| 1. |
- Graham, Emelie Linnéa, 1989-
(författare)
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Insights into key processes governing atmospheric aerosol loadings and their interactions with clouds
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aerosol particles are ubiquitous in the atmosphere and an essential part of the atmospheric radiation balance regulating the Earth’s temperature. Aerosol-cloud interaction still remains the largest single uncertainty in future climate projections. In addition, aerosols are also responsible for air pollution, causing severe health effects. With various origins and short atmospheric lifetimes, aerosols are unevenly distributed in the atmosphere, making simulations of air pollution and future climate scenarios challenging. This thesis aims to improve the understanding of the physical and chemical processes that govern aerosol concentration in the atmosphere, using both field as well as laboratory experiments.Field measurements were performed at a remote station at Mt Åreskutan, central Sweden. Located at 1250 m a.s.l. the station is frequently covered by clouds, allowing for in-cloud measurements. Aerosol particle size distribution measurements revealed a shift towards smaller diameters in the ambient aerosol size distribution after the station had been within a cloud. This is a result of the larger (> 60 nm) particles being more effectively scavenged by clouds as compared with the smaller end of the size distribution. Chemical analysis revealed a similar composition of the cloud water as the particulate matter, suggesting that cloud droplet activation at Mt Åreskutan is primarily dependent on particle size, and the aerosol population to have been internally mixed. Similarly, measurement of hygroscopicity and volatility revealed similar water-solubility and evaporation behaviour for the ambient aerosols and cloud residuals, with the organic fraction representative of aged boreal secondary organic aerosol (SOA) and showing no signs of significant aqueous phase processing.The NArVE laboratory campaign took place in an atmospheric simulation chamber at Paul Scherrer Institute, Switzerland. The experiments traced nitrate-induced SOA formation and ageing of three biogenic precursors, namely α-pinene, isoprene, and β-caryophyllene, using mass spectrometric techniques and evaporation measurements. The volatility of α-pinene SOA from nitrate oxidation was found to be higher than the corresponding ozonolysis products. The nitrate oxidation of isoprene resulted in species with similar volatility to α-pinene, while the β-caryophyllene system produced lower volatility compounds then the other two precursors. Quantitative comparison of the volatility measurements to commonly-used theoretical parameterizations revealed the need for further studies of the impact of the nitrate functional group on molecular volatility. Dark ageing of α-pinene was found to mainly occur through particle phase oxidation forming less volatile species. During the photolysis related to sunrise the molecular composition changed towards more volatile species, while no significant evaporation could be observed for the α-pinene and isoprene systems.A common theme in all these studies was investigating the level of detail needed to theoretically describe the observations. We found that while simple approximations (such as internal mixing and size-independent chemical composition of the particles) are often sufficient to capture trends in atmospheric aerosol properties, more research on (1) the processes taking place on shorter time- and smaller size scales than investigated here and (2) the effects of nitrate group on molecular volatility are warranted.
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| 2. |
- Baró Pérez, Alejandro, 1991-
(författare)
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Aerosol impacts on subtropical low-level clouds: a satellite and modelling perspective
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Complex interactions between aerosols, clouds, and radiation impact Earth's climate. However, several aspects of these interactions remain uncertain, which has led to extensive research over the last decades. This thesis explores some unresolved aspects by focusing on subtropical low-level stratocumulus (Sc) clouds, which have a significant cooling effect on climate. The clouds are also sensitive to varying aerosol conditions, which can influence their formation, properties, and lifetime. Clouds over the South East Atlantic have been studied in detail, using both numerical modeling and satellite observations, to shed light on the interactions between aerosols, clouds, and radiation. This geographical region displays a large and semi-permanent Sc cloud deck and is also subjected to meteorological conditions that bring large amounts of light-absorbing aerosols from biomass fires over the African continent. The biomass-burning plumes also bring enhanced levels of moisture, and the individual influence of the aerosols and the moisture on the low-level cloud properties have been investigated.The analysis of satellite retrievals showed a radiative impact (sensitive to aerosol composition and aerosol optical depth) of moist aerosol layers in the free troposphere over the South East Atlantic; however, it was not possible to observe a clear influence of these humid aerosol layers on the underlying low-level clouds. Aerosol-radiation interactions were implemented in a large eddy simulation (LES) code that was used to model stratocumulus to cumulus transitions (SCT) in weather situations where moist absorbing aerosol layers were in contact with low-level clouds and mixed into the marine boundary layer (MBL). In these simulations, the heating by the absorbing aerosol within the MBL affected the persistence of the Sc clouds by accelerating the SCT, especially during daylight and broken cloud conditions. However, the humidity accompanying the absorbing aerosol was also found to be important -- it reduced the deepening of the MBL when located above the Sc deck and delayed the SCT when in contact with clouds. Furthermore, the additional moisture resulted in a radiative cooling effect that was comparable to the radiative cooling effect caused by the aerosol itself. The simulated SCTs were found to be mostly driven by increased sea surface temperatures, regardless of aerosol conditions. This result was different compared to two other LES models where the SCT was driven by drizzle under the same low aerosol conditions. On a larger scale, it was found that an explicit description of aerosol-cloud interactions in a climate model led to smaller differences between the simulated and mean observed values of the shortwave cloud radiative effect compared to when a non-interactive parameterization was used.
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| 3. |
- Siegel, Karolina, 1990-
(författare)
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Chemical perspectives on aerosol-cloud interactions in the High Arctic
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric aerosol particles have important yet highly uncertain impacts on the Earth’s climate, with the largest uncertainties residing in the interactions between aerosols and clouds. The extent to which aerosols act as cloud condensation nuclei (CCN) depends on the chemical composition and size of the particles. To make correct predictions of cloud formation and the associated climate forcing, more knowledge on the physicochemical properties of aerosols is needed.This thesis investigates the chemical composition and CCN activity of aerosols in the High Arctic using a Chemical Ionization Mass Spectrometer with a Filter Inlet for Gases and Aerosols (FIGAERO-CIMS). The Arctic is the region on Earth with the current largest increase of mean surface temperature due to global warming and with big knowledge gaps in terms of aerosol-cloud-climate interactions.The first two articles focus on the region within the pack ice and marginal ice zone (MIZ) during Arctic late summer. They introduce new insights into the molecular composition of organic submicron (diameter<1 μm) aerosols and the associated hygroscopicity. The composition is shown to include a wide range of carbon and oxygen numbers, with a clear contribution from dimethyl sulfide (DMS) oxidation products. Together with observations of the inorganic aerosol fraction and CCN, the aerosol is shown to be highly hygroscopic, and the activation diameter and CCN number concentration to be possible to predict using κ-Köhler theory.The last two articles present results from a year-long study in Ny-Ålesund, Svalbard. The third article addresses the seasonality of DMS oxidation products, with a focus on the newly discovered compound hydroperoxymethyl thioformate (HPMTF). The analysis shows that gas-phase HPMTF follows the same development pattern in summer as the well-known oxidation product methylsulfonic acid (MSA), indicating a local source of DMS. HPMTF was however not found in significant amounts in the particle phase in either season. In the fourth article, the chemical composition of cloud residuals (particles remaining after drying of cloud droplets) was shown to be clearly influenced by DMS oxidation products (MSA and sulfuric acid) in summer. The importance of MSA and sulfuric acid for Arctic low-level cloud formation has previously been presumed, but not confirmed by in-situ observations.
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| 4. |
- Acosta Navarro, Juan Camilo, 1983-
(författare)
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Anthropogenic influence on climate through changes in aerosol emissions from air pollution and land use change
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Particulate matter suspended in air (i.e. aerosol particles) exerts a substantial influence on the climate of our planet and is responsible for causing severe public health problems in many regions across the globe. Human activities have altered the natural and anthropogenic emissions of aerosol particles through direct emissions or indirectly by modifying natural sources. The climate effects of the latter have been largely overlooked. Humans have dramatically altered the land surface of the planet causing changes in natural aerosol emissions from vegetated areas. Regulation on anthropogenic and natural aerosol emissions have the potential to affect the climate on regional to global scales. Furthermore, the regional climate effects of aerosol particles could potentially be very different than the ones caused by other climate forcers (e.g. well mixed greenhouse gases). The main objective of this work was to investigate the climatic effects of land use and air pollution via aerosol changes.Using numerical model simulations it was found that land use changes in the past millennium have likely caused a positive radiative forcing via aerosol climate interactions. The forcing is an order of magnitude smaller and has an opposite sign than the radiative forcing caused by direct aerosol emissions changes from other human activities. The results also indicate that future reductions of fossil fuel aerosols via air quality regulations may lead to an additional warming of the planet by mid-21st century and could also cause an important Arctic amplification of the warming. In addition, the mean position of the intertropical convergence zone and the Asian monsoon appear to be sensitive to aerosol emission reductions from air quality regulations. For these reasons, climate mitigation policies should take into consideration aerosol air pollution, which has not received sufficient attention in the past.
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| 5. |
- Bulatovic, Ines
(författare)
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Investigating aerosol effects on stratocumulus clouds through large-eddy simulation
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clouds have a large impact on Earth’s radiative budget by reflecting, absorbing and re-emitting radiation. They thus play a critical role in the climate system. Nevertheless, cloud radiative effects in a changing climate are highly uncertain. Atmospheric aerosol particles are another factor affecting Earth’s climate but the magnitude of their influence is also associated with high uncertainty. Therefore, an accurate representation of aerosol-cloud interactions in models is critical for having confidence in future climate projections. This thesis investigates aerosol impacts on cloud microphysical and radiative properties through numerical modelling, more specifically large-eddy simulation (LES). Moreover, the thesis investigates how the simulated cloud response to changes in the aerosol population depends on the model description of different processes. Mixed-phase stratocumulus (MPS) clouds are especially problematic to simulate for models on all scales. These clouds consist of a mixture of supercooled water and ice in the same volume and are therefore potentially thermodynamically unstable. MPS clouds over the central (north of 80° N) Arctic Ocean are particularly sensitive to aerosol changes due to the relatively clean atmospheric conditions in this region. At the same time, the clouds also have an important impact on the Arctic surface radiative budget. Therefore, this thesis mostly focuses on Arctic MPS clouds.Simulations of a typical subtropical marine stratocumulus cloud showed that the aerosol-cloud forcing depends on the model treatment for calculating the cloud droplet number concentration (CDNC). The simulated change in the top of the atmosphere shortwave radiation due to increased aerosol number concentrations was almost three times as large when the CDNC was prescribed compared to when the CDNC was prognostic. Simulations of a central Arctic summertime low-level MPS cloud confirmed that the chemical composition and the size of aerosol particles both can play an important role in determining the efficiency of an aerosol to act as cloud condensation nuclei - and thus influence cloud properties. However, the hygroscopicity of the aerosol particle was only important if the particles were small in size (i.e., if they correspond to the Aitken mode size) or if they were close to hydrophobic. Further, it was also found that Aitken mode particles can significantly change microphysical and radiative properties of central Arctic MPS if the concentration of larger particles (i.e., corresponding to the accumulation mode) is less than approximately 10-20 cm-3. One of the most recent research expeditions in the central Arctic (in the summer of 2018) was characterized by a high occurrence of multiple cloud layers. Namely, the boundary layer structure consisted of two MPS, one located close to the surface and one at the top of the boundary layer. Large-eddy simulations of an observed case with this particular cloud structure showed that the two-layer boundary-layer clouds are persistent unless the aerosol number concentrations are low (< 5 cm-3) or the wind speed is high (≥ 8.5 m s-1). In the model, low aerosol numbers led to a dissipation of the upper cloud layer while the lower cloud layer dissipated if the wind speed was strong. Changes in the optical thickness and cloud emissivity of each individual cloud layer of the two-layer cloud structure were found to substantially impact the surface radiative fluxes.
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| 6. |
- Dimitrelos, Antonios, 1986-
(författare)
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A large-eddy simulation perspective on Arctic airmass transformation and low-level cloud evolution
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Arctic is currently warming faster than other regions of the Earth. Many processes and feedbacks contribute to the enhanced warming. Among these are the radiative effects of clouds. Arctic mixed-phase clouds, which contain both liquid and ice condensate, have high longevity and can exert significant surface warming since the amount of solar radiation in the region is relatively low and the surface reflectivity often is high. In this thesis, we study these clouds utilizing a large-eddy model coupled with one-dimensional thermodynamic sea ice model. The main aim is to understand the interactions between cloud dynamics, microphysics, radiation, and turbulent processes and how these together govern the life cycle and surface warming of the clouds. By comparing a group of models with observations from the summertime high Arctic, we confirm the hypothesis that when aerosol concentrations are low, a small increase in their number concentration can increase the liquid water content of the cloud and in turn, the surface warming. Idealized simulations of moist intrusions into the Arctic show that the surface temperature may increase by more than 15o C if we allow clouds to form during a moist intrusion compared to if the atmosphere is cloud free. The simulations also show that the large-scale divergence rate strongly impacts the maintenance of the liquid layer at the top of these clouds. A main finding of the thesis is that the temperature of the cloud that forms during a moist intrusion is close to the initial dew point temperature. Thus, the surface warming induced by the clouds depends mostly on the initial humidity of the air mass rather than the initial temperature. In addition, the stability of the initial dew point temperature profile largely controls the turbulent state of the cloud. If the profile is unstable, then the cloud can transform from a thin, stable stratus to a deeper stratocumulus cloud, which also enhances the surface warming. Consequently, both the initial amount and the vertical structure of the initial moisture of the intrusion are important for the warming of the sea ice. A change in the number of cloud condensation nuclei does not affect the cloud evolution considerably provided that there is a continuous supply of these nuclei. However, if cloud condensation nuclei sources are absent then the cloud may remain in its stable state. Furthermore, a decrease in the cloud ice condensate, which may be caused by a lack of ice nucleation particles, may delay the transformation of the cloud into a stratocumulus. These results suggest that any future change in aerosol loading and atmospheric moisture transport into the Arctic may alter the surface longwave cloud radiative effect and cause changes in the sea ice evolution.
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| 7. |
- Siegel, Karolina
(författare)
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Chemical composition of summertime High Arctic aerosols
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents new insights into the chemical composition of semi-volatile compounds in aerosol samples collected in the central Arctic Ocean close to the North Pole in September 2018. The central Arctic Ocean is an inaccessible location due to the lack of land areas along with heavy pack ice conditions. Therefore, large knowledge gaps remain to understand the Arctic climate system, and in particular the role of aerosol particles in its pristine atmosphere.The chemical composition of the aerosol samples was analysed on a molecular level using High Resolution Time-of-Flight Chemical Ionization Mass Spectrometry coupled to a Filter Inlet for Gases and AEROsols (FIGAERO-HRToF-CIMS). The analysis revealed a significant signal from compounds that are likely from marine sources. One important precursor for marine aerosols is dimethyl sulfide (DMS), a gas released by phytoplankton and ice algae in the Arctic Ocean. DMS oxidises in the atmosphere to produce oxidation products that can contribute to aerosol growth. Analysis of air mass origin with backward trajectories showed that the highest ambient DMS concentrations originated from marine areas around the pack ice. However, no correlation could be shown within the pack ice between ambient DMS and its oxidation product methanesulfonic acid (MSA) in the particle phase.As FIGAERO-HRToF-CIMS is commonly used in areas with higher particle concentrations and has never been used in the central Arctic before, this thesis further demonstrates its suitability for measurements of aerosol chemical composition in this remote region.
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| 8. |
- Bender, Frida A-M, 1978-
(författare)
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Earth's albedo in a changing climate
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The albedo is a key parameter in the radiative budget of the Earth and a primary determinant of the planetary temperature and is therefore also central to questions regarding climate stability, climate change and climate sensitivity. Climate models and satellite observations are essential for studying the albedo, and the parameters determining it, on large spatial and temporal scales. Although climate models are able to capture the large-scale characteristics of the albedo, a bias is found between modelled and observed global albedo estimates, and on a regional scale particular problematic regions can be identified. Cloud parameters, that are of great importance for determining the albedo, vary widely among models, but lack of observations makes constraining models, and even evaluating models, difficult. The freedom of variability for cloud parameters can be used to make models agree with observations of the better constrained radiative budget. It is shown that tuning a model to different radiative budget estimates by altering cloud parameters can influence the climate sensitivity of the model, but the effect seen is small, compared to the range of climate sensitivities estimated by different models. Despite their different parameterizations of clouds, aerosols etc., models do have fundamental features in common, which can further the understanding of the real climate system. For instance it is found that sensitivity to volcanic forcing is related to climate sensitivity in an ensemble of models. If this relation is valid for the real climate as well, observations of the volcanic sensitivity can help restrict the climate sensitivity. The range of climate sensitivity estimates in models can largely be attributed to variations in cloud response to forcing. It is found that in models with high climate sensitivity changes in cloud cover and cloud reflectivity enhance a positive radiative forcing due to increased CO2 concentrations, feeding back on the warming and in models with low climate sensitivity, cloud response counteracts the positive radiative forcing and warming induced by the same forcing. As a consequence the total albedo response to increased CO2 forcing is found to be stronger (more negative) in high sensitivity models and vice versa. Cloud albedo and its variation between different cloud regimes, is important in this regard, yet not well known. A method based on the relation between cloud fraction and albedo is presented, giving a way to estimate regional cloud albedo, primarily for homogeneous cloud regimes, but possibly also extended to a global scale.
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| 9. |
- Johansson, Erik, 1981-
(författare)
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Improving the understanding of cloud radiative heating
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clouds play an essential role in regulating Earth’s radiation budget by reflecting and absorbing energy at different spectra. As they interact with radiation, they can radiatively heat or cool the adjacent atmosphere and the surface. This heating effect can have a strong implication for the circulation and can change the surface properties by, for example, melting sea ice. The lack of high-resolution global observations has previously been a limitation for our understanding of the vertical structure of cloud radiative heating, and for evaluating the cloud radiative effect in climate models. In this thesis, we will investigate and document cloud radiative heating derived from space-based observations. We will focus on two regions, the Arctic and the Tropics, where cloud radiative heating plays an important, but fundamentally different role.In the Tropics, radiative heating at high altitudes influences the large scale circulation. Stratiform, deep convective, and cirrus clouds have a strong radiative impact in the upper troposphere. We found while investigating the Indian monsoon, that thick stratiform clouds will radiatively heat the upper troposphere by more than 0.2 K/day when the monsoon is most intense during June, July and August. Deep convective clouds cause considerable heating in the middle troposphere and at the same time, cool the tropical tropopause layer (TTL). These two thick cloud types will also cool the surface during the monsoon, weakening the temperature gradient between land and ocean. During these months, cirrus clouds are frequently located inside the TTL. We further find that in the Tropics, the climate model, EC-Earth, can capture the seasonal variations in cloud radiative heating seen in the satellite observations. However, the model overestimates the radiative heating in the upper region and underestimates them in the middle region of the troposphere. This dissimilarity is caused by unrealistic longwave heating and low cloud fraction in the upper and middle of the troposphere, respectively.Radiative heating from cirrus, located inside the TTL, is considered to play an important role in the mass transport from the troposphere to the stratosphere. This heating generates enough buoyancy so that the air can pass the barrier of zero net radiative heating. We find that high thin single-layer clouds can heat the upper troposphere by 0.07 K/day. If a thick cloud layer is present underneath, they will radiatively suppress the high cloud, causing it to cool the adjacent air instead. The optical depth and cloud top height of the underlying cloud are two crucial factors that radiatively impact the high cloud above.Warm moist air is regularly transported from the mid-latitudes into the Arctic by low- and high-pressure systems. As the moist air enters the Arctic, it increases the cloudiness and warms the surface. This surface heating has the potential to affect the ice cover months after the intrusion. We find that during extreme moist intrusions, the surface temperature in the Arctic can rise by more than 5 K during the winter months with an increase in cloudiness by up to 30% downstream from the intrusion. These extra clouds radiatively heat the lower part of the atmosphere and cool the middle part, affecting the stability of the Arctic atmosphere.
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| 10. |
- Karlsson, Linn, 1990-
(författare)
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Aerosol–cloud interactions in a warming Arctic
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric aerosol particles are small liquid or solid particles suspended in the air. They are present in the atmosphere all around us and affect the planetary energy balance by scattering and absorbing radiation and by interacting with clouds. In model projections of future climate, aerosol–cloud interactions contribute a lot of uncertainty. Large-scale climate models particularly struggle with simulating low-level clouds in the Arctic, which is a region that is not only warming at twice the global average rate or higher but also where natural aerosol emissions are expected to change most as a result of the warming. The goal of this thesis was to study aerosol–cloud interactions to help improve our understanding of what role clouds play in the Arctic climate and how they will respond to climate change. Specifically, the project focused on studying the microphysical properties of aerosol particles and cloud nucleating particles—the subset of aerosol particles that participate in cloud formation. This was done both through field experiments in the high Arctic over the pack ice and by analysis of an existing two-year data set from an Arctic research station on Svalbard.The main instrument used in this thesis was a ground-based counterflow virtual impactor (GCVI) inlet, which dries cloud droplets and ice crystals and allows us to characterise the particles that were inside. The Svalbard study is the longest GCVI study to date, and the first to cover more than a full annual cycle. It also involved a detailed evaluation of the GCVI. Using the GCVI inlet and a large array of other instruments, we were able to show that small, so-called Aitken mode particles act as cloud nucleating particles, supporting results from previous studies. However, our measurements showed these particles to be more abundant in the cloud droplets and ice crystals than expected, both over the pack ice and on Svalbard. While some uncertainties remain, these datasets can potentially be used to evaluate and improve model representations of low-level Arctic clouds. In the other parts of this thesis, we found that iodine nucleation and breakup of larger particles are potential formation pathways for Aitken mode particles over the pack ice. However, detailed chemical composition measurements of cloud nucleating particles would be needed to determine whether these formation mechanisms are important for Arctic cloud formation.
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