Recent global aerosol optical depth variations and trends - A comparative study using MODIS and MISR level 3 datasets

Aerosol distribution at global and regional scales is one of the crucial parameters for climate change assessment. The study presents global spatio-temporal variations in aerosol optical depth (AOD) retrieved by Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) sensors at spatial resolutions of 1° × 1° and 0.5° × 0.5° respectively. Spatial correlations between MODIS and MISR AOD are reported on annual scale. The entire globe is divided into six sub-regions, i.e., North-West, North, North-East, South-West, South and South-East. The annual and seasonal mean AOD variations are obtained by analyzing Level 3 monthly datasets for the period 2001-2014. Relative changes (%) in AOD between the first and last seven years (i.e. 2001-2007 and 2008-2014 respectively) are discussed at annual and seasonal levels. AOD trends are calculated using the weighted least squares regression approach and weights are assigned to the AOD values corresponding to the expected errors. Statistically significant trends are reported at 95% confidence level. The results inferred from both the sensors are found to be in quite acceptable agreement. Increasing trends are found in the economically growing parts of Asian landmass and the surrounding oceanic regions, especially Arabian Sea and Bay of Bengal. On the other hand, decreasing trends are observed over parts of Europe, South America and North America. The study also focuses on examining the AOD trends over the ten most populous nations of the world. In terms of aerosol loadings, Bangladesh, Pakistan and Nigeria turned out to be the most polluted amongst the most populous countries. Significantly increasing trends in AOD are reported over the Indian subcontinent.