## 1 Introduction  
Solar radiation reaching the surface of the Earth undergoes attenuation due to the absorption and scattering of the solar rays by the atmospheric constituents [1]. Therefore, the solar radiation levels measured on the surface of the Earth depend on the concentration of water vapor, nitrogen dioxide, ozone, mixed gases and atmospheric aerosols. Although empirical-analytical expressions have been provided in the international literature for the atmospheric transmitances of the first four constituents, such expressions would not have been possible for aerosols if the notion of atmospheric turbidity were not introduced. The atmospheric turbidity expresses the attenuation of solar rays by aerosols. Several indices (called atmospheric turbidity parameters) have been introduced for this purpose; these are the Angstrom turbidity coefficient (\(\beta\)) and exponent (\(\alpha\)) [2; 3; 4], the Schuepp turbidity factor (\(B\)) [5], the Linke turbidity factor (\(T_{\rm L}\)) [6; 7] and the Unsworth-Monteith turbidity coefficient (\(T_{\rm UM}\)) [8].  
There is a growing interest in atmospheric turbidity worldwide as this issue is related to air-pollution studies in cities, solar radiation modelling, atmospheric chemistry, and climate studies. Apart from the pioneering works defining the various atmospheric turbidity parameters mentioned above, numerous studies have been carried out since the 1970s. A sample of such studies is given here.  
[PERSON] [9] attempted to evaluate \(T_{\mathrm{L}}\) for an urban, a suburban, a rural and an arctic station in Canada. [PERSON] et al. [10] reported \(T_{\mathrm{L}}\) levels for a desert environment (Dhahran, Saudi Arabia). [PERSON] et al. [11] developed a model for deriving \(T_{\mathrm{L}}\) by means of updated spectral extraterrestrial irradiances and extinction coefficients of gaseous absorbers. [PERSON] [12] discussed the influence of atmospheric boundary-layer stability on atmospheric turbidity and optical turbulence in a desert environment. [PERSON] et al. [13] studied the variation of the Linke turbidity factor at two Italian localities, while [PERSON] et al. [14] evaluated the same factor for several sites in Africa. [PERSON] et al. [15] studied the atmospheric turbidity levels at two sites in Egypt. Hove and Manyumbu [16] estimated the Linke turbidity factor over Zimbabwe. [PERSON] et al. [17] studied the turbidity coefficients in central Spain, while [PERSON] et al. [18] reported the spatial and temporal variability of atmospheric turbidity over Tunisia. [PERSON] et al. [19] studied the Linke turbidity factor over Warsaw and Belsk, Poland. A recent study [20] has investigated the accuracy of three different methodologies (in essence, different data inventories) in assessing high atmospheric-turbidity episodes over north-central Spain. The authors found that all three methodologies have a 60% coincidence in identifying such episodes.  
In Greece, studies about atmospheric turbidity have been conducted mostly for Athens [21; 22; 23; 24; 25; 26; 27] and Thessaloniki [28]. Most of them refer to the Linke turbidity factor. Therefore, there is a gap in recent turbidity studies for Athens and a complete absence of research for Greece as a whole. This gap is intended to be covered in the present work.  
The structure of the paper is as follows. Section 2 presents the methodology for calculating the Linke and Unsworth-Monteith turbidity parameters (Section 2.1), as well as the data collection (Section 2.2) and the data processing (Section 2.3). Sections 3.1 and 3.2 present the month-hour graphs of \(T_{\mathrm{L}}\) and \(T_{\mathrm{UM}}\) for selected sites under all and clear-sky conditions, respectively. Section 3.3 accounts for the effect of the clearness index on \(T_{\mathrm{L}}\) and \(T_{\mathrm{UM}}\), while Section 3.4 refers to the intra-annual variation of \(T_{\mathrm{L}}\) and \(T_{\mathrm{UM}}\) over Greece under all and clear-sky conditions. Section 4 presents the main conclusions of the study.