6 Effectiveness of trees and plants for

Good quality plantllife is necessary to structurally iml prove air quality. All plants contribute to improvement in air quality. However, some species of plants are more suitable than others and the effectiveness of a species is dependent on the air pollution component. Leaves form the most important place to filter gaseous air poll lution and particulates. Differences in leaf structure and the quantity of foliage play a big part in the difference of effectiveness between species. A great deal of inforl mation is still lacking about the precise effectiveness of different sorts of trees and shrubs to improve the air quality. Table 8 shows estimations of effectiveness based upon the properties of the leaves, and lays out the relative differences in effectiveness between diffel rent species. After all, all plants remove pollution from the air to a greater or lesser degree. Because some plants can stimulate the forming of ozone due to their emission of volatile organic coml pounds, available information about this is summarised in the last column.

6.1 Particulates (PM10)

Effectiveness with regard to particulates is evaluated by means of the following criteria: 1 Conifers are more effective in the removal of

particulates than deciduous trees; 2 Within the category of deciduous trees, those with

coarse hairy foliage are more effective than those with glossy flat leaves; 3 Evergreen species remove more particulates than

those which are not evergreen; 4 Species with a large leaf surface capture more partil

culates than species with a small leaf surface. In this sense, trees are more effective than shrubs.

26

As far as it is known, particulate pollution isnit harmful to plants. When choosing plant types it therefore isnit necessary to take the aspect of sensitivity to particul lates into account.

The effectiveness value is shown in the table on a scale of 1 (least effective) to 3 (most effective).

6.2 Nitrogen oxides

With regard to nitrogen oxides, effectiveness is assessed by means of the following criteria: 1 Deciduous trees are more effective in the absorption

of nitrogen oxides than conifers; 2 Within the category of deciduous trees, those with

glossy flat leaves are more effective than those with coarse hairy leaves; 3 Species with a large leaf surface absorb more nitrogen oxides than species with a small leaf surface. In this sense trees are more effective than shrubs.

The effectiveness value is shown in the table on a scale of 1 (least effective) to 3 (most effective).

Nitrogen oxides can be harmful to plants. If trees and shrubs are planted for the purpose of reducing nitrogen dioxide concentrations, aspects of sensitivity need to be taken into account.

Sensitivity also varies between species. Tree species which, according to Japanese research (Takahashi et al, 2005), are known to be suitable for planting in areas with a lot of nitrogen oxides are indicated with a (+).



improving air quality

6.3 Ozone

The effect trees have on ozone concentrations is very complex. Ozone is formed out of nitrogen dioxide and volatile organic compounds in conditions of high tempel ratures and the presence of sunlight. In these circumsl tances there are numerous ways in which tree species can influence ozone concentration: 1 As a result of the transpiration of water through the leaves, plants subdue temperature rises and inhibit the forming of ozone, in comparison to a situation without plants; 2 Trees and shrubs absorb nitrogen dioxide (column 3 in

table 8) to a greater or lesser degree. The more nitrogen dioxide is absorbed, the less ozone can be formed; 3 Trees and shrubs absorb ozone to a greater or lesser

degree; 4 Trees themselves emit airborne volatile organic coml

pounds in varying quantities (see column 5 in table 8). These organic substances contribute to the forming of ozone. The more of these types of compounds are emitl ted, the more the production of ozone is stimulated.

Estimation of the effectiveness of trees and shrubs to absorb ozone is shown in column 4. With regard to effectiveness, the absorption of ozone runs parallel with that of nitrogen dioxide because the processes to do so are similar. The effectiveness value is also shown in the table on a scale of 1 (least effective) to 3 (most effective).

The tree species which according to English research (Donovan et al, 2005) can be effective on ozone concenl trations in urban areas are shown in brackets by (+) to decrease or (l) to raise.

Species which emit a relatively high amount of volatile organic compounds (column 5 in Table 8) can lead to an increase in ozone levels. Although these species also absorb ozone, the net effect is an increase in ozone levels. If reduction in ozone concentrations is desired it is therefore better to prevent largelscale planting of these species.

27



TABLE 8 Estimation of effectiveness of the most important species for reduction of concentrations

of particulates, nitrogen oxides and ozone in the air.

COLUMN SPECIES

Stated properties also apply to cultivars of the stated species.

COLUMN PARTICULATES (CAPTURE), NITROGEN OXIDES (ABSORPTION) AND OZONE (ABSORPTION)

COLUMN OZONE

Species which are effective for ozone concentration reduction in urban areas (based on English research)

 Species which raise ozone concentration in urban

areas (based on English research) : Least effective

■ ■ ■

: Most effective

COLUMN EMISSIONS OF VOLATILE ORGANIC COMPOUNDS

■ COLUMN NITROGEN OXIDES

Species which absorb a lot of nitrogen dioxide and are not sensitive to it (based on Japanese research).

: Very limited emissions

■ ■ ■

: Considerable emissions

Emissions of volatile organic compounds are not measurable for these species.

SOURCE

Takahashi et al, 2005 Donovan et al, 2005

SPECIES

Stewart and Hewitt, 2002 Nowak et al, 2002

PARTICULATES PM10

NITROGEN OXIDES NO+NO

2

OZONE O

3

EMISSIONS OF VOLATILE ORGANIC COMPOUNDS SHRUBS Amelanchier lamarckii ■ ■ ■ i Berberis xfrikartii * ■ ■ ■ ■ ■ ■ ■ ■ Chaenomeles ■ ■ ■ ■ ■ Corylus colurna ■ ■ ■ ■ ■ ■

+ i Euonymus (deciduous) ■ ■ ■ ■

+

■ ■ ■ i Euonymus (evergreen) ■ ■ ■ ■ ■

+

■ ■ ■ i Hedera ■ ■ ■ ■ i Ilex xmeserveae ■ ■ ■ ■ ■ ■

+ i Lonicera (deciduous) ■ ■ ■ i Lonicera (evergreen) ■ ■ ■ Mahonia ■ ■ ■ ■ ■ ■ ■ ■ ■ Potentilla fruticosa ■ ■ ■ ■ ■ ■ ■ Rosa ■ ■ ■ ■ ■ ■ ■ Spiraea ■ ■ ■ ■ ■ ■ CLIMBING PLANTS Clematis ■ ■ ■ i Fallopia ■ ■ ■ ■ ■ ■ ■ i Hedera ■ ■ ■ ■ ■ i Lonicera ■ ■ ■ ■ ■ i Parthenocissus ■ ■ ■ ■ ■ i Pyracantha ■ ■ ■ ■ ■ ■ ■ ■ i Rosa ■ ■ ■ ■ ■ ■ ■ Wisteria ■ ■ ■ ■ ■ i

28



NITROGEN SPECIES

PARTICULATES

OXIDES PM10

NO+NO

2

EMISSIONS OF VOLATILE ORGANIC COMPOUNDS CONIFERS Ginkgo biloba * ■ ■ ■ ■ ■ ■ ■ ■ Metasequoia glyptostroboides ■ ■ ■ ■ ■ ■ Pinus nigra ■ ■ ■ ■ ■

+

■ Pinus sylvestris * ■ ■ ■ ■ ■ ■ Taxus ■ ■ ■ ■ ■ ■ HEDGES Carpinus betulus ■ ■ ■ ■ ■ ■ ■ ■ ■ Fagus ■ ■ ■ ■ ■ ■ ■ ■ i Ligustrum ■ ■ ■ ■ ■ ■ ■ ■ i DECIDUOUS TREES Acer platanoides * ■ Acer pseudoplatanus * ■ ■ ■ ■ ■ ■ ■

+ i

■ ■ ■ ■ ■ ■

+ i Aesculus ■ ■ ■ ■ ■ ■ ■ ■ i Ailanthus altissima ■ ■ ■ ■ ■ ■ ■ ■ Alnus cordata ■ ■ ■ ■ ■ ■ ■

+ i Alnus glutinosa * ■ Alnus xspaethii ■ ■ ■ ■ ■ ■ ■ ■

+ i

■ ■ ■ ■ ■ ■

+ i Betula ermanii * ■ ■ Betula nigra ■ ■ ■ ■ ■ ■ ■ ■

+

■ ■ ■ ■ ■ ■

+

■ Betula pendula ■ ■ ■ ■ ■ ■ ■ ■

+

■ Betula utilis * ■ ■ ■ ■ ■ ■ ■ ■

+

■ Carpinus betulus * ■ ■ Crataegus xpersimilis * ■ ■ ■ ■ ■ ■ ■ ■

■ ■ ■ ■ ■ ■

+ i Fagus sylvatica * ■ ■ ■ ■ ■ ■ ■ ■ i Fraxinus angustifolia * ■ Fraxinus excelsior * ■ ■ ■ ■ ■ ■ ■ i

■ ■ ■ ■ ■ ■

+ i Fraxinus ornus * ■ ■ ■ ■ ■ ■ ■ i Fraxinus pennsylvanica ■ ■ ■ ■ ■ ■ ■ ■ i Gleditsia triacanthos * ■ ■ ■ ■ ■ ■ ■ ■ i Koelreuteria paniculata ■ ■ ■ ■ ■ ■ ■ ■ Liquidambar styraciflua ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Liriodendron tulipifera ■ ■ ■ ■ ■ ■ ■ i Magnolia kobus ■ ■ ■

+

■ ■ ■ Malus * ■ ■ ■ ■ ■ ■ ■ ■

+ i Parrotia persica ■ ■ ■ ■ Platanus xhispanica * ■ ■ Populus * ■ ■ Prunus * ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■

■ ■ ■

+

■ ■ ■

-

■ ■ ■

■ ■ ■

+

■ ■ ■

+ i Pyrus calleryana * ■ Quercus palustris ■ ■ ■ ■ ■ ■ ■ ■ i

■ ■ ■

+

■ ■ ■

-

■ ■ ■ Quercus robur * ■ Salix alba * ■ ■ ■ ■ ■

+

■ ■ ■

-

■ ■ ■

■ ■ ■

+

■ ■ ■

-

■ ■ ■ Sophora japonica ■ ■ ■ ■ ■ ■ ■ ■ i Sorbus ■ ■ ■ ■ ■ ■ ■ ■

+

■ Tilia cordata * ■ ■ Tilia europaea * ■ Ulmus * ■ ■ ■ ■ ■ ■ ■ ■ ■

■ ■ ■ ■ ■ ■

+

■ ■ ■ ■ ■ ■

+

29

OZONE O

3