Retrieval of Aerosol Size Index From High-Resolution Spectroradiometer Observations

Aerosol characteristics over an urban station were studied by utilizing aerosol optical depths at 61 wavelengths in the region 400-700 nm (5-nm interval) obtained with a high spectral resolution radiometer during the period from April 1993 to April 1995 at the Indian Institute of Tropical Meteorology (IITM), Pune (18°32ʹ N, 73°5f E, 559-m AMSL), India. The retrieved columnar aerosol size index values are compared with those derived from colocated bistatic argon ion Iidar angular scattering measurements. The spectroradiometer-derived size index (daytime) values are found to vary between 2.91 and 4.04, whereas the lidar-derived (nighttime) value is found to vary from 2 to 4.5 during the observational period. The temporal and short-term variations in aerosol size index are presented. The seasonal variation in aerosol size index over the experimental station indicates larger values during winter months and smaller values during pre-monsoon months, which suggests relatively larger-sized particles during pre-monsoon as compared to winter months. The spectroradiometer-derived aerosol optical depths are found to be correlated well (correlation coefficient = 0.75) with the surface-level extinction coefficient.