PHYSICOCHEMICAL PROPERTIES OF ATMOSPHERIC AEROSOLS AND THEIR EFFECT ON ICE CLOUD FORMATION

Atmospheric aerosols play a vital role in the Earth\u27s energy budget-directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei and ice-nucleating particles [1, 2]. The cloud formation potential of aerosol is driven by multiple factors, including surface properties, size distribution, composition, mixing state, phase state, and morphology [3]. The interaction of aerosols with clouds alters the aerosol\u27s physicochemical properties. Those properties can also evolve during transport due to atmospheric processing, in turn, affect the aerosol\u27s ice nucleation and cloud formation activities. This thesis presents experimental studies to understand the role of physicochemical properties of aerosol on the formation of ice. To get a detailed understanding of the aerosol effect on ice nucleation, we conducted controlled ice nucleation experiments on a known surface (muscovite mica) with controlled properties (e.g., surface cations) as well as ice nucleation experiments on complex atmospheric particles, which were characterized with multimodal microspectroscopic techniques. The results from controlled experiments suggest that the ice nucleation activity of a surface can be modified by simply changing the surface cations. In contrast, ice nucleation experiments with complex atmospheric particles indicates a more complicated dependence on the physicochemical properties

Molecular and physical composition of tar balls in wildfire smoke: an investigation with complementary ionisation methods and 15-Tesla FT-ICR mass spectrometry

Tar balls (TBs) are a major carbonaceous product of wildfires and other biomass-burning events that often exceed soot or other elemental forms of carbon in number and mass. Being a recalcitrant fraction of organic carbon, TBs are capable of long-range atmospheric transport, and thus, exert influence not only in the vicinity of wildfires but also in remote regions. Here, we characterised ambient atmospheric aerosol samples with varying TB number fractions collected downwind of Pacific Northwest wildfires using a 15-Tesla Fourier transform-ion cyclotron resonance mass spectrometer (15-T FT-ICR MS). Relative to non-TB aerosol, we found 2006 and 851 molecular formulae exclusively in TB-rich aerosol using laser desorption ionisation (LDI) of samples directly from an aerosol-loaded substrate and electrospray ionisation (ESI) of ACN-extracted aerosol, respectively. Elemental composition from LDI/15-T FT-ICR MS revealed TBs to be abundant in molecular species of low volatility and high viscosity, providing molecular detail that was consistent with key climate and air quality-related properties of TBs. Our findings demonstrate that the TB-specific molecular composition obtained from (−)LDI/15-T FT-ICR MS not only complements (−)ESI analyses, but provides a more apt reflection of the physical properties of TBs as well. We provide proof-of-concept evidence for the potential value of using LDI/15-T FT-ICR MS in routine OA analyses, specifically smoke samples rich in refractory OA, and improve the representation of OA in atmospheric and climate modelling studies that aim to fully understand its impact and occurrence

African smoke particles act as cloud condensation nuclei in the wintertime tropical North Atlantic boundary layer over Barbados

The number concentration and properties of aerosol particles serving as cloud condensation nuclei (CCN) are important for understanding cloud properties, including in the tropical Atlantic marine boundary layer (MBL), where marine cumulus clouds reflect incoming solar radiation and obscure the low-albedo ocean surface. Studies linking aerosol source, composition, and water uptake properties in this region have been conducted primarily during the summertime dust transport season, despite the region receiving a variety of aerosol particle types throughout the year. In this study, we compare size-resolved aerosol chemical composition data to the hygroscopicity parameter κ derived from size-resolved CCN measurements made during the Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) campaigns from January to February 2020. We observed unexpected periods of wintertime long-range transport of African smoke and dust to Barbados. During these periods, the accumulation-mode aerosol particle and CCN number concentrations as well as the proportions of dust and smoke particles increased, whereas the average κ slightly decreased (κ=0.46±0.10) from marine background conditions (κ=0.52±0.09) when the submicron particles were mostly composed of marine organics and sulfate. Size-resolved chemical analysis shows that smoke particles were the major contributor to the accumulation mode during long-range transport events, indicating that smoke is mainly responsible for the observed increase in CCN number concentrations. Earlier studies conducted at Barbados have mostly focused on the role of dust on CCN, but our results show that aerosol hygroscopicity and CCN number concentrations during wintertime long-range transport events over the tropical North Atlantic are also affected by African smoke. Our findings highlight the importance of African smoke for atmospheric processes and cloud formation over the Caribbean.</p

Water Structure on Mica Surfaces: Investigating the Effect of Cations

We studied thin films of water at the mica-air interface using infrared spectroscopy and molecular dynamics simulations. We investigate the influence of ions on interfacial water by exchanging the naturally occurring K+ ion with H+/Na+, Ca2+, and Mg2+. The experiments do not show a difference in the bulk structure (i. e. in the infrared spectra), but indicate that water is more strongly attracted to the Mg2+ mica. The simulations reveal that the cation-water interactions significantly influence the microscopic arrangement of water on mica. Our results indicate that the divalent cations result in strong water-mica interactions, which leads to longer hydrogen bond lifetimes and larger hydrogen bonded clusters of interfacial water molecules. These results have implications for surface-mediated processes such as heterogeneous ice nucleation, protein assembly and catalysis

Vertical Gradient of Size-Resolved Aerosol Compositions over the Arctic Reveals Cloud Processed Aerosol in-Cloud and above Cloud

Arctic aerosols play a significant role in aerosol-radiation and aerosol-cloud interactions, but ground-based measurements are insufficient to explain the interaction of aerosols and clouds in a vertically stratified Arctic atmosphere. This study shows the vertical variability of a size resolved aerosol composition via a tethered balloon system at Oliktok Point, Alaska, at different cloud layers for two representative case studies (background aerosol and polluted conditions). Multimodal microspectroscopy analysis during the background case reveals a broadening of chemically specific size distribution above the cloud top with a high abundance of sulfate particles and core-shell morphology, suggesting possible cloud processing of aerosols. The polluted case also indicates broadening of aerosol size distribution at the upper layer within the clouds with the dominance of carbonaceous particles, which suggests that the carbonaceous particles play a potential role in modulating Arctic cloud properties

Multivalent Surface Cations Enhance Heterogeneous Freezing of Water on Muscovite Mica

Heterogeneous ice nucleation is a crucial phenomenon in various fields of fundamental and applied science. We investigate the effect of surface cations on freezing of water on muscovite mica. Mica is unique in that the exposed ion on its surface can be readily and easily exchanged without affecting other properties such as surface roughness. We investigate freezing on natural (K+) mica and mica in which we have exchanged K+ for Al3+, Mg2+, Ca2+, and Sr2+. We find that liquid water freezes at higher temperatures when ions of higher valency are present on the surface, thus exposing more of the underlying silica layer. Our data also show that the size of the ion affects the characteristic freezing temperature. Using molecular dynamics simulations, we investigate the effects that the ion valency and exposed silica layer have on the behavior of water on the surface. The results indicate that multivalent cations enhance the probability of forming large clusters of hydrogen bonded water molecules that are anchored by the hydration shells of the cations. These clusters also have a large fraction of free water that can reorient to take ice-like configurations, which are promoted by the regions on mica devoid of the ions. Thus, these clusters could serve as seedbeds for ice nuclei. The combined experimental and simulation studies shed new light on the influence of surface ions on heterogeneous ice nucleation

Aerosol Composition, Mixing State, and Phase State of Free Tropospheric Particles and Their Role in Ice Cloud Formation

The prediction of ice cloud formation in the atmosphere remains challenging. Free tropospheric aerosols can act as ice nucleating particles, affecting cloud properties and precipitation. The physicochemical properties of free tropospheric particles are modified upon long-range transport by different atmospheric processes. These modifications affect the ice formation potential of individual particles. We investigated the physicochemical properties of free tropospheric particles collected at the remote Pico Mountain Observatory at 2225 m a.s.l. in the North Atlantic Ocean using multimodal micro-spectroscopy and chemical imaging techniques. We probed their ice nucleation (IN) activity using an IN stage interfaced with an environmental scanning electron microscope. Retroplume analysis, chemical imaging, and micro-spectroscopy analysis indicated that the size-resolved chemical composition, mixing state, and phase state of the particles with similar aging times but different transport patterns were substantially different. Relative humidity-dependent glass-transition temperatures estimated from meteorological conditions were consistent with the observed organic component of the particles\u27 phase. More viscous (solid and semi-solid-like) particles are more ice active in the deposition mode at temperatures ranging from 205 to 220 K than less viscous particles. This study provides a better understanding of the phase and mixing state of long-range transported free tropospheric aerosols and their role in ice cloud formation

Particle phase-state variability in the North Atlantic free troposphere during summertime is determined by atmospheric transport patterns and sources

Free tropospheric aerosol particles have important but poorly constrained climate effects due to transformations of their physicochemical properties during long-range transport. In this study, we investigate the chemical composition and provide an overview of the phase states of individual particles that have undergone long-range transport over the North Atlantic Ocean in June and July 2014, 2015, and 2017 to the Observatory of Mount Pico (OMP) in the Azores. The OMP is an ideal site for studying long-range-transported free tropospheric particles because local emissions have a negligible influence and contributions from the boundary layer are rare. We used the FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) to determine the origins and transport trajectories of sampled air masses and found that most of them originated from North America and recirculated over the North Atlantic Ocean. The FLEXPART analysis showed that the sampled air masses were highly aged (average plume age \u3e10 d). Size-resolved chemical compositions of individual particles were probed using computer-controlled scanning electron microscopy with an energy-dispersive X-ray spectrometer (CCSEM-EDX) and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS). CCSEM-EDX results showed that the most abundant particle types were carbonaceous (∼ 29.9 % to 82.0 %), sea salt (∼ 0.3 % to 31.6 %), and sea salt with sulfate (∼ 2.4 % to 31.5 %). We used a tilted stage interfaced within an environmental scanning electron microscope (ESEM) to determine the phase states of individual submicron particles. We found that most particles (∼ 47 % to 99 %) were in the liquid state at the time of collection due to inorganic inclusions. Moreover, we also observed substantial fractions of solid and semisolid particles (∼ 0 % to 30 % and ∼ 1 % to 42 %, respectively) during different transport patterns and events, reflecting the particles\u27 phase-state variability for different atmospheric transport events and sources. Combining phase state measurements with FLEXPART CO tracer analysis, we found that wildfire-influenced plumes can result in particles with a wide range of viscosities after long-range transport in the free troposphere. We also used temperature and RH values extracted from the Global Forecast System (GFS) along the FLEXPART-simulated path to predict the phase state of the particles during transport and found that neglecting internal mixing with inorganics would lead to an overestimation of the viscosity of free tropospheric particles. Our findings warrant future investigation aiming at the quantitative assessment of the influence of internal mixing on the phase states of the individual particles. This study also provides insights into the chemical composition and phase state of free tropospheric particles, which can help models to reduce uncertainties about the effects of ambient aerosol particles on climate

Case study evaluation of size-resolved molecular composition and phase state of carbonaceous particles in wildfire influenced smoke from the Pacific Northwest

Wildfires are significant sources of carbonaceous particles in the atmosphere. Given the dependence of atmospheric processes on particle physical and molecular properties, the interplay between particle size, phase state and chemical composition is investigated here for aerosol influenced by a 2021 Pacific Northwest wildfire event. Both micro-spectroscopy and high resolution mass spectrometry analyses highlight a similarity in particle compositions independent of both particle size (0.1-0.32 μm particle diameters) and day/night cycle influences. Microscopy techniques revealed similar phase states for periods of both day and night, with increases in liquid-like character for smaller particles. Finally, we apply an evaporation kinetics model on estimated volatility distributions from assigned molecular formulae, similarly revealing a slight increase in liquid-like character for smaller particles with no significant day/night dependency. While the observations here are limited to a case study, the lack of influence from the day/night cycle on chemical composition and phase state of particles in a wildfire influenced plume is of particular note given that dependences are otherwise commonly observed for different environments/sources. This observation, combined with the lack of compositional dependencies for size-resolved wildfire-influenced particles, may have substantial implications for wildfire particle optical properties, transport, and atmospheric models

Cloud condensation nuclei activity of internally mixed particle populations at a remote marine free troposphere site in the North Atlantic Ocean

This study reports results from research conducted at the Observatory of Mount Pico (OMP), 2225 m above mean sea level on Pico Island in the Azores archipelago in June and July 2017. We investigated the chemical composition, mixing state, and cloud condensation nuclei (CCN) activities of long-range transported free tropospheric (FT) particles. FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) simulations reveal that most air masses that arrived at the OMP during the sampling period originated in North America and were highly aged (average plume age \u3e 10 days). We probed size-resolved chemical composition, mixing state, and hygroscopicity parameter (κ) of individual particles using computer-controlled scanning electron microscopy with an energy-dispersive X-ray spectrometer (CCSEM-EDX). Based on the estimated individual particle mass from elemental composition, we calculated the mixing state index, χ. During our study, FT particle populations were internally mixed (χ of samples are between 53 % and 87 %), owing to the long atmospheric aging time. We used data from a miniature Cloud Condensation Nucleus Counter (miniCCNC) to derive the hygroscopicity parameter, κCCNC. Combining κCCNC and FLEXPART, we found that air masses recirculated above the North Atlantic Ocean with lower mean altitude had higher κCCNC due to the higher contribution of sea salt particles. We used CCSEM-EDX and phase state measurements to predict single-particle κ (κCCSEM-EDX) values, which overlap with the lower range of κCCNC measured below 0.15 % SS. Therefore, CCSEM-EDX measurements can be useful in predicting the lower bound of κ, which can be used in climate models to predict CCN activities, especially in remote locations where online CCN measurements are unavailable