust outbreak was over all observing stations. Knowing that the origin of the aerosol layers was the same and that mineral dust is not hygroscopic and preserves its properties during transport, these differences can be explained only by the mixing with continental polluted aerosols, which are more present at Romanian stations than at Greek stations.

Statistical database of the aerosol optical properties

Main air pollution sources over Athens/Aegean Sea (island of Evia)