Air Quality

CO2

All plants produce oxygen (O2) and sequester carbon dioxide (CO2) through photosynthesis. When plant parts (leaves, fruits and roots) die and decompose, that CO2 is released again. Initiatives in which grass clippings are reused and therefore do not remain on site and decompose minimises release of  stored CO2. For example, the fibres might be converted into high-quality building material and the juices are used to combat slipperiness.

Trees and shrubs have a positive net effect on an annual basis because they capture a (often large) part of the converted CO2 in wood for a longer period of time. On average, this is about 10 to 30 kilos of CO2 per tree, depending on the size and age of the tree, the tree species, but also the speed of growth and future-proofing. Companies and people who want to offset their CO2 emissions can do so by planting a tree, for example in an urban forest.

The mineral olivine removes CO2 from the air. It is applicable in outdoor areas. Weathering of olivine (through a chemical reaction with water) forms lime, which also helps against acidification of the soil. Olivine is used in water-permeable semi-pavement, parking lots, verges and railway paths. This can be used to compensate, for example, for CO2 emitted during work. The amount of CO2 cleaned up with olivine can be recorded in a CO₂ certificate.

During summer heat periods, energy consumption skyrockets due to the use of air conditioners; The peak load is extreme on summer days. The extra energy used for cooling then affects CO2 emissions from power plants and other emissions. During hot summer periods, extra smog formation occurs.

Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs) are converted under the influence of sunlight into ozone (O3), which is very harmful to human, plant and animal health. Ozone leads to smog formation. Trees can absorb both ozone and VOCs and therefore improve air quality. Some trees produce their own VOCs, particularly when stressed. Overall, the balance sheet on the urban level is more favourable with greening: more VOCs are captured than produced.

In order to prevent the formation of summer smog as much as possible, it is wise not to plant large numbers of VOC-producing species together.

Gaseous contaminants such as VOCs and nitrogen oxides are absorbed into the leaf through stomata. Deciduous trees are more effective than conifers due to their leaf structure. Deciduous trees with large smooth leaves are more effective than deciduous trees with small, rough or hairy leaves. The total volume of contaminants captured depends on the crown volume and the total leaf area.

Particulate Matter

Very small particulate matter (PM0.1) is absorbed as a gas by the stomata and stored. Coarser particulate matter (especially PM10) is captured on the outside of the leaves; When it rains, it is washed down to the ground, removing it as an air pollutant. Precipitated particulate matter can be dispersed again by the wind.

The finer the structure of the tree crown, the more effectively particulate matter is captured. In quantitative terms, deciduous trees remove the most particulate matter, because they have a large leaf area. But in winter, most deciduous trees have little capacity to capture particulate matter because they lose their leaves. Evergreen conifers are, therefore, generally more effective than deciduous trees when considered all year round. In deciduous trees, species with many small leaves are more effective than species with large leaves. Furthermore, strains with rough, sticky, or hairy leaves are more effective than those with smooth, flat leaves. Because the amount eventually captured depends on the volume of the crown, trees are more effective than shrubs. Many small trees and shrubs can have the same effect as one large tree.

Local traffic is the main source of particulate matter along roads.

This comes not only from exhaust fumes, but also particulate matter due to wear and tear of brakes and tyres. The concentration of particulate matter directly above and next to the road is highest and gradually decreases with vertical distance from the road. As a result of turbulence caused by trucks, a second peak occurs at a height of 5 to 7 meters.

Trees and block hedges along roads can capture 15 to 20% of the particulate matter where concentrations are high. An average urban tree can capture about 100 grams of particulate matter per year. This corresponds to the particulate matter production of 5,500 car kilometres driven.

Mosses are known as ‘particulate matter killers’. Due to a combination of properties, they remove a relatively large quantity of particulate matter. Mosses, unlike plants, get rid of all particulate matter. They consume particulate matter and convert it into biomass. One square metre of moss contains as many as 5 million small leaves and can ‘eat’ about 14 grams of particulate matter per year. There is one condition: the moss must be wet. Dry moss is ineffective.

Influencing Air Quality

Greenery affects air quality in several ways. Greenery affects airflow, it has a shading effect, and it can capture or absorb pollutants. The effect on air flows is much greater than the filtering effect.

Air flows and Air Exchange

Air pollution in the city consists of three parts: the rural background (that which comes from the surrounding area), the city-wide contribution, and the local contribution (e.g. busy traffic points). By far the most important factor to reduce local peak concentrations is the exchange and mixing with air from the broader environment (the dilution effect).

Trees can promote and also block air flow. Aim for open corridors that reach from the countryside to the heart of the city. Do not completely close off neighbourhoods with closed planting around them and make sure that exchange is also possible at district level.

In high-traffic streets, limit the number of large trees close to the source. In these streets, avoid trees with wide crowns (the green tunnel effect) that prevent mixing with clean air. If trees are desired, use species with narrow crowns or small trees at a wide distance from each other.

In very narrow streets (height/width ratio >2) with high buildings (street canyons), use façade greenery. For slightly less narrow streets (height/width 0.5-2), a combination with low planting with perennials close to the road can help to achieve optimal ventilation exchange with the highest possible capture capacity. Or place continuous low hedges (1 to 2 metres high) on both sides of the street or in the entire central traffic reservation. The latter has the most effect on improving air quality on roadside footpaths.

Shielding

Along busy traffic roads, continuous hedges of at least 2 metres (above breathing height) height can (partially) shield the footpaths and cycle paths from air pollution from traffic. In wider streets, taller and wider plantings are better. These must be continuous and closed from top to bottom. A noise barrier with closed plants or a dense row of trees behind it is a good alternative.

Buildings or neighbourhoods can be shielded from adjacent traffic routes by mixed planting of trees, shrubs and perennials that are preferably high, wide and dense.

Filtering

Greenery that aims to capture contaminants from the air should be placed as close as possible to the source of the pollution. This is where the concentrations are highest and where the chance that the contaminants will come into contact with the greenery is also the highest. In order to achieve good filtration, the polluted air must come into contact with as much leaf mass as possible.

To do this, the air must be able to flow through the plantings and tree crowns. Plantings and trees with an optical porosity of at least 30% are the most effective.

Different types of greenery capture different types of pollution. Therefore, make sure you have mixed plantings with a layer of trees, shrubs and herbs or perennials. Mixed plantings complement the effect of trees on the air quality in the city, and can also be used when space is limited or when tall plants are not desired.

Lower concentrations of air pollution are usually measured in parks compared with the streets around them. In addition, the concentration within the park decreases with the distance to the edge of the park. The concentration of negative ions is significantly higher in city parks and in the vicinity of other green spaces. Some attribute a positive health effect to this, but this has not been scientifically proven.