Seasonality of XCO2, XCH4, XCO, and XH2O Using a COCCON EM27/SUN in the Po Valley

We report the installation and first two years of operation of an EM27/SUN Fourier transform spectrometer at the CNR-ISAC facility in Bologna, Italy, the first instrument of its kind operating in the Po Valley, one of the European regions most affected by anthropogenic pollution. The instrument is integrated into the COCCON network, and it retrieves total columns and column-averaged dry-air mole fractions of CO2, CH4, CO, and H2O using the PROFFAST processing chain as per standardized COCCON protocols. The data are quality-controlled using the fitted solar gas shift in the O2 absorption band, and the seasonality of the retrieved products is investigated. To explore the relationship between column-integrated and near-surface measurements, we present an illustrative one-day comparison with a co-located cavity ring-down spectroscopy analyzer for CO2 and CH4, using the ceilometer-derived mixed aerosol layer height to interpret the different diurnal variability between the two techniques, with the CRDS showing substantially larger amplitude swings driven by boundary-layer dynamics and surface emission/sink processes. We further compare the EM27/SUN with TROPOMI satellite retrievals of XCH4 and total-column CO over the full observing period, obtaining low systematic biases (0.01% and 0.60%, respectively) consistent with satellite mission requirements and comparable to those reported for TCCON stations at similar latitudes. As a case study, we show that both instruments captured an anomalous CO enhancement in July 2024 that is consistent with long-range transport of Canadian wildfire emissions, illustrating the potential of ground-based FTIR observations to identify high-altitude pollution intrusions that may not be detected by in situ instrumentation alone. These results demonstrate the potential of the EM27/SUN measurements in Bologna as a reference dataset for satellite validation and as a complementary tool for investigating atmospheric transport events in the Po Valley.