The present study investigates the spectral distribution of global, direct (E<sub>bλ</sub>) and diffuse (E<sub>bλ</sub>) 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<sub>dλ</sub>/E<sub>bλ</sub>]) 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<sub>dλ</sub>/E<sub>bλ</sub> 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<sub>550</sub> data has been used to evaluate the relationship between aerosol load and ratio (E<sub>dλ</sub>/E<sub>bλ</sub>) on incoming solar radiation. A strong dependence of measured diffuse-to-direct irradiance ratio (E<sub>dλ</sub>/E<sub>bλ</sub>) on wavelength was observed. It was found to be decreasing exponentially with wavelength. Value of E<sub>dλ</sub>/E<sub>bλ</sub> was found to be maximum and minimum during Post-monsoon and Pre-monsoon seasons respectively. Also, the maximum values of E<sub>dλ</sub>/E<sub>bλ</sub> ratio were observed at the shorter wavelengths in all the studied seasons. The AOD<sub>550</sub> 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<sub>dλ</sub>/E<sub>bλ</sub> and AOD<sub>550</sub> 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
Present study shows the seasonal variation of the Aerosol Optical Depth (AOD) and aerosols characteristics in an urban and rural environment over Delhi-NCR. Aerosol sampling was carried out using a Mini-Volume sampler at an urban and rural location in Delhi-NCR. A relatively higher PM<sub>2.5</sub> (particulate matter of size < 2.5 μm) concentrations were observed at the urban sampling site than the rural one in the summer as well as winter season. PM<sub>2.5</sub> samples were further analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) in order to understand the morphology and elemental composition of the PM<sub>2.5</sub> aerosols. Summer SEM results showed the dominance of fluffy agglomerate (soot) in urban area whereas the rural area was relatively clean. The winter season SEM results showed the presence of aggregates of smaller particles at urban site whereas flaky, round and irregular shaped particles were observed at the rural site. EDX analysis showed the presence of elements such as C, Cu, Zn, Ga and Fe (representative elements) in varying concentrations at both the urban and rural sampling locations. NASA’s Aqua satellite MODIS sensor AOD data for summer and winter seasons have been used to study the spatial distributions of aerosols over the study region. AOD was found to be relatively higher in urban area as compared to the rural area in both the summer and winter seasons indicating the contribution of high amount of anthropogenic aerosols in the urban atmosphere.