| 1. |
- Khosravi, Maryam, 1975
(författare)
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Diurnal variation of stratospheric short-lived species
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Diurnal variation of stratospheric short-lived speciesMaryam KhosraviChalmers University of TechnologyDepartment of Earth and Space SciencesAbstractThe depletion of ozone in the stratosphere has a direct impact on the amount of ultravioletradiation reaching the Earth’s surface. The ozone abundance and distributionis controlled by the photo-chemical reactions and catalytic cycles involving halogens(chlorine and bromine), odd hydrogen and odd nitrogen species as well as by atmospherictransport.An introduction to ozone related chemistry of the stratosphere and modelling ofshort-lived species using photo-chemical models is presented. A one dimensional (1D)atmospheric model is used in two distinct studies: modeling of short-lived species inthe Arctic lower stratosphere (paper I) and in the tropical mid to upper stratosphere(paper II).The first part of this thesis describes the diurnal variation of chlorine monoxide,ClO, which is the most important short-lived species controlling ozone in the polarlower stratosphere during winter and early-spring. The ClO-dimer cycle, involvingClO and its nighttime reservoir Cl2O2, contributes to about 75%of the polar ozone loss.ClO measurements from an airborne submillimeter radiometer in the Arctic twilighthave been compared with the results from a 1D photo-chemical model (MISU-1D), inorder to validate the model and to test the kinetics of the reactions controlling the partitioningof chlorine species during the course of a day. The results show that crosssections leading to faster photolysis rates of Cl2O2 match best with the ClO observations.This is consistent with the recent version of the chemical kinetics evaluation bythe Jet Propulsion Laboratory. Slower photolysis rates can not be reconciled with theobservations since active chlorine higher than the total available chlorine would be required.The model reproduces higher nighttime ClO than the observations, howeverthe nighttime ClOmodelled using recent JPL recommendations of the thermal equilibriumconstant agree within the uncertainty range of the observations. The sensitivityof the model to the assumed albedo and temperature are also tested. Neither the temperaturenor the albedo uncertainties allow us to reconcile the model with the lowerobserved nighttime ClO. Moreover, it is found that the ClO-BrO cycle decreases ClOmostly around sunrise and sunset.The second part of the thesis presents the partitioning and diurnal variation of chlorine,bromine, hydrogen, nitrogen and oxygen species in the tropics from the stratosphereto the lower mesosphere. Model results of the diurnal variation of HOCl (asone of the chlorine reservoirs), the related short-lived species ClO and HO2 and HCl(as the main chlorine reservoir) for the tropics and three altitudes (35, 45 and 55 km)are compared with measurements from five satellite instruments. The model resultsgenerally agree with the observations both in terms of the absolute values and the differencesbetween day and night.
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| 2. |
- Acosta Navarro, Juan Camilo
(författare)
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Historical anthropogenic radiative forcing of changes in biogenic secondary organic aerosol
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Human activities have lead to changes in the energy balance of the Earth and the global climate. Changes in atmospheric aerosols are the second largest contributor to climate change after greenhouse gases since 1750 A.D. Land-use practices and other environmental drivers have caused changes in the emission of biogenic volatile organic compounds (BVOCs) and secondary organic aerosol (SOA) well before 1750 A.D, possibly causing climate effects through aerosol-radiation and aerosol-cloud interactions. Two numerical emission models LPJ-GUESS and MEGAN were used to quantify the changes in aerosol forming BVOC emissions in the past millennium. A chemical transport model of the atmosphere (GEOS-Chem-TOMAS) was driven with those BVOC emissions to quantify the effects on radiation caused by millennial changes in SOA.The specific objectives of this licentiate thesis are: 1) to understand what drove the changes in aerosol-forming BVOC emissions (i.e. isoprene, monoterpenes and sesquiterpenes) and to quantify these changes; 2) to calculate for the first time the combined historical aerosol direct and aerosol-cloud albedo effects on radiation from changing BVOC emissions through SOA formation; 3) to investigate how important the biological climate feedback associated to BVOC emissions and SOA formation is from a global climate perspective.We find that global isoprene emissions decreased after 1800 A.D. by about 12% - 15%. This decrease was dominated by losses of natural vegetation, whereas monoterpene and sesquiterpene emissions increased by about 2% - 10%, driven mostly by rising surface air temperatures. From 1000 A.D. to 1800 A.D, isoprene, monoterpene and sesquiterpene emissions decline by 3% - 8% driven by both, natural vegetation losses, and the moderate global cooling between the medieval climate anomaly and the little ice age. The millennial reduction in BVOC emissions lead to a 0.5% to 2% reduction in climatically relevant aerosol particles (> 80 nm) and cause a direct radiative forcing between +0.02 W/m² and +0.07 W/m², and an indirect radiative forcing between -0.02 W/m² and +0.02 W/m². The suggested biological climate feedback seems to be too small to have observable consequences on the global climate in the recent past.
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| 3. |
- Grieco, Francesco, 1992
(författare)
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Recovery and validation of Odin/SMR measurements of mesospheric CO and H2O
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Although life on Earth is concentrated in the troposphere, not only the processes happening here are of interest, since tropospheric processes are not the only ones that have an impact on the climate. Coupling mechanisms between different layers are such that changes in middle atmospheric dynamics and composition have an effect on what happens at lower altitudes. Therefore there is the need to extend climate models to include higher altitudes and to perform measurements of the middle atmosphere. Among the species that can be observed in the middle atmosphere, carbon monoxide (CO) and water vapour (H2O) are of high scientific interest thanks to their aptness to being used as circulation tracers. This is due to their long photochemical lifetime which is longer or equal to zonal, meridional and vertical transport time scales. Moreover, H2O plays a decisive role for O3 chemistry. In the mesosphere, meridional circulation is driven by momentum deposition from breaking gravity waves. This originates an annual cycle which affects mostly high latitudes: with measurements it can be observed how CO-rich dry air descends towards the lower mesosphere at the winter pole while moist and CO-poor air is uplifted towards upper mesosphere and lower thermosphere at the summer pole. Gravity waves also give rise to the Semi- Annual Oscillation (SAO) in zonal winds resulting in concentration oscillations that can be observed in CO and H2O measurements, mostly at the tropics. Among the satellite instruments currently performing remote sensing of the middle atmosphere, the Sub-Millimetre Radiometer (SMR) on board the Odin satellite is one of the most long-lived. In fact, SMR has been performing limb sounding of the middle atmosphere for 19 years, providing valuable long term datasets. However, the CO and H2O datasets are both affected by instrumental artifacts that resulted in a misestimation of the two species’ concentration. The two papers included in this thesis focus on identifying the causes and correcting such artifacts. CO observation modes were affected by a malfunctioning of the Phase-Lock Loop (PLL) of the local oscillator (LO) which was causing frequency shifts in the spectra and line broadening. An algorithm that shifts the line back to its theoretical position has been developed and the line broadening has been quantified and taken into account when performing new retrievals. Regarding the H2O observation modes, underestimation of sideband leakage resulted in artifacts in the spectra that caused the misestimation of the retrieved concentration and temperature. For this reason SMR agreed poorly with other instruments measurements. An improvement was brought by assuming a larger sideband leakage than previously thought. For both species new inversions have been performed using the Optimal Estimation Method (OEM) with the Atmospheric Radiative Transfer Simulator (ARTS) as forward model. The recovery and correction of these products resulted in two new long-term and global data sets that are now available to the scientific community to study middle atmospheric dynamics.
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| 4. |
- Angelbratt, Jon, 1981
(författare)
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Trend Analysis and Model Comparison of Ground-Based Solar FTIR Data
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Network for the Detection of Atmospheric Composition Change (NDACC) was initiated in the early 90s to investigate the major atmospheric problem of the day, the destruction of stratospheric ozone. About three years later solar FTIR measurements were started in Harestua, 50 km north of the Norwegian capital Oslo. Since then, much of the medial focus has shifted towards the climate issue but still the stratospheric chemistry is of great importance. Since the FTIR measurements in Harestua, and at several other stations within the NDACC network, have been going on for more than 15 years it is now possible and of great interest to study the long term trends of the measured species.In this thesis, the measured time series of methane (CH4) and nitrous oxide (N2O) have been studied. Reasons for the variability in these two important climate gases were outlined and used in a multiple regression model to estimate reliable long term linear trends. Further, the trend model was used on FTIR data from four European FTIR stations to investigate spatial difference in the trends.A very useful tool in atmospheric science is chemical transport models (CTMs). In this thesis, the EMEP model was used to find possible causes to the negative trends in carbon monoxide (CO) seen within the NDACC network. CO is the dominant sink for the hydroxyl radical (OH) which acts as a detergent of the atmosphere and hence is a key component of the tropospheric chemistry.
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| 5. |
- Frostenberg, Hannah, 1989
(författare)
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Modeling aerosol-cloud interactions in the Arctic
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clouds have a large impact on Earth’s energy balance, especially in the Arctic. Through their warming or cooling effect on the surface, clouds can play a critical role in the onset of melting and freezing of Arctic sea ice, which itself has a large effect on energy and moisture fluxes between the ocean and the atmosphere. One of the factors that determine whether a cloud has a net warming or cooling effect is its microphysical structure determined by the phase of the hydrometeors that the cloud consists of. Mixed-phase stratiform clouds, consisting of both water droplets and ice crystals, often occur in the Arctic between mid-spring and mid-fall. To be able to simulate the Arctic climate, it is crucial that models capture Arctic mixed-phase stratiform clouds (AMPS) and the apportionment between liquid and frozen hydrometeors in these clouds. A good representation of ice nucleation is the necessary first step for accurate modeling of cloud ice. Ice nucleation in mixed-phase clouds occurs heterogeneously with the requirement of ice nucleating particles (INP). This work presents a new heterogeneous freezing parameterization that was tested in a large-eddy simulation of AMPS. Different to other parameterization schemes, this parameterization does not require knowledge about the aerosol concentration and characteristics (type, size, etc.). Instead the parameterization is based on the observation that the frequency of INP concentrations at a specific temperature follows a log-normal distribution and randomly draws INP concentrations from this distribution at the present temperature. It is shown that the new parameterization results in reasonable amounts of cloud ice and that the random drawing of INP concentrations is an important aspect to be investigated when it comes to cloud ice formation.
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| 6. |
- Kanngiesser, Franz, 1992
(författare)
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Modelling optical properties of morphologically complex soot aerosols
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Soot containing aerosol has both adverse impacts on the Earth's climate and on human health. Monitoring soot sources, transport pathways and sinks on global scale requires satellite-borne remote sensing techniques. A detailed understanding of the soot particle's optical properties is important to improve the interpretation of remote sensing data as well as the use of lidar remote sensing data in chemical transport modelling. The calculations of the optical properties were carried out using the discrete dipole approximation (DDA). Aim of this thesis is to identify key morphological features, which affect the depolarisation ratio. As soot particles age in the atmosphere, condensation of other compounds from the gas phase onto the particles results in soot aggregates coated by liquid-phase material. Initially, the soot particles are coated by a thin film (i.e., the coating follows the shape of the aggregate). As more liquid phase material is added, the coating becomes increasingly spherical. It is found that this transition from film coating to radial growth of spherical shells is an important process affecting the linear depolarisation ratio. If this transition occurs first at relatively high amounts of coating, then the depolarisation ratio tends to be high. Conversely, if the coating becomes already spherical at low amounts of coating material, then the depolarisation ratio of the coated soot particles is much lower. The linear depolarisation ratio of thickly coated aggregates was found to be sensitive to changes in the complex refractive index of the coating material, which represents changes in the chemical composition. These differences in the optical properties, even after averaging over a particle size distribution, are large enough to clearly distinguish the coating materials.
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| 7. |
- Andersson, Emma, 1987
(författare)
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On the use of aerosol optical properties and satellite-retrieved trace gases in regional air quality modelling
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The effects of anthropogenic pollutants in the lower atmosphere (troposphere) have been shown, historically and scientifically, to have a deleterious effect on both plant and animal life. Pollutants such as near-surface ozone and soot (black carbon) are two notorious examples with clear anthropogenic sources. Consequently, laws controlling emissions and future projections of air quality, especially around large cities worldwide, are critical to the vibrancy and sustainability of life.The main purpose of this thesis is to summarise two studies involving the regional chemical transport model MATCH and its simulations of carbon monoxide (CO), ozone (O3) and aerosols. The core question addressed in the first study is how the long-range transport representation of carbon monoxide and ozone can be evaluated. Simulating the long-range transport in regional models is important to get a well represented vertical distribution of the long lived (relative to modelling time and domain) species, a category to which the aforementioned gases belong. A methodology for evaluating lateral boundary conditions (LBCs) is tested for LBCs derived from the global EMEP MSC-W model. First, the method directly evaluates the LBCs at the model boundaries with satellite retrievals from MOPITT (CO) and OMI (O3). The second part of the method uses the new LBC to drive MATCH model. MATCH is compared to satellite-retrieved data from the AIRS sensor and ground based observations from the GAW-network. The method of evaluating LBCs highlights the need for better evaluation techniques, rather than only using a direct evaluation at the lateral boundaries. The use of combined ground-based and satellite measurements, especially close to the model boundaries, needs to be further exploited.A similar study, where aerosols concentrations fields are confronted with satellite retrievals, instead of trace gases, cannot be made without the use of an aerosol optics model. An optics model simulates the corresponding radiometric properties as retrieved from satellite measurements. Therefore, the focus of the second study involves the simulation of aerosol optical properties using a new, stand-alone, aerosol optics model. Simulated aerosol fields are provided by using MATCH. The new optics model simulates radiometric properties from particles whose morphology is based on more realistic assumptions. Special consideration is given to fractal aggregates of externally mixed soot and inhomogeneous internally mixed soot. The latter part mixes solid soot with liquid hydrophilic substances, where parts of the soot particle are confined as a solid spherical core and parts of the soot is mixed into a liquid phase coating of for example sulphate or nitrate. The simulations of the new optics model resulted in large impacts on the radiometric properties, comparable to the effects of using aerosol dynamics. This is an important finding since most climate and remote sensing applications, today, use rather coarse and simple models to retrieve aerosol optical properties.
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| 8. |
- Bennet, Patrik, 1995
(författare)
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Characteristics of Vertical Forest Backscatter Profiles Measured using Radar Tomography
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Forests are an integral part of the Earth’s climate system, having shaped the conditions of life throughout millions of years, but the coverage and conditions of forests are under rapid change. To monitor this change and predict the impact on climate, yearly global mapping of forest Above-Ground Biomass (AGB) is needed. Synthetic Aperture Radar (SAR) is a well-suited technique for this purpose, able to sense through dense canopies with high spatial resolution. Tomographic SAR (TomoSAR) resolves the vertical distribution of backscatter by constructing 3D tomograms of forested areas. Here, results obtained in the TomoSense project are presented, where sensitivity of P- and L-band TomoSAR to AGB of a temperate forest is analysed. For the first time, differences between the AGB dependence of spruce and beech vertical backscatter profiles are identified. Moreover, unique observations of ground slope influence on TomoSAR AGB retrieval are presented. The effect for P-band is significant, but not for L-band. For spruce, AGB was estimated for P-/L-band (ground slopes below 10◦), with R2 = 0.86/0.75 and RMSE = 15.6/12.5%. Without separating forest types, R2 = 0.77/0.54 and RMSE = 11.4/12.0%. Finally, unique L-band radar tomography observations using the BorealScat tower-based radar are presented. Time series of the vertical backscatter profile of a boreal forest during the summer of 2018 is analysed. Weekly diurnal cycles up to 1.3 dB are observed, showing clear differences depending on both canopy height layer and polarization. These differences are new results which are likely related to tree transpiration phenomena but needs further study.
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| 9. |
- Ekelund, Robin Nils, 1989
(författare)
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A Database of Microwave Single Scattering Properties of Ice Hydrometeors
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Microwave remote sensing by satellites is important for global observations of ice hydrometeors. Interpretation of the measurements requires sufficiently accurate knowledge of hydrometeors’ interaction with photons, i.e. article scattering and absorption. This presents a challenge for several reasons. Liquid hydrometeors can typically be modelled by spheroids, while the shapes of ice hydrometeors are known to be significantly more complex and variable. Also, the shapes can from a remote sensing perspective generally not be known exactly, as they vary from case to case. Finally, calculating the light scattering properties is challenging and computationally costly. This thesis presents work related to recent efforts in improving the representation of light scattering by ice hydrometeors. A new single scattering database is presented, which includes 34 frequencies in between 1 and 874 GHz, and supports both passive and active microwave applications. A total of 34 different particle models were included, ranging from pristine crystals to aggregates. Complete random orientation is assumed throughout, slightly limiting its usefulness with respect to polarimetric measurements. Most aggregates were generated through simulation of aggregation, by letting particles collide randomly. The database can be considered the most extensive of this type to date, and future versions are intended to include oriented and melting particles. The general intention is to aid existing and future satellite retrievals, and satellite data assimilation into weather prediction models, all requiring accurate modelling of measured radiances. Special attention has been given to the upcoming Ice Cloud Imager (ICI), part of Europe’s next generation of weather satellites. Using the aggregation simulation tools developed for the database, a more dedicated case study was performed, which looked at the impact of different aggregate shape parameters on the resulting scattering properties. Both the amount and aspect ratio of the aggregate constituent crystals was found to have a high impact on both extinction (183, 325 and 664 GHz) and back-scattering (13, 36 and 94 GHz). Effective density and aerodynamic area had a high impact as well. Calculated radar triple frequency signatures were seen to clearly depend on the particle shape, consistent with previous studies. Overall, the results indicate that the particle shape should be considered in both passive and active applications above 13 GHz, and future database development will consider this. A potential application is also retrieval of ice particle shape through remote sensing.
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| 10. |
- Li, Anqi, 1990
(författare)
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New perspectives in mesospheric wave dynamics and oxygen photochemistry
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The mesosphere is the region of the atmosphere between 50km to 100km, where both dynamical and photochemical aspects play important roles for the thermal balance. This thesis focuses on the following three areas for mesospheric studies: wave dynamics, oxygen photochemistry and retrieval using the optimal estimation method. Atmospheric gravity waves are internal disturbances in the medium that propagate horizontally and vertically. Based on linear wave theory, this thesis attempts to enhance our understanding of the relationships between the wave characteristics, the mean flow and the sources. We try to emphasise the frequency change due to the Doppler effect in several reference frames. This thesis proposes a consistent framework for deriving those wave parameters that cannot be obtained from a single type of instrument due to their particular observational geometry. Finally, a plausible interpretation of a readily available ground-based lidar observation is given as an example. Oxygen photochemistry is another important aspect in this thesis. The underlying chemical reactions are affected by disturbances in the local temperature and density, which in turn changes the distribution of the excited oxygen species. In this work, a photochemical model has been implemented, which describes most of the important processes such as O3 photolysis that are related to the production and loss of O(1D), O2(b1Σg+ ) and O2(a1∆g). The observation of airglow emissions provides an opportunity to explore the chemical composition and wave dynamics in the upper mesosphere. The Odin satellite has been routinely measuring O2(a1∆g) airglow emissions since 2001. In this thesis, data collected by OSIRIS are explored. Inversions are carried out in order to retrieve the volume emission rate of O2(a1∆g) as well as the mesospheric ozone density. The resulting ozone profiles are shown to be consistent with other independent ozone datasets collected by instruments aboard the same spacecraft as well as ACE-FTS and MIPAS, despite intrinsically different measurement principles. The overall good agreement between them illustrates the good performance of the retrieval technique. Furthermore, these investigations serve well as a preparatory activity for the upcoming satellite mission MATS, set for launch later this year.
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