Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO2. The primary sources of sulfur dioxide are burning of fossil fuels in power plants, metal smelting, and volcanic emissions. Sulfur dioxide has a short lifetime in air, turning into sulfate aerosol particles in a day near the ground and in a month in the stratosphere. The sulfate can combine with water to create a haze of sulfuric acid aerosol, and if precipitation occurs, acid rain. These particles and/or haze can be very bright and can reflect incoming sunlight.
Large, violent eruptions can send sulfur dioxide high into the Earth's atmosphere. The resulting sulfate aerosols can linger for several years and cool the climate. In 1991, Mount Pinatubo sent millions of tons of SO2 into the atmosphere, and global temperatures, which had been expected to rise because of the greenhouse effect, leveled out.
Although invisible to human eyes, sulfur dioxide can still make its presence known—by irritating them. Sulfur dioxide can inflame mucous membranes of the eyes, nose, and throat, and even skin. The upper respiratory tract is the most susceptible to sulfur dioxide irritation. It is toxic to humans and concentrations as low as 8 ppm (parts per million) will produce coughing (ref http://mattson.creighton.edu/SO2/SO2_Info.html). Sulfur dioxide also leads to acid rain and volcanic smog (vog) that interferes with air transport.
SO2 is currently being measured over the entire earth using the Ozone Monitoring Instrument (OMI) on the AURA spacecraft. The gas is measured in Dobson Units (DU), the number of molecules in a square centimeter of the atmosphere. If you were to compress all of the sulfur dioxide in a column of the atmosphere into a flat layer at standard temperature and pressure (0o C and 1013.25 hPa), one Dobson Unit would be 0.01 millimeters thick and would contain 0.0285 grams of SO2 per square meter.
The OMI instrument is a Dutch-Finnish Instrument, on board NASA's Earth Observing System (EOS) Aura satellite mission. NIVR (the Dutch space agency) is the overall program manager, in coordination with Finnish Meteorological Institute (FMI). The Royal Netherlands Meteorological Institute (KNMI) is the Principal Investigator institute.