Our Air Quality
Urban forests help to improve our air quality. Heat from the earth
is trapped in the atmosphere due to high levels of carbon dioxide
(CO2) and other heat-trapping gases that prohibit it from releasing
the heat into space. This creates a phenomenon known today as the
“greenhouse effect.” Therefore, trees help by removing
(sequestering) CO2 from the atmosphere during photosynthesis to
form carbohydrates that are used in plant structure/function and
return oxygen back into the atmosphere as a byproduct. Roughly half
of the greenhouse effect is caused by CO2. Therefore, trees act
as carbon sinks, alleviating the greenhouse effect.
On average, one acre of new forest can sequester about 2.5 tons
of carbon annually. Young trees absorb CO2 at a rate of 13 pounds
per tree each year. Trees reach their most productive stage of carbon
storage at about 10 years at which point they are estimated to absorb
48 pounds of CO2 per year. At that rate, they release enough oxygen
back into the atmosphere to support two human beings. Planting 100
million trees could reduce an estimated 18 million tons of carbon
per year and consequently save American consumers $4 billion each
year on utility bills.
Trees also reduce the greenhouse effect by shading houses and office
buildings. This reduces the need for air conditioning by up to 30
percent which in turn reduces the amount of fossil fuels burned
to produce electricity. The combination of CO2 removal from the
atmosphere, carbon storage in wood and the cooling effect makes
trees extremely efficient tools in fighting the greenhouse effect.
Planting trees remains one of the most cost-effective means of drawing
excess CO2 from the atmosphere. If every American family planted
one tree, the amount of CO2 in the atmosphere would be reduced by
one billion pounds annually. This equates to almost 5 percent of
the amount that human activity pumps into the atmosphere each year.
It is estimated by the U.S. Forest Service that all the forests
in the United States, combined, sequestered approximately 309 million
tons of carbon each year from 1952 - 1992, offsetting approximately
25 percent of human-caused emissions of carbon during that period.
Over a 50-year lifespan, a tree generates almost $32,000 worth of
oxygen, providing $62,000 worth of air pollution control. This tree
would also be responsible for recycling $37,500 worth of water and
controlling $31,000 worth of soil erosion.
The Worldwatch Institute, in its Reforesting the Earth
paper, estimated that the earth needs at least 321 million acres
of trees planted just to restore and maintain the productivity of
soil and water resources, annually remove 780 million tons of carbon
from the atmosphere and meet industrial and fuel wood needs in the
third world. For every ton of new-wood growth, about 1.5 tons of
CO2 are removed from the air and 1.07 tons of life-giving oxygen
Trees also remove other gaseous pollutants through the stomata
in the leaf surface by absorbing them with normal air components.
Some of the other major air pollutants and their primary sources
• Sulfur Dioxide (SO2) – Sixty percent of sulfur
dioxide comes from coal burning for electricity and home heating
while 21 percent comes from refining and the combustion of petroleum
• Ozone (O3) – Ozone is a naturally occurring oxidant
that exists in the upper atmosphere. O3 may be brought to Earth
by turbulence during severe storms. Also, small amounts are formed
by lightning. Automobile emissions and industrial emissions mix
in the air and undergo photochemical reactions in sunlight releasing
ozone and another oxidant, peroxyacetylnitrate (PAN). Naturally,
high concentrations of these two oxidants build up where there
are many automobiles.
• Nitrogen Oxides (NOx) – Probably the largest producer
of nitrogen oxide is automotive exhaust. These are also formed
by high temperature combustion when two natural air components
are present; nitrogen and oxygen.
• Particulates – These are small particles emitted
in smoke from burning fuel, particularly diesel, which enters
our lungs and causes respiratory problems. With trees present,
there is up to a 60 percent reduction in street-level particulates.
Studies have shown that in one urban park, tree cover removed 48
pounds of particulates, 9 pounds of nitrogen dioxide, 6 pounds of
sulfur dioxide, 0.5 pounds of carbon monoxide and 100 pounds of
carbon – daily. It has also been noted that one sugar maple
along a roadway removes 60mg cadmium, 140mg chromium, 820mg nickel
and 5,200mg lead from the environment in one growing season.