Insitu Measurement Techniques

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Introduction to Measurement Techniques in
Environmental Physics University of Bremen,
summer term 2006 In-situ Measurement
Techniques Andreas Richter ( richter_at_iup.physik.u
ni-bremen.de )
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Overview
 

• some general thoughts on measurements of chemical
  species in the atmosphere
• some standard techniques for in-situ measurements
• some problems related to these techniques
• some applications

 
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Which quantities do we need to measure?
 

• pollutants in the atmosphere, in particular those
  that are regulated by law (e.g. CO, SO2, NOx)
• key species in atmospheric chemistry (e.g. OH,
  O3)
• green house gases (e.g. CH4)
• ozone depleting substances (e.g. halons)
• aerosols gt not treated here

How do we want to measure them?

• in as many places as possible
• as continuously as possible
• as reproducible as possible
• at concentrations covering both background
  conditions and high levels
• at all relevant altitudes in the atmosphere

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Temporal and Spatial Scales

• The requirements on
• the number of measurements
• the sampling frequency
• the geographical distribution of the measurements
• depends on the life time of the species which in
  turns determines the horizontal and vertical
  inhomogeneity found in the atmosphere.
• Example OH vs. CH4

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Abundance Units
kB 1.38 1023 J mol-1 K-1
Beware ppb part per billion 10-9 although
European billon 1012 !
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Pre-treatment of air samples
Problem Often, air samples have to be
pre-treated to concentrate the species of
interest or to remove unwanted interfering
species Filters e.g. from Nylon or Teflon are
used to extract species from airflow for later
analysis Problems interference by particles,
lack of specificity, change of collection
efficiency Denuders removal of a gas from a
laminar airflow by diffusion to the walls of a
coated tube Mist chamber and scrubber
air is passed through a chamber where a mist of
water or other aqueous solution is used to scrub
out a species of interest
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In-situ Absorption Measurements I

• Idea use characteristic wavelength dependence of
  absorption by species of interest
• Absorption measurements in the UV or IR,
  depending on the molecule of interest
• Lambert Beers law for absorber concentration I
  I0 exp? a s
• Reference (I0) by
• comparing measurements with / without absorber
• comparing measurements with reference of known
  absorption
• comparing measurements at different wavelengths
• Selectivity by
• chemical preconditioning
• use of optical filters
• use of interfering absorbers
• use of wavelength sensitive detectors
  (spectrometers)
• use of wavelength specific light source (e.g. in
  Tunable Diode Laser Spectroscopy, TDLS)
• Improved sensitivity by
• multipass cells
• cavity ring-down (CRD)
• concentration (e.g. Matrix Isolation Spectroscopy
  MI)

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In-situ Absorption Measurements II
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In-situ Absorption Measurements III Ozone
Photometer

• Principle of ozone photometer
• absorption measurement at 253.7 nm (Hg line).
• use of ozone scrubber to produce ozone free air
  flow for reference.
• use of second detector to monitor lamp output
• combination of both detector signals to determine
  ozone absorption gt ozone concentration

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Gas Correlation
Idea Achieve highly specific absorption
measurements by using gas of interest as filter
in front of detector. Absorption (or emission)
structures of the gas correlate 100 with the
filter any other absorption pattern is
averaged out. Application CO, CO2, SO2,
CH4 Problems Only for one species, works best
for low pressures (no pressure broadening), p and
T must agree between measurement sample and cell.
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Resonance Fluorescence

• Idea When illuminated with light at a wavelength
  corresponding to an electronic transition,
  photons are absorbed and re-emitted at the same
  wavelength with high efficiency in all
  directions. If the exciting light beam is well
  focused, the fluorescence can be measured
  orthogonal to the incident light beam without
  much interference.
• Application OH, BrO, ClO
• Light source
• for atoms microwave discharge lamp using the
  target species
• for molecules laser (LIF)
• Advantages
• high sensitivity
• highly specific (resonance)

• Problems
• flow must be well characterised (wall losses,
  chemical losses)
• geometry must be well known
• scattering in the instrument must be suppressed
• species of interest (ClO, BrO) must be converted
  to measurement quantity (Br, Cl) by reaction with
  NO and alternating NO addition between ON and OFF
• works only at low pressures

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Chemiluminescence I
Idea In some exothermic reactions, part of the
energy is released as photons that can be
measured by a photomultiplier. Example O3 NO
-gt NO2 O2 NO2 -gt NO2 h? NO2 M
-gt NO2 M The emitted intensity depends on the
effectiveness of quenching which is proportional
to the pressure and the concentrations of O3
and NO. If pressure and one concentration are
kept constant, the intensity is proportional to
the concentration of the other.
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Chemiluminescence II

• Application O3, NO, NO2, NOy, ROx
• NO, O3 direct measurement by adding excess of
  the other species
• O3 also reaction with ethene
• O3 C2H4 gt HCHO others
• HCHO gt HCHO h?
• NO2 photolysis to NO
• or reaction with luminol (interference by PAN
  and O3)
• NOy conversion to NO with CO on gold converter
• ROx conversion to NO2 through chemical
  amplification through NO and CO HO2 NO -gt OH
  NO2OH CO -gt H CO2H O2 M -gt HO2
  Mdetection of NO2 through chemiluminescence of
  organic dye (luminol)
• Advantage High sensitivity
• Problems interference by other species,
  determination of amplification factor (chain
  length) in the case of ROx

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Peroxy Radical Chemical Amplification (RO2SHO2
RO2)
L
Chain Length CL


RO2 NO ? NO2 ... RO
RO2 O2 ? R..COR. HO2
HO2 NO ? NO2 ... OH

OH CO ? HO2 CO2
L
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Gas Chromatography

• Idea
• When passing through a heated column,
  different components have different speeds and
  therefore reach a detector at different times.
• Advantage
• detector does not have to be specific
• many species can be measured at the same time
• Disadvantages
• if detection is to be highly specific, specific
  detector is needed (such as (chemical ionisation)
  mass spectrometer)
• Usually, the samples have been collected in the
  field and are analysed in the lab, which can
  introduce problems from chemical or physical
  losses in the container and on the walls.

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Mass Spectrometry

• Idea
• gas flow is pre-treated (e.g. by gas
  chromatography)
• molecules are ionized
• their charge / mass ratio is used for separation
• amount of ions at different ratios is detected
• Advantages
• very high sensitivity
• Disadvantages
• needs low pressures (high voltages)
• ionization by electron impact often produces many
  fragments gt unambiguous identification of an ion
  not simple as it might not be the parent ion

• ionizer
• chemical ionization
• laser photoionization
• mass filter
• quadrupole
• time-of-flight (TOF)
• detectors
• electron multipliers (channeltrons or
  multichannel plate detectors)

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Reminder Mass Spectrometers

• Sector magnet
• kinetic energy Ekin q U
  ½ m v2
• movement in magnetic field m v2 / r q v B
• mass as function of B filed m q / (2 U) B2
  r2
• Time of flight
• energy of ion q U Ekin
• mass as function of time m 2 q U r2 / s2
• Quadropole
• all but resonant ions are on unstable trajectories

http//physik2.uni-goettingen.de/f-prakt/massenspe
ktrometrie.htm
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Ozone Sondes (ECC)
Idea Titration of ozone in a potassium iodide
(KI) solution according the redox reaction
2 KI O3 H2O ? I2 O2 2 KOH Measurement
of "free" iodine (I2) in electrochemical reaction
cell(s). The iodine makes contact with a platinum
cathode and is reduced back to iodide ions by the
uptake of 2 electrons per molecule of
iodine   I2 2 e- on Pt ? 2 I-   cathode
reaction

• the electrical current generated is proportional
  to the mass flow of ozone through the cell
• continuous operation through pumping of air
  through the solution
• Applications Measurement of vertical O3
  distribution up to the stratosphere
• Problems interference by SO2 (11 negative) and
  NO2 (5-10 positive)
• solution preparation has large impact on
  measurement accuracy
• pump efficiency is reduced at high altitudes

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Summary
 

• in-situ measurements of atmospheric trace gases
  have to cover a wide range of concentrations,
  temperatures and pressures
• they need to cope with the large number of
  species present in any air sample
• many measurement techniques rely on optical
  methods
• chemiluminescence is one typical effect used
• fluorescence is another effect applied to
  measurements
• gas chromatography is used for measurements and
  separation of mixtures
• mass spectrometry is an important tool
• wet chemistry methods are also used
• amplification, concentration, and purification
  (scrubbing) is often needed

Some References to sources used

• http//www.umweltbundesamt.de/messeinrichtungen/2E
  text.pdf
• Barbara J. Finlayson-Pitts, Jr., James N. Pitts,
  Chemistry of the Upper and Lower Atmosphere
  Theory, Experiments, and Applications, Academic
  Press  1999  
• Guy P. Brassuer, John J. Orlando, Geoffrey S.
  Tyndall (Eds) Atmospheric Chemistry and Global
  Change, Oxford University Press, 1999