Methodology for Selecting Near-Surface CH₄, CO, and CO₂ Observations Reflecting Atmospheric Background Conditions at the WMO/GAW Station in Lamezia Terme, Italy

Since 2015, the permanent World Meteorological Organization/Global Atmosphere Watch (WMO/GAW) station of Lamezia Terme (LMT) in Calabria, Southern Italy, has been performing continuous measurements of atmospheric greenhouse gases (GHGs). As a coastal monitoring station, LMT allowed continuous data gathering of carbon dioxide (CO2), carbon monoxide (CO) and methane (CH4) mole fractions in a region characterized by a Mediterranean climate. This work aims to test the adoption of three different methods in the selection of observations representative of the atmospheric background conditions at LMT. In particular, we applied the Background Data Selection (BaDS) method, the smoothed minima baseflow separation method (SM), and the “Wind” method. All the three selection methods appeared to be effective in retaining the background CH4, CO and CO2 data. The “Wind” method, based on the analysis of the local wind regime, selected the lowest number of data. For all the gases considered, the monthly mean values obtained after the implementation of BaDS (SM) were the highest (lowest). Taking into account the complete gases datasets over the period 2015 - 2023, Mann-Kendall and Sen's slope showed annual and seasonal increasing tendencies for CH4 and CO2 with significance levels of α = 0.05 and α = 0.001, respectively. For CO, a decreasing tendency was only observed for the winter season level of α = 0.05. The application of the three selection methods resulted in changes in the calculated annual and seasonal growth rates and non-negligible deviations were also found for the average annual growth rates calculated for the three background datasets. This indicates that the growth rate calculations are sensitive to the choice of background selection method and we recommend that multiple selection methods could be applied to resolve the associated uncertainties.