How does Viridi Air filter 100% green and sustainable outdoor air?

Viridi Air plants air-purifying rows of very specific tree/hedge species. These are trees that have been scientifically proven to purify the air above average. They are planted in a unique way (BBA model) in order to filter out fine dust, ozone, NOx and ammonia. CO2 is captured as well by these trees and they are also beneficial for biodiversity. In brief, what we do is a natural and sustainable way to filter the air of many more pollutants other than just CO2. We are aware that this method is not the ultimate solution to the problem of air pollution. Adjustments must also be made at the source. That is why companies can only participate in tree planting if they have demonstrated, with a product, service or conscious company policy, that they contribute to clean indoor air and/or fewer emissions into the outdoor air. One can choose for Viridi Air Partnerships with or without extra marketing exposure/promotion. See the overview of all Viridi Air Partnerships.
Working exclusively in Belgium and the Netherlands, Viridi Air plants air-purifying rows made up of unique species of trees/hedges, mostly in outlying areas (where there is a lot of wind). The locations where the trees/hedges are planted can be visited by appointment with Viridi Air. During the planting you and your colleagues are welcome to come and lend a hand. Filtering the outside air is done with a unique filtering technique:

  • The Viridi Air external filter principle
  • How much polluted air does Viridi Air filter?
  • Planning outdoor filter projects
  • Realized outdoor filtration projects

How does Viridi Air filter outdoor air with a 100% green and sustainable method?

Viridi Air focuses on filtering particulates, NOx and ozone. The filtering is done by planting trees and hedges, so there will also be an absorption of CO2. The uptake of CO2 is higher in our projects because a high-quality performance model is used. 

Scientifically proven filter technology

The outdoor air is filtered according to the BBA model: a practical application from scientific research by Alterra, Wageningen University. The three elements of the BBA model influence each other and must be properly attuned. Viridi Air has a high-quality application where attention is paid to the power of nature. This entails:

  • B: soil = choosing the right soil in the Netherlands and making it suitable for tree planting. Viridi Air only plants on new soils (e.g., ex-agricultural land) in the Netherlands;
  • B: trees (or hedgerows) = Viridi Air uses a mix of specific coniferous/deciduous trees & hedge species that perform above average in terms of air purification. For conifers, pine trees are an ideal choice. For deciduous trees, you can consider birch, maple and bird cherry. These trees do not grow on the same type of soil. It is therefore necessary to analyse for each project which trees/ hedges should be placed on which soil. We do not plant monocultures and biodiversity is very important to us;
  • A: architecture = the trees or hedges are planted in a special way. The trees or hedges are planted as rows that can optimally capture light and wind and therefore grow well and have an extra air-purifying effect;

Types of trees used

Viridi Air works according to the BBA model in which the type of tree is a very important factor in the success of filtering. Scientific research by (Alterra, Wageningen University) shows that not all Dutch tree/hedge types perform well in terms of air purification. On the contrary, some trees/hedges even emit more frequent substances that contribute to ozone creation (a reaction of the pollution already in the air with a naturally produced substance by the tree/hedge). Viridi Air only uses a range of tree species that have been scientifically proven to filter the air above average. The filtering principle of coniferous and deciduous trees is different, as is their ability to do so. In addition to the degree of filtering, biodiversity is very important in order to keep trees and hedges healthy and future-proof. 

Ultrafine dust

Coniferous trees in particular are used to capture fine dust. We briefly explain the difference at the bottom. 

Coniferous trees

Conifers consist of several branches to which needles are attached. The total contact surface of the needles together is much larger than in a tree consisting of leaves. The needles contain resin, which is very sticky and thus acts as a glue when dust bumps into them. When it rains, or the needle tree is damp, the absorption of fine dust is even higher. Another positive factor is the electrostatic conductivity of the needles versus air speed and the particles in the air. The faster the wind blows past the needles, the higher the electrostatic conductivity and the higher the uptake of airborne particles.
The needles remain green throughout the year, which is a great advantage compared to deciduous trees. In addition, the needles are reusable in filtering function. During a rainstorm or strong wind, the fine dust falls to the ground where it is absorbed and destroyed. A coniferous tree absorbs a gross amount of 10kg of fine dust per year, part of which is returned to the air. A coniferous tree absorbs a net 1.5kg of fine particles. 

Deciduous Trees

Trees that hold leaves have a different way of filtering the air. A leaf consists of several layers of which the outer one has a protective function by the microscopic layer of wax that is on top. This wax layer is sticky. When dust particles from the air hit the leaves, they stick to them because of this wax layer (adsorption). The larger the leaf, the length of time the leaf hangs on the tree during the year AND the number of leaves the tree contains (LAI: Leaf Area Index) influence the air-purifying effect. When considering the size of the leaf, it is important to note that a small tree with large leaves absorbs comparatively less than a large tree with many small leaves.
Looking at the leaf, a rougher surface (when the leaf contains small hairs) and the extent to which it can be electrostatically charged have a positive influence. External factors such as wind and humidity are also important. Higher moisture levels of the leaf AND faster wind which passes over the leaf; improves the mount of particulate matter which will adhere. The wind increases electrostatic conductivity, which attracts the dust that is positively or negatively charged. This is an important finding as it means that trees in sheltered locations, such as cities, have little positive effect on air quality.
In the event of wind or rain, a large proportion falls to the ground and is absorbed there. This also creates space on the leaf to absorb new particles. The reusability of a leaf is therefore high!

The disadvantage is that deciduous trees lose their leaves in the winter, so it is better to use conifers for fine particles. 

CO2 and toxic gases (NO2, NOx)

Both coniferous and deciduous trees can absorb carbon dioxide (CO2) and convert it into oxygen (O2), but for the other gases it is a different story. We explain this briefly at the bottom:
Coniferous trees
Airborne gases such as CO2 and toxic gases are absorbed by conifers, but not as effectively, as gases prefer to make contact with large, flat and smooth surfaces. The needles on conifers have less of these characteristics. They therefore, do absorb polluting gases, but to a limited extent. 

Deciduous trees

The leaves of deciduous trees contain small stomata that open wide under ideal conditions (moisture and sunlight). The further the stomata are open, the better the absorption of gases. CO2 is absorbed and converted to oxygen (O2) through the photosynthesis process. In unfavourable weather conditions, such as drought and little sunshine in the case of a tree, the stomata close further and take up relatively little gas.
In addition to CO2, the air unfortunately also contains many other harmful substances (ozone, NOx, etc.) that trees can capture. The degree of filtering differs per tree. Trees with leaves that have many stomata do a much better job than trees with few stomata. In contrast to particulate matter, leaves that are large, smooth and flat are crucial for the absorption of nitrogen dioxide. Small, rough, hairy leaves absorb less nitrogen dioxide. The wind also has an influence on this (as it does for fine dust). The more wind there is, the better the gases are absorbed. 

Method of planting & architecture

The selection of the right tree and hedge species is not the only important thing; the method of planting, the architecture, is also essential. The first year, when planting, the trees/ hedges are still small and planted close to each other. After a number of years, it is necessary to thin out and trees/hedges will be moved to other projects. Thinning gives the plot its future architecture. Viridi Air does not create traditional wooded areas but rather linear filtering rows.
The filter rows that are planted on the plots leave enough space in between each other. This has an extra purifying effect. Trees get a lot of light because of the space between them, which is necessary for growth, but also for the purifying effect. More space between the rows also means more wind that rubs against them and that helps the purification process by creating electrostatic conductivity and thus attracting fine dust to the leaves or needles.

The plots are planted according to two roughly defined structures/architecture:

  1. Funnel shape: several funnels can be laid out on the plot (depending on the most dominant wind direction). These are V-shapes where the corner is in the middle of the plot. There is thus a relatively large amount of open space so that the wind has free range.
  2. Rows which are laid out on the plot. Usually in multiple L and I shape. This still allows for a space build-up but there is more efficient planting on the plot.

Conclusion

There is a difference between the absorption of fine dust and gasses by coniferous and deciduous trees. Based on the above, the conclusion is that a healthy combination is optimal AND in an environment that is often humid and with a lot of wind. By planting the coniferous and deciduous trees scattered but together, filter rows will extract a lot of pollution. Quantitatively, the annual result (with mature specimens) of this filter principle is an improvement in outdoor air quality:

  • CO2 uptake – 22kg
  • Particulate matter 1,5kg
  • Ozone uptake 500gr
  • NOx uptake 200gr
  • Ammonia absorption 53% of what is emitted in that area

More information about this filter principle is available in the brochure “Trees, a breath of fresh air for the city”. The digital version is available from various sources on the internet (source: WUR).

NOTE: various studies done by placing trees along busy highways or in urban areas have little effect. In fact, they can increase pollution concentrations. This is mainly due to the fact that e.g., in urban areas, there is little wind, so less uptake is possible. Optimal effect requires optimal planting according to our BBA model. 

Scientific studies prove the filtering effect of tree/hedge species on outdoor air

Viridi Air filters outdoor air exclusively using a mix of certain coniferous and deciduous trees. The following scientific studies, among others, have shown this to be highly effective. Read the following articles, among others: