Nephelometer Training for Air Monitoring Professionals

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Nephelometer Training for Air Monitoring
Professionals

• Puget Sound Clean Air Agency
• Matt Harper

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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

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Light and Fine Particles
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Webcam Photos Comparing Low Pollution Day to High
Pollution Day
PM 2.5 2.8 ug/m3 PM 2.5 40.1 ug/m3
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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

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Nephelometer Terminology
sext sscat sabs

• 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?

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Nephelometer Terminology
I Io e sext x

• Beer Lambert law
• Io initial light intensity
• I intensity after distance x
• x distance in meters
• s ext light extinction coefficient

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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.htm
  l

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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.

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Block Diagram
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Picture of Typical Nephelometer
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Parts of the Sampling System

• Heated Probe
• Inlet
• Light Source (Flash lamp)
• Motive Force
• Photo Multiplier Tube
• Detector
• Calibration Gas

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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.

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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.

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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.

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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.

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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

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Nephelometer Data as Visual Range Estimate

• Visual range (km) 3.912 / s ext (Mm-1)

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Estimating PM 2.5
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Aerosol Dependent Factor Equations
PM 2.5 in ug/m3 (m)(s scat in 10-4 m-1)
Seattle Industrial Port Land Use 33.1
Tacoma Industrial Port Land Use 29.5
Kent Valley Residential and Commercial 28.0
Marysville Everett Wood Smoke Impacted Neighborhood 26.3
Tacoma Wood Smoke Impacted Neighborhood 25.4
Darrington Rural Wood Smoke Impact 20.7
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Graphics
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Graphics
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ARM Approval Preparation

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

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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