Seasonal variation in spectral global and direct solar irradiances over a megacity Delhi

Abstract

The present study investigates the spectral distribution of global, direct (E_bλ) and diffuse (E_bλ) irradiances over Delhi with the help of handheld Field Spectroradiometer in the year 2014 – 2015. Seasonal variation in the solar irradiances as well as the diffuse-to-direct ratio ( E_dλ/E_bλ]) was studied during three seasons (post-monsoon, winter and pre-monsoon) having different atmospheric conditions. Observations were carried out under clear sky days during day time when solar zenith angle is low in order to get maximum solar radiation. The ratio E_dλ/E_bλ is used as a function to measure the impact of aerosol load on incoming solar irradiance. NASA's Aqua satellite, MODIS (Moderate Resolution Imaging Spectroradiometer) AOD₅₅₀ data has been used to evaluate the relationship between aerosol load and ratio (E_dλ/E_bλ) on incoming solar radiation. A strong dependence of measured diffuse-to-direct irradiance ratio (E_dλ/E_bλ) on wavelength was observed. It was found to be decreasing exponentially with wavelength. Value of E_dλ/E_bλ was found to be maximum and minimum during Post-monsoon and Pre-monsoon seasons respectively. Also, the maximum values of E_dλ/E_bλ ratio were observed at the shorter wavelengths in all the studied seasons. The AOD₅₅₀ value was found to be maximum during postmonsoon followed by winter and Pre-monsoon seasons. Crop residue burning in addition to low wind speed was responsible for high aerosol load during the post-monsoon season where as inversion layer and calm wind conditions favored high aerosol load during winter season. A strong relation between the ratio E_dλ/E_bλ and AOD₅₅₀ is observed in all the studied seasons, indicating that high aerosol load was responsible for the attenuation of the incoming solar radiation in all the seasons