Balloon-borne study of the upper tropospheric and stratospheric aerosols over a tropical station in India

Abstract

Using sun-scanning/tracking photometers onboard a high-altitude balloon the upper tropospheric and stratospheric aerosol characteristics have been studied over Hyderabad (17.5°N) in April 2001. In the upper troposphere the aerosol extinction coefficients are found to be around 10^-2 km^-1, while in the stratosphere the values are in the range 10^-3-10^-4 km^-1, which are about two orders of magnitude smaller than those measured after the Pinatubo eruption over the same site. The balloon-derived aerosol extinction coefficients are in good agreement with the results obtained from lidar experiments conducted over Mt. Abu (24.6°N). In the upper troposphere the integrated aerosol extinction coefficients at 500 nm are found to show an average increase of about 11 ± 1% yr^-1 during the period 1985-2001. This increase is found to be consistent with the observed increase in the long-term columnar aerosol optical depth data over selected locations in India. The Ångström wavelength exponent is found to be about 1.33 ± 0.12 in the stratosphere, which is comparable to the Junge size parameter values obtained in 1985. The mode radii of stratospheric aerosols are found to be 0.10 ± 0.01 µm in both the 2001 and 1985 experiments, indicating that the background stratospheric aerosol size distribution has not changed between these years, which is also corroborated by the good agreement between the wavelength exponent and size parameter values obtained in 2001 and 1985. The aerosol optical depth at 1 µm in 2001 is about 0.005, which is five times larger than that measured by SAGE in 1979 over the tropics. The stratospheric aerosol optical depth spectra are calculated for both the volcanic and quiescent conditions by taking into account the mode radius and aerosol number density. The estimated and measured aerosol optical depths agree well. We estimate that the sulfur dioxide and aerosol emissions from coal over India have increased by 10% yr^-1 during the past two decades. The increase in emissions from high-speed diesel oil and petrol is higher and is in the range 13- 14% yr^-1 over India. These increases could possibly be responsible for the observed increase in the upper tropospheric aerosol optical depths over India.