Spectral characteristics of urban aerosols and their association with relative humidity

Multi-spectral extinction measurements made with co-located high-spectral resolution radiometer (spectroradiometer) and sunphotometer at the Indian Institute of Tropical Meteorology (IITM), Pune (18°32′N, 73°5′E, 559 m AMSL) on 188 cloud-free days between March 1993 and May 1995 are presented. The observed aerosol optical size spectrum was approximatd by a composite power-law distribution function. The seasonal mean size distributions, inferred from both the radiometers, exhibit a power-law type distribution with different exponents changing at an intermediate size. The mean Junge size exponents (ν1 and ν2) and switching radius (r0) obtained with spectroradiometer are found to be in good agreement with those obtained with sunphotometer. The relationships among aerosol optical depths and derived size distributions, and meteorological parameters (height-integrated) are discussed.