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SCIAMACHY: The Instrument onboard ENVISAT

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CHAPTER 2: What is SCIAMACHY?

2. SCIAMACHY: The Instrument onboard ENVISAT

2.1. Measurement Goals


SCIAMACHY’s approach for passive atmospheric sounding from space was to measure solar absorption spectra at the top of the atmosphere. Atoms, molecules and particles absorb, emit and scatter the incoming solar electromagnetic radiation. The incoming solar radiation is described to a good approximation by the emission from a black body having a temperature of about 5800 K, modulated by atomic absorption lines, the solar Fraunhofer lines. The upwelling radiation at the top of the atmosphere from the Ultraviolet (UV) to Short-Wave Infrared (SWIR) comprises – after travelling through the atmosphere – the solar output, modified by scattering, absorption and emission processes along its light path through the atmosphere and reflected as well as scattered at the Earth’s surface.

SCIAMACHY observed in the wavelength range from 214-2386 nm


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the scattered and reflected spectral radiance in nadir and limb geometry,

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the spectral radiance transmitted through the atmosphere in solar and lunar occultation geometry,

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the extraterrestrial solar irradiance and the lunar radiance.


 

Trace gases, aerosols, clouds and the surface of the Earth modify the light observed by SCIAMACHY via absorption, emission and scattering processes. Inversion of the radiance and irradiance measurements allows retrieval of the amounts and distributions of a significant number of trace gas constituents from their spectral signatures as well as to obtain information on other atmospheric parameters and even selected surface phenomena. This task requires – beside high quality measurements – an accurate understanding and knowledge of the absorption spectroscopy and the scattering of electromagnetic radiation in the atmosphere and at the Earth’s surface.

The targeted atmospheric trace gas species and parameters depend on the main objectives of the SCIAMACHY mission. With the goal to improve our knowledge of:


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global atmospheric composition,

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its change in response to both natural and anthropogenic activity and

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the processes associated to it as well as the related global issues of importance to the chemistry and physics of our atmosphere such as:

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the impact of anthropogenic activity and natural processes on tropospheric ozone, air quality and global warming,

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exchange processes between the stratosphere and troposphere,

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the interaction of stratospheric chemistry and dynamics,

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natural modulations of atmospheric composition resulting from volcanic eruptions, lightning, solar output variations (e.g. solar cycle), or solar proton events.


 

SCIAMACHY’s list of "deliverables" included tropospheric and stratospheric trace gases, such as e.g. O3, NO2, CO, CO2, HCHO, CH4, H2O, SO2, BrO, OClO. By combining nadir and limb observations tropospheric amounts of the constituents down to the ground or cloud top depending on cloud cover could be determined. In addition to the trace gases, information on clouds (cloud top height, cloud optical thickness, ice-water cloud discrimination) and aerosol were deduced from the SCIAMACHY measurements. Particularly interesting, among these, were Polar Stratospheric Clouds (PSC) and Noctilucent Clouds (NLC), also referred to as Polar Mesospheric Clouds (PMC).

The feasibility of these goals for nadir observations, including measurement principles and retrieval concepts was successfully demonstrated with the Global Ozone Monitoring Experiment (GOME) on ESA’s second European Remote Sensing satellite (ERS-2). GOME on ERS-2 was a smaller scale version of SCIAMACHY derived from the original SCIAMACHY concept, measuring in nadir viewing geometry the upwelling radiation at the top of the atmosphere between 240 and 793 nm. ERS-2 had been launched on April 20th, 1995 into a sun-synchronous orbit with an equator crossing time in descending node of 10:30 a.m.. It was deorbited in July 2011, delivering measurements over more than 16 years.


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