Sulphur dioxide (SO2) enters the atmosphere as a results of both natural phenomena and anthropogenic activities, such as fossil fuel combustion, oxidation of organic materials in soils, volcanic eruptions and biomass burning. SO2 contributes to acid rain and it is a key precursor for sulphuric acid aerosol formation. At high concentration, it also adversely affects human health, in particular in combination with fog (smog).

Changes in the abundance of SO2 have an impact on atmospheric chemistry and hence on air quality and on climate. Effects of volcanic eruptions may have an impact on air traffic, as such eruptions are important sources of ash (aerosols) and SO2. Consequently, global observations of SO2 are important for atmospheric and climate research, and for air traffic organisations. Global monitoring of SO2 concentrations is done on the basis of UV-Visible measurements by satellite based nadir-viewing instruments. Local monitoring, notably around volcanoes, is done using ground-based instruments.

SO2 released in the atmosphere due to anthropogenic activities will usually stay in the lower atmosphere, the troposphere, and from there it is transported by winds. It is removed from the troposphere:

• in gas phase by formation of sulphuric acid (H2SO4), which forms condensation nuclei for aerosols and clouds and acidifies the rain;
• directly, by way of an uptake on aerosols and clouds, which leads to dry and wet acid depositions.

The elevation reached by SO2 of volcanic origin depends on the kind of volcano, on the kind of eruption: for some eruptions the SO2 will stay relatively close to the ground, while explosive eruptions may take the SO2 high up into the statosphere, up to 15 km or more. Once in the stratosphere, the lifetime of SO2 is several weeks, during which it produces sulphate aerosols. This makes sulphur dioxide from volcanos one of the two most important sources of stratospheric aerosols.
 

Data products & projects

Within the UV-Vis/DOAS group SO2 is retrieved from measurements made by several instruments (these data pages are maintained outside the group):

instrument	SO2 data access
GOME-1		--
SCIAMACHY	Off-line	NRT
GOME-2		NRT

The data is retrieved and/or made available within the framework of several services and projects (links lead to the respective project websites):

• The Support to Aviation Control Service (SACS) and air quality monitoring services of:
  • TEMIS -- Tropospheric Emission Monitoring Internet Service
  • PROMOTE -- Protocol Monitoring for the GMES Service Element

• GlobVolcano

• AMFIC

• NOVAC