Study of the daily and seasonal atmospheric CH₄ mixing ratio variability in a rural Spanish region using ²²²Rn tracer

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 bottomup 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.