analytical techniques for atmospheric measurement · 6.6 applications of aerosol mass spectrometric...

Analytical Techniques forAtmospheric Measurement

Edited by

Dwayne E. HeardUniversity of Leeds

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Edited by

Dwayne E. HeardUniversity of Leeds

© 2006 by Blackwell Publishing Ltd

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First published 2006 by Blackwell Publishing Ltd

ISBN-13: 978–1–4051–2357–0ISBN-10: 1–4051–2357–5

Library of Congress Cataloging-in-Publication DataAnalytical techniques for atmospheric measurement / edited by Dwayne E. Heard.

p. cm.Includes bibliographical references and index.ISBN-13: 978–1–4051–2357–0 (acid-free paper)ISBN-10: 1–4051–2357–5 (acid-free paper)1. Atmosphere—Measurement. 2. Meteorological instruments.3. Weather forecasting. I. Heard, Dwayne E.QC876.A53 2006551.51′1′028—dc22

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Contents

Preface xii

Acknowledgements xv

Contributors xvi

1 Field Measurements of Atmospheric Composition 1Dwayne E. Heard1.1 The role of field measurements in atmospheric science 1

1.1.1 Our changing atmosphere 11.1.2 The importance of atmospheric chemistry 21.1.3 Why field measurements of atmospheric composition are

important 31.1.4 The challenges of field measurements in the atmosphere 71.1.5 Comparison with calculations from numerical models 9

1.2 Scope, structure and content of this book 131.2.1 Scope and structure of the book 131.2.2 Previous texts describing methods for determining atmospheric

composition 141.2.3 Content of the book: Summary of individual chapters 15

1.3 The measurement of atmospheric composition 191.3.1 Units of concentration 191.3.2 Selection criteria for instruments 201.3.3 Instruments organised by classification of trace species 221.3.4 Instruments organised by analytical technique 22

1.4 Instrument platforms 241.4.1 Ground-based platforms, including vehicle-based mobile

laboratories 251.4.2 Ship-borne platforms 311.4.3 Balloon-borne platforms 321.4.4 Aircraft-borne platforms 331.4.5 Commercial passenger or freight aircraft platforms 341.4.6 Uninhabited aerial vehicles 351.4.7 Rocket platforms 351.4.8 Satellites and other space-borne platforms 36

1.5 Analytical methods not covered elsewhere in this book 411.5.1 LIDAR methods 411.5.2 Matrix isolation electron spin resonance (MIESR) 42

vi Contents

1.5.3 Solid-state and electrochemical sensors 431.5.4 Far-infrared and microwave absorption and emission

spectroscopy 441.5.5 Measurement of fluxes of trace gases and aerosols 45

1.6 Quality assurance and quality control 461.6.1 Precision and accuracy 471.6.2 Calibration of instruments 481.6.3 Intercomparison of instruments 50

1.7 Atmospheric chemistry and policy 521.7.1 Health effects and environmental policy 521.7.2 Monitoring networks 53

1.8 Major field campaigns for measurement of atmospheric composition 571.8.1 The design of field campaigns 571.8.2 Case study of a field campaign: The 2002 NAMBLEX campaign 59

1.9 Instrumented chambers for the study of simulated atmospheres 621.10 Future directions 63Acknowledgements 67Further reading 67References 68

2 Infrared Absorption Spectroscopy 72Alan Fried and Dirk Richter2.1 Introduction 722.2 Fundamentals of infrared absorption spectroscopy 73

2.2.1 Electromagnetic radiation in the infrared 732.2.2 Molecular interactions in the IR 752.2.3 Vibrational bands and rotational lines in the IR 772.2.4 Vibrational bands of atmospheric spectral features in the

mid-IR 812.2.5 Vibrational–rotational spectral line intensities 84

2.3 Quantitative trace gas measurements employing IR absorptionspectroscopy 872.3.1 IR absorption lineshapes and linewidths 872.3.2 Beer–Lambert absorption law and absorbance 94

2.4 Trace gases in the atmosphere 982.5 Measurement approaches employing IR absorption spectroscopy 99

2.5.1 In situ measurements 1032.5.2 Remote measurements 134

2.6 Advances in atmospheric studies employing IR absorptionmeasurements and future directions 1352.6.1 Measurements of CH2O mixing ratios and model comparisons 1352.6.2 Flux measurements of CH4 1362.6.3 Measurements in the Antarctic polar vortex 1372.6.4 Isotopic ratio measurements of water in the UT/LS 1372.6.5 Remote FTIR measurements 1382.6.6 Future directions 138

Contents vii

Acknowledgements 139Further reading 139References 140

3 UV-Visible Differential Optical Absorption Spectroscopy (DOAS) 147John M.C. Plane and Alfonso Saiz-Lopez3.1 Introduction 1473.2 Principles of differential optical absorption spectroscopy 148

3.2.1 The Beer–Lambert law 1483.2.2 Differential absorption spectroscopy 1493.2.3 Wavelength regions and differential absorption cross-sections 1513.2.4 Application to atmospheric measurements 153

3.3 DOAS using artificial light sources 1543.3.1 Broad-band light sources 1543.3.2 Laser sources 163

3.4 DOAS using scattered sunlight 1683.4.1 Spectroscopic principles 1693.4.2 Zenith-viewing geometry: Retrieval of NO3 vertical

profiles 1713.4.3 Multi-axis DOAS 173

3.5 Summary and conclusions 179Further reading 180References 180

4 Fluorescence Methods 189Ezra C. Wood and Ronald C. Cohen4.1 Introduction 1894.2 Basic aspects of fluorescence measurements 190

4.2.1 Molecular and atomic photophysics 1904.2.2 The fluorescence signal 1914.2.3 The background 1934.2.4 The signal-to-noise ratio and detection limit 1944.2.5 General design of fluorescence instruments 195

4.3 The hydroxyl radical 1984.3.1 Instruments for stratospheric and upper tropospheric OH

measurements 1994.3.2 Instruments for lower tropospheric OH measurements 2034.3.3 OH fluorescence as an indirect detection strategy 2064.3.4 Naphthalene 2064.3.5 Measurement of the OH lifetime 207

4.4 The nitrogen oxides 2084.4.1 Photophysics of NO2 2094.4.2 Single-photon NO2 instrument design 2104.4.3 Supersonic expansions 2124.4.4 Calibration of LIF NO2 instruments 213

viii Contents

4.4.5 Intercomparisons 2144.4.6 Thermal dissociation: LIF 2154.4.7 Two-photon LIF detection of NO 2174.4.8 Single-photon LIF detection of NO 2184.4.9 Photofragmentation two-photon LIF detection of NO2 2194.4.10 NO3 and N2O5 219

4.5 Detection of halogen compounds 2204.5.1 Spectroscopy and calibration 2214.5.2 Balloon-borne instruments 2224.5.3 Aircraft measurements 2224.5.4 ClONO2 and ClOOCl 222

4.6 Future directions 223Further reading 223References 224

5 Mass Spectrometric Methods for Atmospheric Trace Gases 229Jonathan Williams5.1 Introduction 229

5.1.1 Generating a mass spectrum 2295.1.2 Historical development of the mass spectrometer for air

measurements 2305.2 Fundamentals 234

5.2.1 The importance of vacuum 2345.2.2 The atomic mass scale 2345.2.3 Resolving power and resolution 236

5.3 Key elements of a mass spectrometer 2375.3.1 Ionisation 2375.3.2 Mass filters 2395.3.3 Detectors 244

5.4 Mass spectrometry used in atmospheric chemistry 2465.4.1 Atmospheric pressure ionisation-chemical ionisation mass

spectrometer (API-CIMS) 2465.4.2 The proton transfer reaction mass spectrometer 2495.4.3 The use of CIMS for radical measurement (OH) 2535.4.4 Coupled GC-MS capabilities 2565.4.5 The isotope ratio mass spectrometer 258

5.5 Future developments 260Acknowledgements 261Further reading 261References 261

6 Mass Spectrometric Methods for Aerosol Composition Measurements 265Hugh Coe and James D. Allan6.1 Introduction to atmospheric aerosols 2656.2 Physical aerosol measurements and sizing definitions 266

Contents ix

6.3 Particle composition 2696.3.1 Offline methods 2696.3.2 Online bulk sampling and analysis methods 271

6.4 Online aerosol mass spectrometry 2726.4.1 Mass spectrometer types 2736.4.2 Inlet technologies 2766.4.3 Particle sizing methods 2786.4.4 Ablation and ionisation techniques 281

6.5 Analysis of ambient data 2916.6 Applications of aerosol mass spectrometric techniques to atmospheric

aerosol sampling 2936.7 Summary 300Further reading 301References 302

7 Chemical Methods: Chemiluminescence, Chemical Amplification,Electrochemistry, and Derivatization 311Andrew J. Weinheimer7.1 Introduction 311

7.1.1 Scope 3117.1.2 Measurement highlights 312

7.2 O3 via heterogeneous chemiluminescence 3127.2.1 Early years (luminol and rhodamine-B) 3137.2.2 Since mid-1980s (eosin Y, coumarin-47, chromotropic acid) 3167.2.3 Summary 317

7.3 O3 via electrochemistry 3177.4 Nitrogen compounds �NO�NO2�NOy� via chemiluminescence with O3 319

7.4.1 The NO+O3 reaction 3197.4.2 The measurement of nitric oxide (NO) 3207.4.3 The measurement of nitrogen dioxide �NO2� 3237.4.4 The measurement of total reactive nitrogen (NOy) 3287.4.5 Routine NOx monitoring 331

7.5 Ozone via homogeneous chemiluminescence 3347.5.1 O3 + Ethene 3347.5.2 O3 + NO 334

7.6 NO2 via luminol 3367.7 Peroxides, HCHO, and HONO via liquid techniques 337

7.7.1 H2O2 via dissolution and chemiluminescence 3377.7.2 Peroxides via dissolution, derivatization, and fluorimetry 3387.7.3 HCHO via dissolution, derivatization, and fluorimetry 3407.7.4 HCHO via dissolution, derivatization, and HPLC 3407.7.5 HONO via liquid techniques 341

7.8 Isoprene via O3 chemiluminescence 3427.9 Peroxy radicals via chemical amplification 344

7.9.1 Using reagent NO and CO, followed by detection of NO2 3447.9.2 Using reagent NO and SO2, followed by CIMS detection of H2SO4 349

x Contents

7.10 Summary and future directions 353Further reading 354References 354

8 Chromatographic Methods 361Jacqueline F. Hamilton and Alastair C. Lewis8.1 Introduction 3618.2 Gas chromatography 363

8.2.1 Sample acquisition 3668.2.2 Sample preparation and injection 3688.2.3 Separation of atmospheric samples 3718.2.4 Detection 3748.2.5 Calibration and quality control 3778.2.6 Instrument deployment in the field 3788.2.7 Examples of applications of chromatography in atmospheric

analysis 3808.3 Liquid chromatography 391

8.3.1 Sample acquisition and preparation 3948.3.2 Separation 3948.3.3 Detection 3958.3.4 Examples of the application of liquid chromatography in air

analysis 3968.4 Future work 397Further reading 402References 402

9 Measurement of Photolysis Frequencies in the Atmosphere 406Andreas Hofzumahaus9.1 Introduction 406

9.1.1 Photolysis reactions in atmospheric chemistry 4069.1.2 Photolysis frequencies: Kinetic definition 4089.1.3 Photolysis frequencies: Physical model 4099.1.4 Actinic flux 4119.1.5 Irradiance 4119.1.6 Actinic flux in the atmosphere 412

9.2 Methods for measuring photolysis frequencies 4169.2.1 Measurement requirements 4169.2.2 General measurement methods 4179.2.3 Actinic-flux sensing techniques 4189.2.4 Radiative transfer models 419

9.3 Chemical actinometry 4209.3.1 Principle of chemical actinometry 4219.3.2 Chemical kinetics considerations 4219.3.3 Optical and photochemical considerations 4269.3.4 Ozone chemical actinometers 4279.3.5 Nitrogen-dioxide chemical actinometers 432

Contents xi

9.4 Actinic-flux spectroradiometry 4379.4.1 Principle of actinic-flux spectroradiometry 4379.4.2 Spectroradiometers for actinic-flux measurements 4399.4.3 Actinic-flux receiver optics 4429.4.4 Determination of spectral actinic fluxes 4449.4.5 Spectroradiometer calibration 4459.4.6 Determination of photolysis frequencies 448

9.5 Actinic-flux filter radiometry 4539.5.1 Filter radiometer set-up 4539.5.2 Determination of photolysis frequencies 4559.5.3 Filter radiometer calibration 4589.5.4 j�O1D� filter radiometers 4609.5.5 j�NO2� filter radiometers 464

9.6 Irradiance radiometry 4669.6.1 Principle of irradiance radiometry 4679.6.2 Physical model–based methods 4689.6.3 Empirical methods 471

9.7 Modelling of photolysis frequencies 4749.8 Sample applications 475

9.8.1 Photochemistry field experiments 4759.8.2 Temporal and spatial distributions 4769.8.3 Instrumental intercomparisons 4789.8.4 Molecular data assessments 480

9.9 Conclusions and outlook 482Acknowledgements 482Appendix 483

A.1 Radiation quantities and their relationships 483A.2 Effective transmission of solar radiation into a chemical

actinometer 485A.3 Spectral spectrometer bandwidth 488A.4 Correction of the angular response of an actinic-flux receiver

�2� sr� 489A.5 Equivalent plane-receiver concept 490

References 493

Index 501

Colour plate section after page 270

Preface

In almost all of the scientific journals where data from field measurements are presented,it is the results, and how they have advanced our understanding of our complex workingsof the atmosphere, that have received the most attention. We must not forget, however,that the instruments making the measurements are themselves state-of-the-art and highlyspecialised, having been first developed in the laboratory by chemists, physicists orengineers, before being applied to the measurement of atmospheric composition. Thereis usually insufficient space in the journal articles for a thorough description of theinstrument, and often only essential details are given. Although there are a numberof journals that specialise in the description of instruments, for example Review ofScientific Instruments, there is no common theme to a given issue, and an instrument tomeasure an important atmospheric constituent will likely be sandwiched between articlescovering instruments from unrelated fields. Published proceedings from conferences orsymposia devoted to instrumentation do offer some coherence, but the articles tendto be specialised, and although appealing to the highly expert reader, they are, in themain, not accessible to scientists wishing to further their knowledge in a new area. Thereare a number of textbooks that contain sections on instruments for measurements ofatmospheric composition, but these usually do not cover recent developments, nor dothey give a broad or detailed coverage of the techniques involved.New analytical techniques or detectors are often invented by researchers with little or

no interest in atmospheric science and are then ‘discovered’ by the atmospheric fieldmeasurement community, who go on to utilise the method to make significant break-throughs. More recently, there has been a stronger collaboration between atmosphericchemists, physicists, engineers and analytical scientists, to develop bespoke instrumentswith the sole intention of measuring new species in the atmosphere. Sometimes a scientistin a core discipline, for example a physical chemist, recognises the need for a particularmeasurement in the atmosphere, and realises that the techniques they have used in thelaboratory for fundamental studies can equally be applied in the field. Once a rarity, thispractice is now becoming more common.

Aims of the book

The major aim of this book is to take the focus away from the results and the advancesresulting from field measurements (very important though they are), and to place the

Preface xiii

emphasis on the instruments themselves: how they work, their strengths and weaknessesfor a particular task, the platforms on which they have been deployed, how they arecalibrated etc. The book explains the fundamental physical principles upon which theinstrumental techniques are based. For instance, what properties of molecules can beexploited to enable their detection? What limits the sensitivity and accuracy of a giveninstrument, and what information can be gained from its use? Instruments developedto make measurements of atmospheric composition are highly specialised, but oftenhave evolved over many years via several incarnations. The book attempts to convey theexcitement of the challenge to quantitatively measure trace atmospheric constituents thatare at the heart of atmospheric chemistry, and responsible for many of the problemsfacing society today; for example, the warming of our atmosphere, the destruction ofthe ozone hole, and the formation of urban air pollution and acid rain. Measurementsof the composition of our atmosphere range from the earth’s surface to the edge ofspace. Unsurprisingly, the most important species to measure, those that exert specialcontrol on our atmosphere, are often the most difficult to measure – only being presentin miniscule concentrations as a result of being so reactive. For some species, for examplethe hydroxyl radical (OH), it has taken over twenty years of instrument development toenable a reliable and accurate measurement. In some cases, just a single measurement hasopened up new areas for study – for example chlorofluorocarbon (CFC) measurementspioneered by Jim Lovelock using the electron capture detector.

Intended audience

The book is designed to appeal to two major types of audience. One class of readers arethose who wish to gain a general understanding of instrumentation for measurement ofatmospheric composition, the fundamental principles upon which techniques depend,their major capabilities, together with highlights of the important results and the advancesin understanding that have resulted – but without wanting a detailed discussion of theunderlying atmospheric chemistry or physics. The chapters have been written so that thisinformation is easily retrievable, and is accessible to the non-expert. These readers willbe final year undergraduates or postgraduate students starting out on a research project,or postdoctoral fellows or faculty (or indeed anyone) who wish to move into or learnmore about composition measurements in the atmosphere. The other class of readersare field scientists or instrument developers who are more experienced, and who willbe interested in the finer detail of specific instruments, and latest developments, andperhaps wish to discover if a particular technique were suitable for a new measurement.The individual chapters within this book contain a high level of instrumental detail notnormally found in the literature due to lack of space in regular journal articles, and alsoinclude a discussion of the strengths and weaknesses of a particular method.

Intended outcomes

Upon completion of this book, it is hoped that you will be able to describe the analyticaltechniques that are used to make quantitative measurements in the atmosphere, and

xiv Preface

understand the physical principles upon which the techniques are based. You will beable to discuss, for each analytical method, issues such as species selectivity, calibration,sensitivity (lower detection limit), time-response, uncertainties (accuracy and precision),interferences and the method of data analysis. You will also be able to give examples ofspecies that can be detected using the different techniques, with details of their detection,and describe the experimental platforms in the field on which instruments have beendeployed, for example from the ground, ships, aircraft, balloons, rockets, satellites andspacecraft. You will be able to describe applications of the techniques used in the fieldto further understand the key chemical processes in the atmosphere facing our societytoday, and will be able to give specific highlights. Finally, it is hoped that you will beable to discuss why measurement of atmospheric composition in the field is such animportant component of atmospheric science.

Acknowledgements

I am indebted to the authors of individual chapters who agreed so readily to contributeto this book, and for writing authoritative accounts of their specialist areas enriched byyears of experience of measurements in the field. I would also like to thank Paul Sayer andSarahjayne Sierra, of Blackwell Publishing, for their encouragement and advice duringthe preparation of this book.

Dwayne HeardLeeds, November 2005

Contributors

James D. Allan School of Earth, Atmospheric and Environmental Sciences,Sackville Street Building, The University of Manchester,Sackville Street, PO Box 88, Manchester, M60 1QD, UK

Hugh Coe School of Earth, Atmospheric and Environmental Sciences,Sackville Street Building, The University of Manchester,Sackville Street, PO Box 88, Manchester, M60 1QD, UK

Ronald C. Cohen Department of Chemistry, University of California, LatimerHall, Berkeley, CA 94720-1460, USA

Alan Fried Atmospheric Chemistry Division, National Center forAtmospheric Research, 3450 Mitchell Lane, Boulder, Colorado80301, USA

Jacqueline F. Hamilton University of York, Department of Chemistry, Heslington, York,YO19 5DD, UK

Dwayne E. Heard School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK

Andreas Hofzumahaus Forschungszentrum Juelich GmbH, Institut fuer Chemieund Dynamik der, Geosphaere, ICG-II: Troposphaere, LeoBrandtstr., D-52425 Juelich, Germany

Alastair C. Lewis University of York, Department of Chemistry, Heslington, York,YO19 5DD, UK

John M.C. Plane University of East Anglia, School of Environmental Sciences,Norwich NR4 7TJ, UK

Dirk Richter Atmospheric Chemistry Division, National Center forAtmospheric Research, 3450 Mitchell Lane, Boulder, Colorado80301, USA

Alfonso Saiz-Lopez School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK

Andrew J. Weinheimer Atmospheric Chemistry Division, National Center forAtmospheric Research, PO Box 3000, Boulder, Colorado, CO80305-3000, USA

Contributors xvii

Jonathan Williams Max-Planck-Institut für Chemie, Abt. Luftchemie, J.J.-Becher-Weg27, D-55128 Mainz, Germany

Ezra C. Wood Department of Chemistry, University of California, Latimer Hall,Berkeley, CA 94720-1460, USA

Chapter 1

Field Measurements of AtmosphericComposition

Dwayne E. Heard

1.1 The role of field measurements in atmosphericscience

1.1.1 Our changing atmosphere

It is difficult to switch on the radio or television, or open a newspaper, without beingexposed to accounts of environmental concern resulting from a change in the compositionof our atmosphere; for example, increased instances of drought or flooding as a resultof the warming of our atmosphere through the increased emissions of greenhouse gases,or the increase in deaths attributable to a deterioration of urban air quality as a resultof increased emissions of harmful gases and particles and their secondary oxidationproducts. Sensational documentaries based on the possible occurrence of some futureapocalyptic event are becoming more widespread, and although in many cases they arenot based on any scientific fact, the awareness of the general public on environmentalissues is raised considerably. In a highly technological and information-rich age, it isbecoming generally accepted that our atmosphere is changing as a result of mankind’sactivities, and if current trends are left unchecked, there may be dire consequences ahead.The key questions are, how quickly is our atmosphere changing and what can be doneto mitigate the consequences? In order to predict the future changes in atmosphericcomposition, and hence the degree of expected climate change through global warming,numerical models are used extensively. The models require three types of input:

(1) An estimation of the future rates of emissions of trace gases. For natural emissions, forexample isoprene from trees and plants, the rate of emission is strongly temperaturedependent and hence there is a positive feedback with any future warming. Foranthropogenic emissions, various scenarios for future use of fossil fuels are estimated,and many assumptions have to be made.

(2) The model must be able to disperse and mix trace gases away from emission sources,for example via advection by the wind or via convective uplift in weather fronts, andon a larger scale through global circulation.

(3) Following their emission, trace gases are removed from the atmosphere by solar-induced photolysis, chemical transformation or by physical deposition to surfaces,forming a large number of chemical intermediates and secondary species, which

2 Analytical Techniques for Atmospheric Measurement

themselves undergo further transformation. It is often the secondary products that aremost harmful to the atmosphere, and it is crucial that the model contains an accuratedescription of the chemical mechanisms that describe the atmospheric degradationof trace gases emitted into the atmosphere.

1.1.2 The importance of atmospheric chemistry

The earth’s atmosphere is oxidising not because of reaction of trace species with O2

but owing to the presence of several key oxidising intermediates, which initiate theatmospheric removal of trace gases in the troposphere, eventually forming CO2 andwater vapour. The most important of these is the hydroxyl radical (OH), which ismostly generated in the daytime as a result of ozone photolysis to form electroni-cally excited oxygen atoms, O�1D�, which react rapidly with water vapour to form OH.The hydroxyl radical reacts with virtually all trace gases, including CO, hydrocarbons,oxygenated volatile organic compounds (VOCs), hydrochlorofluorocarbons (HCFCs),used as replacements for chlorofluorocarbons (CFCs) following their ban after theMontreal Protocol, and SO2. Under normal circumstances, OH concentrations are verylow at night, and the nitrate radical �NO3� replaces OH as the major oxidising species.NO3 is generated by the reaction of NO2 with ozone, and reacts either by hydrogen atomabstraction or by addition to double bonds. Ozone itself is the third major oxidisingspecies, reacting, for example, with unsaturated molecules through addition to doublebonds forming ozonides which decompose to form a variety of unstable intermediates.Common to the oxidation by OH, NO3 or O3 is the formation of intermediate peroxyradicals, RO2, where R is an organic fragment, including R = H, which reacts with nitricoxide, emitted following the burning of fossil fuels, to form nitrogen dioxide �NO2�. NO2

gas is brown, as a result of strong absorption of sunlight in the blue and green parts ofthe spectrum, and is rapidly photolysed by sunlight �� < 400nm� to form ground-stateoxygen atoms, O�3P�, which almost instantaneously combine with O2 to form O3, whichis harmful to humans and plants in high concentrations.The conversion of primary emissions, such as VOCs, eventually to CO2 and water

vapour can be extremely complex, involving several reactions. To give an example,the Master Chemical Mechanism (Jenkin et al., 2003), http://mcm.leeds.ac.uk/MCM/,describes the complete oxidation pathways for the top 135 VOC emissions in the UK, andconsists of 13 600 chemical species and 5900 chemical reactions. An important input tothe model is the rate coefficient for each of these reactions over a range of conditions oftemperature and pressure encountered in the atmosphere, and although many have beenmeasured in the laboratory, the majority are not known and have to be estimated. Inaddition, solar-induced photodissociation and deposition to surfaces (the ground, oceanor aerosols) must be included to completely describe the chemistry of the atmosphere,and it is necessary in the laboratory to measure absorption cross-sections as a functionof wavelength and temperature, and to measure photodissociation quantum yields as afunction of wavelength, temperature and pressure.The focus of this book is not about the chemistry in the atmosphere that is examined

via field measurements, but the techniques by which the field measurements are made.The interested reader is referred to a number of excellent textbooks and review articles

Field Measurements of Atmospheric Composition 3

to discover more about the chemistry of the atmosphere, which in certain locationscan be relatively simple involving a handful of species, but in polluted environmentsrequires thousands of chemical species to adequately describe its complexities (Ehhalt,1998; Finlayson-Pitts & Pitts, 2000; Ravishankara, 2003; Seinfeld & Pandis, 1998; Wayne,2000). Suggestions for further reading are given in Section 1.11.

1.1.3 Why field measurements of atmospheric compositionare important

The composition of the air we breathe was unknown until the second half of the eighteenthcentury, when carbon dioxide, nitrogen and oxygen were all discovered and isolated, withconstituents at lower concentrations, such as methane and ozone, not being discovereduntil almost a century later. Since the first atmospheric measurements of ozone at groundlevel were made by Schönbein in 1858, scientists have continued to develop ever moresensitive instruments to discover and measure thousands of trace gases in the atmosphere.However, it is only in the last forty years or so that the link between concentrations oftrace gases and global environmental issues has been made. Figure 1.1 shows the changein the mean global radiative forcings due to a number of agents in the period 1750–2000(Houghton, 2005; IPCC, 2001). It can be seen that several trace gas species, and also

Greenhouse gases

Aerosols + clouds

Global mean radiative forcing (Wm–2)

Troposphericozone

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Aerosolindirecteffect

N2OBlack

carbonfromfossilfuel

burning

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SulphateBiomassburning

Halocarbons

Figure 1.1 Anthropogenic and natural forcing of the climate for the year 2000, relative to 1750. Notethat water vapour is also a greenhouse gas, but is not shown. Taken with permission from the 2001 reportof the Intergovernmental Panel on Climate Control (Houghton, 2005; IPCC, 2001).


particulate matter (aerosols and clouds), are able to exert a global warming or coolingeffect, or both. The graph is based upon our knowledge of the concentrations of thesespecies, and how they interact with incoming or scattered solar radiation. Long-term andquantitative determination of these species in the atmosphere is crucial to calculate theindividual contributions to global warming and hence climate change, and to informpolicy makers. The concentration of greenhouse gases is increasing all the time, andtheir measurement is crucial to calculate future changes in atmospheric temperature (seeSection 1.7.2). Understanding the trends in chemically and radiatively important gasesand particles, and how they can be controlled in the future, is perhaps the most importantchallenge facing society today.Field measurements are necessary over a wide range of temporal and spatial scales

in order to record any long-term trends, and also to test how well models can predictthe composition of the current atmosphere. Although a complete understanding of thecomplex process within our atmosphere requires an integration of field measurements,computer modelling and laboratory studies, almost all of the major breakthroughs havebeen initiated by field observations. Without the development of a suite of sensitive andaccurate field instrumentation we would not be aware of the links between greenhousegases/aerosols and global warming, the formation of ozone holes in the stratosphere, thedeterioration in air quality on our cities, the changes in the oxidising capacity of theatmosphere, or other threats to our well-being.An example is the discovery of the ozone hole, first observed in the mid-1980s as

a reduction in the overhead column of ozone measured above Antarctica, measuredfrom the ground using a Dobson spectrophotometer that relies on the absorption ofultraviolet (UV) light from the sun (UV absorption is covered in Chapter 3). Thesemeasurements were later confirmed using satellite measurements (see Section 1.4.8), withthe ozone ‘hole’ demonstrating a marked spatial, altitudinal and seasonal dependence.An interesting aside is that the more sophisticated satellite instruments had recordedall the necessary data to observe the formation of the ozone hole, but the data analysissoftware had been programmed not to include data below a certain value! The ozonehole continued to grow in size and intensity, and the observations led to a flurry ofactivity amongst atmospheric chemists, who eventually postulated that heterogeneouschemistry occurring on the surface of polar stratospheric clouds that formed during thelong Antarctic winter was able to generate active forms of chlorine that destroyed ozonewhen the sun returned. The work led to the Nobel Prize for Chemistry being awarded in1995 to three scientists for their elucidation of the mechanism of formation of the ozonehole (but not to the instrument developers!). The postulated mechanism was provenbeyond doubt by further field observations, as shown in Figure 1.2, that indicated a clearanti-correlation between concentrations of ozone and chlorine monoxide intermediates,measured by in situ instruments aboard the ER-2 research aircraft that flew into thestratospheric polar vortex from Patagonia.Another example is the invention of the electron capture detector by Jim Lovelock,

which enabled the detection of CFCs, present only at the part per trillion level (ppt,1 part in 1015), which led to the realisation that mankind was releasing into theatmosphere chemical species for which there were no natural removal mechanisms, anda species which if left unregulated could lead to disastrous consequences for strato-spheric ozone. These field measurements of CFCs, followed by the understanding of the


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3000C

lO m

ixin

g ra

tio, p

pt

O3

mix

ing

ratio

, ppb

O3 mixing ratio

ClO mixing ratio

Figure 1.2 Concentrations of O3 (UV absorption, Chapter 3) and ClO radicals (resonance fluorescencedetection of Cl atoms following conversion of ClO by reaction with NO, Chapter 4) measured simulta-neously aboard the NASA ER-2 aircraft as it flew into the polar vortex during a flight originating in PuntaArenas, Chile. There is almost perfect anticorrelation between rising ClO levels and the destruction ofozone. (Taken with permission from the American Geophysical Union, Anderson et al., 1989.)

destructive nature of their degradation in the atmosphere, led to the implementation ofthe Montreal Protocol in 1987 and subsequent amendments. Concentrations of CFCs arenow beginning to level-off, or are falling (dependent upon their atmospheric lifetime), asshown in Figure 1.3, but it will be at least several decades before critical chlorine loadingsin the stratosphere fall below the threshold for formation of the Antarctic ozone hole.During the heat wave experienced during the summer of 2003, when temperatures in

the UK rose for the first time above 100�F �37�8�C�, concentrations of ozone rose in the SEof England to 150 parts per billion (ppb, 1 part in 109), levels not observed for 20 years,yet at relatively low concentrations of nitrogen oxides (a result of stringent controls onexhaust pipe emissions). Field measurements of isoprene, a biogenic emission from plants,demonstrated concentrations that rose exponentially with temperature, with concentra-tions at the ppb level during the heat wave, sufficient to generate large quantities of ozonefollowing its atmospheric oxidation. These measurements highlight the importance ofincluding changes in the rate of biogenic emissions in any global climate change model.Measurements by satellites have played a crucial role in understanding the effects of

major volcanic eruptions. Stratospheric aerosols affect the atmospheric energy balance byscattering and absorbing solar and terrestrial radiation, and perturb stratospheric chemicalcycles by catalysing heterogeneous reactions which markedly perturb nitrogen, chlorine


(a)

(b)

Figure 1.3 (a) Long-term trend (mixing ratios in pptv) of CFC -11 (CFCl3) measured by ground-basedinstruments of the NOAA CMDL network since 1977. In the stratosphere the maximum is reached about5 years later. (Data courtesy of Jim Butler and Jim Elkins, NOAA.) (b) More recent measurements ofCFC-11 (CFCl3) and CFC-12, from the same CMDL network. The atmospheric lifetimes of CFC-11 andCFC-12 are 45 and 100 years, respectively. The values for the northern hemisphere are slightly higher,because hemispherical mixing is always faster than the transport across the equator. The measurementsare made using gas-chromatographs equipped with an electron capture detector. (Data courtesy of JimButler and Jim Elkins, NOAA.)

and ozone levels. These aerosols are small (0�1�m radius) sulphuric acid solution dropletsproduced primarily through oxidation of sulphur dioxide injected into the stratosphereby volcanic eruptions. The Stratospheric Aerosol and Gas Experiment II (SAGE II)instrument, http://www-sage2.larc.nasa.gov/, launched on the Earth Radiation BudgetSatellite (ERBS) in 1984, monitored the long-term global effects of the Mt. Pinatubovolcanic eruption in June–July 1991, which is estimated to have deposited 3�6×107 tonnesof aerosol material into the stratosphere. The instrument measured the stratospheric


optical depth in the near-infrared �� = 1�02�m�. Measurements just after the eruptionand again a year later showed that the volcanic material initially concentrated in theTropics had spread across the entire globe. The Pinatubo aerosol layer warmed the localsubtropical stratosphere by about 2�5–3�C within three months of the eruption, and astatistically significant global average surface cooling was predicted by the end of 1992.There is a need to continually develop new instruments to measure the spatial and

temporal distributions of trace gases, be these harmful pollutants or short-lived inter-mediates, as these observations are required to monitor trends and as input to modelsand as target species to test the performance of computer models. There has been aphenomenal improvement in the measurement of many trace gases. Only 10 or 15 yearsago, measurement accuracy for some trace species was limited to a factor of two, oftenwith quite long averaging periods. An accuracy of 10% or less is now common, withaveraging times in some cases of less than a second. New trace gases are being discoveredall the time. The most powerful greenhouse gas ever discovered, SF5CF3, was measuredby gas chromatography with detection via a magnetic sector mass spectrometer (seeChapter 5 for details) with mixing ratios of ∼0�005pptv, trapped in air within snow inAntarctica (Sturges et al., 2000). SF5CF3 is thought to be a by-product of the use of SF6by the electronics industry.

1.1.4 The challenges of field measurements in theatmosphere

Many field instruments have been developed to measure a very large number of tracespecies, whose mole fractions (or mixing ratios) in the atmosphere vary from the per centlevel down to parts per quadrillion �ppqv = 10−15�. A crucial property of a trace gas is itsatmospheric lifetime, which is defined as the time taken for the concentration to decayto 1/e (37%) of its initial value once its source is removed. The range of atmosphericlifetimes is truly enormous, varying from less than a second, as is the case for free-radicalssuch as the hydroxyl radical (OH) which mediates virtually all of atmospheric chemistry,to hundreds of years, for example CFCs. Figure 1.4 shows the atmospheric lifetime fora range of species together with the timescales for mixing processes across a range ofspatial scales. The atmospheric lifetime determines the degree of transport away from thesource region, and whether a trace gas is well mixed globally or exhibits significant spatialstructure. The atmospheric lifetime of a trace gas is one of the most critical parameterswhen choosing the sampling strategy and detection method adopted for its measurementin the atmosphere. The atmospheric lifetime, , of a trace gas, X, is the reciprocal of therate of removal, k′, from the atmosphere, = 1/k′ (Kurylo & Orkin, 2003; Ravishankara& Lovejoy, 1994). The rate of removal of X is the sum of the rates of photolysis (J , seeChapter 9), dry or wet deposition to surfaces and reactions in the gas phase. For somespecies photolytic removal or removal on surfaces can be dominant, but for the majorityof trace species, k′ is controlled by the rate of gas phase reactions, in particular with thehydroxyl radical, OH, and is given by:

k′ = kOH+XOH� (1.1)


Micro-scale

Urban orlocal scale

Regional ormesoscale

Synoptic orglobal scale

Inter-hemisphericmixing time

Intra-hemisphericmixing time

Boundary layermixing timeT

ime

scal

e

Space scale

1 m 10 m 100 m 1 km 10 km 100 km 1000 km 10 000 km

1 s

100

s1

hr1

day

1 yr

10 y

r10

0 yr

Long-livedspecies

• CFCs• N2O

• CH4

• CH3CCI3• CH3Br

• CO

• Trop O3• SO2

• H2O2

• C3H6• C5H8

• CH3O2• HO2

• NO3

• OH

• NOx• DMS

• Aerosols

Short-lived species

Moderately long-lived species

Figure 1.4 Lifetimes and distribution of atmospheric trace gases. The graph shows the very wide range oftemporal and spatial scales over which atmospheric trace gases demonstrate variability in the atmosphere.(From Brasseur et al., 1999, with credits to W.L. Chameides.)

where kOH+X is the second-order rate constant for the reaction of OH with X and [OH]is the OH concentration. Thus it is critically important to know the concentration ofOH, the quantitative measurement of which illustrates the serious challenges of fieldmeasurements. The importance of OH as the dominant oxidant in the atmospherewas recognised in the late 1960s/early 1970s, from which time a considerable effortworldwide has been invested in the development of field instruments for its detection.OH concentrations are very low, a typical daily maximum (at solar noon) being a few 106

molecule cm−3�∼0�1pptv�; OH reacts very quickly on surfaces, and will not be transmittedthrough sample lines; OH concentrations vary significantly on short spatial scales; OHlifetimes are very short �∼1 s�, and thus any measurement technique must be in situ, haveexcellent sensitivity, temporal and spatial resolution, and not be subject to interferencefrom other species. For OH, in common with many other field determinations, it isthis last criterion, namely the elimination of interferences, that is the most difficult toachieve. Laser-induced fluorescence (LIF) was suggested in 1972 as a suitable methodfor OH detection (see Chapter 4 for further details). However, while using this methodan interference plagued OH measurements for many years, namely the laser photolysisof O3 to generate O�1D�, with subsequent reaction of O�1D� with atmospheric watervapour to generate OH. The first results in 1974 gave a maximum [OH] of 1�5× 108


molecule cm−3, with nighttime values of 5× 106 molecule cm−3, and at the time thesewere accepted, as model calculations of OH were subject to very large errors becausekinetic data for key reactions relevant to OH were not accurately known. In the mid-1980smeasurements by several instruments using LIF during the NASA GTE/CITE campaignwere later discredited, with an evaluation panel concluding that OH concentrationshad not been measured! The laser-generated interference is now well recognised, andcurrent OH field instruments using LIF rely on a different excitation scheme and pressureregime, and are free from such interferences. Some thirty years or more after the firstOH measurements, several methods are now routinely used to measure OH (Table 1.1),with reported accuracies as good as 10%. An important lesson was learnt by the entirecommunity of the dangers of artefact signals, but this has led to the emergence of veryrigorous acceptance criteria for new instruments to measure OH.The concentration of trace species varies with latitude, longitude, altitude and time

of day, sometimes on very short scales, and a wide range of instruments deployed ona variety of measurement platforms is required. The atmosphere displays a wide rangeof temperatures (from +50�C (323K) at the surface down to −130�C (143K) in themesosphere), pressures (decreasing exponentially from 1 atm at sea level to 10−6 atmat 100 km above sea level), solar intensity and meteorological conditions (wind, snow,hail), and many practical and logistical barriers must be surmounted even to attempt ameasurement.

1.1.5 Comparison with calculations from numerical models

Our understanding of atmospheric chemistry is manifested through atmospheric modelswhich describe the main physical and chemical processes that control how trace gasesand particulate matter are distributed spatially on local, regional and global scales, andalso as a function of time. It is natural therefore to compare field measurements ofatmospheric composition on a variety of temporal and spatial scales, using a plethoraof instrumental techniques and platforms that are the subject of this book, with thecalculations of numerical models. The field measurement alone of a trace species providesonly limited information. Atmospheric models are used for a variety of purposes, forexample to calculate future trends in greenhouse gases, such as methane, that controlfuture changes in atmospheric temperature, or substances harmful to health, such asozone, to define future air quality. Policy makers make extensive use of model predictionsto define emission reduction legislation, which can cost the economy billions of poundsto implement. Whatever the application, the output of the desired model is only asgood as the input (‘garbage in garbage out’), and as one of the model inputs is thechemical mechanism used to describe atmospheric transformation, it must be as accurateas possible.Often the major degradation pathways are not known due to an absence of kinetic

data on rate coefficients or product branching ratios, and these parameters have tobe estimated using structure–activity relationships or numerical calculation (for whichthermodynamic data are often required). In order to be confident about the ability ofa model to calculate future atmospheric composition, it is important to design waysto test the accuracy of the mechanism used, and to identify which parts of it require


Table 1.1 Analytical techniques for selected classes of neutral atmospheric trace constituents

Class ofconstituent

Species Analytical technique (chapter number)

Hydrogencompounds

OH Laser-induced fluorescence (4), UV-visible DOAS (3),chemical ionisation mass spectrometry (5), far-IR emissionspectroscopy (1)

HO2 Fluorescence (after conversion) (4), matrix isolationelectron-spin resonance (1), peroxy radical chemical amplifer(7), far-IR emission spectroscopy (1), microwave absorptionspectroscopy (1)

H2O vapour Fluorescence after photolysis (4), IR absorption spectroscopy(2), surface acoustic wave (1), cavity ring-down spectroscopy(3), dew/frost point hygrometer (1), Lyman-� UV absorption (3)

H2O2 IR absorption spectroscopy (2), liquid chromatography (afterconversion, 8), chemiluminescence and fluorescence (afterconversion, 7)

H2 Gas chromatography, reaction with hot HgO bed, Hgdetermined photometrically (8), mass spectrometry (5)

Nitrogencompounds

NO IR absorption spectroscopy (2), UV-visible DOAS (3),fluorescence (4), chemiluminescence (7), solid-state sensors (1)

NO2 IR absorption spectroscopy (2), UV-visible DOAS (3),laser-induced fluorescence (4), chemiluminescence (afterconversion, 7), sensors (after conversion, 1)

NO3 UV-visible DOAS (3), matrix isolation ESR (1), cavityring-down spectroscopy (3), laser-induced fluorescence (4)

N2O5 Cavity ring-down spectroscopy (3) or LIF after thermolysis toNO3 and NO2 (4)

HONO UV-visible DOAS (3), fluorescence (after conversion, 7)HNO3 IR absorption spectroscopy (2), fluorescence (after conversion,

4), chemical ionisation mass spectrometry (5)Sum NOy Fluorescence (after conversion, 4), chemiluminescence (after

conversion, 7)HO2NO2 Fluorescence (after conversion, 4), chemical ionisation mass

spectrometry (after conversion, 5)Alkyl nitrates Fluorescence (after thermolysis and conversion, 4), Chemical

ionisation mass spectrometry (5), chromatography (8)PAN Chemical ionisation mass spectrometry (5), gas

chromatography with electron-capture detection (8)Other PANs Chromatography (8)N2O IR absorption spectroscopy (2), gas chromatography (8)NH3 IR absorption spectroscopy (2), UV-visible DOAS (3)

Halogenatedcompounds

HCl, HBr, HF IR absorption spectroscopy (2)IO, I2, OIO UV-visible DOAS (3)BrO UV-visible DOAS (3), fluorescence (after conversion, 4)OBrO UV-visible DOAS (3)ClO UV-visible DOAS (3), far-IR absorption spectroscopy (1),

fluorescence (after conversion, 4)

analytical techniques for atmospheric measurement · 6.6 applications of aerosol mass spectrometric...

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