PM Primer
The regulatory background surrounding the monitoring of particulate pollutants rests on the establishment of National Ambient Air Quality Standards (NAAQS). The 1970 Clean Air Act (CAA) (as amended) established the standards for sulfur dioxide, total suspended matter (TSPs), carbon monoxide, ozone, nitrogen dioxide and lead and two types of ambient air quality standards were established: primary standards and secondary standards.
As defined, primary ambient air quality standards are those for which, "the attainment and maintenance of which are requisite to protect the public health." Secondary ambient air quality standards were established to provide, "a level of air quality attainment and maintenance of which is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutants in the ambient air." These primary and secondary standards must be achieved throughout the United States as well as its territories.
In order to meet the standards, the 1970 CAA required each state to develop strategies to achieve and preserve ambient air quality standards. The designed mechanism to achieve these objectives was the creation of the so-called, state implementation plans (SIPs) -- each state and territory must develop and update a plan according to individual needs and requirements. Control strategies established in the SIPs may include actions such as the development of industrial and urban zoning plans, establishment of mass transportation systems, vehicle emission inspection programs and regulations pertaining to emissions originating from stationary sources. Since the original SIPs development phase and subsequent revisions, ambient air quality monitoring still plays a vital role in the development and evaluation of control strategies because the gathered data helps to determine if a geographical area is under attainment or non-attainment status.
Air quality data may be used to generate or validate computer models of air pollution dispersion which are later used in the development of control measures. After the control measures have been implemented, ambient air quality monitoring is required in both attainment and non-attainment areas to determine if the objectives have been met. Areas that have achieved attainment status require monitoring to determine if it has been maintained. Monitoring in non-attainment areas is required to see if control strategies are helping to reach attainment status.
The 1977 CAA Amendments (CAAA) established the concept of prevention of significant deterioration (PSD). The incorporation of PSD required the U.S. Environmental Protection Agency (EPA) to develop regulations to prevent degradation of air quality standards at attainment areas. The CAAA also imposed the incorporation of PSD provisions in the SIPs.
Air quality monitoring data is used to establish the preexisting air quality of the studied geographical area. To meet this goal, air quality monitoring networks are established to measure concentrations of air pollutants in the atmosphere. One of the most widely monitored pollutants is total suspended particle (TSP), also known as particulate matter (PM). PM monitoring efforts continue to be well emphasized. In the atmosphere PM appears to have different formation mechanisms, chemical composition and physical properties.
Particulate matter
Particulate matter identifies a broad class of chemically and physically diverse substances that exist as discrete particles (liquid droplets or solids) and have wide variation in size. Sizes may range from molecular clusters of approximately 0.005 micrometers (µm) in diameter to coarse particles with 100 µm. The particle size or the diameter refers to the aerodynamic diameter -- defined as "the diameter of a spherical particle with equal settling velocity and a material density of 1 gram per cubic centimeter (g/cm3) normalizing particles of different shapes and densities (EPA, 1999)."
| In the atmosphere PM appears to have different formation mechanisms, chemical composition and physical properties. |
Size is an important factor to consider during the development phases of control strategies for particulates, because size determines settling rates, transport and concentration levels in the atmosphere. The difference in particle size causes varying health effects upon the exposed individuals. For example, larger particles (>20 µm) may settle down at faster rates, preventing their transport over longer geographical distances. Particles with a diameter of at least15 µm are easily removed at the nose or throat. On the other hand, particles with a diameter of 10 µm or less may reach the deeper areas of the respiratory system such as the lungs and alveoli. The original 1971 NAAQS established limits for particulates as TSP.
In 1987, EPA added PM10 to TSP as an indicator for particles with a diameter <10 µm. The PM10 standard required annual average PM10 particulate levels to be below 65 milligrams per cubic meter (µg/3) and the peak readings on any one day (24-hours) to be less than 150 µg/m3 (Wes, 1999). On July 17, 1997, EPA published a new regulation concerning PM with a diameter of 2.5 µm or less. The new PM2.5 standard will require ambient annual average PM2.5 particulate levels to be 15 µg/m3, and 65 µg/m3 on any 24-hour event. On May 14, 1999, the U.S. Court of Appeals, District of Columbia Circuit, issued an opinion regarding the previously published (July's 1997) final NAAQS regulation for PM2.5.
On June 28, 1999, EPA and the U.S. Department of Justice filed a petition for rehearing key aspects of the U.S. Court of Appeals, D.C. Circuit's decision (www.epa.gov/oar/oarregul.html). This case, Browner v. American Trucking Association, et al, Docket number 99-1257, 05/22/00, is on appeal before the U.S. Supreme Court. At press time, the court had not rendered a decision. If EPA loses the appeal, the Agency will have to prepare a new, and maybe stronger proposal -- one backed with more scientific data (Curreri, 1999). For the latest update on the PM2.5 and smog (ozone) standard the following Web site may be consulted: www.epa.gov/oar/oarregul.html. The incorporation of the new regulation will impose PM2.5 monitoring requirements and the related costs are over the next five years are estimated to be very large.
Particulate monitoring concepts
There are different sampling approaches or techniques for the collection of suspended particles in the atmosphere. Some of the techniques for PM monitoring include filtration, electrostatic and thermal precipitation, and impaction. One of the most widely used methods involves the utilization of filters. The high volume sampler was formerly the most widely used equipment for monitoring TSP levels in the atmosphere. Nearly 20,000 high volume samplers were installed during the 1970's, but its use decreased considerably -- 450 units -- after the promulgation of the PM10 standard in 1987 (EPA, 1999). Typical atmospheric sampling procedures use process flow diagrams like the one depicted in Figure 1. The process is mainly composed of a sample collection device, contaminant detector, air mover and a device to measure flow rates. Polluted air enters the sampling device and passes through a sample collection apparatus. The collection device isolates the pollutant from the air stream by physical or chemical means.
Of the sampling techniques used to collect suspended particles in the atmosphere, impaction and filtration are the two most widely used. In 1948, a device was developed to collect aerosols in the atmosphere filtering the air sample and suctioning it with a vacuum motor. Due to its high sampling volume, the device was designated as the high-volume sampler. The latest type of high-volume sampler apparatus consists of a seven-inch by nine-inch filter media, enclosed in a standard shelter casing. Typical flow rates are 1.1 to 1.7 m3 /min (39 to 60 cubic feet per minute ft3/min) collecting particles ranging from 25 to 50 µm in diameter. The size of the casing opening and the volume of the filtered air per unit of time determines the average size of the collected particles. Due to that fact, since February 3, 1983, sampling devices must have opening dimensions to provide air velocities between 20 to 35 cm/sec. Sampling filters are made of fiberglass material with typical collection effici
encies of 99 percent
for particles with diameters of 0.3 µm and larger. The samples collected by fiberglass filters are suitable for the analysis of organic and inorganic pollutants and metals. Other filters are made out of cellulose materials.
| Size is an important factor to consider during the development phases of control strategies for particulates, because size dtermines settling rates, transport and concentrtation levels in the atmosphere. |
Sampling protocols in most cases are established over a 24-hour period at least once every six days. Once the sample has been collected, the concentration of particles is determined by the weight difference between the filter's original and final weight divided by sampled air volume.
To calculate the total sampled air volume, the following expression is used:
V = (Qstd) × t
where:
V = total air volume sampled, in standard volume units, std cubic meter m3
Qstd = average standard flow rate, std m3/min
T = sampling time, min
To calculate the total suspended particles in the sample, the following expression is used:
TSP = (W_f -W_i) × (106)/ (V)
where:
TSP = mass concentration of total suspended particulate matter, µg/std m3
Wi = initial weight of clean filter, in grams
Wf = final weight of exposed filter, in grams
V = volume of air sampled converted to standard conditions, std m3
106 = conversion factor grams to micrograms
Based on the fact that ambient conditions (temperature, pressure) may differ from conditions during filter weighting, a correction must be made to calculate the actual particulate matter concentration. The following expression is used for such purposes:
(TSP)_a = (TSP) × (P_3/P_std) × (298/T_3)
where:
(TSP)_a = actual concentration at field conditions, µg/m3
(TSP) = concentration at standard conditions, µg/m3
P_3 = average barometric pressure during the sampling period, µm Hg
P_std = 760 µm Hg (101 kPa)
T_3 = average ambient temperature during sampling period, °K
The calculated concentration is reported in micrograms per cubic meter µg/m3.
For measuring flow, this type of sampler uses either an orifice/pressure indicator, an electronic mass flowmeter or a rotameter. Measuring devices must be calibrated against known flow rates following procedures stated in 40 Code of Federal Regulations (CFR) 50, Appendix B. All the high-volume sampling procedures for suspended particulate matter are specified in 40 CFR 50, Appendix B.
The original TSP standard was substituted for the 1987 PM10 standard. The determination of particulate matter as PM10 is described in Appendix M, 40 CFR 50. The method applies to the in-stack measurement of PM10 from stationary sources. The stationary source sampling method takes a gas sample isokinetically and is fed into a cyclone that separates bigger suspended particles from PM10. Ambient concentrations of PM10 are taken using the concepts of inertial separation and impaction. The PM10 sampler draws air at a constant flow rate into a circular mushroom shaped inlet where the particles are inertially separated by their size fractions.
PM10 particles are connected on a filter after the particles have been inertially separated. The two types of federally approved samplers are the high-volume (1000 liters per minute L / min) and the dichotomous sampler (16.7 L/min). Similar to the TSP sampler, each filter is weighed after temperature and moisture equilibration. Particle concentrations are determined by weight difference from the measured sampled flow rate. The quality assurance procedures associated with PM10 monitoring can be found in Appendices A and B, 40 CFR 58.
The determination of particulate matter as PM2.5 in the atmosphere is described in 40 CFR 50, Section 50.6. Quality assurance procedures are described in 40 CFR 58, Appendix A. The sampler consists of an inlet, down tube, impactor for particle separation, filter holder assembly, air pump and a flow rate control system. The sampler possesses an electrically powered air sampler capable of drawing ambient air for up to 24-hours. The filters used by this type of sampler have a diameter of 46.2 µm made out of polytetrafluoroethylene (Teflon). EPA has designated seven methods for measuring mass concentration of PM2.5. An updated list of EPA designated and referenced methods may be obtained at www.epa.gov/ttn/amtic.
On April 22, 1999, EPA published in the Federal Register (FR) "Revisions to Reference Method for the Determination of Fine Particulate Matter as PM2.5." In the revised rule the establishment of a new national network of particulate monitors was proposed for the next two years. Specific design and performance requirements were detailed in 40 CFR 50, Appendix L, to assure that monitoring data is of the highest quality and comparable both within and between air monitoring agencies. Other specific requirements were published in documents such as, the "Quality Assurance Handbook for Air Pollution Measurement Systems, Vol II," Ambient Air Specific Methods, Section 2.12, EPA/600/R-94/038b (FR/ Vol 64, No.77).
One design requirement detailed in 40 CFR 50, Appendix L, is the use of a protective metal container for transporting filter cassettes from monitoring sites to the conditioning environment. This requirement is important for the post-sampling weight gain procedures. As stated in the rule "this protective container shall be made of metal and contain no loose material that could be transferred to the filter." The real importance is based on the fact that the container's construction material may not cause the transfer of its own material to the filter paper. Other requirements regarding the specific sampling rates are established in 40 CFR 50, Appendix L, Section 9.2.5.
Continuous monitoring of particulates is performed at state and local air monitoring stations. The requirements for this type of monitoring is specified in 40 CFR 58, Appendix D, Section 2.8.1.3.8, and was part of a final rule for ambient air quality surveillance for particulate matter cited in the FR, Vol.62, No.138, July 18, 1997. Under the mentioned regulation, at least one continuous fine particulate sampler is required at PM2.5 sites in metropolitan areas with a population of one million or more.
Sources of information
There are many sources for monitoring information, especially on the Internet. One of the most widely used is the Aerometric Information Retrieval System (AIRS) maintained by EPA's Technology Transfer Network (TTN). The AIRS-TTN Web site was designed to provide technical information from the AIRS's data management system to federal, state and local regulatory personnel, consultants and interested environmental organizations. The AIRS is a computer database containing information about airborne pollution in the United States and various member countries of the World Health Organization (WHO). The system is managed by EPA's Office of Air Quality Planning and Standards (OAQPS), Information Transfer and Program Integration Division (ITPID), located in Research Triangle Park, NC. This system resides in EPA's mainframe, and to retrieve information from it, interested parties must sign up for an account and pay the applicable computer usage charges.
The primary purpose of AIRS is to help EPA officials in the management and improvement of air quality in non-attainment areas as well as attainment areas. The CAA required every state to establish a network of air monitoring stations for criteria pollutants using a methodology set by the OAQPS for their location and operation. The result was the establishment of State and Local Air Monitoring Stations (SLAMS). On an annual basis, each state and territory must develop a report of the monitoring results obtained at the SLAMS. The OAQPS established an additional network of monitors called National Air Monitoring Stations (NAMS), to obtain more detailed information about air quality in regions or areas with particular interest to the agency. NAMS is a subdivision of SLAMS and provides long-term trend data across the nation. The siting of NAMS requires detailed location selection procedures and specific equipment and quality assurance criteria. The data gathered at these sites is compiled in quarterly and annua
l reports.
Another source of information that may be accessed through the TTN Web page is the Ambient Monitoring Technology Information Center (AMTIC) operated by the Monitoring and Quality Assurance Group (MQAG). This site contains information about ambient air quality monitoring programs, specific information regarding monitoring methods, relevant monitoring documents and articles, air quality trends and non-attainment areas and regulations related to ambient air quality monitoring. The MQAG develops national air quality regulations, policy and technical documentation, oversees the SLAMS and provides national technical support in relation to monitoring.
Detailed information on ambient air quality surveillance including general provisions, quality assurance and monitoring methods and siting of instruments may be found in 40 CFR 58, Subpart A & B. Information summarizing requirements for networks description, design and establishment for SLAMS, NAMS and others can also be found in Part 58.
Conclusions
Ambient air monitoring continues to be an integral part of air pollution control management efforts, as mandated by federal laws and regulations. Air quality monitoring data helped to establish preexisting air pollution concentrations as well as present conditions and has also been used to monitor the progress made by the implementation of air pollution control strategies. The determination of ambient air quality concentrations provides direct evidence of pollutants in the atmosphere, and corroborates the information generated by dispersion models and emissions inventories over a geographical area. The design, establishment and maintenance of air quality monitoring networks requires expertise in the specific technical areas as well as a regulatory background.
References
Appendix L, 40 CFR 50, "FRM for the Determination of Fine Particulate Matter as PM2.5 in the Atmosphere," Final Rule.
Appendix M, 40 CFR 50, "FRM for the Determination of Fine Particulate Matter as PM2.5 in the Atmosphere," Final Rule.
Chow, J.C., and Watson, J.G., Nov 6, 1997, "Guideline on Speciated Particulate Monitoring, " Special Report by EPA, RTP, NC.
Curreri, J., July,1999, "AQY2K," Environmental Protection, pp. 30-32.
Federal Register, Vol.62, No.138, Jul.18, 1997, "Revised Requirements for PM2.5 and Ambient Air Quality Surveillance for Particulate Matters," Final Rule, 40 CFR 53 and 58.
Federal Register, Vol.62, No.138, "National Ambient Air Quality Standard for Particulate Matter," Final Rule, 40 CFR 50.
Wes, D., June, 1999, "The PM-2.5 Challenge," Environmental Protection, pp. 35-37.
e-sources
EPA's Office of Air and Radiation (OAR) - Regulations
-- www.epa.gov/oar/oarregul.html
EPA's Office of Air Quality Planning and Standards (OAQPS) - Ambient Monitoring Technology Information Center (AMTIC)
-- www.epa.gov/ttn/amtic
AMTIC - 40 CFR 50
-- www.epa.gov/ttn/amtic/40cfr50.html
AMTIC - 40 CFR 53
-- www.epa.gov/ttn/amtic/40cfr53.html
AMTIC - 40 CFR 58
-- www.epa.gov/ttn/amtic/40cfr58.html
Mario G. Cora, EIT, is a doctoral student, and Yung-Tse Hung, PhD, PE, DEE is professor of Civil Engineering at Cleveland State University's Civil and Environmental Engineering Department, Cleveland, Ohio. Mario Cora may be reached via e-mail at mariocora@hotmail.com.
This article originally appeared in the 11/01/2000 issue of Environmental Protection.