Human impact on the atmospheric sulfur balance

Abstract

The paper reviews the development in our understanding of the atmospheric part of the globalsulfur cycle, including the role played by C.-G. Rossby and his colleagues in the 1950s, andpresents a brief assessment of the current knowledge. Measurements of the concentrations ofsulfur compounds in air, precipitation, ice cores and sea water during the past 25 years, togetherwith recent development in three-dimensional tracer transport modeling, have resulted in areasonably consistent picture of the burdens and fluxes of the main sulfur compounds in theatmosphere. It is clear that man’s activities, in particular the burning of fossil fuels, are havinga large impact on the atmospheric sulfur balance. Even on a global scale, the man-madeemissions of gaseous sulfur compounds are likely to be two to three times as large as the naturalsources. In and around the most heavily industrialized regions this ratio exceeds ten overextended areas. Nevertheless, there are several important issues that need to be resolved. Someof these are directly linked to the urgent problem of reducing the uncertainty in the estimateof direct and, in particular, indirect climate forcing due to man-made sulfate aerosols. One suchissue is the magnitude of the wet scavenging of SO₂ and aerosol sulfate during upward transportinto and within the free troposphere in connection with convective and frontal cloud systemswhich has a decisive influence on the sulfate concentrations in the upper troposphere. Anotheruncertain process is the rate of oxidation of SO₂ in cloud droplets and on aerosol particles. Afundamental question that remains to be answered is to what degree man-made sulfur emissionshave increased the number of aerosol particles that can act as cloud condensation nuclei.