Remote sensing of sulphur dioxide

Abstract

There are many possible methods for the remote sensing of sulphur dioxide but only two are sufficiently sensitive to measure trace amounts: correlation spectrometry and differential lidar. The correlation spectrometer measures line integrals of concentration, or burdens (in atm-μm or ppm-m), by analysing incident radiation in the ultraviolet for absorption by sulphur dioxide. It has already been widely used to measure vertical burdens against a skylight background Measurements of emission rate have been derived from traverses of a plume near its source: they are limited by the accuracy of the associated wind speed rather than by the spectrometer. Comprehensive measurements of horizontal dispersion and its dependence on times of travel and sampling have also been obtained from traverses farther downwind. The instrument has proved particularly valuable as a sensitive plume locator on mobile surveys. The differential lidar provides range-resolved measurements of concentration by reflecting pulses of laser light at two wavelengths with different absorption coefficients from particles along the line of sight. It offers a sensitivity of a few parts per billion (ppb) to ranges in excess of 1 km with resolution in space and time of 100m and 10s. The instrument has already been demonstrated in prototype form and is now being developed for operational use.