Volcanoes inject both gases and particles into the atmosphere. This material can modify atmospheric composition and so change the chemical and radiative balance of the atmosphere. The radiative changes induced by volcanic eruptions result in perturbations of the Earth’s climate over the local to global scale as well as over a range of periods from hours to millennia. In general the study of active volcanism on Earth can be broken down into
The principal gases released by volcanic activity are H2O, CO2, CO, SO2, H2S, S2, HCl, HF, H2. Volcanoes are one of the principal sources of atmospheric SO2 and so are important to the atmospheric sulphur budget. One of the consequences of large eruptions injecting large amounts of SO2 and H2S into the atmosphere is the formation of drops of H2SO4 from the oxidised gases. These aerosol particles alter the atmospheric radiation field by scattering solar radiation, and by absorbing infrared radiation. For example the eruption of Mt Pinatubo in the Philippines in 1991 is estimated to have cooled the average surface temperature of the Earth by 1.5 C.
An eruption also ejects molten and solid rock fragments (tephra) into the air. The large fragments fall back to the ground near the vent but the fragments with a diameter less than 2 mm, called ash, can remain suspended in the atmosphere from days to weeks. In the troposphere ash particles additionally act as cloud condensation nuclei and so have an indirect effect on the radiation budget by modifying the radiative properties of cloud.
Several jet airplanes have been damaged by encounters with drifting clouds of volcanic ash. Casadevall (1994) reported 80 cases in 20 years. The natural variability of volcanism implies that there will be high year-to-year variability about the average. At least two commercial passenger aircraft have had all engines fail following flight into an unanticipated volcanic ash cloud during those 20 years (for example BA flight 9). Most aircraft ash encounters occur where the volcanoes are relatively isolated.
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