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Remote sensing of mole fraction

Remote sensing-based measurements of atmospheric greenhouse gas mole fraction rely on spectroscopic features of greenhouse and ah' quality gas molecules. Two general approaches are used: Methods that disperse the observed light spatially over a relatively broad wavelength range in order to observe single or multiple absoiption bands or those that are based on relatively narrow laser emissions. Dispersive methods range from the grating spectrometers used in the satellite instruments of NASA’s Orbiting Carbon Observatory (Crisp, 2017) to the Fourier Transform IR spectrometer used by the Japan Aerospace Exploration Agency’s GF1G observing Satellite ‘IBUKI’ (Nakajuna, 2017). These specialized instruments feature limited spectral range resulting in good resolution in regions of individual absoiption bands. Instruments with broader spectral range, e.g.. the TROPOMI instrument on the European Space Agency’s Sentinel-5P satellite (ESA, 2019), have a much wider spectral range to observe a larger number of analytes with less spectral resolution. These are focused on the ah quality trace gases, ozone. SO,, NO,, CO, CH4, and atmospheric particulates.

The frequency tunability and stability of single-frequency laser-based methods most often observe features of a single absoiption line hi a baud, similar to the approach described above for CRDS instruments. Remote sensing instruments project the laser beam over paths of hundreds of meters to several kilometers in the atmosphere to make either path-integrated or path-segregated measurements using on-line center, off-line laser frequency tuning. Optical frequency combs are laser sources haring a spectrum of emissions that is spread over an octave or a wavelength range of a factor of 2 (Cimdiff, 2003; J. Hall, 2006; Hanscli, 2006). In the near infrared, this range can extend from к 1 pm to ~ 2 pm, covering absorption features of several small molecules of interest to both greenhouse and air quality gas measurements. Frequency comb observations involving the measurement of multiple absorption bands simultaneously for multiple molecular species (Coddington, 2016) over kilometer ranges har e been reported. To date, laser-based methods have only been used in surface and airborne platform applications.

6.3.1 Low Earth Orbit (LEO) satellite instruments

Several satellite-bome CO, observing instruments are currently on-orbit: NASA’s Orbiting Carbon Observatory (OCO) satellites (Eldering, 2017; Ostennan May, 2015), the Japan Aerospace Exploration Agency’s Greenhouse Gases Observing Satellite (GOSAT) series (JAXA—Japan Aerospace Exploration Agency JAXA. 2019), GHGSat-Claire (GHGSat, Inc., 2019) that monitors both CO, and CH4, and China’s TanSat or CarbonSat (China Meteorlogical Administration, 2018). Recently, there have been several announcements of intent to put additional greenhouse gas observing satellites or groups of them into orbit (EDF, 2018; Bloomberg, 2019). These observing instruments are carried on satellite platforms in LEO, sun-synchronous, polar orbits such that the instruments view Earth’s surface in sunlight constantly with a near constant illumination angle. LEO satellites revisit the same surface location with fixed periodicity. For example, the OCO-2 satellite revisits the same spot on Earth every 16 days.

On-orbit observing instruments use sunlight reflected from Earth’s surface as then light source, resulting in two transits through the atmosphere. This increases observational sensitivity of column- averaged mole fraction. Designed to be sensitive over restricted wavelength regions, such instruments resolve the individual lines across all or most of an absorption band in order to improve mole fraction measurement accuracy. Interference of on-orbit observations of atmospheric column-averaged mole fractions arise for the presence of clouds and/or significant atmospheric particulate concentration in the atmospheric observing path. To account for these variations in effective atmospheric optical depth, the OCO series of instruments contain three spectrometers tuned to two CO, absorption bands, the 1.61 pm and 2.05 pm bands, and one to oxygen’s A absorption band near 765 nin. As an observational calibrant, molecular oxygen is well mixed and of nearly constant concentration and distribution in the atmosphere. This observing channel gives the means for simultaneous observation of atmospheric optical depth/ pathlength and detection of atmospheric scattering effects that interfere significantly with observations in the two CO, observing bands. The OCO series of instruments have associated optical systems that momentarily lock their view onto specific surface locations while flying overhead. For the OCO-2 instrument, a target track pass can last for up to 9 minutes while acquiring as many as 12.960 samples at local zenith angles that vary between 0° and 85°. These samples are obtained from ~ 2 x 3 km footprints on Earth’s surface. Target mode allows these to be closely spaced in a selected region, such as a city, thereby providing spatially dense mole fraction data over selected regions ofx 10 kilometers.

6.3.2 Geosynchronous Orbit (GEO) satellite instruments

A primary characteristic of greenhouse gas observing satellites in LEO is the relatively sparse coverage of the surface and atmospheric column over which measurements are made with a < 1.5° separation between ground tracks, or ~ 160 km (100 miles). Geostationary satellites travel in an orbit that matches the speed of Earth's rotation so that they remain above the same place on Earth’s surface constantly. Platforms in GEO provide observing instrument designs with the opportunity to cover all of the Earth’s surface viewed fr om that orbital location. Geostationary satellites are mainly used for communications, TV broadcasting, and weather observation. NASA, NOAA, and ESA all have satellite missions in geostationary orbit, e.g., the Geostationary Operational Environmental Satellites GOES-16 and 17 observe weather patterns and environmental parameters in the Northern Hemisphere. Although there are no greenhouse gas observing satellites in GEO, NASA has begun planning an observing instrument design with the potential to advance understanding of the global carbon cycle by mapping key carbon gases in the atmosphere from the GEO vantage point, a satellite designated GeoCARB (Moore, 2018). Placed above the Americas, such an instrument in GEO could provide sustained mole fraction observations from between ~ 50° North to ~ 50° South latitude, stretching from the southern tip of Hudson Bay to a few degrees north of Tierra del Fuego. South America. Column averaged mole fraction measurements of CO,. CH4, and CO would be available across the hemisphere. This instrument design will be based on OCO spectrometer designs with the addition of a CH4 channel and using the oxygen А-Baud total column calibration strategy of the OCO satellite series. А-Baud capability also supports observation of solar induced fluorescence (Sim, 2018; Koffi, 2015) that would provide additional information about photosynthetic activity of surface vegetation, potentially providing significantly improved quantification of CO, uptake across the Western Hemisphere. Instrument designs that could supply data at spatial resolutions of 5 to 10 kilometers at an estimated rate of ten million samples per day are being investigated. Such data is anticipated to allow determination of both the exchange of carbon between the atmosphere and the land masses of the Americas and the distribution and dynamics of CO,, CH4, and CO in the atmosphere not available otherwise. The GeoCarb launch date is currently estimated to be in the early 2020s.

 
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