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/al uminium/garnet (Nd:YAG) (532 nm) backscatter lidar system at the Physi cal Research Laboratory, Ahmedabad (23 degrees N; 72.5 degrees E), Ind ia, from April 1992 to May 1994. The results obtained on the integrate d mass densities and aerosol backscatter from 17 to 30 km show a 1/e-f olding time of 9 months, for the Pinatubo aerosol layer to decay. Calc ulations show that if the layer decays at the same rate, then it may t ake about 4.5 years for the stratosphere to attain its background aero sol level over the tropics. However, the peak scattering ratio value s hows a longer decay time of 12.5 months as has been reported earlier i n the case of Fuego and El Chichon eruptions which could be due to fas ter removal of particles below the peak altitude, The aerosol size dis tribution has not undergone any considerable change throughout the dec ay 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 int egrated backscattering coefficient are found similar to the values rep orted in the case of El Chichon eruption. The results obtained on the aerosol extinction coefficients are found to compare well with Stratos pheric Aerosol and Gas Experiment (SAGE) II results.