in-situ measurement techniques - doas

Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 1

Introduction to Measurement Techniques in Environmental Physics

University of Bremen, summer term 2006

In-situ Measurement Techniques

Andreas Richter ( [email protected] )

Date 9 – 11 11 – 13 14 – 16April 19 Atmospheric Remote

Sensing I (Savigny)Oceanography

(Mertens)Atmospheric Remote Sensing II (Savigny)

April 26 DOAS (Richter) Radioactivity (Fischer)

Measurement techniques in Meteorology (Richter)

May 3 In-situ measurement techniques (Richter)

Soil gas ex- change (Savigny)

Measurement Techniques in Soil physics (Fischer)

mailto:[email protected]


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


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 => 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


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


Abundance Units

quantity name units

number of molecules N mol = 6.022 x1023

number density n particles / m3

mass density kg / m3

volumemixing ratio

ppmV = 10-6

ppbV = 10-9

pptV = 10-12

massmixing ratio

ppmm =10-6

ppbm =10-9

pptm = 10-12

column abundance molec/cm2

DU = 10-3 cm at STP

Beware: ppb = part per billion = 10-9 although European billon = 1012 !

kB = 1.38 1023 J mol-1 K-1


Pre-treatment of air samplesProblem: Often, air samples have to be pre-treated to concentrate the species of interest or to

remove unwanted interfering speciesFilters: e.g. from Nylon or Teflon are used to extract species from airflow for later analysisProblems: interference by particles, lack of specificity, change of collection efficiencyDenuders: 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


In-situ Absorption Measurements IIdea: 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)


In-situ Absorption Measurements II


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 => ozone concentrationg a s in

g a s o utd e te c to r 2

d e te c to r 1

o zo ne sc rub b e r3 wa y va lve

UV la m p


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.


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, ClOLight 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


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 -> 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.


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 + 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


Peroxy Radical Chemical Amplification (RO2*=ΣHO2 + RO2)

NORO HO ROCL

* 2

2 2 2

Chain Length CL

L

L

RO2 + NO → NO2 + ... + RO

HO2 + NO → NO2 + ... + OHRO2 + O2 → R..COR.+ HO2

OH + CO → HO2 + CO2


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

timeDisadvantages: • 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.


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 detectedAdvantages: • very high sensitivityDisadvantages: • 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”

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

multichannel plate detectors)


Reminder: Mass SpectrometersSector 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/massenspektrometrie.htm

http://physik2.uni-goettingen.de/f-prakt/massenspektrometrie.htm


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 solutionApplications: 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


Summary

• 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

Some References to sources used

• 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

http://www.umweltbundesamt.de/messeinrichtungen/2Etext.pdf

in-situ measurement techniques - doas

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