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The second generation Collection 005 (C005) MODIS operational algorithm for retrieval of aerosol properties was evaluated and validated for the greater Mediterranean basin (29.5° N-46.5° N and 10.5°W-38.5° E), a region with an atmosphere under siege by air pollution and diminishing water resources that are exacerbated by high aerosol loads and climatic change. The present study aims to quantify the differences between the C005 and the previous (C004) MODIS collections, and re-assess the results of previous studies that have been performed for the region using MODIS C004 aerosol optical depth (AOD) products. Daily data of AOD from EOS-Terra covering the 6-year period 2000-2006 were taken from both C005 and C004 Level-3 datasets, and were inter-compared and validated against ground-based measurements from 29 AERONET stations. The C005 data were found to significantly better agree with the AERONET data than those of C004. The correlation coefficient between MODIS and AERONET was found to increase from 0.66 to 0.76 and the slope of linear regression MODIS/AERONET from 0.79 to 0.85. The MODIS C005 data still overestimate/underestimate the AERONET AOD values smaller/larger than 0.25, but to a much smaller extent than C004 data. The better agreement of C005 with AERONET data arises from the generally lower C005 values, with regional mean AOD values equal to 0.27 and 0.22 for C004 and C005, respectively. This decrease, however, is not uniform over the region and involves a significant decrease over land and a small increase over the ocean for AOD values greater than 0.1 (opposite changes were found under aerosol-clean conditions). Both data sets indicate a decrease in the regional mean AOD over the period 2000-2006, equal to 20% based on C005 and 17% based on C004 datasets, though the intra-annual and inter-annual variation did not change significantly, thus indicating a systematic correction to C004 values. [ABSTRACT FROM AUTHOR]

We used a spectral radiative transfer model to compute the direct radiative effect (DRE) of natural plus anthropogenic aerosols in the solar near-infrared (IR), between 0.85-10µm, namely, their effect on the outgoing near- IR radiation at the top of atmosphere (TOA, ΔFTOA), on the atmospheric absorption of near-IR radiation (ΔFatmab) and on the surface downward and absorbed near-IR radiation (ΔFsurf, and ΔFsurfnet, respectively). The computations were performed on a global scale (over land and ocean) under all-sky conditions, using detailed spectral aerosol optical properties taken from the Global Aerosol Data Set (GADS) supplemented by realistic data for the rest of surface and atmospheric parameters. The computed aerosol DRE, averaged over the 12-year period 1984-1995 for January and July, shows that on a global mean basis aerosols produce a planetary cooling by increasing the scattered near-IR radiation back to space by 0.48Wm-2, they warm the atmosphere by 0.37Wm-2 and cool the surface by decreasing the downward and absorbed near-IR radiation at surface by 1.03 and 0.85Wm-2, respectively. The magnitude of the near-IR aerosol DRE is smaller than that of the combined ultraviolet (UV) and visible DRE, but it is still energetically important, since it contributes to the total shortwave (SW) DRE by 22-31%. The aerosol-produced near-IR surface cooling combined with the atmospheric warming, may affect the thermal dynamics of the Earth-atmosphere system, by increasing the atmospheric stability, decreasing thus cloud formation, and precipitation, especially over desertification threatened regions such as the Mediterranean basin. This, together with the fact that the sign of near-IR aerosol DRE is sometimes opposite to that of UV-visible DRE, demonstrates the importance of performing detailed spectral computations to provide estimates of the climatic role of aerosols for the Earth-atmosphere system. This was demonstrated by sensitivity tests revealing very large differences (up to 300%) between aerosol DREs computed using detailed spectral and spectrally-averaged aerosol optical properties. Our model results indicate thus that the aerosol direct radiative effect on the near-IR radiation is very sensitive to the treatment of the spectral dependence of aerosol optical properties and solar radiation. [ABSTRACT FROM AUTHOR]

In this study, we investigate the aerosol optical properties, namely aerosol extinction optical thickness (AOT), Angström parameter and size distribution over the Eastern Mediterranean Basin, using spectral measurements from the recently established FORTH (Foundation for Research and Technology-Hellas) AERONET station in Crete, for the two-year period 2003-2004. The location of the FORTH-AERONET station offers a unique opportunity to monitor aerosols from different sources. Maximum values of AOT are found primarily in spring, which together with small values of the Angström parameter indicate dust transported from African deserts, whereas the minimum values of AOT occur in winter. In autumn, large AOT values observed at near-infrared wavelengths arise also from dust transport. In summer, large AOT values at ultraviolet (340 nm) and visible wavelengths (500 nm), together with large values of the Angström parameter, are associated with transport of fine aerosols of urban/industrial and biomass burning origin. The Angström parameter values vary on a daily basis within the range 0.05-2.20, and on a monthly basis within the range 0.68-1.9. This behaviour, together with broad frequency distributions and back-trajectory analyses, indicates a great variety of aerosol types over the study region including dust, urban-industrial and biomass-burning pollution, and maritime, as well as mixed aerosol types. Large temporal variability is observed in AOT, Angström parameter, aerosol content and size. The fine and coarse aerosol modes persist throughout the year, with the coarse mode dominant except in summer. The highest values of AOT are related primarily to southeasterly winds, associated with coarse aerosols, and to a less extent to northwesterly winds associated with fine aerosols. The results of this study show that the FORTH AERONET station in Crete is well suited for studying the transport and mixing of different types of aerosols from a variety of sources, especially those associated with major dust events from the Sahara. [ABSTRACT FROM AUTHOR]

A series of sensitivity studies were performed with a spectral radiative transfer model using aerosol data from the Global Aerosol Data Set (GADS, data available at ) in order to investigate and quantify the relative role of key climatic parameters on clear-sky ultraviolet–visible direct aerosol radiative forcing at the top of the atmosphere (TOA), within the atmosphere and at the Earth's surface. The model results show that relative humidity and aerosol single-scattering albedo are the most important climatic parameters that determine aerosol forcing at the TOA and at the Earth's surface and atmosphere, respectively. Relative humidity exerts a non-linear positive radiative effect, i.e. increasing humidity amplifies the magnitude of the forcing in the atmosphere and at the surface. Our model sensitivity studies show that increasing relative humidity by 10%, in relative terms, increases the aerosol forcing by factors of 1.42 at the TOA, 1.02 in the atmosphere and 1.17 at the surface. An increase in aerosol single-scattering albedo by 10%, in relative terms, increased the aerosol forcing at the TOA by 1.29, while it decreased the forcing in the atmosphere and at the surface by factors of 0.2 and 0.69, respectively. Our results show that an increase in relative humidity enhances the planetary cooling effect of aerosols (increased reflection of solar radiation to space) over oceans and low-albedo land areas, whilst over polar regions and highly reflecting land surfaces the warming effect of aerosols changes to a cooling effect. Thus, global warming and an associated increase in relative humidity would lead to enhanced aerosol cooling worldwide. The sensitivity results also demonstrate that an increase in surface albedo due to, for example, a reduction in land vegetation cover, would lead to enhanced atmospheric warming by aerosols leading to a reduction in cloud formation and enhancement of the desertification process. On the contrary, a decrease in surface albedo over polar regions due to, for example, ice-melting associated with global warming, would reduce the planetary warming effect of aerosols over polar areas. Aerosol forcing is found to be quite sensitive to cloud cover, as well as to aerosol optical thickness and the asymmetry parameter, and to the wavelength dependence of the aerosol optical properties. [ABSTRACT FROM AUTHOR]

A deterministic atmospheric spectral radiative transfer model, that uses comprehensive climatological data, is developed to compute the global distribution of mean monthly clear-sky total direct aerosol radiative forcing in the ultraviolet (UV) and visible, between 0.2–0.85 μm, at the top of the atmosphere (TOA), within the atmosphere and at the Earth's surface for winter and summer conditions. The aerosol data were taken from the Global Aerosol Data Set (GADS), given for various fixed relative humidity values and for 11 wavelengths within the UV–visible range, both for natural and anthropogenic aerosols. We first derive global climatologies of extinction aerosol optical thickness (AOT), single-scattering albedo (ωaer) and asymmetry factor ( gaer), for actual relative humidity values within the aerosol layer, based on the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project and the Tiros Operational Vertical Sounder (TOVS) datasets. We include the global distribution of cloud cover using the D2 data from the International Satellite Cloud Climatology Project (ISCCP), to define the clear-sky fraction at the pixel level for each month. Supplementary 10-yr climatological data for surface and atmospheric parameters were taken from NCEP/NCAR, ISCCP-D2 and TOVS. Our present analysis allows the aerosol radiative properties and forcings to vary with space, time and wavelength. The computed mean annual global AOT, ωaer and gaer values are found to be 0.08, 0.96 and 0.73, respectively, at 0.5 μm. On a mean monthly 2.5° pixel resolution, aerosols are found to decrease significantly the downward and the absorbed solar radiation at the surface, by up to 28 and 23 W m−2, respectively, producing a surface cooling at all latitudes in both winter and summer. Aerosols are found to generally increase the outgoing solar radiation at TOA (planetary cooling) while they increase the solar atmospheric absorption (atmospheric warming). However, the model results indicate that significant planetary warming, by up to 5 W m−2, can occur regionally, such as over desert areas, due to strong aerosol absorption. A smaller planetary warming (by up to 2 W m−2) is also found over highly reflecting ice- or snow-covered areas, such as Antarctica and Greenland, as well as over Eastern Europe, Siberia and North America. In general, the aerosol-induced surface cooling exceeds the induced atmospheric warming, except for regions characterized by strong aerosol absorption (e.g. deserts). On a mean annual global basis, natural plus anthropogenic aerosols are found to cool the Earth by 0.6 W m−2 (they increase the planetary albedo by 0.28%), to heat the atmosphere by 0.8 W m−2, while they decrease the downward and net surface solar radiation (surface cooling) by about 1.9 and 1.4 W m−2. [ABSTRACT FROM AUTHOR]

Abstract We use global aerosol optical depth data from CALIOP with a radiation transfer model to investigate the aerosol direct radiative effect (DRE) and its sensitivity to the aerosol vertical resolution. Our study spans three years (2007–2009) and uses cloud data from ISCCP D2 to take into account cloud-aerosol radiative interactions on a monthly 2.5° × 2.5° resolution. The three-year global average all-sky aerosol DRE at the surface, in the atmosphere, and at the top of atmosphere (TOA) was calculated to be −4.23, 2.40, and −1.83 Wm−2, respectively. As expected, local DREs and atmospheric heating rates are shown to vary significantly. The largest magnitudes of the DREs are observed in regions with heavy aerosol load consisting of both natural and anthropogenic particles, such as desert dust, biomass burning and urban/industrial pollution. At TOA the aerosol effect is generally of negative sign, though a planetary heating effect is found in regions characterized by both absorbing aerosol and large surface albedo, such as deserts. Clouds scatter and absorb solar radiation, which generally decreases the aerosol cooling at the surface and the aerosol warming in the atmosphere. However, the latter effect is attenuated due to the enhancement of radiation absorption by the above-cloud aerosols. As a result, clouds decrease the aerosol TOA (planetary) cooling, and sometimes even cause aerosol warming (e.g. over the tropical South Atlantic). The cloud effect on the aerosol DRE depends strongly on the aerosol optical properties and the aerosol load fraction above low clouds. Comparing the effect of the observed aerosol vertical profile against an exponentially decreasing profile, we find a small sensitivity for the surface DRE, but larger for the atmospheric column and the top of the atmosphere. Under all-sky conditions, when continental aerosols are lifted higher in the atmosphere, the outgoing shortwave radiation at TOA decreases, due to the increase of UV and visible radiation absorption by particles while higher oceanic aerosols generally increase the outgoing shortwave radiation through more efficient backscatter and decrease of the NIR radiation absorption by atmospheric gases below aerosol particles. Highlights • A radiation transfer model produces the global aerosol direct radiative effect (DRE) from vertically resolved AOD from CALIOP • We present the 3-year DRE at the surface, in the atmosphere, and at the TOA, on a 2.5° × 2.5°, monthly resolution • We present global annual and seasonal DRE maps at the surface, at the TOA, in the atmosphere, as well as heating rates • We provide vertical profiles for both AOD and heating rates for 9 of the most aerosol-laden areas of the globe • The aerosol-cloud radiative interplay is examined and the significant effect of the above-cloud aerosols is studied • We investigate the DRE sensitivityby comparing exponentially decreasing with height profiles and the ones measured by CALIOP • The sensitivity of DRE to aerosol height is very different for the UV-Vis part of the spectrum compared to the NIR part [ABSTRACT FROM AUTHOR]