Heard D.E. (editor) Analytical Techniques for Atmospheric Measurement

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x Contents

7.10 Summary and future directions 353

Further reading 402

References 402

9 Measurement of Photolysis Frequencies in the Atmosphere 406

Andreas Hofzumahaus

9.1 Introduction 406

9.1.1 Photolysis reactions in atmospheric chemistry 406

9.1.2 Photolysis frequencies: Kinetic definition 408

9.1.3 Photolysis frequencies: Physical model 409

9.1.4 Actinic flux 411

9.1.5 Irradiance 411

9.1.6 Actinic flux in the atmosphere 412

9.2 Methods for measuring photolysis frequencies 416

9.2.1 Measurement requirements 416

9.2.2 General measurement methods 417

9.2.3 Actinic-flux sensing techniques 418

9.2.4 Radiative transfer models 419

9.3 Chemical actinometry 420

9.3.1 Principle of chemical actinometry 421

9.3.2 Chemical kinetics considerations 421

9.3.3 Optical and photochemical considerations 426

9.3.4 Ozone chemical actinometers 427

9.3.5 Nitrogen-dioxide chemical actinometers 432

Contents xi

9.4 Actinic-flux spectroradiometry 437

9.4.1 Principle of actinic-flux spectroradiometry 437

9.4.2 Spectroradiometers for actinic-flux measurements 439

9.4.3 Actinic-flux receiver optics 442

9.4.4 Determination of spectral actinic fluxes 444

9.4.5 Spectroradiometer calibration 445

9.4.6 Determination of photolysis frequencies 448

9.5 Actinic-flux filter radiometry 453

9.5.1 Filter radiometer set-up 453

9.5.2 Determination of photolysis frequencies 455

9.5.3 Filter radiometer calibration 458

9.5.4 jO

D filter radiometers 460

9.5.5 jNO

 filter radiometers 464

9.6 Irradiance radiometry 466

9.6.1 Principle of irradiance radiometry 467

9.6.2 Physical model–based methods 468

9.6.3 Empirical methods 471

9.7 Modelling of photolysis frequencies 474

9.8 Sample applications 475

9.8.1 Photochemistry field experiments 475

9.8.2 Temporal and spatial distributions 476

9.8.3 Instrumental intercomparisons 478

9.8.4 Molecular data assessments 480

9.9 Conclusions and outlook 482

Acknowledgements 482

Appendix 483

A.1 Radiation quantities and their relationships 483

A.2 Effective transmission of solar radiation into a chemical

actinometer 485

A.3 Spectral spectrometer bandwidth 488

A.4 Correction of the angular response of an actinic-flux receiver

2 sr 489

A.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 workings

of 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 highly

specialised, having been first developed in the laboratory by chemists, physicists or

engineers, before being applied to the measurement of atmospheric composition. There

is usually insufficient space in the journal articles for a thorough description of the

instrument, and often only essential details are given. Although there are a number

of journals that specialise in the description of instruments, for example Review of

Scientific Instruments, there is no common theme to a given issue, and an instrument to

measure an important atmospheric constituent will likely be sandwiched between articles

covering instruments from unrelated fields. Published proceedings from conferences or

symposia devoted to instrumentation do offer some coherence, but the articles tend

to be specialised, and although appealing to the highly expert reader, they are, in the

main, not accessible to scientists wishing to further their knowledge in a new area. There

are a number of textbooks that contain sections on instruments for measurements of

atmospheric composition, but these usually do not cover recent developments, nor do

they 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 field

measurement community, who go on to utilise the method to make significant break-

throughs. More recently, there has been a stronger collaboration between atmospheric

chemists, physicists, engineers and analytical scientists, to develop bespoke instruments

with the sole intention of measuring new species in the atmosphere. Sometimes a scientist

in a core discipline, for example a physical chemist, recognises the need for a particular

measurement in the atmosphere, and realises that the techniques they have used in the

laboratory for fundamental studies can equally be applied in the field. Once a rarity, this

practice 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 advances

resulting 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 weaknesses

for a particular task, the platforms on which they have been deployed, how they are

calibrated etc. The book explains the fundamental physical principles upon which the

instrumental techniques are based. For instance, what properties of molecules can be

exploited to enable their detection? What limits the sensitivity and accuracy of a given

instrument, and what information can be gained from its use? Instruments developed

to make measurements of atmospheric composition are highly specialised, but often

have evolved over many years via several incarnations. The book attempts to convey the

excitement of the challenge to quantitatively measure trace atmospheric constituents that

are at the heart of atmospheric chemistry, and responsible for many of the problems

facing society today; for example, the warming of our atmosphere, the destruction of

the ozone hole, and the formation of urban air pollution and acid rain. Measurements

of the composition of our atmosphere range from the earth’s surface to the edge of

space. Unsurprisingly, the most important species to measure, those that exert special

control on our atmosphere, are often the most difficult to measure – only being present

in miniscule concentrations as a result of being so reactive. For some species, for example

the hydroxyl radical (OH), it has taken over twenty years of instrument development to

enable a reliable and accurate measurement. In some cases, just a single measurement has

opened up new areas for study – for example chlorofluorocarbon (CFC) measurements

pioneered 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 are

those who wish to gain a general understanding of instrumentation for measurement of

atmospheric composition, the fundamental principles upon which techniques depend,

their major capabilities, together with highlights of the important results and the advances

in understanding that have resulted – but without wanting a detailed discussion of the

underlying atmospheric chemistry or physics. The chapters have been written so that this

information is easily retrievable, and is accessible to the non-expert. These readers will

be 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 learn

more about composition measurements in the atmosphere. The other class of readers

are field scientists or instrument developers who are more experienced, and who will

be interested in the finer detail of specific instruments, and latest developments, and

perhaps 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 not

normally found in the literature due to lack of space in regular journal articles, and also

include 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 analytical

techniques 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 be

able 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 of

species 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 been

deployed, for example from the ground, ships, aircraft, balloons, rockets, satellites and

spacecraft. You will be able to describe applications of the techniques used in the field

to further understand the key chemical processes in the atmosphere facing our society

today, and will be able to give specific highlights. Finally, it is hoped that you will be

able to discuss why measurement of atmospheric composition in the field is such an

important component of atmospheric science.

Acknowledgements

I am indebted to the authors of individual chapters who agreed so readily to contribute

to this book, and for writing authoritative accounts of their specialist areas enriched by

years of experience of measurements in the field. I would also like to thank Paul Sayer and

Sarahjayne Sierra, of Blackwell Publishing, for their encouragement and advice during

the preparation of this book.

Dwayne Heard

Leeds, 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, Latimer

Hall, Berkeley, CA 94720-1460, USA

Alan Fried Atmospheric Chemistry Division, National Center for

Atmospheric Research, 3450 Mitchell Lane, Boulder, Colorado

80301, 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 Chemie

und Dynamik der, Geosphaere, ICG-II: Troposphaere, Leo

Brandtstr., 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 for

Atmospheric Research, 3450 Mitchell Lane, Boulder, Colorado

80301, USA

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

Andrew J. Weinheimer Atmospheric Chemistry Division, National Center for

Atmospheric Research, PO Box 3000, Boulder, Colorado, CO

80305-3000, USA

Contributors xvii

Jonathan Williams Max-Planck-Institut für Chemie, Abt. Luftchemie, J.J.-Becher-Weg

27, D-55128 Mainz, Germany

Ezra C. Wood Department of Chemistry, University of California, Latimer Hall,

Berkeley, CA 94720-1460, USA

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Chapter 1

Field Measurements of Atmospheric

Composition

Dwayne E. Heard

1.1 The role of ﬁeld measurements in atmospheric

science

1.1.1 Our changing atmosphere

It is difficult to switch on the radio or television, or open a newspaper, without being

exposed to accounts of environmental concern resulting from a change in the composition

of our atmosphere; for example, increased instances of drought or flooding as a result

of 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 result

of increased emissions of harmful gases and particles and their secondary oxidation

products. Sensational documentaries based on the possible occurrence of some future

apocalyptic event are becoming more widespread, and although in many cases they are

not based on any scientific fact, the awareness of the general public on environmental

issues is raised considerably. In a highly technological and information-rich age, it is

becoming generally accepted that our atmosphere is changing as a result of mankind’s

activities, 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 done

to mitigate the consequences? In order to predict the future changes in atmospheric

composition, 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, for

example isoprene from trees and plants, the rate of emission is strongly temperature

dependent and hence there is a positive feedback with any future warming. For

anthropogenic 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, and

on 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