  Aerosol optical characteristics AOD and \AA ngstr\"om exponent is often used
to asses environmental aerosol loading. AOD or Aerosol Optical Depth is an
indirect measure of atmospheric aerosol loading by means of total extinction of
incoming solar radiation due to scattering and absorption whereas \AA ngstr\"om
exponent($\alpha)$ is used to get qualitative understanding of aerosol particle
size. Analysis of long term time series AOD data reveals how AOD vis-\`a-vis
aerosol on a particular place changes over time. Similar study with \AA
ngstr\"om exponent($\alpha)$ gives an idea how particle size distribution is
changing over some area. Such studies cannot be conducted by data measured by
ground based stations alone because they are inadequate in numbers on earth
moreover such data for considerably long period are not available for most
places. To overcome this, radiance data sensed by MODIS instruments on board
Aqua and Terra satellite have been used by many authors. In this study 13 years
of MODIS level 2 data by Terra have been analyzed for four Indian mega-cities
namely Delhi, Mumbai, Kolkata and Chennai using well known Mann-Kendall
statistics and Sen's non-parametric estimation of slopes.
