Isotope measurements are useful for separating different methane sources. However, the lack of widely accepted standards and calibration methods for stable carbon and hydrogen isotopic ratios of methane in air has caused significant measurement offsets among laboratories. We conducted worldwide interlaboratory comparisons, surveyed the literature and assessed them systematically. This study may be of help in future attempts to harmonize data sets of isotopic composition of atmospheric methane.

European methane (CH₄) emissions are estimated for 2006–2012 using atmospheric in situ measurements from 18 European monitoring stations and 7 different inverse models. Our analysis highlights the potential significant contribution of natural emissions from wetlands (including peatlands and wet soils) to the total European emissions. The top-down estimates of total EU-28 CH₄ emissions are broadly consistent with the sum of reported anthropogenic CH₄ emissions and the estimated natural emissions.

Modelling of carbon isotopes ¹³C and ¹⁴C in land and ocean components of Earth system models provides opportunities for new insights and improved understanding of global carbon cycling, and for model evaluation. We compiled existing historical datasets to define the annual mean carbon isotopic composition of atmospheric CO₂ for 1850–2015 that can be used in CMIP6 and other modelling activities.

Fossil fuel CO₂ is the major contributor of anthropogenic CO₂ in the atmosphere, and accurate quantification is essential to better understand the carbon cycle. Such accurate quantification can be conducted based on radiocarbon measurements. In this study, we present radiocarbon measurements from a tall tower site in Switzerland. From these measurements, we have observed seasonally varying fossil fuel CO₂ contributions and a biospheric CO₂ component that varies diurnally and seasonally.

The Ocean Model Intercomparison Project (OMIP) is a model comparison effort under Phase 6 of the Coupled Model Intercomparison Project (CMIP6). Its physical component is described elsewhere in this special issue. Here we describe its ocean biogeochemical component (OMIP-BGC), detailing simulation protocols and analysis diagnostics. Simulations focus on ocean carbon, other biogeochemical tracers, air-sea exchange of CO2 and related gases, and chemical tracers used to evaluate modeled circulation.

Nitrous oxide is the third most important anthropogenic greenhouse gas with an increasing mole fraction. To understand its natural and anthropogenic sources we employ isotope measurements. Results show that while the N₂O mole fraction increases, its heavy isotope content decreases. The isotopic changes observed underline the dominance of agricultural emissions especially at the early part of the record, whereas in the later decades the contribution from other anthropogenic sources increases.

A European-wide ²²²radon/²²²radon progeny comparison study has been conducted at nine measurement stations in order to determine differences between existing ²²²radon instrumentation and atmospheric data sets, respectively. Mean differences up to 45 % were found between monitors. These differences need to be taken into account if the data shall serve for quantitative regional atmospheric transport model validation.

²²²Radon is often used to parameterise atmospheric transport in the lower troposphere. It can be measured via its decay products, which are bound to aerosol. Air sampling through long tubing, which sometimes cannot be avoided at tall tower sites, may then cause severe aerosol and corresponding radon daughter activity loss. We have quantified this loss for 8.2 mm ID Decabon tubing used at European stations and provide a length-dependent correction function for this experimental setting.

We evaluate the capability of the TM5 model to reproduce observations of the boundary layer dynamics and the associated variability of trace gases close to the surface, using ²²²Rn. Focusing on the European scale, we compare the TM5 boundary layer heights with observations from radiosondes, lidar, and ceilometer. Furthermore, we compare TM5 simulations of ²²²Rn activity concentrations, using a novel, process-based ²²²Rn flux map over Europe, with ²²²Rn harmonized measurements from 10 stations.

Using a synthetic dataset, we show how to best determine the mean source signature, δ_S, at high temporal resolution using continuous CO₂ and δ¹³C(CO₂) data. We apply this method to measured data from Heidelberg and find a distinct seasonal cycle of δ_S. Disentangling this record into its source components requires the isotopic end members of CO₂ from the biosphere and those from the fuel mix. They can be estimated from the δ_S record, but only when their relative share is close to 100 %.

Detailed 222Rn flux maps are a prerequisite for the use of radon in atmospheric transport studies. We present a high-resolution 222Rn flux map for Europe, based on a parameterization of 222Rn production and transport in the soil. Spatial variations in 222Rn exhalation rates are determined by soil uranium content, water table depth and soil texture. Temporal variations are related to soil moisture variations as the diffusion in the soil depends on available air-filled pore space.

In this model sensitivity study we compare and evaluate the surrogate tracers CO2, CO, δ13C-CO2 and Δ14C-CO2 for estimating continuous anthropogenic CO2. The results can be used to optimize the measurement network design with respect to the partitioning of total CO2 into biospheric and anthropogenic CO2 contributions. This enables improvement and validation of highly resolved emission inventories using atmospheric observation and regional modeling.