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 SO<sub>2</sub> over the city of Mantova and the power plant Porto Tolle, both situated in the Po-valley, Northern Italy. The SO<sub>2</sub> vertical columns and emission flux were derived from two days of measurements, 26 and 27 September 2003. The SO<sub>2</sub> emission flux from the power plant Porto Tolle was calculated to 1.93×10<sup>25</sup> molec s<sup>-1</sup> on 26 September and in good agreement with official emission data, which quote 2.25×10<sup>25</sup> molec s<sup>-1</sup>. On 27 September the measured flux was much lower (3.77×10<sup>24</sup> molec s<sup>-1</sup>) if ECMWF wind data are used, but of comparable magnitude (2.4×10<sup>25</sup> molec s<sup>-1</sup>) if the aircraft on-board wind measurements are utilised. Official emission data was 2.07×10<sup>25</sup> molec s<sup>-1</sup> indicating only a small change from the previous day. Over the city of Mantova, the observed SO<sub>2</sub> vertical columns were 1.1×10<sup>16</sup> molec cm<sup>-2</sup> and 1.9×10<sup>16</sup> molec cm<sup>-2</sup> 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 1.2×10<sup>16</sup> molec cm<sup>-2</sup> and 2.2×10<sup>16</sup> molec cm<sup>-2</sup> if a well mixed boundary layer of 500m altitude is assumed.