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The reaction CH3NC ⇌ CH3CN, a model reaction for the study of unimolecular isomerization, is important in astronomy and atmospheric chemistry and has long been studied by numerous experiments and theories. In this work, we report the first full-dimensional accurate potential energy surface (PES) of this reaction by the permutation invariant polynomial-neural network method based on 30 974 points, whose energies are calculated at the CCSD(T)-F12a/AVTZ level. Then, ring polymer molecular dynamics is used to derive the free energy barrier of the reaction at the experimental temperature range of 472.55–532.92 K. Reaction kinetics are studied at the high-pressure limit and in the fall-off region by standard transition state theory and the master equation, respectively. The calculated temperature- and pressure-dependent rate coefficients are in good agreement with previous experimental and theoretical results. Furthermore, quasi-classical trajectory simulations are performed on this PES to study the intramolecular energy transfer dynamics at initial vibrational energies of 4.336, 5.204, and 6.505 eV. [ABSTRACT FROM AUTHOR]

This study examines the intermolecular interactions between small molecules and solvents, with a particular focus on pyruvic acid (PA). PA plays a significant role in biochemistry, astrochemistry, and atmospheric chemistry, particularly in aerosol particle formation. Previous studies on PA have been expanded upon by exploring its hydration and complexation with 2,2,2-trifluoroethanol (TFE). The clusters were generated using a supersonic expansion and characterized by broadband Fourier transform microwave spectroscopy. The structures of the clusters were identified by comparing the experimental results with high-level quantum-chemical computations. Among the possible isomers for the hydrated complex, the Tc-(H2O)2 kinetic complex, where PA exhibits an internal hydrogen bond, was favored over the Tt-(H2O)2 form, predicted to be the most stable conformer. Transitions from both the A and E internal rotation substates were observed exclusively in the dihydrate. The complex with TFE did not exhibit splitting due to the internal rotation of the methyl top. This is attributed to the presence of electronegative fluorine groups in TFE, stabilizing the complex through additional CH⋯F interactions, thereby hindering the internal rotation motion of the methyl top. [ABSTRACT FROM AUTHOR]

The curvature of soft interfaces plays a crucial role in determining their mechanical and thermodynamic properties, both at macroscopic and microscopic scales. In the case of air/water interfaces, particular attention has recently focused on water microdroplets, due to their distinctive chemical reactivity. However, the specific impact of curvature on the molecular properties of interfacial water and interfacial reactivity has so far remained elusive. Here, we use molecular dynamics simulations to determine the effect of curvature on a broad range of structural, dynamical, and thermodynamical properties of the interface. For a droplet, a flat interface, and a cavity, we successively examine the structure of the hydrogen-bond network and its relation to vibrational spectroscopy, the dynamics of water translation, rotation, and hydrogen-bond exchanges, and the thermodynamics of ion solvation and ion-pair dissociation. Our simulations show that curvature predominantly impacts the hydrogen-bond structure through the fraction of dangling OH groups and the dynamics of interfacial water molecules. In contrast, curvature has a limited effect on solvation and ion-pair dissociation thermodynamics. For water microdroplets, this suggests that the curvature alone cannot fully account for the distinctive reactivity measured in these systems, which are of great importance for catalysis and atmospheric chemistry. [ABSTRACT FROM AUTHOR]

The relaxed eddy accumulation (REA) method is a widely-known technique that measures turbulent fluxes of scalar quantities. The REA technique has been used to measure turbulent fluxes of various compounds, such as methane, ethene, propene, butene, isoprene, nitrous oxides, ozone, and others. The REA method requires the accumulation of scalar concentrations in two separate compartments that conditionally sample updrafts and downdraft events. It is demonstrated here that the assumptions behind the conventional or two-compartment REA approach allow for one-compartment sampling, therefore called a one compartment or 1-C-REA approach, thereby expanding its operational utility. The one-compartment sampling method is tested across various land cover types and atmospheric stability conditions, and it is found that the one-compartment REA can provide results comparable to those determined from conventional two-compartment REA. This finding enables rapid expansion and practical utility of REA in studies of surface-atmosphere exchanges, interactions, and feedbacks.

The precise determination of surface transport coefficients at liquid interfaces is critical to an array of processes, ranging from atmospheric chemistry to catalysis. Building on our prior results that highlighted the emergence of a greatly reduced surface viscosity in simple liquids via the dispersion relation of surface excitations [Malgaretti et al., J. Chem. Phys. 158, 114705 (2023)], this work introduces a different approach to directly measure surface viscosity. We use modified Green–Kubo relations suitable for inhomogeneous systems to accurately quantify viscosity contributions from fluid slabs of variable thickness through extensive molecular dynamics simulations. This approach distinguishes the viscosity effects of the surface layer vs the bulk, offering an independent measure of surface viscosity and providing a more detailed understanding of interfacial dynamics and its transport coefficients. [ABSTRACT FROM AUTHOR]

Noncovalent bonding between atmospheric molecules is central to the formation of aerosol particles and cloud condensation nuclei and, consequently, radiative forcing. While our understanding of O–H⋯B interactions is well developed, S–H⋯B hydrogen bonding has received far less attention. Sulfur- and nitrogen-containing molecules, particularly amines, play a significant role in atmospheric chemistry, yet S–H⋯N interactions are not well understood at a fundamental level. To help characterize these systems, H2S and methyl-, ethyl-, n-propyl-, dimethyl-, and trimethylamine (MA, EA, n-PA, DMA, and TMA) have been investigated using matrix isolation Fourier transform infrared spectroscopy and high-level theoretical methods. Experiments showed that H2S forms hydrogen bonded complexes with each of the amines, with bond strengths following the trend MA ≈ EA ≈ n-PA < TMA ≤ DMA, in line with past experimental work on H2SO4·amine complexes. However, the calculated results indicated that the trend should be MA < DMA < TMA, in line with past theoretical work on H2SO4·amine complexes. Evidence of strong Fermi resonances indicated that anharmonicity may play a critical role in the stabilization of each complex. The theoretical results were able to replicate experiment only after binding energies were recalculated to include the anharmonic effects. In the case of H2SO4·amine complexes, our results suggest that the discrepancy between theory and experiment could be reconciled, given an appropriate treatment of anharmonicity. [ABSTRACT FROM AUTHOR]

Many applications involving plasma–liquid interactions depend on the reactive processes occurring at the plasma–liquid interface. We report on a falling liquid film plasma reactor allowing for in situ optical absorption measurements of the time-dependence of the ferricyanide/ferrocyanide redox reactivity, complemented with ex situ measurement of the decomposition of formate. We found excellent agreement between the measured decomposition percentages and the diffusion-limited decomposition of formate by interfacial plasma-enabled reactions, except at high pH in thin liquid films, indicating the involvement of previously unexplored plasma-induced liquid phase chemistry enabled by long-lived reactive species. We also determined that high pH facilitates a reduction-favoring environment in ferricyanide/ferrocyanide redox solutions. In situ conversion measurements of a 1:1 ferricyanide/ferrocyanide redox mixture exceed the measured ex situ conversion and show that conversion of a 1:1 ferricyanide/ferrocyanide mixture is strongly dependent on film thickness. We identified three dominant processes: reduction faster than ms time scales for film thicknesses >100 µm, •OH-driven oxidation on time scales of <10 ms, and reduction on 15 ms time scales for film thickness <100 µm. We attribute the slow reduction and larger formate decomposition at high pH to HO 2 − formed from plasma-produced H2O2 enabled by the high pH at the plasma–liquid interface as confirmed experimentally and by computed reaction rates of HO 2 − with ferricyanide. Overall, this work demonstrates the utility of liquid film reactors in enabling the discovery of new plasma-interfacial chemistry and the utility of atmospheric plasmas for electrodeless electrochemistry. [ABSTRACT FROM AUTHOR]

The α-hydroxyethyl radical (CH3·CHOH, 2A) is a key intermediate in ethanol biochemistry, combustion, atmospheric chemistry, radiation chemistry, and astrochemistry. Experimental data on the vibrational spectrum of this radical are crucially important for reliable detection and understanding of the chemical dynamics of this species. This study represents the first detailed experimental report on the infrared absorption bands of the α-hydroxyethyl radical complemented by ab initio computations. The radical was generated in solid para-H2 and Xe matrices via the reactions of hydrogen atoms with matrix-isolated ethanol molecules and radiolysis of isolated ethanol molecules with x rays. The absorption bands with maxima at 3654.6, 3052.1, 1425.7, 1247.9, 1195.6 (1177.4), and 1048.4 cm−1, observed in para-H2 matrices appearing upon the H· atom reaction, were attributed to the OHstr, α-CHstr, CCstr, COstr + CCObend, COstr, and CCstr + CCObend vibrational modes of the CH3·CHOH radical, respectively. The absorption bands with the positions slightly red-shifted from those observed in para-H2 were detected in both the irradiated and post-irradiation annealed Xe matrices containing C2H5OH. The results of the experiments with the isotopically substituted ethanol molecules (CH3CD2OH and CD3CD2OH) and the quantum-chemical computations at the UCCSD(T)/L2a_3 level support the assignment. The photolysis with ultraviolet light (240–300 nm) results in the decay of the α-hydroxyethyl radical, yielding acetaldehyde and its isomer, vinyl alcohol. A comparison of the experimental and theoretical results suggests that the radical adopts the thermodynamically more stable anti-conformation in both matrices. [ABSTRACT FROM AUTHOR]

The prediction accuracy of atmospheric visibility significantly impacts daily life. However, there is a relative scarcity of research on post-processing methods for visibility obtained from the WRF-Chem atmospheric chemistry model results. In order to explore a more accurate method for visibility calculation, we conducted a study on the meteorological conditions in the East China region during a heavy pollution period from October 1 to 23 in the year of 2022. The meteorological data were processed using both the XGBoost (XGB) model and the IMPROVE to calculate visibility. The results indicate that XGB outperforms the IMPROVE in various aspects. The visibility improved from a correlation of 0.56–0.71 with the use of XGB. And in comparison with the IMPROVE equation, XGB demonstrated a statistically significant reduction in RMSE by 1.96 km. Even in regions where the IMPROVE performs poorly, XGB demonstrates superior performance. In regions where the correlation simulated by the IMPROVE equation is less than 0.2 (Anqing and Nanyang), XGB still performs well, achieving correlations of 0.69 (Anqing) and 0.68 (Nanyang). Throughout the entire study period, the average visibility results obtained by XGB deviate by only 0.07 km from the observed values. These findings underscore the importance of incorporating the XGBoost model into WRF-Chem visibility simulations, as it significantly improves the accuracy of visibility predictions. [ABSTRACT FROM AUTHOR]

Long path length FTIR‐smog chamber techniques were used to study the title reactions in 650 Torr of N2, oxygen, or air diluent at 296 ± 3 K. Values of k(Cl + (Z)‐CF2HCF = CHCl)═(6.6 ± 0.7) × 10−11 and k(OH + (Z)‐CF2HCF═CHCl)═(4.1 ± 0.7) × 10−12 cm3 molecule−1 s−1 were measured. The IR spectrum of (Z)‐CF2HCF═CHCl is reported. The atmospheric lifetime of (Z)‐CF2HCF═CHCl is determined by the reaction with OH and is approximately 2.8 days. Reaction of (Z)‐CF2HCF═CHCl with Cl atoms gives HC(O)Cl and CF2HC(O)F as major primary products. Under environmental conditions, the OH radical initiated oxidation gives CF2HC(O)F and HC(O)Cl in yields of (98 ± 8)% and (100 ± 4)%, respectively. Accounting for non‐uniform horizontal and vertical mixing leads to a 100‐year time‐horizon global warming potential value for (Z)‐CF2HCF═CHCl of essentially zero. [ABSTRACT FROM AUTHOR]

Atmospheric energy transport is central to the cooling of primordial magma oceans. Theoretical studies of atmospheres on lava planets have assumed that convection is the only process involved in setting the atmospheric temperature structure. This significantly influences the ability for a magma ocean to cool. It has been suggested that convective stability in these atmospheres could preclude permanent magma oceans. We develop a new 1D radiative-convective model in order to investigate when the atmospheres overlying magma oceans are convectively stable. Using a coupled interior-atmosphere framework, we simulate the early evolution of two terrestrial-mass exoplanets: TRAPPIST-1 c and HD 63433 d. Our simulations suggest that the atmosphere of HD 63433 d exhibits deep isothermal layers which are convectively stable. However, it is able to maintain a permanent magma ocean and an atmosphere depleted in |$\mathrm{H_{2}O}$|⁠. It is possible to maintain permanent magma oceans underneath atmospheres without convection. Absorption features of |$\mathrm{CO_{2}}$| and |$\mathrm{SO_{2}}$| within synthetic emission spectra are associated with mantle redox state, meaning that future observations of HD 63433 d may provide constraints on the geochemical properties of a magma ocean analogous with the early Earth. Simulations of TRAPPIST-1 c indicate that it is expected to have solidified within |$100 \,\mathrm{M}\rm {yr}$|⁠ , outgassing a thick atmosphere in the process. Cool isothermal stratospheres generated by low-molecular-weight atmospheres can mimic the emission of an atmosphere-less body. Future work should consider how atmospheric escape and chemistry modulates the lifetime of magma oceans, and the role of tidal heating in sustaining atmospheric convection. [ABSTRACT FROM AUTHOR]

[Display omitted] Elevated concentrations of formaldehyde and other carbonyl compounds are frequently observed in the marine atmosphere but are often significantly underestimated by atmospheric models. To evaluate the potential impact of marine sources on atmospheric formaldehyde, high-resolution measurements were conducted at a coastal site (∼15 m from the sea) during the summer in Qingdao, China. Observed formaldehyde levels averaged 2.4 ± 0.9 ppbv (1 ppbv = 10–9 L L–1), with peaks reaching 6.8 ppbv. Backward trajectories indicate that formaldehyde concentrations remained high in marine air masses. Formaldehyde exhibited weak correlations with primary pollutants such as NO and CO but showed strong correlations with marine tracers, notably methyl ethyl ketone and 1-butene. Chamber experiments confirmed that the photodecomposition of Enteromorpha released large amounts of formaldehyde and marine tracer species. When normalized to acetylene, the levels of formaldehyde, 1-butene, and MEK increased by factors of 3.8, 8.1, and 3.5, respectively. Results from an observation-based chemical box model simulation, which utilizes the Master Chemical Mechanism (MCM), revealed that formaldehyde contributes 56% to the primary source of HO 2 radicals, while neglecting formaldehyde chemistry would lead to a 15% reduction in coastal ozone production rates. This study interlinks oceanic biology and atmospheric chemistry, advancing the understanding of the ocean's role as a significant source of organic compounds and its contribution to carbon cycling. [ABSTRACT FROM AUTHOR]

The difficulty of quantum chemically computing vibrational, rotational, and rovibrational reference data via quartic force fields (QFFs) for molecules containing aluminum appears to be alleviated herein using a hybrid approach based upon CCSD(T)‐F12b/cc‐pCVTZ further corrected for conventional CCSD(T) scalar relativity within the harmonic terms and simple CCSD(T)‐F12b/cc‐pVTZ for the cubic and quartic terms: the F12‐TcCR+TZ QFF. Aluminum containing molecules are theorized to participate in significant chemical processes in both the Earth's upper atmosphere as well as within circumstellar and interstellar media. However, experimental data for the identification of these molecules are limited, showcasing the potential for quantum chemistry to contribute significant amounts of spectral reference data. Unfortunately, current methods for the computation of rovibrational spectral data have been shown previously to exhibit large errors for aluminum‐containing molecules. In this work, ten different methods are benchmarked to determine a method to produce experimentally‐accurate rovibrational data for theorized aluminum species. Of the benchmarked methods, the explicitly correlated, hybrid F12‐TcCR+TZ QFF consistently produces the most accurate results compared to both gas‐phase and Ar‐matrix experimental data. This method combines the accuracy of the composite F12‐TcCR energies along with the numerical stability of non‐composite anharmonic terms where the non‐rigid nature of aluminum bonding can be sufficiently treated. [ABSTRACT FROM AUTHOR]

The Artificial-Intelligence-based Nonspherical Aerosol Optical Scheme (AI-NAOS) is a newly developed aerosol optical module that improves the representation of aerosol optical properties for radiative transfer simulations in atmospheric models. It incorporates the nonsphericity and inhomogeneity (NSIH) of internally mixed aerosol particles through a deep learning method. Specifically, the AI-NAOS considers black carbon (BC) to be fractal aggregates and models soil dust (SD) as super-spheroids, encapsulated partially or completely with hygroscopic aerosols such as sulfate, nitrate, and aerosol water. To obtain AI-NAOS, a database of the optical properties for the models was constructed using the invariant imbedding T-matrix method (IITM), and deep neural networks (DNN) were trained based on this database. In this study, the AI-NAOS was integrated into the mesoscale version 5.1 of Global/Regional Assimilation and Prediction System with Chinese Unified Atmospheric Chemistry Environment (GRAPES_Meso5.1/CUACE). Real-case simulations were conducted during a winter with high pollution, comparing BC aerosols evaluated using three schemes with spherical aerosol models (external-mixing, core-shell, and volume-mixing schemes) and the AI-NAOS scheme. The results showed that the NSIH effect led to a moderate estimation of absorbing aerosol optical depth (AAOD) and obvious changes in aerosol radiative effects, shortwave heating rates, temperature profiles, and boundary layer height. The AAOD values based on three spherical schemes were 70.4 %, 125.3 %, and 129.3 % over the Sichuan Basin, benchmarked to AI-NAOS results. Compared to the external-mixing scheme, the direct radiative effect (DRE) induced by the NSIH effect reached +1.6 W m−2 at the top of the atmosphere (TOA) and -2.9 W m−2 at the surface. The NSIH effect could enhance the shortwave heating rate, reaching 23 %. Thus, the warming effect at 700 hPa and the cooling effect on the ground were strengthened by 21 % and 13 %, reaching +0.04 and -0.10 K, which led to a change in the height of the planetary boundary layer (PBL) by -11 m. In addition, the precipitation was inhibited by the NSIH effect, causing a 15 % further decrease. Therefore, the NSIH effects demonstrated their non-negligible impacts and highlighted the importance of incorporating them into chemical weather models. [ABSTRACT FROM AUTHOR]

Microorganisms in the atmosphere comprise a small fraction of the Earth's microbiome. A significant portion of this aeromicrobiome consists of bacteria that typically remain airborne for a few days before being deposited. Unlike bacteria in other spheres (e.g., litho-, hydro-, phyllo-, cryospheres), atmospheric bacteria are aerosolized, residing in individual particles and separated from each other. In the atmosphere, bacteria encounter chemical and physical conditions that affect their stress levels and survival. This article goes beyond previous overviews by placing these conditions in the context of fundamental chemical and microphysical concepts related to atmospheric aerosols. We provide ranges of water amounts surrounding bacterial cells both inside and outside clouds and suggest that the small volumes of individual cloud droplets lead to nutrient and oxidant limitations. This may result in greater nutrient limitation but lower oxidative stress in clouds than previously thought. Various chemical and microphysical factors may enhance or reduce microbial stress (e.g., oxidative, osmotic, UV-induced), affecting the functioning and survival of atmospheric bacteria. We illustrate that these factors could impact stress levels under polluted conditions, indicating that conclusions about the role of pollutants in directly causing changes to microbial abundance can be erroneous. The perspectives presented here aim to motivate future experimental and modeling studies to disentangle the complex interplay of chemical and microphysical factors with the atmospheric microbiome. Such studies will help to comprehensively characterize the role of the atmosphere in modifying the Earth' microbiome, which regulates the stability of global ecosystems and biodiversity. [ABSTRACT FROM AUTHOR]

Chemical ionization mass spectrometry (CIMS) is widely used in atmospheric chemistry studies. However, due to the complex interactions between reagent ions and target compounds, chemical understanding remains limited and compound identification difficult. In this study, we apply machine learning to a reference dataset of pesticides in two standard solutions to build a model that can provide insights from CIMS analyses in atmospheric science. The CIMS measurements were performed with an Orbitrap mass spectrometer coupled to a thermal desorption multi-scheme chemical ionization inlet unit (TD-MION-MS) with both negative and positive ionization modes utilizing Br- , O2- , H3O+ and (CH3)2COH+ (AceH+) as reagent ions. We then trained two machine learning methods on these data: (1) random forest (RF) for classifying if a pesticide can be detected with CIMS and (2) kernel ridge regression (KRR) for predicting the expected CIMS signals. We compared their performance on five different representations of the molecular structure: the topological fingerprint (TopFP), the molecular access system keys (MACCS), a custom descriptor based on standard molecular properties (RDKitPROP), the Coulomb matrix (CM) and the many-body tensor representation (MBTR). The results indicate that MACCS outperforms the other descriptors. Our best classification model reaches a prediction accuracy of 0.85 ± 0.02 and a receiver operating characteristic curve area of 0.91 ± 0.01. Our best regression model reaches an accuracy of 0.44 ± 0.03 logarithmic units of the signal intensity. Subsequent feature importance analysis of the classifiers reveals that the most important sub-structures are NH and OH for the negative ionization schemes and nitrogen-containing groups for the positive ionization schemes. [ABSTRACT FROM AUTHOR]

Ambient Volatile Organic Compounds (VOCs) were investigated to determine their characteristics, Ozone Formation Potentials (OFPs), and health risks in two crude oil production plants (Nusaybin and Egil plants) in southeastern Türkiye. Benzene, toluene, ethylbenzene, m + p xylene, o xylene, and 1,3,5-trimethylbenzene were measured at eight passive sampling points in each plant. Samples were analyzed using gas chromatography coupled with a flame ionization detector and a thermal desorption. The concentration of ∑ 6 VOC ranged from 5.03 to 88.43 μg/m3 in the Nusaybin Plant and from 7.70 to 154.35 μg/m3 in the Egil Plant. Toluene and xylenes were predominant in both plants. In the Nusaybin Plant, VOCs were mainly associated with crude oil production, while in the Egil Plant, they were associated with a combination of crude oil production and mobile vehicle activities. The OFP of ∑ 6 VOC was 297.47 μg/m3 in the Nusaybin Plant, and 249.25 μg/m3 in the Egil Plant. M + p xylene, toluene, and 1,3,5-trimethylbenzene together contributed 86% and 84% of the total OFP in the Nusaybin and Egil plants, respectively. Benzene exposure posed a possible cancer risk to oil workers in both plants. Non-cancer health risk was at a potential level in the Egil Plant but negligible in the Nusaybin Plant. This study is expected to enhance knowledge regarding the effects of crude oil production plants on air quality and workplace exposure. [ABSTRACT FROM AUTHOR]

Patterns of ionospheric luminosity provide a unique window into our complex, coupled space environment. The aurora, for example, indicates plasma processes occurring thousands of km away, depositing immense amounts of energy into our polar ionospheres. Here we show observations of structured continuum emission associated with the dynamic aurora. The presence of weak ambient continuum emission has long been recognized. However, studies of its relationship to aurora are scarce and limited by observational constraints. We use spectrally resolved measurements to analyze these previously unexplained emissions, adding critical information about spatial structure, characteristic spectra, and location within auroral dynamics. Our findings demonstrate that the coupling among auroral processes, the plasma, and the neutral atmosphere can unfold at meso-scales and is more complex than previously reported. We suggest that the meso-scale auroral precipitation may, under certain circumstances, effectively couple to atmospheric chemistry and conditions to produce the continuum structure. There is a continuum emission component in the aurorae spectra that is unexplained. Here, the authors show gray-toned continuum emission structures associated with auroral dynamics explains previous continuum emission observations. [ABSTRACT FROM AUTHOR]

As a highly reactive dialkenes similar to isoprene, 1,3‐butadiene (BD) is rapidly oxidized and fully involved in atmospheric oxidation processes. We have established a method for the online measurement of ambient BD using Differential Optical Absorption Spectroscopy (DOAS) technique. Lab testing demonstrated that DOAS technique can accurately measure BD concentrations in high temporal resolution of minute‐level, with accuracy and precision within ±5% and 1%, respectively. Following field measurement, the detection limit for BD reached 90 pptv, the r between DOAS and the online VOCs system results reached 0.885, and the nighttime hourly averaged concentration peaked at 0.75 ppbv. At this BD level, if the NO3 reaction pathway with BD was not considered, the maximum proportional variations in the NO3 concentration simulated by the box model exceeded 10%. In conclusion, DOAS is suitable for long‐term BD measurements, and continuous attention should be given to the potential alteration on atmospheric chemistry caused by this type of easily neglected compounds. Plain Language Summary: 1,3‐Butadiene (BD) is a highly reactive gas that plays an important role in atmospheric chemistry. We introduced a non‐contact optical method, Differential Optical Absorption Spectroscopy (DOAS), to measure BD concentration in the ambient air. Our lab tests showed that DOAS is accurate and precise in measuring BD with high time resolution. We thoroughly evaluated the performance of DOAS and then conducted real atmosphere measurements, comparing the results with other instruments. We also explored the impact of high ambient BD concentrations on atmospheric nighttime chemistry. Overall, DOAS proves to be a reliable method for long‐term BD measurements, and it is important to pay attention to how this kind of compound may alter atmospheric chemistry in the future. Key Points: The lab tests demonstrated that the DOAS technique is suitable for long‐term monitoring ambient BDThe evaluation and comparison with other methods testified the good performance of BD measured by DOAS techniqueHigh ambient BD have the potential to alter nighttime chemistry, with maximum proportional variations in NO3 concentration exceeding 10% [ABSTRACT FROM AUTHOR]

This study offers insights into the complex relationship between chemical species constituting air pollution and chemosensory function. We examined the relationship between chemical species known to contribute to air pollution and assault human health and chemosensory sensitivity. Chemosensory sensitivity data was retrieved from a large-scale study involving 711 urban-dwelling participants inhabiting 10 different regions of the globe. Their olfactory threshold towards phenyl ethyl alcohol (PEA) and olfactory/trigeminal threshold towards Eucalyptol was measured in a multicentre study. We matched the individual chemosensory data with the levels of PM2.5, PM10, O3, NO2, SO2, CO at the location of testing sites, on the exact date of the test, using EMAC (ECHAM5/MESSy for Atmospheric Chemistry) model. Our findings indicate that air pollution negatively affects olfactory function and has cumulative negative effects with aging. The reported patterns are seasonal and increase during Autumn and Winter, and interact with medical conditions related to poorer olfactory function. We extend the current knowledge by demonstrating that olfactory/trigeminal perception is also disrupted by toxic air, albeit in a slightly different manner. The analyzed models promote a more complex perspective on the relationship between air composition and chemosensory sensitivity, but delineate problems related to the interdependence of the levels of chemical species constituting air pollution and using them together to predict chemosensory sensitivity. Conclusions point to the need to investigate the problem of air pollution and chemosensory health from a global perspective, as air quality partly accounts for the differences in chemosensory perception in different regions of the world. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) significantly impact human health due to their persistence, toxicity, and potential carcinogenicity. Their global distribution and regional changes caused by emission changes, especially over areas in developing countries, remain to be understood along with their health impacts. This study implemented a PAH module in the global–regional nested Atmospheric Aerosol and Chemistry Model of the Institute of Atmospheric Physics (IAP-AACM) to investigate the global distribution of PAHs and the change in their health risks from 2013 to 2018 in China. An evaluation against observations showed that the model could capture well the spatial distribution and seasonal variation in Benzo[a]pyrene (BaP), the typical indicator species of PAHs. On a global scale, the annual mean concentrations are the highest in China followed by Europe and India, with high values exceeding the target values of 1 ng m−3 over some areas. Compared with 2013, the concentration of BaP in China decreased in 2018 due to emission reductions, whereas it increased in India and southern Africa. However, the decline is much smaller than for PM2.5 during the same period. The concentration of BaP decreased by 8.5 % in Beijing–Tianjin–Hebei (BTH) and 9.4 % in the Yangtze River Delta (YRD). It even increased over areas in the Sichuan Basin due to changes in meteorological conditions. The total incremental lifetime cancer risk (ILCR) posed by BaP only showed a slight decrease in 2018, and the population in east China still faced significant potential health risks. The results indicate that strict additional control measures should be taken to reduce the pollution and health risks of PAHs effectively. The study also highlights the importance of considering changes in meteorological conditions when evaluating emission changes from concentration monitoring. [ABSTRACT FROM AUTHOR]

The chemistry of atmospheric precipitation serves as an important proxy for discriminating the source(s) of air contaminants in urban environments as well as to discuss the dynamic of atmospheric chemistry exchanges. This approach can be undertaken at time scales varying from single events to seasonal and yearly time frames. Here, we characterized the chemical composition of two single rain episodes (18 July 2018 and 21 February 2019) collected in Wrocław (SW Poland). Our results demonstrated inner variations and seasonality (within the rain event as well as between summer and winter), both in ion concentrations as well as in their potential relations with local air contaminants and scavenging processes. Coupling statistical analysis of chemical parameters with meteorological/synoptic conditions and HYSPLIT back trajectories allowed us to identify three main factors (i.e., principal components; PC) controlling the chemical composition of precipitation, and that these fluctuated during each event: (i) PC1 (40%) was interpreted as reflecting the long-range transport and/or anthropogenic influences of emission sources that included biomass burning, fossil fuel combustion, industrial processes, and inputs of crustal origin; (ii) PC2 (20%) represents the dissolution of atmospheric CO2 and HF into ionic forms; and (iii) PC3 (20%) originates from agricultural activities and/or biomass burning. Time variations during the rain events showed that each factor was more important at the start of the event. The study of both SO42− and Ca2+ concentrations showed that while sea spray inputs fluctuated during both rain events, their overall impact was relatively low. Finally, below-cloud particle scavenging processes were only observed for PM10 at the start of the winter rain episode, which was probably explained by the corresponding low rain intensity and an overlap from local aerosol emissions. Our study demonstrates the importance of multi-time scale approaches to explain the chemical variability in rainwater and both its relation to emission sources and the atmosphere operating processes. [ABSTRACT FROM AUTHOR]

The Brewer‐Dobson circulation (BDC) characterizes the large‐scale meridional overturning mass circulation influencing the composition of the whole middle atmosphere. The BDC consists of two separate parts—a shallow branch in the lower stratosphere and a deep branch higher in the middle atmosphere. Climate models robustly project the advective BDC part to accelerate due to greenhouse gas‐induced climate change and this acceleration strongly influences middle atmospheric chemistry and physics in the projections. A prominent quantity that is being studied as a proxy for advective BDC changes is the net tropical upwelling across pressure levels, particularly in the lower stratosphere. The upper branch of the BDC has received considerably less research attention than its shallow part, although, together with the mean mesospheric pole‐to‐pole circulation, it couples the stratosphere and mesosphere and is responsible for a large portion of the interhemispheric transport in the middle atmosphere. Aiming to fill this gap, we here study climatology and trends in advective mass transport across the stratopause. Results based on the analysis of seven CCMI models include quantification of the climatological cross‐stratopause advective transport, characterization of its interannual variability and long‐term trend as well as detailed analysis of inter‐model differences using a novel decomposition methodology. Our results demonstrate that the changes in circulation speed as well as changes in horizontal and vertical structure in the upper stratosphere and mesosphere jointly shape the projected increasing advective mass flux across the stratopause due to increasing greenhouse‐gas emissions. Plain Language Summary: The Brewer‐Dobson circulation (BDC) is an important circulation pattern, affecting the composition of the entire middle atmosphere. Our study focuses on the less‐studied higher part of the BDC, which connects the stratosphere and mesosphere. Analyzing comprehensive climate model simulations, we found changes in circulation speed, along with horizontal and vertical structure changes of the upper BDC branch as a response to increasing greenhouse gas emissions. Understanding these changes aids improvements of climate predictions and highlights the significant influence of human activities on the atmosphere. Key Points: Advective transport between the stratosphere and mesosphere exhibits a robust annual cycleModels project an average amplification of cross‐stratopause transport of about 1% per decade during all seasonsThe transport decomposition reveals inconsistencies between the models regarding the causes of long‐term changes [ABSTRACT FROM AUTHOR]

Reactive bromine species play important roles in atmospheric chemistry and influence the abundance of climate‐ and air quality‐relevant trace gases. While extensive studies have focused on bromine chemistry in polar regions, the bromine species in extrapolar regions, particularly regarding pollution‐related bromine recycling, have received less attention. In this study, we examine the factors influencing the bromine recycling at a clean marine site of the North Atlantic (Cape Verde, CVAO), a semipolluted coastal site of the South China Sea (Hok Tsui, HT), and a polluted continental site of North China (Wangdu, WD), using an observation‐based model combined with updated halogen chemistry. Our results indicate that aerosol‐related multiphase processes significantly affect bromine recycling in these extrapolar sites. Nitrate photodissociation efficiently recycled bromide into the gas phase in regions characterized by strong aerosol acidity (HT) or high aerosol concentrations (WD). Concurrently, bromine species are lost from the gas phase toward aerosol surfaces, with this process being more effective at higher liquid water content (WD > HT > CVAO). In semipolluted and polluted sites, the aerosol‐related processes resulted in elevated bromine levels in the daytime compared to nighttime, leading to larger effects of bromine species on ozone than previously thought. Our study suggests the need for a better understanding of the complex effects of aerosols on reactive halogen chemistry. Plain Language Summary: Reactive bromine species significantly influence the abundance of climate‐ and air quality‐relevant trace gases. Over the past decades, the sources and atmospheric impacts of bromine species in polar regions have been extensively studied due to their critical role in the Arctic ozone depletion episodes (ODEs). Both observations and laboratory studies have identified aerosols as crucial players in bromine explosions and subsequent Arctic ODEs. Here, using an observation‐constrained multiphase chemical box model with updates in halogen chemistry, we demonstrate that aerosols can provide a medium for bromine species activation or loss in extrapolar marine and continental regions, depending on their compositions and properties. These aerosol‐related processes are not included in current chemistry climate models, potentially hindering the accurate assessment of the impact of halogen species on climate and air quality. Key Points: We updated bromine‐aerosol interaction mechanisms and improved bromine simulations across extrapolar marine and continental regionsAerosols can provide a medium for bromine species activation or loss, depending on aerosol compositions and propertiesOverlooking aerosol‐related processes may underestimate the effects of bromine species on ozone in semipolluted and polluted regions [ABSTRACT FROM AUTHOR]

Particulate nitroaromatic compounds (NACs), among the major atmospheric components of light‐absorbing brown carbon, have garnered increasing attentions due to their impacts on atmospheric environment and ecological health. However, there is a scarcity of comprehensive understanding on their origins and formation pathways. In this study, 14 particulate NACs were measured at high time resolution at the summit of Mount Tai over North China Plain in winter with an average concentration of 30.5 ng m−3. The relatively higher concentrations and distinct diurnal profiles of NACs suggest the occurrence of strong secondary formation driven from polluted urban plumes transported by mountain‐valley breezes, elevated NOx and precursor levels, and strong oxidation capacity. An observationally constrained multiphase chemical box model was developed and employed to elucidate the origins and formation pathways of NACs. They mainly came from the air mass transport from the polluted urban regions to the mountaintop during the day, while the nocturnal NACs formation is dominated by the partitioning of gaseous NACs to the particle phase and heterogenous processes on aerosols. This work provides observational evidence of elevated NACs levels at high mountain site as well as modeling reference for multiphase processes of particulate NACs and the contributions from different formation pathways to a large degree addressing the problem of underestimation in traditional models. Plain Language Summary: Particulate nitroaromatic compounds (NACs), as important atmospheric pollutants, have significant effects on air quality, atmospheric chemistry, and health. Here, high‐resolution particulate NACs observational measurements were conducted at a mountain site in northern China in winter. The distinct diurnal patterns and temporal variation in particulate NACs pollution were examined. The influencing factors including mountain‐valley breezes, photooxidation reactions, gas‐particle partitioning, and heterogenous processes on aerosols were comprehensively investigated. Particularly, a multiphase chemical box model based on MCM‐CAPRAM and constrained by field observations was developed and then employed to unravel the origins and quantitative contributions of fine particulate NACs. This study provides observational and modeling evidences for the relatively high particulate NACs concentrations at mountain site in northern China and reports the non‐negligible and significant contributions of gas‐particle partitioning and heterogeneous processes on the formation of particulate NACs. Key Points: Particulate nitroaromatic compounds (NACs) exhibited daytime enhancement due to mountain‐valley breezes and secondary formationObservation‐based multiphase chemical box model was developed for particulate NACsGas‐particle partitioning and heterogenous processes contributed greatly to particulate NACs [ABSTRACT FROM AUTHOR]

In this work, the rate coefficients for OH radical, k1(T), and Cl atom, k2(T), reaction with allyl 1,1,2,2-tetrafluoroethyl ether, CH2=CHCH2OCF2CHF2, were studied as a function of temperature and pressure in a collaborative effort made between UCLM, Spain, and LAPKIN, Greece. OH rate coefficients were determined in UCLM, between 263 and 353 K and 50–600 Torr, using the absolute rate method of pulsed laser photolysis-laser-induced fluorescence technique, while Cl kinetics were studied in temperature (260–363 K) and pressure (34–721 Torr) ranges, using the relative rate method of the thermostated photochemical reactor equipped with Fourier transform infrared spectroscopy as the detection technique. In both OH and Cl reactions, a negative temperature dependence of the measured rate coefficients was observed, which is consistent with complex association reactions. The temperature dependence of OH rate coefficients was found to be well represented by the following expression: k1(T) = (2.30 ± 0.35) × 10−12 exp[(544 ± 46) K/T] cm3 molecule−1 s−1. In the case of the Cl-initiated reaction, a slight curvature was observed in the Arrhenius plot for k2(T), and the kinetic data were fitted to a modified Arrhenius expression: k2(T) = (4.42 ± 0.32) × 10−16 T2 exp[(610 ± 22) K/T] cm3 molecule−1 s−1. No pressure dependence was observed in either case. These results are consistent with a complex reaction mechanism that is not uncommon in radical association reactions to the unsaturated bond. As part of this work, UV (200–400 nm) and infrared absorption spectra (500–3200 cm−1) were also measured to further evaluate CH2=CHCH2OCF2CHF2 atmospheric impact. Atmospheric lifetimes with respect to OH radical and Cl atom reactions were estimated to be 19.8 h and 38 days, respectively, showing that OH radicals dominate atmospheric oxidation. CH2=CHCH2OCF2CHF2 is a very weak absorber in the solar actinic region, while its relatively low radiative efficiency in the atmospheric IR window, 0.0034 W m−2 ppbv−1, and the short lifetime led to a very low GWP value relative to CO2, 1.2 × 10−2 and 3.3 × 10−3, at time horizons of 20 and 100 years, respectively. [ABSTRACT FROM AUTHOR]

The H2O 1.38-µm band has been used to measure water vapor in the Martian atmosphere since the MAWD (Mars Atmospheric Water Detector) experiment on Viking-1 and -2. Currently, two experiments in orbit around Mars—SPICAM IR (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars InfraRed) on Mars Express and ACS NIR (Atmospheric Chemistry Suite Near InfraRed) on the Trace Gas Orbiter—are measuring water vapor in this spectral range. The spectrometers provide monitoring of the seasonal cycle of the column water vapor abundance and its vertical distribution over several Martian years. The HITRAN (High Resolution Transmission) database was used as a spectroscopic information for water vapor retrievals in these experiments. To take into account the broadening of water vapor lines in the carbon dioxide atmosphere of Mars, a scaling factor of 1.7 was adopted relative to the broadening by air, presented in HITRAN. This could lead to systematic uncertainty in the results, even despite the low pressure in the Martian atmosphere. Recent laboratory measurements of the broadening of water vapor lines in CO2 for the lines of three vibrational bands ν1 + ν3, 2ν2 + ν3, and 2ν1 in the spectral region of 6760–7430 cm–1 have improved the spectroscopic parameters for the carbon dioxide atmosphere. We performed water vapor retrievals with new spectroscopy at 1.38 µm for SPICAM IR nadir measurements and ACS NIR occultation measurements. In the case of SPICAM IR, changes due to spectroscopy were below the sensitivity of the instrument due to the low resolution and signal-to-noise ratio. For the ACS NIR, which is a high resolution spectrometer, the new spectroscopy resulted in systematic deviations of 2–5% depending on the height in the atmosphere, exceeding the random errors of the instrument. [ABSTRACT FROM AUTHOR]

Dramatic reductions in anthropogenic emissions during the lockdowns of the COVID‐19 pandemic provide an unparalleled opportunity to assess responses of the Earth system to human activities. Here, we synthesize the latest progress in understanding changes in short‐lived atmospheric constituents, that is, aerosols, ozone (O3), nitrogen oxides (NOx), and methane (CH4), in response to COVID‐19 induced emission reductions and the associated climate impacts on regional and global scales. The large‐scale emission reduction in the transportation sector reduced near‐surface particulate and ozone concentrations, with certain regional enhancements modulated by atmospheric oxidizing capacity and abnormal meteorological conditions. The methane increase during the pandemic is a combined effect of fluctuations in methane emissions and chemical sinks. Global net radiative forcing of all short‐lived species was found to be small, but regionally, aerosol radiative impacts during the lockdowns were discernible near China and India. Aerosol microphysical effects on clouds and precipitation were reported from modeling assessments only, except for observed reductions in aircraft contrails. There exist moderate climatic impacts of the pandemic on regional surface temperature, atmospheric circulations, and ecosystems, mainly over populous and polluted areas. Novel methodologies emerge in the pandemic‐related research to achieve the synergy between observations from multiple platforms and model simulations and to overcome the enormous hurdles and sophistication in detection and attribution studies. The insight gained from COVID‐19 research concerning the complex interplay between emission, chemistry, and meteorology, as well as the unexpected climate forcing‐responses relationships, underscores future challenges for cleaning up the air and alleviating the adverse impacts of global warming. Plain Language Summary: Dramatic reductions in anthropogenic emissions during the lockdowns of the COVID‐19 pandemic can be harnessed to devise and evaluate effective pollution and climate mitigation policies. In this review, we synthesize the latest progress in understanding changes in short‐lived atmospheric compositions, that is, aerosols, ozone (O3), methane (CH4), and nitrogen oxides (NOx), in response to COVID‐19 induced emission reductions and the associated climate impacts on regional and global scales. Emission perturbations from the transportation sector of short‐lived species led to the reduction of major pollutants globally, but with large regional disparities due to non‐linear atmospheric chemistry and meteorological influence. Localized aerosol reductions and resultant warming over East and South Asia are identifiable, along with aircraft contrail decreases in major flight routes. We suggest that analysis of a large volume of real‐time observational data in consortium with machine‐learning techniques promises a powerful and efficient tool to conduct non‐linear analyses and to develop data‐driven air quality and climate models. Key Points: COVID‐19 impacts on short‐lived species feature the influence of the transportation sector and non‐linear emission‐concentration relationshipAnthropogenic aerosol reduction in Asia induced changes in surface radiation with implications for regional atmospheric circulations and ecosystemsRelatively small perturbations on major climate forcers call for advanced analysis tools, including machine learning [ABSTRACT FROM AUTHOR]

Biogenic volatile organic compounds are emitted by plants and influence human and environmental health. They contribute to the formation of pollutants such as ozone and secondary organic aerosols, thereby influencing air quality and climate. Here we review biogenic volatile organic compounds with focus on biosynthesis, release to the atmosphere, distribution at various scales, tropospheric chemical processes, and secondary organic aerosols. Biogenic volatile organic compounds are emitted primarily through enzymatic pathways in response to environmental factors, varying across plant species and ecosystems. These emissions exhibit heterogeneity at multiple scales, influenced by meteorological conditions and plant structure. [ABSTRACT FROM AUTHOR]

Two decades of ACE-FTS, the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer, version 5.2 (v5.2) ozone data (2004–2023) are evaluated with ozonesonde data from across the globe. The biases between the ACE-FTS and ozonesonde measurements are first estimated by analyzing coincident data pairs. A second approach is taken for the validation by comparing the ACE-FTS and ozonesonde monthly mean time series, with the former generated by sampling the ACE-FTS data within latitude/longitude boxes (i.e., ± 5°/ ± 30°) surrounding the stations and calculating the monthly averages. The biases, correlations, variation patterns, and the mean states of the two time series are compared. The biases estimated in this way exhibit more consistent and smoother features than using the coincident pair method. The ACE-FTS and ozonesonde monthly mean time series are highly correlated and exhibit similar variation patterns in the lower stratosphere at all latitudes. The ACE-FTS instrument drifts for each station are assessed in terms of the long-term linear trends relative to ozonesondes, which, although highly stable, may have their own minor changes with time. The ACE-FTS ozone profiles exhibit in general high biases in the stratosphere for altitudes above ∼ 20 km, increasing with altitude up to ∼ 10 % at around 30 km. For altitudes between 20 km and the tropopause, biases of up to ± 10 % are found, depending on altitude and latitude with the largest biases found in the tropics and southern mid-latitudes. The ACE-FTS instrument drifts are generally non-significant overall in the stratosphere with high variation between the stations. Averaging the individual station instrument drifts within several latitude bands results in small non-significant drifts of within ± 1 %–2 % per decade in the northern mid-latitudes to high latitudes and the southern high latitudes. It also results in a positive but non-significant drift of up to 5 % per decade in the tropics and southern mid-latitudes, with overall uncertainties in this region ranging up to 5 %–10 % per decade (2 σ level) in the low stratosphere. As part of this assessment, an analysis of ozonesonde measurement stability using ACE-FTS as a transfer standard is conducted and finds small step changes in ozonesonde response at some stations. These results are in general agreement with recent findings using other satellite data sources. [ABSTRACT FROM AUTHOR]

Mineral dust from desert areas accounts for a large portion of aerosols globally, estimated at 3–4 billion tons per year. Aerosols emitted from arid and semi-arid areas, e.g., from parched lakes or rivers, are transported over long distances and have effects on a global scale, affecting the planet's radiative balance, atmospheric chemistry, cloud formation and precipitation, marine biological processes, air quality, and human health. Desert dust transport takes place in the atmosphere as the result of a dynamical sequence beginning with dust uplift from desert areas, then followed by the long-range transport and terminating with the surface deposition of mineral dust in areas even very far from dust sources. The Mediterranean basin is characterized by frequent dust intrusion events, particularly affecting Spain, France, Italy, and Greece. Such events contribute to the increase in PM10 and PM2.5 concentration values, causing legal threshold values to be exceeded. In recent years, these events have shown a non-negligible increase in frequency and intensity. The present work reports the results of an analysis of the dust events that in recent years (2018–2023) affected the Mediterranean area and in particular central Italy, focusing on the more recurrent meteorological configurations leading to long-range transport and on the consequent increase in aerosol concentration values. A method for desert intrusion episodes identification has been developed using both numerical forecast model data and PM10 observed data. A multi-year dataset has been analyzed by applying such an identification method and the resulting set of dust events episodes, affecting central Italy, has been studied in order to highlight their frequency on a seasonal basis and their interannual variability. In addition, a first attempt at a meteorological classification of desert intrusions has been carried out to identify the most recurrent circulation patterns related to dust intrusions. Understanding their annual and seasonal variations in frequency and intensity is a key topic, whose relevance is steeply growing in the context of ongoing climate change. [ABSTRACT FROM AUTHOR]

Emissions of land transport and anthropogenic non-traffic emissions (e.g. industry, households and power generation) are significant sources of nitrogen oxides, carbon monoxide and volatile organic compounds (VOCs). These emissions are important precursors of tropospheric ozone and affect air quality. The contribution of the emission sectors to ozone cannot be measured directly but can only be calculated using sophisticated atmospheric chemistry models. For this study we apply the MECO(n) model system (MESSy-fied ECHAM and COSMO models nested n times) equipped with a source attribution method to investigate the contribution of various sources to ground-level ozone in Europe. Compared to previous source apportionment studies for Europe, for the first time we apply a combined NOx–VOC tagging implemented in an online nested global–regional chemistry–climate model to achieve a finer resolution over central Europe (12 km) but concurrently incorporating the effect of long-range transport. We distinguish 10 different source sectors and 4 geographical source regions, analysing especially the contribution from the land transport sector. Our analysis focuses on large ozone events during summer in four different regions, two major polluted regions (Po Valley and Benelux) and two more remote regions (Iberian Peninsula and Ireland). The analysis concentrates on results for summer 2017, during which measurement campaign EMeRGe took place. Measurement data from this campaign are used for model evaluation. Our analysis shows that European land transport emissions contribute largely (42 % and 44 %, respectively) to ground-level NOy mixing ratios over Benelux and the Po Valley. Due to the overall lower ozone production efficiency over Benelux compared to the Po Valley, however, the contributions to ground-level ozone are larger in the Po Valley (12 %) compared to Benelux (8 %). In line with previous publications using different source apportionment methods, our results underline the large importance of long-range transport of ozone, especially from North America (Benelux, Ireland), but also from Africa (Iberian Peninsula), and provide additional information about the sectoral contribution not available before. Our analysis shows that the contributions of European emissions from land transport and anthropogenic non-traffic sectors strongly increase with increasing values of MDA8 (daily maximum 8 h average) ozone over the Po Valley and in the Benelux region. Accordingly, these two sectors drive large MDA8 values in these regions. Inter-comparisons of results for 2018 and with a coarser model resolution (50 instead of 12 km) show that these results are robust with respect to inter-annual variability and model resolution. Comparing our results with results from other source attribution methods we find that the contributions to ozone from individual sectors, which have large NOx but rather low VOC emissions, are estimated to be lower, if their emissions of NOx and VOCs are regarded concurrently. [ABSTRACT FROM AUTHOR]

The viscosity of aerosol particles determines the critical mixing time of gas–particle partitioning of volatile compounds in the atmosphere. The partitioning of the semi-volatile ammonium nitrate (NH4NO3) might alter the viscosity of highly viscous secondary organic aerosol particles during their lifetimes. In contrast to the viscosity of organic particles, data on the viscosity of internally mixed inorganic–organic aerosol particles are scarce. We determined the viscosity of an aqueous ternary inorganic–organic system consisting of NH4NO3 and a proxy compound for a highly viscous organic, sucrose. Three techniques were applied to cover the atmospherically relevant humidity range: viscometry, fluorescence recovery after photobleaching, and the poke-flow technique. We show that the viscosity of NH4NO3 –sucrose– H2O with an organic to inorganic dry mass ratio of 4:1 is 4 orders of magnitude lower than the viscosity of the aqueous sucrose under low-humidity conditions (30 % relative humidity (RH), 293 K). By comparing viscosity predictions of mixing rules with those of the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity (AIOMFAC-VISC) model, we found that a mixing rule based on mole fractions performs similarly when data from corresponding binary aqueous subsystems are available. Applying this mixing rule, we estimated the characteristic internal mixing time of aerosol particles, indicating significantly faster mixing for inorganic–organic mixtures compared to electrolyte-free particles, especially at lower RH. Hence, the assumption in global atmospheric chemistry models of quasi-instantaneous equilibrium gas–particle partitioning is reasonable for internally mixed single-phase particles containing dissolved electrolytes (but not necessarily for phase-separated particles), for most conditions in the planetary boundary layer. Further data are needed to see whether this assumption holds for the entire troposphere at midlatitudes and at RH > 35 %. [ABSTRACT FROM AUTHOR]

Chemical mechanisms form the core of atmospheric models to describe degradation pathways of pollutants and ultimately inform air quality and climate policymakers and other stakeholders. The accuracy of chemical mechanisms relies on the quality of their input data, which originate from experimental (laboratory, field, chamber) and theoretical (quantum chemistry, theoretical kinetics, machine learning) studies. The development of robust mechanisms requires rigorous and transparent procedures for data collection, mechanism construction and evaluation and the creation of reduced or operationally defined mechanisms. Developments in analytical techniques have led to a large number of identified chemical species in the atmospheric multiphase system that have proved invaluable for our understanding of atmospheric chemistry. At the same time, advances in software and machine learning tools have enabled automated mechanism generation. We discuss strategies for mechanism development, applying empirical or mechanistic approaches. We show the general workflows, how either approach can lead to robust mechanisms and that the two approaches complement each other, resulting in reliable predictions. Current challenges are discussed related to global change, including shifts in emission scenarios that result in new chemical regimes (e.g., low-NO scenarios, wildfires, mega- and gigacities) and that require the development of new or expanded gas- and aqueous-phase mechanisms. In addition, new mechanisms should be developed to also target oxidation capacity and aerosol chemistry impacting climate, human and ecosystem health. [ABSTRACT FROM AUTHOR]

For nearly 30 years, the International Network to study Deposition and Atmospheric chemistry in AFrica (INDAAF) programme has measured surface ozone from 14 sites in Africa representative of the main African ecosystems: dry savannas (Banizoumbou, Niger; Katibougou and Agoufou, Mali; Bambey and Dahra, Senegal), wet savannas (Lamto, Côte d'Ivoire; Djougou, Benin), forests (Zoétélé, Cameroon; Bomassa, Republic of the Congo) and agricultural–semi-arid savannas (Mbita, Kenya; Louis Trichardt, Amersfoort, Skukuza and Cape Point, South Africa). The data are collected with passive samplers and archived as monthly averages; quality assurance is maintained by INDAAF's calibration and intercomparison protocols with other programmes employing the same systems. This analysis reports on correlations of INDAAF ozone time series (1995–2020) with local meteorological parameters and with ozone precursors, biogenic volatile organic compounds (BVOCs) and nitrogen oxides (NOx), derived from standard global databases. Mean annual averages of surface ozone range from 3.9 ± 1.1 ppb (Bomassa) to 30.8 ± 8.0 ppb (Louis Trichardt), reflecting a general positive gradient from west central Africa to South Africa. At the decade scale, from 2000 to 2020, the Katibougou and Banizoumbou sites (dry savanna) experienced a significant decrease in ozone of around - 2.4 and - 0.8 ppb per decade, respectively. These decreasing trends are consistent with those observed for nitrogen dioxide (NO2) and BVOCs. An increasing trend is observed in Zoétélé (2001–2020), estimated at + 0.7 ppb per decade, and at Skukuza (2000–2015; + 3.4 ppb per decade). The increasing trends are consistent with increasing biogenic emissions at Zoétélé and NO2 levels at Skukuza. Very few surface O3 measurements exist in Africa, and the long-term results presented in this study are the most extensive for the ecosystems studied. The importance of maintaining long-term observations like INDAAF cannot be overstated. The data can be used to assess ozone impacts on African crops. For the Tropospheric Ozone Assessment Report Phase II (TOAR II), they provide invaluable constraints for models of chemical and climate processes in the atmosphere. [ABSTRACT FROM AUTHOR]

Whistler mode waves scatter energetic electrons, causing them to precipitate into the Earth's atmosphere. While the interactions between whistler mode waves and electrons are well understood, the global distribution of electron precipitation driven by whistler mode waves needs futher investigations. We present a two‐stage method, integrating neural networks and quasi‐linear theory, to simulate global electron precipitation driven by whistler mode waves. By applying this approach to the 17 March 2013 geomagnetic storm event, we reproduce the rapidly varying precipitation pattern over various phases of the storm. Then we validate our simulation results with POES/MetOp satellite observations. The precipitation pattern is consistent between simulations and observations, suggesting that most of the observed electron precipitation can be attributed to scattering by whistler mode waves. Our results indicate that chorus waves drive electron precipitation over the premidnight‐to‐afternoon sector during the storm main phase, with simulated peak energy fluxes of 20 erg/cm2/s and characteristic energies of 10–50 keV. During the recovery phase, plume hiss in the afternoon sector can have a comparable or stronger effect than chorus, with peak fluxes of ∼1 erg/cm2/s and characteristic energies between 10 and 200 keV. This study highlights the importance of integrating physics‐based and deep learning approaches to model the complex dynamics of electron precipitation driven by whistler mode waves. Plain Language Summary: Whistler mode hiss and chorus waves are electromagnetic waves in Earth's magnetosphere that interact with electrons, altering their motion and causing them to precipitate into the atmosphere. Understanding electron evolution is crucial, as precipitating electrons affect ionospheric conductivity and atmospheric chemistry, leading to aurorae and other phenomena. However, direct observations of electron precipitation caused by these waves are scarce, and global simulations are challenging due to the dynamic nature of wave‐particle interactions. This study presents a two‐stage simulation framework that models global wave activities using deep learning and runs physics‐based simulations with the neural network output as inputs. We validate our results by comparing them with the multi‐point POES/MetOp observations during a geomagnetic storm event on 17 March 2013, successfully reproducing the dynamic evolution of the observed precipitation. We found that chorus waves predominantly drive energetic electron precipitation during the storm main phase, while plume hiss causes comparable or stronger precipitation during the recovery phase. Our study highlights the importance of whistler mode waves in electron precipitation, identifies the quantitative contribution of hiss and chorus waves at different storm phases, and demonstrates how deep learning can advance scientific research in understanding the complex dynamics of electron precipitation in Earth's magnetosphere. Key Points: We integrate physical and deep learning approaches to simulate electron precipitation due to whistler mode waves in a storm eventThe simulation captures the dynamics of the electron precipitation observed by POES throughout the storm periodElectron precipitation is primarily driven by chorus waves during the main phase, but plume hiss becomes important in the recovery phase [ABSTRACT FROM AUTHOR]