Comparing Measurements of Carbon in Diesel Exhaust 


Aerosols Using the Aethalometer, NIOSH Method 5040, 


and SMPS 


lam Pou Ng, Hongbin Ma and David Kittelson 

University of Minnesota 

Art Miller 

NIOSH/CDC · Spokane Research Laboratory 

ABSTRACT 

Combustion aerosols consist mainly of elemental and 

organic carbon (EC and QC). Since EC strong ly ab~o~s 

light and thus affects atmospheric visibility and radiation 

balance, there is great interest in its measurement. To 

this end, the National Institute for Occupational Safety 

and Health (NIOSH) published a standard method to 

determine the mass of EC and OC on filter samples. 

Another common method of measuring carbon in 

aerosols is the aethaiometer, which uses light extinction 

to measure "black carbon" or Be, which is considered to 

approximate EC. A third method sometimes used for 

estimating carbon in submicron combustion aerosols, is 

to measure particle size distributions using a scanning 

mobility particle sizer (SMPS) and calculate mass using 

the assumptions that the particles are spherical, 

carbonaceous and of known density. With an eye toward 

evaluating the use of these methods for measuring 

carbon in freshly emitted diesel aerosols, the objective of 

this paper is to compare the results among these 

methods and to investigate the role of OC when 

measuring the mass concentration of elemental carbon 

(or black carbon) in aerosols emitted by a medium duty 

Diesel engine. Results indicate that the aethalometer 

response correlates well with the dat~ from ~Iter 

samples (R2 = 0.99), with a very shght POSitive 

interference at high levels of OC. 

INTRODUCTION 

Diesel engines are widely used in heavy~duty 

applications because of their excellent fuel economy and 

durability, but some concerns have been raised about 

their impact on the environment. [1), [2), [3] One area of 

concern focuses on the optical properties of the Diesel 

exhaust particles and their corresponding influence on 

visibility and solar energy absorption. [4), [5], [6] Such 

particles consist mainly of elemental carbon (EC) an~ 

organic carbon (~C). The EC plays a key role In air 

pollution due to its strong light absorption properties, [1), 

(7) so its measurement is an important and necessary 

aspect of environmental research. 

In order to quantify the carbon in aerosols, the National 

Institute for Occupational Safety and Health (NIOSH) 

recently published the Standard Method 5040, which is a 

method to determine the masses of EC and OC from a 

filter sample (NIOSH 1998). The method is based on a 

thermal-optical technique developed by Sunset 

l aboratories Inc and is aerosol type and size 

independent. (8) Although the method is widely 

accepted, the sample collection and analysis processes 

are time consuming. 

An alternative method for such carbon measurements is 

an instrument called the aethalometer, which has 

become widely used to measure the mass concentration 

of "black carbon" in aerosols. It is related to the ability of 

particles deposited on a filter to attenuate a light beam in 

the aethalometer. [9], [10] Although the instrument 

requires aerosol dependent calibration, it is convenient 

to use and can provide a near real·time measurement. 

The aethalometer calibration process depends upon 

knowledge of the ·specific optical attenuation cross· 

section", commonly referred to as the sigma (u) value. 

111], [12). [13], [14] Since the sigma value is a function of 

aerosol type, using a constant sigma value to calculate 

the BC concentration could cause over Of 

underestimation of the actual carbon concentration in 

the ambient air. Aerosols that were well aged in the 

atmosphere as well as freshly emitted from Diesel 

engines were studied previously. [15], [16] 

The primary thrust of this worn is to investigate the 

response of the aethalometer to freshly emitted Diesel 

exhaust aerosols and to quantify how this response 

varies as particles are treated to remove their organic 

carbon content. This is done by comparing the 

measured values of BC with simultaneous 
measurements of EC and OC derived from filter 

samples. The BC and EC measurements were also 

compared with particle volume data derived from particle 

size stributio~~ simult~neous y measured using a 

scannln.g mobility particle sizer (SMPS). This 

companson was used to elucidate the relationship 

between concentrations of BC, EC, OC and the two 

submicron modes of Diesel exhaust particles, i.e. the 

nuclei and accumulation modes. (2] 

EXPERIMENTAL SETUP AND EQUIPMENT 

EXPERIMENTAL SETUP 

The source of Diesel particulate for this work was a John 

Deere 4045 engine, which is a turbocharged, four 

cylinder, direct injection engine, typical of those used to 

power off-road vehicles. load to the engine was 

p~ovided by a 150kW DC dynamometer, employing a 

Dlgalog controller. The fuel used for all tests was 

standard EPA #2 Diesel fuel with nominal sulfur level of 

350 ppm. The engine speed was 1400 RPM for all 

experiments and four engine load conditions were used. 

The "lightest" load was 40 Nm (10% of full load) and the 

"heaviest" load was 360 Nm (90% of full load). 

Intermediate settings of 50% and 75% were also used. 

The reason for choosing a range of load conditions is 

that many of the particles generated at lighter loads are 

relatively volatile, while those generated at heavy loads 

are mainly non-volatile. The response of the 

measurement methods to volatile and non-volatile 

carbonaceous aerosols was one of the key issues under 

investigation in this work. 

The issue of volatility was addressed using a catalytic 

stripper to remove volatile material from the aerosols. 

The stripper used has a honeycomb structure with a 

catalytic surface coating. [17] The stripper is maintained 

at 300 deg C and as aerosol passes through it, volatile 

species are evaporated and oxidized. Solid particles are 

ideally not affected, but some are lost by diffusion and 

thermophoresls. [17], (18] 

Figure 1 shows the setup for these experiments. 

,

_... 

....

'0, 

- - -­ ......... ' 


c-...._ ... 

.......~~ 

Figure l .Block diagram ofthe experimental selllp f or the 

methods- comparison tests. 

­

To 

better simulate the Diesel aerosol exiting a vehicle 

tailpipe, the raw exhaust was diluted in two stages and 

passed through a dilution tunnel. The dilution ratios 

were subsequently calculated for all tests by measuring 

NO in the raw and diluted exhaust. This was done for 

every test and all data presented in this report have 

been dilution-corrected to represent engine-out 

conditions. The EC/OC measurements were conducted 

at the first stage and the BC measurements were taken 

at the second stage. This was done to prevent saturation 

of the aethalometer, and the effect on aerosol content is 

considered negligible. [19] 

EC/OC MEASUREMENTS 

The mass concentration of the EC and ac were 

determined using the NIOSH Method 5040. Following 

the standard method, (20] samples were collected using 

37 mm plastic filter cassettes containing pre-fired quartz 

filters. The samples were drawn at 3 Ipm from the 

diluted exhaust by drawing aerosol through critical 

orifices using a vacuum pump. For each test condition, 

filter samples were collected in triplicate to ensure 

repeatability of results, and sampling times ranged 

between 150 minutes at 10% load and 20 minutes at 

90% load to ensure adequate filter loading. The filter 

loadings ran ~ed from (3.3 to SO) I-lgcm· 

2 for EC and (0.5 

to 2S) I-lgcm for ac. The EC/OC data presented in this 

report are based on the average of the three samples 

taken at each condition. The samples were sent to a 

NlaSH·contracted lab (DataChem Labs in Salt lake 

City), along with media blanks and field blanks as 

required. The samples were analyzed using the NlaSH 

standard method 5040. [8] ~nd time-we hted average 

values of EC and OC In umts of mgm were derived 

from the analytical results. 

The NIOSH method uses the thermal-optical technique 

to quantify the EC and ac, and is considered a 

reference method. During implementation of the 

method, carbon volatilized below 850 °C in a helium 

atmosphere is defined as ac, while carbon burned off at 

850 °C in an atmosphere of oxygen and helium is 

defined as EC. The optical component of the method 

entails evaluation of the char, i.e. residue left on the filter 

due to pyrolysis of some ac during the first healing 

cycle. This char is oxidized at the beginning of the 

second heating cycle and its mass apportioned as ac. 

An optical technique is used to determine when this char 

is gone from the filter, which is why the method is called 

"thermal-optical". 
AETHALOMETER MEASUREMENTS 

The single channel aethalometer used in this work 

(Magee Scientific model AE-16) employs an optical 

method to measure the black carbon (Be). It measures 

Be using optical absorption of an B80nrn wavelength 

light beam, by aerosol particles that have been 

deposited onto a filter. The instrument is mainly 

sensitive to solid (non-volatile, or "black") carbon, but 

other materials present in the sample may also prevent 

light from reaching the aethalometer sensor. Ught 

attenuation could depend on interactions between 

particle morphology, nature of the deposit on the filter, 

and scattering/absorption of light by non-EC materials, 

any of which could influence the aelhalometer response. 

The attenuation is a function of the amount of deposited 

BC. [9}, [10], (13] Equation (1) shows the definition of the 

attenuation, where 10 and I are the transmitted light 

intensities through the blank and loaded filter, 

respectively. [13] 

10 

ATN=100 x LN {- ) I (1)

Equation (2) relates the attenuation to (BC), i.e. the 

mass concentration per unit area of the filter, which has 

units of J.lgcm·

2

. [21] In the equation, • 0" ~ is the specific 

attenuation cross-section, or so-called sigma value. The 

sigma value is not a 'physical constant' but a value of 

wavelength dependent. The sigma value for black 

carbon (BC) used by the AE-16 operating in near 

infrared at a wavelength of 880 nm is 16.6 m2 g-1

. 

ATN (BC)= - -

" 

(2) 

Using this value, the actual BC concentration of the 

sampled aerosol can be determined in units of J.lgm-3 by 

using equation (3), where the intet flow (F) and sampling 

time (T) are recorded by the aethalometer and the 

sampling spot area (A) is known (Aethalometer manual). 

A BC=(BC] x-­

TxF 

(3) 

SMPS MEASUREMENTS 

Particle size and concentration were determined using 

an SMPS, which measures particle size based on 

electrical mobility. The instrument used for this work 

was built in-house and is based on the "long-column 

DMA" design. [22) Flow rates of aerosol and clean 

sheath air are 1.0 and 10.0 Ipm respectively. During 

measurement, particles are first charged to Boltzmann 

equilibrium using a P0210 source and then pass through 

the column of the differential mobility anatyzer (DMA). 

The differential voltage in the column determines the 

size of particles that exit through a narrow slit at the far 

end of the column. The voltage is scanned from zero to 

10kV and back to zero over a four-minute period. During 

this time a TSI model 3010 condensation particle 

counter (CPC) [23] counts the particles exiting the DMA 

and the counts are corrected for charge distribution and 

voltage ramping effects, yielding a number-weighted 

size distribution for the aerosol. 

EXPERIMENTAL PROCEDURE 

Experimental work was conducted between May 21 and 

May 25, 2004. As a baseline, EC/OC filter samples 

were collected from the exhaust stream without the 

stripper in line, while simultaneous measurements were 

made using the aethalometer and SMPS. Then, the 

stripper was placed before all instruments and the 

sampling repeated using the same engine speed and 

loads. 

The main purpose of this experiment was to compare 

measurements of carbon made with the aethalometer 

(BC concentration) the NIOSH Method 5040 (EC 

concentration), and the SMPS (particle volume 

estimations). Additionally, we wanted to investigate the 

change in the response of the three methods when the 

volatile component of the aerosol was removed. This 

was accomplished by using a catalytic stripper to 

remove the volatile fraction of the aerosol for some tests. 

Over a period of about 20 to 150 minutes while the filter 

samples were being collected, the SMPS recorded the 

size distribution approximately every four minutes, and 

the aethalometer measured BC mass concentration 

every two minutes. At the end of the sampling period, 

the BC and SMPS data were averaged to give one 

(time-weighted average) value of BC and particle size 

distribution for that lest. 

RESULTS AND DISCUSSION 

ESTIMATING EXHAUST VOLUME CONCENTRATION 

USING PARTICLE SIZE DISTRIBUTIONS (SMPS 

DATA) 

An SMPS was used to measure the particle number 

concentration. The size distributions of particles emitled 

by the John Deere engine at various loads are 

presented in Figure 2. Each plot includes a second 

curve which represents the particle size distribution after 

the exhaust has been put through the catalytic stripper 

(CS). Note that in each case the particle number Is 

reduced through the stripper. Since the accumulation 

mode range contains most of the particulate matter 

mass, (2] converting the number concentration to 

volume concentration using the SMPS data is an indirect 

way to estimate the particulate mass before and after 

the stripper. The data of Figure 2 were used to estimate 

particulate volume concentration. 
'_R~ ."_ """ _C 

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0.001 un 1.100 1.000 

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0.0'0 0,100 1.100 

Figure 2.SMPS data (a ll 4 loads bOlh wilh and withoul 

slripper). 

_.­

, 

This was done by using the averaged number weighted 

size distribution for each test condition and applying a 

curve fitting program to generate idealized lognormal 

volume concentration curves, using the assumption of 

spherical particles of unit density (an example is shown 

in Figure 3). 

1.lEoOI • 

l .• '" 

UloGO -

" '00 

~I'" 

Figure 3.Example o/SMPS data showing how a lognormal 

curve was generated in order to calculate volumes. 

However, this method has some 

uncertainty since no volume data beyond 300 nm is 

considered. 

COMPARING CARBON MASS CONCENTRATIONS 

MEASURED BY THE AETHAlOMETER AND BY THE 

NIOSH METHOD 5040 

Black carbon measurements were taken using the AE16 

aethalometer, and filter samples were collected for 

analysis by the NIOSH method 5040. The results for aU 

engine loads and for both raw and post·stripper exhaust 

are shown in Figure 4. Note that only one aethalometer 

measurement was made at both 50% and 90% engine 

load due to time constraints. Note also that the 

concentrations of both Be and EC are much higher at 

higher loads than at the lighter loads. This is expected 

since the higher engine load typically produces more 

solid carbonaceous material. The data in Figure 4 

shows remarkably good agreement between black 

carbon (Be) measured with the aethalometer and 

elemental carbon measured with the NIOSH method. 
DIo.., poorIkIo ......_ .....poMIon 01_,. .... NIOSH f(; 

IJoIuo 000.. _T, EPA 12Il001, \~) 

1"1----------------------------1 ,. ~--------------------1! 

l. ~----~~---l 

. " ~.'" 

Figure 4.Comparison ofAE16-B/ack Carbon and EC (5040) 

mass concentrations for all engine load, with and 

without use ofthe catalytic stripper (bar chart). 

When the two data sets are graphed against each other 

(Figure 5), it is seen that the slope of the EC vs. BC 

curve is about 6% higher with the stripper in place. 

_ .....u_"' .. A£.'.'_ ng IJOM DH,. U"T.lPA f2 "'.,. 1«o'....m. ...... ol .0'" 51%, 15OM., ..d 

.. "'" 
 .. - ........... 
 ,....".. ......, 
 ' . ' 

., ",-".,.,,, , "" ,0"" /" 

/" 

• w~h ....... 

~: ~.;~:-~" 

. .. H • .. .. .. " " ~"""""""""., 

Figure 5.Comparison ofAE16-Black Carbon and EC (5040) 

moss concentrations for all engine load, with alld 

without use ofthe catalytic stripper (linear plots). 

This 

suggests that the AE16 is slightly less responsive to BC 

aft~r OC has been partially removed with the catalytic 

st~lpper. It is.not~ble that this apparent effect is relatively 

minor, consldenng the large variation in OC for the 

various cases (see Figure 6). 

oc ..... co",porlolotl_....... . "., <.10",1< otrIppor 

I_Ooono 's.toT, lI'An fuel, 14011rprftl 

• 

• 

• 

, 

, 

• .. 

.... I 

~ rt 

.... l 

• 

LFigure 6.0C values before and after catalytic stripper at 

different loads. 

It is also notable that the 

overall agreement between BC and EC is better without 

the stripper. This suggests that the AE16 calibration 

may be based on soot that contains some OC. 

It is .commo~ for di~seJ particulate to have an organic 

fraction . ThiS volatile material is a result of exhaust 

temperature at light load being so low that volatile 

organic carbon derived mainly from lubricating oil, 

cannot be completely oxidized. It has been shown that 

under light and medium load conditions the nuclei mode 

consists mainly of volatile heavy hydrocarbons. (24] 

These .volatil~ materials undergo gas to particle 

conversion dUring exhaust cooling and dilution. As they 

deposit onto elemental carbon agglomerates, the 

volume and surface area of the agglomerates increase 

(~5] and it is hypothesized that this may lead to greater 

light absorbance and thus slightly higher BC readings. 

Although typically most of these materials are adsorbed 

and condensed onto existing accumulation mode 

particles, when the ratio of volatiles to solid particles is 

sufficiently high they nucleate to form the nuclei mode. 

The resulting volatile particulate matter may cause 

interference with the aethalometer response. One paper 

[11] has shown that particles could be trapped on the 

filter surface and the backscatter effects may lead to 

errors in BC determination. Also, the aethalometer over 

and under estimating the BC is also found in another 

study. [26) 

Figur.e 7 shows the OC fraction OC/(OC+EC) as a 

function of load with and without the stripper. The plots 

have the expected characteristics i.e. that the OC 

fraction for the case without the stripper is much higher 

than when the stripper in place and that the OC fraction 

decreases as the engine load increases. 
•• 

. 0 

•., 

•., 

•., 

o 10 ~ ~ ~ ~ ~ ~ ~ ~ I~ 

,." 

Figure 7. DC fractions determined USing NfOSH method 5040, 

along with dynamc blank correction. 

COMPARING CARBON MASS CONCENTRATIONS 

MEASURED BY THE THREE METHODS 

Another way to compare the three methods is to look at 

the measured reductions across the stripper, which 

should in theory be equal. 

Table 1 shows that the SMPS reductions across the 

catalytic stripper are typical except for the 90% engine 

load. [17], [18} The reductions are in part attributed to 

diffusion and thermophoretic losses inside the catalytiC 

stripper. 

Table I .The percent mass reduction across the CS for the SMPS. 

NIOSH 5040 and Aethalomeler Data. 

Engine Load 10% 50% 75% 90% 

SMPS 

3045 ± 

3 56 

31 69 ± 

1 95 

35 57 ± 

279 

42 93 ± 

5. 84 

EC 

3868 ± 

1.33 

31 73 ± 

3 00 

35 ± 

4.03 

34 04 t 

5.54 

Aethalometer 

26.63 ± 

144 

222 

26 07 ± 

4.98 

371 5 

The reduction at 90% is higher than expected, 

possibly due to the volume estimation uncertainties 

stated earlier. The reductions of EC at all loads have 

fairly good agreement although the reduction at 10% 

engine load may be artificially high. The reason for this 

high reduction could be the effect of the ac.It has been 

recognized that a fraction of OC is pyrolytically 

converted to EC during analysis. This conversion leads 

to difficulty in accurate speciation of OC and EC. [27] 

The reductions vary somewhat for the Be data. The Be 

reduction at 10% engine load was expected to be higher 

i.e. closer to the EC reductions, since the actual data 

suggest that the AE16 has higher Be reading with OC 

present. The reductions between EC and Be agree 

more as engine load increases. The data indicates that 

at low loads. the aethalometer measures less reduction 

across the stripper than the other two methods. This 

suggests that OC could have influence on both the EC 

and Be measurements at light engine load condition and 

the effect diminishes at higher loads. According to 

Figure 7, the highest OC fraction is at the 10% engine 

load. Although this DC fraction is based on the NIOSH 

method it stands to reason that the same amount of OC 

deposits on the tape of the aethalometer and could 

cause the aethalometer to give a higher response. 

Figure 8 shows the standard deviation for the three 

instruments at different engine loads. 

-- -

$1•••.-.1 I)n;" ... _ ...... -.,,--'_••' 01;",........__ 


• " 

Figure 8.Standard deviation among three measurements of 

EC reductions across the catalytic stripper. for the 

four different engine loads. 

,.0 .. --. 

'"0 

,or-­

1 

!" 

I' .,.. '" 0 

f-­

r-­ f-­ r ,13.1 

r-­ , ·1 3 of-­

0 • 

The standard 

deviation decreases as the OC fraction decreases. It 

appears that the OC effect is decreasing since less OC 

is generated at high engine load. 

CONCLUSION 

The overall correlation between the aethalometer and 

filter sample data is quite good. The measured Be 

concentration is slightly higher than the EC 

concentration with the stripper in place. This suggests a 

slight positive bias in the aethalometer response when 

volatile materials are present. On the other hand, the 

aethalometer results showed reasonable agreement with 

the EC results for the light load condition when no 

stripper was used I.e. when volatile material was in fact 

present. This suggests that the aethalomter calibration 

may account somewhat for OC. 

In using the aethalometer to measure black carbon in 

Diesel aerosols, the high OC content at lower loads may 

­ ­
thus result in OC contributing slightly to the BC 

measurements because of the OC effect. At higher 

engine loads it is not a problem. Since the aethalometer 

softwa re uses a constant extinction cross-section 

(sigma) value to calculate the BC mass concentration, 

the importance of field calibration to determine a unique 

sigma value for various aerosols should be stressed. 

ACKNOWLEDGMENTS 

We gratefully acknowledge NIOSH and the Spokane 

Research Lab for providing a student scholarship for 

part of this work, as well as lab equipment and sample 

analysis during collection of the EC/OC data. Support 

for this project was also received in part from the 

Coordinating Research Council E - 43 project titled 

"Diesel Aerosol Sampling Methodology". Additional 

support was received from the Department of Energy 

National/Renewable Research Laboratory under 

contract DE - FC04 - 01AL66910 titled "Gasoline 

Vehicle Exhaust Particle Sampling Study·. 

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