This is the fourth in a series of diaries about air pollution.
First in the series was about general air pollution facts and how the Clean Air Act works. It can be found here
Second in the series was all about ozone and photochemical smog and can be found here
And third in the series was all about reactive organic compounds and can be found here
Today's diary is all about oxides of nitrogen.
Oxides of nitrogen (NOx) are a group of chemicals that all contain only oxygen and nitrogen. The most common are nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide has little health effects and there is no NAAQS for NO set by the EPA. However, nitrogen dioxide does have health effects and the EPA as long had a NAAQS for NO2. In urban air NO is easily converted to NO2. As discussed in the second diary of this series, NOx is a precursor for the formation of ozone or photochemical smog. NO is colorless, but NO2 is a brown color. In areas with high levels of photochemical smog it is typical to see a brown haze below the temperature inversion in the morning indicating high levels of NO2. Then in the afternoon once the sun has cooked the soup in our air and much of the NOx has combined with ROC's to form ozone, the brown tends to go away and you see a more white haze. When NOx reacts with moisture, nitric acid is formed. This is a form of acid rain. Acid rain is typically more sulfuric acid on the east coast, and more nitric acid on the west coast. This is because there are more sulfur emissions on the east coast and more NOx emissions on the west coast.
Emissions of oxides of nitrogen are almost exclusively from combustion of fossil fuel. Cars and trucks, power plants, boilers, and even your hot water heater are the common sources of NOx. NOx is formed by the high temperatures of combustion that allow oxygen to combine with nitrogen. Control technologies to reduce the amount of NOx emitted by the burning of fossil fuel include:
-switching to a fuel with lower nitrogen content
-Reducing the temperature of combustion by injecting steam or other techniques
-Injection of ammonia to the combustion gasses that result in NOx being converted to water and nitrogen (N2).
The original NAAQS was based on an annual average concentration of 53 ppb. Every area of the country met this standard. In January 2010 the EPA revised the NO2 NAAQS to also include a one hour average standard of 100 ppb. In addition to the new one hour standard EPA is requiring additional near source NO2 monitors be installed nationwide by January 2013. The near source monitors are a new approach from EPA regarding monitoring. Studies have shown that NO2 concentrations near roadways are typically 30 to 100% greater than average city wide concentrations. Past monitoring practice was to locate monitors away from roadways to measure a more "average" exposure of the public.
For quite some time the method for measuring NOx in air has been a technique called chemiluminescence. In this method, NO and total oxides of nitrogen (NOx) are measured directly and then the two concentrations are subtracted to obtain the NO2 concentration. It turns out NO and NOx are easier to measure than NO2. The analyzer's detector can only measure NO. So NOx is measured by first passing the sample through a heated catalytic (molybdenum)converter than does nothing to the NO but converts any NO2 to NO. So the sample flowing out the converter contains the original NO plus the NO formed in the converter from any NO2 that was originally in the sample. Typically the analyzer will have a valve that cycles between routing the sample directly to the detector for NO measurement and then through the converter prior to the detector to measure NOx.
The detector that measures the NO takes the sample and mixes it with high concentration of ozone in a chamber called the reaction chamber. The ozone (O3) quickly reacts with any NO to form NO2. In this reaction a photon of light at a specific wavelength is emitted in the reaction chamber. There is a window between the reaction chamber and a very very sensitive light detector called a photomultiplier tube (PMT). The window is equipped with an optical filter at the precise color(wavelength)of the light generated by the reaction. The PMT detects the light from the NO+O3=NO2 reaction, the more light, the more NO in the sample. The ozone used for this reaction is made in the analyzer by passing room air through a device that is arcing with high voltage. The electric arc converts much of the oxygen in the air to ozone. Obviously, the exhaust of the analyzer that contains high concentration ozone is scrubbed prior to being released into the air.
As with the ozone analyzer, NOx analyzers operate 24/7 creating a continuous stream of concentration data. It is typical for a monitoring site to have both an ozone analyzer and a NOx analyzer. It is interesting that when there is an increase in NO concentration you will always see a decrease in the ozone concentration. While NO is a precursor to the formation of ozone, the initial reaction is NO combines with ozone to produce NO2 and O2, causing the ozone concentration to decrease. This titration of ozone by NO was a plan hatched by the big NO emitters in the Los Angeles basin in the 1980 as a way to reduce the ozone concentration in the LA basin. These big emitters, mostly power plants, tried to sell this idea to the local air pollution control agency (SCAQMD). The emitters proposed eliminating all NOx controls in the LA basin which would cause the extra NO to titrate down the ozone concentration. Their models did show that removing NOx controls in the basin would indeed reduce ozone concentrations in the basin. What they conveniently forgot to mention is that the same models showed huge increases in ozone farther downwind in the inland valleys. This is because it takes 3 or 4 hours for the photochemical reaction of NOx and ROC to form ozone. By the time that reaction occurred, the wind would have blown the emissions out of the LA basin and into into the inland valleys.
Any data used by EPA must come from an EPA certified analyzer so that the measurements between all locations are comparable. Also, strict quality control checks are required to be performed on the analyzer. The most important is injecting test gasses with known concentrations of NO and NO2 into the analyzer to test the accuracy of the analyzer. These test gasses are generated by a calibrations system that precisely blends purified air with high concentration NO from a compressed gas cylinder. The ratio of pure air to concentrated NO is varied to create any concentration needed. NO is stable in a compressed gas cylinder, but NO2 is not so stable. So typically NO2 is created in the calibration system by mixing a known amount of ozone with the NO in the test gas to convert some of the NO to NO2.
So there you have everything you wanted to know about oxides of nitrogen. Please hit me with any questions in the comments or by email. Tomorrow the next in this series will be about particulates. Particulate pollution and measurement is quite different from what we have covered so far. Recent health studies seem to indicate that particulates are the pollutant with the most significant health impacts.