Due to their disproportionately high soil carbon stores, understanding the response of wetlands to changes in climate that have already been observed is a key issue. Of particular importance is determining contemporary changes in their carbon budget, including net ecosystem exchange (NEE). However, due to the high inter-annual variability of this element both the determination of a realistic multiannual mean, and identification of trends, require several decades of measurements. Available NEE data sets rarely meet this requirement. To overcome this problem a modeling approach can be used. However, the use of complex models may be limited by the availability of input data. Consequently, single indicators or simple semi-empirical models using a small set of commonly monitored variables may be a more suitable option. Here we test a variety of NEE partitioning equations at one wetland site to derive an optimal model that could subsequently be tested at other sites. The empirical database utilized to develop this model comprises eddy covariance measurements of COQ fluxes and meteorological data from a site within Biebrza National Park, Poland, between 2013 and 2022. The subsequent multi-year analysis is based on routine temperature and precipitation data from climatological stations in the region. By developing a semi-empirical model, we show that three easily obtained environmental variables, mean monthly temperature, precipitation, and radiation at the top of the atmosphere, function as sufficient indicators to reflect the contemporary multi-year variability of a wetland ecosystem. The modeled NEE for the period 1971-2022 (-133 +/- 383 g-CO2 m-2 y-1) shows higher exchange than measured one for 2013-2022 (-106 +/- 616 g-CO2 m-2 y-1). At the same time, the clear multi-year trend in temperature is not reflected in NEE trends, although annual cycle simulations for different temperature scenarios indicate an increase in NEE (reduced uptake) in a warmer climate. This illustrates the complex influence of various factors on NEE.

Radon gas is the largest source of public exposure to naturally occurring radioactivity, and concentration maps based on atmospheric measurements facilitate compliance with national implementation of the Council Directive 2013/59/Euratom. A number of different metrology projects dealing with the traceability of radon measurements for different reasons and in different conditions have been funded. As a health risk in dwellings and workplaces (MetroRADON, EMPIR), as an atmospheric tracer in a climate change mitigation context (traceRadon, EMPIR), networked in big buildings or future cities (RadonNET, EPM), and at coastal or remote oceanic island sites in a climate change monitoring context (NuClim, EURATOM). Calibrations with small uncertainties that are traceable to the Système international d'unités (SI) (The International System of Units (SI), 2019) are needed for low 222Rn activity concentrations under difficult conditions. Starting with different classical calibration solutions, we then propose and explain a new in-situ operando calibration technique (the pulse calibration method), and provide two examples of these calibration techniques for research grade outdoor air activity concentration detectors.

Project NuClim (Nuclear observations to improve Climate research and GHG emission estimates) aims to use high-quality measurements of atmospheric radon activity concentration and ambient radioactivity to advance climate science and improve radiation protection and nuclear surveillance capabilities. It is supported by new metrological capabilities developed in the EMPIR project 19ENV01 traceRadon. This work reviews the scientific objectives of project NuClim in terms of both climate science and radiological protection, and provides an overview of the NuClim field campaign and the various nuclear measurements being implemented within the scope of the project.

Radon gas is the largest source of public exposure to naturally occurring radioactivity, and concentration maps based on atmospheric measurements aid developers in complying with EU Safety Standard Regulations. But radon can also be used as a tracer to evaluate dispersal models important for supporting successful greenhouse gas (GHG) mitigation strategies. That is why reliable measurements of low-level radon activity concentrations, such as those found in the environment (<20 Bq·m−3), are important for both radiation protection and climate research. Despite the enormous changes in radon metrology that have occurred in recent years activity concentrations below 100 Bq·m−3 had not been subject to metrological research so far. This evokes new challenges which are the development of traceable methods and robust technology for measurements of environmental low-level radon activity concentrations and radon fluxes from the soil. Both are important to derive information on greenhouse gas fluxes in the environment and therefore are important for planning the reduction strategy.In the framework of the EMPIR project 19ENV01 traceRadon, stable atmospheres with low-level activity concentrations of radon have been produced to enable calibration of radon detectors capable of measuring these environmental activity concentrations. The required traceability of the calibration at very low activity concentrations, was not possible in the past.To achieve this goal, low activity sources of radium have been produced with different methods and different characteristics. Sources down to few Bq of 226Ra have been developed and characterized leading to uncertainties as low as 2 % (k = 1), even in case of the lowest activity sources. Additionally, sources with medium and high activities were produced, with advanced production methods like ion implantation of mass separated 226Ra in different target materials.As an outcome, for the first-time traceable methods for measuring low-level atmospheric radon activity concentrations in the range of 1 Bq·m−3 to 50 Bq·m−3 with uncertainties below 5 % (k = 1) are now available. To compare the performance of the sources in application, e.g., to establish a reference atmosphere, a calibration exercise with two highly sensitive, large volume, low radon activity concentration detectors of different design and principle was carried out at the PTB. The details of the calibration procedure of these unique prototypes of detectors are presented. The results of the characterization of the detectors are discussed and as a conclusion new developments in the field of radon metrology are presented.

The impact that biogenic emissions have on aerosol-cloud interactions across the Southern Ocean is poorly quantified. Here we use satellite and ship observations during austral summer to study these interactions. We present observational evidence that biogenic aerosols increase cloud condensation nuclei and cloud droplet number concentrations over the Southern Ocean off East Antarctica, coinciding with very low concentrations of ice-nucleating particles and higher occurrences of supercooled liquid-containing low-level clouds.

Abstract. The Radon Tracer Method (RTM) is an established, independent top-down method that can be used to cross-check bottom-up greenhouse gas (GHG) emission estimates. Furthermore, as uncertainties of Atmospheric Transport Models are reduced, the RTM can provide a convenient means of quantifying continual improvement of inversion-based top-down GHG emission estimates. While the accessibility and perceived simplicity of the RTM drive its popularity, the technique is better suited to assessing long-term relative changes in GHG emissions than absolute changes, due to short-term soil moisture influences on simulated radon flux uncertainty. Considerations for applying the RTM, based on fundamental assumptions of the technique's development, are application and season specific, making the development of a "standard protocol" for its use challenging. After proposing a novel alternative means of applying the nocturnal accumulation RTM, which improves interpretation of findings, we use measurements from a range of contrasting sites to discuss the significance of the technique's eight key considerations: (i) nocturnal window definition, (ii) radon and target gas accumulation thresholds, (iii) radon-to-target gas regression linearity thresholds, (iv) measurement height, (v) the contributing fetch, (vi) spatial and temporal radon flux variability, (vii) RTM temporal resolution, and (viii) application specific selection of a suitable radon monitor. The insight provided by these examples to the flexibility (or otherwise) of the technique's considerations will clarify the implications if users choose to relax or ignore them, potentially making future RTM studies more directly comparable.

Radon (Rn-222) is a unique atmospheric tracer, since it is an inert gaseous radionuclide with a predominantly terrestrial source and a short half-life (3.8232 (8) d), enabling quantification of the relative degree of recent (< 21 d) terrestrial influences on marine air masses. High quality measurements of atmospheric radon activity concentration in remote oceanic locations enable the most accurate identification of baseline conditions. Observations of GHGs under baseline conditions, representative of hemispheric background values, are essential to characterise long-term changes in hemispheric-mean GHG concentrations, differentiate between natural and anthropogenic GHG sources, and improve understanding of the global carbon budget.The EU-funded project NuClim (Nuclear observations to improve Climate research and GHG emission estimates) will establish world-leading high-quality atmospheric measurements of radon activity concentration and of selected GHG concentrations (CO2, and CH4) at a remote oceanic location, the Eastern North Atlantic (ENA) facility, managed by the Atmospheric Radiation Measurement (ARM) programme (Office of Science from the U.S. Department of Energy), located on Graciosa Island (Azores archipelago), near the middle of the north Atlantic Ocean. These observations will provide an accurate, time-varying atmospheric baseline reference for European greenhouse gas (GHG) levels, enabling a clearer distinction between anthropogenic emissions and slowly changing background levels. NuClim will also enhance measurement of atmospheric radon activity concentration at the Mace Head Station, allowing the identification of latitudinal gradients in baseline atmospheric composition, and supporting the evaluation of the performance of GHG mitigation measures for countries in the northern hemisphere.The high-quality nuclear and GHG observations from NuClim, and the resulting classification of terrestrial influences on marine air masses, will assist diverse climate and environmental studies, including the study of pollution events, characterisation of marine boundary layer clouds and aerosols, and exploration of the impact of natural planktonic communities on GHG emissions. This poster presents an overview of NuClim, outlines the project objectives and methodologies, and summarises the relevant data products that will be made available to the climate community.Project NuClim received funding from the EURATOM research and training program 2023-2025 under Grant Agreement No 101166515.

Energy and environmental radioactivity are closely interconnected. The production of energy by nuclear power plants, currently considered a way to meet greenhouse gas emission targets and reduce dependency on fossil fuels, generates radioactive waste, which has to be adequately handled in order to prevent environmental contamination. Furthermore, non-renewable energy production from coal and other fossil fuels increases the release of radionuclides, such as radium (Ra-226), uranium, thorium, and radon, into the environment, potentially causing adverse effects on human health and ecosystems. Thus, monitoring radioactivity levels in the atmosphere is fundamental for assessing environmental risks from energy production activities, protect populations from potential adverse health effects, and balancing health against energy-saving efforts. Project NuClim (Nuclear observations to improve Climate research and GHG emission estimates) aims to increase the capability to assess radioactivity levels in the natural environment and improve radiation protection through a detailed field campaign in the Graciosa Island (Azores archipelago) that combines very detailed meteorological information from the Eastern North Atlantic (ENA) Atmospheric Radiation Measurement (ARM) facility and accurate measurements of atmospheric radon activity concentration and ambient gamma dose rate, down to very low levels/activities. The new measurements will enable improvements in radiation protection, including the distinction between natural and anthropogenic sources of radioactivity, thus contributing to more environmentally responsible and sustainable energy production.

We present a protocol to improve confidence in reported radon activity concentrations, facilitating direct site-to-site comparisons and integration with co-located greenhouse gas (GHG) measurements within a network of three independently managed observatories in the UK. Translating spot measurements of atmospheric GHG amount fractions into regional flux estimates (“top-down” analysis) is usually performed with atmospheric transport models (ATMs), which calculate the sensitivity of regional emissions to changes in observed GHGs at a finite number of locations. However, the uncertainty of regional emissions is closely linked to ATM uncertainties. Radon, emitted naturally from the land surface, can be used as a tracer of atmospheric transport and mixing to independently evaluate the performance of such models. To accomplish this, the radon measurements need to have a comparable precision to the GHGs at the modelled temporal resolution. Australian Nuclear Science and Technology Organisation (ANSTO) dual-flow-loop two-filter radon detectors provide output every 30 min. The measurement accuracy at this temporal resolution depends on the characterization and removal of instrumental background, the calibration procedure, and response time correction. Consequently, unless these steps are standardized, measurement precision may differ between sites. Here we describe standardized approaches regarding (1) instrument maintenance, (2) quality control of the raw data stream, (3) determination and removal of the instrumental background, (4) calibration methods, and (5) response time correction (by deconvolution). Furthermore, we assign uncertainties for each reported 30 min radon estimate (assuming these steps have been followed) and validate the final result through comparison of diurnal and sub-diurnal radon characteristics with co-located GHG measurements. While derived for a network of UK observatories, the proposed standardized protocol could be equally applied to two-filter dual-flow-loop radon observations across larger networks, such as the Integrated Carbon Observation System (ICOS) or the Global Atmosphere Watch (GAW) baseline network.

The atmospheric mixing state and emission rates play decisive roles in public exposure to urban air pollution. This study utilizes atmospheric radon measurements taken with the SM200 “stability monitor,” which reflect changes in the atmospheric mixing state, to evaluate and forecast air quality. Using six months (March–August 2016) of atmospheric radon measurements in Jinhua, China, we classify the nocturnal atmospheric stability conditions into four distinct categories, “well-mixed”, “weakly stable”, “moderately stable”, and “most stable”, by applying a modified radon-based stability technique. We calculate the atmospheric self-cleaning ability index (ASI) and evaluate it with the four-category stability scheme, and the results confirm that the atmospheric radon measurements reliably represent the atmospheric mixing state. Analyzing PM2.5, PM10, SO2, NO2, CO, and O3 measurements from three nearby stations during the campaign, we find that the pollutant concentrations and air quality index (AQI) values assigned using the aforementioned stability scheme are consistent with the defined atmospheric mixing states. We subsequently demonstrate that the modified radon-based stability method is suitable for targeting the most unfavorable air quality conditions and determining where the emissions originated. Finally, we propose a simple ASI-based model for predicting regional severe air pollution.

Consumer-grade economical radon monitors are becoming increasingly popular in private and institutional use, in the contexts of both Citizen Science and traditional research. Although originally designed for screening indoor radon levels in view of radon regulation and decisions about mitigation or remediation—motivated by the health hazard posed by high radon concentrations—researchers are increasingly exploring their potential in some environmental studies. For long time, radon has been used as a tracer for investigating atmospheric transport processes. This paper focuses on RadonEye, currently the most sensitive among low-cost monitors available on the market, and specifically, its potential use for monitoring very low radon concentrations. It has two objectives: firstly, discussing issues of statistics of low count rates, and secondly, analyzing radon concentration time series acquired with RadonEyes outdoors and in low-radon indoor spaces. Regarding the first objective, among other things, the inference radon concentration reported to expected true is discussed. The second objective includes the application of autoregressive methods and fractal statistics to time series analysis. The overall result is that radon dynamics can be well captured using this “low-tech” approach. Statistical results are plausible; however, few results are available in the literature for comparison, particularly concerning fractal methods. The paper may therefore be seen as an incentive for further research in this direction.

We isolated diurnal timescale contributions to a 6-year hourly radon record and incorporated them in ME-2 as a proxy for changes in atmospheric mixing depth in an attempt to improve the source apportionment of fine atmospheric particulate matter (PM2.5). Results from this radon-based implementation of ME-2 are directly compared with the more traditional ME-2 implementation where wind speed is used, as a proxy for changes in mixing depth. The radon-based version more accurately reproduced daily PM2.5 source contributions, as evidenced by better correlations with the results from the corresponding bi-linear model. The versions of ME-2 employed in this study were modified to account for calm wind conditions separately, and a recently updated solution approach was adopted.Source apportionment for the radon-based ME-2 implementation was most successful for the finer, primary emissions (Smoke, Autos, Industry) that are more easily suspended and whose concentrations are more directly tied to changes in atmospheric mixing depth. Incorporation of the diurnal radon signal in ME-2 improved the estimated source strength distributions of the Smoke, Autos and Industry sources with respect to the township of Muswellbrook. It also resulted in a more consistent anti-correlation between these 3 source types and atmospheric mixing depth than for the wind speed case. These results confirm that near surface radon concentration is more closely tied to atmospheric mixing depth (and therefore pollutant concentrations) than wind speed.The measurement site for this study is a small township in a rural setting, with nearby power stations and open-cut coal mines. Consequently, the distribution and characteristics of anthropogenic aerosol sources are very different than for a typical urban or industrial setting. This is reflected in lower correlation between the multi-linear models and the corresponding bi-linear models, indicating that the performance of multi-linear models is affected by the nature of the distribution of sources.

We report on 10 years of hourly atmospheric radon, CO, and SO2 observations at Gosan Station, Korea. An improved radon detector was installed during this period and performance of the detectors is compared. A technique is developed whereby the distribution of radon concentrations from a fetch region can be used to select air masses that have consistently been in direct contact with land-based emissions, and have been least diluted en route to the measurement site. Hourly radon concentrations are used to demonstrate and characterise contamination of remote-fetch pollution observations by local emissions at this key WMO GAW site, and a seasonally-varying 5-hour diurnal sampling window is proposed for days on which diurnal cycles are evident to minimise these effects. The seasonal variability in mixing depth and “background” pollutant concentrations are characterised. Based on a subset of observations most representative of the important regional fetch areas for this site, and least affected by local emissions, seasonal estimates of CO and SO2 in air masses originating from South China, North China, Korea and Japan are compared across the decade of observations.

We apply a simple objective measure of an airshed's degree of ventilation and determine the impact on PM2.5 observations at Lucas Heights, Sydney, Australia. We extend the analysis of previous studies, which considered total PM2.5, by: using positive matrix factorisation to split the aerosol mass by source type; and using Radon-222 measurements as an independent indicator of ventilation and mixing. For this coastal airshed we found that for 64% of the time, conditions could be classified into four categories: local recirculation ( LRC; 15%), stagnation (19.5%), regional recirculation (RRC; 10.9%), or ventilation (18.6%). Mean PM2.5 concentrations under recirculation (in this study separated into; LRC and RRC) were 33% higher than under stagnation and can be double that of concentrations under ventilation. Since the combination of LRC and RRC events account for around 26% of all events, recirculation effects on PM2.5 concentrations are significant. However, we found that airshed ventilation doesn't affect PM2.5 concentrations from all sources evenly. Considering the three main sources of total PM2.5 at this site (vehicle exhaust 26.3%, secondary sulfate 23.7% and aged industrial sulfur 20.6%), conditions leading to the highest concentrations differ. The highest vehicle exhaust concentrations occur under LRC, the highest aged-industrial-sulphur concentrations occur under RRC, and secondary sulfur had similarly high concentrations under LRC and RRC. Under LRC the concentration from vehicle exhaust can be up to a factor of 3.9 greater than under ventilation. On a seasonal basis, RRC flow is most likely to occur in summer and spring (the warmer months of the year when sea breezes are more likely), whereas LRC conditions are more likely to occur in autumn and winter. These findings support those of previous studies, indicating that re-circulation can have a significant effect on PM2.5 concentrations in coastal airsheds, and the degree of impact can vary by source type.

Daily and weekly rainfall stable isotopic data, collected over a 9-year period (for daily) and 14-year period (for weekly) at Lucas Heights, Australia, were compared. Small differences in the local meteoric water lines (LMWLs) were seen when developed using daily, weekly, or monthly data (where the effect of short-term small atmo-spheric variability is reduced by the associated averaging). Although, if a precipitation weighted least squares method was used to derive the LMWLs, the differences between the daily and weekly derived LMWLs were lower.A systematic analysis was then conducted of the impact on the isotopic composition of precipitation from environmental conditions at the sampling site and along the air mass trajectory, the synoptic weather systems, and finally large-scale climate drivers (e.g., El Nin tilde o-Southern Oscillation).The best correlation of 818O was with local temperature, rainout of the air mass, and rainfall amount at the site. The correlation between precipitation amount and 818O values was higher when daily samples were used, than when the precipitation weighted monthly values were considered (-0.36 as opposed to -0.31). A good correlation was also seen between 818O and duration of the precipitation event (r = -0.42). The correlation between 818O and the fraction of convective rainfall was significant, but low (r = 0.11).On a synoptic systems basis, the lowest 8 values were seen in rainfall from offshore low-pressure systems, located to the east or north-east of the site; although, the largest rainfall was seen from low pressure systems arriving from the north-east. The influence of the large-scale climate drivers on the isotopic composition of precipitation was insignificant at this coastal site.When Radon-222 was used to categorise the atmospheric mixing state, rainfall amount was clearly negatively correlated with nocturnal stability, with the lowest amount of rainfall occurring on days with the most stable nocturnal periods. Most of the convective rainfall occurred on these same days, occurring after the warmer period of the day, although the total rainfall amount was low. Analysing the data within the separate radon-derived stability categories improved the correlations with rainfall amount for some of the stability categories.

Radon (Rn) is a radioactive, colourless, odourless, noble gas that decays rapidly. It’s most stable isotope, 222Rn, has a half-life of around 3.8 days. Atmospheric radon measurements play an important role in understanding our atmospheric environments. Naturally occurring radon can be used as an atmospheric tracer for airmass tracking, to assist in modelling boundary layer development, and is important for understanding background radiation levels and personal exposure to natural radiation. The daughter products from radon decay also play an important role when measuring fine particle pollution using beta-attenuation monitors (BAM). Beta radiation from the 222Rn decay chain interferes with BAM measurements of fine particles; thus, some BAMs incorporate radon measurements into their sampling systems. BAMs are ubiquitous in air quality monitoring networks globally and present a hitherto unexplored source of dense, continuous radon measurements. In this paper, we compare in situ real world 222Rn measurements from a high quality ANSTO dual flow loop, dual filter radon detector, and the radon measurements made by a commercial BAM instrument (Thermo 5014i). We find strong correlations between systems for hourly measurements (R2 = 0.91), daily means (R2 = 0.95), hour of day (R2 = 0.72–0.94), and by month (R2 = 0.83–0.94). The BAM underestimates radon by 22–39%; however, the linear response of the BAM measurements implies that they could be corrected to reflect the ANSTO standard measurements. Regardless, the radon measurements from BAMs could be used with correction to estimate local mixed layer development. Though only a 12-month study at a single location, our results suggest that radon measurements from BAMs can complement more robust measurements from standard monitors, augment radon measurements across broad regions of the world, and provide useful information for studies using radon as a tracer, particularly for boundary layer development and airmass identification.

A recently-developed radon-based method for combined classification of both diurnal and synoptic timescale changes in the atmospheric mixing state is applied to 1-year of observations in Ljubljana (capital of Slovenia). Five diurnal-timescale mixing classes (#1 to #5) were defined for each season along with an additional mixing class (#6) in non-summer months, representative of synoptic-timescale changes of the atmospheric mixing state associated with "persistent temperature inversion" (PTI) events. Seasonal composite radiosonde profiles and mean sea level pressure charts within each mixing class are used to demonstrate the link between prevailing synoptic conditions and the local mixing state, which drives changes in urban air quality. Diurnal cycles of selected pollutants (BC, NO2, CO, PM10, SO2 and O3) exhibited substantial seasonality as a result of changing mixing conditions, source types and strengths. For the more well-mixed conditions (classes #2 to #3), surface wind speeds were 3 times higher than during class #6 (PTI) conditions, resulting in a 3-fold reduction of primary pollutant accumulation. Daily-mean PM10 concentrations only exceeded EU and WHO guideline values in winter and autumn for two of the radon-defined mixing classes: (i) class #5 (strongly stable near-surface conditions associated with passing synoptic anti-cyclone systems), and (ii) class #6 (PTI conditions driven by regional subsidence in the presence of the "Siberian High"). Both mixing states were associated with low mean wind speeds (∼0-0.7 m s-1) and strong thermal stratification, as indicated both by pseudo-vertical temperature gradients (∆T/∆z) and radiosonde profiles. Diurnal ∆T/∆z values indicated limited opportunity for convective mixing of pollutants from the basin atmosphere under these conditions. The demonstrated consistency in atmospheric mixing conditions (vertically and spatially) across the diurnal cycle within each of the defined mixing classes suggests the radon-based classification scheme used in conjunction with 3-D urban sensor networks could be well suited to evaluate mitigation schemes for urban pollution and urban climate.

The noble and radioactive gas radon is well known to be the most important source of public exposure to natural environmental radioactivity in indoor environments (workplaces, homes, etc.). Consequently, it is important to identify radon-prone areas, where radon fluxes are high, and also to develop and apply mitigation measures when radon activity concentrations of indoor areas exceed guideline values.However, radon is also known by the climate and atmospheric research communities to be a useful environmental tracer and it is nowadays being used in several studies such as the improvement of atmospheric transport models or the indirect estimation of GHG fluxes by the Radon Tracer Method. These previous applications will benefit from the availability of radon flux maps.Stakeholders and scientists involved in radiation protection and climate analysis may benefit from reliable continuous radon flux measurements to validate and improve existing and future radon flux maps. In the framework of the project traceRadon (EMPIR reference 19ENV01) a full metrology chain has been designed and built for radon flux measurements.The work and the challenges related to this type of measurement will be presented here together with possible guidelines for carrying out continuous radon flux measurements in the field.

Poor air quality is recognised as the most important environmental health issue of our time. Meteorological variables like temperature and wind speed can strongly influence air quality and these variables often show clear annual cycles. It is therefore common to analyse atmospheric pollutants within a seasonal framework. However, the commonly used seasons in Australia do not align well with all of the most important annual weather patterns that influence air quality in the Sydney Basin. We used Indigenous perspectives on ‘seasons’ as identified by the co-authors and combined these with statistical analysis of the local climatology. This enabled us to create a set of locally informed ‘quasi-seasons’ that we named IKALC-seasons (Indigenous Knowledge Applied to Local Climatology). Engaging with the IKALC-seasons improved our understanding of temporal variability of air pollution in western Sydney, mainly due to a better identification of the time of year when cold, still weather conditions result in higher levels of fine particulate pollution, carbon monoxide and nitrogen oxides. Although the IKALC seasons identified in this study are intrinsically local in nature, the methodology developed has broadscale application. This approach can be used to identify the times of year when micrometeorological conditions are most likely to drive poor air quality thereby helping to inform effective decision-making about emission controls.

In the framework of the EMPIR project traceRadon, stable atmospheres with low-level radon activity concentrations have to be produced for calibrating radon detectors designed to measure outdoor air activity concentrations. The traceable calibration of these detectors at very low activity concentrations is of special interest to the radiation protection, climate observation, and atmospheric research communities. Radiation protection networks (such as the EUropean Radiological Data Exchange Platform (EURDEP)) and atmospheric monitoring networks (such as the Integrated Carbon Observation System (ICOS)) need reliable and accurate radon activity concentration measurements for a variety of reasons, including: the identification of Radon Priority Areas (RPA); improving the sensitivity and reliability of radiological emergency early warning systems (Melintescu et al., 2018); for more reliable application of the Radon Tracer Method (RTM) to estimate greenhouse gas (GHG) emissions; for improved global "baseline" monitoring of changing GHG concentrations and quantification of regional pollution transport (Chambers et al., 2016), (Chambers et al., 2018); and for evaluating mixing and transport parameterisations in regional or global chemical transport models (CTMs) (Zhang et al., 2021), (Chambers et al., 2019). To achieve this goal, low activity sources of radium with a variety of characteristics were produced using different methods. Sources ranging from MBq 226Ra down to several Bq 226Ra were developed and characterised during the evolution of production methods, and uncertainties below 2 % (k=1) were achieved through dedicated detection techniques, even for the lowest activity sources. The uncertainty of the lowest activity sources was improved using a new online measurement technique for which the source and detector were combined in the same device. This Integrated Radon Source Detector device, henceforth an IRSD, reaches a counting efficiency approaching 50 % through detection under quasi 2π sr solid-angle. At the time of this study the IRSD was already produced with 226Ra activities between 2 Bq and 440 Bq. To compare the working performance of the developed sources (i.e., to establish a reference atmosphere), study the stability of the sources, and to establish traceability to national standards, an intercomparison exercise was carried out at the PTB facility. Here we present the various source production techniques, the determination of their radium activity, and determination of their radon emanation (including assigned uncertainties). This includes details of the implementation of the intercomparison set-up, and a discussion of the results of the source characterisations.