This is the third is a series on air pollution. The first installment that discussed general air pollution and the Clean Air Act (CAA) is here and the second installment that went into detail about photochemical smog or ozone is here.
Today's installment is about one of the precursors to photochemical smog, reactive organic compounds (ROC).
ROC's and NOx are the precursors that with the sun create photochemical smog or ozone as was discussed in the previous posting. But these compounds also can be primary pollutants with their own health issues all by themselves. The CAA has a national ambient air quality standard (NAAQS) for nitrogen dioxide (NO2) and many of the ROC's, such as benzine are known carcinogens and are regulated by the CAA as a toxic air contaminant.
Reactive organic compounds (ROC) are a collection of hundreds of individual chemicals. The thing they have in common is that they are all hydrocarbons (they contain hydrogen and carbon), they are usually a gas at normal temperatures, and they are photochemically reactive. Each individual ROC species has a different photochemical reactivity. They sometimes go by other names such as volatile organic compounds (VOC) or reactive hydrocarbons (RHC). As with most pollutants, the majority of ROC in our air is from human activities, mostly fossil fuel usage. ROC's can be a result of evaporative emissions, such as many paints, solvents used in dry cleaners, and vapors that escape as you fill your gas tank on your car. ROC's are also emitted by cars and trucks as unburnt fuel due to incomplete combustion. Natural vegetation also emits ROC's. Different vegetation emits different amounts with some types of plants contribution being insignificant while others can be more significant. ROC's from plants are generally not considered carcinogenic, but they can be quite reactive so can play a role in the formation of ozone.
The oil spill in the gulf has created a huge evaporative source of ROC's. The light compounds contained in the crude are evaporating as reach the surface. EPA has dispatched mobile labs to attempt to measure the concentration of these emissions to assess if there is a significant risk to the population from these vapors. So far, they say the levels they have measured "do not present a significant long term danger". I have not had a chance to completely review all the data, but as I do if I see anything that I think is significant, I will definitely post a diary with what I see. Even though I have worked for many years with great folks at EPA, after what they did at ground zero, I am always a bit skeptical. The most well known ROC that is also a toxic is benzene, which is definitely evaporating off the oil spill.
Measuring ROC's is super difficult. The first thing is you are not measuring just one or two compounds, you are trying to measure literally hundreds of compounds. Each compound has its own unique physical and chemical characteristics. The other issue is that in typical urban air, even on a bad day, the individual concentration of each compound is very low, usually a couple of parts per billion (ppb) or less, which as I pointed out in my first posting, is a very small quantity to try and measure.
One approach is to measure all the hydrocarbon compounds together. This is somewhat easy to do with a detector called a flame ionization detector (FID). A FID burns a small flame using hydrogen as fuel and creates a small flow of electrical current when hydrocarbons are present. The more hydrocarbons, the more current. So measuring the total hydrocarbon concentration is pretty easy with an analyzer that uses a FID. The problem is the measurement does not tell us much. First problem is the most abundant hydrocarbon in air is methane and it is completely non-reactive and non-toxic and present in concentrations orders of magnitude greater than all other hydrocarbon species.
A solution to separating the methane from the other hydrocarbons you want to measure is to use a technique called gas chromatography (GC). GC is a very cool concept. The basic idea of a GC is you have a column that contains a solvent that the compounds you want to separate are at least somewhat soluble in. The sample you want to separate is injected into one end of the column and an inert gas (helium, nitrogen, or hydrogen typically) is set flowing through the column as the carrier gas to carry your sample through the column. So any compounds that are not soluble in the solvent in the column will pass through the column at the speed of the carrier gas. The compounds that are at least somewhat soluble will take longer to pass through the column. Each compound, depending on its solubility will take a different time to pass through the column. The time to pass the column is called the retention time. Columns initially were simply a steel tube packed with an inert material (crushed red brick was first used) that has been coated with a solvent. Most modern columns are a thin glass like capillary that has solvent bonded to the inside walls. Capillary columns can be as long as 100 feet. Generally, the longer the column, the better separation you can achieve. Sometimes GC's are configured with a non-specific detector like a FID, in that case you need to rely on the retention time to identify what species is what. Other times a GC can be configured with a detector like a mass spectrometer (mass-spec)that can tell you something about the composition of what it is detecting, in this case you have both the detector and the retention time to confirm the identity of each species you are detecting.
So using a GC solves the problem of how to measure the various hydrocarbon species you are interested in and avoid measuring things you are not interested in, like methane. The other problem is that the concentration of each species is typically very low, below the sensitivity of most detectors like a FID or mass spec. This problem is solved by concentrating your sample using something called a cold trap. The first cold traps were simply a small loop of steel tubing packed with fine glass beads for surface area. The loop was immersed in liquid nitrogen and a fairly large volume of your air sample is passed through the tube. At liquid nitrogen temperatures the nitrogen and oxygen (that make up the vast majority of any air sample) stay a gas and pass through the trap. However, hydrocarbons, CO2, and water turn to a liquid or solid and are trapped in the tube. Once you have passed the volume of air you want through the trap (the more air the lower concentrations you can measure), typically a couple of liters, a valve is switched to connect the trap to the GC, and you quickly heat the trap which turns everything back to a gas and it flows into the GC for separation and ultimately detection by a FID or other detector. The cold trap has evolved over the past couple of decades so often there is a series of cold traps designed to deal with the complications that the water and CO2 can pose.
This sounds pretty strait forward, but believe me it is not. Even with all the improvements we have made over the years, it is still a very difficult measurement to make. Most of the time it is too difficult to set up this apparatus in a field location, so an air sample is collected in a metal sphere and transported to a lab where it is easier to keep everything working correctly for the analysis. I was lucky enough to work with researchers at EPA in the 1980's when this method was being developed and fine tuned. It was a wonderful fun experience and I learned a ton. The cost of this equipment is at least $40K with using just a FID detector and about $20K more if you use a mass-spec detector.
So there is everything you ever wanted to know about ROC's. Please ask questions in comments or email. Tomorrow I will post about the other precursor to ozone, oxides of nitrogen.