ENSC 412612 Week 5 Sampling and Measurement

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ENSC 412/612 Week 5Sampling and Measurement

• Readings Text Ch 9, 10

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Ambient Air Sampling

• Objective of a sampling system is to obtain a
  sample that is representative of the atmosphere
  at a particular place and time and that can be
  evaluated as a mass or volume concentration.
• System should not alter the sample in an
  undesirable way.
• samples should be collected near ground level and
  close to receptors
• samplers can be broadly categorized as either
  active in which air at a known flow rate is drawn
  through a sampling device or passive / static in
  which pollutants are passively sorbed or
  deposited on collector
• active sampling is more accurate (but usually
  more costly) since the volume of air sampled is
  known more exactly and one doesn't need to
  estimate the diffusion of pollutants onto the
  collector as in passive sampling

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Active sampling systems

• An active sampling system usually consists of
• an inlet manifold made of inert materials such
  as glass, Teflon, Stainless steel
• an air mover or pump which creates a vacuum to
  draw in air
• a collection medium may be a liquid or solid
  sorbent for dissolving gases. a filter surface
  for collecting particles or a chamber to collect
  air for analysis
• a flow measurement device to measure the volume
  of air

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Active sampling contd

• A extractive sampling - e.g. SO2 in liquid
  sorbent
• B open face'' filter exposed directly
• C sample collection for lab analysis
• D typical continuous monitor

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Active sampling contd

• there is a close similarity between engineering
  control methods and sampling / analysis both
  are trying to do the same thing (remove
  pollutants from the air)
• Important sampling system characteristics
• collection efficiency
• sample stability
• recovery (fraction of pollutant in the sample
  which is measured)
• minimal chemical interference or transformation

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Sampling systems for gaseous pollutants

• collection medium is usually liquid or solid
  sorbents, or an evacuated flask
• in liquid collection systems, bubblers maximize
  the gas-liquid interface and species react with
  water to form anions or cations
• analytical techniques used often detect the
  anions or cations e.g. SO2NH3 ? HSO3- NH4
• bubblers are low cost and portable
• solid sorbents (e.g. Tenax, activated carbon) can
  sample by trapping species (e.g. HC) on active
  sites
• before the sorbent becomes saturated, it is
  sealed and sent to a lab for analysis

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Sampling systems for gaseous pollutants contd

• the sorbed gas can be recovered by heating while
  passing through an inert gas
• the de-sorbed species is concentrated and
  injected into a gas chromatograph for analysis
• more commonly now, in-situ continuous monitoring
  and analysis is done
• instruments are sophisticated enough that they
  can sample and measure/analyse at the same time
• sample air is drawn in through an intake
  manifold, a small sample taken and analysed
  either discretely or continuously
• the procedure is automated
• in-situ monitors exist for SO2, NO, NO2, O3, CO,
  PM

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Sampling systems for particulate matter

• particles are inherently different from gases due
  to their size and mass
• in sampling particulate matter, we want to know
• mass concentration
• size
• chemical composition
• more recently particle number is also of interest
  - ultrafines

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Sampling systems for particulate matter contd

• the primary approach is to separate particles
  from a known volume and weigh/analyse them ?
  filtration and impaction are most commonly used
• sometimes particles behave unexpectedly near
  inlets, leading to sampling problems Overhead
  Fig 13-3 ? we want situation 13-3B

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• 13-3B represents isokinetic flow, when the wind
  speed is the same as the flow into the sampler

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Sampling systems for particulate matter contd

• airflow near the inlet is not a problem for gases
  because fractionation by different molecules does
  not occur
• for particles, inertial effects may select the
  sample size ? inlets must be as short and
  bend-free as possible
• inertial effects are less important when
  particles are lt 5 um in diameter
• when a specific size range is required (i.e.
  PM10, PM2.5) this inertial separation can be used
  to pre-filter the sample ? the inlet manifold
  must be very carefully designed

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Static sampling

• there is no pump or active air moving component
  to these systems
• examples might be dust-fall buckets (large
  particles) radon detectors, etc.
• systems are exposed to ambient conditions for a
  specified period of time, then the collectors are
  sealed and analysed in the lab, and the analysis
  is related to the average concentration over the
  exposure period
• systems can't usually quantify the amount of
  pollution present over a short period of time
• these systems are low in cost, portable, and
  typically do not require power
• collection is by diffusion of species through the
  air onto a collector, or by permeation through a
  membrane onto the collector

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Diffusion collectors

• diffusion collectors work by diffusion of the
  species down the gradient following a
  flux/gradient relationship Fick's Law
• R - DA dC/dx
• where R is the transport rate, D is a diffusion
  coefficient, A is the area of the inlet opening,
  and dC/dx is the concentration gradient through
  the inlet.

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Diffusion collectors contd

• if the collector removes all of the pollutant
  from the air it contacts, dC is simply equal to C
  the ambient concentration ? this is the principle
  of operation

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Permeation collectors

• in permeation collectors gas molecules pass
  through a membrane and are collected Overhead
  Fig 13-5
• the membrane and collection medium are specific
  to the gas of interest

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Air toxic sampling

• often expensive due to low concentrations and
  reactivity of many toxics - VOC, PAH, Dioxins and
  Furans

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Analysis and Measurement

• in many older types of monitoring equipment,
  analysis and measurement are separate steps from
  sampling - this is not the case for in-situ
  continuous measurements
• AQ measurement techniques usually pass through
  three technological stages
• qualitative identification
• separate collection and quantification (high lab
  costs, low installation costs)
• concurrent collection and quantification (high
  installation costs)

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Analysis of CO

• non-dispersive infra-red (NDIR) photometry
  exploits the fact that CO preferentially absorbs
  IR
• a hot filament IR source is passed through a
  reference cell of non-absorbing gas and a cell
  containing ambient air to a detector cell
  containing CO
• differential energy absorbed on either side
  causes the membrane to move, generating AC
  current Overhead Fig 14-1
• (must remove H2O vapour first)

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Analysis of ozone

• chemiluminescence is most commonly used
• when ozone and ethylene react chemically ?
  excited electron state ? fluoresce releasing
  light which is amplified by a photomultiplier and
  measured - light emitted is proportional to O3
• calibrated by putting a known O3 into the
  device

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Analysis of NO2

• also uses chemiluminescence
• the NO2 is determined as the difference between
  NO and NOX
• the reaction of NO with O3 releases light NO
  O3 ? NO2 O2 h?
• the light signal is proportional to NO
• this is a dual instrument - one side has a
  converter th change NO2 to NO Overhead Fig 14-4
  

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Analysis of SO2

• continuous methods involve many techniques such
  as flame photometry
• air with SO2 is fed to an H2 flame and light
  emissions from electronically excited combustion
  products are detected by a photomultiplier

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Analysis of particulate matter

• Two systems are in common use in BC
• HI-VOL - these systems are very simple,
  consisting of
• an inlet which removes particles greater than 10
  or 2.5 um
• a filter which traps the particles
• a pump system which draws air through the inlet
  and filter
• A known volume of air is drawn through the
  filter for a specified length of time. The
  particulate concentration is found as the mass
  difference of the filter before and after
  sampling divided by the total volume of air drawn
  through the filter. In BC there is a network of
  HI-VOL samplers which run for 24 hours on a six
  day cycle. (giving 24 h averages)
• Also Partisol samplers are commonly used as the
  FRM (Federal Reference Method)

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Partisol 2000 FRM Air Sampler
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Analysis of particulate matter contd

• 2. TEOM - (Tapered Element Oscillating
  Microbalance) - consists of the same three
  elements as the HI-VOL with the following
  additions
• the filter rests on top of a tapered glass
  element which is forced to oscillate
• the frequency of the oscillation is measured and
  its change depends on the change in mass of
  particulate on top of the element - this is the
  principle of measurement
• TEOM's are capable of continuous real-time
  measurements, which makes them ideal for
  regulatory monitoring, and issuing air quality
  advisories, etc.

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• Thermoscientific
• TEOM 1400ab

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Analysis of particulate matter contd

• HARVARD IMPACTOR used in wood-smoke study is a
  cascade impactor
• Here sampler used for outdoor measurement in the
  Noullett et al (2006) study (with heat tape to
  avoid frost) is shown on left, and backpack
  arrangment for personal monitoring on right

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• 4. DATARAM is a light scattering nephelometer or
  photometer.
• individual particles interact with a light beam
  in light sensing chamber intensity of light
  scattered over a forward angle of 45 to 90
  degrees is linearly proportional to to the
  concentration intensity of scattered light is a
  function of both diameter and refractive index of
  the particle.
• pDR 1000 is a passive sampler and does not
  specifically select for a specific size range.
• air surrounding the monitor circulates freely
  through the open sensing chamber by natural
  convection, diffusion and background air motion.
• configuration produces optimal volume response to
  particles in the size range of 0.1 to 10
  micrometers but research shows best correlation
  is with 2.5 size fraction (Lui et al, 2002).
• pDR results not directly comparable to filter
  based measurements and standards based on these
  methods.

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Atmospheric Chemistry

• Air Pollution chemistry refers to the subset of
  atmospheric chemical processes which have impact
  on life, vegetation, water or soil
• can be anthropogenic or natural e.g. Mount St.
  Helens in 1980 was a gigantic point source of SO2
  and PM which was first a regional air pollution
  problem, then a global one

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Chemistry contd

• most chemical transformations in the atmosphere
  are oxidation processes ? those involving C, N, S
  are of most interest
• in the troposphere there is oxidation of
  hydrocarbons, NO, SO2 to form aldehydes, NO2,
  H2SO4 (secondary pollutants) OH Fig 12-1

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• solar radiation interacts with some molecules ?
  free radicals formed by photodissociation (e.g.
  O, H, OH, HO2)
• in areas with photochemical smog, the main
  photoacceptors are aldehydes, NO2, nitrous acid
  (HNO2) and O3

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• Ozone is formed following a cycle
• NO2 h? ? NO O
• O O2 M ? O3 M
• NO O3 ? NO2 O2
• (M is any third body molecule, like N2 or O2)
• The photostationary state expression for O3 is
• O3 k1NO2/NO
• k1 is a rate constant that depends on location
  and time of year
• Most NO must be converted to NO2 before O3 will
  build up in the atmosphere
• Hydrocarbons (VOCs) increase the rate of
  conversion of NO to NO2, allowing O3 to build up
  further

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Readings for next week