The ClimaDat station at Gredos (GIC3) has been continuously measuring atmospheric (dry air) mixing ratios of carbon dioxide (CO₂) and methane (CH₄), as well as meteorological parameters, since November 2012. In this study we investigate the atmospheric variability of CH₄ mixing ratios between 2013 and 2015 at GIC3 with the help of co-located observations of ²²²Rn concentrations, modelled ²²²Rn fluxes and modelled planetary boundary layer heights (PBLHs). Both daily and seasonal changes in atmospheric CH₄ can be better understood with the help of atmospheric concentrations of ²²²Rn (and the corresponding fluxes). On a daily timescale, the variation in the PBLH is the main driver for ²²²Rn and CH₄ variability while, on monthly timescales, their atmospheric variability seems to depend on emission changes. To understand (changing) CH₄ emissions, nocturnal fluxes of CH₄ were estimated using two methods: the radon tracer method (RTM) and a method based on the EDGARv4.2 bottom-up emission inventory, both using FLEXPARTv9.0.2 footprints. The mean value of RTM-based methane fluxes (FR_CH₄) is 0.11 mg CH₄ m⁻² h⁻¹ with a standard deviation of 0.09 or 0.29 mg CH₄ m⁻² h⁻¹ with a standard deviation of 0.23 mg CH₄ m⁻² h⁻¹ when using a rescaled ²²²Rn map (FR_CH₄_rescale). For our observational period, the mean value of methane fluxes based on the bottom-up inventory (FE_CH₄) is 0.33 mg CH₄ m⁻² h⁻¹ with a standard deviation of 0.08 mg CH₄ m⁻² h⁻¹. Monthly CH₄ fluxes based on RTM (both FR_CH₄ and FR_CH₄_rescale) show a seasonality which is not observed for monthly FE_CH₄ fluxes. During January–May, RTM-based CH₄ fluxes present mean values 25 % lower than during June–December. This seasonal increase in methane fluxes calculated by RTM for the GIC3 area appears to coincide with the arrival of transhumant livestock at GIC3 in the second half of the year.

The use of the noble gas radon (²²²Rn) as a tracer for different research studies, for example observation-based estimation of greenhouse gas (GHG) fluxes, has led to the need of high-quality ²²²Rn activity concentration observations with high spatial and temporal resolution. So far a robust metrology chain for these measurements is not yet available. A portable direct atmospheric radon monitor (ARMON), based on electrostatic collection of ²¹⁸Po, is now running at Spanish stations. This monitor has not yet been compared with other ²²²Rn and ²²²Rn progeny monitors commonly used at atmospheric stations. A 3-month intercomparison campaign of atmospheric ²²²Rn and ²²²Rn progeny monitors based on different measurement techniques was realized during the fall and winter of 2016–2017 to evaluate (i) calibration and correction factors between monitors necessary to harmonize the atmospheric radon observations and (ii) the dependence of each monitor's response in relation to the sampling height and meteorological and atmospheric aerosol conditions. Results of this study have shown the following. (i) All monitors were able to reproduce the atmospheric radon variability on a daily basis. (ii) Linear regression fits between the monitors exhibited slopes, representing the correction factors, between 0.62 and 1.17 and offsets ranging between −0.85 and −0.23 Bq m⁻³ when sampling 2 m above ground level (a.g.l.). Corresponding results at 100 m a.g.l. exhibited slopes of 0.94 and 1.03 with offsets of −0.13 and 0.01 Bq m⁻³, respectively. (iii) No influence of atmospheric temperature and relative humidity on monitor responses was observed for unsaturated conditions at 100 m a.g.l., whereas slight influences (order of 10⁻²) of ambient temperature were observed at 2 m a.g.l. (iv) Changes in the ratio between ²²²Rn progeny and ²²²Rn monitor responses were observed under very low atmospheric aerosol concentrations. Results also show that the new ARMON could be useful at atmospheric radon monitoring stations with space restrictions or as a mobile reference instrument to calibrate in situ ²²²Rn progeny monitors and fixed radon monitors. In the near future a long-term comparison study between ARMON, HRM, and ANSTO monitors would be useful to better evaluate (i) the uncertainties of radon measurements in the range of a few hundred millibecquerels per cubic meter to a few becquerels per cubic meter and (ii) the response time correction of the ANSTO monitor for representing fast changes in the ambient radon concentrations.