Characterization of aerosols and its radiative impacts over urban and rural environments—a case study from Hyderabad and Srisailam
Introduction
Industrial societies have begun to appreciate especially during the last few decades. Degradation of air quality in urban areas is one of the most obvious results of modern civilization; there have been other more subtle changes on a global scale. Recently attention has been paid to increased atmospheric concentrations of aerosol particles, which can drive the significant radiative forcing process of the planet (Charlson and Wigley, 1994). Aerosol concentrations in remote areas are small in comparison with most continental areas especially with urban areas. Aerosols in urban regions are chemically and substantially different from aerosols in remote areas with the most obvious differences being the high concentrations of sulfur and heavy metals in urban aerosols. Aerosols influence climate directly by scattering and absorbing solar radiation (Charlson et al., 1992), indirectly by acting as cloud condensation nuclei, thereby affecting the droplet concentrations, optical properties, precipitation rate and lifetime of clouds. Aerosols also influence the long-wave radiation, but to a smaller extent. Back scattering of solar radiation by aerosols enhances the planetary albedo, leading to negative surface forcing (cooling). The presence of absorbing aerosols such as black carbon (BC) can change the sign of forcing from negative to positive (heating) (Heintzenberg et al., 1997). The climate forcing due to aerosols is poorly characterized in climate models because of the lack of comprehensive global database of aerosol concentrations, chemical composition and optical properties. The present study concentrates on aerosol characterization and its radiative impacts at rural and urban areas namely Srisailam and Hyderabad by using the aerosol optical depth (AOD) measurements from MICROTOPS-II sunphotometer during February 2003.
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Study sites
The urban site in the present study is Hyderabad, which is the 5th largest city in India. It has twin cities viz., Hyderabad and Secunderabad with its suburbs extending up to 16 km. The city Hyderabad is situated at 17°10′ to 17°50′N latitude and 78°10′ to 78°50′E of the longitude. With respect to the climate, the normal rainfall of the district is 786.8 mm and the mean temperature varies from 28–42 °C. The rural site in the study is Srisailam lying between the latitudes 15°00′ to 16°15′N and
Short-wave aerosol radiative forcing
Radiative forcing is the change in net flux, either at the top of the atmosphere or at the surface due to a change in environment. Such a change will be produced by changes in atmospheric composition, nature of the constituent species, cloudiness or surface properties. In the case of aerosol direct forcing,where FA and FNA are, respectively, the net flux with and without aerosols. In the present study, we used OPAC model developed by Hess et al. (1998) for urban and rural
Results and discussion
Fig. 2, Fig. 3 show the variation of aerosol optical depth (AOD) at different wavelengths viz., 380, 440, 500, 675, 870 and 1020 nm at the urban and rural environments. AOD was observed to be high at the urban environment compared to the rural environment. AOD measurements over the urban site were 0.76, 0.61, 0.55, 0.37, 0.4 and 0.33 at the corresponding wavelengths viz., 380, 440, 500, 675, 870 and 1002 nm. AOD values at the rural site are relatively low compared to the urban site. Aerosol
Conclusions
Aerosol optical depth (AOD) values derived from ground measurements are in good agreement with satellite derived values. Over the urban environment aerosol forcing at the surface is as high as −42 W m−2 and at TOA is +10 W m−2 whereas at the rural environment aerosol forcing at the surface has been observed to be −11 W m−2 and at TOA it is observed to be +5.7 W m−2. The difference between TOA and the surface forcing over the urban environment is +32 W m−2 and over the rural environment is +5.3 W m−2, which
Acknowledgments
Authors are grateful to Director, NRSA and Dy. Director (RS&GIS), NRSA for their help and encouragement and ISRO-GBP for funding support.
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