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Identifier

000346562

Title

Φυσικές ιδιότητες αιωρούμενων σωματιδίων με κλιματικό ενδιαφέρον στην ατμόσφαιρα της ανατολικής Μεσογείου

Alternative Title

Physical properties of aerosol particles affecting climate in the eastern Mediterranean atmosphere

Author

Καλυβίτης, Νικόλαος Πέτρου

Thesis advisor

Μιχαλόπουλος, Νικόλαος

Abstract

The climatic effect of atmospheric aerosol particles has not yet been fully clarified. The high spatial and temporal variability of aerosol concentrations makes the investigation of their physical and chemical properties in global scale crucial in order to estimate their total climatic effect. Mediterranean Basin aerosol is of special interest for global climate research since a variety of aerosol types can be found there, namely marine aerosols, anthropogenic aerosols and desert dust aerosols. The environmental research station of the University of Crete at Finokalia has been pointed out as an anchor station for the Eastern Mediterranean basin. The present work focuses on the physical properties of atmospheric aerosol particles at the station of Finokalia and three scientific goals have been distinguished: the study of aerosol particles size distributions in the area, the study of aerosol optical properties and the study of mineral dust events over the eastern Mediterranean.
For the first time in the Eastern Mediterranean basin have the aerosol particle size distributions been measured on a regular basis and for a length of time long enough to investigate their temporal variations. Two separate periods of measurements took place, the first from July 2004 until January 2005 and the second from August until July 2005 and during both periods the same tendencies were revealed. The most important observation is the depletion of aerosol particles with smaller diameter than 50 nm. Two patterns of depletion have been observed. The first pattern is characterized by a gradual depletion of the particles. In general, this pattern is not necessarily related to diurnal variation of solar irradiance, as its initiation appeared in the late afternoon or even before sunrise. The second pattern occurred only during day time and the maximum depletion was observed at noon or in the early afternoon while the depletion rate was more rapid. Both the depletion intensity and the frequency of the depletion events, decreased from summer to winter. We compared these observations with simplistic box model simulations of the particle size distribution involving only condensation of sulfuric acid and coagulation on larger particles. Τhe agreement of observations and simulations for the second pattern of depletion suggests that condensation of sulfuric acid vapors and coagulation are the main factors controlling this phenomenon while for the first pattern the depletion of small particles must be attributed to some other semi-volatile species because of the low concentrations of sulfuric acid during nighttime. For the first time in the marine boundary layer of Eastern Mediterranean new particle formation events were observed as a reoccurring phenomenon. Moreover, the observation of nighttime new particle formation events is of great importance because of the scarcity of such observations. Daytime nucleation events were more frequent during winter months while nighttime events in summer. In the second part of this work the optical properties of atmospheric aerosol particles were studied and specifically scattering and absorption coefficients for the period 2004 – 2006. Maximum values of the aerosol scattering coefficient are observed during summer while minimum during winter in agreement to the annual circle of ammonium sulfate and organic matter concentrations. Secondary maxima are observed during the transition periods and are attributed to mineral dust transport from Northern Africa. The hygroscopic growth of aerosol particles plays an important role in the determination of their optical properties since for hygroscopic particles at high relative humidities the scattering coefficient is enhanced by a factor of two or more compared to dry conditions. We were able to reconstruct with quite good accuracy scattering coefficients based on chemical composition of the aerosol particles, assuming that ammonium sulfate and organic matter are the only scattering species, both for dry and ambient conditions. Maximum values for absorption coefficient are observed in summer with a secondary maximum in spring while minimum values are observed n winter with a secondary minimum in early summer.
The above are in accordance with the annual circle of black carbon concentrations. During dust event outbreaks in the area, absorption coefficient at short wavelengths can get extremely high values. In the third part of this work the transport of mineral dust from Northern Africa to Eastern Mediterranean basin is studied, since during such episodes dust particles dominate the atmosphere. Dust particles arrive to Crete mainly when air masses originate from the south. Back trajectories analysis of southerly air masses at altitudes of 1000 and 3000 m over a 5 year period (2000–2005), showed that dust can potentially arrive over Crete, either simultaneously in the lower free troposphere and inside the boundary layer (vertical extended transport (VET)) or initially into the free troposphere with the heavier particles gradually being scavenged inside the boundary layer (free troposphere transport (FTT)). Both pathways present significant seasonal variations, VET transport pattern prevails during autumn and winter, FTT is more frequently met during summer while during spring they are equally observed, but on an annual basis contribute almost equally to the dust transport in the area. During VET the aerosol index (AI) derived from TOMS was significantly correlated with surface PM10, and in general AI was found to be adequate for the characterization of dust loadings over the eastern Mediterranean on a climatological basis. A significant covariance between PM10 and AOT was observed during VET as well, indicating that AOT levels from AERONET may be estimated by PM10 levels at the surface. Surface measurements are thus crucial for the validation of remote sensing measurements and hence are a powerful tool for the investigation of the impact of aerosols on climate.

Language

Greek

Subject

AERONET

Absorption

Aerosol particles

Dust

East Mediterranean

LIDAR

New particle formation

Optical properties

SMPS