Aerosol Optics: Light Absorption and Scattering by Particles in the Atmosphere (Springer Praxis Books / Environmental Sciences)

154 Pages·2007·6.014 MB·English

by  Alexander A. Kokhanovsky

Aerosol Optics
LightAbsorptionandScatteringbyParticlesintheAtmosphere
Dr Alexander A. Kokhanovsky
Aerosol Optics
Light Absorption and Scattering by Particles in the
Atmosphere
Published inassociationwith
PPrraaxxiiss PPuubblliisshhiinngg
Chichester, UK
Dr Alexander A. Kokhanovsky
Institute of Environmental Physics
University of Bremen
Bremen
Germany
SPRINGER–PRAXIS BOOKS IN ENVIRONMENTAL SCIENCES
SUBJECTADVISORYEDITOR:JohnMasonB.Sc.,M.Sc.,Ph.D.
EDITORIALADVISORYBOARDMEMBER:DrAlexanderA.Kokhanovsky,Ph.D.InstituteofEnvironmental
Physics,UniversityofBremen,Bremen,Germany
ISBN 978-3-540-23734-1 Springer Berlin Heidelberg New York
Springer is part of Springer-Science+Business Media (springer.com)
Library of Congress Control Number: 2007935598
Apartfrom any fair dealing for the purposes of research or private study, or criticism
or review, as permitted under the Copyright, Designs and Patents Act 1988, this
publication may only be reproduced, stored or transmitted, in any form or by any
means, with the prior permission in writing of the publishers, or in the case of
reprographic reproduction in accordance with the terms of licences issued by the
Copyright Licensing Agency. Enquiries concerning reproduction outside those terms
should be sent to the publishers.
# Praxis Publishing Ltd, Chichester, UK, 2008
Printed in Germany
The use of general descriptive names, registered names, trademarks, etc. in this
publication does not imply, even in the absence of a specific statement, that such
namesareexemptfromtherelevantprotectivelawsandregulationsandthereforefree
for general use.
Cover design: Jim Wilkie
Project copy editor: Mike Shardlow
Author-generated LaTex, processed by EDV-Beratung, Germany
Printed on acid-free paper
Preface
Theopticalpropertiesofatmosphericaerosolareof importanceforanumberofapplica-
tions, including atmospheric visibility and climate change studies, atmospheric remote
sensingandparticulatemattermonitoringfromspace.Theseapplicationsareinvestigated
at many research centers worldwide using spaceborne, airborne, shipborne, and ground-
basedmeasurements.Bothpassiveandactiveinstruments(e.g.lidars)areused.The pri-
maryinterestliesinthedeterminationoftheverticalaerosolopticalthickness,thesingle
scattering albedo, the absorption and extinction coefficients, the phase function and the
phasematrix.Verticaldistributionsoftheaerosolpropertiesarealsostudiedusingground-
based and spaceborne lidars. Considerable progress in understanding aerosol properties
hasbeenmadeinrecentyears.However,manyproblemsstillremainunsolved.Theyin-
clude,forinstance,directandindirectaerosolforcing,lightinteractionwithnonspherical
aerosolparticles(e.g.,desertdust),andalsotheretrievalofaerosolopticalthicknessand
optical particle sizing using satellite observations.
The area of aerosol research is extensive. Therefore, no attempt has been made to
achieve a comprehensive coverage of the results obtained in the area to date. The
mainfocusofthisbookisthetheoreticalbasisoftheaerosoloptics.Theresultspresented
areverygeneralandcanbeappliedinmanyparticularcases.Thefirstsectionisconcerned
with the classification of the different aerosol particles existing in the terrestrial atmo-
sphere with respect to their chemical composition and their origin (e.g., dust and sea
saltaerosols,smoke,andbiologicalandorganicaerosols).Inthesecondchapter,Iintro-
ducethechiefnotionsofaerosoloptics,suchasabsorption,scattering,andextinctioncoef-
ficients, andalsophase functions and scattering matrices. Numerous examplesofsingle
scattering calculations using Mie theory are presented. Chapter 3 aims to describe tech-
niquesforthecalculationofmultiplescatteringeffectsinaerosolmedia.Theresultsareof
importanceforstudiesoflightpropagationinthickaerosollayers,wherethesinglescat-
teringapproximationcannotbeused.Thediscussioninthissectionisbasedonthesolid
groundofradiativetransfertheory.Bothscalarandvectorversionsofthetheoryarepre-
sented.Chapter4isfocusedontheFourieropticsofaerosolmedia.Inparticular,the re-
duction of contrast due to atmospheric effects and also the optical transfer functions of
aerosol media are considered in detail. This section is of importance for understanding
imagetransferthroughtheterrestrialatmosphere.Thefinalchapterofthebookisfocused
ontheapplicationofopticalmethodsforthedeterminationofaerosolmicrophysicaland
opticalproperties.Suchtopicsasmeasurementofbothdirectanddiffusedsolarlightusing
Sun photometers and satellite remote sensing of atmospheric aerosol are covered. Also
lidar measurements from ground and space are briefly touched upon in this chapter.
VI Preface
Myhopeisthatthisbookwillbeusefultobothstudentsandengineersworkinginthe
areaofaerosolopticsandatmosphericremotesensing.Iamgratefultothemanycollea-
gueswhoareinvisibleauthorsofthisbook.Itisnotpossibletomentionalloftheminthis
prefacebutmyspecialgratitudegoestoEleonoraZegeforherencouragementduringmy
first steps in science and also for shaping my approach to problem solving, to Vladimir
Rozanovforhislong-termcollaborationintheareaofradiativetransfer,andtoWolfgang
vonHoyningen-HueneandJohnBurrowsfornumerousdiscussionsonthephysicalfoun-
dationsofsatelliteremotesensing.IamalsoindebtedtoCliveHorwood,Publisher,forhis
encouragement, his patience, and his skill in the design and production of the book.
Alexander K. Kokhanovsky
Bremen, Germany
January 2008
Table of contents
1. Microphysical parameters and chemical composition of atmospheric
aerosol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Classification of aerosols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Aerosol models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2. Optical properties of atmospheric aerosol . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 Extinction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Absorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.5 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3. Multiple light scattering in aerosol media . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.1 Radiative transfer equation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2 The diffuse light intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3 Thin aerosol layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.4 Semi-infinite aerosol layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.5 Thick aerosol layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.6 Aerosols over reflective surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.7 Multiple scattering of polarized light in aerosol media . . . . . . . . . . . . 65
3.7.1 The vector radiative transfer equation and its numerical
solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.7.2 The accuracy of the scalar approximation . . . . . . . . . . . . . . . . . 72
3.7.3 The accuracy of the single scattering approximation. . . . . . . . . . 78
3.7.4 The intensity and degree of polarization of light reflected from
an aerosol layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4. Fourier optics of aerosol media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.1 Main definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.2 Image transfer through aerosol media with large particles . . . . . . . . . . 89
4.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.2.2 Geometrical optics approximation . . . . . . . . . . . . . . . . . . . . . . 96
VIII Tableofcontents
5. Optical remote sensing of atmospheric aerosol. . . . . . . . . . . . . . . . . . . . . . . 100
5.1 Ground-based remote sensing of aerosols . . . . . . . . . . . . . . . . . . . . 100
5.1.1 Spectral attenuation of solar light . . . . . . . . . . . . . . . . . . . . . . 100
5.1.2 Measurements of scattered light . . . . . . . . . . . . . . . . . . . . . . . 115
5.1.3 Lidar measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.2 Satellite remote sensing of atmospheric aerosol . . . . . . . . . . . . . . . . 121
5.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.2.2 Passive satellite instruments: an overview . . . . . . . . . . . . . . . . 122
5.2.3 Determination of aerosol optical thickness from space . . . . . . . 124
5.2.4 Spatial distribution of aerosol optical thickness . . . . . . . . . . . . 129
5.2.5 Lidar sounding from space . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
Chapter 1. Microphysical parameters and chemical
composition of atmospheric aerosol
1.1 Classification of aerosols
The optical properties of atmospheric aerosol are determined by chemical composition,
concentration,size,shape,andinternalstructureofliquidandsolidparticlessuspendedin
air.Allthesecharacteristicsvaryinspaceandtime.Atanytimenewparticlescanenteror
leavetheatmosphericvolumeunderstudy.Alsoparticlescanbegeneratedinthisvolume
bygas-to-particleconversionprocesses.Verydifferentparticlesarefoundinanelementary
volume of atmospheric air. Depending on the aerosol type, one can identify among the
particlesdifferentminerals,sulfates,nitrates,biologicalparticlessuchasbacteriaandpol-
len, organic particles, soot, sea salt, etc. These particles are very tiny objects with sizes
typicallyaround100nm.Therefore,usuallytheyarenotvisibletothenakedeye.Never-
theless,aerosol particlesconsiderablyreducevisibility, influence climate,andcan cause
health problems in humans.
There are three main sources of particulate matter in the terrestrial atmosphere. Par-
ticlescanentertheatmospherefromthesurface(e.g.,dustandseasalt).Particlescanbe
generatedintheatmospherebygas-to-particleconversions.Sometheparticlesenteratmo-
spherefromspace(cosmicaerosol).Waterandiceaerosolsformclouds.Theyaretreated
in a separate branch of atmospheric science, namely, cloud physics. Clouds will be not
consideredhereinasystematicway.Importantly,aerosolparticlesdonotexistinisolation.
They interact with cloud droplets, ice crystals, and gases. Also the interaction between
aerosolparticles(e.g.,coagulationandcoalescence)isofgreatimportanceforatmospheric
science.
Surface-derivedaerosolconstitutesthemainmassofsuspendedparticulatematterwith
about50%contributiononaglobalscale.Theparticlesbornintheatmospheredominate
theaerosolnumberconcentration.Thecosmicaerosolinfluenceisnegligibleinthelower
atmosphere. However, it can influence atmospheric air properties in the higher atmo-
spheric layers, where the concentration of terrestrial aerosol is low.
Humankindhasimportantinfluencesonaplanetaryscale.Inparticular,theconcentra-
tion of trace gases increased considerably due to industrial activities and transportation.
Thisisalsothecaseforaerosols.Atpresentthecontributionoftheanthropogenicaerosol
tothetotalaerosolmassissignificant(seeTable1.1).Thisleadstoserioushealthproblems
inhighlypopulatedindustrialareas.Alsotheanthropogenicaerosolisamajorsourceof
climatechange.Greenhousegaseswarmtheplanetandtheanthropogenicaerosolactsin
theoppositedirectionglobally.Therefore,cleaningoftheairinmajorcitieswithrespectto
suspended aerosol particles may lead to additional warming with respect to the current
state.
2 1 Microphysicalparametersandchemicalcompositionofatmosphericaerosol
Table1.1. Emissions of main aerosol types. Reported ranges correspond to estimations of different
authors(Landolt-Bornstein,1988)
Aerosoltype Emission(106tonsperyear)
Sea-saltaerosol 500–2000
Aerosolformedinatmospherefromagaseousphase 345–2080
Dustaerosol 7–1800
Biologicalaerosol 80
Smokefromforestfires 5–150
Volcanicaerosol 4–90
Anthropogenicaerosol 181–396
For a correct simulation of light propagation in atmosphere, one needs to know the
microphysical properties and type of aerosol in the propagation channel. This is rarely
known in advance. Therefore, a number of models have been proposed to characterize
averagemicrophysicalcharacteristicsofaerosoldependingonthelocationand,therefore,
onthe proportion ofvarious typesofparticles (e.g., desertandoceanicaerosolmodels).
Itisof importancetohaveaclassificationofmainaerosoltypes.Thenthesetypescan
beusedasbuildingblocksforthedevelopmentofmicrophysicalandopticalaerosolmod-
els.
Atmosphericaerosolsareusuallyclassifiedintermsoftheiroriginandchemicalcom-
position. The main aerosol types are given in Table 1.1.
Sea-saltaerosol(SSA)originatesfromtheoceanicsurfaceduetowavebreakingphenom-
ena. Thelargest droplets fall close totheir areaoforigin. Only the smallest aerosol par-
ticles with sizes from approximately 0.1 to 1lm (e.g., those formed by the bursting of
bubblesattheoceansurface)areofaprimaryimportancetothelarge-scaleatmospheric
aerosolproperties.Theseparticlescanexistintheatmosphereforalongtime.Theyhave
been identified over continents as well.
The shape of sea-salt aerosol particles depends on the humidity. Cubic particles (see
Fig.1.1)arefoundatlowhumidity.Thisisduetothecubicstructureofsodiumchloride,
NaCl,themainconstituteofSSA.NaCliseasilydissolvedinwater.Therefore,cubicforms
transformintosphericalshapesinhigh-humidityconditions.WeseethatSSAisextremely
dynamicwithrespecttothemodificationof itsshape.Itisdifficulttoconstructtheuni-
versalopticalmodelofSSAbecauseoftheconsiderableinfluenceofshapesonthe pro-
cesses oflight interaction with particles. At least twooptical models ofSSAare needed
(i.e., for low- and high-humidity conditions). Yet another problem is associated with the
factthatseasaltisnotdistributeduniformlyintheaerosolparticleformedbytheattraction
ofwatermoleculesinthefieldofhighhumidity.TheconcentrationofNaClmoleculesis
largerclosetothecenterofaparticleascomparedtoitsperiphery.Thisleadstothene-
cessitytoaccountfor theinhomogeneityofaparticleintheoreticalstudiesof itsoptical
characteristics.Themodelsofradiallyinhomogeneousparticlesmustbeusedinthiscase.
Itisknownthattheinternalinhomogeneityofparticlesconsiderablyinfluencestheirabil-
itytoscatterandabsorblight.Unfortunately,therearecomputationalproblemsrelatedto
thecalculationofopticalcharacteristicsinthecaseofnonsphericalinhomogeneouspar-
ticles. This leads to thewidespread use of the homogeneous sphere model of an aerosol