In situ Measurement of the Infrared Spectral Extinction for 

Various Chemical, Biological, and Background Aerosols 

by Kristan P. Gurton, David Ligon, and Rachid Dahmani 

ARL-TR-3071 September 2003 

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ARL-TR-3071 September 2003 

In situ Measurement of the Infrared Spectral Extinction for 

Various Chemical, Biological, and Background Aerosols 

Kristan P. Gurton, David Ligon, and Rachid Dahmani 

Computational and Information Sciences Directorate, ARL 

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In situ Measurement of the Infrared Spectral Extinction for Various Chemical, 

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Kristan P. Gurton, David Ligon, and Rachid Dahmani 

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14. ABSTRACT 

We conducted a series spectral extinction measurements on a variety of aerosolized chemical and biological simulants over the 

spectral range 3–13 μm using conventional Fourier transform infrared (FTIR) aerosol spectroscopy. Samples consist of both 

aerosolized particulates and atomized liquids. Materials considered include Bacillus subtilis endospores (BG), lyophilized 

ovalbumin, polyethylene glycol (PEG), dimethicone (SF-96 grade 50), and three common background materials (Kaolin clay 

[hydrated aluminum silicate], Arizona road dust [primarily SiO2], and diesel soot). Aerosol size distributions and mass density 

were measured simultaneously with the FTIR spectra. As a result all optical parameters presented here are mass-normalized, i.e., 

(m2

/g). 

In an effort to establish the utility of using Mie theory to predict such parameters, a series of calculations were conducted. For 

materials in which the complex indices of refraction are known, e.g., silicone oil (SF-96 grade 50) and Kaolin, measured size 

distributions were convolved with Mie theory and the resultant spectral extinction calculated. Where there was good agreement 

between measured and calculated extinction spectra, absorption, total scattering, and backscatter were also calculated. 

15. SUBJECT TERMS

Aerosol, infrared, extinction, absorption, backscatter FTIR, Bactillus subtilis

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Kristan P. Gurton 

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iii 

Contents 

List of Figures iv 

Acknowledgments v 

1. Introduction 1 

2. Experiment 2 

3. Results 4 

4. Comments 4 

5. References 12

Distribution 15 
iv 

List of Figures 

Figure 1. Measured size distribution for particulates and liquid droplets.......................................3 

Figure 2. Complex indices of refraction for Bacillus subtilis endospores......................................5 

Figure 3. Complex indices for liquid silicone SF-96 grade 50.......................................................6 

Figure 4. Complex indices of refraction for Kaolin clay................................................................7 

Figure 5. Measured (thick red) and calculated (thick blue) spectral extinction for aerosolized 

Bacillus subtilis endospores. Also shown are the Mie theory predicted total scatter (thin 

blue), absorption (orange), and backscatter (green)...................................................................8 

Figure 6. Measured (thick red) and calculated (thick blue) spectral extinction for nebulized 

liquid silicone SF-96 grade 50. Also shown are the Mie theory predicted total scatter 

(thin blue), absorption (orange), and backscatter (green)..........................................................8 

Figure 7. Measured (thick red) and calculated (thick blue) spectral extinction for aerosolized 

Kaolin clay. Also shown are the Mie theory predicted total scatter (thin blue), absorption 

(orange), and backscatter (green)...............................................................................................9 

Figure 8. Measured spectral extinction for PEG-200, diesel soot, Arizona road dust, and 

Ovalbumin..................................................................................................................................9 

Figure 9. Comparison of the measured extinction spectra for Ovalbumin (lyophilized egg 

white) with a Mie calculation using the indices of refraction for Bacillus subtilis 

endospores (BG). .....................................................................................................................11 

Figure 10. Measured extinction for agglomerated Cab-O-sil aerogel at various 

concentrations. The flat spectral extinction is synonymous with very large particles 

(compared with the wavelength)..............................................................................................12 
v 

Acknowledgments 

We would like to thank the Edgewood Chemical and Biological Center for their continued 

support and sponsorship in the area of aerosol research. In particular, we are extremely 

appreciative to Dr. Richard Vanderbeek and Dr. Alan Samuels for their technical expertise and 

the insightful conversations that aided us in the work. 
vi 

INTENTIONALLY LEFT BLANK
1 

1. Introduction 

Currently, there is a great effort underway within both the defense and academic research 

community to develop an optical-based technique that is capable of detecting the presence of 

harmful airborne biological or chemical agents. In order to accomplish this, researchers require a 

set of fundamental parameters that describe the electromagnetic (EM) interaction with micronsized particles (i.e., extinction, absorption, total-scatter, and backscatter coefficients). This 

report is an extension of prior work conducted as a “basic parameters” study designed to provide 

optical cross-sections for a variety of aerosol materials used to simulate the presence of more 

harmful agents (1). For many common naturally occurring aerosols, such parameters are readily 

available (2,3). However, for materials that are derived from biological and/or chemical matter, 

measured in situ optical parameters in the infrared (IR) are woefully lacking. 

In addition, for those who wish to calculate optical cross-sections, a similar knowledge of the 

bulk complex indices of refraction and particle size distributions is required. Often Mie theory 

(only truly appropriate for perfect spheres) are applied to media in which particle geometry is 

quite complex. Validity of these calculations are sometimes questionable particularly when the 

aerosol particle dimensions are comparable to the wavelength in question. Similarly, issues arise 

with experimental techniques used to measure the complex indices of refraction for particulate 

material (4). The preferred method for deriving indices in the IR involve compressing the 

materials into small quasi-transparent pellets. Transmission and near-normal reflectance 

measurements are conducted on these pellets over a given spectral range. A Kramers-Kronig 

relation is then applied to the reflection spectra and resultant complex indices are calculated. 

However, large uncertainties arise in spectral regions where anomalous dispersion for the 

material is great (i.e., regions where the absorption is strong). It is often these regions of strong 

absorption that are of greatest interest to the researcher. As a result, we attempt to address some 

of these issues of accuracy for certain calculated parameters in the IR, when the aerosols are not 

necessarily spherical, and whose indices of refraction are in question. In practice, accurate 

knowledge of both size distribution and complex indices of refraction are usually very difficult to 

acquire. In a report soon to follow we examine to what degree such unknown quantities can be 

“inverted,” using only spectral extinction and/or backscatter (5). 

The materials considered here may be broken into three distinctly different groups. The first 

group is characterized as typical biological simulants, which include Bacillus subtilis var. niger

(BG) endospores (a common soil-based bacteria) and dry powdered ovalbumin (lyophilized egg 

white). 
2 

The second group we considered consisted of two common chemical species, polydimethylsiloxane (SF-96 grade 50) and polyethylene glycol (PEG 200), and represented our chemical 

simulants. These two chemicals were chosen due to their relatively low vapor pressure, which 

allowed good droplet formation with little or no evaporation. 

The final group consisted of several “background” aerosols often present in the atmosphere (i.e., 

Kaolin clay [hydrated aluminum silicate], Arizona road dust [primarily SiO2 and Al2O3] and 

diesel soot). 

2. Experiment 

All samples presented here were provided by the Edgewood Research Development and 

Engineering Center (ERDEC), Aberdeen Proving Ground, MD. However, much of these 

materials are quite common and can be acquired through conventional chemical suppliers. 

Aerosol samples derived from solid material (e.g., BG, Kaolin, and ovalbumin), were milled to 

produce particle distribution within an approximate size range of 0.7 μm < diameter < 10 μm. 

These powdered materials were effectively aerosolized by injecting pressurized dry air through a 

cylindrical nozzle that contained a spiraling array of fine stainless-steel wires. A vortex created 

within the nozzle effectively separated and dispersed the dry particles with minimal 

agglomeration. Care was taken to properly adjust the air pressure to levels that resulted in good 

particle separation while remaining low enough to avoid fragmentation of the particles below 

their milled diameters. 

Liquid samples were dispersed using a pharmaceutical nebulizer. These nebulizers are 

specifically designed to produce particle distributions within the respiratory range of 0.8 μm < 

diameter < 4.0 μm when pressurized to 10 psi. 

We have found that depending on the viscosity of the liquid, generated size distributions can be 

shifted to smaller diameters by slightly increasing the pump pressure. As a result both PEG 200 

and SF-96 grade 50 were atomized using a pressure of 20 psi, which produced a distribution with 

a modal diameter 0.82 μm. Particle-size distributions were measured in situ using a commercial 

particle-size spectrometer, TSI Aerodynamic Particle Sizer Model 3321. Resultant size 

distributions for aerosolized particulates and liquid droplets are shown in Figure 1. Because TSI 

instrument use a “time-of-flight” method to predict an aerodynamic diameter, accurate values for 

the “bulk” density of the aerosol particle is required. Unfortunately, for aerosols that were 

highly amorphous or that contain many different constituents, accurate bulk densities were 

unattainable (e.g., soot and Arizona road dust). As a result, size distributions for those materials 

are not presented. However, antidotal evidence-based aerosol settling rates and videomicroscopy images of particles captured on slides suggest distributions similar to what is seen 

for Kaolin (i.e., a large submicron component with a model diameter between 1 and 5 μm). 
3 

Figure 1. Measured size distribution for particulates and liquid droplets. 

The primary transmission measurements were conducted in a 0.8-m3 aerosol chamber which 

provided an optical path length of 0.61 m. Dispersed aerosols were gently drawn into the 

chamber using a small-area recirculating fan. Continuous monitoring of humidity levels was 

conducted using a filtered dew-point hygrometer that was inserted through the walls of the 

chamber. Because dry air was used to flush the various optical windows, water vapor levels 

were kept to a minimum. Relative humidity levels for most runs were typically < 8%, which is 

representative of a fairly dry environment. 

We obtained IR transmission spectra using a high-resolution (0.02 wave number) Bomem 

DA2.02 Fourier transform IR (FTIR) spectrometer. For this study, the spectrometer was 

operated in a transmission mode (i.e., spectral attenuation was measured by placing the aerosol 

chamber between the source and the interferometer). A broadband IR Nernst glower was 

collimated with a ZnSe condensing lens assembly and projected through the aerosol chamber 

with two BaF2 transmission windows that were fitted with dry-air flushes. Transmitted light was 

coupled to the interferometer with a gold-surfaced f/4 off-axis parabola. Because spectra derived 

from aerosols are devoid of any fine structure often seen in vapor spectra, the FTIR was operated 

at a spectral resolution of 4.0 cm–1. 

Aerosol mass densities (g/m3

) were periodically measured during the run using conventional 

dosimetric techniques (6). Aerosol mass samples were collected by drawing known volumes of 

the air-particle mixture onto polycarbonate filters, nominal pore-size 0.20 μm, for predetermined 

periods of time. Results from the dosimetric sampling were then used to mass normalize the 

measured spectral extinction, (m2/g). 

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

0.1 1 10 100

Aerodynamic Diameter (um) 

Particle

 Count 

(au) SF-96 & PEG-200

Bacillus subtilis (new)

Kaolin

Ovalbumin 
4 

3. Results 

Interferograms were recorded before, during, and after each aerosol dispersion. A Bartlett 

apodization was applied to each interferogram before the background spectra were removed. 

Transmission was converted to extinction using a Beer’s law relation. Results were mass 

normalized by dividing the path-integrated extinction, (1/m), by the corresponding aerosol 

density, (g/m3

). 

For materials in which complex indices of refraction were available from prior studies (Figures 

2–4), Mie theory calculations were conducted using the appropriate size distribution shown in 

Figure 1 (7). Where agreement between the measured and calculated extinction was reasonable, 

the calculated spectral absorption, total scatter, and backscatter are also presented (Figures 5–7). 

For ovalbumin, PEG 200, diesel soot, and Arizona road dust in which no such indices were 

available, only the measured spectral extinction is presented (Figure 8). 

4. Comments 

When comparing measured extinction with Mie calculated values (Figures 5–7) one should keep 

in mind that all the complex indices used in the calculations were gathered from prior studies and 

may not accurately represent the materials used here. Nevertheless, we thought it informative to 

show such comparisons because it is often the case that one must rely on previously published 

constants to compute various optical parameters. 

For the three materials presented with refractive indices (i.e., BG, Kaolin, and SF-96 grade 50), 

the refractive index for SF-96 grade 50 is least likely to vary from one measurement to the next 

because it is a highly standardized chemical where production is tightly monitored. In addition, 

because it is a liquid, we know with certainty that the nebulized particles must be spherical and 

thus most appropriate for applying Mie theory. As one can see in Figure 6, agreement between 

measured and calculated spectral extinction is quite reasonable. Agreement for the location of 

the peaks and valleys for the spectral extinction is quite good. However, agreement between the 

relative magnitude between the two is not as good and is attributed to uncertainties in the size 

distribution (Figure 1). In general, the underpredicted values for the Mie extinction is 

synonymous with a slightly broader size distribution than seen in Figure 1. Nevertheless, where 

agreement is exceptable (e.g., 3–8.5 μm and 10–12 μm), Mie computed absorption, scatter, and 

backscatter should be fairly representative of the true values.
5 

Index of refraction for Bacillus subtilis

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55

1.6

3 4 5 6 7 8 9 10 11 12 13 14

Wavelength (um)

Real 

Index

1.E-03

1.E-02

1.E-01

1.E+00

3 4 5 6 7 8 9 10 11 12 13 14

Wavelength (um)

Imaginary

 Index

Figure 2. Complex indices of refraction for Bacillus subtilis endospores. 
6 

Refractive index for silicone SF-96 grade 50

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Real

Index

0

0.2

0.4

0.6

0.8

1

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Imaginary

 Index

Figure 3. Complex indices for liquid silicone SF-96 grade 50. 
7 

Refractive index for Kaolin

0

0.5

1

1.5

2

2.5

3

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Real 

Index 

0

0.5

1

1.5

2

2.5

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Imaginary 

Index 

Figure 4. Complex indices of refraction for Kaolin clay.
8 

1.E-03

1.E-02

1.E-01

1.E+00

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Mass 

Normalized

 Coefficient

(m^2/gram)

Extinction (measured)

Extinction (M ie theory) 

Total Scatter (M ie theory) 

Absorption (M ie theory)

Backscatter (M ie theory)

Figure 5. Measured (thick red) and calculated (thick blue) spectral extinction for aerosolized Bacillus 

subtilis endospores. Also shown are the Mie theory predicted total scatter (thin blue), 

absorption (orange), and backscatter (green). 

1.E-03

1.E-02

1.E-01

1.E+00

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Mass

Normalized

Coefficient

(m^2/gram)

Extinction (measured) 

Extinction (Mie theory)

Total scatter (Mie theory)

Absorption (Mie theory)

Backscatter (Mie theory)

Figure 6. Measured (thick red) and calculated (thick blue) spectral extinction for nebulized liquid 

silicone SF-96 grade 50. Also shown are the Mie theory predicted total scatter (thin blue), 

absorption (orange), and backscatter (green). 
9 

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Mass

 normaized Coefficient

(m^2/gram)

Extinction (measured)

Extinction (Mie theory)

Total Scatter (Mie theory)

Absorption (Mie theory)

Backscatter (Mie theory)

Figure 7. Measured (thick red) and calculated (thick blue) spectral extinction for aerosolized Kaolin clay. 

Also shown are the Mie theory predicted total scatter (thin blue), absorption (orange), and 

backscatter (green). 

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Extinction

Coefficient 

(m^2/gram)

PEG-200

Diesel soot

Arizona road dust (SiO2)

Ovalbumin

Figure 8. Measured spectral extinction for PEG 200, diesel soot, Arizona road dust, and ovalbumin. 
10 

As for the Bacillus subtilis (BG) samples, little is known about the exact form and conditions in 

which the indices were measured by Querry (7). What is certain is that the BG sample(s) 

provided by ERDEC was grown, cleaned, and milled from a completely different batch than used 

by Querry nearly a decade prior. In lieu of this and given the fact that growth media used for 

producing bacterial endospores can be quite variable, agreement seen in Figure 5 is good. One 

notable region of divergence occurs in the long wave IR between 9 and 10 μm. Although 

difficult to see in Figure 5, the measured spectral extinction shows a modest peak at 9.1 μm, but 

the calculated spectra using Querry’s indices show this peak quite suppressed and shifted toward 

10 μm. Although researchers have attributed this peak to absorption by phosphates and/or 

polysaccharides, we believe a portion of the increased extinction seen in the measured spectra 

between 9 and 10 μm may be due to a residual silicate used during the milling process. 

Perhaps the least certain complex index used was that of Kaolin. It is well known that the optical 

properties for Kaolin are quite variable, depending upon the geographic location where it was 

mined (i.e., Kaolin is routinely sourced from North America, China, and Europe). As a result, 

good agreement between measured and calculated extinction was not expected. As seen in 

Figure 7, the spectral features of the Mie calculated extinction appears suppressed when 

compared to the measured values. 

Figure 8 shows the measured mass-normalized spectral extinction for PEG 200, Arizona road 

dust, ovalbumin, and diesel soot. As mentioned before, complex indices of refraction were not 

available for these materials and as a result no Mie calculations are presented. However, we will 

note several key aspects for the various spectra seen in Figure 8. 

First, the two noticeable features seen in the extinction spectra for diesel soot seen at 4.25 μm 

and the fine structure seen between 5 and 8 μm are due to gaseous absorption of CO2 and water, 

respectively, that were generated during the burning of diesel fuel and proved extremely difficult 

to remove using conventional FTIR background subtraction methods. Prior studies have shown 

this spectra to be devoid of any real structure and decreases monotonically with increasing 

wavelength (8). 

Although the preparation/dissemination of the PEG 200 liquid was relatively straightforward, we 

did notice during several runs the emergence of strong water vapor absorption lines when the 

liquid samples were left in the open for periods of several hours due to the fact that PEG 200 is 

extremely hydroscopic. For this reason, care should be taken when using PEG 200 chemical 

simulant for aerosol research because water uptake for atomized samples is very likely except in 

the most arid of environments. 

The extinction spectrum for ovalbumin turned out to be somewhat surprising. As one can see by 

comparing ovalbumin spectra with the Mie calculated extinction spectra for BG seen in Figure 5, 

the two spectra are nearly identical (Figure 9). 
11 

1.E-02

1.E-01

1.E+00

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Mass 

Normalized

Extinction

 Coefficient

(m^2/gram) Bacillus subtilis (Mie theory) 

Ovalbumin (measured)

Figure 9. Comparison of the measured extinction spectra for ovalbumin (lyophilized egg white) with a 

Mie calculation using the indices of refraction for Bacillus subtilis endospores (BG). 

Although not definitive, based on this comparison, it seems that one might be able to use with 

this comparison the expected complex indices of refraction for ovalbumin should be similar to 

that of BG seen in Figure 2. Where cost and the availability of a good biological simulant are an 

issue, using ovalbumin powder may suffice. This would be particularly useful for field tests in 

which large quantities of a simulant are required. 

The last material shown in Figure 8 is that of a somewhat standardized background aerosol 

called Arizona road dust obtained from Powder Technology Inc., Burnsville, MN. Because 

Arizona road dust is well characterized it has become a popular natural obscurant used in many 

radiative transfer measurements. The particular type used for this study was sized to have a 

nominal diameter of 1.13 μm with a standard deviation 0.385 μm. Chemical analysis shows the 

following compounds as a percentage of mass: 76% SiO2, 15% Al2O3, 4% Fe2O3, 4% Na2O, as 

well as traces of CaO, MgO, and TiO2. 

Finally, as part of this study we were provided with an unusual material called Cab-O-sil. CabO-sil is a synthetic, amorphous, untreated fumed silicon dioxide that is composed of fine submicron SiO2 particles. Used commercially as a thickening agent for food and cosmetics, this 

silica “aerogel” is extremely amorphous (94% of its volume is air) and is sometimes used as a 

fluidizer to improve aerosol dissemination efficiencies. As a result, we were asked to produce 

spectra for this possible residual compound. However, difficulties with extreme agglomeration 

arose during the FTIR measurement. Various attempts to disperse dry well-separated Cab-O-sil 

particles failed. Resultant measured spectra showed little wavelength dependence regardless of 

the concentration (Figure 10). These “flat” spectra are synonymous with the presences of very 

Bacillus subtilis (Mie theory) 

Ovalbumin (measured) 
12 

1.E-02

1.E-01

1.E+00

3 4 5 6 7 8 9 10 11 12 13

Wavelength (um)

Extinction 

(1/m)

Cab0001

Cab0002

Cab0003

Cab0004

Figure 10. Measured extinction for agglomerated Cab-O-sil aerogel at various concentrations. The flat 

spectral extinction is synonymous with very large particles (compared with the wavelength). 

large particles when compared to the wavelength (i.e., the extinction mechanism is primarily due 

to the particle’s geometric cross-section rather than any diffraction effects). Efforts to measure 

the in situ aerosols density also failed because we were unable to collect enough measurable 

mass on the dosimetric filter substrates during the spectral runs. Nevertheless, one can see the 

obvious absorption between 8 and 10 μm due to SiO2, which is also seen in the Arizona road 

dust spectra. 

5. References 

1. Gurton, K. P.; Ligon, D.; Kvavilashvili, R. Measured Infrared Spectral Extinction For 

Aerosolized Bacillus subtilis var. niger Endospores From 3 to 13 um. Applied Optics 2001, 

40 (25), 4443–4448. 

2. Deepak, A. Atmospheric Aerosols: Their Formation, Optical Properties, and Effects; 

Spectrum Press: Hampton VA, 1982. 

3. Gerber, H.; Hindman, E. Light Absorption by Aerosol Particles; Spectrum Press: Hampton 

VA, 1982. 

4. Wieliczka, D. W.; Querry, M. R. Four Techniques to Measure Complex Refractive Indices of 

Liquids and Solids at Carbon Dioxide Laser Wavelengths in the Infrared Spectral Region; 

CRDEC-CR-062; Chemical Research, Development and Engineering Center: Aberdeen 

Proving Ground, MD, 1990. 
13 

5. Ligon, D.; Gillespie, J.; Pellegrino, P. Aerosol Properties From Spectral Extinction and 

Backscatter Estimated by an Inverse Monte Carlo Method. Applied Optics 2000, 39 (24), 

4402–4410. 

6. Bruce, C. Measuring Aerosol Density Using Nephelometry and Dosimetry; Center for 

Atmospheric Sciences Press: Las Cruces, NM, 1987. 

7. Querry, Marvin R. Optical Constants of Minerals and Other Materials from the Millimeter 

to the Ultraviolet; CRDEC-CR88009; Chemical Research, Development and Engineering 

Center; Aberdeen Proving Ground, MD, 1987. 

8. Bruce, C. W.; Gurton, K. P.; Stromberg, T. F. Trans-Spectral Absorption and Scattering of 

Electromagnetic Radiation by Diesel Soot. Applied Optics 1991, 30 (12), 1537–1546. 
14 

INTENTIONALLY LEFT BLANK
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Director

US Army Rsrch Lab

ATTN AMSRL-RO-D JCI Chang

ATTN AMSRL-RO-EN W D Bach

PO Box 12211

Research Triangle Park NC 27709

US Army Rsrch Lab

ATTN AMSRL-D D R Smith

ATTN AMSRL-D J M Miller

ATTN AMSRL-CI-IS-R Mail & Records Mgmt

ATTN AMSRL-CI-IS-T Techl Pub (2 copies)

ATTN AMSRL-CI-OK-TL Techl Lib

ATTN AMSRL-CI-EP K Gurton

ATTN AMSRL-SE-EE A Goldberg

ATTN AMSRL-SE-EM D Beekman

Adelphi MD 20783-1197