The KNMI/ESA Tropospheric EMission Internet Service (TEMIS) publishes daily forecasts of the clear-sky UV index
for today and 8-days ahead and analyses of the daily UV dose of yesterday and before. Near-real time ozone satellite
observations are used to account for the variations in UV radiation as a function of the overhead ozone column. The daily
UV dose depends critically on the assumptions that are made for the cloud correction factor. Cloud cover fraction is the
most uncertain parameter for day-to-day variations in the daily UV dose under non-overcast cloud conditions. The cloud
optical thickness is the most important parameter determining variations in daily UV dose for overcast situations. A good
cloud mask is essential to identify cloud-free scenes. Still further improvements in the daily UV dose can be made,
including the inclusion of the diurnal variation in cloud optical thickness as well as corrections for aerosols.
TROPOMI (Tropospheric Ozone-Monitoring Instrument) is a five-channel UV-VIS-NIR-SWIR non-scanning nadir
viewing imaging spectrometer that combines a wide swath (114°) with high spatial resolution (10 × 10 km<sup>2</sup> ). The
instrument heritage consists of GOME on ERS-2, SCIAMACHY on Envisat and, especially, OMI on EOS-Aura.
TROPOMI has even smaller ground pixels than OMI-Aura but still exceeds OMI's signal-to-noise performance. These
improvements optimize the possibility to retrieve tropospheric trace gases. In addition, the SWIR capabilities of
TROPOMI are far better than SCIAMACHY's both in terms of spatial resolution and signal to noise performance.
TROPOMI is part of the TRAQ payload, a mission proposed in response to ESA's EOEP call. The TRAQ mission will
fly in a non-sun synchronous drifting orbit at about 720 km altitude providing nearly global coverage. TROPOMI
measures in the UV-visible wavelength region (270-490 nm), in a near-infrared channel (NIR) in the 710-775 nm range
and has a shortwave infrared channel (SWIR) near 2.3 μm. The wide swath angle, in combination with the drifting orbit,
allows measuring a location up to 5 times a day at 1.5-hour intervals. The spectral resolution is about 0.45 nm for UVVIS-
NIR and 0.25 nm for SWIR. Radiometric calibration will be maintained via solar irradiance measurements using
various diffusers. The instrument will carry on-board calibration sources like LEDs and a white light source. Innovative
aspects include the use of improved detectors in order to improve the radiation hardness and the spatial sampling
capabilities. Column densities of trace gases (NO<sub>2</sub>, O<sub>3</sub>, SO<sub>2</sub> and HCHO) will be derived using primarily the Differential
Optical Absorption Spectroscopy (DOAS) method. The NIR channel serves to obtain information on clouds and the
aerosol height distribution that is needed for tropospheric retrievals. A trade-off study will be conducted whether the
SWIR channel, included to determine column densities of CO and CH<sub>4</sub>, will be incorporated in TROPOMI or in the
Fourier Transform Spectrometer SIFTI on TRAQ.
The TROPI instrument is similar to the complete TROPOMI instrument (UV-VIS-NIR-SWIR) and is proposed for the
CAMEO initiative, as described for the U.S. NRC Decadal Study on Earth Science and Applications from Space.
CAMEO also uses a non-synchronous drifting orbit, but at a higher altitude (around 1500 km). The TROPI instrument
design is a modification of the TROPOMI design to achieve identical coverage and ground pixel sizes from a higher
altitude. In this paper capabilities of TROPOMI and TROPI are discussed with emphasis on the UV-VIS-NIR channels
as the TROPOMI SWIR channel is described in a separate contribution .
In order to characterize the solar UV radiation reaching the Earths surface it is monitored from space by means of (i) the clear-sky UV index at local solar noon, which is most relevant for operational UV forecasting, and (ii) the daily UV dose including cloud shielding effects, which is most relevant for long-term UV monitoring and assessments of health risks and biological UV effects. Optimal space- based surface UV monitoring combines information from platforms in different orbits. Space-based total ozone column products from polar orbiting platforms can be used adequately for UV monitoring because the diurnal variability in the total ozone column is limited. However, cloud cover and cloud optical thickness typically vary significantly on time scales of minutes to hours, especially over land in relation to convective activity. Because diurnal variations in cloud amount and cloud optical thickness impact dramatically on the daily-integrated UV radiation levels transmitted to the Earths surface, the time variations in (key) cloud parameters over the day need to be captured by observations. Sampling of the diurnal variations in clouds is most efficiently done from geostationary platforms. Here we demonstrate examples of calculations of the clear- sky UV index and the UV daily dose for erythema over Europe based on assimilated total ozone column data derived from observations by GOME aboard ERS-2 and its successor SCIAMACHY aboard ENVISAT, in combination with cloud information retrieved from MVIRI aboard Meteosat-7 and its successor SEVIRI aboard MSG (Meteosat-8). Some first validations with ground-based surface spectral UV data are presented.
Several organizations in the Netherlands are cooperating to develop user requirements and instrument concepts in the line of SCIAMACHY and OMI but with an increased focus on measuring tropospheric constituents from space. The concepts use passive spectroscopy in dedicated wavelength sections in the range of 300 to 2400 nm and wide angle, non-scanning, swath viewing.
To be able to penetrate into the troposphere small ground pixels are used to obtain a fair fraction of cloud-free pixels and to allow precise detection of the sources of polluting gases.
The trace gas products aimed for are O<sub>3</sub>, NO<sub>2</sub>, HCHO, H<sub>2</sub>O, SO<sub>2</sub>, Aerosol (optical depth, type and absorption index), CO and CH<sub>4</sub>, covering science issues on air quality and climate.
The main challenge in the instrument design is to obtain a good signal-to-noise for cloud free pixels and for low ground albedo and light levels. Also the retrieval of separated tropospheric and stratospheric column amounts from a nadir looking instrument is challenging.
The paper discusses the user requirements and compares alternative measurement strategies. It explains the selection of passive UV-Visible-NIR spectroscopy and comes with an instrument concept which provides the current best realisation of the user requirements.