Seasonal variations in aerosol characteristics over an urban location and a remote site in western India

Spatial and temporal variabilities in aerosol optical depth (AOD) and size distribution are investigated using ground based measurements over two different locations in western India, namely, Ahmedabad (urban industrialized location) and Gurushikhar (high altitude remote site) during 2006–2008. AODs over Ahmedabad are more than two times higher than Gurushikhar at all the wavelengths indicating higher aerosol loading over Ahmedabad throughout the study period. Ångström exponent (α) is higher over Ahmedabad than Gurushikhar suggesting higher fine mode aerosol concentration over Ahmedabad. Different values of α are observed when calculated over different wavelength ranges indicating bimodal aerosol size distribution. Discrimination of aerosol size during different seasons is made using the coefficients of polynomial fit (α1, α2) which showed the presence of changing dominant aerosol types as a function of season over both the locations. The α2 – α1 values are used to get confirmation on the dominant aerosol mode over both the locations during different seasons. About 87% and 78% of AOD spectra are found to be dominated by wide range of fine mode fractions or mixture of modes during winter over Ahmedabad and Gurushikhar respectively. During premonsoon ≥80% of AOD spectra are found to be dominated by coarse mode aerosols over both the locations. During monsoon ∼70% of AOD spectra are coarse mode dominant over Ahmedabad. In postmonsoon <40% of AOD spectra are dominated by coarse mode aerosols over both the locations. This characterization of aerosols is important in assessing the response of different aerosol types on radiative forcing and regional climate.