Bio
I am a Doctoral Researcher at the Institute for Atmospheric and Earth System Research (INAR) at the University of Helsinki, Finland, with interests in micro to mesoscale meteorology, computational fluid dynamics and data science. I have a MSc in meteorology from the University of Helsinki, granted in 2020.
I study micro to mesoscale dynamics, processes and interactions within atmospheric boundary layers in coastal cities. In my research, I combine both modelling (large-eddy simulations) and observational (lidar and in-situ measurements) methods at turbulence-resolving temporal and spatial scales.
I work in the urban meteorology research group of INAR in close collaboration with researchers from other INAR research groups and the Finnish Meteorological Institute.
Curriculum Vitae (not yet available)
Email: sasu.karttunen@helsinki.fi
Tel: +358 45 1329800
Address:
Institute for Atmospheric and Earth System Research (INAR) / Physics
Faculty of Science
University of Helsinki, Finland
P.O. Box 68, FI-00014 University of Helsinki
Room: Exactum D118
Teaching
(2023-24) ATM309: Analysis of atmosphere-surface interactions and feedbacks, Autumn, 5 cr (assistant teacher)
(2020-23) FYS2033: Introduction to Atmospheric Flow Dynamics, Autumn, 10 cr (assistant teacher)
Education
2020 Master of Science (meteorology)
Faculty of Science, University of Helsinki
Specialization in dynamic meteorology and remote sensing.
Master’s thesis: The effect of street vegetation configuration on the pedestrian-level aerosol mass concentrations in a wide street canyon (link)
2019 Bachelor of Science (meteorology)
Faculty of Science, University of Helsinki
Work experience
2020– Doctoral researcher
Urban meteorology research group
Institute for Atmospheric and Earth System Research (INAR) / Physics
University of Helsinki
2017–2020 Research assistant
Urban meteorology research group
Institute for Atmospheric and Earth System Research (INAR) / Physics
University of Helsinki
Peer-reviewed first-author articles
Sasu Karttunen, Ewan O'Connor, Olli Peltola and Leena Järvi
Atmos. Meas. Tech., 15, 2417–2432, 2022
Abstract (click to expand): The structure of the urban boundary layer, and particularly the surface layer, displays significant complexity, which can be exacerbated by coastal effects for cities located in such regions. Resolving the complexity of the coastal urban boundary layer remains an important question for many applications such as air quality and numerical weather prediction. One of the most promising new techniques for measuring the structure of the surface layer is fibre-optic distributed temperature sensing (DTS), which has the potential to provide new significant insights for boundary layer meteorology by making it possible to study thermal turbulence with high spatial and temporal resolution. We present 14 weeks of profile measurements with a DTS system at an urban site in Helsinki, Finland, during the winter and spring of 2020. We assess the benefits and drawbacks of using DTS measurements to supplement sonic anemometry for longer measurement periods in varying meteorological conditions, including those found difficult for the DTS method in prior studies. Furthermore, we demonstrate the capabilities of the DTS system using two case scenarios: a study of the erosion of a near-ground cold layer during the passage of a warm front, and a comparison of the near-ground thermal structure with and without the presence of a sea-breeze cell during springtime convective boundary layer development. This study demonstrates the utility of DTS measurements in revealing the internal surface layer structure, beyond the predictions of traditional surface layer theories. This knowledge is important for improving surface layer theories and parametrisations, including those used in numerical weather prediction. The study also highlights the drawbacks of DTS measurements, caused by low signal-to-noise ratios in near-neutral atmospheric conditions, especially when such a system would be used to supplement turbulence measurements over longer periods. Overall, this study presents important considerations for planning new studies or ongoing measurements utilising this exciting and relatively new instrumentation.
Full-text PDF
Sasu Karttunen, Mona Kurppa, Mikko Auvinen, Antti Hellsten and Leena Järvi
Atmos. Env., 6, 100073, 2020
Abstract (click to expand): Street vegetation has been found to have both positive and negative impacts on pedestrian-level air quality, but the net effect has remained unclear. In this study, the effect of street trees on aerosol mass (PM10 and PM2.5) and number in a boulevard-type street canyon with high traffic volumes in Helsinki is examined using the large-eddy simulation model PALM. Including a detailed aerosol module and a canopy module to comprise permeable trees, PALM allows to examine the effect of street trees in depth. The main aim is to understand the relative importance of dry deposition and the aerodynamic impact of street trees on the different aerosol measures at pedestrian-level and to find a suitable street-tree layout that would minimise the pedestrian-level aerosol particle concentrations over the boulevard pavements. The layout scenarios were decided together with urban planners who needed science-based knowledge to support the building of new neighbourhoods with boulevard-type streets in Helsinki. Two wind conditions with wind being parallel and perpendicular to the boulevard under neutral atmospheric stratification are examined. Adding street trees to the boulevard increases aerosol particle concentrations on the pavements up to 123%, 72% and 53% for PM10, PM2.5 and total number, respectively. This shows decreased ventilation to be more important for local aerosol particle concentrations than dry deposition on vegetation. This particularly for PM10 and PM2.5 whereas for aerosol number, dominated by small particles, the importance of dry deposition increases. Therefore the studied aerosol measure is important when the effect of vegetation on pedestrian-level air quality is quantified. Crown volume fraction in the street space is one of the main determining factors for elevated mass concentrations on the pavements. The lowest pedestrian-level mass concentrations are seen with three rows of trees of variable height, whereas the lowest number concentrations with four rows of uniform trees. The tree-height variation allows stronger vertical turbulent transport with parallel wind and largest volumetric flow rates with perpendicular wind. Introducing low (height < 1 m) hedges under trees between the traffic lanes and pavements is found to be a less effective mitigation method for particle mass than introducing tree-height variability, and for particle number less effective than maximising the tree volume in the street canyon. The results show how street trees in a boulevard-type street canyon lead to decreased pedestrian-level air quality with the effect being particularly strong for larger aerosol particles. However, with careful planning of the street vegetation, significant reductions in pedestrian-level aerosol particle concentrations can be obtained.
Full-text PDF
Other peer-reviewed articles
Leena Järvi, Mona Kurppa, Heino Kuuluvainen, Topi Rönkkö, Sasu Karttunen, Anna Balling, Hilkka Timonen, Jarkko V. Niemi and Liisa Pirjola
Sci. Tot. Env., 856 (1), 158974, 2022
Abstract (click to expand): Urban air pollutant concentrations are highly variable both in space and time. In order to understand these variabilities high-resolution measurements of air pollutants are needed. Here we present results of a mobile laboratory and a drone measurements made within a street-canyon network in Helsinki, Finland, in summer and winter 2017. The mobile laboratory measured the total number concentration (N) and lung-deposited surface area (LDSA) of aerosol particles, and the concentrations of black carbon, nitric oxide (NOx) and ozone (O3). The drone measured the vertical profile of LDSA. The main aims were to examine the spatial variability of air pollutants in a wide street canyon and its immediate surroundings, and find the controlling environmental variables for the observed variability's. The highest concentrations with the most temporal variability were measured at the main street canyon when the mobile laboratory was moving with the traffic fleet for all air pollutants except O3. The street canyon concentration levels were more affected by traffic rates whereas on surrounding areas, meteorological conditions dominated. Both the mean flow and turbulence were important, the latter particularly for smaller aerosol particles through LDSA and N. The formation of concentration hotspots in the street network were mostly controlled by mechanical processes but in winter thermal processes became also important for aerosol particles. LDSA showed large variability in the profile shape, and surface and background concentrations. The expected exponential decay functions worked better in well-mixed conditions in summer compared to winter. We derived equation for the vertical decay which was mostly controlled by the air temperature. Mean wind dominated the profile shape over both thermal and mechanical turbulence. This study is among the first experimental studies to demonstrate the importance of high-resolution measurements in understanding urban pollutant variability in detail.
Full-text PDF
Mona Kurppa, Pontus Roldin, Jani Strömberg, Anna Balling, Sasu Karttunen, Heino Kuuluvainen, Jarkko V. Niemi, Liisa Pirjola, Topi Rönkkö, Hilkka Timonen, Antti Hellsten and Leena Järvi
Geosci. Model Dev., 13, 5663--5685, 2020
Abstract (click to expand): High-resolution modelling is needed to understand urban air quality and pollutant dispersion in detail. Recently, the PALM model system 6.0, which is based on large-eddy simulation (LES), was extended with the detailed Sectional Aerosol module for Large Scale Applications (SALSA) v2.0 to enable studying the complex interactions between the turbulent flow field and aerosol dynamic processes. This study represents an extensive evaluation of the modelling system against the horizontal and vertical distributions of aerosol particles measured using a mobile laboratory and a drone in an urban neighbourhood in Helsinki, Finland. Specific emphasis is on the model sensitivity of aerosol particle concentrations, size distributions and chemical compositions to boundary conditions of meteorological variables and aerosol background concentrations. The meteorological boundary conditions are taken from both a numerical weather prediction model and observations, which occasionally differ strongly. Yet, the model shows good agreement with measurements (fractional bias <0.67, normalised mean squared error <6, fraction of the data within a factor of 2 >0.3, normalised mean bias factor <0.25 and normalised mean absolute error factor <0.35) with respect to both horizontal and vertical distribution of aerosol particles, their size distribution and chemical composition. The horizontal distribution is most sensitive to the wind speed and atmospheric stratification, and vertical distribution to the wind direction. The aerosol number size distribution is mainly governed by the flow field along the main street with high traffic rates and in its surroundings by the background concentrations. The results emphasise the importance of correct meteorological and aerosol background boundary conditions, in addition to accurate emission estimates and detailed model physics, in quantitative high-resolution air pollution modelling and future urban LES studies.
Full-text PDF
Works in progress
Implementation and evaluation of a single-layer urban energy balance scheme for the PALM model system
Sasu Karttunen, Ewan O'Connor, Antti Hellsten and Leena Järvi
Resolving micro to mesoscale interactions between urban surface and a sea-breeze circulation using high resolution large-eddy simulations
Sasu Karttunen, Ewan O'Connor, Antti Hellsten, Carl Fortelius, Dmitri Moisseev and Leena Järvi
ORCiD