Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS): Final Report of the ASCENDS Ad Hoc Science Definition Team

Improved remote sensing observations of atmospheric carbon dioxide (CO2) are critically needed to quantify, monitor, and understand the Earth's carbon cycle and its evolution in a changing climate. The processes governing ocean and terrestrial carbon uptake remain poorly understood,especially in dynamic regions with large carbon stocks and strong vulnerability to climate change,for example, the tropical land biosphere, the northern hemisphere high latitudes, and the Southern Ocean. Because the passive spectrometers used by GOSAT (Greenhouse gases Observing SATellite) and OCO-2 (Orbiting Carbon Observatory-2) require sunlit and cloud-free conditions,current observations over these regions remain infrequent and are subject to biases. These short comings limit our ability to understand and predict the processes controlling the carbon cycle on regional to global scales.In contrast, active CO2 remote-sensing techniques allow accurate measurements to be taken day and night, over ocean and land surfaces, in the presence of thin or scattered clouds, and at all times of year. Because of these benefits, the National Research Council recommended the National Aeronautics and Space Administration (NASA) Active Sensing of CO2 Emissions over Nights,Days, and Seasons (ASCENDS) mission in the 2007 report Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. The ability of ASCENDS to collect low-bias observations in these key regions is expected to address important gaps in our knowledge of the contemporary carbon cycle.The ASCENDS ad hoc Science Definition Team (SDT), comprised of carbon cycle modeling and active remote sensing instrument teams throughout the United States (US), worked to develop the mission's requirements and advance its readiness from 2008 through 2018. Numerous scientific investigations were carried out to identify the benefit and feasibility of active CO2 remote sensing measurements for improving our understanding of CO2 sources and sinks. This report summarizes their findings and recommendations based on mission modeling studies, analysis of ancillary meteorological data products, development and demonstration of candidate technologies, anddesign studies of the ASCENDS mission concept

On the ability of space-based passive and active remote sensing observations of CO2 to detect flux perturbations to the carbon cycle

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Atmospheres 123 (2018): 1460–1477, doi:10.1002/2017JD027836.Space-borne observations of CO2 are vital to gaining understanding of the carbon cycle in regions of the world that are difficult to measure directly, such as the tropical terrestrial biosphere, the high northern and southern latitudes, and in developing nations such as China. Measurements from passive instruments such as GOSAT and OCO-2, however, are constrained by solar zenith angle limitations as well as sensitivity to the presence of clouds and aerosols. Active measurements such as those in development for the Active Sensing of CO2 Emissions over Nights, Days and Seasons (ASCENDS) mission show strong potential for making measurements in the high-latitude winter and in cloudy regions. In this work we examine the enhanced flux constraint provided by the improved coverage from an active measurement such as ASCENDS. The simulation studies presented here show that with sufficient precision, ASCENDS will detect permafrost thaw and fossil fuel emissions shifts at annual and seasonal time scales, even in the presence of transport errors, representativeness errors, and biogenic flux errors. While OCO-2 can detect some of these perturbations at the annual scale, the seasonal sampling provided by ASCENDS provides the stronger constraint.NASA Grant Numbers: NNX15AJ27G, NNX15AH13G2018-07-2

Differential absorption lidar for volcanic CO(2) sensing tested in an unstable atmosphere.

Motivated by the need for an extremely durable and portable instrument to quantify volcanic CO2 we have produced a corresponding differential absorption lidar (DIAL). It was tested on a volcano (Vulcano, Italy), sensing a non-uniform volcanic CO2 signal under turbulent atmospheric conditions. The measured CO2 mixing ratio trend agrees qualitatively well but quantitatively poorly with a reference CO2 measurement. The disagreement is not in line with the precision of the DIAL determined under conditions that largely exclude atmospheric effects. We show evidence that the disagreement is mainly due to atmospheric turbulence. We conclude that excluding noise associated with atmospheric turbulence, as commonly done in precision analysis of DIAL instruments, may largely underestimate the error of measured CO2 concentrations in turbulent atmospheric conditions. Implications for volcanic CO2 sensing with DIAL are outlined. © 2015 Optical Society of America

Detectability of CO2 Flux Signals by a Space-Based Lidar Mission

Satellite observations of carbon dioxide (CO2) offer novel and distinctive opportunities for improving our quantitative understanding of the carbon cycle. Prospective observations include those from space-based lidar such as the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission. Here we explore the ability of such a mission to detect regional changes in CO2 fluxes. We investigate these using three prototypical case studies, namely the thawing of permafrost in the Northern High Latitudes, the shifting of fossil fuel emissions from Europe to China, and changes in the source-sink characteristics of the Southern Ocean. These three scenarios were used to design signal detection studies to investigate the ability to detect the unfolding of these scenarios compared to a baseline scenario. Results indicate that the ASCENDS mission could detect the types of signals investigated in this study, with the caveat that the study is based on some simplifying assumptions. The permafrost thawing flux perturbation is readily detectable at a high level of significance. The fossil fuel emission detectability is directly related to the strength of the signal and the level of measurement noise. For a nominal (lower) fossil fuel emission signal, only the idealized noise-free instrument test case produces a clearly detectable signal, while experiments with more realistic noise levels capture the signal only in the higher (exaggerated) signal case. For the Southern Ocean scenario, differences due to the natural variability in the ENSO climatic mode are primarily detectable as a zonal increase

Effect of Instrument Errors in the Retrieval of Anthropogenic Gas Distribution from Satellite

In this thesis the remote sensing is presented as a method for observing the distribution of trace gases in the Earth's atmosphere. In theoretical basis, the focus is set to the absorption spectroscopy, which makes use of absorption fingerprints of molecules to detect different gases. Four current hyperspectral satellite instruments are described that work on the principle of spectroscopy - GOME, SCIAMACHY, OMI and TROPOMI. In the practical part of the thesis analyses of the effect of the instrumental errors on trace gas retrieval are described as well as their results. Analyses have been done in the framework of TROPOMI development project with TIDE software. TIDE simulates the retrieval of trace gases with TROPOMI and was developed especially for this project

Development of China’s first space-borne aerosol-cloud high-spectral-resolution lidar: retrieval algorithm and airborne demonstration

Aerosols and clouds greatly affect the Earth’s radiation budget and global climate. Light detection and ranging (lidar) has been recognized as a promising active remote sensing technique for the vertical observations of aerosols and clouds. China launched its first space-borne aerosol-cloud high-spectral-resolution lidar (ACHSRL) on April 16, 2022, which is capable for high accuracy profiling of aerosols and clouds around the globe. This study presents a retrieval algorithm for aerosol and cloud optical properties from ACHSRL which were compared with the end-to-end Monte-Carlo simulations and validated with the data from an airborne flight with the ACHSRL prototype (A2P) instrument. Using imaging denoising, threshold discrimination, and iterative reconstruction methods, this algorithm was developed for calibration, feature detection, and extinction coefficient (EC) retrievals. The simulation results show that 95.4% of the backscatter coefficient (BSC) have an error less than 12% while 95.4% of EC have an error less than 24%. Cirrus and marine and urban aerosols were identified based on the airborne measurements over different surface types. Then, comparisons were made with U.S. Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) profiles, Moderate-resolution Imaging Spectroradiometer (MODIS), and the ground-based sun photometers. High correlations (R > 0.79) were found between BSC (EC) profiles of A2P and CALIOP over forest and town cover, while the correlation coefficients are 0.57 for BSC and 0.58 for EC over ocean cover; the aerosol optical depth retrievals have correlation coefficient of 0.71 with MODIS data and show spatial variations consistent with those from the sun photometers. The algorithm developed for ACHSRL in this study can be directly employed for future space-borne high-spectral-resolution lidar (HSRL) and its data products will also supplement CALIOP data coverage for global observations of aerosol and cloud properties

Q-Switched 2 Micron Solid-State Lasers and Their Applications

In this chapter, we overview the Q-switched 2 μm solid-state laser development achieved in recent years, including flash- and diode-pumped solid-state lasers based on active and passive modulators. In summary, active Q-switching is still the first choice for obtaining large pulse energy at 2 μm currently, while passive Q-switching based on saturable absorbers (SAs), especially the newly emerging broadband low-dimension nanomaterial, is becoming promising approach in generating Q-switched 2 μm lasers specially with high repetition rate, although the output power, pulse duration, and pulse energy needs further enhancement. Besides, some important applications of 2 μm lasers, such as medicine, laser radar, and infrared directional interference, have also been introduced in brief

Advanced exploitation of Ground-Based measurements for Atmospheric Chemistry and Climate Applications "AGACC"

We live in an era in which human activities are causing significant changes to the atmospheric environment which result in local to global consequences on the ecosystems. Changes in the atmospheric composition impact our climate via chemical and dynamical feedback mechanisms; in many instances they also affect air quality, and the health of the biosphere. Monitoring and understanding those changes and their consequences is fundamental to establish adequate actions for adaptation to and mitigation of the environmental changes. Furthermore, after implementation of regulatory measures like the Montreal Protocol, it is necessary to verify whether the measures are effective. This can only be achieved if we have adequate detection methods and a reliable long record of a series of key geophysical parameters. Thus the AGACC project contributes to the provision of basic new knowledge regarding the atmospheric composition and its changes, based on advanced groundbased monitoring, in combination with satellite and numerical modelling data. Its results are integrated in ongoing international research programmes. The general objective of AGACC has been to improve and extend the groundbased detection capabilities for a number of climate-related target species and, based hereupon, analyse past and present observations to derive new information about the atmospheric composition, its variability and long-term changes. Despite the advent of a growing and more performant fleet of Earth Observation satellites, ground-based observations are still indispensable to (1) guarantee long-term continuity, homogeneity and high quality of the data, and (2) to underpin the satellite data for calibration and (long-term) validation. A first target gas is atmospheric water vapour. It is the key trace gas controlling weather and climate. It is also the most important greenhouse gas in the Earth’s atmosphere. Its amount and vertical distribution are changing, but how and why? Especially in the upper troposphere - lower stratosphere, the radiative effects of changes in the water vapour are significant and should be quantified. The measurement of water vapour is a hot topic since several years. It is a challenge, because water vapour exhibits a large gradient in its concentration when going from the ground to the stratosphere, and because it is highly variable in time and space. For example, we have found that the time scale of the variations of the total water vapour amount at Jungfraujoch is in the order of minutes. In AGACC, we have therefore investigated various experimental techniques to measure the concentration of water vapour in the atmosphere, focusing on the total column as well as on the vertical distribution in the troposphere up to the lower stratosphere. The retrieval of water vapour vertical profiles and total columns from ground-based FTIR data has been initiated at three very different stations where correlative data for verification are available, namely Ukkel (± sea level, mid-latitude), Ile de La Réunion (± sea level, tropical) and Jungfraujoch (high altitude, mid-latitude), with promising results. In particular, at Jungfraujoch, it has been demonstrated that the precision of the FTIR integrated water vapour (IWV) measurements is of order 2%. The capability to retrieve individual isotopologues of water vapour, and to monitor their daily and diurnal variations, has also been demonstrated. This could open new ways to study in the future the role of water vapour in the radiative balance, the global circulation, precipitation etc. We also started joint exploitation of ground-based FTIR and satellite IASI data for water vapour and its isotopologues, in order to exploit fully the potential of the existing instrumentation. A correction method for the radiosoundings at Ukkel has been successfully implemented, resulting in a homogeneous and reliable time series from 1990 to 2008 from which trends in upper troposphere humidity (UTH) and tropopause characteristics have been derived. One observes a rising UTH until September 2001, followed by a decline, accompanied by a descent and heating of the tropopause up to the turning point and an ascent and cooling afterwards. The changes after September 2001 in the upper troposphere can be explained by surface heating and convective uplift. At Jungfraujoch, one does not observe any significant trend in the total water vapour abundance above the station over the 1988-2010 time period, although significant positive summer and negative winter trends have been detected. We have made a quantitative statistical comparison between ground-based FTIR, CIMEL, GPS and integrated (corrected) radio sounding measurements of the IWV at Ukkel. This work is important to better characterize the different sensors in order to exploit together different observations made by different instruments. A second target species is atmospheric aerosol. There is a very large variety of aerosol both from natural or anthropogenic origin. One of the reasons why they are so important is that they affect the optical properties of the atmosphere. In particular, it has been demonstrated in previous studies that the aerosols have a large impact on the quantity of harmful UV-B radiation received at the Earth’s surface. The latest IPCC Report also stressed that the radiative forcing caused by atmospheric aerosols is one of the largest uncertainties in determining the total radiative forcing in the atmosphere. Better monitoring capabilities of aerosol properties can therefore improve our understanding and forecasting of the atmospheric processes and evolution, and in particular of UV-B and climate changes. Several measurement techniques are now operational in the AGACC consortium for the ground-based monitoring of aerosol properties. These are the Brewer spectrometer and CIMEL observations at Ukkel, the latter contributing also to the AERONET network since July 2006, and the newly developed MAXDOAS observations. Unlike CIMEL and Brewer measurements, that provide the total Aerosol Optical Depth, it has been demonstrated that the MAXDOAS measurements also provide additional information about the vertical distribution of the aerosol extinction in the lowest kilometres of the troposphere. A better understanding of the ultimate capabilities of MAXDOAS aerosol remote sensing has been gained through participation to the international CINDI campaign (Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments ) in summer 2009. The combination of Brewer, CIMEL and MAXDOAS instruments gives us a remote-sensing dataset that will enable a more comprehensive characterization of the tropospheric aerosol optical properties. The usefulness of these aerosol observations has already been demonstrated in the improvement of the UVindex predictions for the general public. Another application is their use as input data in the retrieval of vertical profiles of tropospheric pollutants from MAXDOAS measurements, like tropospheric NO2 and formaldehyde. Third we have focused on a few climate-related trace gases. Changing greenhouse gas and aerosol concentrations directly affect the radiative budget of the atmosphere, and therefore climate. But many species known as pollutants like carbon monoxide (CO), nitrogen oxides (NOx) and hydrocarbons, - often related to fossil fuel or biomass burning -, also affect climate through their role in chemical reactions that produce tropospheric ozone, which is a well-known greenhouse gas, or that modify the lifetime of gases like methane, or the oxidation capacity of the atmosphere. Therefore in AGACC, we have focused on the measurement of a number of trace gases that are subject to changing concentrations, that directly or indirectly affect climate, and that are either difficult to monitor or that have not yet been measured from the ground. We have included attempts to observe distinctly some isotopologues, because the isotopic ratios observed in an airmass provide information on its history, and because the FTIR solar absorption measurements provide a rather unique capability hereto. The investigated species are the isotopologues of CH4 and CO, and hydrogen cyanide (HCN), as examples of biomass burning tracers, some hydrocarbons like formaldehyde (HCHO), ethylene (C2H4) and acetylene (C2H2), and HCFC-142b, a replacement product for CFCs and a greenhouse gas. In many cases, retrieval strategies had to be adapted when going from one site to another with different atmospheric conditions, especially when the local humidity and abundances are very different as is the case between Jungfraujoch (dry, high altitude, mid-latitude) and Ile de La Réunion (humid, low altitude, low latitude). Still we have been able to show the feasibility of retrieving particular trace gas information even under difficult conditions. Many of our results have been compared to correlative data, to validate the approach and to gain complementary information. It is also important to note that the retrieval strategies developed in AGACC have regularly been presented to the global Network for the Detection of Atmospheric Composition Change (NDACC) UV-Vis and Infrared communities and have often been adopted by others or even proposed for adoption as a standard in the community (e.g., for hydrogen cyanide (HCN)). In particular: We have been able to study the seasonal variations of HCN at the Jungfraujoch and at Ile de La Réunion, and to show the dominant impact of biomass burning. Formaldehyde was studied in much detail at Ukkel, Jungfraujoch and Ile de la Réunion. The challenge for detection at Jungfraujoch is the small abundance (about 10 times smaller than at Ukkel and Ile de La Réunion); a particular observation strategy was developed successfully, resulting in a time series that already shows the day-to-day and seasonal variations. At Ile de La Réunion, comparisons of FTIR, MAXDOAS, satellite and model data have (1) shown the good agreement between the various data sets, but also, (2), the variability of HCHO (diurnal, seasonal, day-to-day), and (3), thanks to the complementarities of the various data sets, they have enabled us to learn more about the long-range transport of Non-methane Volatile Organic Compounds (NMVOCS, precursors of HCHO) and deficiencies in the models. It was shown that fast, direct transport of NMVOCS from Madagascar has a significant impact on the HCHO abundance and its variability at Ile de La Réunion, and that this is underestimated in the model. Significant progress was made as to the detection of 13CH4 and CH3D from ground-based FTIR observations, both at Jungfraujoch and Ile de La Réunion. To our knowledge, it is the first time that a d13C data set is derived from ground-based FTIR observations. More work is needed to improve the CH3D retrieval at Ile de La Réunion, and to interpret the results, in combination with models. Also for the first time, 12CO and 13CO have been retrieved individually at Jungfraujoch. The d13C time series shows significant seasonal and interannual changes. As to the hydrocarbon ethylene, it is shown that it can be detected at Jungfraujoch only in spectra at low solar elevation, given its small atmospheric abundance. Regarding acetylene, the observed time series at Jungfraujoch and Ile de La Réunion show clear seasonal variations and enhancements due to the impact of biomass burning events, correlated with enhancements in CO, C2H6 and HCN. It is not clear yet whether we can reliably retrieve the concentration of HCFC- 142b, a replacement product that is increasing strongly in the troposphere. New line parameters for the interfering species HFC-134a are required to confirm/infirm the preliminary results. This highlights again the importance of the laboratory work for providing such parameters. Improved line parameters have been obtained for water vapour and its isotopologues, ethylene and formic acid. These AGACC results have been integrated in the international spectroscopic databases. We also showed that line intensities available around 2096 cm–1 for the 13C16O isotopologue of carbon monoxide in the HITRAN database seem to be accurate to 2%. We failed to improve line intensities for the 13.6 μm region of acetylene. The new data sets that have been derived in AGACC from FTIR and MAXDOAS observations have been archived in the NDACC data centre, where they are available for users (generally modelers and satellite teams). In addition, they are stored locally and are available to users upon request. AGACC results have been reported to the international scientific community, via the literature, via integration in geophysical or spectroscopic databases, and via participation to international research initiatives like the Atmospheric Water Vapour in the Climate System (WAVACS) Cost Action, the International Space Science Institute (ISSI) Working Group on Atmospheric Water Vapour, the International Union of Pure and Applied Chemistry (IUPAC) project, the International CINDI campaign, etc. The results have already found important scientific applications. A few examples are worth mentioning: the re-evaluation of methane emissions in the tropics from SCIAMACHY based on the new H2O spectroscopy, and the improved retrievals of HCOOH from the satellite experiments ACE-FTS and IASI, and from the ground. In the longer-term, the AGACC results will no doubt benefit the research in atmospheric sciences –in particular in the monitoring of its composition changes–, which is the fundamental basis of environmental assessment reports for supporting policy makers.Advanced exploitation of ground-based measurements for atmospheric chemistry and climate applications "AGACC

A Greenhouse-Gas Information System: Monitoring and Validating Emissions Reporting and Mitigation

This study and report focus on attributes of a greenhouse-gas information system (GHGIS) needed to support MRV&V needs. These needs set the function of such a system apart from scientific/research monitoring of GHGs and carbon-cycle systems, and include (not exclusively): the need for a GHGIS that is operational, as required for decision-support; the need for a system that meets specifications derived from imposed requirements; the need for rigorous calibration, verification, and validation (CV&V) standards, processes, and records for all measurement and modeling/data-inversion data; the need to develop and adopt an uncertainty-quantification (UQ) regimen for all measurement and modeling data; and the requirement that GHGIS products can be subjected to third-party questioning and scientific scrutiny. This report examines and assesses presently available capabilities that could contribute to a future GHGIS. These capabilities include sensors and measurement technologies; data analysis and data uncertainty quantification (UQ) practices and methods; and model-based data-inversion practices, methods, and their associated UQ. The report further examines the need for traceable calibration, verification, and validation processes and attached metadata; differences between present science-/research-oriented needs and those that would be required for an operational GHGIS; the development, operation, and maintenance of a GHGIS missions-operations center (GMOC); and the complex systems engineering and integration that would be required to develop, operate, and evolve a future GHGIS

Optical Gas Sensing: Media, Mechanisms and Applications

Optical gas sensing is one of the fastest developing research areas in laser spectroscopy. Continuous development of new coherent light sources operating especially in the Mid-IR spectral band (QCL—Quantum Cascade Lasers, ICL—Interband Cascade Lasers, OPO—Optical Parametric Oscillator, DFG—Difference Frequency Generation, optical frequency combs, etc.) stimulates new, sophisticated methods and technological solutions in this area. The development of clever techniques in gas detection based on new mechanisms of sensing (photoacoustic, photothermal, dispersion, etc.) supported by advanced applied electronics and huge progress in signal processing allows us to introduce more sensitive, broader-band and miniaturized optical sensors. Additionally, the substantial development of fast and sensitive photodetectors in MIR and FIR is of great support to progress in gas sensing. Recent material and technological progress in the development of hollow-core optical fibers allowing low-loss transmission of light in both Near- and Mid-IR has opened a new route for obtaining the low-volume, long optical paths that are so strongly required in laser-based gas sensors, leading to the development of a novel branch of laser-based gas detectors. This Special Issue summarizes the most recent progress in the development of optical sensors utilizing novel materials and laser-based gas sensing techniques