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Previous studies showed that, over the global ocean, there is no noticeable hemispheric asymmetry in cloud fraction (CF). This contributes to the balance in solar radiation reaching the sea surface in the northern and southern hemispheres. In the current study, we focus on the tropical Atlantic (30° N–30° S), which is characterized by significant amounts of Saharan dust dominating other aerosol species over the North Atlantic. Our main point is that, over the tropical Atlantic, Saharan dust not only is responsible for the pronounced hemispheric aerosol asymmetry, but also contributes to significant cloud cover along the Saharan Air Layer (SAL). Over the tropical Atlantic in July, along the SAL, Moderate Resolution Imaging Spectroradiometer CF data showed significant cloud cover (up to 0.8–0.9). This significant CF along SAL together with clouds over the Atlantic Intertropical Convergence Zone contributes to the 20% hemispheric CF asymmetry. This leads to the imbalance in strong solar radiation, which reaches the sea surface between the tropical North and South Atlantic, and, consequently, affects climate formation in the tropical Atlantic. During the 10-year study period (July 2002–June 2012), NASA Aerosol Reanalysis (aka MERRAero) showed that, when the hemispheric asymmetry in dust aerosol optical thickness (AOT) was most pronounced (particularly in July), dust AOT averaged separately over the tropical North Atlantic was one order of magnitude higher than that averaged over the tropical South Atlantic. In the presence of such strong hemispheric asymmetry in dust AOT in July, CF averaged separately over the tropical North Atlantic exceeded that over the tropical South Atlantic by 20%. Both Multiangle Imaging Spectroradiometer measurements and MERRAero data were in agreement on seasonal variations in hemispheric aerosol asymmetry. Hemispheric asymmetry in total AOT over the Atlantic was most pronounced between March and July, when dust presence over the North Atlantic was maximal. In September and October, there was no noticeable hemispheric aerosol asymmetry between the tropical North and South Atlantic. During the season with no noticeable hemispheric aerosol asymmetry, we found no noticeable asymmetry in cloud cover. [ABSTRACT FROM PUBLISHER]

The Indian subcontinent occupies 2.4% of the world land mass and is home to ∼17% of the world population. It is characterized by a wide range of population density (P), significant population growth and high levels of air pollution. The quantification of the effect of urbanization on aerosol optical thickness (AOT) trends was carried out by analysing 8-year (March 2000 to February 2008) Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging SpectroRadiometer (MISR) satellite data. Here we show that over extensive areas with differing population densities, which are significant parts of the Indian subcontinent, (1) the higher the averaged population density the bigger the averaged AOT and (2) the larger the population growth the stronger the increasing trends in AOT. Over the regions with P > 100 persons km−2 (more than 70% of the territory), a population growth of ∼1.5% year−1 was accompanied by increasing AOT trends of over 2% year−1. The presence of the aforementioned AOT trends is evidence of air quality deterioration, in particular in highly populated areas with P > 500 persons km−2. This situation could worsen with the continued growth of the Indian population. [ABSTRACT FROM AUTHOR]