The instrument is calibrated by comparing all measurements to a known SO 2 standard mounted in the instrument. Typically, 3-6 traverses are made beneath the plume in order to determine the average SO 2 concentration along a vertical cross section of the plume. Wind speed is determined during flight either by GPS or by comparing true air speed, flying with and against the wind, with true ground speed. Because the location of roads around a volcano are fixed, however, such a configuration is not always possible. For the measurements to be successful, the plume must pass directly over a road.
Wind speed is determined by using a hand-held anemometer or from a portable meteorological station.
The LI-COR is mounted in a small aircraft configured for sampling outside air. From these data, a carbon dioxide emission rate can be calculated. This technique was first employed by USGS scientists at Popocatepetl volcano in Mexico in 1995. More recently, it has been used at several domestic volcanoes. The air intake is located well upstream of an aircraft's engine exhaust to avoid contamination.