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Nephelometer Training for Air Monitoring Professionals

Nephelometer Training for Air Monitoring Professionals. Puget Sound Clean Air Agency Matt Harper. Nephelometer. Goals for the training Nephelometer Terminology and Theory of Operations How do we use Neph Data? Visibility and PM 2.5 Estimates

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Nephelometer Training for Air Monitoring Professionals

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  1. Nephelometer Training for Air Monitoring Professionals Puget Sound Clean Air Agency Matt Harper

  2. Nephelometer • Goals for the training • Nephelometer Terminology and Theory of Operations • How do we use Neph Data? • Visibility and PM 2.5 Estimates • Maintenance Issues, Quality Assurance, and Hands On Operation

  3. Light and Fine Particles

  4. Webcam Photos Comparing Low Pollution Day to High Pollution Day PM 2.5 = 2.8 ug/m3 PM 2.5 = 40.1 ug/m3

  5. Nephelometer Terminology • “Sensitive” range of wavelengths for particle detection and visual range detection is typically 520 – 530 nm • Most commercial nephelometers operate in this range. • freq * l = c

  6. Nephelometer Terminology • Light Extinction Coefficient • Over the Distance axis, light will extinguish for a number of reasons: • Light Scattering Coefficient • Light Absorption Coefficient • Measured in Inverse Meters (m-1) • “How Much is Extinguished Per Meter?” sext = sscat + sabs

  7. Nephelometer Terminology • Beer – Lambert law: • Io = initial light intensity • I = intensity after distance x • x = distance in meters • sext = light extinction coefficient I = Io e – sext x

  8. Photo Multiplier Tube • PMT – Photo Multiplier Tube – Geometrically shaped tube that utilizes multiplication of electrons by secondary emission to measure low light intensities. • Incident Light Intensity is known. • Ending Light Intensity is measured after light goes through the measurement chamber getting scattered by the particles. • PMT allows electron amplification. • http://www.electrontubes.com/pmt/understanding.html

  9. Terms • Shutter Count – Periodically, the light source is blocked by a material with known transmittance (called the Shutter). The Shutter Count is used by the microprocessor to account for variations in the light source. • Dark Count – The Dark Count is what the detector sees when the light source is not on. The Dark Count is used by the microprocessor to detect light leaks into the PMT.

  10. Block Diagram

  11. Picture of Typical Nephelometer

  12. Parts of the Sampling System • Heated Probe • Inlet • Light Source (Flash lamp) • Motive Force • Photo Multiplier Tube • Detector • Calibration Gas

  13. Heat treatment of sample • At above 60% RH, particles can experience hygroscopic growth because the water vapor can condense on the particles making them “Grow” • Nephelometers have heated probes to maintain RH of the sample below 60%.

  14. Automatic Calibration Systems • WA Dept of Ecology has a calibration box • Automatic calibration system specifically designed for Radiance Research Nephelometers and our Envidas For Windows Data Acquisition System. • DAS scheduled and controlled, relay driven system that performs and records an automated check. • Operators view the Cal Check data remotely and can evaluate whether or not a trip to the site is necessary.

  15. Automatic Calibration System • Ecotech nephelometers have a device driven automated calibration check capability. • Customizable for most applications. • Cal Check Data is automatically acquired by the Envidas For Windows DAS.

  16. Nephelometer Calibrations • Regular Calibration Checks • Zero and Span using a gas with a known s scat • We have moved away from R-134a • CO2 has a much lower risk of Global Warming Potential. • Common Issues • Spider Webs often will form inside samplers • High PM periods will cause more frequent calibrations until cleaning can be done.

  17. Maintenance • Lamp Replacement • Shutter Count • Partial Failure on LED style lamps • Full Failure Indications on Radiance Research • Heater Replacement at a regular interval • Heater types • PMT and Sampling Chamber Cleaning at a regular interval • We typically do this once every 2 years

  18. Nephelometer Data as Visual Range Estimate Visual range (km) = 3.912 / s ext (Mm-1)

  19. Estimating PM 2.5

  20. Aerosol Dependent Factor Equations PM 2.5 in ug/m3 = (m)(sscat in 10-4 m-1)

  21. Graphics

  22. Graphics

  23. ARM Approval Preparation • PM 2.5 Estimation Method has performed within ARM specifications when inputting nephelometer and FRM data into EPA’s Precision and Bias tool • 1 Year of Collocated data not yet collected.

  24. Summary and Hands On Operation • Radiance Research and Ecotech Nephelometer available for some hands on training • Terminology and Theory of Operations • Visibility and PM 2.5 Estimates • Maintenance, Quality Assurance, and Hands On operation

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