Ward Burgess
David Collins
Dewey Dixon
Autumn Leaves: A Contributor to Air Pollution?
In the United States we have been told that humans are responsible for all of the
pollution and filth on our planet. But humans are not to be held responsible for all of the
damage. When the production of smog is concerned, humans have an accomplice: plant
life. Specifically, chemicals emitted by colorful autumn leaves (volatile organic
compounds, or VOCs) react with nitrogen oxides produced by various industrial
processes. To find out more about this interesting phenomenon, go to
www.chemweb.com. Log in, and click on The Alchemist. On this page, there will be a
search option. Use “autumn leaves” as the keyword and you will arrive at an article
titled: Autumn Leaves: a Major Contributor to Air Pollution. This article explains how
VOCs react with human-produced nitrogen oxides to produce tropospheric ozone, the
major component of photochemical smog. It should be known that the production of this
tropospheric, or ground-level, ozone, occurs naturally in a process that proceeds as
follows: Sunlight induces the dissociation of nitrogen dioxide into NO and O. The free
radical O atom immediately combines with atmospheric oxygen to produce ozone (O3).
The ozone then reacts with the NO already generated to regenerate nitrogen dioxide
along with O2. A simple mechanistic model for this process is shown below; for more
information concerning the process, visit the following websites:
http://www.uni-giessen.de/biostress/ozone_and_vegetation.htm#o3-troposphere
http://www.environment.sa.gov.au/epa/pdfs/is_no21.pdf
(1) NO2 + hv → NO + O
(2) O + O2 → O3
(3) O3 + NO → NO2 + O2
However, when VOCs enter the stage, a problem is encountered: VOCs react with
NO as shown below:
(4) NO + VOC → NO2 + other products
If reaction 4 proceeds, the amount of O3 in the air builds up because there is a
scarcity of NO, which is necessary for the decomposition of ozone.
At this point, one might say, “Wait a minute. I thought people were trying to save the
ozone. Why are we now trying to get rid of it?” The answer to that question is that, while
ozone in the stratosphere is of great benefit to humans in that it absorbs harmful
ultraviolet radiation, tropospheric ozone is of great harm to man and plant life. See
http://registrar.ies.ncsu.edu/ol_2000/module6/ozone/formation/formation.htm for more
information concerning the two different types of ozone. Tropospheric ozone is a real
problem, and levels are expected to rise in the future. Levels are now closely monitored
over land and even over the sea by means of special buoy-mounted ozone detectors. For
more detail concerning this interesting technology, go to the end of the article at:
http://www.whoi.edu/science/MCG/people/ehintsa/Ozone/background.html
Besides being the main component of smog, ozone is irritating to the respiratory
system and concentrations as low as 0.1 parts per million can cut the rate of
photosynthesis in half. If ozone concentrations were to increase by an order of
magnitude, the effect on plants and animals would be catastrophic. Plants are
indispensable components in cycles that produce gases necessary for life to exist. A lower
rate of photosynthesis leads to stunted plant growth. Crop yields can be sharply reduced.
In addition, it can irritate the respiratory system in a number of ways. For more
information on the debilitating effects of ground-level ozone, log onto:
http://royal.okanagan.bc.ca/mpidwirn/atmosphereandclimate/smog.html
The following links will be helpful in providing additional information
concerning this subject:
(1) http://www.gsf.de/eurotrac/tropchem.htm
(2) http://artico.lma.fi.upm.es/chemistry.html
The following questions should test one’s understanding of the subject. Answers
are at the bottom of the page.
(1)
Briefly explain why tropospheric ozone levels drop off at night. (Chapter 4,
section 19 of text)
(2)
In the Northern Hemisphere, the levels of tropospheric ozone are highest
during the months of May through September, which is referred to as the
“ozone season.” Why?
(Chapter 4, section 19; the following link should also help.)
http://registrar.ies.ncsu.edu/ol_2000/module6/ozone/formation/formation.htm
(3)
Oxygen radicals are produced in step (1) of the reaction mechanism on the
previous page. Predict the manner in which the nitrogen-oxygen bond is
broken. (Chapter 4, section 17)
(4)
Many chemical reactions require a high temperature to proceed in appreciable
quantities. Why doesn’t this one require a high temperature? (Chapter 4,
section 19)
According to Alex Guenther of the National Center for Atmospheric Research,
VOCs really are not the main problem. He states that if the production of nitrogen oxides
were kept under control, VOCs would be perfectly harmless. This idea is thought-
provoking, but one must keep in mind practicality of such measures. So the question
remains: Is smog totally the fault of humans, or do plants share some of the blame? You
make the call.
Answers to questions:
(1) The production of ozone is a light-catalyzed process, so nitrogen dioxide dissociation
into nitric oxide and oxygen radical at night is virtually halted. Thus ozone and nitric
oxide react to form diatomic oxygen and nitrogen dioxide. So the amount of ozone in the
troposphere falls.
(2) The intensity of sunlight is greatest during this time period; these conditions are most
conducive to the production of ozone.
(3) Nitrogen dioxide undergoes a homolytic cleavage as it forms nitric oxide and oxygen.
(4) This reaction is a photochemical reaction; only light is required.