Kaupunkimetsien vaikutuksia ilmanlaatuun teiden lähiympäristössä

Trees and other vegetation have been suggested as an efficient means in improving urban air quality. While it is well known that vegetation is able to remove air pollutants by dry deposition on their leaves and other surfaces, whether or not this removal affects local ambient pollutant concentrations is still not fully understood. Overall, the challenge in this field of research is due to the complexities of changing environmental conditions and green infrastructure design. A plethora of factors under these categories determine the relative significance of vegetation-related pollution deposition (improves air quality) and dispersion inhibition (deteriorates air quality), which are considered to be the most important processes defining the effects of vegetation on local air quality. This thesis provides new empirical evidence regarding the efficacy of urban parks and forests in improving air quality in near-road environments in urban and peri-urban areas. Levels of nitrogen dioxide (NO2), anthropogenic volatile organic compounds (AVOCs), gaseous polycyclic aromatic hydrocarbons (PAHs) and particulate matter (PM) were measured in tree-covered and open habitats using several sampling methods and an unprecedented number of site replication. Sampling events were carried out either at single points in open and tree-covered habitats, or at several measuring points along transects perpendicular to roads. PM was measured in many size-fractions to observe if vegetation affects nanoparticles, fine particles and coarser particles differently. Results were ambiguous, as consistent vegetation-induced reductions in the parameters investigated were found only for PM deposition obtained with passive collector, the average reductions being 36–49 %. Less distinct reductions were observed for AVOCS, coarser PM and NP, being markedly less consistent and often providing results of a merely tentative nature. Coarser PM (> 2.5 µm) concentration data from active sampling are more reliable in terms of exposure estimation as opposed to the passive PM deposition data, which may be altered by wind speed differences between open and tree-covered habitats. Active sampling results were similar with a reduction observed using passive PM collectors, yet the reduction was less prominent: 12 % (2.5–10 µm) and 33 % (> 10 µm). However, larger particles are of lesser importance in causing health problems, whereas the most hazardous fraction, PM2.5 (< 2.5 µm), was not reduced by tree cover. PM results were broadly in line with the deposition mechanisms of particles, where nano-sized particles and coarser particles are more prone to dry deposition than PM2.5. Furthermore, instead of being reduced, levels of NO2 and gaseous PAHs were at times significantly elevated within tree cover. Consistent distance attenuation differences between open and tree-covered transects did not emerge, unexpectedly, meaning that there was no difference in pollutant decay between the study habitats. These studies did not show clear connections between the measured vegetation properties and the observed pollutant concentrations. Overall, the general consensus according to which urban vegetation is effective in mitigating air pollution, was not supported. Instead, these data imply that the importance of urban vegetation in reducing local air pollution is limited and often over-estimated. Consequently, urban parks and forests appear to mitigate air pollution problems only occasionally, thereby not serving as a sustainable solution for local air pollution problems. A more detailed mechanistic understanding of the transformation, deposition and dispersion of atmospheric pollutants in terms of urban vegetation as affecting local concentrations is needed in order to create landscape designs where unintended consequences of vegetation are avoided and the truly potential set-ups for air quality improvement are recognized. Ultimately, air quality effects are merely one aspect in the wider spectrum of ecosystem services provided by urban vegetation; assessing the most beneficial and cost-efficient combinations of ecosystem services to be supported at a given location requires a holistic approach.Puita ja muuta kasvillisuutta on ehdotettu tehokkaaksi keinoksi parantaa kaupunkien ilmansaasteongelmia. Kasvillisuus sitoo saasteita pinnoilleen, mutta vielä ei täysin ymmärretä, miten sitominen vaikuttaa paikallisiin ilmansaastepitoisuuksiin. Kasvillisuuden ilmanlaatuvaikutuksen tutkimus on haastavaa johtuen monimutkaisista ja muuttuvista tekijöistä liittyen sekä kasvillisuusrakenteisiin että muihin ympäristön olosuhteisiin. Nämä tekijät määrittävät kahden merkittävimmän prosessin suhteellisen merkityksen: saasteiden sitoutuminen kasvillisuuteen (parantaa ilmanlaatua) sekä saasteiden leviämisen estyminen (huonontaa ilmanlaatua). Väitöskirjani sisältää uutta tietoa kaupunkipuistojen ja -metsien vaikutuksesta ilmanlaatuun teiden lähiympäristössä. Tutkimuksissa tarkasteltiin typpidioksidia (NO2), ihmisperäisiä haihtuvia orgaanisia yhdisteitä (AVOC), kaasumaisia polyaromaattisia hiilivetyjä (PAH) sekä hiukkasia (PM) puustoisilla ja avoimilla alueilla. Mittauskampanjat toteutettiin joko mittaamalla yksittäistä pistettä kullakin puustoisella ja avoimella alueella, tai useammilla mittauspisteillä jonomittauksina tieltä poispäin. Hiukkasmittauksissa tarkasteltiin useita kokofraktioita: nanohiukkasia, pienhiukkasia ja karkeampia hiukkasia. Tulokset eivät olleet yksiselitteisiä, ja johdonmukainen ilmansaastevähennys (36–49 %) puustoisilla alueilla todettiin ainoastaan passiivisella hiukkaskeräimellä, jonka tuloksiin karkeammat hiukkaset vaikuttavat dominoivasti. Alustavia, merkittävästi vähemmän johdonmukaisia ilmansaastevähennyksiä havaittiin AVOC-yhdisteille, karkeammille hiukkasille (aktiivikeräimillä) ja nanohiukkasille. Karkeampien hiukkasten (> 2.5 µm) tuloksista aktiivimittauksin saadut ovat luotettavampia altistuksen arvioinnissa verrattuna passiivikeräinten tuloksiin, joihin ovat voineet vaikuttaa tuulierot avoimella ja puustoisella alueella. Aktiivikeräinten tulokset olivat samansuuntaisia, mutta selvästi maltillisempia kuin passiivikeräinten, puustoisen alueen saastevähennyksen ollessa 12 % (2.5–10 µm) ja 33 % (> 10 µm). Suuremmat hiukkaset aiheuttavat kuitenkin vähemmän vakavia terveyshaittoja verrattuna pienhiukkasiin (< 2.5 µm), joiden pitoisuudet eivät vähentyneet puustoisilla alueilla. Hiukkastulokset olivat karkeasti ottaen linjassa hiukkasten yleisten asettumis- eli laskeumamallien kanssa; nanohiukkaset ja karkeat hiukkaset asettuvat pinnoille herkemmin kuin pienhiukkaset. NO2:n ja kaasumaisten PAH-yhdisteiden pitoisuudet olivat ajoittain merkittävästi korkeampia puustoisilla alueilla. Johdonmukaisia eroja ilmansaasteiden laimenemisessa avoimilla ja puustoisilla alueilla ei havaittu jonomittauksissa. Myöskään kasvillisuusmuuttujien ja ilmansaastepitoisuuksien välillä ei havaittu yhteyttä. Yleinen väite, jonka mukaan kaupunkikasvillisuus on tehokas ilmanlaadun parantaja, ei saanut vahvaa tukea tutkimuksistani. Sen sijaan tulokseni viittaavat siihen, että kaupunkivihreän merkitys ilmansaasteiden torjunnassa on hyvin rajallinen ja usein yliarvioitu. Puustoiset alueet parantavat ilmanlaatua vain ajoittain, eivätkä ne siten muodosta kestävää ja pitkäaikaista ratkaisua paikallisiin ilmansaasteongelmiin. Ilmansaasteiden hallitsemisen kannalta toimiva kaupunkisuunnittelu tarvitsee tuekseen yksityiskohtaista tietoa ilmansaasteiden muuntumisesta, laskeumasta, sekä leviämisestä ja laimenemisesta, jotta vältetään kasvillisuuden negatiiviset vaikutukset ilmanlaatuun, ja toisaalta tunnistetaan parhaat mahdolliset ratkaisut altistumisen minimoimiseksi. Ilmanlaatuvaikutukset ovat vain yksi ulottuvuus kaupunkikasvillisuuden tuottamassa ekosysteemipalveluiden kirjossa. On tärkeää tunnistaa paikallinen potentiaali erilaisten ekosysteemipalveluiden tuottamisessa, jotta kokonaisuus on kustannustehokas, ja mikä tärkeintä, mahdollisimman suureksi hyödyksi kaupungin asukkaille

Urban forests near roads do not reduce gaseous air pollutant concentrations but have an impact on particles levels

Peer reviewe

Impacts of urban roadside forest patches on NO2 concentrations

Although it is commonly believed that trees can improve air quality, recent studies have shown that such pollution mitigation can be negligible – or that tree canopies can even increase pollutant concentrations near their sources compared to adjacent treeless areas. We explored the impacts of urban roadside forest patches on the concentrations of nitrogen dioxide in summer and winter in the Helsinki Metropolitan Area, Finland, and especially investigated if canopy cover can result in increased concentrations of NO2 below the canopy. Our results, however, did not show significantly higher – or lower – NO2 concentrations underneath tree canopies compared to levels above canopies. Neither did NO2 levels at the below-canopy sampling height differ significantly between forest patches and adjacent open, treeless areas. The lack of a canopy effect may derive from the rather small size of the forest patches, and – compared to previous studies with similar design – divergent tree species composition forming a dense canopy structure. Our results corroborate previous studies that the potential ecosystem services offered by urban near-road forests are more likely due to benefits other than those related to the removal of air pollutants.Peer reviewe

Effects of forests on particle number concentrations in near-road environments across three geographic regions

Trees and other vegetation have been advocated as a mitigation measure for urban air pollution mainly due to the fact that they passively filter particles from the air. However, mounting evidence suggests that vegetation may also worsen air quality by slowing the dispersion of pollutants and by producing volatile organic compounds that contribute to formation of ozone and other secondary pollutants. We monitored nanoparticle (>10 nm) counts along distance gradients away from major roads along paired transects across open and forested landscapes in Baltimore (USA), Helsinki (Finland) and Shenyang (China) − i.e. sites in three biomes with different pollution levels − using condensation particle counters. Mean particle number concentrations averaged across all sampling sites were clearly reduced (15 %) by the presence of forest cover only in Helsinki. For Baltimore and Shenyang, levels showed no significant difference between the open and forested transects at any of the sampling distances. This suggests that nanoparticle deposition on trees is often counterbalanced by other factors, including differing flow fields and aerosol processes under varying meteorological conditions. Similarly, consistent differences in high frequency data patterns between the transects were detected only in Helsinki. No correlations between nanoparticle concentrations and solar radiation or local wind speed as affecting nanoparticle abundances were found, but they were to some extent associated with canopy closure. These data add to the accumulating evidence according to which trees do not necessarily improve air quality in near-road environments.Trees and other vegetation have been advocated as a mitigation measure for urban air pollution mainly due to the fact that they passively filter particles from the air. However, mounting evidence suggests that vegetation may also worsen air quality by slowing the dispersion of pollutants and by producing volatile organic compounds that contribute to formation of ozone and other secondary pollutants. We monitored nanoparticle (>10 nm) counts along distance gradients away from major roads along paired transects across open and forested landscapes in Baltimore (USA), Helsinki (Finland) and Shenyang (China) − i.e. sites in three biomes with different pollution levels − using condensation particle counters. Mean particle number concentrations averaged across all sampling sites were clearly reduced (15 %) by the presence of forest cover only in Helsinki. For Baltimore and Shenyang, levels showed no significant difference between the open and forested transects at any of the sampling distances. This suggests that nanoparticle deposition on trees is often counterbalanced by other factors, including differing flow fields and aerosol processes under varying meteorological conditions. Similarly, consistent differences in high frequency data patterns between the transects were detected only in Helsinki. No correlations between nanoparticle concentrations and solar radiation or local wind speed as affecting nanoparticle abundances were found, but they were to some extent associated with canopy closure. These data add to the accumulating evidence according to which trees do not necessarily improve air quality in near-road environments.Peer reviewe

The impact of urban trees on concentrations of PAHs and other gaseous air pollutants in Yanji, northeast China

It is generally conceived that trees can clean polluted air in urban areas sufficiently enough to be considered providers of a vital ecosystem service, although there have not been many field studies showing this in practice in the neighbourhood scale. Using passive sampling methods, we investigated the effect of urban park trees on the concentrations of gaseous polycyclic aromatic hydrocarbons (PAHs), nitrogen dioxide (NO2), ground-level ozone (O3) and sulfur dioxide (SO2) in early summer in the temperate zone city of Yanji, northeast China. Concentrations of total gaseous PAHs and certain PAH constituents were higher and concentrations of O3 lower in tree-covered areas compared to nearby open areas, while tree cover did not affect the concentrations of NO2 and SO2. The higher PAH concentrations under tree canopies may associate with air-soil gas exchange and the trapping of polluted air under canopies. Lower O3 concentrations in tree-covered areas may result from a combination of absorption of O3 by tree canopies, and lower temperatures and solar radiation under tree canopies compared to open areas.Peer reviewe

Greenbelts do not reduce NO2 concentrations in near-road environments

Trees are believed to improve air quality, thus providing an important ecosystem service for urban inhabitants. However, empirical evidence on the beneficial effects of urban vegetation on air quality at the local level and in boreal climatic regions is scarce. We studied the influence of greenbelt-type forest patches on NO2 levels (i) in front of, (ii) inside and (iii) behind greenbelts next to major roads in the Helsinki Metropolitan Area, Finland, during summer and winter using passive collectors. Concentrations of NO2 were significantly higher in front of greenbelts compared to road sides without greenbelts. The more trees there were inside greenbelts the higher the NO2 level in front of greenbelts, likely due to the formation of a recirculation zone of air flow in front of greenbelts. Similarly, NO2 levels were higher inside greenbelts than in open areas without them, likely due to reduced air flow inside greenbelts. NO2 levels behind greenbelts were similar to those detected at the same distance from the road but without greenbelts. Our results suggest that, regardless of season, roadside greenbelts of mostly broadleaf trees do not reduce NO2 levels in near-road environments, but can result in higher NO2 levels in front of and inside greenbelts.Trees are believed to improve air quality, thus providing an important ecosystem service for urban inhabitants. However, empirical evidence on the beneficial effects of urban vegetation on air quality at the local level and in boreal climatic regions is scarce. We studied the influence of greenbelt-type forest patches on NO2 levels (i) in front of, (ii) inside and (iii) behind greenbelts next to major roads in the Helsinki Metropolitan Area, Finland, during summer and winter using passive collectors. Concentrations of NO2 were significantly higher in front of greenbelts compared to road sides without greenbelts. The more trees there were inside greenbelts the higher the NO2 level in front of greenbelts, likely due to the formation of a recirculation zone of air flow in front of greenbelts. Similarly, NO2 levels were higher inside greenbelts than in open areas without them, likely due to reduced air flow inside greenbelts. NO2 levels behind greenbelts were similar to those detected at the same distance from the road but without greenbelts. Our results suggest that, regardless of season, roadside greenbelts of mostly broadleaf trees do not reduce NO2 levels in near-road environments, but can result in higher NO2 levels in front of and inside greenbelts.Peer reviewe

Gaseous polycyclic aromatic hydrocarbon concentrations are higher in urban forests than adjacent open areas during summer but not in winter – Exploratory study

Peer reviewe

Trees in urban parks and forests reduce O3, but not NO2 concentrations in Baltimore, MD, USA

Trees and other vegetation absorb and capture air pollutants, leading to the common perception that they, and trees in particular, can improve air quality in cities and provide an important ecosystem service for urban inhabitants. Yet, there has been a lack of empirical evidence showing this at the local scale with different plant configurations and climatic regions. We studied the impact of urban park and forest vegetation on the levels of nitrogen dioxide (NO2) and ground-level ozone (O3) while controlling for temperature during early summer (May) using passive samplers in Baltimore, USA. Concentrations of O3 were significantly lower in tree-covered habitats than in adjacent open habitats, but concentrations of NO2 did not differ significantly between tree-covered and open habitats. Higher temperatures resulted in higher pollutant concentrations and NO2 and O3 concentration were negatively correlated with each other. Our results suggest that the role of trees in reducing NO2 concentrations in urban parks and forests in the Mid-Atlantic USA is minor, but that the presence of tree-cover can result in lower O3 levels compared to similar open areas. Our results further suggest that actions aiming at local air pollution mitigation should consider local variability in vegetation, climate, micro-climate, and traffic conditions.Peer reviewe