Measuring Particles and Aerosols in the Atmosphere.

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Measuring Particles and Aerosols in the
Atmosphere.
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Aerosol Production and Composition.
Airborne particles are one of the most obvious
forms of pollution, they are the visible smog
frequently seen in the urban and industrial
environment. The famous London smogs were due to
high particle/aerosol loading caused by coal
fires (carbonaceous soot) in the 1940s 50s
whereas the Los Angeles smogs were photochemical
in nature. 1940s London Primary
Los Angeles Secondary
The main property that determines the behaviour
of airborne particles is the aerodynamic
diameter this is very similar to the geometric
diameter.
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Particle/Aerosol Modes.
Nucleation Mode (lt0.2 ?m diameter). Particles
emitted from processes involving condensation of
hot vapours (e.g. incinerators smelters), or
particles freshly formed within the atmosphere by
GAS TO PARTICLE conversion (e.g. sulphuric acid
particles from sulphur dioxide oxidation).
Nucleation mode particles have a rather transient
experience, as the will rapidly coagulate into
larger particles hence in many situations this
mode is not found. Accumulation Mode (0.2 - 2.0
?m diameter). This mode comprises of particles
grown from the nucleation mode by coagulation or
condensation of vapours. These are the most
stable and long lived atmospheric particles with
a lifetime of 7-30 days, as they are not subject
to gravitational settling, scavenging by rain or
any of the other mechanisms which remove smaller
and larger particles. Coarse Mode (gt2.0 ?m
diameter). These are particles are generally
formed mechanically by attrition processes. They
have short atmospheric lifetimes due to their
high settling speeds (large size). Examples Soil
dust, sea spray, many industrial dusts, bacteria,
spores, pollen volcanic eruption.
Aerosols are measured in either mass
concentration (e.g. ?g m-3) or number
concentration (cm-3)
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Size Ranges of Atmospheric Aerosols.
Drizzle
Mist
Clouds and fog
Metal dust/fumes
Colloidal silica
Nebulizer
Tobacco smoke
Cloud condensation nuclei (CCN)
Sea Salt
Coal Dust
Gas molecules
Spray dried milk
Milled flour
Pollen
Particle Diameter (?m)
coarse
nucleation
accumulation
Fine particles
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Typical Diesel Particle Size DistributionsNumber,
Surface Area, and Mass Weightings are also shown
Issues Associated with Size and Composition
Measurements Using a nano-MOUDI, David Kittelson,
Win Watts, and Jason Johnson CRC Workshop on
Vehicle Exhaust Particulate Emission Measurement
Methodology, San Diego, California, 21 October
2002
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Sea Spray Particles
Bubbles produced usually by action of wind on the
sea - whitecapping - but also in surf zone and
when bubbles released by other processes such as
warming of sea waters, release of gas from
volcanoes and natural gas deposits etc.
Film drops - several hundred 1 µm Jet drops -
1-5 / bubble 5-10µm Spindrift gt 30µm
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Health Implications.
Much attention is now focused on finer particles
known as PM10, these are ALL particles less than
10 microns (lt10 ?m). These fine particles can be
breathed more deeply into the lungs and are more
likely to have a toxic effect than larger
particles. Increasing concern now surrounds even
finer particles known as PM2.5, PM1.0 and the
smallest PM0.1.


Much of the particulate in the urban environment
is carbonaceous soot from combustion, however
such material has a strongly adsorbent surface
and organic combustion products can stick to
the surface and thus can be inhaled, the finest
particles can penetrate into the alveolar in the
lungs and then the organics can enter the blood
stream.


Measurements of PM10 have only been carried out
in this country for the last few years - too
short a period to be able to identify any
significant trends. PM2.5 monitoring has only
recently commenced in a few areas of London.
Again, it is likely that improvements brought
about by a decrease in coal burning and improved
technology are at least partly offset by
increased numbers of vehicles on the road. The
increased market share of diesel vehicles, which
typically emit more PM10 particles than petrol
vehicles, exaggerates this.
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Climatic Effects of Aerosols. IPCC 2007
An important thing to measure!

• Depending upon their size and composition they
• can scatter and/or absorb solar and terrestrial
  radiation (known as the direct aerosol effect)
• can act as cloud condensation nuclei thereby
  modifying the microphysical, optical and lifetime
  of clouds (indirect aerosol effect) and modify
  atmospheric profiles of temperature and relative
  humidity (semi-direct aerosol effect)
• In the so-called 1st indirect effect the
  available liquid water is spread over more
  droplets leading to smaller droplets and a higher
  cloud albedo.
• In the 2nd effect the cloud lifetime, cloud
  fractional cover and drizzle efficiency may be
  altered. These changes to cloud properties then
  affect the radiation indirectly.

A VERY VERY important thing to measure!
Forster, et al., 2007 Changes in Atmospheric
Constituents and in Radiative Forcing. In
Climate Change 2007 The Physical Science Basis.
Working Group IFourth Assessment Report of the
IPCC
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The Basics of an Aerosol Sampling System.
Filter (single or multiple) or in-situ
measurement (e.g. TEOM, OPC, CPC)
Transmission
Size Selection
Calibrated flow control
Collecting or sensing medium
Inlet design (PM10 or PM2.5)
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Streamlines Showing Sample Flowlines
Isokinetic Sampling. When sampling from a moving
stream on air (ducts, stacks and aircraft) it is
critical that the probe is aligned parallel to
the direction of flow. The velocity of the air
flow inside the probe is equal to the free-stream
velocity forward on the inlet. Isokinetic
sampling ensures that no particles are lost
through wall collisions. It does NOT however
guarantee that there is complete transmission of
ALL particles to the sampling device.
In general the sample flow inside the probe is
maintained using a pump, this means you must
calculate the pump flow required to match the
bulk flow outside the probe (U0 U)
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Non-Isokinetic Sampling (Anisokinetic)

• Misalignment
• The sample probe is NOT parallel with the bulk
  flow (??0), large particles with greater momentum
  overshoot the inlet. Resulting in a biased
  distribution entering the inlet.
• Super-isokinetic
• When the sample flow exceeds the bulk flow
  (UgtU0), as with Misalignment the larger particles
  are likely to be lost as their momentum is too
  large to allow them to be directed into the
  inlet.
• Sub-isokinetic
• This occurs when the bulk flow is greater than
  the sample flow (UgtU0). Smaller and lighter
  particles are drawn around the probe whereas the
  heavier particles enter the inlet.

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Particle Measuring Methods
Mass Loadings - filter methods - cascade
impactors - cyclones
Size and Number concentrations - optical
particle counters - mobility analysers -
microscopy
Composition - volatility technique -
particle mass spectrometers
Aerosol Scattering/Absorption -
nephelometers - aethalometers
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Measuring the Mass Loading
Filter Sampling - Simple, low resolution. Casca
de Sampling - More size fractions, still low
resolution TEOM - Single size range BUT much
high temporal resolution

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Filter Sampling
The most simple method of collecting particulate
material is to draw air through a filter. The
pore size of the filter can be used to add a
degree of selectivity in particle size
collected. Very large sample volumes are
required, typically 50-1500 m3, hence known as
Hi-Vols, samples take 12 - 24 hours to
collect. Total flow rate must be measured to
allow production of a concentration. Filters must
be measured before exposure and kept cool once
they have been used when analysing for VOC (PAH
PCB). Analysis options- Weigh filter gives a TSP
value (total suspended particulate), comparison
with a simple colour chart, but there are
different charts for different particle types
(coal - black, diesel - brown) and when a
comprehensive analysis is required use
soxhlet/SFE followed by LC/GC. Also look at
particle base material using SEM.
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Cascade Impactor.
Cascade impactors can be thought of as a stack of
sub-isokinetic inlets. Impaction plates collect
progressively smaller size fractions of
particles. Low resolution since a lot of sample
must be collected for subsequent analysis. As
with single filters analysis can be comparison
with a simple colour chart, weighing or
extraction followed by LC/GC
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Cascade Impactor and Hi-Vol Sampler.
2 stage filter holder (1 - glass fibre 2 - PUF)
Pump
Flow meter
To pump
Timer
Impaction cells
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Virtual Impactor.
Uses sub-isokinetic inlet to split the size
fractions
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Cyclone Particle Collectors
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TEOM - Taped Element Oscillating Microbalance.

• A filter is mounted on the top of a glass stalk
  which vibrates horizontally at high frequency
  (200-600 Hz).
• The sample flow passes through the filter, where
  particulate matter collects, and then continues
  through the hollow tapered element on its way to
  an active volumetric flow control system and
  vacuum pump.
• As particulate material is deposited on the
  filter the resonant frequency of the element
  changes.
• An inertial balance directly measures the mass
  collected on the exchangeable filter cartridge by
  monitoring the corresponding frequency change.
• Mass calibration may be verified, using an filter
  of known mass.
• Active flow control is maintained and set points
  are constantly adjusted in accordance with
  ambient temperature and pressure.

vibration
Glass stalk
Filter
Air drawn down central tube by calibrated pump
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Size Selection - TEOM
By changing the sampling head (inlet dimensions)
it is possible to monitor different size ranges
(PM10, PM2.5 PM1.0)
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Condensation Particle Counter (CPC).
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Optical Particle Counters
Dump spot
Particle size affects scattering of the light beam
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Aerosol Instruments come is all sorts of
shapes (FSSP, OPA, ADA)
Ice Detector
Specialised aircraft instrument detects
reflection of light in many directions and uses
this to construct a shape for the ice, this
gives much information beyond just the size
(reflectivity, composition, age..)
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Differential Mobility Analyser (DMA).

• Particles are charged as they enter (having first
  all been neutralised to standardise).
• The central rod is at a high potential.
• Particles are discriminated by their electrical
  mobility (function of size and charge).
• Only a narrow size cut passes through the inlet
  to the instrument.

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Scanning Mobility Particle Sizer (SMPS).

• Aerosol sample first passes through a
  single-stage inertial impactor that removes large
  particles (they can contribute to data inversion
  errors caused by multiple charging).
• This aerosol passes through a bipolar ion
  neutralizer, which produces a high concentration
  of positive and negative ions
• Charged and neutral particles enter the
  Differential Mobility Analyzer (DMA), which
  separates particles according to their electrical
  mobility.
• Particles with negative charge are repelled
  towards and deposited on the outer wall of the
  DMA.
• Particles with a neutral charge exit with the
  excess air.
• Particles with a positive charge move across the
  flow, towards the negatively charged center rod
  at a speed proportional to their electrical
  mobility.
• Only particles within a narrow range of
  electrical mobility have the correct trajectory
  to pass through an open slit in the center rod
  near the DMA exit.
• After exiting the DMA, the classified particles
  enter a Condensation Particle Counter (CPC),
  which provides a measurement of particle number
  concentration.
• By ramping the voltage of the charged rod or disk
  over a user-selected period of time, the entire
  particle size distribution is measured to a high
  degree of accuracy.

__________
Central rod negatively charged
Remember Mass Spec??
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Complete Maritime Aerosol Spectrum
Obtained using a number of instruments
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Composition - Volatility Techniques.
By cycling the particles through a range of
temperatures, the presence of specific aerosol
components may be identified.
While lacking the precision of more conventional
chemical analysis techniques, this method enables
size-differentiated composition of the aerosol
particles to be determined in near real-time.
New techniques such as aerosol particle mass
spectrometers may offer more detail but the
volatility approach is well-suited to studies
where aerosol particles principally consist of a
limited number of inorganic species.
Volatilities (deg C) MSA 50 H2SO4
100 (NH4)2SO4 200 NaCl -650 Elemental C - 800
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Marine Boundary layer 1500 ft Constant level
of gt3nm particles (ultrafine) Less of the larger
particles (but can see composition transition)
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Aerosol Time-of-Flight Mass Spectrometer
Particle size range 0.3 to 3µm high
resolution sizing, with 0.1 to 3µm
optical detection Display resolution 32
channels per decade of particle size
(logarithmic), 34 channels total Mass range
1 to 400 Da Total inlet flow rate
(volumetric) 0.9 0.05 l/min
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NdYAG laser desorption / ionisation