This diary is the fifth installment in a short series about air pollution. Links to the previous diaries are listed below:
-Air Pollution 101 - Basics and the Clean Air Act
-Air Pollution 102 - All about Ozone
-Air Pollution 103 - Reactive Organic Compounds
-Air Pollution 104 - Oxides of Nitrogen
Today's diary is all about airborne particulates.
Particulates are microscopic particles that are small enough to stay airborne for long periods of time. Particle size and composition is quite variable. These particles can be a liquid or a solid. Some particles are emitted directly, primary particulate. Some particles are formed from gaseous pollutants, secondary particulate. The gaseous pollutant sulfur dioxide can convert to a particle of ammonium sulfate or even sulfuric acid mist in the atmosphere. Likewise, the gaseous pollutant NOx can also convert to ammonium nitrate particulate. The secondary sulfate and nitrate particulates can make up the majority of the fine particles in urban areas. Sulfate is more common on the east coast and nitrate is more common on the west coast.
Particles with diameters larger than 10 micron are filtered out by our nose and throat and never make it to our lungs, so in most cases have no health impact (there are a couple of exceptions). Particles 10 microns to 2.5 microns are considered coarse particulate. Particles less than 2.5 micron are considered fine particulate, and particles less that 0.1 micron in diameter are considered super fine particulate. Coarse particulates are typically composed of earth crustal materials. Fine and super fine particulates are typically the result of combustion, usually combustion of fossil fuels. Categories of particulate based on diameter are used by EPA in defining the NAAQS for particulates. PM10 is the concentration of all particles with a diameter less than 10 micron. PM2.5 is the concentration of all particles with a diameter of 2.5 micron and smaller. If you need a point of reference on how small 10 micron is, the average human hair is about 60 micron in diameter, so we are talking very small particles.
Some particulate is from natural sources like dust storms that can create huge concentrations of coarse particulate. In most areas of the country, especially urban areas the majority of particulate is due to human activities. Cars and trucks, power plants, and fireplaces are typical sources of particulate.
Prior to 1987 the NAASQ for particulates was based on "total suspended particulates". This included particles greater than 10 micron in diameter which gets filtered out by our bodies before they make it to our lungs. In 1987, EPA realized that a standard based on only particles that would make it to our lungs would be more appropriate and changed the NAAQS for particulates to PM10 (less than 10 micron). Then in 1997 EPA added a NAAQS for PM2.5 (less than 2.5 micron). In 2006 EPA dramatically reduced the NAAQS standard for PM2.5 as a result of new health data demonstrating health effects at lower concentrations that previous data demonstrated. Both the PM10 and PM2.5 NAAQS are based on the mass of particles regardless of the particle composition.
In addition to NAAQS for particulates based on size and not composition (PM10 and PM2.5) EPA has long had a NAAQS for lead particles. This is the case where particles greater than 10 micron have health impacts. The lead particles larger than 10 micron that are filtered out by the body are still absorbed into the body and blood stream, unlike most other particles. As a result the lead NAAQS is based on a total suspended particulate measurement. The lead standard has recently been reduced to 1/10th the previous standard based on better understand of lead impacts on health.
Over the past 10 years or so numerous studies have demonstrated serious risk from particulates. This new health data has made many researches feel that particulates are the most serious air pollutant in terms of direct health effects. Numerous studies from all over the world have shown the connection between both short term and long term exposure and mortality. A number of studies suggest that for every 10 ug/m3 increase in fine particulate concentration, mortality rates increase by about 1 percent. Studies show that world wide, 7-10 percent of lower respiratory illness in children is due to short term particulate episodes. This figure rises to 21 percent in the more polluted urban areas. Many researchers feel that there is no lower bound of "safe" level of particulates. While some studies suggest the composition of the particles make a difference in terms of health effects, many studies show serious health issues regardless of particle compositon.
Measuring particulates began with a very simple method but recently new techniques are taking over offering a better understanding of particulates. The early measurements were made with a device called a hi-volume sampler. It was essentially a vacuum cleaner that sucked air through a filter paper. The way it worked was a filter paper (usually made of glass fiber) about the size of a sheet of paper was very carefully weighed. Then the filter was installed on the sampler so the sampler would suck air through the filter, trapping any particles in the air on the filter paper. After enough air had been passed through the filter (normally the sampler is operated for 24 hours) the filter paper is removed and re-weighed. The pre-sampled weight is subtracted from the post-sampled weight to obtain the mass of the particles that were trapped on the filter. The sampler was equipped with a way to measure the flowrate of the air being sucked through the filter. So the mass of the trapped particles divided by the total volume of air passed through the filer gives you the concentration of particulates averaged over the time the sampler was running.
When the PM10 NAAQS was created, the hi-volume sampler was equipped with a head that the air enters prior to encountering the filter that uses the particles inertia to only allow particles less than 10 micron to pass through to the filer. This technique is really quite simple; the incoming air is passed through a series of nozzles that are offset requiring the air to make a tight turn to pass through the device. The velocity of the air through the nozzles is such that particles with a diameter greater than 10 micron can't make the bend and strike a plate that is greased to capture them so they don't make it to the filter.
When the PM2.5 NAAQS was created, the reference method EPA developed was similar to the hi-volume sampler but instead used a smaller teflon filter and lower flowrate through the filter. This type of sampler is called a low-vol sampler. The technique to separate the particles less than 2.5 micron is the same as was used for the PM10 samplers.
These filter samplers were utilized almost exclusively until recently. The disadvantage of the filter methods is that they are manual, requiring labor to set and retrieve and weigh the filter. Results are not usually available for a few days or more. They also normally will only produce data that is the average over a 24 hour period. The advantage of the filter samplers is they are fairly simple and that the filters can be analyzed to determine the chemical composition of the particles on the filter. In normal monitoring operations, unless there is a nearby source of lead or other toxic particulates the analysis is limited to total mass.
Newer technology particulate samplers have recently evolved that allow PM10 or PM2.5 to be measured automatically 24/7 on a continuous basis. These methods are replacing the filter methods that have been the standard for some time. The continuous methods offer immediate data without having to wait for the lab to weigh and process the filter. These continuous methods also provide data on hourly averages rather than 24 hour averages allowing for a better understanding by being able to correlate the wind shifts to particle concentrations.
The first method for real time particulate measurement was the tapered element oscillating microbalance (TEOM). This instrument used a small filter element on the end of a tapered element that is similar to a small tuning fork. The air would pass through the small filter and deposit the particles on the small filter. The tapered element was made to vibrate. If you have ever played with a tuning fork, as you touch the tuning fork the pitch or frequency changes. The tapered element with the filter on the end works the same, as particles are deposited on the filter the mass increases. As the mass increases it changes the frequency that the tapered element oscillates. The instrument senses the change in frequency of the element and converts it to a change in mass, thereby being able to calculate the particulate concentration. The TEOM was approved by EPA, but then a few years later the California Air Resources Board (CARB) discovered a problem with the TEOM. It turns out that EPA did the testing in North Carolina where most of the particles are crustal in composition. CARB tested it in urban areas where almost half of the particulate was ammonium nitrate and the TEOM measurement was about 50% low. The TEOM filter and element need to be at a constant temperature, so they are heated to 50 degrees C. This high temperature ended up volatizing the nitrate particulate which allowed it to pass through the filter and not be counted in the measurement. The manufacture has come out with changes to the instrument as an attempt to fix the problem.
The other method for continuous particulate measurement is a beta attenuation monitor (BAM). In this method the instrument has a filter rolled up in a long narrow tape. The tape is loaded into the instrument like film on a movie projector. The BAM has a very small amount of radioactive material that emits beta rays. Beta rays are quite easily blocked by anything with sufficient mass, in fact the more mass the less beta rays make it through. The BAM advances the tape to a fresh spot, with the beta source above the tape and a beta detector below the tape; it measures how much beta energy passes through the blank filter tape. Then the BAM lowers a nozzle onto the tape and passes the air sample through the tape for about an hour. Then the BAM again measures the amount of beta energy that passes through the tape that has particles deposited on it. The BAM calculates the mass of the tape prior to sampling and after sampling, which allows it to calculate the particulate concentration for the hour period. The BAM moves the tape ahead and repeats the process.
So now you have everything you wanted to know about particulate air pollution and how to measure it. Please hit me with any questions in the comments or by email. Next, I will post a diary on the last criteria pollutant, sulfur dioxide.