Spectral aerosol optical depth and Angstrom parameters in the polluted Athens atmosphere

Using ground-based spectral solar extinction data taken in the Athens atmosphere during a field survey, experimental and modeled aerosol optical depths AOD have been retrieved. The Angstrom parameters α and β were derived through spectral extinction curves determined from a log-log plot fit in four spectral intervals, e.g. UV, VIS, UV-VIS and VIS-NIR. The results reveal that exponent α derived in the shorter wavebands, is correlated with β in the VIS-NIR range the correlation is weaker, however. It has also found that α does not depend on the Angstrom law fits, while β does. Retrieved experimental AOD obtained at longer wavelengths exhibits a daily pattern similar to turbidity β by contrast, AOD at shorter wavelengths follows the temporal variation of α. © Springer-Verlag/Wien 2005

Study on an intense dust storm over Greece

Springtime constitutes the most favorable period for Sahara dust outbreaks and transport over Eastern Mediterranean. This study investigates the aerosol properties during April 2005 using remote-sensing and ground-based measurements. Three dust events with high aerosol optical depth (AOD) values have been observed during the measuring period, with duration of two days, i.e. 11-12, 16-17 and 25-26 April 2005. In this paper we mainly focus on the intense dust event of 16-17 April 2005, when a thick dust layer transported from Libya affected the whole Greek territory. Very high AOD values obtained from Aqua-MODIS sensor were observed over Greece (mean 2.42 ± 1.25) on 17 April, while the respective mean April value was 0.31 ± 0.09. The AOD at 550 nm (AOD550) values over Crete were even larger, reaching ∼4.0. As a consequence, the PM10 concentrations over Athens dramatically increased reaching up to 200 μg m-3. On the other hand, the fine-mode fraction values obtained from Terra-MODIS showed a substantial decrease in the whole Greek area on 17 April with values below 0.2 in the Southern regions. The intense dust layer showed a complex behavior concerning its spatial and temporal evolution and allowed us to study the changes in the optical properties of the desert dust particles along their transport routes due to the mixing processes with other aerosol types. The results from different measurements (ground-based and remote-sensing) did not contradict each other and, therefore, are adequate for monitoring of dust load over the Eastern Mediterranean. © 2008 Elsevier Ltd. All rights reserved

Aerosol climatology and discrimination of different types over Athens, Greece, based on MODIS data

A long-term (2000-2005) monitoring of aerosol data from the moderate resolution imaging spectroradiometer (MODIS) is analyzed focusing on the Greater Athens Area (GAA) in the Eastern Mediterranean region. The MODIS aerosol optical depth standard product (AOD at 550 nm) and its respective ratio attributed to fine-mode (FM) particles are employed to evaluate the seasonal variability of the aerosol properties over Athens. The climatological trend of both parameters in the period 2000-2005 is nearly absent, while remarkable year-to-year variability can be observed. The seasonal analysis reveals a significant AOD variability over Athens, with minimum values in winter (AOD550 ∼ 0.2), and maximum in summer (AOD550 ∼ 0.45). Regarding the FM fraction, maximum values are present in spring and minimum in summer, thus revealing the dominance of FM and coarse-mode particles, respectively. For the whole data set, a method is implemented to distinguish the main aerosol types (urban/industrial (hereafter UI), clean maritime (hereafter CM type) and desert dust (hereafter DD) over Athens, based on both AOD and FM values. Because of the mixing processes in the atmosphere the majority of the cases (46.6%) belong to a mixed (hereafter MT) aerosol type. The UI aerosols are more frequent in spring (41.2%) and less in winter (9.1%), while the coarse particles, probably DD, more frequent in summer (35.8%) and less in winter (3.5%). In contrast, the clean atmospheric conditions are more frequent in winter (23.9%), when the mixing processes are also well established (63.5%). For each aerosol type, the mean AOD550 and FM values are also computed. Their seasonal variability exhibits a clear summer maximum for UI, CM and MT aerosols, while the DD exhibits maximum in spring. As regards the FM values of the different aerosol types they exhibit a rather constant variation with small fluctuations from season to season. © 2007 Elsevier Ltd. All rights reserved

Characterising the long-range transport mechanisms of different aerosol types over Athens, Greece during 2000-2005

This study analyses the weather conditions, the main pathways and transport mechanisms favouring the presence of specific aerosol types over Athens, Greece. On the basis of the aerosol optical depth at 550 nm (AOD 550) and fine mode (FM) values from a Terra-MODIS dataset in the period 2000-2005, three main aerosol types are identified (urban/industrial, UI; clean maritime, CM; and desert dust, DD), each one corresponding to different optical characteristics and source regions. The UI aerosols are associated with polluted air masses from Europe, the CM aerosols with clean Atlantic air masses and the DD aerosols with air masses from North African arid regions carrying significant amount of dust in certain cases. The comparison of the three aerosol types with the air masses from their favourable sector constitutes a first 'quick-validation' of the identification scheme. Thus, the incidence of transport from Europe explain 81% of the variability in the observed UI type, while the 73% of the Atlantic air masses correspond to the CM type; the 50% of the African air masses can be considered as DD aerosols over Athens. The mean synoptic meteorological patterns, favouring the presence of each aerosol type, are also investigated. Further analysing the air-mass trajectories at three altitudes, the transport mechanisms of the aerosol types are identified. The results clearly show that the UI aerosols are mainly transported within the boundary layer, while the CM conditions are associated with Atlantic air masses at higher altitudes. Moreover, the DD aerosols are transported either in the upper atmosphere or in the whole atmospheric column. This is among the first studies conducted over Athens aiming at investigating the weather conditions, pathways and transport mechanisms that favour the presence of aerosols of different characteristics. © 2011 Royal Meteorological Society

Seasonal variation of columnar aerosol optical properties over Athens, Greece, based on MODIS data

A long-term (2000-2005) data set of aerosol optical properties obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) is analyzed focusing on the Greater Athens Area in the Eastern Mediterranean region. The MODIS aerosol optical depth standard product (AOD at 550 nm) and its respective ratio attributed to fine-mode particles (FM) are employed to evaluate the inter-annual and seasonal variability of the aerosol properties over Athens. Based on AOD550 and FM values three specific aerosol types are discriminated corresponding to different aerosol load and optical properties. The aerosol types considered correspond to urban/industrial aerosols, coarse-mode particles and clean maritime conditions. This study focuses on the seasonal and year-to-year fluctuation of the number of occurrences as well as the AOD550 and FM values of each aerosol type. The coarse-mode particles are observed mainly in the summer, while spring is the most favorable season for the occurrence of urban/industrial aerosols. On the other hand, clean maritime conditions occur mainly in the winter. The AOD550 values for the coarse-mode particles are higher in spring, while the urban/industrial and clean maritime aerosols exhibit slightly higher values in the summer. The seasonal distribution of the aerosol properties is related to anthropogenic and dust emissions in the spring/summer period, but is modified by atmospheric dispersion and precipitation in late autumn/winter. The main conclusion of the study is that the coarse-mode particles exhibit much stronger inter-annual and seasonal variability compared to the urban/industrial aerosols. Finally, three cases corresponding to each aerosol type are analyzed with the aid of synoptic weather maps, air mass trajectories and MODIS data. © 2007 Elsevier Inc. All rights reserved

Modification of solar radiation components under different atmospheric conditions in the Greater Athens Area, Greece

The influence of the atmospheric turbidity on the spectral distribution of solar irradiance components is investigated using ground-based spectroradiometric measurements taken in Athens area during May 1995. It is found that both the diffuse-to-global and diffuse-to-direct-beam irradiance ratios exhibits a strong wavelength dependence and exponential curves associated with 99% of the variance can fit each parameter. These exponential curves are further modified as function of the solar zenith angle and atmospheric turbidity conditions. It is found that the slope of the curves strongly depends on the processes attenuating irradiance and aerosol optical characteristics in the short wavelengths. New relations are proposed, which allow the spectral distribution of diffuse irradiance to be estimated as a function of the measured broadband global and diffuse solar irradiances. The diffuse-to-direct-beam ratio, which is an indicator of the atmospheric transmittance, exhibits a strong wavelength and aerosol-loading dependence. The observed differences between turbid urban and clean rural atmospheres constitute a manifestation of contrasting air properties and influence solar irradiance spectra. © 2005 Elsevier Ltd. All rights reserved

Application of SPCTRAL2 parametric model in estimating spectral solar irradiances over polluted Athens atmosphere

Results obtained using SPCTRAL2 parametric model for the polluted urban atmosphere of Athens, Greece, were analysed and compared with ground level experimental spectral solar irradiance measurements and spectrally integrated solar irradiances in two discrete narrow bands, ultraviolet and visible. For the aerosol characterization, the aerosol optical depth evaluated at 500 nm was used as the basic input parameter. The algorithm used seems to reproduce the experimental solar spectral irradiances adequately depending on the aerosol model used. The results obtained have been explained through mean bias and root mean square statistical deviations and the resultant influence of the aerosol volume spectra. © 2003 Elsevier Ltd. All rights reserved

Climatology of the Sistan Levar wind: Atmospheric dynamics driving its onset, duration and withdrawal

Levar wind is the dominant meteorological and climatic feature in east Iran, blowing from Central Asia to the northern coast of the Arabian Sea. It is also known as “120-days wind” due to its mean duration in the summer season. Although violent and responsible for massive dust storms in southwest Asia, long-term climatology and atmospheric dynamics that facilitate genesis and dissipation of Levar have not been well documented. This study uses the two-phase regression method for determining the onset, dissipation and duration of Levar during a 41-years period (1972–2012), based on identification of the change points in wind data series at Zabol meteorological station. The mean duration of the Levar period is estimated to 135 ± 24 days, with a remarkable inter-annual variability. The mean Levar onset is determined around 22 May ±23 days, while the withdrawal on 8 October ±25 days. On average, the onset and dissipation of Levar occur between wind speeds of 4.6–7.7 ms−1 in spring and 3.9–6.7 ms−1 in autumn, respectively. A comprehensive analysis is performed for the first time to examine meteorological dynamics that are associated with onset, duration and withdrawal of Levar. The early onset years are characterized by stronger winds during May, while no considerable changes in wind are detected between early and late withdrawal years. Changes in the mean sea-level pressure (MSLP) dipole between the Caspian Sea and India/Pakistan forces the onset of Levar. Therefore, early (late) onset (withdrawal) of Levar are driven by changes in MSLP over these areas, while local dynamics and topography also play an important role. Levar seems to be modulated by CasHKI (Caspian Sea Hindu Kush Index), while changes in the Caspian-Sea High (majorly) and in the Indian/Pakistan monsoon thermal low (secondarily) affect the Levar intensity. © 2021 Elsevier B.V

Predicting land susceptibility to atmospheric dust emissions in central Iran by combining integrated data mining and a regional climate model

This study aims to predict land susceptibility (a term that indicates the degree of sensitivity of land to detachment of soil particles by wind) to dust emissions in Yazd province, central Iran, by combining a new integrated data mining (DM) model and the RegCM4 climatic model. The study further determines the relative importance of key factors controlling dust emissions by applying 12 individual DM models. The integrated model is based on the individual models returning Nash Sutcliffe coefficient (NSC) values > 90% for the spatial modelling of land susceptibility to dust emissions and using the area under the curve (AUC) for validation. 13 key factors controlling dust emissions are mapped including soil characteristics, climatic variables, vegetation cover, a Digital Elevation Model (DEM), geology and land use. Based on Spearman clustering analysis and multi-collinearity tests (tolerance coefficient -TC and variance inflation factor -VIF), the effective factors for dust emissions are classified into nine clusters and no multi-collinearity is found among the effective factors. DEM, NDVI (normalized difference vegetation index), geology and calcium carbonate are identified as the most important factors controlling dust emissions. Seven individual models return NSC in the range of 90–98% and are used to generate the integrated model for the final mapping of land susceptibility to dust emissions. Among 851 pixels located in the dust sources, 30% (255 pixels) and 70% (596 pixels) are randomly selected as validation and training datasets, respectively for the new integrated model. Using this model, 9%, 17%, 7% and 67% of the study area correspond to low, moderate, high and very high susceptibility classes, while the validation results in AUC = 99.3%. Simulations with the RegCM4 model reveal high consistency regarding the spatial distribution of the most susceptible areas and dust emissions. Overall, combining DM approaches and physical models is useful in aeolian geomorphology studies