Transcript In-situ Measurement Techniques

Introduction to Measurement Techniques in
Environmental Physics
University of Bremen, summer term 2014
In-situ Measurement Techniques
Andreas Richter ( [email protected] )
9– 11
Introduction / DOAS
April 23
11 – 13
14 – 16
Absorption Spectroscopy Measurements of Trace
Gases
April 30
Measurement Techniques
in Oceanography
Radioactivity
Satellite Image Analysis
May 7
Cavity Ring Down
Spectroscopy
Fourier Transform
Spectroscopy
Meteorological
Measurements
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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Overview
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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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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Which quantities do we need to measure?
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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)
greenhouse gases (e.g. CH4)
ozone depleting substances (e.g. halons)
aerosols => not treated here
How do we want to measure them?
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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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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 turn
determines the horizontal
and vertical
inhomogeneity found in
the atmosphere.
Example: OH vs. CH4
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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Abundance Units
quantity
name
units
number of molecules
N
mol = 6.022 x1023
number density
n
particles / m3
mass density

kg / m3
volume
mixing ratio

ppmV = 10-6
ppbV = 10-9
pptV = 10-12
mass
mixing ratio

ppmm =10-6
ppbm =10-9
pptm = 10-12
column abundance
molec/cm2
DU = 10-3 cm at STP
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!
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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 Beer’s law for absorber concentration: I = I0 exp{ α 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)
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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In-situ Absorption Measurements II
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In-situ Absorption Measurements III: Ozone Photometer
detec tor 2
gas out
detec tor 1
UV lam p
3 way valve
gas in
ozone sc rubber
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 => ozone
concentration
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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.
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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.
NO2
O3
Example:
O3 + NO -> NO2* + O2
NO2*
-> NO2 + h
NO2* + M -> 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.
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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 => HCHO* + others
HCHO* => 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 -> OH + NO2
OH + CO -> H + CO2
H + O2 + M -> HO2 + M
detection 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
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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Peroxy Radical Chemical Amplification (RO2*=ΣHO2 + RO2)
L
Chain Length CL
RO2 + NO →
NO2 + ... + RO
HO2 + NO →
NO2 + ... + OH
RO + O2 → R..COR.+ HO2
OH + CO → HO2 + CO2
L
NO2
RO  HO2   RO2 
CL
*
2
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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.
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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Mass Spectrometry
ionizer:
• chemical ionization
• laser photoionization
mass filter:
• quadrupole
• time-of-flight (TOF)
detectors:
• electron multipliers (channeltrons or
multichannel plate detectors)
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 => unambiguous
identification of an ion not simple as it might
not be the “parent ion”
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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
(Lozentz force = Zentripetal force)
mass as function of B field:
m = q / (2 U) B2 r2
Time of flight:
energy of ion:
mass as function of time:
q U = Ekin
m = 2 q U t2 / s2
Quadropole
all but resonant ions are on unstable trajectories
http://physik2.uni-goettingen.de/f-prakt/massenspektrometrie.htm
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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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 (1:1 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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Example: Pollution Monitoring in Bremen
Network:
• 8 stations
• separated into “background” and trafic
related
BLUES = Bremer
LuftUeberwachungs System
Quantities measured:
SO2:
UV fluorescence
CO :
gas correlation
NOx:
chemiluminescence
PM:
gravimetric
O3:
UV absorption
benzene: gas chromatography
• 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?gsid=bremen02.c.2619.de
Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2014
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Example: Pollution Monitoring in Bremen
Measurements in Bremerhaven:
CO
NO2
O3
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?
http://www.umwelt.bremen.de/de/detail.php?gsid=bremen02.c.2619.de
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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
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http://www.umweltbundesamt.de/messeinrichtungen/2Etext.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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