Seasonal heterogeneity in aerosol optical properties over the subtropical humid region of northern India

First-ever measurements and study of aerosol optical properties from a subtropical humid region of the eastern part of the central Indo-Gangetic Plain (IGP), Gorakhpur (26.75°N, 83.38°E, 85 m amsl) is presented. Four years, March 2014–February 2018, of columnar aerosol optical measurements, using a Multi-wavelength solar radiometer (MWR), are analyzed to examine temporal and seasonal heterogeneity in aerosol optical properties, and to ascertain aerosol types over the study region along with their seasonal contribution. Average columnar aerosol optical depth at 500 nm (AOD500) is 0.65 ± 0.27, with highest seasonal mean (0.73 ± 0.30) during pre-monsoon (PM) and lowest seasonal mean (0.59 ± 0.25) during monsoon (M) season. During PM season, more than 50% of AOD500 values are greater than 0.7 indicating high aerosol loading over the study region. Large variability in Ångström exponent (α) and atmospheric turbidity (β) from 0.12 to 2.26 and 0.05 to 1.34, with mean value of 0.92 ± 0.27 and 0.37 ± 0.14, respectively, indicates heterogeneous aerosol emission sources and turbid atmosphere over the study region. Higher α (>1) during post-monsoon (PoM) and winter (W) seasons indicate predominance of fine mode aerosols whereas lower α (