Atmospheric aerosols in the Indo-Gangetic Plain (IGP) depicts high spatial and temporal heterogeneity in their radiative properties. Despite the fact that significant advancement in terms of characterizing aerosols radiative and physiochemical properties in the IGP have been made, information regarding the organic content towards total absorbing aerosol budget is lacking. In the present study we have analyzed two years of aerosol spectral light absorption measurements from the central-IGP, Gorakhpur (26.75°N, 83.38°E, 85m amsl), in order to study their seasonal behavior and to quantify their magnitude in terms of absorbing aerosols loading and source speciation. Remote sensing data in the form of 'Cloud corrected Fire Count' from MODIS Terra and 'Absorption Aerosol Index' from OMI satellites platform have been used to identify absorbing aerosol source regions. Spectral absorption analysis reveals a four-fold enhancement in absorption in the winter (W) and the post-monsoon (PoM) seasons at UV wavelengths as compared to 880 nm on account of increased biomass aerosol contribution to total absorbing aerosol load. Despite having higher fire events and absorption aerosol index, both indicating high biomass burning activities, in the pre-monsoon (PM) season, aerosols from the biomass sources contribute ~ 27% during the W and the PoM seasons as against ~17% in the PM season to the total absorbing aerosol content. This is due to near stagnant wind conditions and shallow height of air masses travelling to the central IGP in the W and the PoM seasons.
Gorakhpur (26.75°N, 83.38°E and 85 m amsl), is strategically located in the central Indo-Gangetic Plain
(IGP), near the foot hills of Himalayas and hence is an ideal place for studying long-range transport as well as
local sources of aerosols and its radiative implications. Here we present results from two years, October 2013
until September 2015, of measurements of spectral aerosol optical depth (AOD) utilizing ground based, Multi-
Wavelength Radiometer (MWR), and satellite, MODIS Terra remote sensing platforms. Mean AOD at 500 nm
(AOD<sub>500</sub>) is 0.63±0.35, associated with a moderate Angstrom exponent (α) of 1.03±0.22 is found for the study
period using MWR measurements. Highest AOD<sub>500</sub> is found during the pre-monsoon months of May and June
while lowest AOD<sub>500</sub> in the post-monsoon months of October and November. The MWR observations have
been compared with MODIS Terra derived AOD and a good correlation of 0.74 is found. We used HYSPLIT
Lagrangian trajectory model to investigate long-range transport of aerosols to the study region. Aerosol sources
in winter season are from the North-West part of the study region while that during pre-monsoon season lies in
the south-westerly arid regions. This finding is also reflected in the α values which are high during winter months
suggesting significant urban and biomass-burning contribution. α values are low and the turbidity coefficient
(β) is high during pre-monsoon months indicating long-range transport of coarse dust particles carried by south
westerly winds from the westerly desert regions.