This news item was created by students Ian Dubman, Michelle Meiklejohn, Sabrina Kerr, Jelena Kocergin and Jennifer R Campbell as part of their Chemistry 210 Semester Project in WS99 under the guidance of Prof. Rainer Glaser.

Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 7. Structure and Synthesis of Alkenes.


For each of the following questions, please refer to the following article:
RETURN OF THE KILLER TREES
By Henry Fountain, New York Times, November 3, 1998)


Editorial Comments

When most people hear the word "aerosols," immediately air pollution and the ozone layer come to mind. Indeed, the complex chemical interactions of aerosols with the ozone layer have important implications for our environment, and a great site with more information on this subject can be found at the University of Cambridge's Ozone Hole Tour. Yet, surprisingly, not all organic aerosols have such an adverse effect on the ozone. Unfortunately, organic aerosols do not get the media attention of its unfriendly counterpart, so their role and impact on our environment is a bit more obscure.

It is well-known that plants emit certain organic compounds into the atmosphere, the most ubiquitous among these being oxygen. It is lesser known, however, that plants emit characteristic compounds that are unique for each particular species. These biogenic emissions range from small molecules such as NOx to larger non-methane hydrocarbons. Collectively, these aerosols would be called Aitken nuclei or cloud condensation nuclei.

You might ask, "Yeah, but if these aerosols are not tearing up ozone layer then why are they important?" The answer to this crucial question is that all climates depend on the lack of or presence of clouds. Clouds depend on the Aitken nuclei as a site for water droplets to condense. Thus, as the amount of aerosols in the atmosphere increase the number of clouds increases with it. As stated in the article, increased concentration of certain kind of trees lead to increased concentration of aerosols.

Eucalyptus globulus was the target species for a study by Euripides Stephanou and Nikos Michalopoulos at the University of Crete. It was found that the most significant biogenic emission of these species is alpha-pinene. This molecule is oxidized in the atmosphere to produce cis- and trans-pinonic acid, the main components of the aerosols mentioned in the article. How do we know the aerosols were in fact emitted by the eucalyptus globulus? It was determined that the aerosols were produced by the eucalyptus trees from measuring the mean diameter of the Aitken nuclei when their concentration was found to be greatest. Interestingly enough, the concentration maxima mean was found to occur in the afternoon, corresponding to a peak time for photosynthesis. The mean diameter of the Aitken nuclei, at this time, was found to be 20-40nm. Particles with this diameter have a lifetime of only a few minutes, nullifying the possibility of long-range transport.

Thus, by the emission of these organic aerosols, trees increase the concentration of Aitken nuclei in the atmosphere which directly effect the global cloud cover. The net effect that cloud cover has on the climate is still being studied intensely, but all-in-all we can say that the biogenic emissions of plants impacts our environment.


Pertinent Text References
Chapter 7: Structure and Synthesis of Alkenes


Questions

Question 1: In order for clouds to form, cooling air must be:
A) saturated and have no condensation nuclei
B) saturated with condensation nuclei
C) unsaturated and no condensation nuclei
D) unsaturated with condensation nuclei

A. (B) is correct answer. Air is full of water vapor and has little particle of condensation nuclei to condense on to form clouds.


Question 2: Write the oxidation reaction relevant to this article(hint: use Chem Finder to find the structures of the molecules)

A.
+ <=====>
Alpha Pinene
Ozone
Pinonic Acid

Question 3: Continuous measurements of aerosol concentration were perform in the experiment. When was the concentration the highest and why?

A. The concentration was the highest in the afternoon. The reason for that is that hydrocarbons must be oxidized into organic acids by the sun and the sunlight is more intense in the afternoon.


Question 4: In the article it is suggested that aerosols may be a climate-creator. Explain what climate effect could be caused by aerosols and how.


A. Aerosols can influence climate in two ways. Directly, increase in aerosol concentration causes more sunlight to be reflected back into space. Since more solar radiation is reflected, less energy is available for earth, so the results of direct influence of aerosols on climate is general cooling.
Aerosols can also influence climate indirectly, through cloud formation. Increase in aerosol concentration produces thicker clouds. As a result of this, two things can occur: 1) Cooling - since reflected solar radiation goes back to space. 2) So-called "blanket effect" - thicker clouds trap in more long wave radiation(radiation in infra red part of the spectrum); radiation emitted from the ground is reflected back to the surface, and clouds act as a blanket.
Which effect will be dominant(cooling or blanket effect) is not clear. It changes from season to season and place to place, so it is hard to predict what change aerosols will actually cause in indirect way.


Question 5: From reading this article and its different links, do you think biogenic emissions should be given much consideration when discussing the problem of pollution? Why?


A. In the article, it is suggested that aerosols participation in pollution is not as important as their role in climate effect. However, this is an open ended question with no right or wrong answer. Based on your knowledge about aerosols you should make decision and justify your answer.