Study of aerosol types and seasonal sources using wavelength dependent Angstrom exponent over North-East India: Ground-based measurement and satellite remote sensing

The spectral estimates of Aerosol Optical Depth (AOD, tau) were made by operating a Microtops-II sun photometer in the spectral range 0.380-0.870 mu m over Tripura in north-eastern India and analyzed to infer the aerosol types and source characteristics in different seasons. The Angstrom exponent (alpha) derived from spectral AOD in different wavelength (lambda) range and subsequent second order derivative of Angstrom exponent (i.e., alpha') and their curvature analysis in the In tau versus In lambda relationship has revealed the crucial information related to the dominance of different aerosol types and their characteristics in different seasons. The average AOD (mean +/- standard deviation) at 0.5 mu m in winter, pre-monsoon, monsoon and post-monsoon seasons are observed to be 0.70 +/- 0.28, 0.74 +/- 0.18, 0.55 +/- 0.20, 0.44 +/- 0.19 respectively; while the corresponding seasonal mean values of alpha (over 0.380-0.870 mu m spectral range) are found to be 1.09 +/- 0.17, 0.92 +/- 0.24, 0.51 +/- 0.27, 0.89 +/- 0.38 respectively. Examination of the Angstrom exponents derived from satellite retrieved AOD by Moderate Resolution Imaging Spectroradiometer (MODIS) instrument operating onboard Terra satellite, along with MODIS fine mode aerosol fraction and aerosol types indicates broad seasonal features of aerosol size spectrum over the study region similar to those observed from ground-based measurements. The estimation of the values of alpha at different spectral ranges indicate that winter season is mainly influenced by the fine-mode aerosols having fine-mode fraction (FMF) similar to 0.7; whereas coarse-mode aerosols dominate in the monsoon season having FMF similar to 0.3. The pre-monsoon and post-monsoon seasons exhibit the presence of mixed type of aerosols, with slightly greater fraction of fine-mode aerosol in pre-monsoon. Curvature analyses of Angstrom exponent put insight into the consistency of observed features of seasonal aerosol types. (C) 2018 COSPAR. Published by Elsevier Ltd. All rights reserved.