The Small Whiskbroom Imager for atmospheric compositioN monitorinG (SWING) is a compact remote sensing instrument dedicated to mapping trace gases from an unmanned aerial vehicle (UAV). SWING is based on a compact visible spectrometer and a scanning mirror to collect scattered sunlight. Its weight, size, and power consumption are respectively 920 g, 27 cm × 12 cm × 8 cm, and 6 W. SWING was developed in parallel with a 2.5 m flying-wing UAV. This unmanned aircraft is electrically powered, has a typical airspeed of 100 km h.sup.-1, and can operate at a maximum altitude of 3 km. We present SWING-UAV experiments performed in Romania on 11 September 2014 during the Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaign, which was dedicated to test newly developed instruments in the context of air quality satellite validation. The UAV was operated up to 700 m above ground, in the vicinity of the large power plant of Turceni (44.67° N, 23.41° E; 116 m a. s. l.). These SWING-UAV flights were coincident with another airborne experiment using the Airborne imaging differential optical absorption spectroscopy (DOAS) instrument for Measurements of Atmospheric Pollution (AirMAP), and with ground-based DOAS, lidar, and balloon-borne in situ observations. The spectra recorded during the SWING-UAV flights are analysed with the DOAS technique. This analysis reveals NO.sub.2 differential slant column densities (DSCDs) up to 13±0.6×10.sup.16 molec cm.sup.-2 . These NO.sub.2 DSCDs are converted to vertical column densities (VCDs) by estimating air mass factors. The resulting NO.sub.2 VCDs are up to 4.7±0.4×10.sup.16 molec cm.sup.-2 . The water vapour DSCD measurements, up to 8±0.15×10.sup.22 molec cm.sup.-2, are used to estimate a volume mixing ratio of water vapour in the boundary layer of 0.013±0.002 mol mol.sup.-1 . These geophysical quantities are validated with the coincident measurements.