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⁻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 NO₂ differential slant column densities (DSCDs) up to 13±0.6×10¹⁶ molec cm⁻². These NO₂ DSCDs are converted to vertical column densities (VCDs) by estimating air mass factors. The resulting NO₂ VCDs are up to 4.7±0.4×10¹⁶ molec cm⁻². The water vapour DSCD measurements, up to 8±0.15×10²² molec cm⁻², are used to estimate a volume mixing ratio of water vapour in the boundary layer of 0.013±0.002 mol mol⁻¹. These geophysical quantities are validated with the coincident measurements.

The Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaigns took place in Romania in September 2014 and August 2015. They focused on two sites: the Bucharest urban area and large power plants in the Jiu Valley. The main objectives of the campaigns were to test recently developed airborne observation systems dedicated to air quality studies and to verify their applicability for the validation of space-borne atmospheric missions such as the TROPOspheric Monitoring Instrument (TROPOMI)/Sentinel-5 Precursor (S5P). We present the AROMAT campaigns from the perspective of findings related to the validation of tropospheric NO₂, SO₂, and H₂CO. We also quantify the emissions of NO_x and SO₂ at both measurement sites. We show that tropospheric NO₂ vertical column density (VCD) measurements using airborne mapping instruments are well suited for satellite validation in principle. The signal-to-noise ratio of the airborne NO₂ measurements is an order of magnitude higher than its space-borne counterpart when the airborne measurements are averaged at the TROPOMI pixel scale. However, we show that the temporal variation of the NO₂ VCDs during a flight might be a significant source of comparison error. Considering the random error of the TROPOMI tropospheric NO₂ VCD (σ), the dynamic range of the NO₂ VCDs field extends from detection limit up to 37 σ (2.6×10¹⁶ molec. cm⁻²) and 29 σ (2×10¹⁶ molec. cm⁻²) for Bucharest and the Jiu Valley, respectively. For both areas, we simulate validation exercises applied to the TROPOMI tropospheric NO₂ product. These simulations indicate that a comparison error budget closely matching the TROPOMI optimal target accuracy of 25 % can be obtained by adding NO₂ and aerosol profile information to the airborne mapping observations, which constrains the investigated accuracy to within 28 %. In addition to NO₂, our study also addresses the measurements of SO₂ emissions from power plants in the Jiu Valley and an urban hotspot of H₂CO in the centre of Bucharest. For these two species, we conclude that the best validation strategy would consist of deploying ground-based measurement systems at well-identified locations.

The Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaigns took place in Romania in September 2014 and August 2015. They focused on two sites: the Bucharest urban area and the power plants in the Jiu Valley. Their main objectives were to test recently developed airborne observation systems dedicated to air quality studies and to verify the concept of such campaigns in support of the validation of spaceborne atmospheric missions such as TROPOspheric Monitoring Instrument (TROPOMI)/Sentinel-5 Precursor (S5P). We show that tropospheric NO₂ vertical column density (VCD) measurements using airborne mapping instruments are valuable for satellite validation. The signal to noise ratio of the airborne NO₂ measurements is one order of magnitude higher than its spaceborne counterpart when the airborne measurements are averaged at the TROPOMI pixel scale. A significant source of comparison error appears to be the time variation of the NO₂ VCDs during a flight, which we estimated at about 4 x 10¹⁵ molec cm^-2 in the AROMAT conditions. Considering the random error of the TROPOMI tropospheric NO₂ VCD (σ), the dynamic range of the NO₂ VCDs field extends from detection limit up to 37σ (2.6 x 10¹⁶ molec cm^-2) or 29σ (2 x 10¹⁶ molec cm^-2) for Bucharest and the Jiu Valley, respectively. For the two areas, we simulate validation exercises of the TROPOMI tropospheric NO₂ product using airborne measurements. These simulations indicate that we can closely approach the TROPOMI optimal target accuracy of 25 % by adding NO₂ and aerosol profile information to the mapping data, which constrains the investigated accuracy within 28 %. In addition to NO₂, we also measured significant amounts of SO₂ in the Jiu Valley, as well as a hotspot of H₂CO in the center of Bucharest. For these two species, we conclude that the best validation strategy would consist in deploying ground-based measurement systems at key locations which the AROMAT observations help identify.

The TROPOMI/S5p instrument was launched in October 2017, aiming to measure from space the atmospheric composition for air quality and ozone monitoring. Since 30 April 2018, TROPOMI/S5p routinely delivers NO2 tropospheric VCDs in quasi-real-time. The first comparisons between this operational TROPOMI product and measurements from the ground and aircraft generally show good correlations but also a negative bias over polluted areas. Such a bias is expected from the low spatial resolution of the CTM used in the operational TROPOMI retrieval and several studies reported a better agreement with local measurements of NO2 VCDs when using a higher resolution model for the satellite AMFs, in practice, changing the original TM5-MP for the CAMS Ensemble. We compare mobile-DOAS measurements with the two aforementioned versions of the TROPOMI retrievals (TM5-MP and CAMS). Our Mobile-DOAS measurements were performed with the BIRA-IASB Mobile-DOAS during 19 clear sky days. We sampled polluted and clean areas during TROPOMI overpasses in Belgium and Germany between June 2018 and September 2020. Beside studying the effect of the CTM model on the comparisons, we investigate the general added-values of such mobile-DOAS measurements for the validation of TROPOMI/S5p and forthcoming missions.

African megacities suffer from air pollution and the problem is expected to worsen in the near-future, with the ongoing explosive demographic growth in these areas. The sources of pollutants in Africa are different from those found in Europe. Agricultural burnings and charcoal-based cooking largely contribute to the NO2 and HCHO burdens. However, many large African cities, such as the City of Kinshasa, capital of the Democratic Republic of Congo, do not have local pollution monitoring capabilities. In these polluted places, space-based measurements are often the only source of data available regarding air quality. Together with the validation of TROPOMI in a poorly sampled area, this context motivated ground-based DOAS observations in Kinshasa since 2017. We first operated a single-axis instrument, which we upgraded as a MAX-DOAS instrument in December 2019. We describe the observation site in Kinshasa, the MAX-DOAS instrument, and the retrievals which use the algorithmic tools developed within the FRM4DOAS project. We compare the MAX-DOAS database (2019-2023) of ground-based observations of aerosol optical thickness (AOT), NO2 and HCHO with MODIS and TROPOMI, and with simulations using the GEOS-CHEM Chemistry and Transport Model. Such comparisons enable to assess the quality of the satellite products and model performances around Kinshasa. The ground and satellite-based observations have different sensitivities to the trace gas profiles. Combining the observations and model datasets sheds light on the true atmospheric state above Kinshasa. Another objective of this work is to constrain emission inventories in central Africa.

In August 2023 and September 2024, King’s College London (KCL) conducted airborne campaigns in Canada to investigate emissions from wildfires and the oil industry. The British Antarctic Survey (BAS) Twin Otter aircraft was equipped with an array of in-situ and remote sensing instruments, including the SWING instrument. Developed by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), SWING is a compact whiskbroom imager designed to map trace gases that absorb in the UV-visible spectral range (300-550 nm).We present the integration of SWING into the BAS Twin Otter and its operations during the airborne campaigns. On 14 and 19 August 2023, the aircraft sampled the plume from a wildfire in Ontario, measuring NO2. On 11 September 2024, the aircraft flew over a fire in Saskatchewan, where we detected NO2 together with HCHO and HONO. In the same 2024 campaign, we observed NO2 emissions from flaring at oil facilities in Alberta: Fort McMurray, Fort McKay, and from chemical plants near Edmonton. These airborne measurements are compared with satellite-based air quality data from TROPOMI and TEMPO, for which we are close to the northern edge of the field of view. We also estimate the HONO/NO2 from the fire and investigate how such measurements may help to quantify the emissions from wildfires and from natural gas flaring.