| 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. |
- Gramlich, Yvette, 1993-
(författare)
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Chemical composition of Arctic aerosols and their link to clouds
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Arctic is a place particularly vulnerable to climate change, warming at an accelerated rate compared to the rest of the Earth. In this remote environment, the atmosphere, the ocean, the ice, and the land are all interlinked and are shaping a very complex system. This is why the interplay between aerosols and clouds and their role in the warming Arctic is still not fully understood.To address this issue, a better understanding of the sources, properties, and fate of aerosol particles in the Arctic is needed. By means of in situ observations of aerosols and clouds at the Zeppelin Observatory on Svalbard, this thesis aims to shed light on aerosol-cloud interactions in the Arctic. These measurements were conducted within the framework of the one-year long Ny-Ålesund Aerosol Cloud Experiment (2019-2020). A special focus of this thesis is on the chemical composition of aerosol particles from a molecular-level perspective, where measurements from a filter inlet for gases and aerosols coupled to a chemical ionization mass spectrometer were used.To identify the properties of the aerosol particles serving as cloud condensation nuclei (CCN) or ice nucleating particles (INP), cloud droplets and ice crystals were sampled with a ground-based counterflow virtual impactor inlet. The measured particles are called cloud residuals. The observations show that the cloud residuals have sizes in the Aitken and accumulation mode (as small as 10 nm in diameter). The chemical composition of these cloud residuals followed largely the expected annual cycle of aerosol particles in the Arctic, suggesting that most of the aerosol particles can act as CCN or INP in the Arctic. Anthropogenic signatures were present in the cloud residuals in the winter and spring, whereas in the summer a large contribution from methanesulfonic acid (MSA) was present, indicating natural source regions.The thesis also investigated how the oxidation products of dimethyl sulfide, MSA, sulfuric acid, and hydroperoxymethyl thioformate (HPMTF) are related to each other in the gaseous and particulate phase. HPMTF was observed to be present mainly in the gas phase, where it followed the gas phase signal of MSA in the summer. However, it was not present in significant amounts in the particle phase. In the presence of clouds, the gas phase levels of HPMTF decreased, indicating the uptake by cloud droplets.Another source of aerosol particles investigated are those from biomass burning (BB) emissions. The BB aerosol showed a largely similar molecular-level chemical composition of the organic aerosol compared to the rest of the year; however, a clear change to a largely organic dominated bulk aerosol composition was observed. Back trajectories suggested mainly Eastern Europe and Siberia as the source regions for the BB events. Using BB tracer compounds in combination with the back trajectories suggested that agricultural fires from Eastern Europe have a larger impact on the Arctic aerosol population, where mass and number enhancements compared to times not influenced by BB were found to reach up to one order of magnitude.The results from this thesis show that aerosol particles from natural emissions are an important source for Arctic aerosol particles. Especially, emissions from marine biological activity are relevant for the growth of aerosol particles to sizes in the CCN active regime in the summer.
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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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