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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]

Exposure to outdoor fine particulate matter (PM 2.5 ) is a leading risk factor for mortality. We develop global estimates of annual PM 2.5 concentrations and trends for 1998-2018 using advances in satellite observations, chemical transport modeling, and ground-based monitoring. Aerosol optical depths (AODs) from advanced satellite products including finer resolution, increased global coverage, and improved long-term stability are combined and related to surface PM 2.5 concentrations using geophysical relationships between surface PM 2.5 and AOD simulated by the GEOS-Chem chemical transport model with updated algorithms. The resultant annual mean geophysical PM 2.5 estimates are highly consistent with globally distributed ground monitors ( R 2 = 0.81; slope = 0.90). Geographically weighted regression is applied to the geophysical PM 2.5 estimates to predict and account for the residual bias with PM 2.5 monitors, yielding even higher cross validated agreement ( R 2 /yr), India (1.13 ± 0.15 μg/m 3 /yr), Europe (-0.15 ± 0.03 μg/m 3 /yr), India (1.13 ± 0.15 μg/m 3 /yr) for India over 2005-2013 and the negative trend (-3.37 ± 0.38 μg/m 3 /yr). The positive trend (2.44 ± 0.44 μg/m 3 /yr) for India over 2005-2013 and the negative trend (-3.37 ± 0.38 μg/m 3 /yr) for China over 2011-2018 are remarkable, with implications for the health of billions of people.