Seasonal Variation In Spectral Global And Direct Solar Irradiances Over A Megacity Delhi

The present study investigates the spectral distribution of global, direct (E-b lambda) and diffuse (E-b lambda) 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-d lambda/E-b lambda) 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-d lambda/E-b lambda 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) AOD(550) data has been used to evaluate the relationship between aerosol load and ratio (E-d lambda/E-b lambda) on incoming solar radiation. A strong dependence of measured diffuse-to-direct irradiance ratio (E-d lambda/E-b lambda) on wavelength was observE(d lambda) It was found to be decreasing exponentially with wavelength. Value of E-d lambda/E-b lambda was found to be maximum and minimum during Post-monsoon and Pre-monsoon seasons respectively. Also, the maximum values of E-d lambda/E-b lambda ratio were observed at the shorter wavelengths in all the studied seasons. The AOD(550) value was found to be maximum during post-monsoon 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-d lambda/E-b lambda and AOD(550) 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