Facing Climate Change: Urban Gardening and Sustainable …
407
as the solubility of the pollutant in water increases. Hydrogen fluoride, sulphur dioxide, nitrogen dioxide, and ozone which are soluble and reactive are readily sorbet
pollutants. Nitric oxide and carbon monoxide, which are very insoluble, are absorbed
relatively slowly or not at all by vegetation.
During daylight periods when plant leaves are releasing water vapor and taking
up carbon dioxide, other gases, including trace pollutant gases, in the vicinity of the
leaf are also taken up through the stomata [178]. The rate of pollutant gas transfer
from the atmosphere to interior leaf cells is regulated by the resistance posed by
atmospheric, stomatal, and mesophilic factors, more specifically wind speed, leaf size
and geometry, gas viscosity and diffusivity. Stomatal resistance is regulated by the
stomatal aperture, which is influenced by water deficit, carbon dioxide concentration,
and light intensity. Mesophilic resistance is regulated by gas solubility in water, gasliquid diffusion, and leaf metabolism [179]. Because the rate of pollutant uptake
is regulated by numerous forces and conditions, the rate of removal under field
conditions is highly variable.
If leaf characteristics, wind speed, atmospheric moisture, temperature, and light
intensity are quantified, the pollutant uptake rate can be estimated. Under growthchamber conditions, wind velocity, canopy height, and light intensity were shown
to affect the rate of pollutant removal by vegetation. As previously stressed, light
plays a critical role in determining physiological activities of the leaf and stomatal
opening, and as such exerts a great influence on the foliar removal of pollutants.
Under conditions of adequate soil moisture, however, pollutant uptake by vegetation
was judged almost constant throughout the day, as the stomata were fully open.
Pollutants were absorbed most efficiently by plant foliage near the canopy surface
where light-mediated metabolic and pollutant diffusivity rates were greatest.
Sulphur and nitrogen dioxides were taken up by respiring leaves in the dark,
but uptake rates were greatly reduced relative to rates in the light. The Removal of
sulphur dioxide is also described by tree uptake process. Because of its high solubility
in water, large amounts of sulphur dioxide are absorbed into external tree surfaces
when they are wet. In the dry condition, sulphur dioxide is readily absorbed by tree
leaves and is rapidly oxidised to sulphate in mesophyll cells [178].
18.3.1 All Plants Absorb Carbon dioxide, but Trees are Best
While all living plant matter absorbs CO 2 as part of photosynthesis, trees process
significantly more than smaller plants due to their large size and extensive root
structures.
In essence, trees, as kings of the plant world, have much more “woody biomass”
to store CO 2 than smaller plants, and as a result, are considered nature’s most efficient “carbon sinks”. It is this characteristic which makes planting trees a form of
climate change mitigation. According to the U.S. Department of Energy (DOE), tree
species that grow quickly and live long are ideal carbon sinks. Unfortunately, these
two attributes are usually mutually exclusive. Given a choice, foresters interested in
maximizing the absorption and storage of CO 2 (known as “carbon sequestration”)
407
as the solubility of the pollutant in water increases. Hydrogen fluoride, sulphur dioxide, nitrogen dioxide, and ozone which are soluble and reactive are readily sorbet
pollutants. Nitric oxide and carbon monoxide, which are very insoluble, are absorbed
relatively slowly or not at all by vegetation.
During daylight periods when plant leaves are releasing water vapor and taking
up carbon dioxide, other gases, including trace pollutant gases, in the vicinity of the
leaf are also taken up through the stomata [178]. The rate of pollutant gas transfer
from the atmosphere to interior leaf cells is regulated by the resistance posed by
atmospheric, stomatal, and mesophilic factors, more specifically wind speed, leaf size
and geometry, gas viscosity and diffusivity. Stomatal resistance is regulated by the
stomatal aperture, which is influenced by water deficit, carbon dioxide concentration,
and light intensity. Mesophilic resistance is regulated by gas solubility in water, gasliquid diffusion, and leaf metabolism [179]. Because the rate of pollutant uptake
is regulated by numerous forces and conditions, the rate of removal under field
conditions is highly variable.
If leaf characteristics, wind speed, atmospheric moisture, temperature, and light
intensity are quantified, the pollutant uptake rate can be estimated. Under growthchamber conditions, wind velocity, canopy height, and light intensity were shown
to affect the rate of pollutant removal by vegetation. As previously stressed, light
plays a critical role in determining physiological activities of the leaf and stomatal
opening, and as such exerts a great influence on the foliar removal of pollutants.
Under conditions of adequate soil moisture, however, pollutant uptake by vegetation
was judged almost constant throughout the day, as the stomata were fully open.
Pollutants were absorbed most efficiently by plant foliage near the canopy surface
where light-mediated metabolic and pollutant diffusivity rates were greatest.
Sulphur and nitrogen dioxides were taken up by respiring leaves in the dark,
but uptake rates were greatly reduced relative to rates in the light. The Removal of
sulphur dioxide is also described by tree uptake process. Because of its high solubility
in water, large amounts of sulphur dioxide are absorbed into external tree surfaces
when they are wet. In the dry condition, sulphur dioxide is readily absorbed by tree
leaves and is rapidly oxidised to sulphate in mesophyll cells [178].
18.3.1 All Plants Absorb Carbon dioxide, but Trees are Best
While all living plant matter absorbs CO 2 as part of photosynthesis, trees process
significantly more than smaller plants due to their large size and extensive root
structures.
In essence, trees, as kings of the plant world, have much more “woody biomass”
to store CO 2 than smaller plants, and as a result, are considered nature’s most efficient “carbon sinks”. It is this characteristic which makes planting trees a form of
climate change mitigation. According to the U.S. Department of Energy (DOE), tree
species that grow quickly and live long are ideal carbon sinks. Unfortunately, these
two attributes are usually mutually exclusive. Given a choice, foresters interested in
maximizing the absorption and storage of CO 2 (known as “carbon sequestration”)