Conclusions
The effect of vegetation on urban air quality depends on vegetation
design and on level of air pollution in the area. This review
identified the following vegetation design considerations based on
air quality arguments:
1. Dilution of emissions with clean air from aloft is crucial; the
vegetation should thus preferably low and/or close to surfaces.
2. Proximity to the pollution source increases concentrations of air
pollutants and thus deposition; vegetation should be close to
the source.
3. Air passing above, and not through, vegetation is not filtered;
barriers should be high enough and porous enough to let the air
through, but solid enough to allow the air to pass close to the
surface.
Other interesting findings are that deposition of coarse particles
is more efficient at high wind speeds, while the opposite is true for
ultrafine particles; and that vegetation density often changes due to
strong winds. To improve deposition, the vegetation should be
hairy and have a large leaf area index, but still be possible to
penetrate.
7.1. Research outlook
Dispersion and deposition related to vegetation in urban areas
are both interesting and vivid research areas. This review suggests
that these areas be further combined, as the environmental problem
in which they interact, urban air quality, is crucial to human
health and results are rapidly transferred into policy. Thus, results
from one area must be modified with results from the other before
action is taken in urban planning.
The effect of non-foliated vegetation during wintertime needs
further studies, as they might have an impact e.g. in northern
countries with air quality problems during winter and spring. In
these areas, air inversion during wintertime often limits dilution, so
pollution levels might be high. There is also a yearly variation, with
different particle sizes being most important during different parts
of the year.
The deposition process differs substantially between different
particle sizes and detailed interactions with various vegetation elements
require combined studies of different particle sizes
together with different plant species.
Barriers are important for experimental data collection due to
their simple geometry, which is a requirement for detailed deposition
and dilution studies. The possibility of studying roads where
a barrier is present along only a part of the road, providing surroundings
and emissions that are similar for stretches with and
without vegetation, is very important. Barrier studies can give great
insights into differences between pollutants and between different
kinds of vegetation.
The description of the vegetation is important, as recommendations
can scarcely include all available species, but must group
them in some way. Parameters such as hairiness, stickiness, LAI,
thickness of leaves etc., but also porosity and the species in question,
are described in the literature. Vegetation can interact with air
pollutants in more ways than these, however, e.g. through emissions
from vegetation and active uptake of water and nutrients.
Therefore studies of vegetation effects need to include these other
factors before vegetation implementation in urban planning can be
efficient.
This review examined the deposition and dispersion of particle
pollution of all size classes and showed that the effects of urban
vegetation on local air quality are complex, so different disciplines
must work together to identify these effects. Such work must be
described in great detail, as we do not yet understand all the parameters
influencing the effects of vegetation on air pollution.