White Paper on PM Light Extinction Measurements

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

Background: EPA is considering establishing a secondary PM NAAQS designed to reduce the welfare impacts associated with visibility degradat ion caused by PM. 1 This NAAQS may use a PM light extinction indi cator instead of the traditional PM mass concentration. PM light extinction is t he 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 absorpt ion, but their contributions would not be subject to this secondary PM NAAQS. Li ght extinction and its component parts (e.g. PM and gaseous light extinction) are measur ed in inverse distance units, which for convenience is often expressed as inverse megameters (i.e., 1/10 6 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 instrumentat ion for PM light exti nction monitoring, thought it does not attempt to describe the pr ocess in detail. This document does not include any information on site selection criteria, inlet probe exposure or network design, which will be de scribed elsewhere.

Measurement Goal: The proposed overall goal is to determine by measurement daylight hourly averaged light extinction by PM 10 (particle with diameter less than 10 μ m) for 550nm wavelength light 2 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 meas ure light extinction that can meet the goals above. Table 1 shows the in strumental techniques that can make these measurements, and summarizes their advantages and disadvantages for the purposes of meeting the goal. Some of the instruments directly measur e the total light extinction. Others measure either light sca ttering or light absorption, so their selection would require the use of two instruments to produc e light extinction. Note that there are prototype instruments (not explicitly identified in T able 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 ba sis of commercially available instruments that have been in wide spr ead use for many years (Beu ttell and Brewer, 1949, Hansen, et al., 1984). Other approaches are used in mo re 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.

1commercial 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, Ar nott, 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 in struments are suitabl e for meeting the light extinction measurement goals. That is not to say that m odifications 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 usin g 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 meas urement 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 relations hip that has light scattering and light absorption proportional to wave length to the power of - α (i.e. λ - α ). Where α is the Angstrom exponent which can r ange from 0.5 to 2.5 for part icle scattering and from 1 for black carbon to up to 2 or more for br own carbon (Moosmüller, et al., 2009). The closer the wavelength of t he measurement of light scattering and absorption is to specified wavelength of 550nm, the less sensit ive is the wavelength adjustment to the value of the Angstrom exponent. Some monitoring methods may include multiple wavelengths which provide s upplemental information that can help in identifying the types and sources of the PM and can be us ed to estimate the Angstrom exponent corresponding to each measurement. Assessm ents of measurement uncertainty should include wavelength adju stment uncertainty.

Except for the open path transmissometer, all of the instrumental approaches being considered for measuring li ght extinction, scattering and absorption pull air into the
instrument to make a measurement. This c an 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

2particles by diffusion within the sample line or instrument. It can also be caused by changes in the temperature that affect the relative humidit y causing hygroscopic particle to shrink in response to lower relative humid ity 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 desi gn (e.g., reduce the number and severity of impaction surfaces in the inlet line and instru ment, 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 def iciencies. 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 PM 10 - 2.5 by impaction contributes little bias
in locations where PM 10 – 2.5 concentrations are small, but ma y be a significant source of bias where it’s concentrations are high. This could lead to di fferent measurement options or requirements depending on monitoring location.

Measurement biases are inherent to severa l 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). Adjust ments for this are not an issue for small particles (e.g. PM 2.5 ), which scatter light symmetrical ly, but are more problematic where a significant but unknown fraction of the PM 10 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 larg e particles, which can be a significant
measurement bias where there are high concentrations of PM 10 – 2.5 . Smaller truncation angles would help minimize the effect di rectly. Another approach would be to separately measure the light scattering from PM 2.5 and PM 10 so that the difference
between the two measurement s can be adjusted separatel y to account for the PM 10 – 2.5 truncation biases.

Another well-document ed measurement bias is associated with filter transmission methods for monitoring particle absorption (Wei ngartner, 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 pr oximity to other parti cles 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 meas urements as input to the adjustment. Use of the filter transmission approach for light extinction should include an assessment of the effectiveness of adjustment approache s that would be needed in order to meet the overall accuracy/precisi on goals of the measurement.

3The above described measurement issues and any others that affect the ability of
methods to meet the overa ll measurement goals need to be in cluded in an evaluation of any method proposed for use in monitoring for the possible secondary PM NAAQS. This evaluation would necessarily in clude an error analysis based on documented effects and expected ambient c onditions. It would also likely involve laboratory and/or field measurement including intercomparison s with other measurement methods. As has been cited above, the scient ific 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 c onsiderations that will be important in selecting the monitoring approach that will be used for this possible secondary PM NAAQS. These include, instrument re liability, cost (capital and maintenance), availability, ease of use, m anufacturer’s operator support, etc. Additional factors that may weigh in on the selection of a moni toring 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 exponent s for climate research and can aid in determining source attributi on, for example distinguishing diesel from wood smoke or dust from black carbon (F ialho, 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 perhap s cost advantages over the currently available commercial inst ruments (e.g., photoacoustic versus filter transmission for light absorption or cavity approaches compared to transmissometer for light extinction). Prototypes of these instruments could be in cluded 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 monito ring network it is not clear whether instrumental approaches that are not curre ntly 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 exam ple filter transmission measurement to
estimate black or soot carbon concentration s (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 NAAQ S may be inconsistent with their current use. For example, a monitoring site situat ed 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

4measurement goal of ambient PM light extinction monitoring. Aside from the capital cost savings, use of familiar instrumentat ion may have the benefit of reduced training costs. From a national pros pective, 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 informati on needs and schedule, which is not yet available, so the following is necessarily lacking in specificity.

Cataloging and assessing available informa tion on the monitoring techniques is a reasonable first step. This would exp and upon the information in this document, especially that contained in Table 1, by including information on specific instruments (make and model for commercially availabl e instruments), published performance specifications, validation test ing, and other operational expe rience (e.g., feedback from current users). This would include an assessm ent of each instrument’s ability to meet the measurement goal (e.g., including a prop agation of errors analysis and performance in past laboratory or ambient intercompar ison studies). Some of the descriptive information has been compiled by others in liter ature 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 ar e familiar with ambient optical monitoring.

A reasonable second step would be to sponsor an invitational measurement intercomparison study where instrument manu facturers and/or devel opers are invited to set up their instruments at one or more comm on locations, for example a suitable wellequipped state/local air monitoring sites (Park, et al., 2006). The purpose would be to gain firsthand ambient monitoring experie nce with the operations of candidate instruments and generate a collocated measur ement data set for intercomparison of results and assessment of instrument perform ance. The locations and timing would be selected to provide challenging ambient condi tions (e.g., high coarse PM impacts, high humidity, etc.). This step could be comp leted within six to tw elve 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 promisi ng of the methods to be tested for a year by typical state and local agencies prior to a more substantia l 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 c ould be valuable for the next PM NAAQS review cycle. The deployment of a prototype network is a specific recommendation of

5the CASAC PM subcommittee. The cost and timing to set this up depends on the number of sites, equipment selected and coo peration by the stat e and local agencies.

References:

Allen, G.A., J. Lawrence, P. Koutrakis, Fi eld Validation of a Se mi-Continuous Method for Aerosol Black Carbon (Aethalometer) and Temporal Patterns of Summertime Hourly
Black Carbon Measurements in Southwestern PA, Atmospheric Environment, 33:817­ 823, 1999.

Anderson, T.L., D.S. Covert, S.F. Marsha ll, M.L. Lauchks, R. J. Charlson, A.P. Waggoner, J.A. Ogren, R. Cal dow, R.L. Holm, F.R. Quant, G.J. Sem, A. Wiedensohler,
N. A. Ahlquist, T.S. Bates, Performance Ch aracteristics of a Hi gh-Sensitivity, ThreeWavelength, Total Scatter/Backscatter N ephelometer, 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 Measurement s with a 7-Wavelength Aet halometer: Evaluation with a Photoacoustic Instrument and 3-Wavelengt h Nephelometer, Aerosol Science and Technology, 39:17-29, 2005.

Arnott, W.P., I.J. Arnold, J.W. Walker, Hi ghly Sensitive Compact Instrument for
Photoacoustic Measurement of Aerosol Li ght Absorption, and Polarization Diverse Forward, Backward and Total light Scatteri ng, 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 and Technology, 41: 447-462, 2007.

Beuttell, R.G., A.W. Brewer, Instruments for Measurement of the Vi sual Range, Journal of Science and Instruction, 26: 357-359, 1949.

Bond, T.C., T. L. Anderson, D. Campbell, Calibration an d Intercomparison of FilterBased Measurements of Visible Light Abso rption 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 Managem ent Association, 56: 398-410, 2006.

Chow, J.C., J.G. Watson, P. Doraiswa my, L.-W.A. Chen, D.A. sideman, D.H. Lowenthal, K. Park, W.P.Arnott, N. Motallebi, Aerosol Light Absorptionk, Black Carbon,

6and 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 Ae rosol 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 Carb on and Aethalometer Black Carbon Measured at Multiple Monitoring Sites, Atmospheric Environm ent, 38: 5193-5204, 2004.

Kebabian, P.L., W.A. Robins on, A. Freedman, Optical Ex tinction monitor Using CW Cavity Enhanced Detection, Review of Scientific Instrume nts, 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. Bay nard, C.A. Brock, E.R. Lovejoy, Uncertainty in Light Scattering Measurements by TS I Nephelometer: Resu lts from Laboratory Studies and Implications for Ambient Meas urements, Aerosol Science and Technology, 43:1064-1074, 2009.

McMurry, P.H., A Review of Atmospheric Aerosol Measurements, Atmospheric Environment, 34: 1959-1999, 2000.

Moosmüller, H., W.P. Arnott, C. F. Rogers, Methods for Real Time, In Situ Measurement of Aerosol Light Absorption, Journal of the Air and Waste Management Association, 47:157-166, 1997.

7Moosmüller, H., W.P. Arnott, Angular Truncation Errors in Integrating Nephelometry,
Review of Scientific Instru ments, 74: 3492-3501, July, 2003.

Moosmüller, H., R. Varma, W.P. Arno tt, Cavity Ring-Down and Cavity-Enhanced Detection Techniques for the Measurement of Aerosol Extinction, Aerosol Science and Technology, 39: 30-39, 2005.

Moosmüller, H., R.K, Chakr abarty, W.P. Arnott, Aerosol Light Absorption and its Measurement: A Review, Journal of Quantitative Spectrosc opy 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 I llumination and Truncation of Three Different Integrating nephelometers: Implications for Empirical Si ze-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. B ode, A. Petzold, A. D. Hans en, Comparison of Continuous
and Filter-Based Carbon Measurements at the Fresno Supersite, Air and Waste Management Association Journal , 56: 474-491, April, 2006.

Petzold, A., M. Schönlinner, Multi-Angle Ab sorption Photometry – A New Method for the Measurement of Aerosol Light Absorption and Atmospheric Black Carbon, Aerosol Science, 35: 421-441, 2004.

Radney, J.G., M.H. Bazargan, M.E. Wrigh t, D.B. Atkinson, Labor atory Validation of
Aerosol Extinction Coefficient Measurements by a Field-Deployable Pulsed Cavity RingDown Transmissometer, Aerosol Sci ence and Technology, 43: 71-80, 2009.

Snyder, D.C., J.J. Schauer, An Inter-C omparison of Two Black Carbon Aerosol
Instruments and a Semi-Continuous Elemen tal Carbon Instrument in the Urban Environment, Aerosol Science and Technology, 41: 463-474, 2007.

Virkkula, A., N.C. Ahlquist, D.S. Covert, W.P. Arnott, P.J. Sheridan, P.K. Quinn, D. J. Coffman, Modification, Calibration and a Field Te st of an Instrument for Measuring Light Absorption by Particles, Aerosol Sc ience and Technology, 39: 68-83, 2005.

Watson, J.G., Visibility: Science and Regu lation, Journal of the Air and Waste Management Association, 52: 628-713, 2002.

Weingartner, E., H. Saathoff, M. Schnaiter, N. Streit, B. Bitnar, U. Baltensperger. Absorption of Light by Soot Particles: Dete rmination of the Absorption Coefficient by Means of Aethalometers, Aeroso l Science, 34: 1445 – 1463, 2003.

8Table 1 Summary of likely candidate light extinction, scattering and absorption methods.MethodApproachAdvantagesDisadvantagesExtinction MeasurementsOpen path transmissometer(Malm, 1979)Path-averaged lightextinction is determined bymeasuring the amount oflight transmitted over aknown distance (typically0.1km to 10km). Themeasurement rangedepends on the site pathlength.1. No modification of theaerosol due to samplehandling.2. Path-averagedmeasurements may bemore representative thanpoint measurements.3. Short and long pathversions are commerciallyavailable.4. Can measure at selectedwavelengths1. Cannot exclude particlesexceeding 10μm diameter,including fog, precipitation,etc.2. Calibration is problematicfor long-path instruments.3. Siting requirements forlong-path instrument canbe difficult to meet.4. Cost can be high (~$25k).Cavity ring down (CRD) andCavity Attenuation PhaseShift (CAPS)(Moosmüller, et al., 2005,Kebabian, et al., 2007,Baynard, et al., 2007, Arnott,et al., 2009, Radney, et al.,2009)Light extinction of sampledair is determined by the rateof decay of light intensity(CRD) or waveformdistortion (CAPS) as a pulseof laser light bounces backand forth between mirrors.1. Can exclude ultra-coarseparticles (larger than 10μmdiameter).2. Can be calibrated with wellcharacterized standards.1. Coarse particle samplingloss is a concern.2. Relative humidity changesdue to sample heating orcooling are a concern.3. Laser-dependentwavelengths (e.g., 531nm,but not 550nm)4. Not commercially availableat this time.Scattering MeasurementsIntegrating nephelometerSampled air is illuminated by1. Can exclude ultra-coarse1. Coarse particle sampling(Beuttell and Brewer, 1949,a light source and the lightparticles (larger than 10μmloss is a concern.Anderson, et al., 1996,scattered between ~0° anddiameter).2. Relative humidity changesHeintzenberg, et al., 2006,~180° is directly measured.2. Can calibrate with welldue to sample heating orMüller, et al., 2009)characterized standards.3. Several commerciallycooling are a concern.3. Angular truncation (i.e.,9MethodApproachAdvantagesDisadvantagesavailable instruments.doesn’t measure all the4. Has been used by manyway from 0° to 180°)researchers for decades incauses underestimatedfield studies and routinecoarse particle scattering.monitoring networks.Absorption MeasurementsFilter transmission(Hansen, et al., 1984,Petzold and Schönlinner,2004)Light transmission ismeasured through a filterwhile it collects particlesfrom the air. The change intransmission over time isrelated to the change in lightabsorption by the particlescollected on the filter.1. Can exclude ultra-coarseparticles (larger than 10μmdiameter).2. Several commerciallyavailable instruments.3. Has been used by manyfor decades in field studiesand routine monitoringnetworks.1. Multiple light scattering byfilter fibers and particlescaused by concentratingparticles on the filterintroduces biases thatneed to be corrected togive atmospheric PM lightabsorption.2. Most of the existing unitsare single wavelength thatmeasure in the infrared,not near 550nm.Photoacoustic(Moosmüller, et al., 1997)Light is pulsed through asample chamber whereabsorbing particles convertthe light to heat expandingthe air at the pulsedfrequency that is measuredacoustically.1. Can exclude ultra-coarseparticles (larger than 10μmdiameter).2. No corrections needed formultiple scattering effectscaused by filtration.3. Commercial units areavailable1. Currently availablecommercial units areexpensive (~$40k).2. Laser-dependentwavelengths (e.g.available at 531nm, butnot 550nm)10