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Aerosols have many natural and human-induced sources such as smoke from forest fires, wind-blown dust, pollution or volcanic eruptions. When present in large concentrations, aerosols can reflect significant amounts of solar radiation back to space and cause cooling at the Earths surface. Depending upon the chemical composition of aerosol particles, they can absorb radiation from the sun or emitted from the Earth, causing the atmosphere to warm. Aerosols can also strongly influence atmospheric chemical processes, including those that control ozone. Because the characteristics of aerosols can vary considerably, understanding how aerosols affect climate is one of the major problems confronting atmospheric scientists.
SAGE II observed the long-term global effects of the June 1991 eruption of the Mt. Pinatubo volcano in the Philippines. The eruption produced large quantities of aerosols in the upper atmosphere. The top-left graphic from SAGE II data shows a relatively aerosol-free atmosphere before the eruption. The top-right graphic reveals that aerosols in the tropics increased by almost a factor of 100 immediately following the eruption. The bottom-left graphic shows that aerosols had spread into the Earths mid-latitudes three months later. The bottom-right graphic illustrates how volcanic aerosols slowly decreased in the atmosphere over several years. The effects of Mt. Pinatubo lingered for up to 10 years following the eruption. The global distribution of aerosols as shown in these images is one of many important stratospheric processes that SAGE III will monitor.
The SAGE III instrument will measure the distribution of aerosols from the middle troposphere through the stratosphere. For example, the SAGE II satellite instrument, the predecessor to SAGE III launched in 1984, observed dispersal of volcanic aerosols following the massive eruption of Mt. Pinatubo in 1991. These measurements were crucial in linking a decline in the globally averaged surface temperature in mid-1992 of about 1 degree Fahrenheit to the large aerosol concentrations from the volcanic eruption. Aerosols from Mt. Pinatubo also strongly influenced the observed ozone trendan effect that would not have been detected without measurements like those from SAGE II. The data provided unique insight into the complex flow of air in the stratosphere that is needed to gain a better understanding of how the upper atmosphere will respond to climate change.