<p><strong>Abstract.</strong> We present inverse modelling (<q>top-down</q>) estimates of European methane (CH<sub>4</sub>) emissions for 2006–2012 based on a new quality-controlled and harmonized in-situ data set from 18 European atmospheric monitoring stations. We applied an ensemble of seven inverse models and performed four inversion experiments, investigating the impact of different sets of stations and the use of <q>a priori</q> information on emissions. <br><br> The inverse models infer total CH<sub>4</sub> emissions of 26.7 (20.2–29.7) Tg CH<sub>4</sub> yr<sup>−1</sup> (mean, 10th and 90th percentiles from all inversions) for the EU-28 for 2006–2012 from the four inversion experiments. For comparison, total anthropogenic CH<sub>4</sub> emissions reported to UNFCCC (<q>bottom-up</q>, based on statistical data and emissions factors) amount to only 21.3 Tg CH<sub>4</sub> yr<sup>−1</sup> (2006) to 18.8 Tg CH<sub>4</sub> yr<sup>−1</sup> (2012). A potential explanation for the higher range of <q>top-down</q> estimates compared to <q>bottom-up</q> inventories could be the contribution from natural sources, such as peatlands, wetlands, and wet soils. Based on seven different wetland inventories from the <q>Wetland and Wetland CH<sub>4</sub> Inter-comparison of Models Project</q> (WETCHIMP) total wetland emissions of 4.3 (2.3–8.2) CH<sub>4</sub> yr<sup>−1</sup> from EU-28 are estimated. The hypothesis of significant natural emissions is supported by the finding that several inverse models yield significant seasonal cycles of derived CH<sub>4</sub> emissions with maxima in summer, while anthropogenic CH<sub>4</sub> emissions are assumed to have much lower seasonal variability. <br><br> Furthermore, we investigate potential biases in the inverse models by comparison with regular aircraft profiles at four European sites and with vertical profiles obtained during the <q>Infrastructure for Measurement of the European Carbon Cycle (IMECC)</q> aircraft campaign. We present a novel approach to estimate the biases in the derived emissions, based on the comparison of simulated and measured enhancements of CH<sub>4</sub> compared to the background, integrated over the entire boundary layer and over the lower troposphere. This analysis identifies regional biases for several models at the aircraft profile sites in France, Hungary and Poland.</p>

Abstract. We present the CarbonTracker Europe High-Resolution (CTE-HR) system that estimates carbon dioxide (CO2) exchange over Europe at high resolution (0.1 × 0.2∘) and in near real time (about 2 months' latency). It includes a dynamic anthropogenic emission model, which uses easily available statistics on economic activity, energy use, and weather to generate anthropogenic emissions with dynamic time profiles at high spatial and temporal resolution (0.1×0.2∘, hourly). Hourly net ecosystem productivity (NEP) calculated by the Simple Biosphere model Version 4 (SiB4) is driven by meteorology from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5) dataset. This NEP is downscaled to 0.1×0.2∘ using the high-resolution Coordination of Information on the Environment (CORINE) land-cover map and combined with the Global Fire Assimilation System (GFAS) fire emissions to create terrestrial carbon fluxes. Ocean CO2 fluxes are included in our product, based on Jena CarboScope ocean CO2 fluxes, which are downscaled using wind speed and temperature. Jointly, these flux estimates enable modeling of atmospheric CO2 mole fractions over Europe. We assess the skill of the CTE-HR CO2 fluxes (a) to reproduce observed anomalies in biospheric fluxes and atmospheric CO2 mole fractions during the 2018 European drought, (b) to capture the reduction of anthropogenic emissions due to COVID-19 lockdowns, (c) to match mole fraction observations at Integrated Carbon Observation System (ICOS) sites across Europe after atmospheric transport with the Transport Model, version 5 (TM5) and the Stochastic Time-Inverted Lagrangian Transport (STILT), driven by ECMWF-IFS, and (d) to capture the magnitude and variability of measured CO2 fluxes in the city center of Amsterdam (the Netherlands). We show that CTE-HR fluxes reproduce large-scale flux anomalies reported in previous studies for both biospheric fluxes (drought of 2018) and anthropogenic emissions (COVID-19 pandemic in 2020). After applying transport of emitted CO2, the CTE-HR fluxes have lower median root mean square errors (RMSEs) relative to mole fraction observations than fluxes from a non-informed flux estimate, in which biosphere fluxes are scaled to match the global growth rate of CO2 (poor person's inversion). RMSEs are close to those of the reanalysis with the CTE data assimilation system. This is encouraging given that CTE-HR fluxes did not profit from the weekly assimilation of CO2 observations as in CTE. We furthermore compare CO2 concentration observations at the Dutch Lutjewad coastal tower with high-resolution STILT transport to show that the high-resolution fluxes manifest variability due to different emission sectors in summer and winter. Interestingly, in periods where synoptic-scale transport variability dominates CO2 concentration variations, the CTE-HR fluxes perform similarly to low-resolution fluxes (5–10× coarsened). The remaining 10 % of the simulated CO2 mole fraction differs by >2 ppm between the low-resolution and high-resolution flux representation and is clearly associated with coherent structures (“plumes”) originating from emission hotspots such as power plants. We therefore note that the added resolution of our product will matter most for very specific locations and times when used for atmospheric CO2 modeling. Finally, in a densely populated region like the Amsterdam city center, our modeled fluxes underestimate the magnitude of measured eddy covariance fluxes but capture their substantial diurnal variations in summertime and wintertime well. We conclude that our product is a promising tool for modeling the European carbon budget at a high resolution in near real time. The fluxes are freely available from the ICOS Carbon Portal (CC-BY-4.0) to be used for near-real-time monitoring and modeling, for example, as an a priori flux product in a CO2 data assimilation system. The data are available at https://doi.org/10.18160/20Z1-AYJ2 (van der Woude, 2022a).

Abstract. Using laser absorption spectrometry for the measurement of stable isotopes of atmospheric CO2 instead of the traditional Isotope Ratio Mass Spectrometry (IRMS) method decreases sample preparation time significantly, and uncertainties in the measurement accuracy due to CO2 extraction and isobaric interferences are avoided. In this study we present the measurement performance of a new dual-laser instrument developed for the simultaneous measurement of the δ13C, δ18O and δ17O of atmospheric CO2 in discrete air samples, referred to as the Stable Isotopes of CO2 Absorption Spectrometer (SICAS). We compare two different calibration methods: the ratio method (RM) based on measured isotope ratio and a CO2 mole fraction dependency correction (CMFD), and the isotopologue method (IM) based on measured isotopologue abundances. Calibration with the RM and IM is based on three different assigned whole air references calibrated on the VPBD scale. An additional quality control tank (QC) is included in both methods to follow long-term instrument performance. Measurements of the QC tank show that best performance is achieved with the RM for both the δ13C and δ18O measurements with mean residuals of 0.007 ‰ and 0.016 ‰ and mean standard errors of 0.009 ‰ and 0.008 ‰ respectively, during periods of optimal measurement conditions. The δ17O standard error in the same measurement period is 0.013 ‰. In addition, intercomparing a total of 14 different flask samples covering a CO2 mole fraction range of 344–439 ppm with the Max Planck Institute for Biogeochemistry shows a mean residual of 0.002 ‰ and a standard deviation of 0.063 ‰ for δ13C, using the RM. The δ18O could not be compared due to depletion of the δ18O signal in our sample flasks because of too long storage times. Finally, we evaluated the potential of our Δ17O measurements as a tracer for Gross Primary Production (GPP) by vegetation through photosynthesis. Here, a measurement precision of

Abstract. Atmospheric flask samples are either collected at atmospheric pressure by simply opening a valve of a pre-evacuated flask, or pressurized with the help of a pump to a few bar above ambient providing large air samples for analysis. Under humid conditions, there is a risk that water vapour in the sample leads to condensation on the walls of the flask, notably at higher than ambient sampling pressures. Liquid water in sample flasks is known to affect the CO2 mixing ratios and also alters the isotopic composition of oxygen (17O and 18O) in CO2 via isotopic equilibration. Hence, for accurate determination of CO2 mole fractions and its stable isotopic composition, it is vital to dry the air samples to a sufficiently low dew point before they are pressurized in flasks to avoid condensation. Moreover, the drying system itself should not influence the mixing ratio and the isotopic composition of CO2, nor of the other constituents under study. For the "Airborne Stable Isotopes of Carbon from the Amazon" (ASICA) project focusing on accurate measurements of CO2 and its singly-substituted stable isotopologues over the Amazon, an air drying system was needed capable of removing water vapour from air sampled at a dew point better than −2 °C, high flow rates up to 12 L/min, and without the need for electrical power. Since to date, no commercial air drying device is available that meets these requirements, we designed and built our own consumable-free, power-free, and portable drying system based on multi-tube Nafion™ gas sample driers (Perma Pure, Lakewood, USA). The required dry purge air is provided by feeding the exhaust flow of the flasks sampling system through a dry molecular sieve (type 3A) cartridge. In this study we describe the systematic evaluation of our Nafion-based air sample dryer with emphasis on its performance concerning the measurements of atmospheric CO2 mole fractions and the three singly-substituted isotopologues of CO2 (16O13C16O, 16O12C17O and 16O12C18O), as well as the trace gas species CH4, CO, N2O, and SF6. Experimental results simulating extreme tropical conditions (saturated air at 33 °C) indicated that the response of the air dryer is almost instantaneous and that approximately 85 L of air, containing up to 4 % water vapour, can be processed staying below a −2 °C dew point temperature (at 275 kPa). We estimated that least 8 flasks can be sampled (at an overpressure of 275 kPa) with a water vapour content below −2 °C dew point temperature during a typical flight sampling up to 5 km altitude over the Amazon, whereas the remaining samples would stay well below 5 °C dew point temperature (at 275 kPa). The performance of the air dryer on measurements of CO2, CH4, CO, N2O, and SF6, and the CO2 isotopologues 16O13C16O and 16O12C18O was tested in the laboratory simulating real sampling conditions by compressing humidified air from a calibrated cylinder, after being dried by the air dryer, into sample flasks. We found that the mole fraction and the isotopic composition difference between the different test conditions (including the dryer) and the base condition (dry air, without dryer) remained well within or very close to, in the case of N2O, the WMO recommended compatibility goals for independent measurement programs, proving that the test condition induced no significant bias on the sample measurements.

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