Physical and optical properties of aerosols over an urban location in western India: implications for shortwave radiative forcing

Abstract

We discuss results on implications of seasonal and interannual variabilities in aerosol parameters measured over Ahmedabad, an urban location in western India, for the regional-scale shortwave aerosol direct radiative forcing. Results on physical and optical properties of aerosols are discussed in a companion paper. A discrete ordinate radiative transfer model has been used to carry out the radiative transfer computations. Two different approaches are followed to generate spectral values of aerosol parameters required as input for the radiative transfer computations, and the estimated values are found comparable for both methods. Magnitudes of surface forcing are found to be highest during postmonsoon (-63 ± 10 W/m²), followed by dry (-54 ± 6 W/m²) and lower values during premonsoon (-41.4 ± 5 W/m²) and monsoon (-41 ± 11 W/m²) seasons. In case of TOA, radiative forcing are found to be negative during dry (-26 ± 3 W/m²) and postmonsoon (-22), while positive values are obtained during monsoon (14) and premonsoon (8). Large differences between TOA and surface forcing during monsoon and premonsoon indicate large absorption of radiant energy (~50 W/m²) within the atmosphere during these seasons. Different properties of aerosols and differences in their vertical distribution give rise to different heating rates within the atmosphere for different seasons. Heating rates at the surface are found to be highest during postmonsoon (5.6°K/day) but decreases sharply with increase in height. Atmosphere is heated strongly at higher levels between 1 and 2 km during monsoon. Results from several sensitivity studies have emphasized the importance of solar zenith angle and other related factors in modulating the values of aerosol radiative forcing.