This is a second in a short series about air pollution. The first posting Here discussed the general background on air pollution and how the Clean Air Act (CCA)works.
This second installment will focus on the criteria pollutant ozone.
Ozone is the major component of photochemical smog. Photochemical smog results when reactive organic compounds - ROC (like the gasoline that evaporates away when you fill your gas tank up) react with oxides of nitrogen - NOx (almost entirely from the internal combustion engine). Interestingly this reaction can only occur with the presence of sun light. Actually the sun drives the reaction. Because ozone is such a major component of photochemical smog, the National Ambient Air Quality Standard (NAAQS) for photochemical smog is in terms of ozone concentration. Ozone is considered a secondary pollutant because most of the ozone in the air is a result of this reaction between ROC's and NOx. This reaction takes 3 or 4 hours so the precursor chemicals (ROC+NOx) are typically emitted in one location but after a few hours of movement by the wind, the location where the ozone actually is formed is quite a ways away. For example in Los Angeles there are lots of ROC and NOX emitted along the coast, yet when the wind is blowing from the ocean the ozone concentration is typically pretty low. Yet as you move downwind to the inland valleys like San Bernadeno and Pasadena the sun has a chance to drive the reaction and enough time has passed that the precursors have converted to photochemical smog. Also as one might expect, ozone is typically highest in the middle of the day when the sun's energy is the highest, driving the reaction to occur even faster.
Ozone is simply three atoms (O3) of oxygen together rather than the normal configuration of two atoms (O2)that we call oxygen. Ozone is extremely reactive, it really really wants to give up that extra atom of oxygen and become the boring but stable oxygen that we breath. When ozone comes into contact with most surfaces, the ozone molecules will oxidize the surface by each molecule giving up an oxygen atom. So when you breath in air with high ozone concentrations each ozone molecule will likely give up an oxygen atom to the soft tissue of your lungs. Oxidize is burning, that is essentially what it does to your lungs.
Now I have to point out a common confusion regarding ozone. The ozone in photochemical smog is at ground level. There also is naturally occurring ozone present high in the atmosphere where we do not breath. This natural occurring ozone is often called the ozone layer. The ozone layer is very important to life on earth. Without the ozone layer the harmful ultra-violet energy from the sun would make life impossible on earth. The ozone molecules in the ozone layer have a property that they absorb ultra-violet light very well (remember this it connects to measuring ozone). Damage to the ozone layer caused by certain chemicals we loved to use and just release into the air (CFC's) appears to be recovering thanks to regulations that have dramatically minimized CFC emissions (Montreal Protocol). So we have good ozone and bad ozone. But it is still the exact same chemical, O3, just formed by different processes, and occur in different places.
Efforts to reduce photochemical smog in the US have been one of the greatest efforts by air pollution regulatory agencies in the past three decades. It has not been easy. One challenge to understanding what is going on is as I pointed out where the ozone forms is normally a long ways from where the original precursor pollutants were emitted. Another complication is that while NOx is relatively easy to measure, ROC's are very difficult to measure. ROC's include hundreds of different chemicals that all contain oxygen and carbon that are mostly emitted by the evaporation of fossil fuels, but vegetation does emit measurable amounts of a couple types of ROC compounds. The concentration of each one is typically fairly small further complicating measurement. Also, each individual ROC compound has a different reactivity, so some ROC's are more of an issue than others.
Initial efforts to reduce ozone in the US focused on reducing the amount of ROC's and not controlling the NOx side of the equation as much. The initial thought was that the limiting precursor was ROC, so focusing on that would give us the most bang for our buck. It wasn't until the mid 1980's that we came to understand that NOx reductions in most cases were as much if not more beneficial than reduction in ROC emissions. Because it is so difficult to accurately measure ROC's, I think this lead to our initial lack of understanding of the dynamics of this reaction.
Current efforts to reduce ozone precursors include:
-catalytic converters on cars to scrub ROC's
-vapor recovery on gas station fill pumps to trap the ROC vapors that get pushed out of your tank when you fill it with gas.
-Tweaks to car engines to minimize the creation of NOx
-Scrubber devices on large NOx sources like power plants to capture most of the NOx.
-Changing to different types of solvent used in paints and other products that are less or non reactive.
These efforts have been amazingly successful. In southern California, famous for smog, the levels today are less than half than what they were when I first started measuring air pollution, 30 years ago. This has occurred in spite of more people, more cars, driving cars more, more factories, more electrical power plants, etc. If I wasn't the one who made many of those measurements, I would not have believed it. We still have areas where the ozone is above health standards, so we have more work to do, but it is really amazing how this has all worked.
Measuring ozone today is done with a microprocessor controlled analyzer called a photometer. The photometer measures how much ultra-violet light is absorbed by the air sample. Remember I pointed out that the naturally occurring ozone in the ozone layer protects us from the harmful UV light. The same characteristic that makes ozone in the ozone layer absorb harmfull UV energy is how we measure it. It turns out that ozone has a very strong absorption band in the UV part of the spectrum. The photometer has a tube with a UV lamp at one end and a UV detector at the other end. An air sample is passed through the tube and the detector measures how much UV light makes it from the lamp to the detector. Actually, the analyzer first routes air that has passed through a scrubber that removes all ozone to make a baseline measurement, then the air does not pass through the scrubber and the amount of UV light that passes through the tube is measured again. The ozone concentration is calculated by something called Beer-Lamberts law from these measurements. Most of these analyzers make that measurement every few seconds. These analyzers are located in an air monitoring station where typically other pollutants as well as wind conditions are also measured. The ozone analyzer operates 24/7 producing a continuous stream of ozone concentration data. For any data to be used for regulatory purposes, it must be measured using an EPA "certified" analyzer. These analyzers cost about $10K each.
The EPA has regulations that specify how these analyzers are to be operated if the data is to be used. These regulations specify quality control tests that must be performed on the analyzer on a regular basis. One important test is actually testing, or calibrating how accurate the analyzer is at measuring ozone. This is accomplished by injecting test gas with a known concentration of ozone into the analyzer and seeing how close the analyzer says the concentration of ozone is to the known concentration of the test sample. Most ozone analyzers are tested with two or three concentrations of ozone automatically in the middle of every night. The results of these auto-calibration tests are the first thing I review when I get to work each morning. The test gas is made in another instrument called a dynamic gas calibrator. Ozone is made in the calibrator by passing purified air through a chamber with high energy UV lamp. The energy of the UV lamp works just like the sun naturally does in converting oxygen (O2) to ozone (O3) high up in the ozone layer. We have a way to calibrate the lamp so we know how much ozone it will produce and we can program the calibrator to automatically make the test gas at a specific concentration and inject it into the ozone analyzer. These calibration systems are $20K or so. Another regulation is that because ozone reacts with most surfaces, all surfaces that touch the air being analyzed can only be made of glass or Teflon. So we use only tubing made of Teflon and even the particle filter that keeps particles from clouding the optics of the photometer is made of Teflon. Each analyzer is normally independently checked once a year. A person other than the one responsible for operating it uses a different calibration system to test the analyzer's accuracy.
So you can see that ozone is quite a complicated pollutant. It is amazing what interesting chemical reactions we have created with our giant chemistry set, earth. As always, please hit me with questions in the comments or by email. Tomorrow I will post about the ozone precursors ROC and NOx. Not only are these gasses the precursors to photochemical smog, but they are themselves pollutants with health effects of their own.