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. 
Army Research Laboratory 

Adelphi, MD 20783-l 197 

ARL-TR-2343 February 2001 

Extinction, Absorption, 

Scattering, and Backscatter for 

Aerosolized Bacillus Subtilis 

Var. Niger Endospores From 

3to13pm 

Kristan P. Gurton, David Ligon, and Ramaz Kvavilashvili 

Computational and Information Sciences Directorate 

Approved for public release; distribution unlimited. 
Abstract 

Spectral extinction was measured in situ for aerosolized Bacillus 

subtilis var. niger (BG) endospores with the use of Fourier-transform 

infrared (FTIR) spectroscopy from 3.0 to 13.0 pm. Corresponding 

aerosol-size distributions were measured with the use of a 

commercially available elastic light-scattering probe and verified by 

direct particle capture and subsequent counting via video 

microscopy. Aerosol mass density was monitored simultaneously 

with conventional dosimetry and used to mass-normalize the 

measured spectral extinction. Mie theory calculations based on 

measured distributions and available complex indices of refraction 

agreed well. Also present are resultant Mie calculations for the 

absorption, total scattering, and backscattering. Included are the 

real and imaginary components of the complex index of refraction for 

BG as measured by Milham and Quarry. Both calculated and 

measured cross sections suggest that for wavelengths longer than 

6.0 pm, the total extinction is primarily due to absorption. Finally, to 

offer a comparison, we present measured spectral extinction for three 

additional aerosols often found in the lower atmosphere, i.e., water 

fog, diesel soot, and Arizona road dust. 
Contents 

1. Introduction 

2. Experiment 

3. Results 

4. Discussion 

Acknowledgments 

References 

Distribution 

Report Documentation Page 

Figures 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

1 

2 

5 

8 

12 

13 

15 

19 

Measured BG size distribution with corresponding log-normal 

curve fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Single BG endospores shown here to be reasonably spherical ..*... 4 

FTIR transmittance for various aerosol concentrations of BG . . . . . . . . . 5 

Measured and calculated mass-normalized extinction, absorption, 

total scatter, and backscatter cross sections for aerosolized BG 

endospores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Real and imaginary indices of refraction for BG . . . . . . . . . . . . . . . . . . . . . . . . 7 

Comparison of measured mass-normalized extinction for BG with 

three common background aerosols, i.e., water fog, Arizona road 

dust, and diesel soot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Comparison of FTIR measurements conducted on various forms of 

BG, i.e., aerosolized BG, thin-film slurry, and bulk powdered BG . . . 9 

Example of two simple “form-preserving” operations . . . . . . . . . . . . . . . 10 

. . . 

111 
1. Introduction 

Infrared (IR) spectral extinction for common inorganic aerosols found in 

the lower atmosphere has been studied and reported on extensively [l-5]. 

However, research involving the optical properties for intact viruses, fungi, 

and bacterial aerosols is still extremely rare and woefully lacking [6-81. 

Though much of the aggregate bioaerosol material in the lower atmosphere 

is mundane in nature, reported diseases in humans, animals, and plants 

have been linked to certain airborne bacteria [9-131. Tong and Lighthart 

have reported ambient total atmospheric bacterial (TAB) concentrations for 

a rural environment that range from 0.1 to 0.001 airborne bacteria per cm3 

(cells/cm3) [14,15]. 

The ability of one to detect the presence of a harmful bioaerosol from a safe 

distance is an ever-increasing topic of interest. Proposed optical methods 

that involve IR light are usually restricted to one or both of the atmospheric 

transmission window regions, i.e., 3 to 5 pm and/or 8 to 12 pm. These methods usually involve either passive or active illumination schemes, e.g., 

hyperspectral IR imaging or a CO2 lidar-type arrangement [ 16-201. Whether 

these (or other) optical techniques are appropriate for the remote detection 

and identification of harmful bioaerosols remains a topic of discussion. 

Nevertheless, for the utility of any particular approach to be properly evaluated, certain optical parameters at IR wavelengths for well-characterized 

bioaerosols are badly needed. 

Methods for determining the electromagnetic interaction with bioaerosols 

usually involve either direct in situ light measurements or particles that 

exhibit some spherical symmetry (or that are Rayleigh); Mie or a comparable theory may be used to calculate the scattered and absorbed fields. For 

the latter approach in which the optical interaction is computed, the complex refractive indices for the biomaterials are assumed to be well known. 

Even when these refractive indices exist (which is rarely), accurate predictive calculations are usually extremely difficult to compute, since these types 

of particles are typically inhomogeneous and often nonspherical. 

We directly measured the IR spectral extinction for aerosolized Bacillus 

subtilis var. niger (BG) endospores from 3 to 13 ym using conventional 

Fourier-transform infrared (FTIR) spectroscopy. Size distributions, aerosol 

densities, and particle morphology were measured simultaneously. We then 

compared these results with Mie theory calculations using complex indices 

of refraction provided by Milham and Quarry [20]. For regions in which 

there is good agreement between the measured and calculated extinction, 

we also present the total scatter, absorption, and backscatter components 

predicted by the Mie theory. Spectral extinction for both bulk powder and 

thin-film forms of BG is also presented. Finally, we contrast the extinction 

spectra for BG with three atmospheric aerosols commonly found in the environment, i.e., water fog, diesel soot, and Arizona road dust. 

1 
2. Experiment 

The BG endospores used in this study were provided by the Edgewood 

Research Development and Engineering Center (ERDEC), Aberdeen Proving Ground, MD, and were produced in large quantities for use as a biological warfare (BW) simulant. This material, often referred to as “militarygrade” BG, was assayed and known to contain 12.7 x lOlo colony-forming 

units per gram (cfu/g). Ion chromatography was performed on both washed 

and unwashed suspensions. Results showed small quantities of sulfate ions 

with lesser amounts of POae3, F; and Cl- ions, which have been attributed 

to small quantities of residual growth media. Prior ultraviolet fluorescent 

studies have associated certain anomalous results to these sulfate ions [22]. 

IR extinction measured here for both washed and unwashed BG samples 

showed no appreciable differences between the spectra. 

The primary transmission measurement was conducted in a 0.5-m3 aerosol 

chamber, which provided an optical path length of 0.61 m. Dry and hydrated BG endospores in aerosol form were dispersed separately with a 

variety of techniques that have proven effective in prior studies. We generated hydrated endospore droplets using two pharmaceutical nebulizers that 

atomized various concentrations of a BG/water solution. To simulate conditions similar to the open atmosphere, we evaporated the encapsulating 

water droplets by directing the BG/water spray into a plenum of heated 

dry air. The resultant spore aerosol was gently drawn into the chamber with 

a small-area recirculating fan. We continuously monitored relative humidity using a filtered dew-point hygrometer that was inserted through the 

walls of the chamber. 

. 

Dry powdered BG was effectively aerosolized and sprayed into the chamber. Pressurized air was used to inject the endospore powder 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 

bacterial spores with minimal agglomeration. Care was taken to properly 

adjust the air pressure so that spore coatings remained reasonably intact. 

We obtained IR transmission spectra using a high-resolution (0.02 

wavenumber) Bomem DA2.02 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 offaxis parabola. 

Particle-size distributions were measured with a commercial particle-size 

spectrometer. We used Particle Measuring Systems, Inc. @‘MS), particle spectrometer (model CSASP-100) to monitor in real time the aerosol size distribution. We determined particle-shaped characteristics by analyzing 
Figure 1. Measured BG 

size distribution with 

corresponding lognormal curve fit. 

photographs of captured particles that were collected on shielded glass 

slides. Using these photographs, we directly counted representative samples 

and generated a size distribution that agreed well with the distributions 

measured using the PMS. The resultant size distribution with a corresponding log-normal fit (r modal = 0.89 pm; standard deviation, cr= 0.15). A typical BG run is shown in figure 1. Photographs also showed that the endospore 

aerosol consisted of mostly single or minimally agglomerated particles that 

appeared reasonably spherical (see fig. 2). Aerosol mass densities (g/m3) 

were periodically measured by collecting the aerosol on polycarbonate filters while sampling known volumes of air for predetermined periods of 

time. Results from the dosimetric sampling were then used to massnormalize the measured extinction (m*/g). 

To offer a comparison, we measured the spectral extinction for three additional “background” aerosols commonly found in the environment, i.e., diesel soot, water fog, and Arizona road dust (SiO*). Environmental aerosols 

were generated by either burning diesel fuel (soot), nebulizing distilled water 

(fog), or dispersing dry dust in a similar manner as just described (Arizona 

road dust). Spectra were measured and mass-normalized in the same fashion as the BG aerosol. 

1.0x1 

1.0x1 

-2 

c 1.0x1 

; a, 1.0x1 

3 

r 

a” 1.0x1 

04 

Cont. = 1.9 e4 

03 

Particle diameter (pm) 
Figure 2. Single BG 

endospores shown 

here to be reasonabl 

spherical. 

4 

.Y 
3. Results 

Figure 3. FUR 

transmittance for 

various aerosol 

concentrations of BG. 

Figure 4. Measured 

and calculated massnormalized (m2/g) 

extinction, absorption, 

total scatter, and 

backscatter cross 

sections for 

aerosolized BG 

endospores. 

Interferograms were recorded before, during, and after each aerosol dispersion. We applied a Bartlet apodization to each interferogram before performing background rationing. Forward-scattering corrections were applied 

to the raw transmission and were foLmd to be insignificant at the wavelengths above 6 pm. As the wavelength becomes comparable (or smaller) to 

the endospore diameter, the forward-scattering correction increases monotonically and was found to be as much as 9 percent at 3 urn [23]. Figure 3 

shows a typical graph of the recorded transmittance from 3 to 13 pm for a 

series of concentrations of BG aerosol. Transmission was converted to extinction with a Beer’s law relation. Results were mass-normalized by dividing the raw extinction (l/m) by the corresponding aerosol mass density 

(g/m3) measured during the dosimetric portion of the experiment (see fig. 4). 

1.05 

1 .oo 

0.95 

0.90 

0.85 

a 0.80 

$ 0.75 

.E 0.70 

5 0.65 

fi 0.60 

+ 0.55 

0.50 

0.45 

0.40 

0.35 

0.30 

Wavelength (pm) 

Extinction (Mie calculation) 

Extinction (measured) 

Absorption &lie calculation) Scatterkg (Mie calculation) 

6 7 8 9 

Wavelength (pm) 

10 11 12 13 

5 
6 

Figure 4 shows both measured and calculated cross sections. Mie theory 

cross sections were calculated by convolving the measured size distribution with the complex indices of refraction (provided by Milham and Quarry) 

shown in figure 5 [20]. As one can see, the measured extinction shows similar form to the calculated spectra, albeit at slightly reduced levels. What is 

interesting and worth noting is that beyond 6 ym, the Mie calculations show 

that almost all the extinction for the BG aerosol is due to absorption. 

Particle number densities were calculated by integrating the measured size 

distribution with the estimated mass per particle (bulk density for BG taken 

to be 1.45 g/cm3) and equating it to the measured aerosol mass density [25]. 

Particle number densities for a typical run ranged from a low of 2 x lo3 

endospores/cm3 for periods in which a significant amount of settling had 

occurred to peak values on the order of 1 x lo6 endospores/cm3 for periods 

shortly after the initial dispersion. Note that certain spectral features became difficult to resolve when particle number densities fell much below 

lo3 endospores/cm3 (based on our relatively small 0.61-m path length). 

It is sometimes convenient to represent the coefficients shown in figure 4 on 

a “per-spore” basis. Using the measured size distribution and assuming the 

endospores and their agglomerates are reasonably spherical, we calculate a 

value of approximately 5 x lo-l2 g/spore, which can then be used to convert the mass-normalized quantities shown in figure 4 (m2/g) to cross sections per spore (m2/spore). As an example, we find from figure 4 the massnormalized extinction at 9.32 pm to be 1.16 x 10-i m2/g. Multiplying this 

value by the conversion factor, 5 x lo-i2 g/spore, we calculate the extinction 

cross section per spore to be 0.583 x lo-l2 m2/spore. This compares well 

with the Mie theory calculated value of 0.525 x lo-l2 m2/spore. 

To contrast these results, we measured the extinction spectra for three atmospheric aerosols, i.e., water fog, diesel soot, and Arizona road dust. A 

similar set of measurements was repeated for the three environmental aerosols. Care was taken to ensure that generated size distributions (especially 

for the water fog) were similar to those measured in the field [26]. Massnormalized extinction spectra for the three aerosols are compared with BG 

and are shown in figure 6. Absorption caused by residual water vapor produced the fine structure seen between 5 and 8 l..trn in both the fog and soot 

spectra, and the apparent spike seen near 4.25 ,um in all spectra (especially 

for soot) was identified as residual CO,. 
Figure 5. (a) Real and 

(b) imaginary indices 

of refraction for BG. 

Figure 6. Comparison 

of measured massnormalized extinction 

(m2/g) for BG with 

three common 

background aerosols, 

i.e., water fog, Arizona 

road dust, and diesel 

soot. 

(4 

(b) 

8 

z .- .?- 

1.25 U 

1.20 

2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.511.512.5 13.5 

1 

0.1 

iz ‘2 

g 0.01 

0.001 

s ‘G 

2 '= 1.0x1 

ii 

u 

w $j 1.0x1 

E 

E 

dY 1.0x1 

z 

2 

1.0 x 1 

Wavelength (pm) 

L 

5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.511.51 

Wavelength (Fm) 

3 4 5 6 7 8 9 10 11 12 13 

Wavelength (pm) 

7 
4. Discussion 

8 

As seen in figure 6, BG shows relatively moderate extinction at the IR wavelengths considered (on a mass-normalized basis) when compared to the 

various background aerosols. As expected, the spectra for aerosolized BG 

are relatively smooth and devoid of any sharp identifiable “line” structure. 

While many of the IR bands of intact microorganisms still have to be assigned unambiguously, preliminary species identification conducted on biological thin-films has been reported [27]. 

The most characteristic features associated with the presence of certain spore 

proteins occur in the range between 5.6 and 6.6 pm and are attributed to the 

so-called “amide I and II” absorption bands. Unfortunately, this highly characteristic region lies in the middle of the most opaque portion of the atmosphere and is probably of little value when possible remote detection techniques are considered (i.e., because of strong water vapor absorption, transmission from 5 to 8 urn is nearly impossible) [28]. Naumann and Helm have 

tentatively identified the smooth broadly peaked region between 8 and 

11 pm as a superposition of many fine absorption bands caused by C-O-C 

and C-O-P stretching of predominantly polysaccharide or phophodiester 

[29]. Naumarm and Helm assert that the peak around 9.3 mm is due primarily to symmetric molecular vibrations of phosphate diesters. Beyond 11 

ym, the region exhibits a variety of weak but extremely characteristic features attributed to aromatic ring vibrations of phenylalanine, tyrosine, tryptophan, and various nucleotides. With the exception of a few weak peaks 

around 13 pm (resulting from XH, rocking modes of fatty acid chains), 

specific assignment is impossible. Although this region appears relatively 

featureless, Naumann and Helm have successfully used this G- to 13-pm 

region for taxonomical identification of bacterial thin-films. Because of this, 

the region is often referred to as the “bacterial fingerprint region.” 

Because much of the existing information (albeit limited) involving IR spectra 

for biological materials is derived from thin-film samples, we thought it 

interesting to compare differences among extinction spectra for various 

forms of BG. Figure 7 shows a comparison of the spectral extinction for 

aerosolized BG, bulk powdered BG (measured with attenuated total reflection (ATR)), and thin-film spectra taken from Yabushita and Wada [27]. As 

expected, much of the spectral information is similar from one form to another for regions where absorption is predicted to dominate. Also seen in 

figure 7 are the effects caused by scattering. As the ratio of the spore diameter to wavelength becomes larger, scattering begins to dominate in the aerosol extinction. Certain absorption features seen near 3 jt.rn for the bulk and 

thin-film spectra are completely obscured by particulate scattering and are 

not seen in the aerosol spectra. 

As one can see in figures 4,6, and 7, most aerosol spectra tend to be devoid 

of any easily identifiable characteristic features. This makes reliable identification difficult with the use of conventional spectra correlation techniques. 

Naumann and Helm have demonstrated the ability to rapidly classify and 
Figure 7. Comparison 100 

of FTIR measurements 

conducted on various ...‘.‘... Thin-film Yabushita 

forms of BG, i.e., 

aerosolized BG (top), 

- thin-film slurry lo- \L I”r\/ 

(middle), and bulk 

r,. 

‘\ 

powdered BG -3 “l.,, ‘-v” \-#---A- /-n-k 

25 

(bottom). 

‘.i= s l- : ._ 

. . . 

E 

: ‘.. : :,. 

..‘. ‘.,,I..“’ 

._..., -- _. . __ 

.- -‘. _- 

2 

w 

0.1 - 

0.01 2.5 

I 1 

6.5 8.5 

Wavelength (pm) 

I , 

10.5 12.5 

group intact bacterial specimens using a technique called “cluster analysis” 

[6]. Their approach is based on treating spectra as images or patterns and 

applying classic pattern recognition algorithms to discriminate subtle differences among similar spectra. A variety of simple mathematical operations is applied to weighted wavelength regions based on certain predetermined criteria. In addition, when comparing one spectral form to another, 

one must consider carefully how the aerosol spectra were recorded. Bryson 

and Flanigan have reported on an obvious but interesting effect witnessed 

during a series of FTIR aerosol field measurements [28]. They report that 

for wavelengths in which aerosols are highly absorbing, as is for BG above 

6 pm, measured transmission may appear as either an extinction or emission 

spectra (one being the mirror image of the other), depending on whether 

the background is considered hot or cold relative to the aerosol. The symmetry relation between absorption and emission spectra is a direct result of 

Kirchhoff’s law that relates aerosol absorption efficiencies to particle emissivity [29]. Under such conditions, it would be desirable to consider a “form”- 

preserving operation that is independent of aerosol density and/or background conditions. 

As an example, we apply two simple form-preserving operations on the 

measured extinction spectra, i.e., first and second derivatives (see fig. 8). 

Figure 8 shows the original extinction for each aerosol measured at various 

concentrations (top spectra in each frame). Below each extinction curve, we 

show the resultant first and second derivatives. One fairly easy parameter 

to “key on” would be the spectral position of where the first and second 

derivatives cross zero (shaded narrow regions). Minor variance in defining 

these zeros arises when attenuation is extremely weak because of low aerosol concentrations. 

9 
BG raw transmission 

. 

._ _. : 7 _. :,;;- ,. . __ ___ _--- I_- _ . ,__: _ :; . . . . - ,~F~,.;;,..~.:X 

._. -. ,! ; ,: ,,., :,;;.<yg;;2:;-- y.. ..q I 

. --- -I..: Soot raw transmission 

Fog raw transmission I 

b 

i I 

derivative 

-..------i 

- .?A_ 

---c ..- 

E-- r. - ; 

Arizona road 

dust raw 

% First 

derivative . . . ..Y ,,’ 

Figure 8. Example of two simple “form-preserving” operations (i.e., first and second derivatives) that one 

might consider to discriminate subtle differences among similar spectra. Each set of three frames consists 

of original transmission spectra measured for various aerosol densities (top) and corresponding first 

(middle) and second (bottom) derivatives. 

10 
The question remains: “Can one remotely identify the presence of a harmful bioaerosol using conventional IR transmission measurements like those 

conducted here?” The answer to this question is complex and a function of 

many parameters, e.g., optical depth, sensor response, ambient atmospheric 

conditions, etc. As a result, we make no attempt to give a definitive answer. 

However, certain key aspects should be addressed when considering such 

a scenario. 

First, because aerosol extinction spectra have a tendency to be relatively 

smooth, low-resolution spectroscopy will usually suffice. Although the spectrometer used for this study was set at a resolving power of 4 cm-i, no spectral features were observed that could not be resolved at a substantially 

lower setting, for example, 20 cm-‘. Second, a reasonable amount of 

bioaerosol material must obviously be present (relative to ambient conditions) to discern noticeable changes in the transmission. 

To get an “order of magnitude” estimate of the bioaerosol concentrations 

necessary to be spectroscopically measurable, let us first assume a simple 

transmisometer-type arrangement in which a homogenous bioaerosol cloud 

obscures a sufficiently powerful broadband IR source. Assuming an endospore cloud 100 m in extent obscures a similarly dimensioned optical 

path for a collimated beam (diameter 4 cm), we estimate a minimum detectable particle density (to achieve a reasonable amount of attenuation) to 

be on the order of 10 to 20 particles/cm3. 

Up to this point, we have assumed that clear conditions and effects caused 

by the intervening atmosphere were negligible, i.e., spectral masking because of path radiance and molecular absorption by gaseous C02, H20, and 

0,. When these effects are considered and the distance from the spectrometer to the biocloud is in excess of 5 km or more, the minimum detection 

limits stated in the previous paragraph increase several fold. This seems to 

restrict the type of approach one might consider to the “plume identification” realm in which the measurement is conducted near to the source and 

that the bioaerosol cloud is sufficiently dense. Based on this and our experience in measuring aerosol spectra, we believe that the remote detection of a 

bioaerosol using IR extinction spectra will be limited to active illumination 

techniques in which the source of radiation is sufficiently strong enough to 

overcome the detrimental effects caused by atmospheric absorption and 

path/ground radiance. 

11 
Acknowledgments 

We would like to thank William Loerop and Dorothea Paterno (ERDEC, 

Aberdeen Proving Ground, MD) for providing their expertise and the 

bioaerosol materials necessary for this study. We would also like to thank 

Dennis Flanigan (also with ERDEC) for an enlightening conversation on 

this topic. 

12 
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13 
19. J. I? Carrico, “The DOD chemical-biological stand-off detection program: A 

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. 

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14 
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I I 

TITLE AND SUBTITLE Extinction, Absorption, Scattering, and Backscatter for 

Aerosolized Bacillus Subtilis Var. Niger Endospores From 3 to 13 pm 

5. FUNDING NUMBERS 

DA PR: B53A 

PE: 61102A 

8. AUTHOR(S) Kristan P. Gurton, David Ligon, and Ramaz Kvavilashvili 

. PERFORMING ORGANIZATION NAME(S) AND ADDRESS 

U.S. Army Research Laboratory 

Attn: AMSRL- CI-EP email: kgurton@arl.army.mil 

2800 Powder Mill Road 

Adelphi, MD 20783-l 197 

. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS 

U.S. Army Research Laboratory 

2800 Powder Mill Road 

Adelphi, MD 20783-1197 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

ARL-TR-2343 

10. SPONSORlNGAnONtTORlNG 

AGENCY REPORT NUMBER 

1. SUPPLEMENTARY NOTES 

ARL PR: OFEJGO 

AMS code: 61110253All 

2a. DlSTRlBUTlONlAVAtLABtLlTY STATEMENT Approved for public release; 

distribution unlimited. 

12b. DISTRIBUTION CODE 

3. ABSTRACT (Maximum 200 words) 

Spectral extinction was measured in situ for aerosolized Bacillus subtilis var. niger (BG) 

endospores with the use of Fourier-transform infrared (FTIR) spectroscopy from 3.0 to 13.0 pm. 

Corresponding aerosol-size distributions were measured with the use of a commercially available 

elastic light-scattering probe and verified by direct particle capture and subsequent counting via 

video microscopy. Aerosol mass density was monitored simultaneously with conventional 

dosimetry and used to mass-normalize the measured spectral extinction. Mie theory calculations 

based on measured distributions and available complex indices of refraction agreed well. Also 

present are resultant Mie calculations for the absorption, total scattering, and backscattering. 

Included are the real and imaginary components of the complex index of refraction for BG as 

measured by Milham and Quarry. Both calculated and measured cross sections suggest that for 

wavelengths longer than 6.0 pm, the total extinction is primarily due to absorption. Finally, to 

offer a comparison, we present measured spectral extinction for three additional aerosols often 

found in the lower atmosphere, i.e., water fog, diesel soot, and Arizona road dust. 

“.S~BJECTTERMS Bioaerosol, infrared extinction, aerosol 15. NUMBER OF PAGES 

23 

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OF REPORT OF THIS PAGE 

Unclassified Unclassified 

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Unclassified 

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