Single-particle characterization of summertime Antarctic aerosols collected at King George Island using quantitative energy-dispersive electron probe X-ray microanalysis and attenuated total reflection Fourier transform-infrared imaging techniques

Direct Observation of Particle-To-Particle Variability in Ambient Aerosol pH Using a Novel Analytical Approach Based on Surface-Enhanced Raman Spectroscopy

The pH of atmospheric aerosols is a key characteristic that profoundly influences their impacts on climate change, human health, and ecosystems. Despite widely performed aerosol pH research, determining the pH levels of individual atmospheric aerosol particles has been a challenge. This study presents a novel analytical technique that utilizes surface-enhanced Raman spectroscopy to assess the pH of individual ambient PM2.5-10 aerosol particles in conjunction with examining their hygroscopic behavior, morphology, and elemental compositions. The results revealed a substantial pH variation among simultaneously collected aerosol particles, ranging from 3.3 to 5.7. This variability is likely related to each particle's unique reaction and aging states. The extensive particle-to-particle pH variability suggests that atmospheric aerosols present at the same time and location can exhibit diverse reactivities, reaction pathways, phase equilibria, and phase separation properties. This pioneering study paves the way for in-depth investigations into particle-to-particle variability, size dependency, and detailed spatial and temporal variations of aerosol pH, thus deepening our understanding of atmospheric chemistry and its environmental implications.

Inputs, amplification and sinks of perfluoroalkyl substances at coastal Antarctica

The sources, biogeochemical controls and sinks of perfluoroalkyl substances, such as perfluoroalkyl acids (PFAAs), in polar coastal regions are largely unknown. These were evaluated by measuring a large multi-compartment dataset of PFAAs concentrations at coastal Livingston and Deception Islands (maritime Antarctica) during three austral summers. PFAAs were abundant in atmospheric-derived samples (aerosols, rain, snow), consistent with the importance of atmospheric deposition as an input of PFAAs to Antarctica. Such PFAAs deposition was unequivocally demonstrated by the occurrence of PFAAs in small Antarctic lakes. Several lines of evidence supported the relevant amplification of PFAAs concentrations in surface waters driven by snow scavenging of sea-spray aerosol-bound PFAAs followed by snow-melting. For example, vertical profiles showed higher PFAAs concentrations at lower-salinity surface seawaters, and PFAAs concentrations in snow were significantly higher than in seawater. The higher levels of PFAAs at Deception Island than at Livingston Island are consistent with the semi-enclosed nature of the bay. Concentrations of PFOS decreased from 2014 to 2018, consistent with observations in other oceans. The sink of PFAAs due to the biological pump, transfer to the food web, and losses due to sea-spray aerosols alone are unlikely to have driven the decrease in PFOS concentrations. An exploratory assessment of the potential sinks of PFAAs suggests that microbial degradation of perfluoroalkyl sulfonates should be a research priority for the evaluation of PFAAs persistence in the coming decade.

Strong Acids or Bases Displaced by Weak Acids or Bases in Aerosols: Reactions Driven by the Continuous Partitioning of Volatile Products into the Gas Phase

Aerosols are ubiquitous in the atmosphere and profoundly affect climate systems and human health. To gain more insights on their broad impacts, we need to comprehensively understand the fundamental properties of atmospheric aerosols. Since aerosols are multiphase, a dispersion of condensed matter (solid particles or liquid droplets, hereafter particles) in gas, partitioning of volatile matter between the condensed and the gas phases is one defining characteristic of aerosols. For example, water content partitioning under different relative humidity conditions, known as aerosol hygroscopicity, has been extensively investigated in the past decades. Meanwhile, partitioning of volatile organic or inorganic components, which is referred to as aerosol volatility, remains understudied. Commonly, a bulk solution system is treated as a single phase, with volatility mainly determined by the nature of its components, and the composition partitioning between solution and gas phase is limited. Aerosols, however, comprise an extensive gas phase, and their volatility can also be induced by component reactions. These reactions occurring within aerosols are driven by the formation of volatile products and their continuous partitioning into the gas phase. As a consequence, the overall aerosol systems exhibit prominent volatility. Noteworthily, such volatility induced by reactions is a phenomenon exclusively observed in the multiphase aerosol systems, and it is trivial in bulk solutions due to the limited extent of liquid-gas partitioning. Take the chloride depletion in sea salt particles as an example. Recent findings have revealed that chloride depletion can be caused by reactions between NaCl and weak organic acids, which release HCl into the gas phase. Such a reaction can be described as a strong acid displaced by a weak acid, which is hardly observed in bulk phase. Generally, this unique partitioning behavior of aerosol systems and its potential to alter aerosol composition, size, reactivity, and other physicochemical properties merits more attention by atmospheric community.This Account focuses on the recent advancements in the research of component reactions that induce aerosol volatility. These reactions can be categorized into four types: chloride depletion, nitrate depletion, ammonium depletion, and salt hydrolysis. The depletion of chloride or nitrate can be regarded as a displacement reaction, in which a strong acid is displaced by a weak acid. Such a reaction releases highly volatile HCl or HNO₃ into the gas phase and leads to a loss of chloride or nitrate within the particles. Likewise, ammonium depletion is a displacement reaction in which a strong base is displaced by a weak base, resulting in release of ammonia and substantial changes in aerosol hygroscopicity. In addition, aerosol volatility can also be induced by salt hydrolysis in a specific case, which is sustained by the coexistence of proton acceptor and hydroxide ion acceptor within particles. Furthermore, we quantitatively discuss these displacement reactions from both thermodynamic and kinetic perspectives, by using the extended aerosol inorganic model (E-AIM) and Maxwell steady-state diffusive mass transfer equation, respectively. Given the ubiquity of component partitioning in aerosol systems, our discussion may provide a new perspective on the underlying mechanisms of aerosol aging and relevant climate effects.

Photochemical reactions on aerosols at West Antarctica: A molecular case-study of nitrate formation among sea salt aerosols

Environmental implications of climate change are complex and exhibit regional variations both within and between the polar regions. The increase of solar UV radiation flux over Antarctica due to stratospheric ozone depletion creates the optimal conditions for photochemical reactions on the snow. Modeling, laboratory, and indirect field studies suggest that snowpack process release gases to the atmosphere that can react on sea salt particles in remote regions such as Antarctica, modifying aerosol composition and physical properties of aerosols. Here, we present evidence of photochemical processing in West Antarctica aerosols using microscopic and chemical speciation of individual atmospheric particles. Individual aerosol particles collected at the Brazilian module Criosfera 1 were analyzed by scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) combined with computer-controlled scanning electron microscopy (CCSEM) with energy-dispersive X-ray (EDX) microanalysis. The displacement of chlorine relative to sodium was observed over most of the sea salt particles. Particles with a chemical composition consistent with NaCl-NO₃ contributed up to 30% of atmospheric particles investigated. Overall, this study provides evidence that the snowpack and particulate nitrate photolysis should be considered in dynamic partition equilibrium in the troposphere. These findings may assist in reducing modeling uncertainties and present new insights into the aerosol chemical composition in the polar environment.

Enrichment of perfluoroalkyl substances in the sea-surface microlayer and sea-spray aerosols in the Southern Ocean

Sea-spray (or sea-salt) aerosol (SSA) formation and their subsequent atmospheric transport and deposition have been suggested to play a prominent role in the occurrence of ionizable perfluoroalkyl substances (PFAS) in the maritime Antarctica and other remote regions. However, field studies on SSA's role as vector of transport of PFAS are lacking. Following a multiphase approach, seawater (SW), the sea-surface microlayer (SML) and SSA were sampled simultaneously at South Bay (Livingston Island, Antarctica). Average PFAS concentrations were 313 pg L^-1, 447 pg L^-1, and 0.67 pg m^-3 in SW, the SML and SSA, respectively. The enrichment factors of PFAS in the SML and SSA ranged between 1.2 and 5, and between 522 and 4690, respectively. This amplification of concentrations in the SML is consistent with the surfactant properties of PFAS, while the large enrichment of PFAS in atmospheric SSA may be facilitated by the large surface area of SSA and the sorption of PFAS to aerosol organic matter. This is the first field work assessing the simultaneous occurrence of PFAS in SW, the SML and SSA. The large measured amplification of concentrations in marine aerosols supports the role of SSA as a relevant vector for long-range atmospheric transport of PFAS.

Pivotal Role of Snow Deposition and Melting Driving Fluxes of Polycyclic Aromatic Hydrocarbons at Coastal Livingston Island (Antarctica)

The atmosphere-land-ocean dynamics of semivolatile organic compounds in polar regions is poorly understood, also for the abundant and ubiquitous polycyclic aromatic hydrocarbons (PAHs). We report the concentrations and fluxes of PAHs in a polar coastal ecosystem (Livingston Island, Antarctica). From late spring (December 2014) to late summer (February 2015), we sampled air, snow, coastal seawater, plankton, and the fugacity in soils and snow. The concentrations of PAHs in seawater were low but increased during the austral summer. The PAH concentrations in snow were significantly higher than in coastal seawater. Soil-air fugacity ratios showed a net volatilization of PAH when soils were covered with lichens, and close to air-soil equilibrium for bare soils. Concentrations in surface snow were also close to equilibrium with atmospheric PAHs. Conversely, there was a net diffusive deposition of PAHs to coastal seawater during late spring, but a net volatilization from seawater during late summer. Volatilization fluxes were correlated with seawater temperature and salinity, consistent with a key role of snowmelt to the fluxes and dissolved phase concentrations during the austral summer. The comprehensive assessment provided here shows that the fugacity amplification in snow is transferred to soils and coastal seawater supporting PAH concentrations and fluxes.

Role of Snow Deposition of Perfluoroalkylated Substances at Coastal Livingston Island (Maritime Antarctica)

Perfluoroalkyl substances (PFAS) are ubiquitous in the environment, including remote polar regions. To evaluate the role of snow deposition as an input of PFAS to Maritime Antarctica, fresh snow deposition, surface snow, streams from melted snow, coastal seawater, and plankton samples were collected over a three-month period (December 2014-February 2015) at Livingston Island. Local sources of PFASs were significant for perfluoroalkyl sulfonates (PFSAs) and C7-14 perfluoroalkyl carboxylates (PFCAs) in snow but limited to the transited areas of the research station. The concentrations of 14 ionizable PFAS (∑PFAS) in freshly deposited snow (760-3600 pg L^-1) were 1 order of magnitude higher than those in background surface snow (82-430 pg L^-1). ∑PFAS ranged from 94 to 420 pg L^-1 in seawater and from 3.1 to 16 ng g_dw^-1 in plankton. Ratios of individual PFAS concentrations in freshly deposited snow relative to surface snow (C_SD/C_Snow), snowmelt (C_SD/C_SM), and seawater (C_SD/C_SW) were close to 1 (from 0.44 to 1.4) for all perfluorooctanesulfonate (PFOS) isomers, suggesting that snowfall does not contribute significantly to PFOS in seawater. Conversely, these ratios for PFCAs ranged from 1 to 33 and were positively correlated with the number of carbons in the PFCA alkylated chain. These trends suggest that snow deposition, scavenging sea-salt aerosol bound PFAS, plays a role as a significant input of PFCAs to the Maritime Antarctica.

Effect of Structural Heterogeneity in Chemical Composition on Online Single-Particle Mass Spectrometry Analysis of Sea Spray Aerosol Particles

Knowledge of the surface composition of sea spray aerosols (SSA) is critical for understanding and predicting climate-relevant impacts. Offline microscopy and spectroscopy studies have shown that dry supermicron SSA tend to be spatially heterogeneous particles with sodium- and chloride-rich cores surrounded by organic enriched surface layers containing minor inorganic seawater components such as magnesium and calcium. At the same time, single-particle mass spectrometry reveals several different mass spectral ion patterns, suggesting that there may be a number of chemically distinct particle types. This study investigates factors controlling single particle mass spectra of nascent supermicron SSA. Depth profiling experiments conducted on SSA generated by a fritted bubbler and total ion intensity analysis of SSA generated by a marine aerosol reference tank were compared with observations of ambient SSA observed at two coastal locations. Analysis of SSA produced by utilizing controlled laboratory methods reveals that single-particle mass spectra with weak sodium ion signals can be produced by the desorption of the surface of typical dry SSA particles composed of salt cores and organic-rich coatings. Thus, this lab-based study for the first time unifies findings from offline and online measurements as well as lab and field studies of the SSA particle-mixing state.

Study of particulate matter from Primary/Secondary Marine Aerosol and anthropogenic sources collected by a self-made passive sampler for the evaluation of the dry deposition impact on built heritage

Dry deposition is one of the most dangerous processes that can take place in the environment where the compounds that are suspended in the atmosphere can react directly on different surrounding materials, promoting decay processes. Usually this process is related with industrial/urban fog and/or marine aerosol in the coastal areas. Particularly, marine aerosol transports different types of salts which can be deposited on building materials and by dry deposition promotes different decay pathways. A new analytical methodology based on the combined use of Raman Spectroscopy and SEM-EDS (point-by-point and imaging) was applied. For that purpose, firstly evaporated seawater (presence of Primary Marine Aerosol (PMA)) was analyzed. After that, using a self-made passive sampler (SMPS), different suspended particles coming from marine aerosol (transformed particles in the atmosphere (Secondary Marine Aerosol (SMA)) and metallic airborne particulate matter coming from anthropogenic sources, were analyzed. Finally in order to observe if SMA and metallic particles identified in the SMPS can be deposited on a building, sandstone samples from La Galea Fortress (Getxo, north of Spain) located in front of the sea and in the place where the passive sampler was mounted were analyzed.

Single Airborne Dust Particles using Plasmonic Metal Films with Hole Arrays

An airborne dust particle is trapped in the hole of a plasmonic metal film with a patterned array of holes (mesh) by pumping air through the mesh. Both scatter-free infrared spectra and scanning electron images are obtained on the same individual airborne dust particle, showing the feasibility of multiple, nondestructive experiments on a single, subwavelength particle. Ultimately, this may help to elucidate the effect of shape, orientation, and crystallinity on IR dust particle spectra.