In early April 2009, La Cumbre Volcano on Isla Fernandina in the Galapagos Islands erupted, producing an ash plume and lava flows. The eruption also produced a substantial cloud of sulfur dioxide that extended far west of the islands, over the Pacific Ocean.
The Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite captured this image on April 14, 2009. In this image, sulfur dioxide is measured in Dobson Units. If you compressed all the sulfur dioxide in an atmospheric column into a flat layer at 0 degrees Celsius and standard atmospheric pressure (1 atmosphere), a Dobson Unit would be 0.01 millimeters thick and would contain 0.0285 grams of sulfur dioxide per square meter. In other words, Dobson Units measure the number of molecules in a square centimeter of the atmosphere. In this image, the greatest concentrations of sulfur dioxide appear in red, and the smallest concentrations appear in lavendar.
As the sulfur dioxide cloud moved away from the volcano, its intensity dissipated, evidenced by the red hues near the summit, and blues, greens, and lavenders farther away. Near the center of the image, the plume seems to intensify, but this is actually an artifact of the data gathering process. This image is compiled from two separate overpasses of the Aura satellite, and the center of this image shows observations taken at the satellite swath edge. At its swath edge, a satellite looks through a thicker layer of atmosphere than it does when it looks straight down, and this longer line of sight exaggerates the apparent intensity of haze, aerosols, and gases such as sulfur dioxide.
Another NASA mission, the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), detected a substantial low-altitude sulfate aerosol plume from Isla Fernandina. Located at an altitude of roughly 3 kilometers above sea level, the sulfate aerosol likely resulted from chemical reactions involving the sulfur dioxide from Isla Fernandina.
At high concentrations at ground level, sulfur dioxide can cause haze, respiratory irritation and, over time, damage to buildings. At high altitudes, sulfur dioxide can affect climate. Oxidation transforms this gas into sulfate aerosol particles, which create a bright haze that reflects sunlight back into space and prevents it from reaching the Earth.