Laboratoire Interuniversitaire des Systèmes Atmosphériques

2,6-Dimethylphenol has three internal rotors: two methyl rotors taking up positions ortho to the phenol-OH group and the OH rotor itself. We recorded the broadband microwave spectrum of 2,6-dimethylphenol over the 7.5-17.5 GHz range under jet-cooled conditions in the gas phase. In contrast to the tunneling doublets observed in phenol, a-type rotational transitions in 2,6-dimethylphenol appear as equally spaced triplets, with the two outer components split by ± 48.54 MHz relative to the central line. We fit the spectrum using a model in which OH tunneling either occurs or is quenched, depending on whether the two methyl rotors are in AA/EE states (tunneling present) or AE/EA states (tunneling quenched). Tunneling of the OH group is quenched in the AE/EA methyl rotor states due to OH/CH3 coupling that modulates the methyl rotor barrier height by almost a factor of 2, depending on the OH orientation relative to the methyl group. This, in turn, changes the energy of the AE and EA methyl rotor states, producing an effective asymmetry in the OH tunneling coordinate that is significantly greater than the tunneling splitting. This localizes the OH torsional wave functions in one or the other of the two wells. By contrast, the a-type rotational transitions in the AA and EE methyl rotor states possess a tunneling splitting similar to that observed in phenol. We compare and contrast the case of state-dependent quenching of tunneling encountered here to the reduction in tunneling splitting that can occur in asymmetric vibrational states.

Abstract. We present a novel methodology, AEROCHEMPro/GRASP (AEROsol CHEMical PROfiling), for retrieving vertical concentration profiles of aerosol chemical species by synergistically combining co-located measurements from a multiwavelength lidar and a multi-angular polarimeter. AEROCHEMPro represents the first retrieval framework for remote sensing of the vertically-resolved aerosol chemical composition, and it is based on an improved, vertical profiling version of the GRASP (Generalized Retrieval of Aerosol and Surface Properties) chemical component framework. The methodology is developed within the context of the Atmosphere Observing System (AOS) international initiative, which proposes a spaceborne observing system to advance our knowledge of aerosols, clouds, convection, and precipitation. Moreover, the retrieval strategy remains broadly applicable to future satellite missions and observing systems involving combined lidar and multi-angular polarimeter aerosol remote sensing. Based on a statistically optimized and physically-constrained inversion, AEROCHEMPro/GRASP delivers three distinct aerosol vertical profiles: (i) a fine mode composed of black carbon, brown carbon, inorganic salts, and associated water uptake; (ii) a coarse mode representing mineral dust, decomposed into iron oxide and quartz species; and (iii) a hydrophilic coarse mode consisting of sea salt particles and their associated water content. The approach explicitly retrieves these three aerosol profiles, along with the fractional abundance of each of the six mentioned aerosol chemical species and their water content. This retrieval of aerosol chemical composition vertical profiles offers, for the first time, a direct observational link between aerosol optical measurements and their speciation resolved in altitude. We demonstrate the feasibility and performance of this technique through an innovative retrieval experiment, where synthetic lidar and polarimeter observations are generated using the MOCAGE chemical transport model and a comprehensive radiative transfer simulator. These pseudo-observations include multi-wavelength attenuated backscatter and depolarization ratios, along with polarized radiances across multiple viewing angles and spectral bands. Results from global-scale transects spanning marine, urban, dust, and complex mixture-dominated regions show that the retrieval captures with good fidelity the main features of vertical aerosol composition and their bulk optical properties. Fine-mode species are well retrieved, particularly in the boundary layer, even for atmospheres with complex mixtures of multiple aerosol species. Dust is accurately retrieved and well defined in terms of vertical extent, load, and composition, and sea salt concentrations are very well reproduced. Some limitations for deriving water content in the upper atmospheric layers are remarked. AEROCHEMPro/GRASP reliably derives optical properties such as aerosol optical depth (AOD), single scattering albedo (SSA), and lidar ratio (LR), demonstrating the approach's robustness. This new retrieval approach represents a significant advancement in spaceborne aerosol remote sensing, as it provides vertically resolved chemical speciation that is directly related to chemical transport model results. It offers new opportunities to improve our understanding of aerosol processes and their effects on climate and air quality.

Abstract. Accurate representation of secondary ice production (SIP) is essential for describing the microphysics of deep convective clouds, yet the dominant mechanisms and their efficiencies remain uncertain. In this study, we use the University of Manchester bin microphysics parcel model to investigate four SIP parameterisations, including rime splintering, ice–ice collisional breakup, and two modes of droplet freezing fragmentation. Air parcel trajectories are simulated through deep convective clouds observed during the Deep Convective Microphysics Experiment (DCMEX) field campaign. The results show that fragmentation between supercooled droplets and more massive ice particles (mode 2) plays a key role in explaining the high ice particle concentrations observed. We further present a systematic assessment of how different entrainment representations, including adiabatic, homogeneous, and inhomogeneous mixing, influence secondary ice production. Homogeneous and inhomogeneous mixing with aerosol entrainment provide reasonable agreement with cloud-core and cloud-edge microphysical properties observed during DCMEX, respectively. The entrainment of external aerosols is found to accelerate the collision–coalescence process under homogeneous mixing, leading to earlier ice enhancement, while having little effect under inhomogeneous mixing.

Herein, the governing role of pyrolysis in the development of active sites in cobalt phthalocyanine (CoPc)-functionalized carbon for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) has been investigated. CoPc was mixed with two different carbon substrates, i.e, Vulcan XC72R and Ketjenblack EC-600JD (KJB), and then subjected to pyrolysis. The influence of pyrolysis temperature (25–800 °C) on the progression of morphological features, textural properties, and surface chemistries in both sets of samples was thoroughly analyzed using state-of-the-art in situ and ex situ characterization techniques. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray absorption spectroscopy (XAS) confirmed the preservation of original CoPc-based species dispersed in carbonaceous frameworks until 600 °C, and afterward, the evolution of CoO and metallic Co nanoparticles was observed. X-ray photoelectron spectroscopy (XPS) indicated a transformation in the superficial active moieties over the course of pyrolysis. Using a rotating disk electrode (RDE) and a rotating ring disk electrode (RRDE), the electrocatalytic performances of the derived electrocatalysts were analyzed toward HER and ORR, respectively, in alkaline media. The samples pyrolyzed at 600–700 °C showed peak electrochemical performances. Overall, KJB-based samples demonstrated slightly superior ORR activities, whereas Vulcan-based samples showed enhanced HER kinetics. The influence of the pyrolysis temperature and the nature of the carbon support on the evolution and transformation of active structures have been studied by developing a relationship among processing parameters, evolved active moieties, and electrochemical activities. Such insights can help in the optimization of structural attributes and performance of the electrocatalysts for diverse electrocatalytic applications.

Abstract. We use an offline radiative transfer model driven by IAGOS aircraft observations to estimate the tropospheric ozone radiative forcing (RF) at decadal time scale (three time intervals with a common starting period in 1994–2004 and ending periods: 2011–2016, 2017–2019 and 2020–2023), over 11 selected Northern Hemispheric regions. We found a positive trend in the tropospheric ozone column (TOC) for the three time intervals, even if decadal trends are progressively reduced from 2011–2016 (ΔTOC +3.6 ± 2.0 DU, +14.8 ± 11.5 %, 2.5 ± 1.4 DU per decade) to 2017–2019 (ΔTOC +3.9 ± 3.9 DU, +17.5 ± 22.1 %, 2.0 ± 2.0 DU per decade) to 2020–2023 (ΔTOC +3.5 ± 2.8 DU, +13.8 ± 12.1 %, 1.5 ± 1.2 DU per decade). The progressively reduced average TOC decadal trends in 2020–2023 and 2017–2019, with respect to 2011–2016, originate from stagnation in ΔTOC in more recent time intervals (especially in the COVID and post-COVID crisis periods) and decreases of trends of the more radiative efficient upper tropospheric ozone. These average trend reductions are accompanied with reductions of the tropospheric ozone RF, from 2011–2016 (41.6 ± 24.0 mW m−2 per decade) to 2017–2019 (27.6 ± 36.7 mW m−2 per decade) to 2020–2023 (14.6 ± 25.6 mW m−2 per decade). The total tropospheric ozone RF sensitivity varies between 18.4 ± 7.4 mW m−2 DU−1, in 2011–2016, and 11.1 ± 10.4 mW m−2 DU−1, in 2020–2023. Our decadal RF estimates vary from being ∼ 25 %–90 % larger (2011–2016 and 2017–2019 ending periods) to ∼ 30 % smaller (2020–2023 ending period) than the most recent global average RF estimates with online modelling. Our study underlines the importance of the evolution of ozone vertical profiles for the tropospheric ozone RF.

<strong class="journal-contentHeaderColor">Abstract.</strong> The accurate representation of aerosol physicochemical properties is important when describing aerosol-cloud interactions. The Deep Convective Microphysics Experiment (DCMEX) aimed to improve the representation of microphysical processes in deep convective systems. As part of this project, an airborne campaign (July to August 2022) was conducted to characterize the thermodynamics-dynamic-aerosol-cloud system over the isolated Magdalena Mountains in New Mexico, US. Backward dispersion analyses identified a transition in dominant airmass origins from Northwest (NW) continental to Southeast (SE) aged oceanic flow, coinciding with substantial changes in meteorological conditions and aerosol characteristics. The SE-flow period generally exhibited lower lifting condensation levels, enhanced convection, higher boundary layer humidity, and more frequent and intense precipitation, compared to the NW-flow period. During the SE-flow period, aerosol size distributions showed a more pronounced bimodality, characterized by increased Aitken-mode concentrations and fewer but larger accumulation-mode particles, indicating enhanced cloud processing. Submicron aerosols exhibited larger sulfate fractions, more oxidized organics, and greater hygroscopicity. Correspondingly, clouds presented larger droplet sizes and liquid water contents. A bin-microphysics parcel model was also employed to simulate the development of cloud droplets, constrained by airborne observations. Model-observation comparisons highlight the critical role of aerosol entrainment in reproducing the observed broad cloud droplet spectra extending toward small sizes. The results underscore the combined influence of meteorological conditions and aerosol characteristics on cloud microphysics under different flow regimes and emphasize the importance of aerosol entrainment in the development of deep convective clouds. This study provides valuable constraints for improving parameterizations of aerosol-cloud interactions in convective systems.

Abstract. Atmospheric simulation chambers are powerful tools for investigating atmospheric processes and form the basis for model parameterisations. Ensuring the atmospheric relevance of experimental conditions is crucial for understanding and predicting the impacts of secondary organic aerosols (SOA) on air quality and climate. However, chamber studies are often conducted under simplified conditions, which may limit their applicability to real-world scenarios. Here, we investigated the impact of CO on the mass yields and chemical composition of SOA particles formed from a biogenic volatile organic compound (VOC, α-pinene), an anthropogenic intermediate-volatility organic compound (IVOC, n-dodecane), and their mixture in the presence of nitrogen oxides (NOx = NO2 + NO) in the Manchester Aerosol Chamber (MAC). This photochemical system better represents polluted atmospheric conditions. The results show that the influence of CO differed between single- and mixed-precursor systems. In the single-precursor systems, CO significantly suppressed SOA particle mass yields, whereas no such suppression was observed in the mixture. Moreover, compared with the single-precursor systems, CO exerted a diminished impact on the organic peroxy (RO2) radical reaction pathways in the mixture, with the extent of this change differing between α-pinene and n-dodecane. These findings demonstrate that variations in reaction conditions can lead to different responses in SOA particle properties between the single- and mixed-precursor systems, highlighting the importance of conducting laboratory experiments under atmospherically relevant conditions.

<strong class="journal-contentHeaderColor">Abstract.</strong> In this study, we present two optimized analytical methods for the quantification of molecular markers to attribute the contribution of various Volatile Organic Compounds (VOC) oxidation products to Secondary Organic Aerosol (SOA). Those involve Ultrahigh Performance Liquid Chromatography Electrospray Ionization coupled to Ion Mobility Time of Flight Mass Spectrometry (UPLC/ESI-IMS-QTOFMS) and Gas Chromatography Mass Spectrometry (GC-MS). Liquid extraction was performed for both techniques, with an extra derivatisation step with N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) containing 1&thinsp;% trimethylchlorosilane (TMCS) for GC-MS analysis, enhancing the compound detection capacity. Between the two techniques, 23 biogenic and anthropogenic markers were identified, with five common species detected. Recoveries &gt; 80&thinsp;% were observed for nitro-containing compounds and &gt; 66&thinsp;% for aromatic and non-aromatic acids except for 3-methyl-1,2,3-butanetricarboxylic acid. Limits of detection &lt; 5&thinsp;ng were observed by UPLC/ESI-IMS-QTOFMS analysis for 4-nitrophenol and 2-methyl-4-nitrophenol, while GC-MS (with BSTFA derivatisation) analysis allowed better detection of lower mass compounds (for example limit of detection for 2-methylerythritol was 0.10&thinsp;ng). While UPLC/ESI-IMS-QTOFMS allow the analysis of high molecular weight compounds at high resolution and sensitivity, GC-MS analysis focus on compounds of lower mass and higher polarity, together, these complementary methods provide a comprehensive tool for the quantification of organic markers arising from the airborne transformation of compounds of both biogenic and anthropogenic origins.