Radon-222 (222Rn) is a short-lived radioactive noble gas (half-life T1/2=3.8 d), and it is a progeny of radium-226 (226Ra), which is a member of the primordial uranium-238 decay series. It is naturally produced in all soils, and as a gas it can escape from the soil air into the atmosphere. The exhalation rate of 222Rn from continental surfaces depends on the soil properties, such as grain size distribution, porosity, and 226Ra content, and it varies with soil moisture and other meteorological parameters e.g.,. The 222Rn flux from (ocean) water surfaces is negligible compared to that from continental surfaces. Therefore, 222Rn can serve as tracer to distinguish continental from marine air masses e.g.,. While the half-life of radon is short relative to synoptic timescales (approximately 4–12 d), preventing it from accumulating in the atmosphere over longer timescales, it is long compared to mixing timescales in the atmospheric boundary layer (1 h), allowing it to act as a quasi-conservative tracer for studying boundary layer mixing and transport processes. These properties make 222Rn a suitable tracer for atmospheric boundary layer mixing or transport model validation studies e.g., and even for estimation of regional fluxes of other soil-borne trace gases by applying the radon tracer method e.g.,, assuming that the spatial and temporal distribution of the 222Rn exhalation rate from continental surfaces is known.

Owing to its applicability as an atmospheric transport tracer, 222Rn observations are recommended as a supplementary measurement component at all ICOS atmosphere stations . For precise atmospheric 222Rn observations three fundamentally different measurement principles are available: (1) a two-filter dual-flow-loop detector developed by ANSTO, Australia , (2) an electrostatic deposition monitor developed by INTE-UPC, Spain , both measuring 222Rn directly, and (3) static or moving filter detectors, which accumulate 222Rn progeny on a filter and calculate the atmospheric 222Rn activity concentration based on assumptions about the secular equilibrium between 222Rn and its progeny in the atmosphere e.g.,. This latter 222Rn measurement principle is applied in the static filter system of the Heidelberg Radon Monitor HRM,, which measures the atmospheric 222Rn progeny 214Po at German ICOS atmosphere stations and also at sites outside of Germany, including Neumayer station at the Antarctic coast and the Amazon Tall Tower Observatory in Brazil ATTO,.

While the static filter HRM is a compact and easy to handle instrument, it has some important caveats, which arise from the fact that 222Rn progeny and not 222Rn itself is measured. In the atmosphere, newly formed 222Rn progeny, such as 214Po, is attached to aerosols and thus potentially affected by dry or wet deposition processes as well as loss in intake lines if sampling is conducted through long tubing . Moreover, the state of radioactive equilibrium between progeny and soil-borne 222Rn in the atmosphere depends on the state of atmospheric stability and height above ground . There may also be some delay in reaching secular radioactive equilibrium of its progeny after rapid increases in the 222Rn activity concentration. In the past, careful comparison measurements were conducted to evaluate the height dependence of the disequilibrium , which turned out to be negligible for measurement heights above ca. 80 ma.g.l. An empirical correction function to compensate for progeny loss in standard 8 mm inner diameter Decabon tubing was determined by extensive laboratory experiments and was applied, e.g., for the HRM progeny measurements at the German ICOS atmosphere stations to estimate 222Rn activity concentrations from the measured 214Po data. A remaining uncertainty is the potential progeny loss due to aerosol scavenging and subsequent wet deposition at very high ambient air humidity, which has not been investigated in detail yet.

In this paper we present the atmospheric 214Po/222Rn activity concentration data from German ICOS stations spanning 11 years, from 2014 to 2024, and evaluate these data with respect to potential aerosol scavenging effects, depending on ambient air humidity. Such effects are most frequent at sites where the air intake can reach into clouds, i.e., when a tall tower is located on the top of a mountain or hill. We have therefore developed a relative humidity (RH) flagging threshold for the HRM that is to be applied to all stations based on re-evaluation of earlier comparison campaign data as well as preliminary regional transport model estimations of the atmospheric 222Rn activity concentration for the individual sites. The 222Rn data published along with this paper are flagged according to these thresholds.

Daily mean 214Po data from all stations are shown in the left panel of Fig. , while the data remaining after relative humidity flagging (where available) according to the procedure described in Sect.  are shown in the right panel of Fig. . Only mean values for days where more than 70 % of the hourly data are available or remained after flagging are displayed. Where no RH data are available, the 214Po data have been flagged. At some of the sites there are large gaps in the 214Po data, which are due to initial handling and logistical problems or occasional HRM failures. After flagging, data gaps become more frequent, especially during winter at high RH and at mountain stations like SSL and TOH, where the high-RH events are more prevalent in general. On average, we observe the highest 214Po (and 222Rn) activity concentrations at KIT located in the upper Rhine valley. Even the mountain stations SSL and HPB, which usually have longer air mass transport away from the 222Rn source and hence lower concentrations, show elevated concentrations compared to the rest of the network, with both of them and KIT located in the south of Germany. The lowest activity concentrations are found at GAT, STE, and TOH, with JUE showing slightly higher 214Po activities. All these stations are situated in the northern half of Germany. This south–north gradient of the 222Rn concentration is consistent with the radon flux estimates for Europe derived from uranium soil content, soil properties, and moisture content by . Another factor contributing to this gradient may be the increased influence of radon-depleted marine air masses on northern stations. These stations are situated closer to the basins of the North Sea and the Baltic Sea with negligible radon flux from the surface. On the other hand, air masses reaching the southern stations potentially have more time for radon emanating from the soil to mix into them, leading to higher observed concentrations. The only exception to this longitudinal scheme is the LIN station that, however, has a region of elevated radon soil emission values to the south of the station.

At all stations we find a clear seasonal cycle, with the highest values in summer and autumn and lowest activity concentrations in winter and spring (Fig.  shows KIT, SSL, and LIN; Fig.  in the Appendix depicts all stations). This seasonality is mainly caused by the seasonality of the radon exhalation rate from soils, which, on the seasonal timescale, is mainly driven by soil moisture . The high 222Rn values in summer and autumn are also due to stable atmospheric conditions in these seasons during nights (cf. Fig. ) as well as to more frequent high-pressure systems bringing continental air masses to Germany.

The mean diurnal cycles of 222Rn from January to December 2022 are displayed in Fig. . Only in summer and early autumn (i.e., from April to September) do we observe on average significant diurnal variations at the tower stations in flat terrain, with maximum values early in the morning after sunrise, i.e., shortly before the atmosphere becomes well-mixed and 222Rn that was accumulated close to the ground is mixed into higher levels of the troposphere. At HPB the morning maximum occurs with a delay of a few hours, as ground-level radon is transported not only through vertical convection but also by upslope winds from the surrounding valleys to the sampling site. A similar diurnal variation is observed at SSL in some of the summer and autumn months. This behavior of 222Rn was observed earlier by . During late autumn, winter, and early spring, systematic diurnal variations are missing at all sites as the variations of the 222Rn activity concentration are much more dominated by synoptic variations. This could be explained by smaller differences in the daily and nightly mixing height distributions during late autumn and winter months , which would greatly diminish the radon diurnal amplitude.

The 222Rn observations are available at the ICOS Carbon Portal (10.18160/Q2M8-B1HJ, ).

Currently, there are eight ICOS stations within Germany measuring atmospheric 214Po/222Rn activity concentrations; these are fairly even distributed over the country with five tall towers in flat terrain and three stations on mountaintops (two of which sample air from more than 90 ma.g.l.). Observations are conducted with the static filter Heidelberg Radon Monitor, measuring atmospheric 222Rn progeny. At all tower stations sampling air from intake heights more than 90 ma.g.l., progeny can be assumed to be in equilibrium with atmospheric 222Rn. Only at very high relative humidity >98 % is this not necessarily the case, which we were able to demonstrate by comparison with simulated 222Rn activity concentrations. We, therefore, recommend excluding the data measured with HRM during RH conditions of 98 % and above at all sites in any applications where 222Rn values are of interest (and not of its progenies), for example as a tracer for boundary layer transport, e.g., for atmospheric transport model validation or for the application of the radon tracer method RTM. In this latter application, the missing data at high RH will probably not cause additional uncertainty in RTM-based flux estimates because these very humid atmospheric situations with marine air reaching the stations normally show little correlation between greenhouse gas (GHG) concentrations and 222Rn, and such situations would have been discarded. In the case of atmospheric transport model validation, however, it is essential to use appropriately flagged radon data, where the equilibrium between radon and its progenies can be ensured, even if this results in data gaps. The scavenging of aerosols in small droplets, followed by wet deposition and rain-out, can cause a significant reduction in radon progenies, which leads to discrepancies with the radon signal. These findings further highlight the limitations of the HRM and the need for direct 222Rn measurements at high-altitude sites where clouds frequently coincide with the intake heights. The quantity of flagged data ranges from 3 % to 55 % in the wetter winter months (December–February, average 24 %), with the highest flagged percentage in TOH (51 %) and SSL (55 %) and the lowest in KIT (9 %) and JUE (3 %). During the drier summer months (June–August), the percentage of flagged observations is significantly lower (average 7 %), with the same stations for the highest and lowest quantities (high: TOH – 13 %, SSL – 16 %, low: KIT – 1 %, JUE – 1 %).

In view of the rather small percentage of the data where the 214Po data measured with the HRM were most probably not in secular equilibrium with 222Rn (with the exception of SSL and TOH stations during winter months), and since the absolute agreement between the HRM and other instruments measuring 222Rn directly was of the order of ±10 % during intercomparison campaigns, we conclude that our method is suitable for sampling heights above 80–90 ma.g.l. for the stations in flat terrain. The situation may be different for lower intake heights, where we cannot generally assume radioactive equilibrium between 222Rn and its progenies. For example, in the case of our station in Heidelberg, where air is collected from the roof of the building at 30 ma.g.l., we have to apply a mean correction for disequilibrium, multiplying the 214Po activity concentration with a corresponding factor that was based on comparison measurements with an ANSTO detector i.e., 1.11. A correction for disequilibrium must also be applied to the Schauinsland 214Po data based on comparison with direct 222Rn measurements . As the disequilibrium between 214Po and 222Rn depends on atmospheric stability , applying just one mean factor adds uncertainty to the 222Rn data. Measuring 222Rn gradients below 80–90 ma.g.l. is, therefore, not recommended with the HRM, unless a careful comparison with an instrument measuring 222Rn directly has been conducted for each height level in order to apply a disequilibrium correction based on direct observations. Furthermore, for high-altitude stations (e.g., TOH or SSL) with a high prevalence of RH values greater than 98 %, it is essential to closely examine the typical meteorological conditions prior to HRM deployment or to consider the utilization of other radon instruments that are not prone to aerosol scavenging effects.