Monday, December 6, 2010

A Brief Introduction to Low-Level Ozone

Somewhat hidden among the concern regarding greenhouse gases and global warming is the steady increase in tropospheric ozone concentration. The increase in this low-level ozone (recall that the traditional ozone layer is located much higher in the atmosphere at the stratosphere) comes indirectly from most of the pollutants that are driving global warming. Typically sunlight reacts with hydrocarbons and nitrogen oxides forming ozone and other products. Since increased concentrations of hydrocarbons, nitrogen oxides and other volatile organic compounds (VOCs) are being released into the atmosphere ozone concentrations are increasing as well. The funny thing about this change in ozone concentration is the disparity in the reaction it receives. Most, even those that are aware of global warming, do not realize and/or care that low-level ozone is increasing and the potential danger it poses; however, some that are aware of it tend to completely and utterly overreact believing that the increasing ozone is in some way worse than global warming as a whole.

One important reason that ozone poses such a threat, beyond the obvious ‘it is toxic to various life forms’, is that the lifecycle of low tropospheric ozone tends to peak during the growing season due to the requirement of sunlight to drive the formation of new ozone from various pollutants. Although ozone concentrations peak during the growing season the overall concentration relative to the last year is still typically higher. The best way to understand how ozone concentrations are increasing is to visualize an oscillatory curve with a positive slope, a monthly x-axis, a peak for a given cycle during the growing season (May-September) and a trough during the Winter months (December-March) similar to the below figure. Note that the below figure is only designed to illustrate the pattern of ozone increase not to demonstrate any specific concentration change.

While it is true that increasing levels of tropospheric ozone damages various crops the scope of this damage is important to consider. For example a recent report out by NASA concluded that almost 2 billion dollars worth of damage to this year’s soybean crop could be attributed to tropospheric ozone.1 Following certain crops like soybean and tobacco is relevant because these crops, for currently undetermined reasons, have a greater level of sensitivity to ozone than other crops like corn and sorghum.

On its face 2 billion dollars may seem like a lot of money, and it is, but when one takes into consideration that the total value of the U.S. soybean crop over the last few years has been over 27 billion dollars,2 the severity of the situation loses momentum. In addition even though ozone pollution has increased over the last decade, soybean crops have also increased both in yield and monetary value. These realities demonstrate the importance of properly accessing the overall damage. Simply put to worry about ozone pollution over global warming is inappropriate.

With that said to assume that the damage potential of ozone pollution will remain static or only increase insignificantly over time is also inappropriate. Fortunately for all parties the simplest way to derail any potential significant problem with ozone pollution is the same strategy to prevent further environmental damage through global warming, significantly reduce greenhouse gas emissions. Unfortunately with the current political climate in the United States the probability that these reductions proceed at a timely pace is highly improbable. Some have suggested that including the dangers of ozone and its wide toxic breadth in the discussion of the threat provided by excess carbon emissions would facilitate greater urgency, but such a contention seems far-fetched.

The problem is that including ozone in the problems catalyzed by carbon emissions is similar to suggesting to an individual that they are going to be decapitated, set on fire, injected with cyanide and pierced through the heart instead of just decapitated, injected with cyanide and pierced through the heart, either way the person is going to die; adding one additional method is meaningless. Thus, with limited ability for action through reducing the source of the problem once again society must look towards technology to provide a means to stem the tide until more permanent action can be taken.

Under natural processes most ozone is eliminated in one of two ways. First, the ozone reacts with nitric oxides or hydrocarbons like aldehydes in the atmosphere. Normally the interacting agent is a nitric oxide creating a hydroxyl free radical that typically leads to the formation of peroxyacyl nitrates. Second, the ozone drops out of the air and is grounded to typically be absorbed by nearby fauna or soil. At lower concentrations this absorption is rather meaningless because the absorbing structure has sufficient recovery time; however, at higher concentrations absorption occurs at a higher turnover reducing the ability of the absorbing structure to recover. A general review regarding the general chemistry of tropospheric ozone can be found here.

Developing a device that would collect and store ozone from the atmosphere for later ground-based neutralization has a theoretically high level of application difficulty. Therefore, the best strategy may be to neutralize the ozone in the air itself. While such a statement may also seem to be quite difficult, there may be a simple work-a-round by tapping into a previously detrimental element. The Montreal Protocol was one of the most successful international treaties ever heavily limiting the utilization of and eventual release of Chlorofluorocarbons (CFCs) into the upper atmosphere to prevent further ozone layer degradation. The reason CFCs were so destructive to the ozone layer is that they chemically react as a free radical catalysis, which allows a single CFC molecule to dissociate thousands of ozone molecules, eventually forming one O- free radical and one molecule of oxygen. What if a device could be designed to utilize CFCs in a controlled fashion to facilitate the dissociation of low atmosphere ozone?

To begin the development of such a device a few important points need to be considered. First, it would be best if this device operated at worst with a trace emission power supply and at best produced no emissions in its operation. The reason for this objective is rather obvious because if the device produces air pollutants through the combustion of fossil fuels there is a high probability that it will end up producing more ozone than removing. Second, a further understanding of how CFCs react with ozone must be discussed to ensure an efficient and effective ozone interaction design is created.

When CFCs react with ozone the entire chemical reaction proceeds in 3 steps. The first reaction involves stripping the CFC molecule of one chloride ion after the CFC molecule is struck by UV radiation. The second reaction occurs between the newly freed chloride ion and ozone forming chlorine oxide and oxygen. The third reaction reforms the chloride ion through the reaction of the chlorine oxide molecule and free radical oxygen recreating another chloride ion and oxygen. The general treatment of the chloride ion as a destructive catalyst of ozone is one of the reasons why CFCs were so destructive to the stratospheric ozone layer.

Looking at the reaction scheme the important element to CFCs driven ozone destruction is the chloride ion. Therefore, actually utilizing CFCs is not necessary, instead only chloride ions are required. Due to the catalytic nature of these chloride ions relative to ozone molecules it is important to avoid directly releasing them into the atmosphere where they could migrate to higher altitudes and cause damage to stratospheric ozone. So the idea is to keep the chloride ions in an isolated environment away from the general atmosphere, but available to react with low atmospheric ozone. One means to accomplish this goal is to construct a blimp, which can move through the low atmosphere and carry a chloride ion storage methodology.

A number of storage possibilities exist for the chloride ions. For example one method would involve creating a ‘wind tunnel’ where air could pass through a portion of the blimp in only one direction. In this ‘wind tunnel’ one could place a number of gas-permeable membranes doped with chloride ions which theoretically could react with ozone in the air stream as it passes through the ‘wind tunnel’ converting the ozone to oxygen. Testing would have to be done to see if the formation of the chlorine oxide intermediate would dislodge it from the membrane, but if the doping was appropriate this concern should be minute.

Although more exotic methods could involve a spinning drum method using liquid chloride which would flow down with gravity, the problem with this particular methodology is that if the device was damaged liquid chloride would prove to be more detrimental if it make contact with anything. Overall, regardless of the design, technological ideas to address increasing low atmospheric ozone concentrations need to be theorized and small-scale tests need to be put into the field. At a future time this concept will be investigated further on this blog.



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