Analysis and Evaluation of Sulfur Dioxide and Equivalent Black Carbon at a Southern Italian WMO/Gaw Station Using the Ozone to Nitrogen Oxides Ratio Methodology as Proximity Indicator

The measurement and evaluation of atmospheric background levels of greenhouse gases (GHGs) and aerosols is useful to determine long-term tendencies and variabilities, and pinpoint peaks attributable to anthropogenic emissions, and exceptional natural emissions such as volcanoes. At the Lamezia Terme (code: LMT) World Meteorological Organization – Global Atmosphere Watch (WMO/GAW) observation site located in the southern Italian region of Calabria, the “Proximity” methodology based on the ratio of tropospheric ozone (O3) to nitrogen oxides (NOx) has been used to discriminate local and remote atmospheric concentrations of GHGs, with the former being heavily affected by anthropogenic emissions while the latter is more representative of atmospheric background conditions. This study applies, to eight continuous years of measurements (2016-2023), the Proximity methodology to sulfur dioxide (SO2) for the first time, and also extends it to equivalent black carbon (eBC) to assess whether the methodology can be applied to aerosols. The results indicate that SO2 follows a peculiar pattern, with LOC (local) and BKG (background) levels being generally lower than their N-SRC (near source) and R-SRC (remote source), thus corroborating previous hypotheses on SO2 variability at LMT by which the Aeolian Arc of volcanoes and maritime traffic could be responsible for these concentration levels. The anomalous behavior of SO2 is assessed using the Proximity Progression Factor (PPF) introduced in this study, which provides a value representative of changes from local to background concentrations. This finding, combined with an evaluation of known sources on a regional scale, has allowed to attempt, for the first time, to implement degrees of spatial resolution to the Proximity methodology, which provides “qualitative” distances based on air mass aging. Furthermore, the results confirm the potential of using the Proximity methodology for aerosols, as eBC shows a pattern consistent with local sources of emissions, such as wildfires and other forms of biomass burning, being responsible for observed peaks.