Improving air quality through the capture of particulate matter

Description

Vegetation filters various pollutants from the air. Suspended particulate matter comes into contact with leaves and branches, settles on them and will then be washed to the ground by rain. The leaves can absorb gaseous pollutants such as ozone and nitrogen oxides through their stomata. A wax layer (the cuticle) on the leaves can absorb volatile components, such as PCBs and dioxins, through adsorption. Ammonia (NH₃) is deposited in the form of ammonium (NH₄+) on the leaves, washes off there during rain and is converted into nitric acid in the soil, causing the soil becomes acidic.

We limit ourselves to the capture of particulate matter because this pollutant is responsible for approximately 60% of the total burden of disease caused by environmental pollution (measured in terms of years of healthy life lost) (MIRA, background document environment, human and health, 2007) and because for other pollutants, little information is available, which is certainly not useful for all land use types in the tool.

Required information:

• Number of hectares per vegetation type
• PM₁₀ concentrations in the area.

Qualitative valuation

The contribution of vegetation to filtering pollutants depends on the type of vegetation, the type of contamination, the location and placement of the vegetation. Of all vegetation types, trees are the most effective at capturing harmful substances, followed in order of decreasing effectiveness by shrubs, herbaceous plants and grasses respectively. Therefore, a score was drawn up based on the deposition rate of particulate matter on vegetation (ECOPLAN, 2016).

Quantitative valuation

The core of the analysis is the estimation of deposition rates. These are based on VITO's own modeling with OPS and testing against data from the literature (Nowak, 2014, Schaubroeck, 2015; Liquete 2015). For grasses and deciduous trees, the VITO modeling fits well with measurements and model studies from the literature. For coniferous forests, the OPS estimates have been adjusted upwards based on a recent, detailed model study by Ghent University for coniferous forests in Flanders, and this value is more in line with those from the literature. The key figures for shrubs and water are more uncertain. For water, the estimate from OPS has been adjusted downwards, and an average of the value from OPS and the JRC model MAPPE (Liquete 2015) has been used.

For ecosystem types for which no measurement data were available, we extrapolated the existing figures for fields, grassland, shrubs and forests, depending on the vegetation type in the ecosystem in question (e.g. heathland was equated with shrubs).

The following formula is used to determine the net capture:

Formula

Capture in kg/ha.year = (deposition rate (cm/s) x PM₁₀ concentration (µg/m³) x 3.1536) x (1-50% resuspension)

In general, these values are 3 to 4 times lower than in the Nature Value Explorer 1.0. This is partly explained by different estimates of the deposition rates and because we now take better account of the current air quality (concentrations of particulate matter have decreased compared to the situation in older studies) and resuspension of 50%.

Monetary valuation

The most recent key figures have been calculated for the Environmental Prices manual by CE Delft (2017). The figures build on studies and key figures on the damage to human health and well-being, damage to buildings and machines and damage to ecosystems due to particulate matter emissions, which were developed in the context of European and Dutch study programs.

Based on this manual, we use a unit cost between € 33.76 and € 73.36 with a central value of € 47.35 (2019 prices) per kg PM₁₀.

Assumptions

• Resuspension: varies from 0% (not included), over 50% to 75% (Schaubroeck, 2015). Most models use 50% resuspension (i-tree (Nowak 2015), Oosterbaan 2006, 2011; Liquete 2015)
• The above-mentioned studies all use approximately the same principles with regard to the effect of vegetation and deposition rates. Deposition rates on buildings are not included
• The dose-effect relationships for estimating the consequences of air pollution by particulate matter can be applied to the particulate matter captured by vegetation.
• Smaller particles are more harmful and have a higher share of air pollution (60%), but they are less well captured by vegetation.
• Caution is required when using the figures for vegetation close to traffic and in heavily built-up areas. The available studies indicate that in these environments several factors have opposing effects. Especially the fact that on the one hand vegetation can capture more pollutants (at higher concentrations) but on the other hand vegetation can also locally limit dilution (street canyon effect) due to effects on local weather. We go into this in more detail in the Nature Value Explorer city.

Numbers to use

Table: qualitative, quantitative and monetary valuation of the capture of particulate matter

Vegetation type	Qualitative score	Deposition rate (cm/s)	Value (€/kg)
Grasslands and tall herbs	4	0.20	€47.35
Deciduous forests	7	0.5	€47.35
Coniferous forests	10	0.7	€47.35
Mixed forests	8	0.6	€47.35
Heathland	5	0.3	€47.35
Shrubs	6	0.344	€47.35
Wetlands reeds	4	0.263	€47.35
Wetlands other vegetation	3	0.2	€47.35
Flat plains and marshes	3	0.2	€47.35
Rivers and lakes	2	0.10	€47.35
Crops	3	0.2	€47.35
Meadow	4	0.2	€47.35
High density orchard	5	0.3	€47.35
Traditional orchard	7	0.5	€47.35
Wasteland or agricultural road	2	0.1	€47.35
Sparsely vegetated land	2	0.1	€47.35
Urban land use	1	0	€47.35

Translation to an indicator

We translate the number of kg that the vegetation captures more or less in the new scenario, into the number of people who would emit this capture and into the number of kilometers you would drive by car to emit the amount that is additionally captured by the vegetation.

Average PM₁₀ emissions for an average European per year = 4.9 kg Average PM₁₀ emissions per average car km = 0.0349 g (COPERT)

An example

For the example, we refer to the Dutch version of the manual.

More detailed models/tools

The VMM measuring network measures the particulate matter concentrations in Flanders at a fixed number of places. In addition, computer models are also used to further map air quality. VITO has extensive expertise in this area. On the one hand, this can be to obtain an estimate of the concentrations at each location in Flanders based on the measured values. On the other hand, different types of models are also used for predictions and scenario analyzes (which measure leads to the best air quality?). In a number of these dispersion models, the filtering effect of vegetation and ecosystems is also taken into account. These models make it possible to accurately estimate the extent to which ecosystems contribute to improving air quality. Differences throughout the year and the effect of seasons or extreme weather conditions can also be included in model studies. In general, one can say that such modeling studies include location-specific information in greater detail, resulting in more accurate estimates and more targeted recommendations. Such model studies require a certain amount of time and budget. For more information, contact VITO (felix.deutsch@vito.be).