Turbulenttinen kuljetus ja ilmanlaatu Helsingin ilmassa

There is a growing need to understand the exchange processes of momentum, heat and mass between an urban surface and the atmosphere as they affect our quality of life. Understanding the source/sink strengths as well as the mixing mechanisms of air pollutants is particularly important due to their effects on human health and climate. This work aims to improve our understanding of these surface-atmosphere interactions based on the analysis of measurements carried out in Helsinki, Finland. The vertical exchange of momentum, heat, carbon dioxide (CO2) and aerosol particle number was measured with the eddy covariance technique at the urban measurement station SMEAR III, where the concentrations of ultrafine, accumulation mode and coarse particle numbers, nitrogen oxides (NOx), carbon monoxide (CO), ozone (O3) and sulphur dioxide (SO2) were also measured. These measurements were carried out over varying measurement periods between 2004 and 2008. In addition, black carbon mass concentration was measured at the Helsinki Metropolitan Area Council site during three campaigns in 1996-2005. Thus, the analyzed dataset covered far, the most comprehensive long-term measurements of turbulent fluxes reported in the literature from urban areas. Moreover, simultaneously measured urban air pollution concentrations and turbulent fluxes were examined for the first time. The complex measurement surrounding enabled us to study the effect of different urban covers on the exchange processes from a single point of measurement. The sensible and latent heat fluxes closely followed the intensity of solar radiation, and the sensible heat flux always exceeded the latent heat flux due to anthropogenic heat emissions and the conversion of solar radiation to direct heat in urban structures. This urban heat island effect was most evident during winter nights. The effect of land use cover was seen as increased sensible heat fluxes in more built-up areas than in areas with high vegetation cover. Both aerosol particle and CO2 exchanges were largely affected by road traffic, and the highest diurnal fluxes reached 109 m-2 s-1 and 20 µmol m-2 s-1, respectively, in the direction of the road. Local road traffic had the greatest effect on ultrafine particle concentrations, whereas meteorological variables were more important for accumulation mode and coarse particle concentrations. The measurement surroundings of the SMEAR III station served as a source for both particles and CO2, except in summer, when the vegetation uptake of CO2 exceeded the anthropogenic sources in the vegetation sector in daytime, and we observed a downward median flux of 8 µmol m-2 s-1. This work improved our understanding of the interactions between an urban surface and the atmosphere in a city located at high latitudes in a semi-continental climate. The results can be utilised in urban planning, as the fraction of vegetation cover and vehicular activity were found to be the major environmental drivers affecting most of the exchange processes. However, in order to understand these exchange and mixing processes on a city scale, more measurements above various urban surfaces accompanied by numerical modelling are required.Kaupungit muokkaavat voimakkaasti liikemäärän, lämmön ja eri aineiden massan vaihtoa ilmakehän ja maan pinnan välillä. Muokkautuneet vaihtoprosessit aiheutuvat ihmisten toiminnasta ja maan käytön muutoksista, ja näillä prosesseilla on useita vaikutuksia ihmisten hyvinvointiin. Erityisesti hiilidioksidin (CO2) ja ilmakehän pienhiukkasten vaikutukset ilmastoon, ja jälkimmäisen tapauksessa myös ihmisten terveyteen, ovat hyvin tiedossa ja näiden, kuten monien muidenkin ilman epäpuhtauksien, ihmisperäiset lähteet sijaitsevat kaupunkialueilla. Useimmat pienhiukkasten ja CO2:n lähteet ja nielut on tunnistettu, mutta näiden jakautumisessa ja voimakkuuksissa on vielä paljon epäselvyyttä. Epäselvää on myös paikallisen meteorologian, erityisesti turbulenttisen sekoittumisen, vaikutukset hiilidioksidin ja pienhiukkasten leviämiseen. Tämä työ pyrkii parantamaan ymmärrystämme näistä vaihto- ja sekoittumisprosesseista kaupunkialueilla Helsingissä suoritettujen pitkäaikaisten mittauksien avulla. Mittaukset kattavat liikemäärän, lämmön, CO2:n ja pienhiukkasten turbulenttisen vaihdon Helsingin Kumpulassa sijaitsevalla mittausasemalla SMEAR III. Lisäksi asemalla mitattiin erikokoisten pienhiukkasten pitoisuuksia sekä ilmanlaadun kannalta tärkeiden kaasujen, typen oksidien (NOx), hiilimonoksidin (CO), otsonin (O3) ja rikkidioksidin (SO2), pitoisuuksia. Mittaukset suoritettiin vuosina 2004 2008. Lisäksi pienhiukkasten mustan hiilen (noki) massapitoisuutta mitattiin kolmessa mittauskampanjassa vuosien 1996 2005 välillä Pääkaupunkiseudun yhteistyövaltuuskunnan (YTV) Vallilan mittausasemalla. Kaupunkialustan havaittiin olevan lämmön lähde erityisesti talvisin, jolloin rakennuksien lämmitykset lisäävät lämmön emissioita. Nämä lämmön emissiot vomistavat ilmakehän turbulenttista sekoittumista. Liikenteen vaikutus CO2:n ja pienhiukkasten päästöihin oli selvä, ja niiden käyttäytyminen seurasi tiiviisti Helsingissä mitattuja liikennemääriä. Mittausaseman ympäristö toimi molempien lähteenä, paitsi kesäpäiväisin, jolloin kasvillisuuden hiilinielu ylitti CO2:n ihmisperäiset päästöt. Liikenne vaikutti eniten pienimpien, ihmisille vaarallisimpien, hiukkasten lukumääräpitoisuuksiin, kun taas meteorologian merkitys korostui suurempien hiukkasten tapauksessa. Turbulenttinen sekoittuminen vaikutti kaikenkokoisiin hiukkasiin ja sen vaikutus riippui hiukkasten koosta. Pienimpien hiukkasten pitoisuudet alenivat niiden sekoittuessa suurempaan ilmatilavuuteen. Suurempien hiukkasten pitoisuudet taas kasvoivat niiden noustessa pinnoilta. Mustan hiilen pitoisuudet riippuivat myös voimakkaasti paikallisesta liikenteestä ja pitoisuuksien havaittiin pysyneen melko samalla tasolla tutkitun kymmenen vuoden aikana. Tämän tutkimuksen tuloksia voidaan hyödyntää kaupunkisuunnittelussa, sillä liikenteen ja kasvillisuuden määrän havaittiin vaikuttavan suurimpaan osaan vaihtoprosesseja. Tässä työssä esitetyt mittaukset kattoivat kuitenkin vain pienen osan Helsinkiä. Jotta voisimme saada kokonaiskuvan kaupungin alueella tapahtuvista vaihto- ja sekoittumisprosesseista, tarvitsemme lisää mittauksia useissa eri mittauspisteissä mallinnuksella täydennettynä

Assessing the effect of vegetation layout on aerosol particle concentrations within an urban street canyon using large-eddy simulation

Non peer reviewe

On the application of spectral corrections to particle flux measurements

To study the environmental fate of nanoparticles, reliable measurements of particle fluxes in the atmosphere are of importance. The eddy-covariance (EC) technique can be used to calculate surface fluxes. In this study, the EC technique has been applied to calculate particle fluxes in Helsinki (Finland) and Cabauw (the Netherlands). For reliable estimations of the surface fluxes, particle flux measurements need to be corrected for attenuation at the highest frequencies. This attenuation is caused by the relatively long response time of scalar sensors and measurement set-up. The attenuation can be estimated using a theoretical or an empirical approach. Horst [Horst, Boundary-Layer Meteorology, 1997, 82(2)] developed a simple formula to estimate the attenuation, based on the empirical approach. The empirical approach relies on the assumption of spectral similarity between two scalars. In this paper the effect of the spectral similarity assumption is investigated. It is shown that in order to apply the Horst formula reliably, a decent estimate of the sensor response time is required. Furthermore, it is shown that in order to apply the empirical method, a fast sensor response is required. It is concluded that theory does not predict the position of the peak in the cospectrum well, which means that for measurement set-ups not yet operational, this requirement is not easily evaluated. The assumption of spectral similarity seems reasonable for particle fluxes and heat fluxes, when compared to similarity between fluxes of other scalars. An altered assumption of spectral similarity has been applied, where similarity is assumed only at frequencies higher than the peak frequency in the cospectrum. This assumption leads to a better estimate for the attenuation, when applied to the Helsinki data. It does not lead to an improvement for the Cabauw data set, due to the large response time of the measurement set-up.Peer reviewe

Environmental and crown related factors affecting street tree transpiration in Helsinki, Finland

We investigated the drivers of street tree transpiration in boreal conditions, in order to better understand tree water use in the context of urban tree planning and stormwater management. Two streets built in Helsinki in 2002, hemiboreal zone that had been planted either with Tilia x vulgaris or Alnus glutinosa f. pyramidalis were used as the study sites. Tree water use was measured from sap flow over the 2008-2011 period by the heat dissipation method. Penman-Monteith based evapotranspiration models of increasing complexity were tested against the tree water use measurements to assess the role of environmental and tree related factors in tree transpiration. Alnus and Tilia respectively used 1.1 and 0.8 l of water per m(2) of leaf area per day under ample water conditions, but the annual variation was high. The Penman-Monteith evapotranspiration estimate and soil water status changes explained over 80 % of the variation in tree transpiration when the model was parameterized annually. The addition of tree crown surface area in the model improved its accuracy and diminished variation between years and sites. Using single parameterization over all four years instead of annually varying one did not produce reliable estimates of tree transpiration. Tree transpiration, scaled to different canopy cover percentages, implied that the columnar Alnus trees could transpire as much as all annual rainfall at or less than 50 % canopy cover.Peer reviewe

Quantifying the coastal urban surface layer structure using distributed temperature sensing in Helsinki, Finland

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.Peer reviewe

Numerical framework for the computation of urban flux footprints employing large-eddy simulation and Lagrangian stochastic modeling

Conventional footprint models cannot account for the heterogeneity of the urban landscape imposing a pronounced uncertainty on the spatial interpretation of eddycovariance (EC) flux measurements in urban studies. This work introduces a computational methodology that enables the generation of detailed footprints in arbitrarily complex urban flux measurements sites. The methodology is based on conducting high-resolution large-eddy simulation (LES) and Lagrangian stochastic (LS) particle analysis on a model that features a detailed topographic description of a real urban environment. The approach utilizes an arbitrarily sized target volume set around the sensor in the LES domain, to collect a dataset of LS particles which are seeded from the potential source area of the measurement and captured at the sensor site. The urban footprint is generated from this dataset through a piecewise postprocessing procedure, which divides the footprint evaluation into multiple independent processes that each yield an intermediate result. These results are ultimately selectively combined to produce the final footprint. The strategy reduces the computational cost of the LES-LS simulation and incorporates techniques to account for the complications that arise when the EC sensor is mounted on a building instead of a conventional flux tower. The presented computational framework also introduces a result assessment strategy which utilizes the obtained urban footprint together with a detailed land cover type dataset to estimate the potential error that may arise if analytically derived footprint models were employed instead. The methodology is demonstrated with a case study that concentrates on generating the footprint for a building-mounted EC measurement station in downtown Helsinki, Finland, under the neutrally stratified atmospheric boundary layer.Peer reviewe

Large-eddy simulation of the optimal street-tree layout for pedestrian-level aerosol particle concentrations – A case study from a city-boulevard

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.Peer reviewe

Study of Realistic Urban Boundary Layer Turbulence with High-Resolution Large-Eddy Simulation

This study examines the statistical predictability of local wind conditions in a real urban environment under realistic atmospheric boundary layer conditions by means of Large-Eddy Simulation (LES). The computational domain features a highly detailed description of a densely built coastal downtown area, which includes vegetation. A multi-scale nested LES modelling approach is utilized to achieve a setup where a fully developed boundary layer flow, which is also allowed to form and evolve very large-scale turbulent motions, becomes incident with the urban surface. Under these nonideal conditions, the local scale predictability and result sensitivity to central modelling choices are scrutinized via comparative techniques. Joint time–frequency analysis with wavelets is exploited to aid targeted filtering of the problematic large-scale motions, while concepts of information entropy and divergence are exploited to perform a deep probing comparison of local urban canopy turbulence signals. The study demonstrates the utility of wavelet analysis and information theory in urban turbulence research while emphasizing the importance of grid resolution when local scale predictability, particularly close to the pedestrian level, is sought. In densely built urban environments, the level of detail of vegetation drag modelling description is deemed most significant in the immediate vicinity of the trees

Simulating urban soil carbon decomposition using local weather input from a surface model

Non peer reviewe

Eddy covariance methodologies revisited for urban measurements

Non peer reviewe