Sulphur dioxide (SO2) enters the Earth’s atmosphere through both natural and anthropogenic processes. It plays a role in chemistry on a local and global scale and its impact ranges from short term pollution to effects on climate. Only about 30% of the emitted SO2 comes from natural sources; the majority is of anthropogenic origin. SO2 emissions affect human and animal health and air quality. SO2 has an effect on climate through radiative forcing, via the formation of sulphate aerosols. Volcanic SO2 emissions may also pose a threat to aviation, along with volcanic ash. The S5P sensor TROPOMI samples the Earth’s surface with a revisit time of one day and with an unprecedented spatial resolution of 7x7 km. This allows the resolution of fine details and S5P will arguably be a valuable tool to better study anthropogenic SO2 emissions but also volcanic emissions, from degassing to eruptive processes. Non-linear effects could largely be accounted for by applying a direct fitting technique in the retrieval scheme. However, for near real time delivery of TROPOMI Level 2 products (i.e. typically 3 hours after measurement), direct fitting algorithms are out of reach; Even when limiting this technique to large volcanic events, a large eruption, covering 1% of the ground pixels with high SO2 content, the processing would take some 30 times longer than processing all pixels with a DOAS technique . The proposed algorithm for the retrieval of SO2 vertical column is based on a DOAS scheme. With this technique, two main steps are involved: First, the effective slant column amount Ns (corresponding to the integrated SO2 concentration along the mean atmospheric optical path) is derived through a least-squares fit of the measured Earth reflectance spectrum to laboratory absorption cross-sections. Second, slant columns are converted into vertical columns by means of air mass factors (AMF) obtained from suitable radiative transfer calculations, accounting for the presence of clouds and aerosols, surface properties and best-guess SO2 vertical profiles
USER DOCUMENTATION: ATBD
The Algorithm Theoretical Basis Document (ATBD) can be found by clicking here.
PUM & SAMPLE DATA
The Product User Manual (PUM) and Sample Data Files are being updated based on a new release of the processing software and will be available for download by mid-May 2017.
After launch, preliminary product results will be provided during the Commissioning Phase and the Operational Phase.