Substantial contribution of transported emissions to organic aerosol in Beijing.

Haze in Beijing is linked to atmospherically formed secondary organic aerosol, which has been shown to be particularly harmful to human health. However, the sources and formation pathways of these secondary aerosols remain largely unknown, hindering effective pollution mitigation. Here we have quantified the sources of organic aerosol via direct near-molecular observations in central Beijing. In winter, organic aerosol pollution arises mainly from fresh solid-fuel emissions and secondary organic aerosols originating from both solid-fuel combustion and aqueous processes, probably involving multiphase chemistry with aromatic compounds. The most severe haze is linked to secondary organic aerosols originating from solid-fuel combustion, transported from the Beijing-Tianjing-Hebei Plain and rural mountainous areas west of Beijing. In summer, the increased fraction of secondary organic aerosol is dominated by aromatic emissions from the Xi'an-Shanghai-Beijing region, while the contribution of biogenic emissions remains relatively small. Overall, we identify the main sources of secondary organic aerosol affecting Beijing, which clearly extend beyond the local emissions in Beijing. Our results suggest that targeting key organic precursor emission sectors regionally may be needed to effectively mitigate organic aerosol pollution.

Regime shift in secondary inorganic aerosol formation and nitrogen deposition in the rural United States.

Secondary inorganic aerosols play an important role in air pollution and climate change, and their formation modulates the atmospheric deposition of reactive nitrogen (including oxidized and reduced nitrogen), thus impacting the nitrogen cycle. Large-scale and long-term analyses of secondary inorganic aerosol formation based on model simulations have substantial uncertainties. Here we improve constraints on secondary inorganic aerosol formation using decade-long in situ observations of aerosol composition and gaseous precursors from multiple monitoring networks across the United States. We reveal a shift in the secondary inorganic aerosol formation regime in the rural United States between 2011 and 2020, making rural areas less sensitive to changes in ammonia concentrations and shortening the effective atmospheric lifetime of reduced forms of reactive nitrogen. This leads to potential increases in reactive nitrogen deposition near ammonia emission hotspots, with ecosystem impacts warranting further investigation. Ammonia (NH3), a critical but not directly regulated precursor of fine particulate matter in the United States, has been increasingly scrutinized to improve air quality. Our findings, however, show that controlling NH3 became significantly less effective for mitigating fine particulate matter in the rural United States. We highlight the need for more collocated aerosol and precursor observations for better characterization of secondary inorganic aerosols formation in urban areas.

Explosive eruption style modulates volcanic electrification signals.

Volcanic lightning detection has proven useful to volcano monitoring by providing information on eruption onset, source parameters, and ash cloud directions. However, little is known about the influence of changing eruptive styles on the generation of charge and electrical discharges inside the eruption column. The 2021 Tajogaite eruption (La Palma, Canary Islands) provided the rare opportunity to monitor variations in electrical activity continuously over several weeks using an electrostatic lightning detector. Here we show that throughout the eruption, silicate particle charging is the main electrification mechanism. Moreover, we find that the type of electrical activity is closely linked to the explosive eruption style. Fluctuations in the electrical discharge rates are likely controlled by variations in the mass eruption rate and/or changes in the eruption style. These findings hold promise for obtaining near real-time information on the dynamic evolution of explosive volcanic activity through electrostatic monitoring in the future.

Energetics of the OH radical H-abstraction reactions from simple aldehydes and their geminal diol forms.

CONTEXT: Carbonyl compounds, especially aldehydes, emitted to the atmosphere, may suffer hydration in aerosols or water droplets in clouds. At the same time, they can react with hydroxyl radicals which may add or abstract hydrogen atoms from these species. The interplay between hydration and hydrogen abstraction is studied using density functional and quantum composite theoretical methods, both in the gas phase and in simulated bulk water. The H-abstraction from the aldehydic and geminal diol forms of formaldehyde, acetaldehyde, glycolaldehyde, glyoxal, methylglyoxal, and acrolein is studied to determine whether the substituent has any noticeable effect in the preference for the abstraction of one form or another. It is found that abstraction of the H-atom adjacent to the carbonyl group gives a more stable radical than same abstraction from the geminal diol in the case of formaldehyde, acetaldehyde, and glycolaldehyde. The presence of a delocalizing group in the Cα (a carbonyl group in glyoxal and methylglyoxal, and a vinyl group in acrolein), reverts this trend, and now the abstraction of the H-atom from the geminal diol gives more stable radicals. A further study was conducted abstracting hydrogen atoms from the other different positions in the species considered, both in the aldehydic and geminal diol forms. Only in the case of glycolaldehyde, the radical formed by H-abstraction from the -CH2OH group is more stable than any of the other radical species. Abstraction of the hydrogen atom in one of the hydroxyl groups in the geminal diol is equivalent to the addition of the •OH radical to the aldehyde. It leads, in some cases, to decomposition into a smaller radical and a neutral molecule. In these cases, some interesting theoretical differences are observed between the results in gas phase and (simulated) bulk solvent, as well as with respect to the method of calculation chosen. METHODS: DFT (M06-2X, B2PLYP, PW6B95), CCSD(T), and composite (CBS-QB3, jun-ChS, SCVECV-f12) methods using Dunning basis sets and extrapolation to the CBS limit were used to study the energetics of closed shell aldehydes in their keto and geminal-diol forms, as well as the radical derived from them by hydrogen abstraction. Both gas phase and simulated bulk solvent calculations were performed, in the last case using the Polarizable Continuum Model.

Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming.

Human activities affect the Earth's climate through modifying the composition of the atmosphere, which then creates radiative forcing that drives climate change. The warming effect of anthropogenic greenhouse gases has been partially balanced by the cooling effect of anthropogenic aerosols. In 2020, fuel regulations abruptly reduced the emission of sulfur dioxide from international shipping by about 80% and created an inadvertent geoengineering termination shock with global impact. Here we estimate the regulation leads to a radiative forcing of +0.2±0.11Wm-2 averaged over the global ocean. The amount of radiative forcing could lead to a doubling (or more) of the warming rate in the 2020 s compared with the rate since 1980 with strong spatiotemporal heterogeneity. The warming effect is consistent with the recent observed strong warming in 2023 and expected to make the 2020 s anomalously warm. The forcing is equivalent in magnitude to 80% of the measured increase in planetary heat uptake since 2020. The radiative forcing also has strong hemispheric contrast, which has important implications for precipitation pattern changes. Our result suggests marine cloud brightening may be a viable geoengineering method in temporarily cooling the climate that has its unique challenges due to inherent spatiotemporal heterogeneity.

Decreases in Epoxide-Driven Secondary Organic Aerosol Production under Highly Acidic Conditions: The Importance of Acid-Base Equilibria.

Isoprene has the highest atmospheric emissions of any nonmethane hydrocarbon, and isoprene epoxydiols (IEPOX) are well-established oxidation products and the primary contributors forming isoprene-derived secondary organic aerosol (SOA). Highly acidic particles (pH 0-3) widespread across the lower troposphere enable acid-driven multiphase chemistry of IEPOX, such as epoxide ring-opening reactions forming methyltetrol sulfates through nucleophilic attack of sulfate (SO42-). Herein, we systematically demonstrate an unexpected decrease in SOA formation from IEPOX on highly acidic particles (pH < 1). While IEPOX-SOA formation is commonly assumed to increase at low pH when more [H+] is available to protonate epoxides, we observe maximum SOA formation at pH 1 and less SOA formation at pH 0.0 and 0.4. This is attributed to limited availability of SO42- at pH values below the acid dissociation constant (pKa) of SO42- and bisulfate (HSO4-). The nucleophilicity of HSO4- is 100× lower than SO42-, decreasing SOA formation and shifting particulate products from low-volatility organosulfates to higher-volatility polyols. Current model parameterizations predicting SOA yields for IEPOX-SOA do not properly account for the SO42-/HSO4- equilibrium, leading to overpredictions of SOA formation at low pH. Accounting for this underexplored acidity-dependent behavior is critical for accurately predicting SOA concentrations and resolving SOA impacts on air quality.

Large contribution of in-cloud production of secondary organic aerosol from biomass burning emissions.

Organic compounds released from wildfires and residential biomass burning play a crucial role in shaping the composition of the atmosphere. The solubility and subsequent reactions of these compounds in the aqueous phase of clouds and fog remain poorly understood. Nevertheless, these compounds have the potential to become an important source of secondary organic aerosol (SOA). In this study, we simulated the aqueous SOA (aqSOA) from residential wood burning emissions under atmospherically relevant conditions of gas-liquid phase partitioning, using a wetted-wall flow reactor (WFR). We analyzed and quantified the specific compounds present in these emissions at a molecular level and determined their solubility in clouds. Our findings reveal that while 1% of organic compounds are fully water-soluble, 19% exhibit moderate solubility and can partition into the aqueous phase in a thick cloud. Furthermore, it is found that the aqSOA generated in our laboratory experiments has a substantial fraction being attributed to the formation of oligomers in the aqueous phase. We also determined an aqSOA yield of 20% from residential wood combustion, which surpasses current estimates based on gas-phase oxidation. These results indicate that in-cloud chemistry of organic gases emitted from wood burning can serve as an efficient pathway to produce organic aerosols, thus potentially influencing climate and air quality.

Tethered balloon system and High-Resolution Mass Spectrometry Reveal Increased Organonitrates Aloft Compared to the Ground Level.

Atmospheric particles play critical roles in climate. However, significant knowledge gaps remain regarding the vertically resolved organic molecular-level composition of atmospheric particles due to aloft sampling challenges. To address this, we use a tethered balloon system at the Southern Great Plains Observatory and high-resolution mass spectrometry to, respectively, collect and characterize organic molecular formulas (MF) in the ground level and aloft (up to 750 m) samples. We show that organic MF uniquely detected aloft were dominated by organonitrates (139 MF; 54% of all uniquely detected aloft MF). Organonitrates that were uniquely detected aloft featured elevated O/C ratios (0.73 ± 0.23) compared to aloft organonitrates that were commonly observed at the ground level (0.63 ± 0.22). Unique aloft organic molecular composition was positively associated with increased cloud coverage, increased aloft relative humidity (∼40% increase compared to ground level), and decreased vertical wind variance. Furthermore, 29% of extremely low volatility organic compounds in the aloft sample were truly unique to the aloft sample compared to the ground level, emphasizing potential oligomer formation at higher altitudes. Overall, this study highlights the importance of considering vertically resolved organic molecular composition (particularly for organonitrates) and hypothesizes that aqueous phase transformations and vertical wind variance may be key variables affecting the molecular composition of aloft organic aerosol.

Gas-Phase Formation of Sulfurous Acid (H2SO3) in the Atmosphere.

Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of 53 - 17 + 7 ${{53}_{-17}^{+\ 7}}$ %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH+CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O+H2SO3) <3×10-18 cm3 molecule-1 s-1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.

New Estimates of Nitrogen Fixation on Early Earth.

Fixed nitrogen species generated by the early Earth's atmosphere are thought to be critical to the emergence of life and the sustenance of early metabolisms. A previous study estimated nitrogen fixation in the Hadean Earth's N2/CO2-dominated atmosphere; however, that previous study only considered a limited chemical network that produces NOx species (i.e., no HCN formation) via the thermochemical dissociation of N2 and CO2 in lightning flashes, followed by photochemistry. Here, we present an updated model of nitrogen fixation on Hadean Earth. We use the Chemical Equilibrium with Applications (CEA) thermochemical model to estimate lightning-induced NO and HCN formation and an updated version of KINETICS, the 1-D Caltech/JPL photochemical model, to assess the photochemical production of fixed nitrogen species that rain out into the Earth's early ocean. Our updated photochemical model contains hydrocarbon and nitrile chemistry, and we use a Geant4 simulation platform to consider nitrogen fixation stimulated by solar energetic particle deposition throughout the atmosphere. We study the impact of a novel reaction pathway for generating HCN via HCN2, inspired by the experimental results which suggest that reactions with CH radicals (from CH4 photolysis) may facilitate the incorporation of N into the molecular structure of aerosols. When the HCN2 reactions are added, we find that the HCN rainout rate rises by a factor of five in our 1-bar case and is about the same in our 2- and 12-bar cases. Finally, we estimate the equilibrium concentration of fixed nitrogen species under a kinetic steady state in the Hadean ocean, considering loss by hydrothermal vent circulation, photoreduction, and hydrolysis. These results inform our understanding of environments that may have been relevant to the formation of life on Earth, as well as processes that could lead to the emergence of life elsewhere in the universe.

Anomalous trends in global ocean carbon concentrations following the 2022 eruptions of Hunga Tonga-Hunga Ha'apai.

We report on observed trend anomalies in climate-relevant global ocean biogeochemical properties, as derived from satellite ocean color measurements, that show a substantial decline in phytoplankton carbon concentrations following eruptions of the submarine volcano Hunga Tonga-Hunga Ha'apai in January 2022. The anomalies are seen in remotely-sensed ocean color data sets from multiple satellite missions, but not in situ observations, thus suggesting that the observed anomalies are a result of ocean color retrieval errors rather than indicators of a major shift in phytoplankton carbon concentrations. The enhanced concentration of aerosols in the stratosphere following the eruptions results in a violation of some fundamental assumptions in the processing algorithms used to obtain marine biogeochemical properties from satellite radiometric observations, and it is demonstrated through radiative transfer simulations that this is the likely cause of the anomalous trends. We note that any future stratospheric aerosol disturbances, either natural or geoengineered, may lead to similar artifacts in satellite ocean color and other remote-sensing measurements of the marine environment, thus confounding our ability to track the impact of such events on ocean ecosystems.

Transpacific Transport of Asian Peroxyacetyl Nitrate (PAN) Observed from Satellite: Implications for Ozone.

Peroxyacetyl nitrate (PAN) is produced in the atmosphere by photochemical oxidation of non-methane volatile organic compounds in the presence of nitrogen oxides (NOx), and it can be transported over long distances at cold temperatures before decomposing thermally to release NOx in the remote troposphere. It is both a tracer and a precursor for transpacific ozone pollution transported from East Asia to North America. Here, we directly demonstrate this transport with PAN satellite observations from the infrared atmospheric sounding interferometer (IASI). We reprocess the IASI PAN retrievals by replacing the constant prior vertical profile with vertical shape factors from the GEOS-Chem model that capture the contrasting shapes observed from aircraft over South Korea (KORUS-AQ) and the North Pacific (ATom). The reprocessed IASI PAN observations show maximum transpacific transport of East Asian pollution in spring, with events over the Northeast Pacific offshore from the Western US associated in GEOS-Chem with elevated ozone in the lower free troposphere. However, these events increase surface ozone in the US by less than 1 ppbv because the East Asian pollution mainly remains offshore as it circulates the Pacific High.

Missing Measurements of Sesquiterpene Ozonolysis Rates and Composition Limit Understanding of Atmospheric Reactivity.

Emissions of biogenic reactive carbon significantly influence atmospheric chemistry, contributing to the formation and destruction of secondary pollutants, such as secondary organic aerosol and ozone. While isoprene and monoterpenes are a major fraction of emissions and have been extensively studied, substantially less is known about the atmospheric impacts of higher-molecular-weight terpenes such as sesquiterpenes. In particular, sesquiterpenes have been proposed to play a significant role in ozone chemical loss due to the very high ozone reaction rates of certain isomers. However, relatively little data are available on the isomer-resolved composition of this compound class or its role in ozone chemistry. This study examines the chemical diversity of sesquiterpenes and availability of ozone reaction rate constants to evaluate the current understanding of their ozone reactivity. Sesquiterpenes are found to be highly diverse, with 72 different isomers reported and relatively few isomers that contribute a large mass fraction across all studies. For the small number of isomers with known ozone reaction rates, estimated rates may be 25 times higher or lower than measurements, indicating that estimated reaction rates are highly uncertain. Isomers with known ozone reaction rates make up approximately half of the mass of sesquiterpenes in concentration and emission measurements. Consequently, the current state of the knowledge suggests that the total ozone reactivity of sesquiterpenes cannot be quantified without very high uncertainty, even if isomer-resolved composition is known. These results are in contrast to monoterpenes, which are less diverse and for which ozone reaction rates are well-known, and in contrast to hydroxyl reactivity of monoterpenes and sesquiterpenes, for which reaction rates can be reasonably well estimated. Improved measurements of a relatively small number of sesquiterpene isomers would reduce uncertainties and improve our understanding of their role in regional and global ozone chemistry.

Typhoon- and pollution-driven enhancement of reactive bromine in the mid-latitude marine boundary layer.

Tropospheric reactive bromine is important for atmospheric chemistry, regional air pollution, and global climate. Previous studies have reported measurements of atmospheric reactive bromine species in different environments, and proposed their main sources, e.g. sea-salt aerosol (SSA), oceanic biogenic activity, polar snow/ice, and volcanoes. Typhoons and other strong cyclonic activities (e.g. hurricanes) induce abrupt changes in different earth system processes, causing widespread destructive effects. However, the role of typhoons in regulating reactive bromine abundance and sources remains unexplored. Here, we report field observations of bromine oxide (BrO), a critical indicator of reactive bromine, on the Huaniao Island (HNI) in the East China Sea in July 2018. We observed high levels of BrO below 500 m with a daytime average of 9.7 ± 4.2 pptv and a peak value of ∼26 pptv under the influence of a typhoon. Our field measurements, supported by model simulations, suggest that the typhoon-induced drastic increase in wind speed amplifies the emission of SSA, significantly enhancing the activation of reactive bromine from SSA debromination. We also detected enhanced BrO mixing ratios under high NOx conditions (ppbv level) suggesting a potential pollution-induced mechanism of bromine release from SSA. Such elevated levels of atmospheric bromine noticeably increase ozone destruction by as much as ∼40% across the East China Sea. Considering the high frequency of cyclonic activity in the northern hemisphere, reactive bromine chemistry is expected to play a more important role than previously thought in affecting coastal air quality and atmospheric oxidation capacity. We suggest that models need to consider the hitherto overlooked typhoon- and pollution-mediated increase in reactive bromine levels when assessing the synergic effects of cyclonic activities on the earth system.

Isomer-resolved unimolecular dynamics of the hydroperoxyalkyl intermediate (•QOOH) in cyclohexane oxidation.

The oxidation of cycloalkanes is important in the combustion of transportation fuels and in atmospheric secondary organic aerosol formation. A transient carbon-centered radical intermediate (•QOOH) in the oxidation of cyclohexane is identified through its infrared fingerprint and time- and energy-resolved unimolecular dissociation dynamics to hydroxyl (OH) radical and bicyclic ether products. Although the cyclohexyl ring structure leads to three nearly degenerate •QOOH isomers (ß-, γ-, and δ-QOOH), their transition state (TS) barriers to OH products are predicted to differ considerably. Selective characterization of the ß-QOOH isomer is achieved at excitation energies associated with the lowest TS barrier, resulting in rapid unimolecular decay to OH products that are detected. A benchmarking approach is employed for the calculation of high-accuracy stationary point energies, in particular TS barriers, for cyclohexane oxidation (C6H11O2), building on higher-level reference calculations for the smaller ethane oxidation (C2H5O2) system. The isomer-specific characterization of ß-QOOH is validated by comparison of experimental OH product appearance rates with computed statistical microcanonical rates, including significant heavy-atom tunneling, at energies in the vicinity of the TS barrier. Master-equation modeling is utilized to extend the results to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclohexane combustion.

Urban ozone variability using automated machine learning: inference from different feature importance schemes.

Tropospheric ozone is an air pollutant at the ground level and a greenhouse gas which significantly contributes to the global warming. Strong anthropogenic emissions in and around urban environments enhance surface ozone pollution impacting the human health and vegetation adversely. However, observations are often scarce and the factors driving ozone variability remain uncertain in the developing regions of the world. In this regard, here, we conducted machine learning (ML) simulations of ozone variability and comprehensively examined the governing factors over a major urban environment (Ahmedabad) in western India. Ozone precursors (NO2, NO, CO, C5H8 and CH2O) from the CAMS (Copernicus Atmosphere Monitoring Service) reanalysis and meteorological parameters from the ERA5 (European Centre for Medium-Range Weather Forecast's (ECMWF) fifth-generation reanalysis) were included as features in the ML models. Automated ML (AutoML) fitted the deep learning model optimally and simulated the daily ozone with root mean square error (RMSE) of ~2 ppbv reproducing 84-88% of variability. The model performance achieved here is comparable to widely used ML models (RF-Random Forest and XGBoost-eXtreme Gradient Boosting). Explainability of the models is discussed through different schemes of feature importance, including SAGE (Shapley Additive Global importancE) and permutation importance. The leading features are found to be different from different feature importance schemes. We show that urban ozone could be simulated well (RMSE = 2.5 ppbv and R2 = 0.78) by considering first four leading features, from different schemes, which are consistent with ozone photochemistry. Our study underscores the need to conduct science-informed analysis of feature importance from multiple schemes to infer the roles of input variables in ozone variability. AutoML-based studies, exploiting potentials of long-term observations, can strongly complement the conventional chemistry-transport modelling and can also help in accurate simulation and forecast of urban ozone.

Development and Testing of a Rocket-Based Sensor for Atmospheric Sensing Using an Unmanned Aerial System.

Measurements of the vertical structure of the lower atmosphere are important to the understanding of air quality. Unmanned Aerial Systems (UASs, drones) can provide low cost, repeatable measurements of the temperature, pressure, and relative humidity. A set of inexpensive sensors controlled with an Arduino microprocessor board were tested on a UAS against a meteorology grade sensor. Two modes of operation for sampling were tested: a forward moving sampler and a vertical ascent sampler. A small particle sensor (Sensiron SPS30) was integrated and was capable of retrieving vertical aerosol distributions during an inversion event. The thermocouple-based temperature probe and the relative humidity measurement on the Bosch BME280 sensor correlated well with the meteorological sensor. The temperature and relative humidity sensors were then deployed on a rocket sounding platform. The rocket sounding system performed well up to a height of 400 m. The inexpensive sensors were found to perform adequately for low-cost development and uses in education and research.

Evaluating the multi-variable influence on O3, NO2, and HCHO using BRTs and RF model.

This study addresses significant knowledge gaps in understanding the complex interplay between atmospheric chemistry and synoptic conditions. Using emerging machine learning techniques-Boosted Regression Trees (BRTs) and Random Forest (RF) models-we investigate the influence of synoptic conditions on pollutant levels. Several BRTs and RF models are developed to estimate surface concentrations of ozone (O3), nitrogen dioxide (NO2), and formaldehyde (HCHO). By considering a range of algorithmic structures and explanatory variables for each pollutant, the research aims to identify the most skillful predictive approaches and influential factors governing pollutant levels. The design seeks to highlight key determinants of concentration patterns without constraining the investigation to pre-defined model structures or explanatory variable sets. Introducing a novel methodology, Correlation Coefficient Differential Evaluation (C2DE), we quantitatively assess the influence of explanatory variables. C2DE reveals significant contributions from spatial variables (i.e., trajectory clusters at varying altitudes), formaldehyde to nitrogen dioxide ratio (FNR), and meteorological parameters. Specifically, spatial variables contribute approximately 28 % to O3 concentrations, while the FNR accounts for around 5.2-9.8 % of the overall influence. For NO2 and HCHO, spatial variables contribute around 26.5 % and 32.1 %, respectively. Moreover, when considering the combined influence of meteorological parameters, these collectively explain about 45.34 %, 35.31 %, and 45.41 % of the variations in O3, NO2, and HCHO concentrations, respectively. Thus, C2DE provides valuable insights into the relative contributions of these factors, aiding in the comprehensive evaluation of air quality dynamics. This underscores the need for a multifaceted approach to comprehending and effectively addressing air pollution before devising its control strategies.

Astrobiological Potential of Venus Atmosphere Chemical Anomalies and Other Unexplained Cloud Properties.

Long-standing unexplained Venus atmosphere observations and chemical anomalies point to unknown chemistry but also leave room for the possibility of life. The unexplained observations include several gases out of thermodynamic equilibrium (e.g., tens of ppm O2, the possible presence of PH3 and NH3, SO2 and H2O vertical abundance profiles), an unknown composition of large, lower cloud particles, and the "unknown absorber(s)." Here we first review relevant properties of the venusian atmosphere and then describe the atmospheric chemical anomalies and how they motivate future astrobiology missions to Venus.

Phase Transitions in Organic and Organic/Inorganic Aerosol Particles.

The phase state of aerosol particles can impact numerous atmospheric processes, including new particle growth, heterogeneous chemistry, cloud condensation nucleus formation, and ice nucleation. In this article, the phase transitions of inorganic, organic, and organic/inorganic aerosol particles are discussed, with particular focus on liquid-liquid phase separation (LLPS). The physical chemistry that determines whether LLPS occurs, at what relative humidity it occurs, and the resultant particle morphology is explained using both theoretical and experimental methods. The known impacts of LLPS on aerosol processes in the atmosphere are discussed. Finally, potential evidence for LLPS from field and chamber studies is presented. By understanding the physical chemistry of the phase transitions of aerosol particles, we will acquire a better understanding of aerosol processes, which in turn impact human health and climate.