International Conference on Ozone Protection Technologies

12-13 November 1997

Baltimore, Maryland

Laboratory Evaluation of Bicarbonate Powders as Fire Suppressants

Mark D. Reeda, James W. Fleming. Bradley A. Williams, and Ronald S. Sheinsonb

NAVAL RESEARCH LABORATORY

Navy Technology Center for Safety and Survivability

Combustion Dynamics Section. Code 6185

Washington, DC, 20375-5342 L'SA

(202) 404-8101, Fax (202) 767-1716

E-mail: sheinson@ccfsun.nrl.navymil

and

Adam Chattaway, Neil Laverty. and David 1. Spring

KIDDE INTERNATIONAL

Mathisen Way, Colnbrook. Slough, SL3 OHB UK

The Combustion DynamIcs Section of the Navy Technology Center for Safety and

Survivability is actively involvcd in addressing US Navy needs for fire protection, concentrating on

the effectiveness of current and future fire extinguishing agents, with the responsibility to

recommend and help design Navy shipboard total flooding fire suppression systems. We are

currently examining several replacement agents and alternative suppression technologies including

the use of aerosols and powders.

This paper describes recent collaborative work with Kidde International on the flame

extinction properties of various bicarbonate powders. We have examined the concentration of

bicarbonate powders required to extinguish both laboratory counterflow propane-air diffusion flames

and small obscured n-heptane pan fires. Extinction information as a function of powder composition

and size has been obtained. Within experimental uncertainty, smaller particles were more effective

than larger ones. Potassium bicarbonate was more effective than sodium bicarbonate for each

particle size studied. Similar dependence on size and powder composition was observed in both the

counterflow diffusion flame and small chamber pan fire tests. Powder extinction data are given as

well as implications of the findings.

NRL effort supported in part by the U.S. Department of Defense SERDP Next Generation Fire

Suppression Technology Program and by the U. S. Naval Sea Systems Command 03R16

aFire Protection Engineering Department, WPI (Thesis Advisor - Prof. Robert Zalosh) WPIINRL

Intern 1996/1997.

bAuthor to whom correspondence should be addressed.

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_______ n _

INTRODUCTION

The phaseout of halon production, as required by the Montreal Protocol, continues to

challenge researchers to find an effective alternative to the superior fire fighting agents of the halon

family. Halon 1301 has been used quite effectively in many applications where a clean agent is not

absolutely required. With the phaseout of halons and the lack of a suitable backfit replacement it is

now more critical to design fire protection systems appropriate for the application.

There are certainly many applications where fire extinguishment by a gaseous suppression

agent is highly desired and there are halon replacement clean agents commercially available.

Gaseous hydrofluorocarbon (HFC) agents and inert gases are the front runners in the race for a halon

replacement, but none reach the level of overall effectiveness and safety as Halon 1301. Although

HFC's cause no significant damage to the ozone layer, they do produce hydrogen fluoride, an

extremely corrosive byproduct of the fire suppression process. There is still a need for an efficient

and economical replacementlalternative for many spaces currently protected by Halon 1301.

Flame extinguishment by dry chemicals, most notably alkali metal salts, is not a new

technology. Agents such as sodium bicarbonate have been used in hand extinguishers since the

1940's. Small particle size alkali metal salts are known to have excellent extinguishing capabilities

[1], more effective on a mass basis than even gaseous agents such as Halon 130I [2-4]. Highly

efficient dry chemical agents can provide excellent fire protection while consuming minimal space.

The most common alkali metal salt agents in use are sodium bicarbonate (NaHC03) and

potassium bicarbonate (KHC03). NaHC03 is widely used in fire extinguishers because of its low

cost. Both agents also have markets in such applications as paint spray booths, fuel filling stations,

restaurant cooking areas, and in explosion protection. In addition to high efficiency, the powders

have many other advantages including low toxicity and corrosivity. The impact on the environment

from NaHC03 or KHC03 is minimal. They have zero ozone depletion potential (ODP) and global

warming potential (GWP). The powder left by the dry chemicals following suppression remains a

drawback, limiting their application to areas that can tolerate the residue. Decreased visibility

during application is also a concern. Particle suspension is another limitation, especially for

applications requiring a total flooding agent. In these applications the particles must remain

suspended in the fire threat area sufficiently long to eliminate the chances of a reignition. The agent

must also flow around objects to extinguish obstructed fires in a similar manner to gaseous agents.

For conventional sized extinguishing powders (i.e. 20-100 ~m) the majority of the particles are too

massive and are not capable of suspension for extended periods of time. Pyrotechnic generation [I]

can yield micron sized aerosol. Also, very small, aerosol sized particles of non-pyrotechnic

potassium bicarbonate powder have been developed [3] to address the questions of flow and

suspension, and are currently being tested in Europe for use in scenario specific applications [5].

EXPERIMENT AL

The flame extinguishing performance of powdered agents was evaluated in a propane-air

counterflow diffusion flame at NRL and in an obscured n-heptane pan fire within a fire test chamber

at Kidde. Extinction testing was conducted with both sodium bicarbonate (NaHC03) and potassium

bicarbonate (KHC03). Preparation of the tested samples was performed by Kidde in separate runs

for both laboratories using the same method and base stock powders. Prior to particle size

fractionation the KHC03 sample was ground using a ball mill (Fritsch Pulverisette type 06.102) to

reduce the large chunks of powder into small enough sizes to permit sieving. Due to the lower mean

particle size of the NaHC03 (Dessikarb) powder as received, it was fractionated without any prior

grinding. Both powders were then mechanically sieved (Fritsch Analysette type 03.502) in a shaker

to size ranges of <38 ~m, 38-45 ~m, 45-53 j.1m,53-63 j.1m,and 63-75~. Silica (2% by mass) was

334
added to the powder samples as a drying/anti-caking additive. All powder samples flowed very

freely with no agglomeration noticed. A scanning electron microscope (Cambridge S200) was used

to examine the powdered test samples following sieving. Photomicrographs of the NaHC03 showed

a very even distribution of particle sizes throughout each size range. Conversely, photomicrographs

of the KHC03 showed that there was a significant percentage of very small particles (i.e. «38 ~m)

in each size bin.

Experiments were conducted in a counterflow diffusion burner [6]. The counterflow

diffusion flame is laminar and one-dimensional and allows for variation of the strain rate (see

below). Extinction testing enables a fundamental study of the extinction mechanism and the

influence of such factors as size, chemical composition, and strain rate. The burner consists of two

axisymmetric, 1 cm 10 burner tubes separated by I cm as shown in Figure 1. A flame is established

in the region between the two tubes where the opposed fuel and oxidizer flows meet and diffuse. Air

and propane were used for the oxidizer and fuel, respectively, with the air flow through the top and

the propane through the bottom tube. The flame can be described in terms of the strain rate. The

strain rate is defined as the maximum velocity gradient on the oxidizer side of the flame and has

units of s-l. Laser- Doppler velocimetry (LDY) is used to measure the velocity profile along the

tube centerline. In an uninhibited flame, extinction is achieved when the air and fuel flow rates are

increased such that the velocity gradient exceeds the critical value referred to as the extinction strain

rate. For an uninhibited propane-air flame this has a value of 560 s-I. For these flames, extinction

was typically only achieved along the centerline, leaving an annular flame.

Powder concentrations for flame extinction were measured at three different flame

conditions characterized by the respective flows of air and fuel. Strain rates determined for the

flames using an LDV system were 180, 310, and 480 s-1 for what are referred to as low, medium,

and high strain, respectively. For the powder extinction concentration tests, the flame was

established approximately midway betw~en the two tubes at one of the predetermined strain rates.

The powder was injected and mixed into the air stream from above the flame to best simulate an

actual fire suppression situation. The agent delivery system consisted of a vibrating, variable orifice

device. The orifice was changed for gross adjustment and the vibration frequency for fine

adjustment. The powder flow was quantified by light scattering using a modulated He-Ne laser

beam just below the air exit tube. The powder scattered the laser light. which was collected by

optics at 90° and processed in a lock-in analyzer synchronized to the frequency of the modulated

beam. The powder flow was correlated to the light scattering by collecting and weighing powder in

the absence of the flame. For flame extinction, the powder flow was introduced into the air side of

the flame and gradually increased until the flame extinguished. Extinction occurred only along the

centerline as in the uninhibited flame. The addition of powder to the flame dramatically increased

the excited C2 emission from the flame. A suitable optical filter was used to block this emission.

Data were collected by a computer using an analog to digital board (Scientific Solutions Lab

Tender). The board was 8 bit, bipolar, with an input range of -5 to +5 volts. The data collection rate

of the computer was set at 2 Hz. In addition to the He-Ne scattering signal collected by the system, a

second channel recorded the intensity of the central, luminous zone of the flame as imaged into a

photomultiplier tube. Extinction of the flame was marked by a dramatic decrease in the light

intensity signal. The scattering signal was evaluated at the time corresponding to the flame emission

decrease to determine the powder scattering signal at extinction. Calibration of the He-Ne scattering

was performed following extinction measurements at each strain and for each particle size bin.

The powders were also tested in a 287 liter fire test chamber [3] modified as shown in Figure

2. The fire is obscured from above and on the sides by metal plates as shown. This test design was

chosen to evaluate the total flooding ability of the powders. The suppressant tested was weighed to

335
0.01 g and loaded into the reservoIr. It was then pressurized to 140 psig with nitrogen and dispersed

through a simple nozzle (sho\\ n In insert) in the roof of the chamber. Water and n-heptane were

added to the fire pan and the battles put in place. The fire was then ignited and the chamber sealed ..

At 45 s after ignition the fan was switched off and the fan door closed; at t=55 s. the test chamber

side vents were closed; at t=hO s. the suppressant was discharged. If the fire remained lit at t=75 s,

the test was classified as a failed suppression.

For each test. fresh water and fuel were used. Between tests, the chamber was cleaned of

residual suppressant. The dispersion apparatus was cleaned with a compressed air line when

different particle size fractions were being evaluated to prevent cross-contamination. By changing

the amount of suppressant used in the subsequent tests according to the results of those previous, it

was possible to find an increasingly smaller range around the "critical" mass measured for extinction

of the flame.

RESULTS

Counterflow Diffusion Flame

Extinction mass concentration measurements for NaHC03 and KHC03 powders in the air

flow of a propane-air countertlow diffusion flame as a function of the measured strain for each size

bin are tabulated in Table I and plotted in Figure 3. In general, they show that the extinction mass

concentration varies in\'crsel~' \\ nh the strain rate of the flame and directly with the particle size of

the agent. Figure 4 is a plut of the extinction mass concentration ratio ofNaHC03 to KHC03 versus

strain for each particle bin size tested. The figure shows that KHC03 is approximately 2.5 times

more effective than NaHCO 3 on a mass basis in extinguishing the flame (3 times more effective on a

molar basis).

A number of factors have been postulated to contribute to the inhibition of fires by powders.

The powder adds a great deal of heat capacity to the flame, and solid particles are often effective

infrared radiators. kading to radiational cooling of the flame zone. In addition, some chemicals

(including the alkali metal bicarbonates) [7] will undergo endothermic decomposition at elevated

temperatures. releasing carbon dioxidc. The residual metal hydroxide may itself vaporize and

decompose at a much higher temperature. Furthermore, there exists the possibility of free radical

scavenging processes involving either the solid surface of the particle (heterogeneous) or gaseous

by-products of the solid's decomposition (homogeneous) [8-10].

The exact mode of the alkali metal bicarbonates' effectiveness is still not completely

understood. The difference in effectiveness between sodium and potassium could be due either to

differences in the decomposition temperature and enthalpy, or to differences in catalytic scavenging

cycles between the two elements.

One point worth noting is that for nearly all of the possible modes of operation, the particles'

effectiveness should scale with surface area. This includes heat transfer, radiation,

decomposition/vaporization. and surface catalysis. Therefore small particles are expected to be

more effective at fire suppression. in accord with our experimental observations. This dependence

does not. however. allow us to dctermine the primary means by which these particles extinguish

fires.

The size of the particles in the flame is an important characteristic in the effectiveness of an

agent. The sooner a particle begins the decomposition phase. the sooner it can act out its role in

suppressing the flame. whether it be physical or chemical. As shown in Figure I, in a stagnation

point flow field the streamlines turn radially upon impingement by the opposing flow. Particles will

either follow these streamlines if small enough, or will deviate from the streamlines and begin to fall

336
---------

downward for the case of partIcles large enough that the gravitational force overcomes the drag force

Imposed by the carrier gas. Thus. the residence time of each particle in the flame is a function of the

trajectory that it follows and the resulting velocity. Also, the rate of heat absorption by a solid

particle is proportional to its surface area and a smaller particle presents more surface area per unit

mass to the flame than a large one. Within the experimental uncertainty there was an increase in

extinction mass concentration (1.ti. decrease in effectiveness) with increasing particle size, which has

also been seen previously at :\RL [1.111. This is seen in Figure 3 for both agents.·

Plots of the extinction mass concentration as a function of average particle diameter and

surface area can be found in Figures 5a and 5b, respectively. The figure contains data for the

medium strain case for both po\ •...ders tested. The average particle diameter was calculated as the

arithmetic mean of each partIcle size bin. assuming an even distribution. This average particle

diameter was then used in the c:llculation of the average particle surface area. In Ref. [11], the

authors concluded that the tire extinguishing effectiveness of dry chemicals can be fully explained

on the basis of particle size :lnd distribution. They found that the effectiveness of the agent increased

gradually as the particle diameter decreased until reaching a critical diameter. At this diameter there

\Vas a dramatic increase in the thme suppression efficiency and the effectiveness remained constant

for all particles below the lImit SILC. The limit sizes were reported as 16 ~m and 22 ~m for NaHC03

and KHC03, respectively. We noticed a dramatic increase in the efficiency at about '45 ~m for the

~aHC03 sample at medium stram. A sizable increase was also noted for KHC03, but at a much

larger particle size of about bS I-lm. The higher value found for KHC03 may be influenced by the

large percentage of very small p:lrtlc les in all size bins of this agent. The data show that the required

extinction concentration incre:lscs with the diameter and surface area. A linear correlation between

extinction mass concentration and diameter was not found at the medium strain tested as seen in

Figure 5a. There does appear to be a linear correlation at the medium strain case for the extinction

mass concentration versus aver3ge particle surface area as seen in Figure 5b. For this strain, the

particle surface area available to interact with the flame more closely correlates with suppression

effectiveness than does the particle diameter.

For the high and 10\•.•.strain cases the correlation between surface area and effectiveness was

complicated by other factors. For the low strain flame, the largest particles were not suspended near

the stagnation plane by the opposmg fuel flow, but fell directly through the flame and into the

bottom tube. A dramatic drop off in effectiveness was observed for the two largest particle bins,

which may have resulted from the reduced residence time. For the high strain case, the amount of

powder required to extinguish the !lame was so small that the particle delivery was difficult to

accurately measure and control \v,th our seeding apparatus. leading to uncertain and poorly

reproducible values. It \\as thus difficult to quantify a relationship between particle size and

effectiveness for these two str.:uns. but smaller particles were once again more effective.

Effectiveness did correlate more closely with surface area than with diameter. Note that the

determination of surface area in this study was based on the assumption of a spherical particle of the

average diameter for the sieve fraction. A more accurate determination of the effect of surface area

on suppression will require direct measurement of surface area (i.e. by gas adsorption). These

observations are. however. consistent with "real world" experience that smaller particles are

preferable due to both increased surface area and more favorable suspension properties [3].

Pan Fire Chamber Test

Extinction mass concentration measurements for NaHC03 and KHC03 powders on an

obscured n-heptane pan fire in a 287 liter chamber are tabulated in Table 2 and plotted in Figure 6.

The results show KHCO) to be more efficient in suppressing these fire than NaHC03. Smaller

337
particles are also more effie ient in suppressing fires than are large particles. It is difficult to

determine the dependence of surface area on the effectiveness because of the many interrelated

variables affecting the chamber test results.

The extinction mass ratio (Na/K) for the two powders tested was determined to be 2.1, 2.8,

and 1.9 for the size ranges of < 38 11m.45-53 !-lmand 63-75 !-lm, respectively. The values are not

monotonic with particle diameter. This may be due to the poor sieving of the KHC03 into the

particular size ranges. This may also be due to the greater hygroscopicity of KHC03 compared with

NaHC03. There is a great number of small particles visible in the photomicrographs of the 45-53

11msieve fraction of KHCO 3. perhaps explaining the apparently much higher efficiency associated

with this sample (i.e. 2.8) compared with the other two size ranges (i.e. 2.1 and 1.9). The 63-75 J.1ITl

sieve fraction for KHC03 was seen to comprise some very large particles, with one axis much

greater than 75 11m. and some very small particles. Though these effects will, to some extent, cancel

one another, this cannot be a basis for scientific comparison.

SUMMARY

We have examined the tire suppression properties of bicarbonate powders in two different

environments. Similar dependence on size and powder composition was found in both studies.

KHC03 was shown to be more efTective on a mass basis than NaHC03 for all particle sizes tested.

It was also shown that the effectiveness of an agent varies inversely with the particle size for the

ranges tested; smaller particles are more effective in suppressing the flame than the larger ones.

However. the results are not readily transferable to full scale fire applications. Factors such as the

throw (i.e. fire penetration). suspension. and flow as a function of particle size are all important

considerations in determining the optimum particle size for each application. Additional testing is

recommended to help evaluate these characteristics. Better understanding of these characteristics

will help to determine the applicability of dry chemical agents as an alternative to Halon 1301.

The testing reported here is preliminary work of an ongoing collaborative effort between the

Naval Research Laboratory and Kidde International. Additional testing by both is planned to better

evaluate the effectiveness of powdered extinguishing agents. Experiments are underway at NRL to

determine the size distributIOn of the samples used. Kidde is also currently involved in the

assessment of particle sizes using a commercial particle analyzer. Extinction testing with better

characterized particle size distnbutions is planned. These results will be used to gain a better

understanding of flame extinguishment by dry chemicals and to help determine the characteristics of

the optimum agent.

REFERENCES

I. Sheinson, R.S., "Fire SuppressIOn by Fine Solid Aerosol," Proceedings of the International CFC

and Halon Alternatives Conference. Washington, O.c., 24-26 October, 1994, pp. 414-421.

2. Hamins. A., Gmurczyk. G.. Grosshandler, W., Rehwoldt, R.G., Vazquez, 1., Cleary, T., Presser,

c., and Seshadri. K.. "Flame Suppression Effectiveness," Evaluation of Alternative In-Flight Fire

Suppressants for Full-Scale Testrn~ in Simulated Aircraft Engine Nacelles and Dry Bays,

National Institute of Standards and Testing, No. NIST SP 861, April 1994, pp. 345-465.

3. Chattaway, A., Dunster. R.G .. Gall. R., and Spring, OJ., "The Evaluation of Non-Pyrotechnically

Generated Aerosols as Fire Suppressants," Proceedings of the Halon Alternatives Technical

Working Conference. Albuquerque, NM, 9-11 May 1995, pp. 473-483.

4. Milne, T.A., Green, c.L., and Benson, O.K., "The use of the Counterflow Diffusion Flame in

Studies of Inhibition Effectiveness of Gaseous and Powdered Agents," Combustion and Flame,

15, 1970, pp. 255-263.

338
5. Chattaway, A, Gall. R., and Spring, 0.1., "Dry Chemical Extinguishing Systems," Proceedings of

{he Halon Options Techmcal Working Conference. Albuquerque, NM, 6-8 May, 1997, p.216.

6. Papas, P., Fleming, J. W .. J.nd Sheinson, R. S., "Extinction of Non premixed Methane- and

Propane-Air Counterflow Flames Inhibited with CF4, CF3H, and CF3Br", Twenty-Sixth

Symposium (InternallOnalJ on c'umbustion, The Combustion Institute, Pittsburgh, 1996, p. 1405.

7. Weast, R.C., Handbook olChem/sIry and Physics, The Chemical Rubber Company, Cleveland,

Ohio, 46th ed., 1965.

8. Rosser, W.A., Inami, S.H. and Wise, H., "The Effect of Metal Salts in Premixed Hydrocarbon-Air

Flames:' Combustion and Flame. 7, June 1963, pp. 107-119.

9. Iya, K.S., Wollowitz, S., J.nd Kaskan, W.E., "The Mechanism of Flame Inhibition by Sodium

Salts," Fifteenth SymposiUm (International) on Combustion, The Combustion Institute,

Pittsburgh, PN, (1975), pp. 3::!9-336.

IO.Birchall, J.D., "On the Mechanism of Flame Inhibition by Alkali Metal Salts," Combustion and

Flame, 14, 1970, pp. 85-96.

11.Ewing, c.T., Faith, F.R., Ifughes, J.T., and Carhart, H.W., "Flame Extinguishment Properties of

Dry Chemicals: Extinction Concentrations for Small Diffusion Pan Fires," Fire Technology, May

1989, pp. 134-149.

12.Williams, B.A., Fleming, J.W .. and Sheinson. R.S., "Extinction Studies of Hydrofluorocarbons in

Methane/Air and Propane/Air Counterflow Diffusion Flames: The Role of the CF3 Radical,"

Proceedings ofIhe Halon OptIOns Technical Working Conference, Albuquerque, NM, 6-8 May,

1997, pp.31-42.

Measured Extinction Mass Concentration for Specified Size Bin (glm3)

Strain<38 (s-I ) 38-45 45-3 6375 5-63 j..lmj..lm j..lm

150 27 52730 70

155356808 310

774I ~

12I 7010837

122051183310623I I

110

310 56

15

Table 1: Propane-air countertlow diffusion flame extinction mass concentration for the indicated

agent. Halon 130 I data derived from Ref. [12].

Extinction Mass Concentration for Specified Size Bin (g/mJ) 

63-75 45-53<8j..lmj.lmj.lm

13082 52

6929 25

Table 2: Obscured pan tire mass extinction concentration for the indicated agent.

339
o..am8dles

Figure 1: Schematic of the counterflow diffusion flame apparatus from Ref. [6].

Figure 2: Schematic of 287 Iiter obscured fire test chamber.

340
800 ------------ 400

~ 700 -

E--

~

~ 600 - ::

.~....

~

..= 500 - ::

QJ

<;J :: '

0400 J... U

'"

~

::;; 300 J...

:: I

.~ ..

t: 200 ~ ::

....

;<

~ 100 -

o

150

---&-t,3-75 microns I

-€--~3-63 microns i

~4~-53 microns I

-e- 'S-45 microns I

-&- <: is mIcrons

.-+- Halon 1301

250 350 450

Strain Rate (S·I)

(a) NaHC03

550

350 .-..

ME

~300

::

.~..-

E 250 ..-

::

QJ

<;J

§2oo

U

'"'"

= 150

::;;

::

o

~loo

.5..-

;<

~ 50

o

150

~63-75 microns

--e- 53-63 microns

"""*- 45-53 microns

~ 38-45 microns

---tr- <38 microns

-'-Halon 1301

250 350 450

Strain Rate (S·I)

(b) KHC03

550

Figure 3: Extinction mass concentration as a function of strain rate for each particle size

range of (a) NaHCO, and (b) KHC03 powders in a propane-air counterflow diffusion

flame. Halon data from Ref. [12].

7

c .S- 6 T (': -

QJ '"'C 5 ~ <;J Uc

0 .S4 ~ "J

- "J

(':

(': ~3

~ - c

.S

2 - - <;J - .5 1 -- >O!

~ 0 150

200 250

-+- 63-75 microns

-€--53-63 microns

~ 45-53 microns

-e- 38-45 microns

~<38 microns

300 350

Strain Rate (S·l)

400 450 500

Figure 4: Extinction mass concentration ratio (NalK) as a function of strain rate for

each particle size range in a propane-air counterflow diffusion flame.

341
o ------------.-----

90

.-. .•.. )"T""It.1

'"'5 80 - -...

~

-; 70 - o~

E- 60 - c

~ 50 - C

Q

U 40 _ '"'"

(:IS

~ 30 -

c

A NaHC03

o KHC03

o ~----

90 -----

-

~ 10

'"'s 80- -...

~

-; 70 -:- o~

E 60 - -

c

~ 50 - C

Q

U'" 40 '"

~

~ 30 - C

Q

0': 20 _ C"j c

+

R2 = 0.74

.u. ••••• -{C03

, 0 KHC03

20 -

c

.~-

C"j

c

~ 10 ~

o 20 40 60 80

(a) Average Particle Diameter

(microns)

o 4000 8000 12000 16000

(b) Average Particle Surface Area

(microns2)

Figure 5: Extinction mass concentration as a function of (a) average particle diameter of the

indicated size range and (b) average particle surface area, for a medium strain (310 S·I)

propane-air counterflow diffusion flame.

140

'"'- _A NaHC03 t

- 120 - _oKHC03

c

o~ 100 . (:IS I.

:;---~-

~ 80-

c

Q

U

~ 60- (:IS

~

oS

- 40 ~ C"j

c

0': 20- Io'C

~

o -----------

o 20 40 60 80

(a) Average Particle Diameter

(microns)

140 -_._-- - '"' A NaHC03

.§ 120 o KHC03 C)!) -co~ 100 - -

~I.

-

~C 80 --'- C"j UCQ'"

60 -- '" (:IS ~ ~ c

40 -

o~

cC"j 20 - 0- -Io'C

~

0-·---·- ------------ o 4000 8000 12000 16000

(b) Average Particle Surface Area

(microns . 2)

Figure 6: Extinction mass concentration as a function of (a) average particle diameter for the

indicated size range and (b) average particle surface area, for an obscured n-heptane pan fire.

342