Introduction to Measurement Techniques in Environmental Physics University

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 ) Date 9 – 11 11 – 13 14 – 16 April 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) Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 1

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 Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 2

Which quantities do we need to measure? • • • pollutants in the atmosphere, in particular those that are regulated by law (e. g. CO, SO 2, NOx) key species in atmospheric chemistry (e. g. OH, O 3) green house gases (e. g. CH 4) 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 Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 3

Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 4

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. CH 4 Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 5

Abundance Units quantity name units number of molecules N mol = 6. 022 x 1023 number density n particles / m 3 mass density kg / m 3 volume mixing ratio ppm. V = 10 -6 ppb. V = 10 -9 ppt. V = 10 -12 mass mixing ratio ppmm =10 -6 ppbm =10 -9 pptm = 10 -12 column abundance molec/cm 2 DU = 10 -3 cm at STP k. B = 1. 38 1023 J mol-1 K-1 Beware: ppb = part per billion = 10 -9 although European billon = 1012 ! Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 6

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 2006 7

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 = I 0 exp{ α s} • Reference (I 0) 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 2006 8

In-situ Absorption Measurements II Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 9

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 Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 10

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, CO 2, SO 2, CH 4 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 2006 11

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, Br. O, Cl. O 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 (Cl. O, Br. O) 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 2006 12

Chemiluminescence I Idea: In some exothermic reactions, part of the energy is released as photons that can be measured by a photomultiplier. Example: O 3 + NO -> NO 2* + O 2 NO 2* -> NO 2 + h NO 2* + M -> NO 2 + M The emitted intensity depends on the effectiveness of quenching which is proportional to the pressure and the concentrations of [O 3] 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 2006 13

Chemiluminescence II Application: O 3, NO 2, NOy, ROx • NO, O 3: direct measurement by adding excess of the other species • O 3: also reaction with ethene: O 3 + C 2 H 4 => HCHO* + others HCHO* => HCHO + h • NO 2: photolysis to NO or reaction with luminol (interference by PAN and O 3) • NOy: conversion to NO with CO on gold converter • ROx: conversion to NO 2 through chemical amplification through NO and CO HO 2 + NO -> OH + NO 2 OH + CO -> H + CO 2 H + O 2 + M -> HO 2 + M detection of NO 2 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 2006 14

Peroxy Radical Chemical Amplification (RO 2*=ΣHO 2 + RO 2) L Chain Length CL RO 2 + NO → NO 2 +. . . + RO HO 2 + NO → NO 2 +. . . + OH RO 2 + O 2 → R. . COR. + HO 2 OH + CO → HO 2 + CO 2 L Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 15

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 2006 16

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” Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 17

Reminder: Mass Spectrometers Sector magnet: kinetic energy: Ekin = q U = ½ m v 2 movement in magnetic field: m v 2 / r = q v B mass as function of B filed: m = q / (2 U) B 2 r 2 Time of flight: energy of ion: mass as function of time: q U = Ekin m = 2 q U r 2 / s 2 Quadropole all but resonant ions are on unstable trajectories http: //physik 2. uni-goettingen. de/f-prakt/massenspektrometrie. htm Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 18

Ozone Sondes (ECC) Idea: Titration of ozone in a potassium iodide (KI) solution according the redox reaction: 2 KI + O 3 + H 2 O I 2 + O 2 + 2 KOH Measurement of "free" iodine (I 2) 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: I 2 + 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 O 3 distribution up to the stratosphere Problems: interference by SO 2 (1: 1 negative) and NO 2 (5 -10% positive) • solution preparation has large impact on measurement accuracy • pump efficiency is reduced at high altitudes Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 19

Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 20

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/2 Etext. 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 Introduction to Measurement Techniques in Environmental Physics, A. Richter, Summer Term 2006 21