The stratospheric sulfur burden: an assessment based on gas and particle phase measurements

Sulfur, by far the most common element found in stratospheric aerosol, appears in the particles due to the low vapor pressure of sulfuric acid which leads to quick condensation on pre-existing particles or homogeneous nucleation in regions of high sulfuric acid concentrations. The sulfuric acid arises through several oxidations of sulfur dioxide (SO2), ultimately by OH. Due to abundance of water, compared to sulfuric acid, the particles quickly take on water and grow. The SO2 appears in the stratosphere either through direct injection, or the photolysis and subsequent oxidation of carbonyl sulfide (OCS). Both SO2 and OCS are transported into the stratosphere through the upwelling caused by tropical convection, the Brewer Dobson circulation, and sporadic volcanic eruptions. Volcanic eruptions are particularly important for SO2. Due to the abundance of sulfur in stratospheric aerosol and to the reliance of that aerosol on gas phase sources of sulfur, an estimate of the sulfur burden from both phases is important to assess stratospheric aerosol and requires profile measurements of stratospheric aerosol, SO2 and OCS. Regular profile measurements of particle phase sulfur began in the 1970s at selected locations using lidar and balloon-borne instruments. The particle measurements became global in the 1980s with satellite-borne instruments. Regular measurements of gas phase sulfur began with ground based measurements at selected locations in the 1980s and became global in the 2000s with satellite-based instruments. Here details of estimations of the sulfur component from the particle measurements of either aerosol extinction, backscatter, limb scatter, or size distribution are presented along with results from gas phase measurements of SO2 and OCS. The temporal and spatial coverage of the measurements will be described, as well as inherent measurement uncertainties arising from instrumental factors, and the assumptions required. Integrals of the sulfur mixing ratios derived will provide a map of the stratospheric sulfur burden available from measurements. Interpolation and extrapolation of the sulfur burden can be used to provide a rudimentary temporal history of the global stratospheric sulfur burden since the beginning of this century, and perhaps earlier.