Envisat was an ESA satellite (2002-2012), devoted to environmental studies, notably in the areas of atmospheric chemistry and ocean, ice and landsurface studies. Up to now, it is the largest Earth Observation spacecraft ever built. It carried ten sophisticated optical and radar instruments to provide continuous observation and monitoring of the Earth's land, atmosphere, oceans and ice caps.
One of the instruments onboard was the German-Dutch-Belgian SCIAMACHY instrument which was built to measure the chemical composition of the Earth's atmosphere. Dutch industry and institutes were responsible for the development of the optical unit of SCIAMACHY including the detectors for ultraviolet up to short wave infrared (SWIR) radiation.
The main scientific objective of SCIAMACHY was to improve our knowledge and understanding of a variety of global environmental issues related to the chemistry and physics of the Earth atmosphere:
- depletion and/or recovery of stratospheric ozone
- climate changes through increased levels of anthropogenic greenhouse gases
- tropospheric air pollution from industrial activities, transport, biomass burning
SRON designed, constructed and tested the detector modules of the SCIAMACHY instrument. SRON further chaired the Calibration subgroup of the Science Advisory Group, and participates in the SCIAMACHY Quality Working Group in particular to contribute to the in-flight monitoring and improvement of the calibration data. Based on the measurements with the SWIR modules SRON deduced the SWIR trace gas abundancies and developed the scientific data products for carbon monoxide (CO), methane (CH4) and heavy water (HDO/H2O). These data products are then validated and used for scientific research for example to estimate surface fluxes using atmospheric inverse modeling.
SCIAMACHY measured the reflectivity of the Earth atmosphere in the spectral range 240-2400 nm. In this wavelength range many species absorb the solar light and thus leave their characteristic 'fingerprints' in the observed reflectance. These include ozone, NO2, CO2, CH4, H2O, CO, SO2 and many others. Their abundances were retrieved from the depth of their absorption bands in the reflectance spectrum. SCIAMACHY was the first instrument in space to measure some of these species, notably CO2, CH4 and CO, down to the surface where most of the emissions occur. SCIAMACHY views the Earth in nadir, limb and solar occultation mode. The latter two provide more height information on the distribution of the various species.
The SCIAMACHY measurements of nitrogen dioxide (NO2), an important indicator of air pollution, were the first to clearly show the air pollution over the Netherlands in the European context. This alarmed environmentalists and politicians in the Netherlands, who then put air quality high on the Dutch political agenda. SCIAMACHY measurements also detected elevated concentrations of carbon monoxide (CO) over Australia which could be traced back to emissions from South American wildfires some 13000 km away.
One of the important breakthroughs of SCIAMACHY was the registration of enhanced methane concentrations in the Tropics (Frankenberg et al, 2005. Methane is a relatively potent greenhouse gas and an important factor in global warming, so these findings were very relevant. They inspired a lot of new research, to further investigate speculations about missing sources of methane in tropical rainforests. Meanwhile, some of them, such as trees in wetland environments and tank bromeliads, have indeed been confirmed by in situ measurements.
SCIAMACHY was performing CH4 measurements in a time when the global methane mixing ratios transitioned from more or less stable conditions to a pronounced increase, which is ongoing at present. SCIAMACHY CH4 retrievals have been used to investigate the cause of this transition from stable to increasing global CH4 mixing ratios, which started in 2007. According to our analysis [Houweling et al, 2014], the global surface fluxes started increasing roughly half a year earlier. Between two year periods before and after this time, the global emissions increased by 30-35 Tg/yr, or about 6% of the global emissions in this period. The largest fraction of this increase came from the tropical zone (30oS-30oN). A more detailed identification of the origin of the increase is not supported by the data, as indicated by the 2-sigma uncertainty margins. With the more advanced Sentinel 5p TROPOMI instrument we expect to further improve the precision of our emission estimates.
Last but not least, the SCIAMACHY instrument made the first global observations of heavy water with sensitivity towards the lowest layers of the atmosphere (Frankenberg et al, 2009), where most of the water vapour resides. This has enabled us to deepen our current understanding of the hydrological cycle, which is a main factor controlling the Earth's climate. Satellite measurements of heavy water provide an important new way of testing climate models, supporting their development, and improving their skill in predicting future climate change.
ENVISAT is one of the giant satellites for new Earth observation research and SCIAMACHY the first instrument for carbon-cycle research with sensitivity down to the Earth surface.