Determining the particle chemical morphology is crucial for unraveling reactive uptake in atmospheric multiphase and heterogeneous chemistry. However, it remains challenging due to the complexity and inhomogeneity of aerosol particles. Using a scanning transmission X-ray microscopy (STXM) coupled with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and an environmental cell, we imaged and quantified the chemical morphology and hygroscopic behavior of individual submicron urban aerosol particles. Results show that internally mixed particles composed of organic carbon and inorganic matter (OCIn) dominated the particle population (73.1 +/- 7.4%). At 86 % relative humidity, 41.6 % of the particles took up water, with OCIn particles constituting 76.8 % of these hygroscopic particles. Most particles exhibited a core-shell structure under both dry and humid conditions, with an inorganic core and an organic shell. Our findings provide direct observational evidence of the core-shell structure and water uptake behavior of typical urban aerosols, which underscore the importance of incorporating the core-shell structure into models for predicting the reactive uptake coefficient of heterogeneous reactions.

Abstract Characteristics of individual particles and aerosol mixing state strongly influence atmospheric chemistry, radiation, and cloud formation yet remain poorly constrained. Here, we investigated the spatiotemporal evolution of size-resolved particle composition in a coastal environment by applying a new capability using a microscopy-based particle sampler coupled to an unmanned aerial vehicle (UAV). With sampling resolutions up to 30 m and 1 min, over 100 000 particles were analyzed by computer-controlled scanning electron microscopy with energy-dispersive X-ray spectroscopy. The observations revealed a pronounced temporal transition in particle chemistry from sea-breeze-influenced aerosol dominated by aged sea salt and dust (46–83%) to continentally impacted aerosol dominated by organic-sulfur-containing particles (57–89%). Fine-resolution vertical profiles further showed that this transition was governed by the complex interactions among boundary layer dynamics, regional transport, and secondary aerosol formation in the coastal atmosphere. The aerosol mixing state showed pronounced diurnal variability and size dependence but weak vertical variation. Formation of organic-sulfur-containing particles shifts the aerosol population toward external mixing, decreasing the mixing state index from 53% to 27%. These findings provide new insights into aerosol transformation and demonstrate the value of high spatiotemporal single-particle characterizations for understanding aerosol environmental and climatic impacts.

Subtropical mangroves are hot spots of blue carbon sequestration. As a dominant species of benthic fauna, crabs are an ecological engine of the mangrove ecosystem, which regulates the carbon transfer from primary producers to either sediment or to consumers of higher trophic levels. With the purpose to disentangle the mechanisms that shape carbon transfer in the mangrove ecosystem, we studied primary producers, sediments, suspended particles, and crabs in Zhangjiang Estuary Mangrove National Nature Reserve, China. Using the compound-specific isotope analysis of amino acids by gas chromatography-combustion-isotope ratio mass spectrometry, the stable carbon isotope values of amino acids were investigated from these mangrove samples. Broad isotope gradients of both amino acids and bulk organic carbon were observed in mangrove samples ranging from −30.0‰ to −14.7‰ and from −32.8‰ to −15.6‰. In the sediment, heterogeneous δ13C distribution and elevated relative abundance of amino acids suggest the selective deposition of nitrogen-enriched organic matter from major primary producers and microalgae in suspended particles. Metabolic modifications of molecular isotope signatures in crabs was observed when comparing the amino acid δ13C values of crabs from different environments. Using a Bayesian mixing model, a mixed diet of mangrove crabs was quantified, which indicates a preferential ingestion of suspended particulate materials (SPM) (24

Abstract. Mineral dust is a major source of ice nucleating particles (INPs) for mixed-phase clouds and thereby influences cloud properties, precipitation, and radiative forcing. However, the ice nucleation properties of natural dust and their response to atmospheric aging remain poorly constrained. Here, we investigated immersion freezing (IMF) of surface-collected dust from Tengger Desert, China, and examined the effects of internal mixing with ammonium sulfate (AS), NaCl, H2SO4, and HNO3, as well as heat treatment. Median freezing temperature (T50) of Tengger Desert dust increased from 251.6 K to 256.7 K as dust mass fraction increased from 1.4x10-4 wt% to 1.0x10-1 wt%. A minor increase in feldspar content (~2 % in surface area) quantified by single particle analysis enhanced the ice nucleation active site density (Ns) by approximately one order of magnitude. Internal mixing with AS enhanced IMF activity, with the enhancements of T50 (0.4 K – 4.8 K), Ns, and heterogeneous ice nucleation rate coefficient (Jhet) exhibiting sigmoidal response to AS concentration from 5x10-5 M to 5x10-2 M. This enhancement is attributed to NH4+ adsorption on dust surface. NaCl and acids, as well as heat treatment, had no impact on the overall IMF activity in terms of T50, although modest changes in Ns and Jhet were observed above 256 K. We developed the experimentally constrained parameterizations of Ns and Jhet for Tengger Desert dust with and without aging, which can be used in cloud modeling to improve our understanding of how atmospheric aging modifies the ice nucleation of natural dust.

Abstract. Heterogeneous ice nucleation initiated by atmospheric ice-nucleating particles (INPs) is a key microphysical process for cloud formation. Detecting the ice nucleation ability (INA) and concentration of INPs is essential for improving global climate models. Droplet freezing techniques (DFTs) are among the widely used tools for measuring the immersion freezing of INPs, which is a predominant ice nucleation process in mixed-phase clouds. To enhance the efficiency and accuracy of DFTs, we developed a Freezing Ice Nucleation Detection Analyzer at Westlake University (FINDA-WLU) with an improved hardware setup, user-friendly software, precise droplet freezing detection, and rigorous temperature calibrations. The temperature uncertainty of FINDA-WLU is about ±0.60 °C, considering both vertical heat transfer efficiency and horizontal temperature heterogeneity. The system is tested with Milli-Q ultrapure water and reference materials like Arizona Test Dust and Snomax®, and the results are consistent with previous studies. We also use the FINDA-WLU to measure INPs in precipitation samples collected in China. Overall, FINDA-WLU proved to be a reliable and precise method for measuring INA and INP concentrations.

Surface nanostructure and hydrophobicity critically govern interfacial wetting behavior and ice nucleation. This work employs molecular dynamics simulations to investigate the desublimation of water vapor on subcooled surfaces, systematically evaluating nanostructural parameters (height [h], structure width [a], and gap width [b]) and surface hydrophobicity. Optimal dewetting transition occurs at h = 15.696 Å with b having a more significant impact on the transition process compared to a, establishing a quantifiable dewetting boundary. During ice nucleation, Wenzel-state surfaces with high lattice matching facilitate rapid hexagonal ice formation within nanogaps, generating vertically aligned ice layers. Low-matching surfaces exhibit delayed nucleation in interfacial liquid films, producing angled ice layers with reduced adhesion strength. In the Cassie–Baxter state, nucleation starts within the liquid membrane, and the adhesion of ice is further reduced due to the angle formed between the ice layer and the wall surface. This study offers insights for rational design of anti-icing surfaces through synergistic optimization of lattice matching, nanostructural geometry, and wetting states.

Dust storms have great impacts on air quality and climate. Dust can influence cloud microphysical properties and determine their radiative forcing and precipitation. Asian dust storms (ADS) are important sources of global aerosol. However, the physiochemical characteristics of dust from ADS at a single particle level are less understood, and the exact particles that can serve as ice nucleating particles (INPs) remain unclear. Here, we present the physicochemical properties and ice nucleation ability of dust particles collected in Beijing during two major ADS in March 2021. The particles from two ADS were classified into Illite, Kaolinite, Feldspar, Quartz, Chlorite, Mixed-dust, and Non-dust particles, which contributed 28.6% f 3.3 %, 20.0% f 3.9%, 12.3 % f 2.3 %, 11.1% f 2.8%, 9.8% f 0.8%, 13.7% f 1.8%, and 4.4% f 1.7% in number, respectively. On average, the ADS particles formed ice crystals via deposition ice nucleation from relative humidity with respect to ice (RHice) RH ice ) of 112 % f 1 % at 250 K to 154 % f 15 % RH ice at 205 K. Part of the samples also formed ice via immersion freezing between 230 K and 250 K. Among the 149 identified INPs, Clay-like particles (Chlorite, Illite, and Kaolinite) contributed 71.1 % f 6.2 % in number and followed by Mixed-dust-like particles (16.9 % f 8.7 %) and Feldspar-like particles (10.4 % f 6.3 %). Enrichment factor of each particle type in INPs is calculated as the ratio of its number fractions in INPs and the aerosol population. It ranges from 0.6 f 0.7 to 1.3 f 2.2. The contribution of each particle type to INP was correlated with its fraction in the population. These results imply that each particle type can serve as INP. Clay-like particles are the dominant INPs during the ADS. We conducted ice nucleation kinetic analysis and provided parameterizations of heterogeneous ice nucleation rate coefficient and contact angle for ADS. These parameterizations can be used in the modeling study to evaluate the impact of ADS in atmospheric ice crystal formation in clouds.

Atmospheric particles can impact cloud formation and play a critical role in regulating cloud properties. However, particle characteristics at the single-particle level and their ability to act as ice-nucleating particles (INPs) over the marine atmosphere are poorly understood. In this study, we present micro-spectroscopic characterizations and ice nucleation properties of particles collected during a cruise from South Korea to Antarctica in 2019. Most of the samples were dominated by fresh sea salt, aged sea salt, and sea salt mixed with sulfate particles, with total number percentages ranging from 48 % to 99 % over the western Pacific and the Southern Ocean. The mixing-state index of the particle population ranged from 50 % to 95 % over the Northern Hemisphere and Southern Hemisphere. Multiphase processes on sea salt particles resulted in chlorine deficiency. This selective aging process made the marine particle population more externally mixed. Ice nucleation onset conditions primarily for the deposition mode were measured and the investigated particles showed diverse ice nucleation abilities. The fresh sea salt particles with organic coatings exhibited the highest ice nucleation ability at a relative humidity with respect to ice as low as 121 %. The sea salt mixed sulfate particle was enriched in INPs by a factor of 1.9. Aging processes affected both the mixing state of the particles and their ice nucleation abilities. Our analysis shows that assuming an internally mixed particle population in the marine atmosphere can lead to errors of several orders of magnitude in predicting ice nucleation rates.

The freezing behavior of liquid water, resulting in solid phases such as glassy structures, hexagonal crystals, or cubic crystals, is influenced by surface supercooling. This study identifies three distinct freezing modes of water nanodroplets on platinum surfaces through molecular dynamics simulations. At surface temperatures below 175 K, water nanodroplets freeze into a solid phase characterized by a glassy structure. Between 176 and 195 K, nucleation begins within the amorphous solid, resulting in crystalline ice formations. In the temperature range of 196-209 K, nucleation and growth occur within the liquid nanodroplets, culminating in the formation of ice crystals exhibiting cubic and hexagonal morphologies. Interestingly, the water temperature rises by 20-37 K during ice crystal nucleation and growth, and the proportion of cubic ice relative to hexagonal ice decreases as the temperature rises. Additionally, a specific threshold involving molecular distance and kinetic energy is essential for the formation of regular ice crystals, thereby confining heterogeneous nucleation to a defined range of surface temperatures. This study offers valuable insights that could inform strategies for controlling ice crystallization in various applications.

Sea spray aerosol (SSA) is a widely recognized important source of ice-nucleating particles (INPs) in the atmosphere. However, composition-specific identification, nucleation processes, and ice nucleation rates of SSA-INPs have not been well constrained. Microspectroscopic characterization of ambient and laboratory-generated SSA confirms that water-borne exudates from planktonic microorganisms composed of a mixture of proteinaceous and polysaccharidic compounds act as ice-nucleating agents (INAs). These data and data from previously published mesocosm and wave channel studies are subsequently used to further develop the stochastic freezing model (SFM) producing ice nucleation rate coefficients for SSA-INPs. The SFM simultaneously predicts immersion freezing and deposition and homogeneous ice nucleation by SSA particles under tropospheric conditions. Predicted INP concentrations agree with ambient and laboratory measurements. In addition, this holistic freezing model is independent of the source and exact composition of the SSA particles, making it well suited for implementation in cloud and climate models.

As a novel and unique large-scale instrument, the synchrotron radiation facilities have been progressively applied to the research of atmospheric science. This article introduces the theoretical principles, key technologies, and recent major research results of key synchrotron radiation-based experimental methods. The main synchrotron radiation technologies include the atmospheric pressure X-ray photoelectron spectroscopy ( APXPS), the near edge X-ray absorption fine structure ( NEXAFS) and the scanning transmission X-ray microscopy ( STXM). A key component ( environmental cell) commonly used in all three technologies is explained in detail. This article classifies the collected research according to experimental types, i. e., APXPS experiments, liquid jet experiments and STXM experiments. The main topics include: ( 1) ice surface, (2) salt surface, ( 3) acidic solution, ( 4) organic solution, ( 5) halite solution, ( 6) ozonolysis, ( 7) soot, ( 8) ice nuclei, (9) hygroscopicity and ( 10) reaction mechanism. The development of synchrotron radiation facilities has provided strong support for the research of aerosol science and atmospheric heterogeneous chemistry, giving atmospheric scientists the ability to explore unknown fields and latitudes. It is foreseeable that more and more important atmospheric processes and mechanisms will be revealed by technologies based on synchrotron radiation, which will also reflect the great potential and value of synchrotron radiation devices in the field of atmospheric and environmental science.

Particles can undergo different phase transitions in the atmosphere including deliquescence, liquid-liquid phase separation (LLPS), melting, and freezing. In this study, phase transitions of particles/droplets containing polyethylene glycol with a molar mass of 400 g mol-1 (PEG400) and ammonium sulfate (AS), i.e., PEG400-AS particles/droplets, were investigated at different organic-to-inorganic dry mass ratios (OIRs) under typical tropospheric temperatures and water activities (aw). The investigated droplets (60-100 μm) with or without LLPS in the closed system froze through homogeneous ice nucleation. At temperatures lower than 200 K, multiple ice nucleation events were observed within the same individual droplets at low aw. Droplets with and without LLPS shared similar lambda values at the same OIR according to the lambda approach indicating they form ice through the same mechanism. A parameterization of lambda values was provided which can be used to predict freezing temperature of aqueous PEG400-AS droplets. We found that adding AS reduces the temperature dependence of aw in aqueous PEG400 droplets. Assuming incorrectly that aw is temperature-independent for a constant droplet composition leads to a deviation between the experimental determined ice nucleation rate coefficients for droplets at OIR > 1 and the predicted values by the water-activity-based ice nucleation theory. We proposed a parameterization of temperature dependence of aw to minimize the deviations of the measured melting temperatures and nucleation rate coefficients from the corresponding predictions for aqueous PEG400-AS system.

We investigated the ice nucleation activities of humic‐like substances (HULIS), an important component of organic aerosol (OA), derived from atmospheric and biomass burning aerosols, and produced from aqueous‐phase chemical reactions. Respective HULIS can effectively trigger heterogeneous IN under mixed‐phase cloud conditions. HULIS ice active entities (IAE) were aggregates in size between 0.02 and 0.10 μm. At −20°C, the IAE numbers per unit HULIS mass varied from 213 to 8.7 × 10 4 mg −1 . Such results were different than those detected in aquatic humic substances (HS) from previous studies, implying using HS as surrogates may not robustly estimate the IAE concentrations in the real atmosphere. Combining the abundance of atmospheric HULIS with the present results suggests that HULIS could be an important IAE contributor in the atmosphere where other ice nucleating particle species, such as dust and biological particles, are either low in concentration or absent.

Abstract. Biological aerosols play an important role in atmospheric chemistry, clouds, climate, and public health. Here, we studied the morphology and composition of primary biological aerosol particles (PBAPs) collected in the Lesser Khingan Mountain boreal forest of China in summertime using transmission electron microscopy and scanning electron microscopy. Of all detected particles > 100 nm in diameter, 13 % by number were identified as PBAPs. In addition, 57 % of the PBAPs were identified as bacteria, followed by brochosomes (24 %) and fungi (19 %). The dominant size of bacteria was 1–4 μm, fungi was 2–4 μm, and brochosomes was 300–500 nm. The number size distribution of PBAPs coupled with the mass concentrations of PM2.5 and PM10 were used to estimate the total mass concentration of PBAPs, which is approximately 1.9 μg m−3 and accounts for 47 % of the in situ PM2.5–10 mass. C, N, O, P, K, and Si are detected in all PBAP particles, and P represented a major marker to identify PBAPs. Moreover, there is a higher frequency and concentration of PBAPs at night compared with day. Bacterial and fungal particles displayed weak hygroscopicity with a growth factor of ~ 1.09 at RH = 94 %. Electron microscopy shows that approximately 20 % of the bacterial particles were internally mixed with metal, mineral dust, and inorganic salts in the boreal forest air. This work provides a database for both further understanding physicochemical state of individual PBAP particles from natural sources and expanding the scope of atmospheric implications.

Weijun Li, Lei Liu, Qi Yuan, Liang Xu, Yanhong Zhu, Bingbing Wang, Hua Yu, Xiaokun Ding, Jian Zhang, Dao Huang, Dantong Liu, Wei Hu, Daizhou Zhang, Pingqing Fu, Maosheng Yao, Min Hu, Xiaoye Zhang, Zongbo Shi Department of Atmospheric Sciences, School of Earth Sciences, Zhejiang University, Hangzhou 310027, China State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China. College of Life and Environmental Sciences, Hangzhou Normal University, 310036, Hangzhou, China Department of Chemistry, Zhejiang University, Hangzhou 310027, China Institute of Surface-Earth System Science, Tianjin University, 300072, Tianjin, China Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto 862-8502, Japan State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China Key Laboratory of Atmospheric Chemistry, Chinese Academy of Meteorological Sciences, Beijing,

Predicting the formation of ice in the atmosphere presents one of the great challenges in physical sciences with important implications for the chemistry and composition of the Earth’s atmosphere, the hydrological cycle, and climate. Among atmospheric ice formation processes, heterogeneous ice nucleation proceeds on aerosol particles ranging from a few nanometers to micrometers in size, commonly referred to as ice nucleating particles (INPs). Research over the last two decades has demonstrated that organic matter (OM) is ubiquitous in the atmosphere, present as organic aerosol (OA) particles or as coatings on other particle types. The physicochemical properties of OM make predicting how OM can contribute to the INP population challenging. This review focuses on the role of OM in INPs, summarizing and highlighting recent advances in our understanding of the ice nucleation process gained from theoretical, laboratory, and field studies. Examination of ice residuals and INPs with analytical techniques demonst...

In the Amazon basin, particles containing mixed sodium salts are routinely observed and are attributed to marine aerosols transported from the Atlantic Ocean. Using chemical imaging analysis, we show that, during the wet season, fungal spores emitted by the forest biosphere contribute at least 30% (by number) to sodium salt particles in the central Amazon basin. Hydration experiments indicate that sodium content in fungal spores governs their growth factors. Modeling results suggest that fungal spores account for ~69% (31-95%) of the total sodium mass during the wet season and that their fractional contribution increases during nighttime. Contrary to common assumptions that sodium-containing aerosols originate primarily from marine sources, our results suggest that locally-emitted fungal spores contribute substantially to the number and mass of coarse particles containing sodium. Hence, their role in cloud formation and contribution to salt cycles and the terrestrial ecosystem in the Amazon basin warrant further consideration.

Few measurements of aerosol chemical composition have been made during the winter–spring transition (following polar sunrise) to constrain Arctic aerosol–cloud–climate feedbacks. Herein, we report the first measurements of individual particle chemical composition near Utqiaġvik (Barrow), Alaska, in winter (seven sample days in January and February 2014). Individual particles were analyzed by computer-controlled scanning electron microscopy with energy dispersive X-ray spectroscopy (CCSEM-EDX, 24 847 particles), Raman microspectroscopy (300 particles), and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS, 290 particles). Sea spray aerosol (SSA) was observed in all samples, with fresh and aged SSA comprising 99 %, by number, of 2.5–7.5 µm diameter particles, 65–95 % from 0.5–2.5 µm, and 50–60 % from 0.1–0.5 µm, indicating SSA is the dominant contributor to accumulation and coarse-mode aerosol during the winter. The aged SSA particles were characterized by reduced chlorine content with 94 %, by number, internally mixed with secondary sulfate (39 %, by number, internally mixed with both nitrate and sulfate), indicative of multiphase aging reactions during transport. There was a large number fraction (40 % of 1.0–4.0 µm diameter particles) of aged SSA during periods when particles were transported from near Prudhoe Bay, consistent with pollutant emissions from the oil fields participating in atmospheric processing of aerosol particles. Organic carbon and sulfate particles were observed in all samples and comprised 40–50 %, by number, of 0.1–0.4 µm diameter particles, indicative of Arctic haze influence. Soot was internally mixed with organic and sulfate components. All sulfate was mixed with organic carbon or SSA particles. Therefore, aerosol sources in the Alaskan Arctic and resulting aerosol chemical mixing states need to be considered when predicting aerosol climate effects, particularly cloud formation, in the winter Arctic.

Predicting the formation of ice in the atmosphere presents one of the great challenges in physical sciences with important implications for the chemistry and composition of the Earth's atmosphere, the hydrological cycle, and climate. Among atmospheric ice formation processes, heterogeneous ice nucleation proceeds on aerosol particles ranging from a few nanometers to micrometers in size, commonly referred to as ice nucleating particles (INPs). Research over the last two decades has demonstrated that organic matter (OM) is ubiquitous in the atmosphere, present as organic aerosol (OA) particles or as coatings on other particle types. The physicochemical properties of OM make predicting how OM can contribute to the INP population challenging. This review focuses on the role of OM in INPs, summarizing and highlighting recent advances in our understanding of the ice nucleation process gained from theoretical, laboratory, and field studies. Examination of ice residuals and INPs with analytical techniques demonstrates that OM participates in atmospheric ice crystal formation. Molecular dynamic simulations provide insight into the microscopic processes that initiate ice nucleation. The amorphous phase state of OM in the supercooled and metastable regime is identified as a key factor in assessing the particles' nucleation pathways and rates. A theoretical model is advanced, based on particle water activity, to holistically predict amorphous phase changes and ice nucleation rates of particles coated by OM. The goal of this review is to synthesize our current understanding and propose future research directions needed to fully evaluate how OA particles contribute to INPs in the atmosphere.