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 abundance of NO₂ in the boundary layer relates to air quality and pollution source monitoring. Observing the spatiotemporal distribution of NO₂ above well-delimited (flue gas stacks, volcanoes, ships) or more extended sources (cities) allows for applications such as monitoring emission fluxes or studying the plume dynamic chemistry and its transport. So far, most attempts to map the NO₂ field from the ground have been made with visible-light scanning grating spectrometers. Benefiting from a high retrieval accuracy, they only achieve a relatively low spatiotemporal resolution that hampers the detection of dynamic features. We present a new type of passive remote sensing instrument aiming at the measurement of the 2-D distributions of NO₂ slant column densities (SCDs) with a high spatiotemporal resolution. The measurement principle has strong similarities with the popular filter-based SO₂ camera as it relies on spectral images taken at wavelengths where the molecule absorption cross section is different. Contrary to the SO₂ camera, the spectral selection is performed by an acousto-optical tunable filter (AOTF) capable of resolving the target molecule's spectral features. The NO₂ camera capabilities are demonstrated by imaging the NO₂ abundance in the plume of a coal-fired power plant. During this experiment, the 2-D distribution of the NO₂ SCD was retrieved with a temporal resolution of 3 min and a spatial sampling of 50 cm (over a 250 × 250 m² area). The detection limit was close to 5 × 10¹⁶ molecules cm⁻², with a maximum detected SCD of 4 × 10¹⁷ molecules cm⁻². Illustrating the added value of the NO₂ camera measurements, the data reveal the dynamics of the NO to NO₂ conversion in the early plume with an unprecedent resolution: from its release in the air, and for 100 m upwards, the observed NO₂ plume concentration increased at a rate of 0.75–1.25 g s⁻¹. In joint campaigns with SO₂ cameras, the NO₂ camera could also help in removing the bias introduced by the NO₂ interference with the SO₂ spectrum.

The performance of an acousto-optical tunable filter (AOTF) depends on several key parameters, such as the diffraction efficiency and the applied RF signal at the external network of the transducer. The latter has a specific bandwidth in which its efficiency is high. The goal of this paper is to experimentally investigate the relationship between the electrically matched RF bandwidth and the achieved diffraction efficiency, especially at the edges of the RF frequency band. The experiment revealed that the impedance matching is not completely correlated to the diffraction efficiency. Temperature measurements on the AOTF under test show that, at one edge of the tuning range, the RF signal is converted into a noninteracting acoustic mode.

Auroras can be observed at the north and south pole of the Earth. At these locations, the Earth magnetic field enters the atmosphere, causing ionization of the atmospheric constituents. Due to this phenomenon, different optical wavelengths are emitted from these constituents, causing the typical spectacular displays. Several optical wavelengths are representative for the auroral emission. The purpose of the described instrument is to measure the degree of linear polarization of the northern lights, in order to detect and confirm previous observations in which only a low level of polarization was seen. The instrument is capable of resolving the polarization measured at any wavelength in the visible region, using two optical chains, each containing an acousto-optical tunable filter (AOTF). The latter is responsible for the selection of the desired optical wavelength and projects the two polarized beams onto an optical detection system. The driving of the AOTF, using a radio-frequency (RF) signal, is a key-element in establishing trustworthy results. The design of this double RF chain, consisting of a generator, an amplifier, and the necessary circumjacent electronics, is described in the study. In addition, the optical setup of the instrument, together with the detection system is explained. The instrument will be used in the harsh environment of the pole regions, so it must be able to withstand these weather conditions. Hence, a thermal compensation system is necessary and is also described.

This paper presents a comprehensive analysis of Class-E series-tuned radio-frequency power amplifiers (RFPAs), focusing on their design and optimization for high efficiency and performance. However, achieving optimal performance involves navigating trade-offs among efficiency, bandwidth, harmonic suppression, output power capability, and device stress. This work examines the trade-offs involved in the series-tuned ($L_s-C_s$) network and establishes the bounds for its quality factor using computer-aided harmonic balance (HB) simulations. Additionally, it explores optimal harmonic termination strategies to enhance the performance and efficiency of the design. Finally, a novel methodology using harmonic termination is proposed, simplifying the design process by eliminating the need for traditional load-pull extraction methods.