Browsing by master's degree program "Master's Programme in Atmospheric Sciences"

The parameterization of deep convection is simulated poorly over the Central and East Pacific. This could lead to issues in predicting the annual total precipitation in the tropics, such as the existence of a double intertropical convergence zone over the equatorial Pacific. Resolving tropical deep convection instead of parameterization leads to the presence of more linear systems. Observations over Atlantic indicate that shear-perpendicular lines (squall lines) propagate faster than shear- parallel lines, mainly due to their connection with the low-level vertical wind shear (VWS). The study examines the different movement speeds of mesoscale convective systems (MCSs) over the East- and West Pacific to determine whether this could explain the reason why climate models have problems with predicting deep convection. A higher proportion of fast moving MCSs (squall lines) could contribute to the prediction problems in the tropics. The MCS motion is determined by the sum of the mean wind and propagation speed. In squall lines, the MCS motion is mainly influenced by the propagation, which is associated with the low-level VWS. Therefore, the effect of the low-level VWS on the fast- and slow moving MCSs is also investigated. The Global High-Resolution Mesoscale Convective System Database is used, which provides infor- mation about the time, location and movement of MCSs. Additionally, ERA5 wind data is used to obtain the mean wind and VWS. Two specific areas over the northern equatorial Pacific are chosen to compare the different types of MCSs. These areas are over the East Pacific (120°W - 140°W and 5°N - 12°N) and West Pacific (140°E - 160°E and 0°N - 7°N). The movement speed is used to categorize the MCSs into three groups: slow moving MCSs (< 3 m/s), moderate moving MCSs (3 m s−1 − 7 m s−1) and fast moving MCSs (> 7 m/s). The study reveals that the share of the fast moving MCSs is 9.8% over the East Pacific and 13.8% over the West Pacific. This is only a 4 percent point difference between the two areas. Therefore, it is not shown that the fast moving MCSs contribute to the existing issues that models have in predicting the annual total precipitation over the East Pacific. Moreover, approximately 85-90% are categorized as slow- to moderate moving MCSs. Hence, the influence of fast moving MCSs is relatively small when compared to the other types. A difference is seen in the mean wind and VWS over the East Pacific, but do not explain the MCS motion vector. Therefore, the difference between fast- and slow moving MCSs cannot be explained by only the monthly averaged mean wind and low-level VWS over the East Pacific. Over the West Pacific, the mean wind direction and VWS are about the same in direction and speed. Therefore, the difference between fast- and slow moving MCSs is not explained by the low-level VWS over the West Pacific.

The purpose of this master's thesis is 1) to determine the meteorological structure and evolution of extra-tropical cyclones that develop in an idealized aqua-planet simulation, which utilizes a newly developed way to represent the large-scale initial state of the atmosphere; and 2) to examine the Lorenz energy cycle of this simulation. The simulation is run with the OpenIFS numerical weather model for 15 days, and the Lorenz energy cycle terms are computed for each time step of the simulation over a volume of the atmosphere, which extends from the 1000-hPa pressure surface to the 200-hPa pressure surface between 30 N and 75 N. The intensity of cyclones is evaluated with the TRACK program. In general, the simulation produces text-book type realistic cyclones whose structures resemble those of the Norwegian cyclone model. Cyclones which form upstream of the firstly developing original cyclone are smaller and less intense than the ones which form downstream of it. The most intense cyclone is the first cyclone which develops downstream of the original cyclone. None of the cyclones undergo their full life-cycle and therefore the simulation could be run for more than 15 days in the future. Over the course of the simulation, the available potential energy of the zonal mean flow decreases due to conversion to eddy available potential energy by a northward meridional heat flux. Vertical heat flux acts to inhibit this energy conversion, but its effect is minor. Eddy available potential energy is converted to eddy kinetic energy by the rising of warm air and the sinking of cold air within the developing cyclones and anticyclones. Eddy kinetic energy is converted to the kinetic energy of the zonal mean flow by momentum fluxes of the zonal wind component. The kinetic energy of the zonal mean flow is converted back to the available potential energy of the mean flow by the mean meridional overturning. In addition, friction dissipates the kinetic energy of the eddies and the zonal mean flow. Overall, the results of this thesis show that Lorenz energy cycle terms can be computed for an idealized aqua-planet simulation and that the computed terms can be used as diagnostics for evaluating the development of extra-tropical cyclones.

The Differential Mobility Particle Sizer (DMPS) is a widely used instrument in the size distribution measurements of sub-micron aerosol particles. The particles are size classified based on their electrical equivalent diameter by a Differential Mobility Analyser (DMA) in the DMPS. Only charged particles can be measured with a DMPS. An aerosol charger is hence required since most ambient particles are neutral. An estimation of the size dependent charge distribution of the aerosol particles is required to reach a representative size distribution of the whole aerosol population from the raw measurement data. The charged fractions of the population are conventionally calculated by applying bipolar charging (i.e., an ion atmosphere including ions of both polarities charges aerosols by diffusion and ion attachment onto the particles) theories. Fixed charger ion properties have been used to derive approximation to these theories. The charger ion properties in ambient “real-world” measurements are, however, not fixed but proven to vary substantially. These variations may lead to significant differences when comparing to the approximations. A new method for aerosol charging that reduces the uncertainties that originate from unknown properties of the charger ions was tested in this thesis. The method consists of the introduction of a known trace compound (hereafter called doping) into the aerosol sample flow upstream of a bipolar charger. The effect of doping on charger ion mobility distribution was successfully tested in laboratory experiments. A possible enhancement of charging efficiencies of nanoparticles was studied. The charger ion doping was also tested in atmospheric measurements, where it showed an effect on charger ion properties.

The mean temperature of Earth has been rising due to human-influenced climate change. Climate change has been mostly caused by the rise of greenhouse gases from anthropogenic sources. After carbon dioxide (CO2), the second most important anthropogenic greenhouse gas to climate change is methane (CH4). Approximately half of the methane emissions come from natural sources, including wetlands. The northern high latitude wetlands store large amounts of carbon in permafrost, and the thawing of permafrost could release more methane into the atmosphere. However, there is still much uncertainty related to the methane emissions from the northern high latitude wetlands. The emissions on these wetlands have an annual cycle related to the freezing and thawing of the soil with the highest emissions during summer and the lowest during winter. Climate change can affect the duration and timing of the freezing and thawing periods leaving the winter period shorter. In this thesis, the melting season for the northern high latitude wetlands was defined for four regions: non-permafrost, sporadic, discontinuous and continuous permafrost as well as two smaller regions: Hudson Bay lowlands and Western Siberian lowlands for the years 2011-2020. The melting period was defined with a new method of using the SMOS F/T soil thawing data, which has not been done before this study. The data includes daily information on the freezing state of the soil in the northern latitudes. The melting period methane emissions were defined from the inversion model Carbon Tracker Europe -CH4. The relationship between the emissions, melting period length and mean temperature was studied. Emissions during the spring melting season were detected in all the permafrost regions defined in this study. The fluxes grew stronger as spring progressed and the soil and snow melted. The melting period methane emissions were relatively small compared to the annual emissions (a few per cent of the annual budget). However, the emissions were a little larger than autumn emissions. To understand the melting season emissions better, different drivers in addition to air temperature, like the melting of the permafrost, should be studied in relation to the CH4 emissions.

Clouds and aerosols are among the key components of Earth's energy budget, and a major source of uncertainty in climate models, affecting the predictability of the future climate. This thesis focuses on the microphysical processes governing cirrus clouds, wispy clouds composed of ice crystals. Understanding these processes is crucial due to the extensive global coverage of cirrus clouds and their potential warming effect on the atmosphere. The study investigates ice nucleation, the process by which ice crystals form in the atmosphere. Ice nucleation occurs via two main pathways: homogeneous freezing and heterogeneous nucleation. Homogeneous freezing is a process where droplets spontaneously freeze without an aid of an ice nucleating particle (INP). It occurs in highly supersaturated conditions and at cold temperatures below -38°C. Heterogeneous nucleation occurs when INPs act as surfaces to trigger freezing at temperatures below 0°C. The study is conducted using UCLALES-SALSA Large Eddy Simulation (LES) model, which offers high spatial and temporal resolution for atmospheric simulation. The aim is to investigate ice nucleation with five well-established parameterizations. Simulations produced with these parameterizations are compared with cirrus cloud properties measured during the MACPEX campaign. Among heterogeneous nucleation mechanisms, deposition ice nucleation is considered as a primary contributor to the formation of cirrus clouds in the upper troposphere and used as a mechanism to generate ice in the model study. Heterogeneous nucleation requires the presence of INPs which are assumed to be mineral dust is used as it known to dominate ice nucleation. Results show good agreement between modeled and measured data for ice concentration (Ni) and ice water content (IWC). The comparison between parameterizations revealed a relatively similar performance, with variations in Ni and IWC falling within the same order of magnitude. However, conclusive determination of the best-performing parameterization within the temperature and humidity ranges of the study was challenging. The study sheds light on the fundamental difficulties when using parameterizations with ice nucleation processes in cirrus clouds without accurate initial conditions and knowledge about the history of ice nucleation of the measured cirrus clouds. Also, the importance of proper validation of each parameterization by using different scenarios was emphasized.

Earth’s energy budget describes the balance between the net incoming and outgoing energy fluxes, and the energy balance approach can be used to better understand the basic physical mechanisms of climate change. Anthropogenic changes in the atmospheric composition, such as increases in greenhouse gases, drive changes in climate system which in turn can cause rising of the global temperatures. Various feedbacks, associated with increase in atmospheric water vapor content, changes in clouds and reduced snow/ice cover, affect the pattern of surface warming by altering the fluxes of energy. By studying the energy balance at the top of the atmosphere and at the surface, we gain useful information about the climate system’s response to changes in the atmospheric composition. In this thesis, data for 23 climate models in the fifth phase of the Coupled Model Intercomparison Project (CMIP5) was used. The present-day distributions and future projections of the simulated changes (under RCP8.5 emission scenario, Representative Concentration Pathway) for 14 radiative and non-radiative energy budget components, along with the changes in surface temperature and cloud cover were studied, with baseline period of 1981-2010 and a comparison scenario period of 2071-2100. The geographical distributions of the multimodel mean changes and their global averages were analysed. Additionally, the intermodel consistency of the simulated changes was studied with the intermodel standard deviations and the ratio of multimodel mean change to the intermodel standard deviation. Furthermore, the intermodel correlation between the change in surface temperature and each energy budget variable was discussed. A general finding was that the multimodel mean surface temperature increases everywhere, more over land than oceans, and that the warming is amplified over the northern polar regions. The changes were largest for the thermal radiation fluxes, and the dominating contribution to the surface warming was concluded to be the change in clear- sky atmospheric re-radiation component. However, increase in absorbed shortwave radiation, presumably due to reduced ice/snow cover and increase in atmospheric water vapor content, was also found to be substantial, and there was a strong negative correlation between the clear-sky downward shortwave radiation flux and the change in temperature over the low-to-mid latitudes. The comparison of contribution of the changes in longwave and shortwave fluxes to global warming in the near-future and long-term climate model projections could be an interesting subject for future studies. Additionally, the changes in the surface energy fluxes were found to modify the pattern of surface warming.

The effects of atmospheric aerosol particles on Earth’s radiative balance are mainly cooling, which is mostly due their indirect effects with clouds. In the Arctic more than half of the cloud condensation nuclei (CCN) production is originated from secondary aerosols, and to further the understanding of Arctic climate and its changes due to the global warming, it is necessary to better understand the secondary aerosol processes there. Highly oxygenated organic molecules (HOM) are known to be important for the formation and especially for the growth of newly formed secondary aerosols to climate relevant sizes. Because of the low volatilities of HOM, they can condense onto the smallest particles, which is crucial for the growth of the new particles. Volatile organic compounds (VOC), especially monoterpenes, are known to be sources of HOM in boreal forest, but in the Arctic where the vegetation is scarce the sources of HOM have not yet been identified. The processes related to secondary aerosol formation in the Arctic are still not fully understood. Especially the observations of HOM and their sources are lacking. Recent studies in Ny-Ålesund, Svalbard showed that multiple aerosol precursors are found to be present in the Arctic atmosphere, as well as contributing to the early stages of the formation of secondary particles. However, more molecular scale observations of aerosol precursors are still needed to form a full picture of the Arctic climate processes. In this thesis, the different aerosol precursors and their contributions to the new particle formations in high Arctic location Ny-Ålesund, Svalbard were analysed. Chemical compositions of HOM were identified for the first time from Arctic atmosphere, and their contributions to new particle formation in high Arctic location were investigated. Because of the high concentrations of HOM during the observed NPF events, it can be suggested that they were contributing to the nucleation of aerosol particles. Particle growth rate calculation shows that the HOM present in the study site were responsible for up to 50% of the total growth of the newly formed particles. VOC flux measurements done in same location were also analysed, and Arctic tundra in Svalbard was found out to be a source of at least four different VOC. Furthermore, the identified HOM were linked to the VOC flux measurements, suggesting a possible link between Arctic VOC and HOM.

After 2013, the environmental protection department in China has significantly reduced on-road emission through the upgrade of emission standards, the improvement of fuel quality and economic tools. However, the specific effect of the control policies on emission and air quality is still difficult to quantify. This is mainly due to the data shortage on vehicle emission factors and vehicle activities. In this research, we developed the 2008-2018 on-road emissions inventory based on Emission Inventory Preparation Guide (GEI) and existing vehicle activity database. Our estimates suggest that CO and PM2.5 showed a relatively significant decrease, by 66.2% and 58.8%. However, the trend of NOx (5.8%) and NMVOC (-4.8%) was relatively stable. The Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD) and Sichuan Basin (SCB) regions all showed a uniform trend especially in NOx. For Beijing-Tianjin-Hebei, the significant decline in NOx might be caused by earlier implementations in emission standard and fuel quality. In addition to this, we designed additional evaporation emission scenarios to verify the application of GEI in quantify emission impact on secondary pollutant (PM2.5 and O3). The results indicate that evaporation emission contributed to Maximum Daily Average 8-hour (MDA8) O3 concentration by about 3.5%, for Beijing, Shanghai and Nanjing. This value can reach up to 5.9%, 5.3% and 7.3%, but the impact on PM2.5is extremely limited. Our results indicate the feasibility of GEI in improving and lowering the technical barrier of on-road emission inventory establishment at the same time and its further application in quantifying on-road emission contribution to air quality. Besides that, it shows a strong potential in on-road policy environmental assessment and short-term air quality assessment.

The aim of this work is to develop and optimise an atmospheric inverse modelling system to estimate local methane (CH4) emissions in peatlands. Peatlands are a major source of CH4 regionally in boreal areas and they have significance on a global scale as a soil carbon storage. Data assimilation in the inverse modelling system is based on an ensemble Kalman filter (EnKF) which is widely used in global and regional atmospheric inverse models. The EnKF in this study is an implementation of the EnKF used in the global atmospheric inversion model CarbonTracker Europe-CH4 (CTE-CH4) applied to local setting in the peatland. Consistency of the methodology with regional and global models means that it is possible to expand the system in scale. Siikaneva fen in Southern Finland is used as a testbed for the optimisation of the system. Prior natural CH4 fluxes in Siikaneva are acquired from the HelsinkI Model of MEthane buiLd-up and emIssion for peatland (HIMMELI) which simulates exchange of gases in peatlands. In addition to the peatland fluxes, anthropogenic fluxes at the site are estimated as well in the inversion. For the assimilation of atmospheric CH4 concentration observations, the CH4 fluxes are transformed into atmospheric concentration with a simple one-dimensional box model. The optimisation of the system was done by changing parameters in the model which affect the data assimilation. In model optimisation tests it was discovered that the performance of the modelling system is unstable. There was large variability in the produced estimates between consecutive model runs. Model evaluation statistics did not indicate improvement of the estimates after the inversion. No exact reason for the unstability was able to be determined. Posterior estimates of CH4 fluxes for years 2012–2015 did not differ much from prior estimates and they had large uncertainty. However, evaluation against flux measurements showed reasonable agreement and posterior concentration estimates were within the uncertainty range of the observed concentration.

Ozone, an important and ubiquitous trace gas, protects lives from harm of solar ultraviolet (UV) radiation in the stratosphere but behaves as a toxic compound in the troposphere to living organisms. Also, tropospheric ozone is a vital oxidant or source of daytime oxidant (i.e., OH radical) for e.g., different volatile organic compounds (VOCs). Affecting global radiation balance directly or indirectly by acting as cloud condensation nuclei and having negative impact to human health, aerosols are widely studied for over a century. Highly oxygenated organic molecules (HOM) were proved to be a large source of secondary organic aerosol (SOA) and their oxidation formation pathways from VOCs can also trigger the production of ozone once involving NOx (=NO+NO2) and UV light. The highly nonlinear relationship among ozone, NOx, and VOCs (O3-NOx-VOC sensitivity or O3 formation sensitivity) has been researched since last century. The complex system was recently reflected during COVID-19 lockdowns: reduction of NOx increased the ozone production. This is because the system was in VOC-limited regime, where reducing VOCs is the most efficient way to reduce O3. However, the determination of O3 formation regimes (either VOC-limited or NOx-limited) is challenging in different environmental conditions. The intrinsic connection between HOM and O3 formation provide a new insight: the proportions of VOCs and NOx not only affect the O3 formation regimes but also impact the distribution of HOM species. Therefore, in this study, we try to unveil the indicating role of HOM species on the O3 formation sensitivity by chamber experimental works with a nitrate chemical ionization mass spectrometer (CI-APi-TOF) and gas monitors. Injected NOx and VOCs step by step, the experiments were designed to make the atmosphere-mimicking system change between those two regimes. The ratio between HOM-dimers and HOM organic nitrate monomers was selected as the indicator for O3 formation sensitivity due to their closely connected chemical reactions, involving peroxy radicals. Furthermore, a simple box model was developed for simulating chamber results and obtaining O3 isopleths to visually show the O3 formation regimes. Through experimental and model results, it can be inferred that ratios below 0.2 consistently correspond to the VOC-limited regime, whereas ratios above 0.5 consistently correspond to the NOx-limited regime. This study demonstrates that the ratio based on HOM species could additionally indicate the O3 formation sensitivity of ambient air when we use CI-APi-TOF to investigate the chemical compounds and aerosol formation, helping to elaborate the O3 pollution in the real troposphere.