On the reduction of urban particle concentration by vegetation a review

Abstract

In order to assess the filtration performance of plants with respect to atmospheric dust, deposition on vegetation has been investigated by a number of different methods (field studies, numerical and physical modelling) over the past few years. The intention of this review is to assess the extent to which a reduction in particle concentration (especially PM 10) can be accomplished by existing vegetation or targeted planting on the basis of international publications. The range of this assessment however is limited to the quantitative filtration potential of urban vegetation. Deposition velocity was taken as a measure of filtration performance as a particle deposited on a plant is in effect taken from the atmosphere. As regarding published deposition velocities, there are differences of an order of magnitude between measured values and the results of model calculations. The average published value (∼1 cm s −1) corresponds to a reduction in pollutant concentration (PM 10) of about 1 % in urban areas. In addition, analyses carried out for a busy arterial road show that very large vegetation areas (in excess of 10,000 m 2) would be needed to compensate for local emissions of particles (PM 10) by vehicles at a deposition velocity of 1 cm s −1. However, current in-situ measurements indicate deposition velocities considerably higher than 1 cm s −1 and, for PM 1 , velocities above 10 cm s −1. If these results were confirmed by further measurements, local planting campaigns covering small areas could also be beneficial for a reduction of particle concentrations. Zusammenfassung Um die Filterwirksamkeit von Pflanzen gegenüber Partikeln zu bestimmen, wurde in den vergangenen Jahren mit einer Vielzahl von Methoden (Feldstudien, Modellierung, Windkanal) die Deposition auf Vegetationsoberflächen untersucht. Die Intention dieses Reviews ist es auf der Basis von internationaler Literatur zu bewerten, inwieweit eine Reduktion der Immissionskonzentration von Partikeln (insbesondere von PM 10) durch vorhandene Vegetation bzw. durch gezielte Pflanzmaßnahmen möglich ist. Diese Einschätzung beschränkt sich jedoch vom Umfang her auf das quantitative Filterungspotential städtischer Vegetation. Als Maß für die Filterleistung wurde die Depositionsgeschwindigkeit herangezogen, da mit der Deposition auf den Pflanzen auch eine Entnahme aus der Luft einhergeht. Die veröffentlichten Depositionsgeschwindigkeiten zeigen Unterschiede von bis zu einer Größenordnung zwischen den gemessenen Werten und den Ergebnissen aus Modellrechnungen. Berechnet man mit einem Mittelwert der veröffentlichten Werte (∼1 cm s −1) das Reduktionspotential für den städtischen Raum, erhält man eine Verringerung der Immissionskonzentration (PM 10) von ca. 1 %. Analysen für eine belastete Ausfallstraße zeigen außerdem, dass bei einer Depositionsgeschwindigkeit von 1 cm s −1 beispielsweise eine Kompensation der lokalen Kfz-Emissionen von PM 10 erst bei sehr großen Vegetationsflächen (> 10 000 m 2) möglich ist. Aktuelle in-situ Messungen weisen jedoch auf Depositionsgeschwin...

The Concentrations and Reduction of Airborne Particulate Matter (PM10, PM2.5, PM1) at Shelterbelt Site in Beijing

“…Roadside tree canopies may adjust air quality through changing air dispersion as well as by capturing air pollutants. Recent studies have found that a street tree canopy has a positive or negative effect on air quality at the pedestrian level [37,38]. The positive effect is the obstacle posed by plants to air flow, reducing air exchange especially in the vertical direction and increasing air pollutant concentrations.…”

Spatial Evaluation of Multiple Benefits to Encourage Multi-Functional Design of Sustainable Drainage in Blue-Green Cities

“…For green streets within street canyons, designers may want to limit the number of trees in bioretention cells and suspended pavement systems and focus on using stormwater measures based on herbaceous vegetation (e.g., bio-retention cells and bio-swales). The close proximity of the vegetation to the PM 10 emission sources maximizes the efficiency of interception and deposition; also, the smaller size of vegetation in installations such as street gardens does not significantly hinder air flow within the street [42].…”

Forestry strategies against PM2.5 pollution in Beijing

“…As resuspension decreases rapidly with time after deposition, it is normally already included in measured velocities. In modelling, it is also normal practice to integrate re-suspension into the deposition velocity (Litschke and Kuttler 2008). A vegetative cover could reduce re-suspension of soil material or previously deposited atmospheric particles due to reduced wind speeds in canopies and to the fixation of particles by roots (Langner 2008).…”

“…Trees may mitigate the damaging effects of airborne particles through removal and subsequent lowering of concentrations (Beckett et al 2000, Litschke and Kuttler 2008, Prajapati 2012, Schaubroeck et al 2014). Farmer (1993 reported that trees are biological filters for airborne particles due to their large leaf surfaces relative to ground covered.…”

“…Therefore, in addition to reducing emissions, other measures to remove airborne particles will be necessary. Reducing air pollutants by increasing urban vegetation could prove to be an effective long-term solution for the improvement of air quality (Litschke and Kuttler 2008). …”