Airborne multi-axis DOAS measurements of tropospheric SO2 plumes in the Po-valley, Italy

During the second FORMAT (FORMaldehyde as A Tracer of oxidation in the troposphere) campaign in 2003 the airborne multi-axis DOAS instrument (AMAXDOAS) performed scattered-light spectroscopic measurements of SO2 over the city of Mantova and the power plant Porto Tolle, both situated in the Po-valley, Northern Italy. The SO2 vertical columns and emission flux were derived from two days of measurements, 26 and 27 September 2003. The SO2 emission flux from the power plant Porto Tolle was calculated to 1.93×1025 molec s−1 on 26 September and in good agreement with official emission data, which quote 2.25×1025 molec s−1. On 27 September the measured flux was much lower (3.77×1024 molec s−1) if ECMWF wind data are used, but of comparable magnitude (2.4×1025 molec s−1) if the aircraft on-board wind measurements are utilised. Official emission data was 2.07×1025 molec s−1 indicating only a small change from the previous day. Over the city of Mantova, the observed SO2 vertical columns were 1.1×1016 molec cm−2 and 1.9×1016 molec cm−2 on 26 and 27 September, respectively. This is in good agreement with ground-based measurements of 5.9 ppbv and 10.0 ppbv which correspond to 1.2×1016 molec cm−2 and 2.2×1016 molec cm−2 if a well mixed boundary layer of 500 m altitude is assumed.