Mass absorption indices of various types of natural aerosolparticles in the infrared

Klaus Fischer

The mass absorption index of aerosol particles has been measured in the 2-17-,um wavelength region. The

measurements were performed on films of aerosol particles that were collected by an automatic jet impac-

tor at polluted and various uncontaminated remote sites. All but marine aerosols possess strong absorp-

tion bands in the transparent part of the atmospheric long-wave spectrum, indicating marked influence of

aerosol particles on the radiation budget of the atmosphere.

1. Introduction

A. GeneralRadiant energy is dissipated in the atmosphere by

scattering and absorption. Gases mainly, as well asparticulate matter suspended as aerosol particles inthe air, participate in these processes. The influenceon the radiation budget of the atmosphere caused byscattering and absorption of gases is known-from ex-perimental data and model computations. However,there are only a few detailed publications on the cor-responding effect of aerosol particles. This is partlydue to the presumption that the effect of aerosol par-ticles on the radiative fluxes and their divergenceshould be negligibly small compared to the influenceof atmospheric gases.

Disagreement between measurements of the globaland sky radiation and model-based computations re-vealed the presence of an additional absorber compa-rable in strength to the known absorbers in the atmo-sphere like H2 0 vapor, 03, 02, N2 0, C0 2, and CO.1-5

This consequence is studied by investigations of theshort-wavelength spectrum, 0.4 < X < 2.4 ,jm, whichcontains the major part of the solar energy input to

-the troposphere as well as by experiments in the so-called long-wave window of the atmosphere, 8 < X <12 Am, where the heat radiative transfer in the loweratmosphere is most intense.

If this extra absorption were attributed to aerosolparticles, their influence on the energy budget of the

The author is with the Johannes Gutenberg-Universitit, Insti-

tut fir Meteorologie, D-6500 Mainz, Germany.Received 10 May 1974.

atmosphere and consequently on the circulation andclimate would not be neglected. Especially thoseaerosol particles produced in excess in heavily in-dustrialized areas are of real importance as of poten-tial climate-changing quantity (SMIC-Report, 1971).Although over the past few years attention has beenincreasingly focused on possible global temperaturechanges due to absorption by particulate matter inthe atmosphere, only minimal data on the absorbingproperties of different types of aerosol particles areavailable.

There are reports of measurements performed onlyon aerosol substances like soot,6 sulfuric acid-watersolutions and ammonium sulfate,7 and clay mineralsand further samples of soil.8 Dust particles of desertorigin9 and residues of various kinds of precipitation1 0 -12 were investigated by application of the KBr meth-od. This technique also does not deal with aerosolparticles themselves but rather with their productsafter effective processes of preparation. Further-more, this technique should be used with reservationin obtaining quantitative results.13

A more realistic approach is presented in thispaper dealing with the absorption of films of aerosolparticles that are deposited on substrates of rolledAgCl monocrystals by an automatic jet impactor.

According to their size distribution the concentra-tion of aerosol particles generally decreases by apower function of an exponent of ca. 3 in the radiusrange 10-1 jim to 101 gm,1 4 i.e., for the atmosphericaerosol most of the mass is concentrated below theradius of about r = 1 Aim. By correct adjustment ofthe impactor the vast majority of aerosol particlesimpinging upon the substrate are of radii r < 1 jim.

On the basis of Mie computations Bergstrom15 haspredicted that for absorbing particles of radii r < 1jim, scattering of infrared radiation is dominated byabsorption, and the absorption per unit mass is a

December 1975 / Vol. 14, No. 12 / APPLIED OPTICS 2851

A3 n-ik

ni

Jo

-l-- -

air

R2 3

substrate

n

t J

aerosol film air

The last two yield multiple reflections within thesubstrate-film system.

The transmitted radiation J is given by

J = j0 "'. tj 2-n/n 3 ,

where16t T23-T 31-exp (-i6)/1 + p23 p31exp (-2i6),= 2-7r-h3-x/X,

Tij, Pij = Fresnel coefficients concerning the mediamarked i, j (= 1, 2, 3), and17

Jo , = .. h3 IT12 12 IT2 2ON 1 - IPi212 -IP2 3 ! 2

Fig. 1. Representation of aerosol-film-measurement (cf. text)

constant with radius and linear with k up to values ofk = 1.25. Similarly, as the inhomogeneities of thepresent films are formed by particles in question, ex-tinction of incident infrared radiation is caused byabsorption mainly, i.e., the optical conditions in com-parison with the wavelength of the infrared irradia-tion may be considered as quasihomogeneous, justi-fying-in principle-the application of Lambert'sformula of absorption. This was confirmed by a testyielding no signals of scattered radiation by the films.

Considering multiple reflections within the sub-strate-film system, measurements of the transmit-tance yield the absorption of the particle film. Theoptical constants thus determined are mean quan-tities refering to certain collectives of aerosol parti-cles.

B. List of Symbols

a = complex refractive index,n = real part of the complex refractive

index,k = imaginary part of the complex refrac-

tive index = absorption index,K = 47rkh/X = absorption coefficient,k/p = mass absorption index,K/p = mass absorption coefficient,p = density of aerosol particles,A = wavelength.

11. ExperimentA uniform film (thickness x X) of closely spaced

small aerosol particles (complex refractive index h3 )is deposited on a thick transparent substrate (refrac-tive index n2). Jo is the irradiance by the beamnormally incident onto the substrate side of the sys-tem (Fig. 1). T and t are the amplitudes of the ra-diation passing through the substrate and the aerosolfilm, respectively. J is the irradiance by the trans-mitted beam.

In addition to the absorption by the aerosol filmthe transmission of radiation is reduced by reflec-tions at the boundaries air-substrate (R 12), sub-strate-aerosol film (R2 a), and aerosol film-air (R31).

is the radiation penetrating into the aerosol film.Because of dispersion the factor Jo" is strongly

dependent on 13 (A), the unknown index of refractionof aerosol, i.e., the measurement of absorbed radia-tion must not be related to the transmittance of theblank substrate.

Suppose there is an aerosol film of thickness x' = x+ Ax (Ax/x < 1) composed of particles of an identi-cal collective:

j = Jo". It12 n/ 3 .

Because of identical boundary conditions Jo" is re-placed by the quotient

J/JP = I 1/ I'l = f exp(47T-k- x/X),

where

{1 + p2 3 p31 exp[-K(x + Ax)] - 4P 23 'P3 i'sin2 [2 T n3( + Ax)/X]*exp[-K(x + ax)]

f = {1 + P2 3 .P3iexPV-KX]}2 - 4 P2 3

-p3j-sin2[2-7-n3X/X] exp[-Kx]

(1)

The periodic terms of both numerator and denomi-nator of f represent multiple-beam interference with-in the aerosol film. They are of minor influence be-cause of absorption (k > 0.04),18 so that with the re-fractive indices of air, of AgC1 as the substrate, and(tentatively) with the real part of the refractive indexof aerosol particles 3,1O ll f oscillates about 1 only toa few percent in dependence on the additional filmthickness Ax. Furthermore, it should be noted thatthe radiation interfering is only partly coherent, asthe conditions of homogenity are not strictly valid inthe aerosol film.

Therefore with the above quantities of ni, n2, h3Eq. (1) is approximated by

J/J' exp(4.7 k Ax/x). (2)

The mass absorption index is given by

k/p = - n J/J'/4T-(m/F - mtIF'),

where p is the density of aerosol particles; m, m', F, F'are mass, respectively, area of the samples.

Thus by measurement of the transmittance of twothin aerosol films the thicknesses of which are slight-ly different, the absorption of aerosol particles col-lected by a jet impactor can be determined.

2852 APPLIED OPTICS / Vol. 14, No. 12 / December 1975

Instrumental apparatusfor ir - measurements

"Nernst" -bar

spherical mirror

chopperblocking filter

Ebert -monochromator

elliptical mirror

Golay cell

sample

parabolic mirror

Fig. 2. Optical setup.

Ill. Instrumental Apparatus

A. Collection of Aerosol Particles

The collections of aerosol particles were performedby an automatic jet impactor' 9 producing uniformthin layers of aerosol particles. The impactor wasadjusted so that (according to model-based calcula-tions2 0 ) the probabilities of deposition are 0.5 andclose to 1 for particles of radii r = 0.1 gim and r > 0.16Aim, respectively. Particles of radii r > 10 jim wereseparated by a preimpactor. The collections by thisimpactor device yield with a good approximation rep-resentations of those collectives of aerosol particlesthat due to their concentration and size distributionparticipate in the radiative transfer in the atmo-sphere. 21 The jet impactor consists of several paral-lel-acting nozzles. The air mass passing through iscontroled by flowmeters so that films of differentthicknesses of identical collectives of aerosol particlesare provided by small differences of the air flow.

B. Optical Setup

A Nernst glower (Fig. 2) is imaged by a sphericalmirror onto the entrance slit of an 0.25-m Ebert mo-nochomator. The diffracted radiation is directed bya paraxial parabolic mirror through the sample to thereceiver system consisting of a 900 paraxial ellipticalmirror and a Golay cell. The signals are detected bya phase-sensitive technique.

IV. Samples of Aerosol ParticlesSamples of aerosol particles of different origins

were submitted to the investigation of the mass ab-sorption index.(1). Urban air particles

Mainz University (145 m), 3 km west of the city(88 m).

(2) Clean air particles(a) Mace Head (30 m), a rocky promontory on the

Atlantic coast, 80 km west of Galway, Rep. ofIreland.

(b) Sphinx (3573 m), an exposed rock in the vicini-ty of Jungfraujoch (3454 m), Switzerland.

(c) Mitzpeh Ramon (950 m), a remote site insouthern Negev, Israel.

V. Results

A. Mass Absorption IndicesFigure 3 to 5 present plots of the mass-absorption-

indices of aerosol particles that were collected at theabove-mentioned urban and remote sites. The sam-ples were investigated by the described two-filmtechnique.

The absorption spectra are characterized by bandpatterns of various types of ions. The stretching vi-brations of the NH 4 + ion give rise to a broad, compli-cated band with components at 3.17 Am, 3.29 ,im,3.45 Am, and 3.52 jim. The N-H bending vibration isdistinguished by an intense band at 7.12 ,am with twoweak shoulders on the high frequency wing. Similar-ly, the broad band centered at 9.22 Am and the in-tense one at 16.36 jim are assigned to the correspond-ing types of vibration of (SO 4

2-) ions.22

Band patterns of this gross classification are foundin the spectra of urban aerosol (Fig. 3) as well as inthose of particles collected at the remote sites onJungfraujoch and on the Atlantic coast with wind

-. ~ wavenumber [cm-TI4000, 2000. 1 50 .800 700 600

O.24 Mainz 024

U22 x-K 233-1973 022-- 12--13-9- 1973.

Q20l Q,20

0 mass absorption index T 0.18

[c m 3 1]

0.14 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0.140.12 Z012

0014 - - 0.00.06- Q~~~~~~~~~~~~~~~~~~~~~~~~~~~00

Q02 ~~~~wavelength )X [m] 00

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Fig. 3. Mass absorption index k/p [cm3/g] vs wavelength X [m].Two samples of urban (Mainz, Germany) air particles.

December 1975 / Vol. 14, No. 12 / APPLIED OPTICS 2853

\

*- wvenumber [m-4000 2000 1500 1000 800 700 600

-- < Mace Head (Ireland) 30.11.-412-71&--- Mace Head 4.12.-7-12-71x-x Jungfraujoch (Switzerland) 14-5.672

Q02

mass absorption

[cm3/g] . i

I'

CrZ,,,A4

1I

0.18

0,16

Qan

0.12

0Oo

Oaoe

OD6

1N0

.02

2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17wavelength x [AMy

Fig. 4. Mass absorption index k/p [cm3 /gJ vs wavelength A [gm].Samples of clean air particles collected at an altitude of 3573 mnear Jungfraujoch, Switzerland and on the west coast of Ireland.

from land (Mace Head, 4 December to 7 December1971) (Fig. 4), suggesting that the main chemical con-stituent of aerosol particles may be (NH4 )2 SO4 .However, it should be noted that more than 10% ofthe mass of aerosol particles is of organic com-pounds.23 The broad absorption band in the S-Ostretching region, for instance, envelops bands of fur-ther sulfur-oxygen compounds like sulfone (R-SO2 -R) and sulfoxide (R-SO-R) and furthermore coin-cides with the C-O and C-N stretching region.2 4 Sothe strong band at 7.7 pm of sample Mainz, 23 March1973 (meteorological inversion, low visibility), whichis positioned in the 0-H and C-H bending region,24

cannot be identified, as specific correlated absorptionbands are covered. The influence of liquid water ad-sorbed by aerosol particles may be demonstrated byweak bands at 3 gm and 6.1 gm,25 although it shouldbe emphasized that the latter coincides with thestretching region of, for example, C-N, C-C, and C-O.

Generally, the marked intense background absorp-tion of urban aerosol is explained by the numeroussuperimposing bands of organic compounds like sootand further carbon-mixed polymerides of pollutedair.

The difference in absorption between continentaland marine aerosol is demonstrated in Fig. 4. Theweak absorption of marine aerosol is determined bythe predominant content of such marine salts asNaCl and KCI, although there are organic constitu-ents, too. 14 The sampling site was located in ascarcely populated region without any local humanactivity. Nevertheless the intensity of the typicalsulfur absorption band is comparable to that of theurban aerosol.

In the absorption spectrum of particles collectedon Jungfraujoch (Fig. 4) CaSO4 is indicated by the15-gm band26 as an additional contribution to possi-ble (NH4 )2 SO4 content. Furthermore, the high fre-quency wing of the sulfur band is superimposed by aband that in combination with an intense band be-

tween 9.2 • X < 9.8 um can be assigned to the doubletof the SiO stretching vibration.2 4 These bands ap-pear in the absorption spectra of quartz2 7 and in a setof Si-containing minerals.2 8

This type of sulfate-silicate aerosol was discoveredwith different mixing ratios at a remote site in thesouthern part of the Negev desert of Israel (Fig. 5).The particles collected during a dust storm (Sharav)are of quartz origin according to their spectrum.The spectrum of the next day's sample indicatesoverlapping of several bands, which therefore cannotbe assigned to a certain clay mineral.2 9 The decreasein the absorption index of aerosol particles during thecourse of time from 15 May to 24 May 1973 is quali-tatively confirmed by a different method, which in-vestigates absorption by measuring the emitted ra-diation of the sky in the atmospheric 9-Am window. 3 0

B. DensityThe density p of the present aerosol particles is un-

known as yet. Because of the small mass of the cleanair collections the technique previously developed' 9

is being improved.In Table I the mean densities of several types of

aerosol particles3 1 are presented. These data on thedensity combined with corresponding values of themass absorption index may yield an estimation con-cerning the absorption index of the present aerosolparticles.

VI. Remarks

The values of the mass absorption index presentedindicate that the presumption has to be revised thatthe effect of absorption by aerosol particles on theradiative fluxes and their divergence in the atmo-sphere is negligibly small. This presumption holdsonly for those parts of the spectrum where the opticaldepth of aerosol is exceeded by the one of gases by

.- wavenumber [cm ]4000 2000 1500 ., 00 800 700 600

Mitzpeh - Ramon (Israel)CA o- 14-15-5-73 (Sharov) 018

0.o 15-16.5-73 ,, A16

OJA1 mass absorptionindex * 18-24573 // 14

W2 [cm3 J 1412

Samples~ ~ ~ ~~~~~~~~~- of deetarprile0olce8nHihNg sal

ON 0.m' a0d

20 3~ 4Q0 941 2 31 5 7

wavelength X [pJm] -

Fig. 5. Mass absorption index k/p [ein3 /g] vs wavelength X [pm].Samples of desert air particles collected on High Negev, Israel.Particles collected during the Sharav of 14-15 May 1973 are of rel-atively weak absorption and show two absorption bands between 8

gm and 10 gmi.

2854 APPLIED OPTICS / Vol. 14, No. 12 / December 1975

.I

.

Table I. Mean Density of Different Types ofAerosol Particlesa

Marine/Contin.(Sahara

dust over Mountain UrbanMarine Atlantic) (1200 m) (Mainz)

Decreasinghumidity 2.35 2.53 2.68

1.81Increasing

humidity 2.41 2.59 2.77

a Relative humidity 35%.31

some orders of magnitude. But it is not necessarilyvalid in the atmospheric window regions, essentiallythose of 0.4-2.4 Am and 8-12 Am. The small windowregions between 3 m and 5 m are less effective, asthere is little energy in the terrestrial radiation andstill less in the solar radiation. Most aerosols exceptmarine aerosols absorb intensely in the atmosphericlong-wave window due to characteristic bands be-longing to the main constituents of natural aerosolparticles-generally sulfates in continental and back-ground aerosols.3 214 Compounds of Si-O structurecontained in certain minerals are also important con-stituents affecting the radiation budget of the long-wave window region, since desert areas, for example,the Sahara, are sources of continuous long-rangetransport of mineral dust in the atmosphere.3 3 34

The effect of aerosol particles on the cooling ratesof the atmosphere strongly depends on the prevailingwater vapor conditions. In the tropical parts of theatmosphere the influence on cooling rates is negligi-bly small because of the overwhelming contributionof water vapor, whereas for mid-latitudes this influ-ence can only be neglected in the case of low particleconcentrations in the air. In arctic regions the influ-ence of aerosol particles on the cooling rates exceedsthat of the water vapor continuum for 8 pm < X < 13pm. 35

Furthermore, short-wave radiation is absorbed byaerosol particles.3 6 -3 9 Computations revealed thatheating rates were of the same order of magnitude asthose due to water vapor caused by absorbent atmo-spheric aerosol particles. 4 0 Recent computations3 0

indicate approximate equivalence between heatingrates in the short-wave region and cooling rates in thetransparent long-wave part of the spectrum.

Whether this compensating effect between radia-tive flux divergence due to short-wave radiation andlong-wave radiation can generally be expected willonly be revealed by future comprehensive investiga-tions of atmospheric physics.

This study was supported by grant SFB 73 fromDeutsche Forschungsgemeinschaft.

Sincere thanks are given to Thomas C. O'Connor,University College, Galway, Rep. of Ireland and toJoachim Josef, Tel-Aviv University, Israel, for theirpersonal assistance in providing facilities at samplingsites.

Table II. Mass Absorption Index of Different Types of Aerosol Particlesa

k/ p (cm 3/g)

Marinec Marine/Cont.c Desert (Negev)Urbanb (west coast (west coast Mountain 14-15 May 1973 15-16

X (gin) (Mainz) of Ireland) of Ireland) (Jungfrau joch) (Sharav) May 1973 18-24 May 1973

3 0.046 0.032 0.055 0.007 0.011 0.034 0.0194 0.048 0.032 0.022 0.013 0.011 0.052 0.0215 0.038 0.036 0.018 0.013 0.010 0.045 0.0196 0.044 0.028 0.025 0.015 0.016 0.046 0.0177 0.089 - - 0.028 0.009 0.051 0.0328 0.047 0.019 0.017 0.012 0.017 0.025 0.0208.5 0.066 0.022 0.029 0.028 0.041 0.045 0.0199 0.135 0.026 0.125 0.063 0.066 0.091 0.0679.5 0.129 0.034 0.111 0.094 0.088 0.150 0.064

10 0.079 0.032 0.039 0.055 0.004 0.132 0.04610.5 0.063 0.025 0.025 0.041 0.006 0.085 0.04011 0.051 0.021 0.016 0.036 0.001 0.066 0.03411.5 0.047 0.011 0.017 0.033 0.001 0.054 0.03112 0.045 0.010 0.027 0.035 0.012 0.048 0.03113 0.050 0.007 0.034 0.038 0.034 0.061 0.03614 0.054 - - 0.042 0.041 0.071 0.04515 0.060 0.009 0.033 0.072 0.031 0.085 0.05216 0.109 0.002 0.082 0.063 0.091 0.126 0.08817 0.109 - - 0.097 0.066 0.163 0.091

a Relative humidity 35%.Mean values of all samples.These samples were collected on polyaethylene.

December 1975 / Vol. 14, No. 12 / APPLIED OPTICS 2855

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New Task Groupsfor Research Directorate

NSF's Advisory Committee for Research metOctober 31 and November to receive reports result-ing from work during the past 6 months by its threetask groups and to begin work on new tasks. Subjectsof the three reports were: Criteria for the Allocationof NSF Resources Between Individual Research Proj-ects and Major Research Facilities in Certain Fields;Fostering Interaction Among Research Organiza-tions; and Evaluation and Support of Inter-disciplinary and Multidisciplinary Proposals. Copiesmay be obtained from the Committee ManagementStaff, K-720, National Science Foundation, Wash.,D.C. 20550. Four new task groups were formed andthe following five new tasks were assigned: The SocialSciences as a Research Area in the National Interest;Review and Evaluation of Committee Operation;Evaluation of a Post Grant Evaluation Experiment;Larger But Fewer Research Grants; and ResearchEquipment. Task groups will meet several timesduring the ensuing year in open sessions. Theirreports will be presented at the next full meeting ofthe committee next year.

2856 APPLIED OPTICS / Vol. 14, No. 12 / December 1975