Introduction to Measurement Techniques in Environmental Physics

1. Introduction to Measurement Techniques in Environmental Physics University of Bremen, summer term 2006 In-situ Measurement Techniques Andreas Richter (richter@iup.physik.uni-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 => 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

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 turns 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 European billon = 1012 !

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 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)

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 => 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 -> 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.

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 => 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 + NO2OH + CO -> H + CO2H + 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

15. Peroxy Radical Chemical Amplification (RO2*=ΣHO2 + RO2) L Chain Length CL RO2 + NO→ NO2 + ... + RO RO2 + 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 => 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 • 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

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 (1:1 negative) and NO2 (5-10% positive) • solution preparation has large impact on measurement accuracy • pump efficiency is reduced at high altitudes

21. 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/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