Assessing pollutant ventilation in city planning alternatives using a large-eddy simulation

This thesis is a case study of the impact of urban planning on local air quality along a planned city boulevard in western Helsinki. The aim of this study is to analyse ventilation and dispersion of traffic-related air pollutants inside street canyons and courtyards in four alternative city block design versions. In particular, whether the format and variation of building height can improve air quality in future planned neighbourhoods and as such, help improve the decision-making process in city planning. The study employs a large-eddy simulation (LES) model PALM with embedded Lagrangian stochastic particle and canopy models to simulate transport of pollutants (air parcels) and the aerodynamic impact of street trees and a surrounding forest on pollutant transport. The embedded models are revised by the author to take into account the horizontal heterogeneity of the particle sources and plant canopy. Furthermore, three-dimensional two-way self-nesting is used for the first time in PALM in this study. High-resolution simulations are conducted over a real urban topography under two contrasting meteorological conditions with neutral and stable stratification and south-western and eastern wind direction, respectively. The comparison of the different boulevard-design versions is based on analysing the temporal mean particle concentrations, the turbulent vertical particle flux densities and the particle dilution rate. Differences in flux densities between the versions show a strong dependence on urban morphology whereas the advection-related dilution rate depends on the volume of unblocked streamwise street canyons. A suggestive ranking of the versions is performed based on the horizontal mean values of the analysis measures (separately for the boulevard, the other street canyons, the courtyards and the surroundings). Considering both meteorological conditions, the design version with variable building height and short canyons along the boulevard outperforms the other design versions based on the ranking. This is especially pronounced in stable conditions. Surprisingly, variability in building shape did not induce clear improvements in ventilation. This is the first high-resolution LES study conducted over a real urban topography applying sophisticated measures to assess pollutant dispersion and ventilation inside street canyons and courtyards.Tässä pro gradu -työssä tutkittiin kaupunkisuunnittelun vaikutusta paikalliseen ilmanlaatuun Helsingin yleiskaavassa 2050 nykyiselle Hämeenlinnanväylälle suunnitteilla olevalla kaupunkibulevardilla kaupungin länsiosassa. Tavoitteena oli verrata liikenteen päästöjen kulkeutumista ja tuulettumista pois katukuiluista ja sisäpihoilta käyttäen neljää erilaista kaupunkikorttelimalli, ja tutkia voiko rakentamisratkaisuilla parantaa ilmanlaatua. Tutkimus tehtiin käyttäen suurten pyörteiden simulaatiomallia (large-eddy simulation, LES) PALM. Lisäksi malliin on liitetty Lagrangelainen stokastinen hiukkasmalli, jolla simuloitiin hiukkasten (ilmapakettien) kulkeutumista ilmavirtauksen mukana, sekä kasvillisuusmalli, joka huomioi katupuiden ja läheisen metsän aerodynaamisen vaikutuksen ilmanvirtaukseen ja näin ollen myös hiukkasten kulkeutumiseen. Tutkimusta varten hiukkas- ja kasvillisuusmalleja kehitettiin ottamaan huomioon sekä hiukkaslähteiden että puiden sijainnin horisontaalisen vaihtelevuuden. Lisäksi mallisimulaatioissa hyödynnettiin ensimmäistä kertaa ominaisuutta, jossa ison laskenta-alueen sisällä on määritelty pienempi ja tiheähilaisempi laskenta-alue, mahdollistaen samanaikaisesti sekä suuren laskenta-alueen että riittävän laskentaerotuskyvyn. Mallisimulaatiot tehtiin todellisen kaupunkitopografian yllä käyttäen korkeaa laskentaerotuskykyä kahdessa eri säätilanteessa, neutraalisti ja stabiilisti kerrostuneelle rajakerrokselle lounais- ja itätuulella. Eri kortteliversioita arvioitiin tarkastelemalla keskimääräisiä hiukkaspitoisuuksia, turbulenttista pystysuuntaista hiukkasvuotieheyttä sekä hiukkaspitoisuuksien laimenemisnopeutta. Erot hiukkasvuotiheyden voimakkuudessa määräytyivät rakennusten muodon ja orientaation mukaan, kun taas laimenemisnopeus riippui virtauksen esteettömyydestä. Kortteliversiot luokiteltiin laskemalla analyysimuuttujien aluekeskiarvot erikseen bulevardille, muille katukuiluille, sisäpihoille ja ympäristölle, ja asettamalla kortteliversiot paremmuusjärjestykseen kaikkien muuttujien perusteella. Molemmat meteorologiset olosuhteet huomioiden, korttelimalli vaihtelevalla rakennuskorkeudella ja suurella määrällä risteyksiä bulevardia pitkin osoittautui luokittelun mukaan parhaaksi vaihtoehdoksi. Tämä havaittiin etenkin stabiilissa säätilanteessa, joka yleensä johtaa huonoon ilmanlaatuun Helsingissä. Yllättäen, rakennusmuodon vaihtelevuus ei osoittanut voimistavan tuulettumista. Tämä on ensimmäinen todellisen kaupunkitopografian yllä suoritettu korkean laskentaresoluution LES-tutkimus, jossa päästöjen kulkeutumista ja tuulettumista arvioidaan käyttäen useita suoria ja aiempaa monimutkaisempia mittareita. Tuloksia tullaan hyödyntämään kaupunkisuunnittelussa ja päätöksenteon tukena, kun tavoitteena on rakentaa mahdollisimman puhtaita elinympäristöjä

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

Non 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

Ventilation and Air Quality in City Blocks Using Large-Eddy Simulation—Urban Planning Perspective

Buildings and vegetation alter the wind and pollutant transport in urban environments. This comparative study investigates the role of orientation and shape of perimeter blocks on the dispersion and ventilation of traffic-related air pollutants, and the street-level concentrations along a planned city boulevard. A large-eddy simulation (LES) model PALM is employed over a highly detailed representation of the urban domain including street trees and forested areas. Air pollutants are represented by massless and passive particles (non-reactive gases), which are released with traffic-related emission rates. High-resolution simulations for four different city-block-structures are conducted over a 8.2 km domain under two contrasting inflow conditions with neutral and stable atmospheric stratification corresponding the general and wintry meteorological conditions. Variation in building height together with multiple cross streets along the boulevard improves ventilation, resulting in 7-9% lower mean concentrations at pedestrian level. The impact of smaller scale variability in building shape was negligible. Street trees further complicate the flow and dispersion. Notwithstanding the surface roughness, atmospheric stability controls the concentration levels with higher values under stably stratified inflow. Little traffic emissions are transported to courtyards. The results provide urban planners direct information to reduce air pollution by proper structural layout of perimeter blocks.Peer reviewe

Effect of radiation interaction and aerosol processes on ventilation and aerosol concentrations in a real urban neighbourhood in Helsinki

Large-eddy simulation (LES) is an optimal tool to examine aerosol particle concentrations in detail within urban neighbourhoods. The concentrations are a complex result of local emissions, meteorology, aerosol processes and local mixing conditions due to thermal and mechanical effects. Despite this, most studies have focused on simplification of the affecting processes such as examining the impact of local mixing in idealised street canyons or treating aerosols as passive scalars. The aim of this study is to include all these processes into LES using the PALM model system and to examine the importance of radiative heating and aerosol processes in simulating local aerosol particle concentrations and different aerosol metrics within a realistic urban neighbourhood in Helsinki under morning rush hour with calm wind conditions. The model outputs are evaluated against mobile laboratory measurements of air temperature and total particle number concentration (Ntot) as well as drone measurements of lung-deposited surface area (LDSA). The inclusion of radiation interaction in LES has a significant impact on simulated near-surface temperatures in our study domain, increasing them on average from 8.6 to 12.4 ∘C. The resulting enhanced ventilation reduces the pedestrian-level (4 m) Ntot by 53 %. The reduction in Ntot due to aerosol processes is smaller, only 18 %. Aerosol processes particularly impact the smallest particle range, whereas radiation interaction is more important in the larger particle range. The inclusion of radiation interaction reduces the bias between the modelled and mobile-laboratory-measured air temperatures from −3.9 to +0.2 ∘C and Ntot from +98 % to −13 %. With both aerosol and radiation interaction on, the underestimation is 16 %, which might be due to overestimation of the ventilation. The results show how inclusion of radiative interaction is particularly important in simulating PM2.5, whereas aerosol processes are more important in simulating LDSA in this calm wind situation.</p

Boundary-layer height and surface stability at Hyytiälä, Finland, in ERA5 and observations

We investigate the boundary-layer (BL) height at Hyytiala in southern Finland diagnosed from radiosonde observations, a microwave radiometer (MWR) and ERAS reanalysis. Four different, pre-existing algorithms are used to diagnose the BL height from the radiosondes. The diagnosed BL height is sensitive to the method used. The level of agreement, and the sign of systematic bias between the four different methods, depends on the surface-layer stability. For very unstable situations, the median BL height diagnosed from the radiosondes varies from 600 to 1500 m depending on which method is applied. Good agreement between the BL height in ERAS and diagnosed from the radiosondes using Richardson-number-based methods is found for almost all stability classes, suggesting that ERAS has adequate vertical resolution near the surface to resolve the BL structure. However, ERAS overestimates the BL height in very stable conditions, highlighting the ongoing challenge for numerical models to correctly resolve the stable BL. Furthermore, ERAS BL height differs most from the radiosondes at 18:00 UTC, suggesting ERAS does not resolve the evening transition correctly. BL height estimates from the MWR are also found to be reliable in unstable situations but often are inaccurate under stable conditions when, in comparison to ERAS BL heights, they are much deeper. The errors in the MWR BL height estimates originate from the limitations of the manufacturer's algorithm for stable conditions and also the misidentification of the type of BL. A climatology of the annual and diurnal cycle of BL height, based on ERA5 data, and surface-layer stability, based on eddy covariance observations, was created. The shallowest (353 m) monthly median BL height occurs in February and the deepest (576 m) in June. In winter there is no diurnal cycle in BL height; unstable BLs are rare, yet so are very stable BLs. The shallowest BLs occur at night in spring and summer, and very stable conditions are most common at night in the warm season. Finally, using ERA5 gridded data, we determined that the BL height observed at Hyytiala is representative of most land areas in southern and central Finland. However, the spatial variability of the BL height is largest during daytime in summer, reducing the area over which BL height observations from Hyytiala would be representative.Peer reviewe

Applying machine learning to replicate large-eddy simulation results on urban pollutant dispersion

Non peer reviewe

Determinants of spatial variability of air pollutant concentrations in a street canyon network measured using a mobile laboratory and a drone

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 lab-oratory 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 mo-bile 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 con-trolled 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.Peer reviewe

Uncertainty of eddy covariance flux measurements over an urban area based on two towers

The eddy covariance (EC) technique is the most direct method for measuring the exchange between the surface and the atmosphere in different ecosystems. Thus, it is commonly used to get information on air pollutant and greenhouse gas emissions, and on turbulent heat transfer. Typically an ecosystem is monitored by only one single EC measurement station at a time, making the ecosystem-level flux values subject to random and systematic uncertainties. Furthermore, in urban ecosystems we often have no choice but to conduct the single-point measurements in non-ideal locations such as close to buildings and/or in the roughness sub-layer, bringing further complications to data analysis and flux estimations. In order to tackle the question of how representative a single EC measurement point in an urban area can be, two identical EC systems - measuring momentum, sensible and latent heat, and carbon dioxide fluxes - were installed on each side of the same building structure in central Helsinki, Finland, during July 2013-September 2015. The main interests were to understand the sensitivity of the vertical fluxes on the single measurement point and to estimate the systematic uncertainty in annual cumulative values due to missing data if certain, relatively wide, flow-distorted wind sectors are disregarded. The momentum and measured scalar fluxes respond very differently to the distortion caused by the building structure. The momentum flux is the most sensitive to the measurement location, whereas scalar fluxes are less impacted. The flow distortion areas of the two EC systems (40-150 and 230-340 degrees) are best detected from the mean-wind-normalised turbulent kinetic energy, and outside these areas the median relative random uncertainties of the studied fluxes measured by one system are between 12 % and 28 %. Different gap-filling methods with which to yield annual cumulative fluxes show how using data from a single EC measurement point can cause up to a 12 % (480 g C m(-2)) underestimation in the cumulative carbon fluxes as compared to combined data from the two systems. Combining the data from two EC systems also increases the fraction of usable half-hourly carbon fluxes from 45 % to 69 % at the annual level. For sensible and latent heat, the respective underestimations are up to 5 % and 8 % (0.094 and 0.069 TJ m(-2)). The obtained random and systematic uncertainties are in the same range as observed in vegetated ecosystems. We also show how the commonly used data flagging criteria in natural ecosystems, kurtosis and skewness, are not necessarily suitable for filtering out data in a densely built urban environment. The results show how the single measurement system can be used to derive representative flux values for central Helsinki, but the addition of second system to other side of the building structure decreases the systematic uncertainties. Comparable results can be expected in similarly dense city locations where no large directional deviations in the source area are seen. In general, the obtained results will aid the scientific community by providing information about the sensitivity of EC measurements and their quality flagging in urban areas.Peer reviewe

Implementation of the sectional aerosol module SALSA2.0 into the PALM model system 6.0 : Model development and first evaluation

Urban pedestrian-level air quality is a result of an interplay between turbulent dispersion conditions, background concentrations, and heterogeneous local emissions of air pollutants and their transformation processes. Still, the complexity of these interactions cannot be resolved by the commonly used air quality models. By embedding the sectional aerosol module SALSA2.0 into the large-eddy simulation model PALM, a novel, high-resolution, urban aerosol modelling framework has been developed. The first model evaluation study on the vertical variation of aerosol number concentration and size distribution in a simple street canyon without vegetation in Cambridge, UK, shows good agreement with measurements, with simulated values mainly within a factor of 2 of observations. Dispersion conditions and local emissions govern the pedestrian-level aerosol number concentrations. Out of different aerosol processes, dry deposition is shown to decrease the total number concentration by over 20 %, while condensation and dissolutional increase the total mass by over 10 %. Following the model development, the application of PALM can be extended to local-and neighbourhood-scale air pollution and aerosol studies that require a detailed solution of the ambient flow field.Peer reviewe