PINATUBO VOLCANIC AEROSOL LAYER DECAY OBSERVED AT AHMEDABAD (23-DEGREES-N), INDIA, USING NEODYMIUM-YTTRIUM ALUMINUM GARNET BACKSCATTER LIDAR

Pinatubo volcanic aerosol layer is studied with a neodymium:yttrium/aluminium/garnet (Nd:YAG) (532 nm) backscatter lidar system at the Physical Research Laboratory, Ahmedabad (23 degrees N; 72.5 degrees E), India, from April 1992 to May 1994. The results obtained on the integrated mass densities and aerosol backscatter from 17 to 30 km show a 1/e-folding time of 9 months, for the Pinatubo aerosol layer to decay. Calculations show that if the layer decays at the same rate, then it may take about 4.5 years for the stratosphere to attain its background aerosol level over the tropics. However, the peak scattering ratio value shows a longer decay time of 12.5 months as has been reported earlier in the case of Fuego and El Chichon eruptions which could be due to faster removal of particles below the peak altitude, The aerosol size distribution has not undergone any considerable change throughout the decay phase of the layer from 1 year to 3 years after the eruption. Also, about 2 years after the eruption the estimated layer mass and the integrated backscattering coefficient are found similar to the values reported in the case of El Chichon eruption. The results obtained on the aerosol extinction coefficients are found to compare well with Stratospheric Aerosol and Gas Experiment (SAGE) II results.