Lecture Notes from CHM 1341
22 July 1996

Environmental Chemistry

Natural Cycles

Environmental issues are almost synonymous with chemical pollution. We all speak of destruction of the environment as if Nature were fragile; She is most definitely NOT. Species may be fragile, but natural cycles and even Life as a generic process is almost immune to our thoughtlessness.

Some of the natural cycles are beyond our (current) ability to damage, and that irritates us no end; "everybody talks about the Weather, but nobody does anything about it!" So the weather, intimately linked to the Hydrological Cycle, resists our best efforts to damage it.

The Hydrological Cycle is the familiar Evaporation / Transport / Precipitation game which has been going on since the Earth cooled sufficiently to permit water condensation. We can certainly make Herculean efforts to mess it up, e.g., pollute the rain with industrial acids, dam or redirect the rivers (it's been a long time since the Colorado ran to the Gulf of California), etc., but rain keeps falling nonetheless.

It might be worried that Global Warming (see below) would dry up the rain; not a bit of it. It'll wreck havoc on current rainfall patterns, but there'll still be high (clear) and low (cloudy) pressure atmospheric regions...indeed, exactly 50% of both (check out any satellite image of the Earth!)...so the rain will come down somewhere. For every molecule of air ascending in a Low, there must be one descending in a High; otherwise, we'd've lost the atmosphere by now! So the only way to lose rain is to get too hot to condense water; not bloody likely.

Furthermore, the Giant Gyres (oceanic currents) which transport more solar energy around the world than does the weather (!) will continue to respond to tropic heating, arctic cooling, and the Earth's rotation, none of which we've a snowflake's chance in Hell of influencing. So does all this mean that our meddling is crashingly irrelevant? That Nature just comes to yet another equilibrium?

Absolutely!

But you won't like it. It could easily be an equilibrium in which the cockroaches inherit the Earth. And they're not the Meek we had in mind!

So it's important to understand the current equilibrium in which Humanity is in ascendance. And only if we want to stay that way, should we become environmentally conscious.

Take a look at this Earth with its tan stretches of North African deserts seen on the upper limb. They didn't used to be there. Instead, ancient Man grazed then overgrazed his sheep in Sahara grasslands and turned them thereby into the Sahara Desert! It didn't stop raining; it stopped raining there! There wasn't any vegetation left to hold the water to support an evaporative cycle. The rain moved off into the Indian Ocean.

Now the Bedou to whom this happened simply changed their lifestyle; it would never have occured to them that something was amiss, so gradually did the grasslands recede. But with the advent of mechanization, we can see the same effect in a generation; witness the American Dust Bowl.

But as a species, we're not terribly fond of such surprises. So it behooves us to learn to avoid them rather than to react to them.

The Carbon Cycle

Unlike the water cycle, the Carbon Cycle seems to be responding measureably on a global scale to our interference. This cycle involves the growth and decay of vegetation. In spring, plants take up extra CO2 from the atmosphere, reducing it be a few percent, and then in the winter, as vegetation decays, soil bacteria release the CO2 to the atmosphere again.

The atmospheric CO2 influencing not only the biosphere but the geosphere as well. Dissolved in rain water, CO2 becomes carbonic acid, H2CO3, at a pH of about 5.6. This mild acidity slowly dissolves the mountains, carrying their calcium ions along with the resulting hydrogencarbonate ions, HCO3-, down to the sea. There, at a pH of near 8, calcium carbonate, CaCO3, is used by corals and shelled marine life to armor themselves. Their dead bodies become the limestone which geologic forces uplift into mountains to continue the cycle.

It would appear that CO2 is nicely balanced between the short photosynthetic and the long geological deposit cycles, but, in fact, it is on a definite rise! Fossil fuels in which prehistoric carbon has been trapped, are being burned in exponentially increasing quantities, shifting those equilibria.

The oceans cannot bury the resultant CO2 fast enough to keep up. And the plants cannot take up the slack because we are torching the rain forests to make room (and crops) for more equatorial peoples.

So far the increase (although exponentiating) is only about 30%. And plants (still left) everywhere should be rejoicing in this new resource! Unfortunately, the CO2 has a side effect. Like water, it is an excellent absorber of infrared radiation. Those and other IR-absorbing molecules in the atmosphere interfere with the Earth's heat balance in a very fortunate way. Low entropic, high frequency photons from the sun fall on the Earth and are converted to high entropic, low frequency (IR) photons which radiate back into space. This radiation balance would leave us too cold but for the "Greenhouse Gases" like water and CO2 which intercept the IR photons on their way out and reradiate ½ of them back to Earth. (The other ½ get reradiated on an out-bound path, perhaps to be recycled by other CO2 molecules.)

This is a good thing, because it renders our climate much warmer than it would be without the Greenhouse Effect. But it becomes a bad thing if it gets out of hand, as it has on Venus, where the 100 atm of CO2 keeps the cytherean surface at 800°F! (Cytherean refers to Cyprus, the legendary birthplace of Venus. Remember Botticelli?)

And it's getting out of hand.

CO2 is rising; that must raise the average equilibrium temperature of the Earth. But the rise is not yet sufficiently dramatic to have anyone but the scientists screaming. Still there are unquestioned effects of a global warming to the point where nations of the world (especially island nations slated to vanish as polar caps recede) are calling for reductions in Greenhouse Gases. Just this month (July '96), the United States reversed its stand against any such action and is now calling for a slowdown in production of such gases, primarily fossil fuel combustion product CO2.

This is a bold move, guaranteed to bring out the Oil Lobby!

While the current global temperature increase is estimated to be only about ½°C, the CO2 is increasing exponentially which means that it won't be long before the increase will be in the 3-5°C range associated with prehistoric climate changes! Even before that, relatively subtle effects will inundate coastal cities, and population maps will show you that that's where most of humanity lives. More significantly, the rainfall pattern shifts mentioned above will render once fertile regions arid (and once arid regions fertile, no doubt, but the current Have-Rains are not going to be amused).

Ozone Cycle

Meanwhile back at the stratosphere, the
         UV-C photons
     O2  ------------> O + O

and  O + O2 + M  ------> O3 + M*   (M is any molecule capable
                                   of absorbing dHrecombination)
         UV-B photons
and  O3  ------------> O2* + O
is working cyclically (see the UV-B absorption generates more O atom to make more O3, etc., etc.) to intercept the vast majority of solar UV light before it reaches sea-level. It's that absorbance too which generates hot molecules (M* and O2*) which make the stratosphere warmer than the expansion worked-out atmospheric gases have a right to be at that altitude.

This temperature inversion is what keeps even the most powerful thunderstorms (capable of lofting softball-sized hail, mind you) from rising above the troposphere. They flatiron out (into the "anvil") at the "tropopause" which marks the beginning of the stratosphere; so watch a thunderhead next storm and you'll see where the stratosphere starts!

Thus, ozone protects us from hard UV and even nastier storms than we might otherwise have.

We don't return the favor.

That cyclic destruction of UV via regeneration of "odd oxygen" (O and O3), can be choked by "scavenging" of odd oxygen...in other words, by molecules which destroy odd oxygen.

One such molecule is "natural" in that it arises from biogenic sources other than human activities. It is NOX, so called to encompass both NO and NO2. These molecules are soluble in rain water (see ACID RAIN below), and are thus efficiently rained out of the troposphere (where they arise in lightning stokes). But another, N2O, a product of decay in soil, survives rainout to enter the stratosphere and
     N2O + O -----> 2 NO

     NO + O3 -----> NO2 + O2

     NO2 + O -----> NO + O2
eat odd oxygen in a catalytic (regenerative) cycle. Our stratospheric jet travel adds NOX directly into the stratosphere where it is most damaging. Fortunately, the fleet of SSTs never materialized; otherwise this would have been a grim scenario.

Unfortunately, NOX isn't the only ozone predator. One unknown to Earth, until developed by chemists as a non-toxic refrigerant (compared to the NH3 we used to use), was produced (and released) in large quantities. It is actually a class of molecules called chlorofluorocarbons (CFCs) typified by CCl2F2. It is not only chemically inert (non-toxic) but also virtually insoluble in water (doesn't rainout) and thus migrates eventually to the stratosphere. There
            UV-C photons
     CCl2F2 ------------> .CClF2 + .Cl

     Cl + O3 -----> ClO + O2

     ClO + O -----> Cl + O2
catalytically destroys odd oxygen as well.

ClO has been found in the infamous Ozone Hole over Antarctica (and now the Arctic) in direct correlation with significant decline in O3 there. That decline is working its way away from the poles and will soon be felt worldwide as an increase in surface UV light. While individuals may well worry about the 2% increase in skin cancer for every 1% increase in UV, even more serious disruptions are in store. Life only left the oceans after the Ozone Layer had rendered the surface non-germicidal! Life forms, including crops, will suffer as will all who depend upon them. There's no sunblock for wheat.

More chilling still is the expectation that were the nations of the Earth to stop making CFCs today, the Ozone Layer wouldn't recover from the assault for almost a century. This is due to the absence of convective mixing in the stratosphere; it is, as its name implies, stratified in altitude, not mixed as is the troposphere.

Fortunately, even Dupont Chemicals now recognizes the importance of ceasing production of this stuff. New, non-ozone-polluting refrigerants have been created to take over from CFC. So we need only wait 3 generations to get our UV security blanket back.

Acid Rain and Smog

Compared to Global Warming and UV Frying, acid rain and urban air pollution are really small potatoes! They result in unpleasant molecules which the hydrological cycle cleanses easily. So their effects are largely local; countries foul their own nest but rarely their neighbors. There's a certain poetic justice in that. The closely-packed European nation states might think otherwise since their proximity means that they're fouling one another fairly regularly.

Still, as cases of chemical reactions gone unintendedly haywire, they're fine examples. Take acid rain, for example. Both NO2 and SO3 are anhydrides of strong acids via
     3 NO2 + H2O -----> NO + 2 HNO3  (note disproportionation redox)
and
       SO3 + H2O -----> H2SO4
which cause more than corrosive mischief in the environment. Their acidity (pH 3 is common) not only kills fish and virtually everything else in granite-bottom lakes (no muck there to neutralize the onslaught), but it also changes the acid-base equilibrium balance of all compounds. Amphoteric (acid/base schizophrenic) aluminum is released from its oxides to poison plant biochemistry; whole forests die as a result.

NO2 comes naturally from lightning strokes through N2, but internal combustion engines churn out more. SO3 originates in volcanoes, but even more is produced in the smelting and (sulfur-rich) coal-burning industries. Of course, this pollution could be stopped tomorrow since the pollutants can be trivially neutralized at their source. Matter of economics.

Ironically, smog results from our friend ozone in the wrong place. Stratospheric ozone is absolutely essential, but tropospheric ozone is a nuisance to all and a killer of the young and the elderly both of whose lungs are not as robust as are those of rest of the population. Smog starts with the morning rush of NOX, CO, and unburnt hydrocarbons on our nation's commuter highways. The photons of the rising sun slowly oxidize (via OH radical intermediates) the NO to brown NO2 (smog's colorant), and the NOX chemistry creates not only ozone but also organic ozonides which are just as lethal.

The fix is less compelling, more economically disruptive, but necessary for other reasons in the long run. The culprit here is the burning of fossil fuels. They are a finite resource; too few dinosaurs are dying, at present, to replace these reserves! (Damned inconsiderate of them to have kicked off 80 million years ago.) So we'll be obliged to find another mobile energy source. Batteries are the current cool alternative, but I hold out for hydrogen fuel cells. The reaction product there, pure water, is the most benign possible. And pound for pound, standing first in the Periodic Table, hydrogen gives you the most dH bang.

So start investing in mossy metal batteries...a stock tip for the next century!

Have a Happy Summer!

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Chris Parr
University of Texas at Dallas
Programs in Chemistry, Room BE3.506
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Richardson, TX  75083-0688

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Last modified 13 September 1996.