Temporal variations and the effect of volcanic eruptions on atmospheric turbidity over India

Atmospheric Turbidity data collected over a period of about 23 years (1975-97) at Indian ‘Global Atmospheric Watch’ stations have been analysed to determine the major causes of time variation. The most extreme perturbations have been associated with significant volcanic eruptions of El-Chichon, Mexico (17° N, 146° E) in April, 1982 and Mt. Pinatubo, Philippines (15° N, 120° E) in June, 1991. The increase in atmospheric turbidity values following these eruptions was discernible for 1.5 - 2 years after the eruption. Annual mean values of the turbidity coefficients computed for the two consecutive decades, 1976-85 and 1986-95, show a general increase of turbidity during the later epoch at all stations except Kodaikanal. Analysis of long-term monthly means shows the seasonal variations and influence of monsoon over turbidity coefficients. A considerable shift of atmospheric turbidity values from the lower band values (0.0 - 0.1) to higher band (0.1 - 0.2) also witnessed from the percentage frequency analysis.