White Paper on PM Light Extinction Measurements 

Prepared by EPA Staff for the AAMMS Advisory Meeting, February 24 and 25, 2010 

Background: EPA is considering establishing a secondary PM NAAQS designed to 

reduce the welfare impacts associated with visibility degradation caused by PM.1 This 

NAAQS may use a PM light extinction indicator instead of the traditional PM mass 

concentration. PM light extinction is the fractional loss of light by scattering and 

absorption by PM per unit of distance through the atmosphere. Gases also contribute 

to light extinction by both scattering and absorption, but their contributions would not be 

subject to this secondary PM NAAQS. Light extinction and its component parts (e.g. 

PM and gaseous light extinction) are measured in inverse distance units, which for 

convenience is often expressed as inverse megameters (i.e., 1/106

 meters and 

abbreviated Mm-1). 

Purpose: The purpose of this document is to identify the overall measurement goal and 

describe measurement approaches that might be consistent with this goal. It envisions 

an evaluation process for approving instrumentation for PM light extinction monitoring, 

thought it does not attempt to describe the process in detail. This document does not 

include any information on site selection criteria, inlet probe exposure or network 

design, which will be described elsewhere. 

Measurement Goal: The proposed overall goal is to determine by measurement 

daylight hourly averaged light extinction by PM10 (particle with diameter less than 10μm) 

for 550nm wavelength light2 under ambient conditions with an overall accuracy and 

precision < 10% (RMS) in a range from 10Mm-1 to 1000Mm-1 for relative humidity 

conditions < 90%. 

Measurement Options: There are several ways to measure light extinction that can 

meet the goals above. Table 1 shows the instrumental techniques that can make these 

measurements, and summarizes their advantages and disadvantages for the purposes 

of meeting the goal. Some of the instruments directly measure the total light extinction. 

Others measure either light scattering or light absorption, so their selection would 

require the use of two instruments to produce light extinction. Note that there are 

prototype instruments (not explicitly identified in Table 1) that combine two 

measurement approaches in one device (Kebabian, et al., 2007, Arnott, et al., 2009). 

Some of the approaches in Table 1 are the basis of commercially available instruments 

that have been in wide spread use for many years (Beuttell and Brewer, 1949, Hansen, 

et al., 1984). Other approaches are used in more limited availability research or pre1

 The first review draft of the Urban-Focused Visibility Assessment is at 

http://www.epa.gov/ttn/naaqs/standards/pm/data/20100121UFVAforCASAC.pdf 2

 Humans are most sensitive to light at about 550nm wavelength. 

1 
commercial production instruments (Baynard, et al., 2007), some of which show 

significant promise to mitigate shortcomings of the more commonly available methods 

(Kebabian, et al., 2007, Arnott, et al., 2009). 

Based upon their understanding of the state of technology, EPA staff believes that a 

combination of currently available integrating nephelometer light scattering and filter 

transmission light absorption measurement instruments are suitable for meeting the 

light extinction measurement goals. That is not to say that modifications to these 

techniques or use of alternative approaches/instrumentation would not ultimately be 

shown to produce superior monitoring systems. Ideally, the monitoring requirements for 

a possible future PM secondary NAAQS using a PM light extinction indicator would 

have the flexibility to permit the use of improved monitoring approaches as they are 

developed and evaluated. 

Method Limitations/Evaluation: In order to be considered for use in making 

measurements for a secondary PM NAAQS, the performance of candidate methods 

needs to be evaluated with respect to the measurement goal. Practical limitations of 

measurement techniques need to be considered in translating the overall goal into 

general specifications. These can be grouped by topic to organize a discussion of the 

issues. The topics include wavelength, sample integrity, and measurement biases. The 

following is a brief discussion of well known method limitations that should be 

considered in evaluations of candidate measurement systems. 

Many instrumental techniques for measuring light extinction, scattering and absorption 

do not make the measurement at 550nm. Measurements made at other wavelengths 

can be adjusted using the power law relationship that has light scattering and light 

absorption proportional to wavelength to the power of -α (i.e. λ-α

). Where α is the 

Angstrom exponent which can range from 0.5 to 2.5 for particle scattering and from 1 

for black carbon to up to 2 or more for brown carbon (Moosmüller, et al., 2009). The 

closer the wavelength of the measurement of light scattering and absorption is to 

specified wavelength of 550nm, the less sensitive is the wavelength adjustment to the 

value of the Angstrom exponent. Some monitoring methods may include multiple 

wavelengths which provide supplemental information that can help in identifying the 

types and sources of the PM and can be used to estimate the Angstrom exponent 

corresponding to each measurement. Assessments of measurement uncertainty should 

include wavelength adjustment uncertainty. 

Except for the open path transmissometer, all of the instrumental approaches being 

considered for measuring light extinction, scattering and absorption pull air into the 

instrument to make a measurement. This can have the effect of modifying the particle 

size distribution thereby biasing the optical measurement. Modifications to the particle 

size distribution can be caused by loss of larger particles by impaction or smaller 

2 
particles by diffusion within the sample line or instrument. It can also be caused by 

changes in the temperature that affect the relative humidity causing hygroscopic particle 

to shrink in response to lower relative humidity or grow in response to higher relative 

humidity. The best approach is to minimize particle loss and temperature changes that 

affect particle size distributions by design (e.g., reduce the number and severity of 

impaction surfaces in the inlet line and instrument, insulate inlet lines and minimize the 

effects of sample heating by the light source and electronics) rather than attempting to 

correct the data for these deficiencies. The uncertainty and biases associated with 

sample modification should be assessed for the PM conditions where the method is 

proposed to be used. For example, loss of PM10 - 2.5 by impaction contributes little bias 

in locations where PM10 – 2.5 concentrations are small, but may be a significant source of 

bias where it’s concentrations are high. This could lead to different measurement 

options or requirements depending on monitoring location. 

Measurement biases are inherent to several of the methods under consideration. 

Integrating nephelometers ideally measure the light scattered from 0° (forward 

scattering) to 180° (backscattering), but most have 5° to 10° truncation at both ends of 

the range (Moosmüller and Arnott, 2003). Adjustments for this are not an issue for 

small particles (e.g. PM2.5), which scatter light symmetrically, but are more problematic 

where a significant but unknown fraction of the PM10 is from larger particles which have 

enhanced forward scattering (Müller, et al., 2009, Massoli, et al., 2009). This results in 

under-measuring the light scattering by large particles, which can be a significant 

measurement bias where there are high concentrations of PM10 – 2.5. Smaller truncation 

angles would help minimize the effect directly. Another approach would be to 

separately measure the light scattering from PM2.5 and PM10 so that the difference 

between the two measurements can be adjusted separately to account for the PM10 – 2.5 

truncation biases. 

Another well-documented measurement bias is associated with filter transmission 

methods for monitoring particle absorption (Weingartner, et al., 2003, Virkkula, et al., 

2005, Snyder and Schauer, 2007, Chow, et al., 2009). Particles collected by filtration 

absorb more light than when they were suspended in the atmosphere because the light 

is subject to a much greater degree of multiple scattering by the filter fibers and other 

particles that they are in much closer proximity to other particles than when suspended 

in the atmosphere. Adjustment approaches to account for this effect have been 

developed (Bond, et al.,1999, Arnott, et al., 2005). One such adjustment approach 

requires simultaneous PM light scattering measurements as input to the adjustment. 

Use of the filter transmission approach for light extinction should include an assessment 

of the effectiveness of adjustment approaches that would be needed in order to meet 

the overall accuracy/precision goals of the measurement. 

3 
The above described measurement issues and any others that affect the ability of 

methods to meet the overall measurement goals need to be included in an evaluation of 

any method proposed for use in monitoring for the possible secondary PM NAAQS. 

This evaluation would necessarily include an error analysis based on documented 

effects and expected ambient conditions. It would also likely involve laboratory and/or 

field measurement including intercomparisons with other measurement methods. As 

has been cited above, the scientific literature includes numerous descriptions of 

previous efforts to explore these issues. As a result some of the evaluation results and 

many of the methods needed for conducting such evaluations are already available. 

Other Factors: There are a number of practical considerations that will be important in 

selecting the monitoring approach that will be used for this possible secondary PM 

NAAQS. These include, instrument reliability, cost (capital and maintenance), 

availability, ease of use, manufacturer’s operator support, etc. Additional factors that 

may weigh in on the selection of a monitoring approach are alternate uses for the 

primary and/or supplemental data generated by the approach. For example separate 

measurements of PM scattering and absorption can be used to determine single particle 

albedo (used in climate research), can aid in source attribution, and may be of help for 

PM health effects research. Similarly, multiwavelength scattering and absorption can 

be used to estimate the Angstrom exponents for climate research and can aid in 

determining source attribution, for example distinguishing diesel from wood smoke or 

dust from black carbon (Fialho, et al., 2005 and 2006). 

Several of the instruments in Table 1 exist as prototypes, but are not currently available 

commercially. These include instruments with technical and perhaps cost advantages 

over the currently available commercial instruments (e.g., photoacoustic versus filter 

transmission for light absorption or cavity approaches compared to transmissometer for 

light extinction). Prototypes of these instruments could be included in evaluations, but it 

is unlikely that commercial instruments would be available in less than a year. Until 

EPA determines a schedule for deploying a monitoring network it is not clear whether 

instrumental approaches that are not currently commercially available should be 

considered. 

Some of the instruments listed in the table are already in use by some air quality 

monitoring agencies for other purposes, for example filter transmission measurement to 

estimate black or soot carbon concentrations (Allen, et al., 1999, Jeong, et al., 2004) or 

heated nephelometer for dry light extinction (Chow, et al., 2006). While a decision to 

approve the use of existing instrumentation would save the procurement costs of new 

instruments, their use for a secondary NAAQS may be inconsistent with their current 

use. For example, a monitoring site situated near a freeway or bus terminal with a filter 

transmission measurement for black carbon is unlikely to be at a good location for 

visibility monitoring. Also use of heated nephelometers would not be consistent with the 

4 
measurement goal of ambient PM light extinction monitoring. Aside from the capital 

cost savings, use of familiar instrumentation may have the benefit of reduced training 

costs. From a national prospective, the use of already deployed instrumentation should 

be assessed to determine the potential benefits. 

Next Steps: There are a number of reasonable next steps to determining the monitoring 

approach for a possible secondary PM NAAQS. Which steps are taken and when they 

are accomplished depends on EPA’s information needs and schedule, which is not yet 

available, so the following is necessarily lacking in specificity. 

Cataloging and assessing available information on the monitoring techniques is a 

reasonable first step. This would expand upon the information in this document, 

especially that contained in Table 1, by including information on specific instruments 

(make and model for commercially available instruments), published performance 

specifications, validation testing, and other operational experience (e.g., feedback from 

current users). This would include an assessment of each instrument’s ability to meet 

the measurement goal (e.g., including a propagation of errors analysis and performance 

in past laboratory or ambient intercomparison studies). Some of the descriptive 

information has been compiled by others in literature review articles (Horvath, 1993, 

McMurry, 2000, Watson, 2002, Moosmüller, et al., 2009). This step could be completed 

within two to six months by those who are familiar with ambient optical monitoring. 

A reasonable second step would be to sponsor an invitational measurement 

intercomparison study where instrument manufacturers and/or developers are invited to 

set up their instruments at one or more common locations, for example a suitable wellequipped state/local air monitoring sites (Park, et al., 2006). The purpose would be to 

gain firsthand ambient monitoring experience with the operations of candidate 

instruments and generate a collocated measurement data set for intercomparison of 

results and assessment of instrument performance. The locations and timing would be 

selected to provide challenging ambient conditions (e.g., high coarse PM impacts, high 

humidity, etc.). This step could be completed within six to twelve months with the 

cooperation of state and local air agencies, and with limited cost if instrument 

manufacturers covered their own cost to participate. 

A final step that might be prudent prior to making a decision on a monitoring approach 

would be to deploy a modest prototype network (e.g., 10 to 20 sites). It would allow one 

or a few of the most promising of the methods to be tested for a year by typical state 

and local agencies prior to a more substantial commitment to a specific approach. It 

would also generate a light extinction data set more quickly than waiting for a fully 

deployed national monitoring capability that could be valuable for the next PM NAAQS 

review cycle. The deployment of a prototype network is a specific recommendation of 

5 
the CASAC PM subcommittee. The cost and timing to set this up depends on the 

number of sites, equipment selected and cooperation by the state and local agencies. 

References: 

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for Aerosol Black Carbon (Aethalometer) and Temporal Patterns of Summertime Hourly 

Black Carbon Measurements in Southwestern PA, Atmospheric Environment, 33:817­

823, 1999. 

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Waggoner, J.A. Ogren, R. Caldow, R.L. Holm, F.R. Quant, G.J. Sem, A. Wiedensohler, 

N. A. Ahlquist, T.S. Bates, Performance Characteristics of a High-Sensitivity, ThreeWavelength, Total Scatter/Backscatter Nephelometer, Journal of Atmospheric and 

Oceanic Technology, 13: 967-986, 1996. 

Arnott, W.P., K. Hamasha, H. Moosmüller, P.J.Sheridan, J.A Ogren, Towards Aerosl 

Light-Absorption Measurements with a 7-Wavelength Aethalometer: Evaluation with a 

Photoacoustic Instrument and 3-Wavelength Nephelometer, Aerosol Science and 

Technology, 39:17-29, 2005. 

Arnott, W.P., I.J. Arnold, J.W. Walker, Highly Sensitive Compact Instrument for 

Photoacoustic Measurement of Aerosol Light Absorption, and Polarization Diverse 

Forward, Backward and Total light Scattering, Report to NSF available through EPA, 

2009. 

Baynard, T., E.R. Lovejoy, A. Pettersson, S.S. Brown, D. Lack, H. Osthoff, P. Massoli, 

S. Ciciora, W.P. Dube, A.R. Ravishankara, Design and Application of a Pulsed Cavity 

Ring-Down Aerosol Extinction Spectrometer for Field Measurements, Aerosol Science 

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Beuttell, R.G., A.W. Brewer, Instruments for Measurement of the Visual Range, Journal 

of Science and Instruction, 26: 357-359, 1949. 

Bond, T.C., T. L. Anderson, D. Campbell, Calibration and Intercomparison of FilterBased Measurements of Visible Light Absorption by Aerosols, Aerosol Science and 

Technology, 30: 582-600, 1999. 

Chow, J.C., J.G. Watson, K. Park, D.H. Lowenthal, N.F. Robenson, K.A. Magiano, 

Comparison of Particle Light Scattering and Fine Particle Mass in Central California., 

Journal of the Air and Waste Management Association, 56: 398-410, 2006. 

Chow, J.C., J.G. Watson, P. Doraiswamy, L.-W.A. Chen, D.A. sideman, D.H. 

Lowenthal, K. Park, W.P.Arnott, N. Motallebi, Aerosol Light Absorptionk, Black Carbon, 

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and Elemental Carbon at the Fresno Supersite, California, Atmospheric Research, 93: 

874-887, 2009. 

Fialho, P., A.D.A. Hansen, R.E. Honrath, Absorption Coefficients by Aerosols in Remote 

Areas: A New Approach to Decouple Dust and Black Carbon Absorption Coefficients 

Using Seven-Wavelength Aethalometer Data, Aerosol Science, 36: 267-282. 2005. 

Fialho, P., M.C. Freitas, F. Barata, B. Vieira, A.D.A. Hansen, R.E. Honrath, The 

Aethalometer Calibration and Determination of Iron Concentration in Dust Aerosol, 

Aerosol Science, 37: 1497-1506, 2006. 

Hansen, A.D.A., H. Rosen, T. Novakov, The Aethalometer – an instrument for the RealTime Measurement of Optical Absorption by Aerosol Particles, The Science of the Total 

Environment, 36: 191-196, 1984 

Heintzenberg, J., A. Wiedensohler, T.M. Tuch, D.S. Covert, P. Sheridan, J.A. Ogren, J. 

Gras, R. Nessler, C. Kleefeld, N. Kalivitis,V. Aaltonen, R.-T. Wilhelm, M. Havlicek, 

Intercomparisons and Aerosol Calibrations of 12 Commercial Integrating 

Nephelometers of Three Manufacturers, Journal of Atmospheric and Oceanic 

Technology, 23: 902-914, 2006. 

Horvath H., Atmospheric Light Absorption – A Review, Atmospheric Environment, 27A: 

293-317, 1993. 

Jeong, C.-H., P.K. Hopke, E. Kim, D.-W. Lee, The Comparison Between ThermalOptical Transmittance and Elemental Carbon and Aethalometer Black Carbon 

Measured at Multiple Monitoring Sites, Atmospheric Environment, 38: 5193-5204, 2004. 

Kebabian, P.L., W.A. Robinson, A. Freedman, Optical Extinction monitor Using CW 

Cavity Enhanced Detection, Review of Scientific Instruments, 78: 063102, 2007. 

Malm, W.C., Considerations in the Measurement of Visibility, Journal of the Air Pollution 

Control Association, 29: 1042-1052, 1979. 

Massoli, P., D.M. Murphy, D.A. Lack, T. Baynard, C.A. Brock, E.R. Lovejoy, Uncertainty 

in Light Scattering Measurements by TSI Nephelometer: Results from Laboratory 

Studies and Implications for Ambient Measurements, Aerosol Science and Technology, 

43:1064-1074, 2009. 

McMurry, P.H., A Review of Atmospheric Aerosol Measurements, Atmospheric 

Environment, 34: 1959-1999, 2000. 

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of Aerosol Light Absorption, Journal of the Air and Waste Management Association, 

47:157-166, 1997. 

7 
Moosmüller, H., W.P. Arnott, Angular Truncation Errors in Integrating Nephelometry, 

Review of Scientific Instruments, 74: 3492-3501, July, 2003. 

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Detection Techniques for the Measurement of Aerosol Extinction, Aerosol Science and 

Technology, 39: 30-39, 2005. 

Moosmüller, H., R.K, Chakrabarty, W.P. Arnott, Aerosol Light Absorption and its 

Measurement: A Review, Journal of Quantitative Spectroscopy and Radiative Transfer, 

110, 844-878, 2009. 

Müller, T., A. Nowak, A. Wiedensohler, P. Sheridan, M. Laborde, D.S. Covert, A. 

Marinoni, K. Imre, B. Henzing, J.-C. Roger, S. martins dos Santos, R. Wilhelm, Y.-Q, 

Wang, G. de Leeuw, Angular Illumination and Truncation of Three Different Integrating 

nephelometers: Implications for Empirical Size-Based Corrections, Aerosol Science and 

Technology, 43:581-586, 2009. 

Park, K, J.C. Chow, J.G. Watson, D.L. Trimble, P. Doraiswamy, W.P. Arnott, 

K.R.Stroud, K. Bowers, R. Bode, A. Petzold, A. D. Hansen, Comparison of Continuous 

and Filter-Based Carbon Measurements at the Fresno Supersite, Air and Waste 

Management Association Journal, 56: 474-491, April, 2006. 

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Science, 35: 421-441, 2004. 

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Instruments and a Semi-Continuous Elemental Carbon Instrument in the Urban 

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8 
Table 1 Summary of likely candidate light extinction, scattering and absorption methods. 

Method Approach Advantages Di

Extinction Measurements 

Open path transmissometer (Malm, 1979) 

Path-averaged light extinction is determined by measuring the amount of light transmitted over a known distance (typically 0.1km to 10km). The measurement range depends on the site path length. 

1. No modification of the 

aerosol due to sample handling. 

2. Path-averaged 

measurements may be more representative than point measurements. 

3. Short and long path 

versions are commercially available. 

4. Can measure at selected 

wavelengths 

1. Cannot 

exceedincludinetc. 

2. Calibra

for long

3. Siting r

long-pabe diffic

4. Cost ca

Cavity ring down (CRD) and Cavity Attenuation Phase Shift (CAPS) (Moosmüller, et al., 2005, Kebabian, et al., 2007, Baynard, et al., 2007, Arnott, et al., 2009, Radney, et al., 2009) 

Light extinction of sampled air is determined by the rate of decay of light intensity (CRD) or waveform distortion (CAPS) as a pulse of laser light bounces back and forth between mirrors. 

1. Can exclude ultra-coarse 

particles (larger than 10μm diameter). 

2. Can be calibrated with well 

characterized standards. 

1. Coarse 

loss is 

2. Relativ

due to cooling 

3. Laser-d

wavelebut not 

4. Not com

at this t

Scattering Measurements 

Integrating nephelometer Sampled air is illuminated by 1. Can exclude ultra-coarse 1. Coarse (Beuttell and Brewer, 1949, a light source and the light particles (larger than 10μm loss is Anderson, et al., 1996, scattered between ~0° and diameter). 2. RelativHeintzenberg, et al., 2006, ~180° is directly measured. 2. Can calibrate with well due to Müller, et al., 2009) characterized standards. 

3. Several commercially 

cooling 

3. Angula
Method Approach Advantages Di

available instruments. doesn’t 

4. Has been used by many way fro

researchers for decades in causes field studies and routine coarse monitoring networks. 

Absorption Measurements 

Filter transmission (Hansen, et al., 1984, Petzold and Schönlinner, 2004) 

Light transmission is measured through a filter while it collects particles from the air. The change in transmission over time is related to the change in light absorption by the particles collected on the filter. 

1. Can exclude ultra-coarse 

particles (larger than 10μm diameter). 

2. Several commercially 

available instruments. 

3. Has been used by many 

for decades in field studies and routine monitoring networks. 

1. Multiple 

filter fibcaused particleintroduneed to give atabsorp

2. Most o

are sinmeasunot nea

Photoacoustic (Moosmüller, et al., 1997) 

Light is pulsed through a sample chamber where absorbing particles convert the light to heat expanding the air at the pulsed frequency that is measured acoustically. 

1. Can exclude ultra-coarse 

particles (larger than 10μm diameter). 

2. No corrections needed for 

multiple scattering effects caused by filtration. 

3. Commercial units are 

available 

1. Curren

commeexpens

2. Laser-d

waveleavailabnot 550