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Aerosol Sampling

Aerosol Sampling. Danielle Hall April 17 th , 2008. Introduction. Why is aerosol sampling important? What are some of the things that we are looking for when we sample? How is sampling air different than sampling soil or water? What are some considerations?. Aerosol Measurement.

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Aerosol Sampling

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  1. Aerosol Sampling Danielle Hall April 17th, 2008

  2. Introduction • Why is aerosol sampling important? • What are some of the things that we are looking for when we sample? • How is sampling air different than sampling soil or water? What are some considerations?

  3. Aerosol Measurement • Direct-Reading Measurements • Optical Particle Counters • Condensation particle counter • Time-of-flight particle sizers • Quartz crystal microbalance • Beta gauge

  4. Piezobalance Hinds. Aerosol Technology. 1999

  5. Beta Gauge Hinds. Aerosol Technology.1999

  6. Aerosol Measurement • Collection and Analysis Measurements • Most common collection media are filters. • Filters are subsequently analyzed by either gravimetric, chemical or biological methods. • Inertial separating devices classify particles based on their inertial properties (size) such as cyclones and impactors. • Classifying by size allows for the determination of the particle size distribution.

  7. Aerosol Sampling System • Aerosol Sampling system consists of: • Sample inlet • Transport system • Sample storage • It is desirable to have a representative sample from the original environment at the time of measurement. What are some factors that could affect this condition?

  8. Factors that affect sampling • Deposition in the inlet • Deposition during transport and storage • Agglomeration of particles • Evaporation and/or condensation of aerosol • Re-entrainment of deposited aerosol • All of these factors should be accounted for so that the measurements made can be corrected and to ensure accuracy.

  9. Sample Extraction • The withdraw of an aerosol from the environment requires making a particle enter the sampling system. The efficiency at which this is done is called the inlet efficiency and is dependent on: • Ambient gas velocity • Inlet geometry (size and position) • Sampling gas velocity • Particles aerodynamic diameter

  10. Sample Extraction • inlet=aspiration * transmission • aspiration: Concentration of particles of a certain size entering the inlet over their concentration in the ambient= C/C0 • transmission: Number of particles of a certain size transmitted through sampling system over the total number aspirated. • trans,inter • trans,grav

  11. Sample Extraction: Inlet Types • Blunt Sampler: Sampler and inlet configuration present a large physical obstruction to the flow. • Difficult to characterize sampling efficiency because of: • Particle bounce at the inlet • Deposition on the lip or face • Re-entrainment of material into the inlet What is a reason that a Blunt Sampler would be used? • Thin Walled: Presents little or no affect on the ambient air flow. • Ratio of external to internal diameter < 1.1

  12. Sample Extraction: Isokinetic Isokinetic Sampling: • The nozzle is parallel with the gas flow • The sample velocity is equal to the gas flow velocity • Has near 100% aspiration efficiency Zang, Y. Indoor Air Quality Engineering. 2004. CRC Press

  13. Sample Extraction: Anisokinetic Superisokinetic Subisokinetic Uintlet > Ustreamline Uinlet < Usteamline Belyaev and Levin: For stk<6 Zang, Y. Indoor Air Quality Engineering. 2004. CRC Press

  14. Sample Extraction:Anisokinetic Transmission Efficiency Sub-isokinetic (Liu et al.) For 0.01≤Stk≤100 and 1≤U0/U≤10 Super-isokinetic (Hangal and Willeke) (Inertial losses in the vena contracta) For 0.02≤Stk≤4 and 0.25≤U0/U≤1

  15. Sample Extraction: Misalignment Durham and Lundgren: Zang, Y. Indoor Air Quality Engineering. 2004. CRC Press Hinds, W. Aerosol Technology. 1999. John Wiley & Sons, Inc.

  16. Sample Extraction:Anisoaxial Transmission Efficiency Iv Inertial losses in the vena contracta Iw direct impaction on the inner wall Downward sampling (nozzle faces upward) Upward Sampling (nozzle faces downward)

  17. Sample Extraction: Example • What is the aspiration efficiency for 20 m particles of standard density sampled at 8L/min by a 10-mm diameter thin-walled probe from a duct having a flow velocity of 10 m/s? Assume the probe is correctly aligned.

  18. Sample Extraction: anisokinetic & anisoaxial Hangal and Willeke (1990) For 0.02≤Stk≤4 and 0.05≤U0/U≤2 and O≤≤60 For 0.02≤Stk≤.2 and 0.05≤U0/U≤2 and 45≤≤90

  19. Sample Transport • Possible Mechanisms for loss are: • Diffusion • Interception • Electrostatic • Thermophoretic • Settling • Impaction

  20. Sample Transport Settling Loss: Inertial deposition in Bend: Laminar Turbulent

  21. Sampling from still air • Bias can occur due to the settling of particles. • Criterion for negligible sampling bias (Davies): Particle settling Particle inertia

  22. Aerosol Sampling Recommendations • In general, to obtain a representative sample: • Isokinetic when possible • Gas velocity should be large compared to the particles settling velocity • Inlet diameters should be on the order of 1cm • Electrostatic effects should be minimized. • Tubing should be kept as short and straight as possible.

  23. Ambient Air Sampling • Objectives • Determine compliance with air quality standards • Enhance the understanding of the chemical and physical properties of air pollution • Evaluate Health Effects of air pollution • All of these objectives will require different sampling methodologies.

  24. Sampling System • Size-Selective Inlet • Denuders • Filter • Flow controller • Pump

  25. Size-Selective Inlets • Inertial classifiers are used to remove particles larger than a specific diameter by impaction. • Three factors that will affect a particles impaction: • Velocity of the air • Size of the particle • Size of the collection body

  26. Denuder • Serve to remove gases while passing 95% of particles. • Various types of denuders: tubular, parallel plate, annular, honeycomb and cloth. • Gases diffuse to the surface of the denuder. Often a coating is applied to retain specific gases. • Why do we want to remove gases?

  27. Filters • Filter media is based on the objective of the sampling i.e. what chemical is being sampled, the loading capacity, the required sampling efficiency, cost, availability and the type of analytical method being used. • Most common types of filters are: Teflon, quartz fiber, nylon, cellulose, Teflon-coated glass fiber, etched polycarbonate and glass fiber.

  28. Ringed-Teflon membrane filters • Composed of a thin, porous polytetrafluoroethylene (PTFE) Teflon sheet stretched across a ring. • Commonly used for mass and elemental analysis. • Typically analyzed by X-ray and proton-induced fluorescence. • Very chemical resistant and hydrophobic. • Not good for high flow rates because of the fragile filter structure.

  29. Quartz fiber filters • Comprised of woven quartz filaments. • Used for ion chromatography, carbon analyses, atomic absorption and particulate measurements (PM 10). • Advantage: very high filter efficiency, low moisture uptake, low trace contaminant levels, low artifact formation. • Disadvantage: adsorb hydrocarbon gases during sampling which can create a sampling artifact

  30. Filters cont… • Nylon • Consist of porous nylon sheets. • Used for collection of nitric acid. • Cellulose • Consist of tightly woven paper mat. • Advantages: Inexpensive, convenient extraction of particulates. • Disadvantages: Not good for submicron sized particles, very sensitive to humidity. • Nitric acid, ammonia, sulfur dioxide and nitrogen dioxide gases are measured by this filter. www.advantecmfs.com

  31. Filters cont… • Teflon-Coated glass fiber filters • Used for ion analysis and for specific organic compounds. • Advantage: Low moisture uptake, minimizes chemical transformation artifacts. • Disadvantages: Cannot be used to measure carbon, nitrate artifact. • Etched polycarbonate membrane filters • Pores of uniform diameter are created in a thin polycarbonate sheet. • Used for size selective measurements. • Good for analysis by electron microscopy. • Disadvantage: carry electrostatic charge. www.sterlitech.com

  32. Flow Measurement and Control • Flow measurement needs to be very precise because it is needed to determine particle concentrations as well as maintain the size-selective inlet properties. • Air is pulled through the sampling system by means of pump. • Mass flow controllers are used to monitor the flow during sampling.

  33. Stack Sampling Method Diagram of EPA Method 5 Stack-sampling system (Hinds 1999)

  34. How to create a sampling system • Identify the monitoring objectives • Identify the particle size, chemical composition, sampling frequency and duration needed to meet objective. • Estimate the amount of sample that will be collected and the detection limits of the analysis method. • Purchase or modify a sampling system that will satisfy these objectives. • Always have a QA/QC plan to ensure accuracy of measurements

  35. Overview • Types of Aerosol Samplers • Aerosol Sampling Components • Sampling Efficiency • Ambient Air Sampling • What did you learn today?

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