A sharper view of air quality from space
The Tropospheric Monitoring Instrument (TROPOMI) is a satellite-borne spectrometer designed to increase our understanding of trace gases and aerosols that affect climate. Its wide field of view and broad range of wavelengths will help atmospheric scientists better constrain the sources and sinks of these constituents and improve understanding of the related chemical processes. The instrument will be launched in 2015 on the European Space Agency's (ESA) Sentinel 5 Precursor (S5P) satellite (see Figure 1).1
TROPOMI is a Dutch national initiative that builds on the successes of the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) and the Ozone Monitoring Instrument (OMI).2, 3 TROPOMI will move in a polar sun synchronous orbit and circle Earth 15 times a day, with a 1/15 view of the earth's equator during each pass. The instrument combines OMI's wide field of view push-broom concept with SCIAMACHY's broad wavelength range, allowing for very good spatial resolution in combination with daily global coverage. Trace gases are measured via their absorption of specific wavelengths, and TROPOMI's broad wavelength range will allow it to measure a large range of gases, including the most important trace gases, aerosols, and clouds (see Figure 2).
TROPOMI also represents a major improvement in spatial resolution and sensitivity compared with its predecessor instruments. The ground pixel size of TROPOMI is 7×7km2, an area reduced by a factor of eight compared with OMI and by a factor of 80 compared with SCIAMACHY. The reduced area covered by a single pixel means that TROPOMI will cover a specific geographical region with more pixels than did previous instruments, giving it higher resolution. The sensitivity is an order of magnitude improved in the sense that the instrument is now designed to meet its signal-to-noise requirements for very dark scenes. These are sea or ocean scenes with albedo of ∼2%, whereas OMI was designed for ‘nominal’ scenes with albedo 30%.
| Spectrometer | Band | Spectral properties (nm) | ||
|---|---|---|---|---|
| Range | Resolution | Sampling | ||
| UV | 1 | 270–300 | 1.0 | 0.065 |
| 2 | 300–320 | 0.5 | 0.065 | |
| UVIS | 3 | 310–405 | 0.55 | 0.2 |
| 4 | 405–500 | 0.55 | 0.2 | |
| NIR | 5 | 675–725 | 0.5 | 0.1 |
| 6 | 725–775 | 0.5 | 0.1 | |
| SWIR | 7 | 2305–2385 | 0.25 | <0.1 |
The instrument layout is shown in Figure 3. It has an ultraviolet–visible–near-infrared (UVN) module with three bands: ultraviolet (UV); UV-visible (UVIS); and near-infrared (NIR)—see Table 1—and a cooled shortwave infrared (SWIR) module with a single band. The figure shows how the UVN module and telescope and the SWIR module are mounted on a common base plate. The telescope and UVN module have a common structure, and the light from the telescope is fed into the SWIR module by relay optics.
The spectral properties of each of the spectrometers are shown in Table 1. The spatial resolution is 7×7km2 with the exception of bands 1 and 6. Band 1 has a larger ground pixel to allow good signal-to-noise given the low radiances for these wavelengths. Band 6 is used to obtain the most important cloud products and is produced at higher spatial resolution to ensure maximum spatial overlap of the cloud products with the other bands.
TROPOMI will fly as the single instrument on ESA's S5P mission. It is currently (January 2012) passing its preliminary design review status and about to have its first sub-system hardware testing. TROPOMI will continue to expand and improve the ozone layer data record, including the gases chemically active in the ozone destruction processes in the (ant-)arctic. Using its enhanced spatial resolution and sensitivity, it will make a major step forward in tracking air-quality-related products (tropospheric NO2, ozone, and aerosols) and thereby provide sub-urban-scale daily global maps of anthropogenic emissions. The methane measurements will show releases from warming permafrost in the northern areas, which may very well develop as another important climate gas next to CO2. We expect it to take a major step forward in measuring climate change and air quality from space and mapping anthropogenic emissions on a global scale.
The TROPOMI instrument is cofunded by the Netherlands Space Office and ESA, with ESA also providing (through Astrium Ltd.) the mission in the framework of the Global Monitoring for Environment and Security program (GMES). Dutch Space (Leiden) is the prime contractor and system integrator of TROPOMI. The Netherlands Organization for Applied Research (Delft) and the Netherlands Institute for Space Research (SRON) (Utrecht) are national team members working on the UVN and SWIR spectrometer channels. For the ESA-contributed elements, Dutch Space has subcontracted companies from GMES participating member states. The principal investigator is from Koninklijk Netherlands Meteorological Institute (De Bilt) with support from SRON. Both are represented in the S5P Mission Advisory Group.
Johan de Vries has worked in the area of atmospheric remote sensing for over 20 years, and has been involved in SCIAMACHY, GOME (Global Ozone Monitoring Experiment), OMI, and TROPOMI. His current work focuses on TROPOMI but includes other instrument developments such as ESA's follow-on of TROPOMI, the Sentinel 5 UVNS.
Robert Voors has worked in the field of remote sensing since 2000, first at the Royal Netherlands Meteorological Institute and since 2008 at Dutch Space. He holds a PhD in astrophysics from the University of Utrecht (1999).
