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Insitu Measurement Techniques

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Title: Insitu Measurement Techniques


1
Introduction to Measurement Techniques in
Environmental Physics University of Bremen,
summer term 2008 In-situ Measurement
Techniques Andreas Richter ( richter_at_iup.physik.u
ni-bremen.de )
2
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

 
3
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

4
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5
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
    turn determines the horizontal and vertical
    inhomogeneity found in the atmosphere.
  • Example OH vs. CH4

6
Abundance Units
kB 1.38 1023 J mol-1 K-1
Beware ppb part per billion 10-9
although British billion used to be 1012 as it
still is in most other languages!
7
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
8
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)

9
In-situ Absorption Measurements II
10
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

11
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.
12
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

13
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.
14
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

15
Peroxy Radical Chemical Amplification (RO2SHO2
RO2)
L
Chain Length CL


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

OH CO ? HO2 CO2
L
16
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.

17
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)

18
Reminder Mass Spectrometers
  • Sector magnet
  • kinetic energy Ekin q U
    ½ m v2
  • movement in magnetic field m v2 / r q v B
  • (Lozentz force Zentripetal force)
  • mass as function of B field m q / (2 U) B2
    r2
  • Time of flight
  • energy of ion q U Ekin
  • mass as function of time m 2 q U t2 / s2
  • Quadropole
  • all but resonant ions are on unstable trajectories

http//physik2.uni-goettingen.de/f-prakt/massenspe
ktrometrie.htm
19
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

20
Example Pollution Monitoring in Bremen
  • Network
  • 8 stations
  • separated into background and trafic related
  • Quantities measured
  • SO2 UV fluorescence
  • CO gas correlation
  • NOx chemiluminescence
  • PM gravimetric
  • O3 UV absorption
  • benzene gas chromatography

BLUES Bremer LuftUeberwachungs System
  • data not meant for science applications
    (automated, no rigorous scientific data analysis)
  • data aimed at surveillance of air quality,
    control of thresholds, support for legislation

http//www.umwelt.bremen.de/de/detail.php?gsidbre
men02.c.2619.de
21
Example Pollution Monitoring in Bremen
  • Some results
  • strong improvement for CO since 1990
  • significant improvements for NOx
  • no improvement at all for ozone
  • Interpretation
  • emission control (cars, power plants, ...) has
    the desired effects for CO
  • improvements in NOx emissions are partly offset
    by increased number of cars
  • ozone levels increase due to other factors
  • long range transport of pollutants?
  • import from the stratosphere?

Measurements in Bremerhaven
CO
NO2
O3
http//www.umwelt.bremen.de/de/detail.php?gsidbre
men02.c.2619.de
22
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23
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

 
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