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This document describes the work conducted within the atmospheric component of INTAROS Task 6.5. The main contributions for the work reported herein were produced by partner MPG, with additional contributions from UB. The East Siberian Arctic Shelf hosts vast carbon reservoirs at risk of degradation and may be a strong emitter of methane to the atmosphere. Yet, estimates of its annual methane emissions and their key controls are highly uncertain. In the presented project, we estimated these emissions with a geostatistical inverse model from atmospheric observations over seventeen months in Tiksi (Russia), Barrow (Alaska) and Ambarchik (Russia). Our simulations yielded annual methane emissions of 0.3 – 1.5 Tg CH4, which is on the low end of previously reported estimates (0 – 17 Tg CH4 yr-1). Our geostatistical approach allows us to test the compatibility of a large number of spatiotemporal emissions patterns with the atmospheric signals. In this context, we specifically tested the suitability of novel data products from the INTAROS database to improve model performance. Our model attributes highest emissions to shallow waters and to ice-free and potentially freeze-up periods, but also finds substantial emissions during the ice-covered period. We do not detect substantial emissions of stored methane during ice breakup. Our results suggest that mixing and stratification of the water column and cracks in sea ice could be among the dominant controls of methane emissions from the shelf to the atmosphere. Other explanations are possible and discussed, including limitations of our study. The information provided by the INTAROS database led to minor improvements in the explained variability of atmospheric greenhouse gas time series, indicating the high quality of the novel products. However, since parameter selection basically replaced existing oceanic variables by better performing new ones for the same parameter but did not add a previously omitted parameter to the highest-ranking models, we could not gain novel process insights. Our study suggests that the relevance of the shelf for the global atmospheric methane burden is currently small, but also reveals limitations of the Arctic atmospheric greenhouse gas observation network.

This document describes the work conducted within INTAROS Task 3.5. This task comprises contributions from seven different partners, operating novel terrestrial or atmospheric observations across different methodological and geographical areas in the Arctic. This document is intended to: − Demonstrate that all partners delivered their results/products according to the requirements listed in the description of work, − Summarize changes in the work plan in the context of COVID-19 related problems, mostly linked to travel restrictions and inaccessibility of field sites. These issues reduced the findings that can be reported here, however, without substantially compromising the overarching objectives, − Describe, on a activity-by-activity level, the novel observations implemented by each partner, the datasets obtained, and their suitability for continuous, long-term operations in an improved future Arctic observing system, − Document the ‘lessons learned’ while designing, installing and operating the novel observation techniques, − Provide a concise interpretation of the specific datasets that were obtained within Task 3.5 observational campaigns, − Summarize data availability, including links to publicly accessible repositories where data can be retrieved by the scientific community, − Give an outlook on plans and options regarding the future operation of the specific observation systems.

This document Final implementation of the observing system: Data delivery and report on results of the observing systems for the Coastal Greenland, describes the final results and future perspectives, including data accessibility, of INTAROS WP3 Task 3.1 for the Coastal Greenland reference site, which is identified as a key location for freshwater output from the Greenland ice sheet to the ocean. The work reported here describes a range of improvements on our monitoring capabilities for the entire journey of the freshwater flux from the Greenland ice sheet, through the fjords and into the ocean, including its impact on the marine ecosystem. Below is a short summary of the results reported by each partner in Task 3.1 of INTAROS. GEUS: Data on snow water equivalent (SWE) successfully retrieved from SnowFox instruments and processed from four sites co-located with PROMICE weather stations, power supply issues were identified, and mitigation measures taken. High-precision vertical and horizontal positional data was successfully retrieved from the new GNSS unit, capable of recording e.g., ice ablation by accurately recording changes in elevation. The deployment successfully tested a range of possible issues and a modified version of the device has already been implemented for landslide monitoring. A rugged, precise and low-power tilt and azimuth sensor was developed and tested in the lab. Field test deployment is ongoing.Corrected rain datasets from the ice sheet have been successfully retrieved and compared to results from regional climate models and reanalysis products. All the demonstrated measurements above will be rolled out for additional PROMICE, GC-Net and GEM weather stations over the coming years and data made openly available through their respective databases. AU: Two years of CTD data so far successfully retrieved from the marine instruments deployed in Aug 2018 and again in 2019, with a third year expected by Aug 2021. It is planned to continue this monitoring as a part of the GEM monitoring programme. SurfacebpCO2and carbonate parameters were observed during two coastal cruises in West Greenland 2016 and East Greenland 2018, respectively, to study the impact of freshening on CO2-uptake of the ocean, yielding 746 observations of the pCO2in the upper 50 m distributed among 120 stations. FMI: Laboratory instrumentation and procedures to characterize the thermal and angular response of pyranometers, to increase the accuracy of the solar irradiance and albedo measurements were successfully applied to CNR1 and CNR4 net-radiometers of the PROMICE network. The method is expected to be applied to similar radiometers of the PROMICE, GC-Net and GEM monitoring networks in the future. UPM: Test flights delivered 200 km of good radar profiles over ice, proving the concept of the new helicopter-borne radar system in the field. The processed radar profiles have successfully yielded bedrock returns in usually difficult glaciological settings near glacier fronts, although partly disrupted by occurrence of meltwater and reflections off nearby nunataks. CNRS-Takuvik: More than 1900 profiles have been acquired so far with unprecedented sets of data with series measured under ice during wintertime. Takuvik intends to continue measuring sea-ice properties in Qikiqtarjuaq and other coastal ecosystems in Baffin Bay. The sensors developed at Takuvik will be integrated to a sea-ice endoscope in development at Université Laval, which will optimize the acquisition of data in situ over a wide range of sea-ice geometries.

This document reports the activities of the Task 3.5, Deliverable 3.9 “First implementation of the observing system: data delivery and report on results of the distributed observing systems for atmosphere and land”. D3.9 focuses on (a) atmospheric observation of the main greenhouse gases (CO2and CH4) using ground, mobile and airborne eddy covariance observations to determine the atmospheric concentrations and fluxes in North slope of Alaska and Sweden; (b) trace gases monitoring of N2O, SF6, CO, O2/N2 using a flask sampler for the automated collection of air samples under standardized conditions; (c) effects of snow cover on surface energy balance and permafrost thermal regime; (d) observation with a spectro-albedometer at high temporal resolution, and VNA-based radar system to monitor soil, snow and surface vegetation proper-ties;(e)multiple airborne campaigns conducted in the Alaskan North Slope,Mackenzie River Delta, Canada,and the Lena River Delta, Siberia,assessing the composition and height of the atmospheric boundary layer as well as greenhouse gas concentrations; (e) development of a low-maintenance atmospheric observatory onboard of the Swedish research icebreaker Oden that include measurements of incoming broad-band radiation, surface temperature, cloud-base lidars and eddy covariance fluxes of CO2and CH4 Referenced materials and products in this report were compiled with inputs from the INTAROS partners of Task 3.5

This document, describes the first implementation and results of use of INTAROS WP3 (Task 3.1) for the Coastal Greenland reference site, which is identified as a key location for freshwater output from the Greenland ice sheet to the ocean. The activities reported here from Task 3.1 describe the first field deployments and results for both offshore, onshore and on-ice instrumentation to monitor identified environmental variables, including: • snow cover on sea ice - for impact of freshening on the marine ecosystem • surface pCO2 and ocean acidification - for impact of freshening on CO2-uptake of the ocean • snow-water equivalent on the ice sheet - for reducing uncertainty in meltwater output to the ocean • precise positioning of ice sheet stations - for calibration for satellite-derived ice velocity maps and numerical weather prediction • new radiometer tilt and azimuth instrument for improved radiation correction – for improving radiometer data corrections • rain gauges on ice sheet stations – for capturing the transition from snow to rain of the precipitation on the Greenland ice sheet • instrument characterization of in-situ ice sheet albedo measurements – for better validation of satellite albedo products • Improvement of ice-penetrating radar system - for generating ice thickness data over ice-sheet outlet glaciers • passive acoustics for characterization of both physical and biological parts of the marine ecosystem – for studying the ice-driven dynamics of Arctic coastal ecosystems • improvement of under-ice monitoring – for observations of spring bloom and bio-optical/-geochemical properties

To understand the quality of the existing observing system in the Arctic to capture important elements of change over the Arctic we performed a gap analysis with respect to the Arctic Ocean, the Arctic atmosphere and the high-latitude carbon-monitoring network. The main points of the findings are: 1) The ocean observing system: The satellite altimeter system is a critical system to monitor the high-frequency variability. Due to the presence of sea ice in winter time, most of the area can be observed only every 5-10 days, leading to large observing gaps. Closing the gap can be done with new arrays of bottom pressure sensor ssuch as tide gauges or moorings in the ocean bottom. In addition, high-frequency transport measurements are required in the Fram, Davis Straights, the Barents Sea Opening, and north of the Laptev Sea. On the seasonal cycle, bottom pressure observations from GRACE are required to monitor the mass related variability and sea-ice observations are crucial for monitoring the halosteric related variability. On decadal time scales,it is important to have a sufficient hydrographic observing component capable of capturing temperature and salinity changes over the entire Arctic Ocean from the surface to the bottom. New algorithms that can recover sea level from sea ice covered areas may help to improve current satellite altimeter systems,and to improve the ability to monitor the Beaufort Gyre. 2) The atmosphere observing system: The density of the existing radiosonde observation network is not the most critical factor for the quality of T850 forecast. Instead,the results pointed out that stations on small islands in the middle of the Atlantic Ocean are critical for the quality of analysis. The Central Arctic Ocean and the Northern North-Atlantic would prob-ably benefit most from new sounding stations. Efforts to improve the quality of radiosonde observations, especially in Russia, would be very beneficial for the quality of T850 forecasts in the Arctic and sub-Arctic. Current data assimilation systems are probably not adequate to optimally exploit the information from the existing observational network. 3) GHG fluxes observing system: The existing network of pan-Arctic atmospheric monitoring sites provides continuous, well-calibrated observations on atmospheric greenhouse gas mixing ratios, generating basic information to quantify surface-atmosphere greenhouse gas exchange processes for most regions in Canada, Europe and Western Russia; also the Arctic Ocean receives good overall data coverage. Regions showing limited data coverage include the Russian Far East, Western Alaska, and the Eastern Canadian Provinces. Areas where footprint coverage gaps exist seasonally include parts of Western Russia and Central Siberia. Investments in observational infrastructure in any of these areas would be beneficial to increase the overall coverage of the pan-Arctic atmospheric network for greenhouse gases.

This document includes the description and assessment of atmospheric in situ observing systems, data collections and satellite products. On the basis of the assessment, critical knowledge gaps are identified and recommendations to solve them are provided. This document is intended to: − define the current gaps in knowledge in atmospheric fields that are critical for operational weather forecasts, and for the understanding of processes that need to be better represented in climate models. − suggest where the focus in future Arctic atmospheric observations should be.