Greenhouse Gases 1

The GHG-CCI project (http://www.esa-ghg-cci.org/) is one of several projects of the European Space Agency’s (ESA) Climate Change Initiative (CCI). The goal of the CCI is to generate and deliver data sets of various satellite-derived Essential Climate Variables (ECVs) in line with GCOS (Global Climate Observing System) requirements. The “ECV Greenhouse Gases” (ECV GHG) is the global distribution of important climate relevant gases – specifically atmospheric CO₂ and CH₄ - with a quality sufficient to obtain information on regional CO₂ and CH₄ sources and sinks. The main goal of GHG-CCI is to generate long-term highly accurate and precise time series of global near-surface sensitive satellite observations of CO₂ and CH₄.  SCIAMACHY on ENVISAT and TANSO-FTS/GOSAT are currently the two main satellite instruments used within the GHG-CCI project as their spectral radiance observations in the near-infrared range of the electromagnetic spectrum permit retrievals of CO₂ and CH₄ columns that are sensitive down to the Earth's surface and because multi-year time series can be derived from these data. In addition other satellite instruments such as IASI/METOP and MIPAS/ENVISAT are also used. In the presentation an overview about the latest data products will be given and selected highlights from using the GHG-CCI data products to address important questions related to the sources and sinks of CO₂ and methane will be presented.

The Methane Remote Sensing Lidar Mission (MERLIN), in phase B until end 2015, is a joint cooperation between France and Germany on the development, launch and operation of a space LIDAR dedicated to the retrieval of total methane (CH₄) atmospheric columns. Atmospheric methane is the second most anthropogenic gas, contributing 20% to climate radiative forcing but also plying an important role in atmospheric chemistry as a precursor of tropospheric ozone and low-stratosphere water vapour.

For the first time, measurements of atmospheric composition will be performed from space thanks to an IPDA (Integrated Path Differential Absorption) LIDAR (Light Detecting And Ranging), with a precision (target 20 ppb for a 50km aggregation along the trace) and accuracy (target 3 ppb) sufficient to improve the constraints on methane fluxes compared to current observation networks. The very low systematic error target is ambitious compared to current methane space mission, but achievable because of the differential active measurements of MERLIN, which guarantees almost no contamination by aerosols or water vapour cross-sensitivity. As an active mission, MERLIN will deliver data for all seasons and all altitudes, day and night.

Here, we present the MERLIN mission and its objectives in terms of reduction of uncertainties on methane surface emissions. To do so, we propose an OSSE analysis (observing system simulation experiment) to estimate the uncertainty reduction brought by MERLIN. The originality of our system is to transfer both random and systematic errors from the observation space to the flux space, thus providing more realistic error reductions than usually provided in OSSE only using the random part of errors. To do so, a precise analysis of causes of errors has been done for the MERLIN mission and is also presented.

The sources and sinks of the greenhouse gases (GHG) carbon dioxide (CO₂) and methane (CH₄), are the main drivers of climate change, the most important environmental challenge in the 21^st century. Despite its importance, Europe has no satellite system planned to quantify anthropogenic emissions and biogenic fluxes of CO₂. The CarbonSat concept was developed to close this important observational gap and is built on the SCIAMACHY, GOSAT, and OCO-2 research programs and lessons learned. CarbonSat was selected by ESA as an Earth Explorer Opportunity candidate mission (EE8), is developed up to Phase A/B, but not recommended by ESAC for implementation as Earth Explorer. Nevertheless there is still a strong need for CarbonSat. The unique feature of the CarbonSat concept is its “GHG imaging capability”, which is achieved by combining high spatial resolution (6 km²) and good spatial coverage (up to 240 km wide swath, with contiguous ground sampling). CarbonSat aims to deliver spatially-resolved, time varying global estimates of dry column mixing ratios of CO₂ and CH₄ with high precision (goal ~1 ppm and ~10 ppb, respectively) and relative accuracy (<0.5 ppm and < 5 ppb, respectively on regional scales).  Benefiting from its imaging capabilities along and across track, CarbonSat will be the first satellite mission to systematically image small scale emission hot spots of CO₂ (e.g., cities, volcanoes, industrial areas) and CH₄ (e.g., fossil fuel production, landfills, seeps) and to discriminate and quantify their emissions. CarbonSat will provide at least an order of magnitude larger number of cloud-free CO₂ soundings than OCO-2 thereby revealing also regional details of natural fluxes, allowing discriminating among regions and across seasons the largest carbon sinks and the areas that may loose CO₂ or CH₄ to the atmosphere. This will allow a better separation of natural and anthropogenic GHG sources and sinks from the local to the global scale. 

Recent results from CarbonSat scientific studies documenting the expected data quality and potential application areas will be presented.

The Greenhouse gases Observing SATellite (GOSAT) has operated for about seven years since January 23, 2009. During the past seven years, almost all of the GOSAT standard data products were opened to general users, and many of them went through several updates. Also, some of the GOSAT research data products have been delivered to the registered researchers. The column concentrations of major greenhouse gases (GHGs), namely carbon dioxide (XCO₂) and methane (XCH₄), were retrieved from GOSAT spectral data, and their precisions (excluding biases) are now at about 0.5% and 0.7 %, respectively. These concentration data were used to estimate the monthly surface fluxes of CO₂ and CH₄ on sub-continental and ocean-basin scales in the first four years of GOSAT operation (2009 – 2013). The concentration data were also utilized to monitor GHGs’ temporal and spatial changes. Various reports on the results of GOSAT data analysis have appeared in peer-reviewed journals so far.

Although GOSAT went through some technical difficulties in the functioning of its solar paddle, sensor pointing mechanism, and cryo-cooler of thermal infrared detector in 2014 and 2015, JAXA has overcome these difficulties somehow. However, the characteristics of the Fourier transform spectrometer onboard were therefore altered to some degree. The influence of this alteration on the retrieved concentrations has been detected.

In this presentation, we will summarize the seven-year-long GHG observation by GOSAT and present the global distributions of the GHG concentrations and the surface flux estimates. We will explain the changes in the characteristics of the GOSAT data products three times after June 2014, February 2015, and September 2015 owing to the influence of the technical difficulties. Also, we will touch on the current status of researches conducted within the framework of the GOSAT Research Announcement.

This paper aims at contributing to the ongoing effort for progressing on the space-based observing system capability to monitor atmospheric concentrations and anthropogenic point source emissions of GHG. Among all recent and current work and initiatives proposed by European scientists, space agencies, industries, it is proposed here to discuss two separated but complementary studies, potentially able to provide new insights concerning two key issues:

Defining instrument/mission solutions able to address very demanding requirements in a limited and cost effective budget.

Progressing on the capability to accurately derive local-scale GHG emissions from space-based instruments with imaging capabilities.

1. The preliminary analysis of a carbon dioxide and methane point source detection concept, GEM (Gas Emission Mission, see the corresponding companion abstract submitted for poster presentation) has been supported by ESA in the context of its “Space and Energy” cross-agency theme of activities. Starting from user needs from the Energy Sectors, mission objectives have been derived, including the detection and quantification of surface fluxes at local scales for anthropogenic emissions of CH₄ and CO₂ from power plants, combustion devices, coal mines, and network infrastructures.

In order to properly address such challenging objective, the proposed mission strategy aims at measuring the space/time structures of the column average mixing ratio XCO₂ and XCH₄ in the atmosphere with very high quality : high horizontal resolution (1 km) and imaging capability (typically 20 km x 20 km) for capturing the concentration distribution (plume) related to point source emissions; high spectral and radiometric performances for achieving high accuracy in the atmospheric content of the two target gases (XCO₂ : random error 1 ppm, systematic error 0.2 – 0.5 ppm; XCH₄: random error 5 - 10 ppb; systematic error 1 - 3 ppb).

In order to define an instrument concept with a reduced mass/reduced power budget, innovative strategies for coverage and pointing from LEO platform, for dealing with clouds, and for L2 products retrieval, are proposed. Pro and cons for achieving the expected performances in terms of level 2/level 3 products (GHG atmospheric content and their spatial gradients associated with atmospheric signatures of anthropogenic GHG sources) are discussed.

2. A critical aspect for the comprehensive assessment of the performances and how these performances fit with the final services and user needs is the derivation of level 4 products (GHG surface emissions) from atmospheric content measurements. In the context of the LOGOFLUX scientific study supported by ESA for the preparation of the CARBONSAT mission, dedicated work has been initiated to analyze the capabilities of a space mission measuring GHG atmospheric concentration with imagery capabilities (as proposed in the CARBONSAT concept or in the GEM preliminary study), to estimate emissions of point sources (individual power plants, landfills, etc.). A Large Eddy Simulation (LES) model was used to estimate the spatio-temporal variability of CO₂/CH₄ near very strong point sources. In support to the review and consolidation of state-of-the-art point source flux inversions approaches, LES simulations demonstrate and quantify the highly intermittent nature of the gas plume in the vicinity of the emission source. The quantification of this variability, as well as the characterisation of the differences between the LES and classical Gaussian approaches, have been done in order to well characterise and consolidate flux retrieval performances at local scales from point sources. A comprehensive assessment of error sources and of the corresponding error budget is discussed.