Paul J. J. Tol

Global column‐averaged methane mixing ratios from 2003 to 2009 as derived from SCIAMACHY: Trends and variability

After a decade of stable or slightly decreasing global methane concentrations, ground-based in situ data show that CH_4 began increasing again in 2007 and that this increase continued through 2009. So far, space-based retrievals sensitive to the lower troposphere in the time period under consideration have not been available. Here we report a long-term data set of column-averaged methane mixing ratios retrieved from spectra of the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) instrument onboard Envisat. The retrieval quality after 2005 was severely affected by degrading detector pixels within the methane 2ν_3 absorption band. We identified the most crucial problems in SCIAMACHY detector degradation and overcame the problem by applying a strict pixel mask as well as a new dark current characterization. Even though retrieval precision after the end of 2005 is invariably degraded, consistent methane retrievals from 2003 through 2009 are now possible. Regional time series in the Sahara, Australia, tropical Africa, South America, and Asia show the methane increase in 2007–2009, but we cannot yet draw a firm conclusion concerning the origin of the increase. Tropical Africa even seems to exhibit a negative anomaly in 2006, but an impact from changes in SCIAMACHY detector degradation cannot be excluded yet. Over Assakrem, Algeria, we observed strong similarities between SCIAMACHY measurements and ground-based data in deseasonalized time series. We further show long-term SCIAMACHY xCH_4 averages at high spatial resolution that provide further insight into methane variations on regional scales. The Red Basin in China exhibits, on average, the highest methane abundance worldwide, while other localized features such as the Sudd wetlands in southern Sudan can also be identified in SCIAMACHY xCH_4 averages.

Characterization results of the TROPOMI Short Wave InfraRed detector

The TROPOMI Earth-observing instrument is the single payload on board ESAs Sentinel-5 Precursor mission. It is the successor of the Sciamachy instrument (ESA ENVISAT) and the OMI instrument (NASA EOS/Aura), and combines and improves the best of both instruments. TROPOMI copies the push broom observation geometry of OMI allowing for daily global coverage due to its instantaneous field of view of 108 degrees, or 2600 km swath on ground. From Sciamachy the 2305 - 2385 nm Short-Wave Infra-Red (SWIR) observational band is copied with which methane and carbon monoxide are observed. This paper reports on the development of the SWIR detector module and the detailed characterization of the 1000x256 SWIR Saturn detector array produced by Sofradir (F) as measured with the SRON-developed Front-End Electronics. The detailed characterization comprises not only the regular properties such as dark current, noise and photo-response, but also more complex characteristics including non-linearity and memory. Characterization of the detection module was performed for all operational parameters: detector temperature (135 - 145 K), bias voltage and integration time. Thanks to the detector-characterization program, the operational clocking of the detector could be optimized, resulting in significantly improved performance.

Breadboarding activities of the TROPOMI-SWIR module

The TROPOMI instrument concept is part of the TRAQ mission proposal to ESA in response to the Call for Ideas in 2005. TRAQ (TRopospheric composition and Air Quality) has been accepted for a further pre-phase A study for the next Earth Explorer core Mission. A very similar instrument has been proposed for the CAMEO platform to the US National Research Council decadal study, which has also been accepted for further study. TROPOMI is a nadir-viewing grating-based imaging spectrometer using the Dutch OMI and SCIAMACHY heritage. It includes an UV-VIS-NIR module that consists of three UV-VIS channels continuously covering the 270-490 nm range to determine O3, NO2, HCHO, SO2, aerosols and a NIR-channel covering 710-775 nm for cloud detection and information on the aerosol height distribution using the oxygen A band. TROPOMI also includes a SWIR module covering 2305-2385 nm that mainly focuses on determination of CO and CH4 total columns. All species are measured with sensitivity down to the Earths surface, thus addressing issues of anthropogenic emissions and their impact on air quality and climate. In the TRAQ mission, unique diurnal time sampling with up to 5 daytime observations over midlatitude regions (Europe, North-America, China) is foreseen by using a non-sun-synchronous, medium-inclination drifting orbit and a 2600 km wide observational swath. Several more general aspects related to the TROPOMI instrument are discussed in a separate paper in this conference. This paper focuses on the development of the SWIR module. A breadboard model (BBM) has been designed and constructed which is as much as possible functionally flight representative. Critical technologies to be demonstrated with the BBM are the SWIR HgCdTe-based 2D focal plane array, the on-board SWIR calibration LED, and in particular, the SRON/TNO developed silicon-based immersed grating that allows a hugely reduced instrument volume. In the presentation the results of a performance analysis of the TROPOMI-SWIR channel will be discussed, as well as results of the detector characterization program on a representative off-the-shelf FPA, and details of the photolithographic production of the immersed grating.

A set of innovative immersed grating based spectrometer designs for METIS

We present innovative, immersed grating based optical designs for the SMO (Spectrograph Main Optics) module of the Mid-infrared E-ELT Imager and Spectrograph, METIS. The immersed grating allows a significant reduction of SMO volume compared to conventional echelle grating designs, because the diffraction takes place in high refractive index silicon. Additionally, using novel optimization techniques and technical solutions in silicon micromachining offered by the semiconductor industry, further improvements can be achieved. We show optical architectures based on compact, double-pass Three Mirror Anastigmat (TMA) designs, which appear advantageous in terms of one or several of the following: optical performance, reduction of volume, ease of manufacturing and testing. We explore optical designs, where the emphasis is put on manufacturability and we investigate optical solutions, where the ultimate goal is the highest possible optical performance. These novel, silicon immersed grating based design concepts are applicable for future earth and space based spectrometers.

Characterization and correction of stray light in TROPOMI-SWIR

The shortwave infrared (SWIR) spectrometer module of the Tropospheric Monitoring Instrument (TROPOMI), on board the ESA Copernicus Sentinel-5 Precursor satellite, is used to measure atmospheric CO and methane columns. For this purpose, calibrated radiance measurements are needed that are minimally contaminated by instrumental stray light. Therefore, a method has been developed and applied in an on-ground calibration campaign to characterize stray light in detail using a monochromatic quasi-point light source. The dynamic range of the signal was extended to more than 7 orders of magnitude by performing measurements with different exposure times, saturating detector pixels at the longer exposure times. Analysis of the stray light indicates about 4.4 % of the detected light is correctable stray light. An algorithm was then devised and implemented in the operational data processor to correct inflight SWIR observations in near-real time, based on Van Cittert deconvolution. The stray light is approximated by a far-field kernel independent of position and wavelength and an additional kernel representing the main reflection. Applying this correction significantly reduces the stray-light signal, for example in a simulated dark forest scene close to bright clouds by a factor of about 10. Simulations indicate that this reduces the stray-light error sufficiently for accurate gas-column retrievals. In addition, the instrument contains five SWIR diode lasers that enable long-term, in-flight monitoring of the stray-light distribution.

Design, simulation and test of silicon immersed gratings: key to compact spectrometers in the short-wave infrared

We present results of our integrated approach to the development of novel diffraction gratings. At SRON we manufacture prism-shaped silicon immersed gratings. Diffraction takes place inside the high-refractive index medium, boosting the resolving power and the angular dispersion. This enables highly compact spectrometer designs. We are continuously improving the cycle of design, simulation and test to create custom gratings for space and ground-based spectroscopic applications in the short-wave infrared wavelength range. Applications are space-based monitoring of greenhouse and pollution gases in the Earth atmosphere and ground-based SWIR spectroscopy for, a.o., characterization of exo-planet atmospheres [1]. We make gratings by etching V-shaped grooves in mono-crystalline silicon. The groove facets are aligned with the crystal lattice yielding a smooth and highly deterministic groove shape. This enables us to predict the polarized efficiency performance accurately by simulation. Feeding back manufacturing tolerances from our production process, we can also determine reliable error bars for the predicted performance. Combining the simulated values for polarized efficiency with ray-tracing, we can optimize the shape of the grating prism to eliminate unwanted internal reflections. In this contribution we present the architecture of our design and simulation platform as well as a description of test setups and typical results.

Results of TROPOMI-SWIR in-flight calibration (Conference Presentation)

TROPOMI (the TROPOspheric Monitoring Instrument) is the single instrument aboard the ESA’s Copernicus Sentinel-5 Precursor (S-5P) satellite launched on October 13, 2017 . S-5P is dedicated to monitoring our atmosphere over a period of at least 7 years. Through spectral absorption features within reflected sunlight, TROPOMI measures total column densities of a large number of gases relevant for air quality and climate. This is done at unprecedented spatial resolution of approximately 3.5-7x7 km2. At this resolution, air pollution at scales of midsized cities can be distinguished from its surroundings. In addition, it can be used to estimate other air pollutant quantities such as aerosols. Each gas has a unique spectral fingerprint across the TROPOMI channels, which range from the UV to the infrared. The Short Wavelength Infrared channel, or SWIR, measures reflected sunlight between wavelengths of 2305 to 2385 nanometers. This channel has been designed to accurately determine methane (CH4) and carbon monoxide (CO) columns To obtain scientifically useful values for both gas species, measured spectra require the SWIR channel to be calibrated to a very high precision and accuracy. The requirements are particularly important for the CH4 product to achieve the ambitious S-5P mission goals. A reliable CH4 product will prove invaluable to the atmospheric scientific community in the next decade. The first six months after the launch of S5-P were used a.o. to execute an in-flight calibration campaign. This campaign was performed as a complement to the on-ground calibration campaign. It was designed to (a) take calibration measurements that were unable to be done on-ground, (b) validate on-ground calibration and (c) prepare for the instrument calibration monitoring during regular operations. Here, we report the calibration results obtained by the SRON Level 1 team for the TROPOMI-SWIR channel. This includes the correct method and settings for background corrections (both dark flux and offset), determination of the initial pixel quality map, the evolution of the background correction and quality map during the first 10 months of flight, validation of the on-ground calibration of the instrumental spectral response function (ISRF) and stray light correction.

Application of new techniques in the calibration of the TROPOMI-SWIR instrument (Conference Presentation)

The Tropospheric Monitoring Instrument (TROPOMI) on-board the Sentinel-5 Precursor satellite is an Earth-observing spectrometer with bands in the ultraviolet, visible, near infrared and short-wave infrared (SWIR). It provides daily global coverage of atmospheric trace gases relevant for tropospheric air quality and climate research. Three new techniques will be presented that are unique for the TROPOMI-SWIR spectrometer. The retrieval of methane and CO columns from the data of the SWIR band requires for each detector pixel an accurate instrument spectral response function (ISRF), i.e. the normalized signal as a function of wavelength. A new determination method for Earth-observing instruments has been used in the on-ground calibration, based on measurements with a SWIR optical parametric oscillator (OPO) that was scanned over the whole TROPOMI-SWIR spectral range. The calibration algorithm derives the ISRF without needing the absolute wavelength during the measurement. The same OPO has also been used to determine the two-dimensional stray-light distribution for each SWIR pixel with a dynamic range of 7 orders. This was achieved by combining measurements at several exposure times and taking saturation into account. The correction algorithm and data are designed to remove the mean stray-light distribution and a reflection that moves relative to the direct image, within the strict constraints of the available time for the L01b processing. A third new technique is an alternative calibration of the SWIR absolute radiance and irradiance using a black body at the temperature of melting silver. Unlike a standard FEL lamp, this source does not have to be calibrated itself, because the temperature is very stable and well known. Measurement methods, data analyses, correction algorithms and limitations of the new techniques will be presented.

Performance of silicon immersed gratings: measurement, analysis, and modeling

The use of Immersed Gratings offers advantages for both space- and ground-based spectrographs. As diffraction takes place inside the high-index medium, the optical path difference and angular dispersion are boosted proportionally, thereby allowing a smaller grating area and a smaller spectrometer size. Short-wave infrared (SWIR) spectroscopy is used in space-based monitoring of greenhouse and pollution gases in the Earth atmosphere. On the extremely large telescopes currently under development, mid-infrared high-resolution spectrographs will, among other things, be used to characterize exo-planet atmospheres. At infrared wavelengths, Silicon is transparent. This means that production methods used in the semiconductor industry can be applied to the fabrication of immersed gratings. Using such methods, we have designed and built immersed gratings for both space- and ground-based instruments, examples being the TROPOMI instrument for the European Space Agency Sentinel-5 precursor mission, Sentinel-5 (ESA) and the METIS (Mid-infrared E-ELT Imager and Spectrograph) instrument for the European Extremely Large Telescope. Three key parameters govern the performance of such gratings: The efficiency, the level of scattered light and the wavefront error induced. In this paper we describe how we can optimize these parameters during the design and manufacturing phase. We focus on the tools and methods used to measure the actual performance realized and present the results. In this paper, the bread-board model (BBM) immersed grating developed for the SWIR-1 channel of Sentinel-5 is used to illustrate this process. Stringent requirements were specified for this grating for the three performance criteria. We will show that –with some margin– the performance requirements have all been met.