Measuring diesel particulate matter in underground mines using

submicron elemental carbon as a surrogate

J.D. Noll, S.E. Mischler, G.H. Schnakenberg, Jr. & A.D. Bugarski

U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control

and Prevention, National Institute for Occupational Safety and Health, Pittsburgh, PA

ABSTRACT: Elemental carbon (EC) is used as a surrogate for regulating the exposure to diesel particulate

matter (DPM) of underground metal/non-metal miners. EC was chosen as a surrogate because EC is selective to

DPM and is a major component of DPM. Using EC as a surrogate also gives one the advantages of no sampling

artifacts and being able to sample at all locations in the mine. Currently, EC represents DPM well in underground

mines. Some control technologies have been shown to possibly alter the relationship between DPM and EC and

characteristics of DPM. Therefore, future work will investigate the relationship between DPM and EC as new

control technologies are implemented.

1 INTRODUCTION

1.1 Background

Long-term exposure to diesel exhaust has become a

concern because diesel emissions are believed to be

a potential carcinogen (NIOSH, 1988). In addition,

acute overexposure to diesel exhaust has been linked

to deleterious health effects such as eye and nose irritation, headaches, nausea, and asthma (Kahn and Orris,

1988; Rundell et al., 1996; Wade, 1993). Measurements have shown that underground miners can be

exposed to over 100 times the typical environmental

concentrations of diesel exhaust and over 10 times the

concentrations measured in other work environments

where diesel engines are common (Cantrell and Watts,

1997; Nauss, 1998; Haney, 1992).

In the United States, the Mine Safety and Health

Administration (MSHA) has promulgated rules to

limit the exposure of metal/non-metal underground

miners to diesel particulate matter (DPM) to

500μg/m3 for an interim limit and 200μg/m3 for a

final limit (MSHA, 2001, 2005). One issue that had to

be overcome was how to measure for DPM.

1.2 Difficulties in measuring DPM in

underground mines

DPM is a complex mixture of particulate elemental

carbon (EC) or soot, particulate and particle bound

organic carbon (OC), sulfates, some metals, etc.

(Kittelson, 1998). Mass measurements of DPM are

prone to interferences from other sources of aerosols

(mineral dust, cigarette smoke, etc.) in the mining

environment and are not sensitive enough for the

concentrations near the proposed final limit (below

200μg/m3). Therefore, a surrogate was needed to

determine DPM exposure.

Initially, total carbon (TC) was considered by

MSHA to be the most adequate surrogate for

DPM because TC accounts for over 80% of the

DPM (MSHA, 2001; Pierson and Brachaczek, 1983;

Kittelson, 1998). However, the EC and OC particles

from mineral dust and OC aerosols from the other

sources commonly present in underground mines,

such as environmental tobacco smoke and oil mist,

were found to interfere with the TC analysis.

A size selective sampler (Cantrell and Rubow, 1991;

McCartney and Cantrell, 1992; Cash et al., 2003, Noll

et al., 2005) has been shown to effectively segregate

the coarse mineral dust from the generally submicron

DPM. Unfortunately, the size selective samplers are

not very efficient in removing cigarette smoke and oil

mist (OC aerosols that generally belong to the same

size category as diesel aerosols). Therefore, cigarette

smoke and oil mist cannot always be avoided when taking personal samples. For example, when TC vs. EC

was plotted from personal samples below 400μg/m3

from smokers and non-smokers from two stone mines

(samples corrected for adsorbed vapor phase OC),

there is greater variability in the TC and EC relationship as samples are potentially exposed to more

interferences (smokers). The R2 goes from about 0.7

to 0.9 when the data from smokers are taken out.

Figure 1 also shows that at the same EC values when

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0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350

non-smokers

smokers

TC (g/m3)

EC (g/m3)

Figure 1. TC vs. EC of personal samples from smokers and

non-smokers in two stone mines.

the majority of the samples were exposed to potentially more cigarette smoke that a larger TC value was

measured. This is probably due to the additional OC

from the cigarette smoke.

At this time, no method to correct for these interferences has been found. NIOSH did some studies on

using solanesol as a possible surrogate for cigarette

smoke but solanesol was found to be too unstable

(Tucker and Pretty, 2005).

As a result, MSHA turned to using EC as a surrogate for DPM for the interim limit since no other

sources of submicron EC are known to exist in the

metal/non-metal mining environment and EC is a

major component of DPM (MSHA, 2005). This paper

gives an overview of the sampling and analytical

methods presently used to measure EC and some issues

when using EC as a surrogate.

2 SAMPLING AND ANALYTICAL METHODS

2.1 Sampling methods

2.1.1 Personal sampling

For taking personal samples, a personal sampling

pump, a 10-mm Dorr-Oliver cyclone, a submicron

impactor, and two quartz filters placed in tandem (in

series) are used. The pump is operated at 1.7 lpm.

The submicron impactor segregates the mineral dust

from the DPM. This is necessary when using EC as a

surrogate because it avoids overloading a filter with

mineral dust, some mineral dusts contain EC, and

it provides a cleaner sample. When impactors were

first being used, the mining industry brought up concerns with cassette performance and possible defects

(MSHA, 2003). The submicron impactor has been

shown to be efficient in collecting DPM while allowing less than 10% of mineral and coal dust to penetrate

in both laboratory and field studies (Noll et al., 2005;

Birch and Noll, 2004; Noll and Birch, 2004). For example, at a dust concentration equivalent to 8-hr time

weighted average of 3 mg/m3, a sub micron impactor

was shown to collect only 2% of the respirable dust

(Noll et al., 2005).

2.1.2 High volume sampling

When testing the efficiencies of control technologies in underground mines, the concentration of DPM

can be low. To collect enough material to be at and

above the limit of quantification for NIOSH Analytical Method 5040, one might have to sample for many

hours. When doing research in the field, one does not

always have this luxury. A high-volume (HV) sampling train was developed by Bugarski et al. (2003,

2005) to collect sufficient material for the analysis of

TC and EC using the Method 5040 while still avoiding

mineral dust for control technology studies. The HV

flow rate was achieved by merging flows from five

classifiers, each consisting of a 10-mm Dorr-Oliver

cyclone followed by a U.S. Bureau of Mines (BOM)

single stage diesel impactor, into a single stream. Flow

rates between 1.7 and 2.0 lpm were maintained through

each cyclone and impactor pair. At this sampling flow

rate only particles with geometric mean smaller then

0.8μm were deposited on the filters. All five classifiers were attached to a symmetrical plenum that

distributed a total flow rate between 8.5–10 lpm uniformly among the five streams. Each of the classifier

assemblies was connected to the plenum chamber by a

3-foot long section of conductive tubing. The outlet of

the plenum was directly connected to a stainless steel

25 mm diameter filter holder containing two stacked

25 mm tissue quartz fiber filters. As seen in Table 1,

the high volume method gave similar results as the personal sampling methods when collecting samples in an

isolated zone of a metal mine (these were when no control technologies or just alternative fuels were used).

This high volume sampling method used mass flow

controllers or critical orifices to control the flow rate

and needed electricity to run. However, electricity may

not always be available at sampling locations in the

underground mine being tested. Therefore, we are

currently testing a more portable assembly using a

high flow personal pump with a cyclone with a 1μm

cutpoint at around 8 lpm (see Figure 2).

2.2 NIOSH analytical method 5040

After a sample is collected, it is sent to a laboratory and

analyzed for elemental and total carbon using NIOSH

Method 5040 (Birch, 2004). This analytical method

analyzes for OC and EC in two different stages. In the

first stage, the OC is measured by ramping the oven

temperature over four progressively higher temperature steps programmed into the instrument, with the

last step being at about 870 ◦C in a pure helium (He)

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Table 1. High volume (9.7 lpm) sampling vs. standard sampling at 1.7 lpm.

EC concentration between high volume sampling and

personal sampling method

HV Standard method

(μg/m3) (μg/m3) % difference

181 172 5

205 167 20

287 257 11

319 289 10

401 353 13

414 376 10

311 296 5

213 198 7

HV – high volume sampling

Standard method – personal sampling method

% difference = (HV − standard method)/average(HV −

standard method) × 100

Figure 2. A high volume setup consisting of a cyclone with

a 1μm cutpoint at 8 lpm, filter to collect particulate and a

high flow rate (5–15 lpm) personal pump.

atmosphere. The EC does not evolve because there is

no oxygen (O2) available for it to react. The evolved

OC is oxidized to carbon dioxide (CO2), reduced to

methane (CH4), and finally measured using a flame

ionization detector (FID). In the second stage, the EC

is measured by reducing the oven temperature to about

600 ◦C and then again raising the temperature to about

900 ◦C in a He/O2 atmosphere (O2 is now present to

react with the EC to form CO2). The EC is then measured in the same way as the OC. TC is simply the

sum of OC and EC. The NIOSH Method 5040 also

optically corrects for pyrolysis (charring) of OC.

NIOSH 5040 has been shown to meet the NIOSH

accuracy criteria and has given good results for interlaboratory testing (Birch, 2004).

3 EC AS A SURROGATE

3.1 Selectivity of EC

As mentioned earlier, submicron EC (after using an

impactor) is selective to DPM in the underground

metal/non-metal mining environment. This gives us

the advantage of sampling for DPM and taking personal samples in all areas of the mine. If one tried to use

TC as a surrogate, one might have to develop a complicated sampling strategy to avoid the interferences.

A complicated sampling strategy could preclude areas

with the highest concentration of DPM.

In addition to avoiding interferences, known OC

sampling artifacts when using quartz filters (Eatough

et al., 1995; Kirchstetter et al., 2001; Turpin et al.,

1994) do not affect EC based methods. Quartz filters are known to adsorb vapor-phase OC (not part

of DPM). We routinely observe between 30–40μg/m3

vapor-phase OC adsorbed by the quartz filter in mining samples. This could cause a 19–25% error in a TC

result at the proposed final limit of 160μg/m3 TC if

not corrected. There is also a possibility of some OC

semi-volatiles evaporating and causing a negative bias

relative to the TC results. These artifacts do not affect

the EC results.

Coal dust also has less of an effect on EC results

than TC or DPM mass results since a smaller portion of coal dust is EC (Birch and Noll, 2004; Noll

and Birch, 2004). In both field and laboratory studies,

when using a sub micron impactor, coal dust contributed relatively minimal (<25%) to the EC results

when measuring 160μg/m3 TC concentrations, but in

a few instances, even when using the impactor, the

coal dust contributed significantly (>25%) to the TC

values when measuring DPM at 160μg/m3 TC (Birch

and Noll, 2004; Noll and Birch, 2004).

3.1.1 How is DPM represented by EC

The strong points of using EC as a surrogate are that it

is a major part of DPM, is selective to DPM, and can be

sampled and measured accurately. One potential issue

when using EC as a surrogate for DPM is that the EC

fraction of DPM may change depending upon various

factors such as engine duty cycle, fuel type, control

technology, etc.

Some preliminary data demonstrates that, at this

time, EC represents DPM in underground mines well

but may change as new control technologies are implemented. Bugarski et al. 2003, 2005 collected DPM

mass (measured using TEOM), TC, and EC samples

in an isolated zone in a metal mine. The isolated zone

was a section in the mine where only clean air entered

and ventilation could be controlled. This allowed measuring the diesel emissions from a vehicle without

interferences. As seen in Figures 3a and 3b, a linear relationship existed between DPM and EC and

107
0

0 200 400 600 800 1000

0

100

200

300

400

500

600

0 100 200 300 400 500

1400

DPM mass (g/m3) 1200

1000

800

600

400

200

EC (g/m3)

1200

TC (g/m3)

EC (g/m3)

(a)

(b)

y = 1.04 x + 90

R2

 = 0.91

y = 1.10x + 52.15

R2 = 0.98

Figure 3. (a) DPM (mass) vs. EC for vehicles in the isolated

zone. (b) TC vs. EC for vehicles in the isolated zone study.

between TC and EC for the types of vehicles tested

in this mine (Noll et al., 2005).

This data was obtained when the vehicles ran

No. 2 diesel fuel, diesel oxidation catalytic converters (DOCs), biodiesel, or water emulsified fuels.

These conditions are present in operating underground

mines.

This relationship was confirmed when sampling in

four mines during actual production (see Figure 4)

(Noll et al., 2005). The sampling was done at the

main exhausts of the mine where oil mist and cigarette

smoke should be so diluted compared to diesel that

they have minimal effect on the TC results. SKC DPM

cassettes were used to prevent the collection of mineral dust. The tandem filter was used to correct for the

adsorption of vapor phase OC (Eatough et al., 1995;

Kirchstetter et al., 2001; Turpin et al., 1994).

DPM mass was not measured during these field

studies but since DPM consists of over 80% TC, most

DPM and EC samples were shown to have a linear

relationship.

It is important to note that the relationship between

TC and EC looks very well with the above data

when interferences were minimized. However, as

shown in Figure 1, when taking personal samples,

cigarette smoke can provide a substantial interference. Even when taking precautions, the variability

0

200

400

600

800

1000

1200

1400

1600

1800

0 200 600 800 1000 1200 1400 1600

TC (g/m3)

400

EC (g/m3)

y = 1.1212x + 41.58

R2

 = 0.99

Figure 4. TC vs. EC for data from four different mines during actual production (trying to avoid interferences).

between TC and EC increases at the lower concentrations (<230μg/m3) of TC which could be the result

of interferences or at least partially.

Some preliminary data has shown that as certain

control technologies are implemented in underground

mines, the DPM and EC relationship and even the characteristic of the DPM may change (Bugarski et al.,

2003). In the Bugarski et al. 2003, 2005 studies, diesel

particulate filter (DPF) systems were shown to preferentially reduce EC over OC and non carbon DPM

(e.g. a platinum (Pt)-catalyzed DPF reduced EC by

95% and OC by 80%) which could cause the relationship between DPM and EC to change when these DPF

systems are implemented. The characteristics of DPM

were also shown to change (e.g. the mass of particulate

decreased but the number of nanoparticles increased

when some DPF systems were used). Even though the

DPM and EC relationship changed, EC still could be

measured accurately and followed the trend of DPM

(as DPM decreased so did the EC). The DPF system

that preferentially reduced EC over OC reduced both

substantially; therefore the concentration of DPM or

the contributions of the DPM from vehicles with these

control technologies may be low enough that the potential change in DPM and EC relationship may not be a

concern.

We do not know at this time the magnitude that the

different control technologies effects will contribute to

the future DPM and EC relationship in underground

mines. We do not know what type of control technologies will be eventually implemented in the mines,

how the characteristic of DPM could change, and at

what concentration. Future work will be needed as new

control technologies are introduced into the mines to

determine the characterization of DPM.

4 CONCLUSION

EC is selective to DPM in underground mines and

can be sampled and measured accurately. Using EC

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as a surrogate allows one to measure in all areas of

the mine. It also gives the benefit of being free of OC

sampling artifacts and makes it easier to sample in coal

mines. Under present conditions, EC represents DPM

well and is a major component. As some control technologies are implemented, such as diesel particulate

filter systems, the relationship between DPM and EC

may change, may vary between mines, and the characteristics of DPM itself may change. EC can still be

measured accurately and it will still follow the trend of

DPM. Future work will be needed as new control technologies are introduced into the mines to determine the

characterization of DPM. At this time the benefits of

using EC as a surrogate outweigh the problems.

Disclaimer: Mention of a company name or product does not constitute endorsement by the Centers

for Disease Control and Prevention. The findings and

conclusions in this report are those of the authors and

do not necessarily represent the views of the National

Institute for Occupational Safety and Health.

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