Methane (CH 4 ) impacts climate as the second strongest anthropogenic greenhouse gas and air quality by influencing tropospheric ozone levels. Space-based observations have identified the Four Corners region in the Southwest United States as an area of large CH 4 enhancements. We conducted an airborne campaign in Four Corners during April 2015 with the next-generation Airborne Visible/Infrared Imaging Spectrometer (near-infrared) and Hyperspectral Thermal Emission Spectrometer (thermal infrared) imaging spectrometers to better understand the source of methane by measuring methane plumes at 1-to 3-m spatial resolution. Our analysis detected more than 250 individual methane plumes from fossil fuel harvesting, processing, and distributing infrastructures, spanning an emission range from the detection limit ∼ 2 kg/h to 5 kg/h through ∼ 5,000 kg/h. Observed sources include gas processing facilities, storage tanks, pipeline leaks, and well pads, as well as a coal mine venting shaft. Overall, plume enhancements and inferred fluxes follow a lognormal distribution, with the top 10% emitters contributing 49 to 66% to the inferred total point source flux of 0.23 Tg/y to 0.39 Tg/y. With the observed confirmation of a lognormal emission distribution, this airborne observing strategy and its ability to locate previously unknown point sources in real time provides an efficient and effective method to identify and mitigate major emissions contributors over a wide geographic area. With improved instrumentation, this capability scales to spaceborne applications [Thompson DR, et al. (2016)

Abstract. Carbon dioxide (CO 2 ) is the most important anthropogenic greenhouse gas (GHG) causing global warming. The atmospheric CO 2 concentration increased by more than 30% since pre-industrial times -primarily due to burning of fossil fuels -and still continues to increase. Reporting of CO 2 emissions is required by the Kyoto protocol. Independent verification of reported emissions, which are typially not directly measured, by methods such as inverse modeling of measured atmospheric CO 2 concentrations is currently not possible globally due to lack of appropriate observations. Existing satellite instruments such as SCIAMACHY/ENVISAT and TANSO/GOSAT focus on advancing our understanding of natural CO 2 sources and sinks. The obvious next step for future generation satellites is to also constrain anthropogenic CO 2 emissions. Here we present a promising satellite remote sensing concept based on spectroscopic measurements of reflected solar radiation and show, using power plants as an example, that strong localized CO 2 point sources can be detected and their emissions quantified. This requires mapping the atmospheric CO 2 column distribution at a spatial resolution of 2×2 km 2 with a precision of 0.5% (2 ppm) or better. We indicate that this can be achieved with existing technology. For a single satellite in sun-synchronous orbit with a swath width of 500 km, each power plant (PP) is overflown every 6 days or more frequent. Based on the MODIS cloud mask data product we conservatively estimate that typically 20 sufficiently cloud free overpasses per PP can be achieved every year. We found that for typical wind speeds in the range of 2-6 m/s the statistical uncertainty of the retrieved Correspondence to: M. Buchwitz (michael.buchwitz@iup.physik.unibremen.de) PP CO 2 emission due to instrument noise is in the range 1.6-4.8 MtCO 2 /yr for single overpasses. This corresponds to 12-36% of the emission of a mid-size PP (13 MtCO 2 /yr). We have also determined the sensitivity to parameters which may result in systematic errors such as atmospheric transport and aerosol related parameters. We found that the emission error depends linearly on wind speed, i.e., a 10% wind speed error results in a 10% emission error, and that neglecting enhanced aerosol concentrations in the PP plume may result in errors in the range 0.2-2.5 MtCO 2 /yr, depending on PP aerosol emission. The discussed concept has the potential to contribute to an independent verification of reported anthropogenic CO 2 emissions and therefore could be an important component of a future global anthropogenic GHG emission monitoring system. This is of relevance in the context of Kyoto protocol follow-on agreements but also allows detection and monitoring of a variety of other strong natural and anthropogenic CO 2 and CH 4 emitters. The investigated instrument is not limited to these applications as it has been specified to also deliver the data needed for global regionalscale CO 2 and CH 4 surface flux inverse modeling.

Abstract. MAMAP is an airborne passive remote sensing instrument designed to measure the dry columns of methane (CH 4 ) and carbon dioxide (CO 2 ). The MAMAP instrument comprises two optical grating spectrometers: the first observing in the short wave infrared band (SWIR) at 1590-1690 nm to measure CO 2 and CH 4 absorptions, and the second in the near infrared (NIR) at 757-768 nm to measure O 2 absorptions for reference/normalisation purposes. MAMAP can be operated in both nadir and zenith geometry during the flight. Mounted on an aeroplane, MAMAP surveys areas on regional to local scales with a ground pixel resolution of approximately 29 m × 33 m for a typical aircraft altitude of 1250 m and a velocity of 200 km h −1 . The retrieval precision of the measured column relative to background is typically 1 % (1σ ). MAMAP measurements are valuable to close the gap between satellite data, having global coverage but with a rather coarse resolution, on the one hand, and highly accurate in situ measurements with sparse coverage on the other hand. In July 2007, test flights were performed over two coal-fired power plants operated by Vattenfall Europe Generation AG: Jänschwalde (27.4 Mt CO 2 yr −1 ) and Schwarze Pumpe (11.9 Mt CO 2 yr −1 ), about 100 km southeast of Berlin, Germany. By using two different inversion approaches, one based on an optimal estimation scheme to fit Gaussian plume models from multiple sources to the data, and another using a simple Gaussian integral method, the emission rates can be determined and compared with emisCorrespondence to: T. Krings (thomas.krings@iup.physik.unibremen.de) sions reported by Vattenfall Europe. An extensive error analysis for the retrieval's dry column results (XCO 2 and XCH 4 ) and for the two inversion methods has been performed. Both methods -the Gaussian plume model fit and the Gaussian integral method -are capable of deriving estimates for strong point source emission rates that are within ± 10 % of the reported values, given appropriate flight patterns and detailed knowledge of wind conditions.

Abstract. Localized anthropogenic sources of atmospheric CH 4 are highly uncertain and temporally variable. Airborne remote measurement is an effective method to detect and quantify these emissions. In a campaign context, the science yield can be dramatically increased by real-time retrievals that allow operators to coordinate multiple measurements of the most active areas. This can improve science outcomes for both single-and multiple-platform missions. We describe a case study of the NASA/ESA CO 2 and MEthane eXperiment (COMEX) campaign in California during June and August/September 2014. COMEX was a multi-platform campaign to measure CH 4 plumes released from anthropogenic sources including oil and gas infrastructure. We discuss principles for real-time spectral signature detection and measurement, and report performance on the NASA Next Generation Airborne Visible Infrared Spectrometer (AVIRIS-NG). AVIRIS-NG successfully detected CH 4 plumes in realtime at Gb s −1 data rates, characterizing fugitive releases in concert with other in situ and remote instruments. The teams used these real-time CH 4 detections to coordinate measurements across multiple platforms, including airborne in situ, airborne non-imaging remote sensing, and groundbased in situ instruments. To our knowledge this is the first reported use of real-time trace-gas signature detection in an airborne science campaign, and presages many future applications. Post-analysis demonstrates matched filter methods providing noise-equivalent (1σ ) detection sensitivity for 1.0 % CH4 column enhancements equal to 141 ppm m.

The quantification of emissions of the greenhouse gas methane is essential for attributing the roles of anthropogenic activity and natural phenomena in global climate change. Our current measurement systems and networks, whilst having improved during the last decades, are deficient in many respects. For example, the emissions from localised and point sources such as landfills or fossil fuel exploration sites are not readily assessed. A tool developed to better understand point sources of the greenhouse gases carbon dioxide and methane is the optical remote sensing instrument MAMAP (Methane airborne MAPper), operated from aircraft. After a recent instrument modification, retrievals of the column-averaged dry air mole fractions for methane <i>X</i>CH₄ (or for carbon dioxide <i>X</i>CO₂) derived from MAMAP data have a precision of about 0.4% or better and thus can be used to infer emission rate estimates using an optimal estimation inverse Gaussian plume model or a simple integral approach. <br><br> CH₄ emissions from two coal mine ventilation shafts in western Germany surveyed during the AIRMETH 2011 measurement campaign are used as examples to demonstrate and assess the value of MAMAP data for quantifying CH₄ from point sources. While the knowledge of the wind is an important input parameter in the retrieval of emissions from point sources and is generally extracted from models, additional information from a turbulence probe operated on-board the same aircraft was utilised to enhance the quality of the emission estimates. Although flight patterns were optimised for remote sensing measurements, data from an in situ analyser for CH₄ were found to be in good agreement with retrieved dry columns of CH₄ from MAMAP and could be used to investigate and refine underlying assumptions for the inversion procedures. <br><br> With respect to the total emissions of the mine at the time of the overflight, the inferred emission rate of 50.4 kt CH₄ yr^&minus;1 has a difference of less than 1% compared to officially reported values by the mine operators, while the uncertainty, which reflects variability of the sources and conditions as well as random and systematic errors, is about ±13.5%