Ambient Aerosol Sampling

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Ambient Aerosol Sampling
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What parameters might we need to measure using an
aerosol sampler in the environment?
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Introduction

• Reasons for ambient aerosol sampling
• Regulations
• Deposition/Visibility
• Determine properties of pollutants
• Determine source of aerosol

Why do we need to know these parameters?
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Note a sampling system may be designed for one
purpose, but may also meet multiple sampling
goals!
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Learning Objectives!

• Requirements of ambient aerosol samplers
• Components required to meet those requirements
• Discuss existing sampling units

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Sampling System Requirements

• General Requirements
• Well-defined size fractions - Why?
• Filter media which are compatible with the
  intended analysis method(s)
• Stable sample volumes that do not overload the
  filter yet provide sufficient deposit for
  analysis
• Sampling surfaces that do not react with the
  measured species What might react?
• Available, cost effective, and practical hardware

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Source Apportionment Models

• Also called receptor models
• These use chemical/physical characteristics of
  gasses and particles to identify and quantify
  contributions from a specific source
• In order to distinguish contributions from one
  source to another, characteristics must be
• Present in different proportions in different
  source emissions
• Proportions must remain relatively constant
• Changes in these proportions between the source
  and receptor must be negligible

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Some Specific Requirements

• PM and TSP
• Wind tunnel testing must be performed for the
  inlet
• Must have sufficient sampling efficiency (gt99)
  and high alkalinity standards for filter media
• Must have stability of sample flow rates
• Precision of gravimetric analysis must be high
• PM2.5
• Samples are taken for differing amounts of time
  concurrent with visible haze during daylight
  hours for visibility standards

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Components of a Sampling Unit

• One of more size-selective inlets
• Sampling surfaces
• Filters
• Filter holders
• Flow movement and control device

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Sampling Inlets

• Designed to remove particles which
  exceed a specified aerodynamic diameter
  characterized by the 50 cut point (dp50) and a
  standard deviation
• Sampling effectiveness curves are created to
  determine the fraction of aerosol penetrating the
  inlet at given conditions
• Experimentally determined using known
  concentrations of similarly sized particles at
  different velocities in a wind tunnel

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Sampling Inlets (cont.)

• Operate on several principals
• Direct Impaction consists of a set of circular
  jets positioned about an impaction plate
• Virtual Impaction impaction surface is replaced
  with an opening that isolates larger particles
• Cyclonic Flow centrifugal force impacts
  particles onto a cylinder wall
• Selective Filtration uniform pore sizes select
  particles below a certain size
• Elutriation particles are drawn into a
  stilled-air chamber, smaller particles with a
  slower settling velocity will be drawn upwards
  while larger particles will settle faster than
  the upwards flow, not to be collected
• Impaction and cyclonic flow are most commonly
  implemented, why?
• Note inlets must be independent of ambient wind
  speed and direction!

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Sampling Surfaces

• Most are made out of aluminum (usually
  oxidized), stainless steel, or plastic
  (polycarbonate or polyvinyl chloride)
• Plastic surfaces can acquire charges which may
  attract suspended particles, though in most
  sampling units these charges are negligible due
  to the unit size
• It is important that the surfaces do not react
  with the influent
• Reactive gasses (especially acids) can create a
  problem here, a denuder may be used to separate
  them

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Filters

• Used as the collection media once a given cut
  size has entered through the inlet
• Several qualities are taken into consideration
• Mechanical stability
• Chemical stability
• Particle or gas sampling efficiency
• Flow resistance
• Loading capacity
• Blank values
• Artifact formation
• Compatibility with analysis methods
• Cost and availability
• No one type of filter meets all of these
  categorical requirements, and the correct filter
  type must be selected for appropriate use
• Several types of filters will be outlined

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Filters (cont.)

• Cellulose Fiber
• Tightly woven paper mat
• Meets all categorical requirements except
  sampling efficiency and water vapor artifacts
• Sampling efficiency is highly dependent on the
  weave
• Material is hygroscopic, so care must be taken
  when considering differential mass
• Glass Fiber
• Tightly woven mat of borosilicate glass filaments
• Meets all categorical requirements except
  artifact formation and blank levels
• High alkalinity in fibers allow for collection of
  acidic species

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Filters (cont.)

• Teflon-coated Glass Fiber
• Glass fiber filter coated with Teflon
• Meets all categorical requirements except blank
  element and carbon levels
• Teflon Membrane
• Porous Teflon sheet which is either stretched
  across a plastic ring or supported by a loosely
  woven Teflon mat
• Meets all categorical requirements except flow
  resistance and carbon blank levels
• Small pore size makes high-volume sampling nearly
  impossible

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Filters (cont.)

• Etched Polycarbonate Membrane
• Thin polycarbonate sheet through which pores of
  uniform diameter have been produced by
  radioactive particle penetration and chemical
  etching
• Meets categorical requirements in all fields
  except sampling efficiency
• Sampling efficiency is generally lt 80 despite
  small pore size
• Best filter for electron microscopy when
  isolating a single particle type
• Filter retains significant electric charge and
  should be discharged before use

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Filters (cont.)

• Quartz Fiber
• Tightly woven mat of quartz filaments
• Meets all categorical requirements except
  artifact formation
• Formation of compounds on this material is
  significantly lower than glass fiber
• Readily absorbs hydrocarbons and should be heated
  to 800 Celsius to remove prior to use
• Nylon Membrane
• Thin sheets of porous nylon
• Used almost exclusively for the collection of
  nitric acid

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Filter Holders

• This device holds the filter media in place and
  protects it from contamination prior to, during,
  and after sampling
• These holders must
• Mate to the sampler and to the flow system
  without leaks
• Be composed of inert materials which do not
  absorb acidic gasses
• Allow a uniformly distributed deposit to be
  collected
• Have a low pressure drop across the empty holder
• Accommodate the sizes of commonly available air
  sampling filters (37 mm or 47 mm)
• Be durable and reasonably priced

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Filter Holders (cont.)

• In-line
• These filter holders focus the air stream towards
  the center of the filter
• Open-faced
• Spread air flow before filter surface

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Advantages? Disadvantages?
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Flow Movement and Control Devices

• Air is passed through a sampling system by vacuum
  using a pump
• There are four ways to measure the flow through
  the system
• Manual volumetric
• Automatic mass
• Differential pressure volume
• Critical orifice volume

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Flow Movement and Control Devices (cont.)

• Manual Volumetric
• This method of flow control is accomplished when
  the system is preset by the user using a valve or
  other control device and then relies on known
  information about the system to gauge the flow
• The flow may change by up to 10 for most systems
  as the filter becomes loaded when using this
  method since the valve remains constant once a
  sample has initiated
• Automatic Mass
• These flow controllers measure heat transfer
  between two points in the gas stream this heat
  transfer is proportional to the flux of gas
  molecules between the two points
• PV nRT

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Flow Movement and Control Devices (cont.)

• Differential Pressure Volume
• Maintains a constant pressure across an orifice
  by a diaphragm-controlled valve located between
  the filter and the orifice
• As pressure increases due to filter loading, the
  diaphragm becomes unseated and allows more
  pressure onto the filters surface to compensate
• Critical Orifice Volume
• A small circular orifice between the filter and
  the pump regulates flow when the pressure
  downstream of the orifice is less than 53 of the
  upstream pressure, the air velocity attains the
  speed of sound and it will remain constant
  regardless of increased flow resistance
• These flow control devices are useful only for
  low flow rates (lt20 LPM) and large pumps with
  large pressure drops

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What Now?

• Once a filter has been loaded with particles,
  there are several ways to test it. Here are a
  few
• Mass
• Radiation
• Optical

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Sampling Systems
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Sampling System TEOM

• Provides direct measurement of PM collected on a
  filter using tapered element oscillating
  microbalance technology
• Can change inlets to target PM10, PM2.5, PM1, or
  TSP (total suspended particle) monitoring
• Collection filters can be analyzed for heavy
  metals
• Active volumetric flow control using pressure and
  temperature sensors
• This device is recognized by the US EPA for PM10
  and PM2.5 measurements
• TEOM mass detectors operate using basic inertial
  mass, that is, inertial impaction on the filters
  give a real-time particulate mass

Tapered element oscillating microbalance
technology
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Sampling System Beta Gauge

• Provides particulate mass measurement on a real
  time scale
• Utilizes beta electrons to load the particles on
  a sliding filter with beta emissions, which are
  then compared to a blank area on the filter and
  can be used to determine differential mass

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Confusion in Ambient Sampling?

• Some samplers, especially at the inlet, have
  different levels of collection efficiency of
  particles
• Also, EPA allows a tolerance on PM10 particles of
  .5 µm
• These factors compound on each other and can lead
  to misinterpreted data in sampling methods
• For example,
• The Wedding high-volume cyclonic inlet had a d50
  of 9.6 µm
• The Sierra-Anderson high-volume direct impaction
  inlet had a d50 of 10.2 µm
• Which one would you pick?

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Summary