Environ. Sci. Technol. 2004, 38, 4414-4422
Equivalence of Elemental Carbon by
Thermal/Optical Reflectance and
Transmittance with Different
Temperature Protocols
JUDITH C. CHOW,* JOHN G. WATSON,
L.-W. ANTONY CHEN,
W. PATRICK ARNOTT, AND
H A N S M O O S M Ü L L E R
Desert Research Institute, 2215 Raggio Parkway,
Reno, Nevada 89512
KOCHY FUNG
Atmoslytic Inc., 24801 Alexandra Court,
Calabasas, California 91302
Charring of organic carbon (OC) during thermal/optical
analysis is monitored by the change in a laser signal
either reflected from or transmitted through a filter punch.
Elemental carbon (EC) in suspended particulate matter
collected on quartz-fiber filters is defined as the carbon
that evolves after the detected optical signal attains the
value it had prior to commencement of heating, with the rest
of the carbon classified as organic carbon (OC). Heretofore,
operational definitions of EC were believed to be caused
by different temperature protocols rather than by the method
of monitoring charring. This work demonstrates that thermal/
optical reflectance (TOR) corrections yield equivalent OC/
EC splits for widely divergent temperature protocols. EC
results determined by simultaneous thermal/optical
transmittance (TOT) corrections are 30% lower than TOR
for the same temperature protocol and 70-80% lower than
TOR for a protocol with higher heating temperatures and
shorter residence times. This is true for 58 urban samples
from Fresno, CA, as well as for 30 samples from the
nonurban IMPROVE network that are individually dominated
by wildfire, vehicle exhaust, secondary organic aerosol,
and calcium carbonate contributions. Visual examination of
filter darkening at different temperature stages shows
that substantial charring takes place within the filter, possibly
due to adsorbed organic gases or diffusion of vaporized
particles. The filter transmittance is more influenced by the
within-filter char, whereas the filter reflectance is
dominated by charring of the near-surface deposit that
appears to evolve first when oxygen is added to helium in
the analysis atmosphere for these samples. The amounts
of charred OC (POC) and EC are also estimated from
incremental absorbance. Small amounts of POC are found
to dominate the incremental absorbance. EC estimated
from absorbance are found to agree better with EC from
the reflectance charring correction than with EC from the
transmittance charring correction.
* Corresponding author phone: (775)674-7050; fax: (775)674-7009;
e-mail: [email protected].
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Introduction
Organic carbon (OC) and elemental carbon (EC) (in some
cases also called black carbon; BC) are measured on source
and ambient quartz-fiber filter samples by several thermal
carbon analysis methods that apply: (i) solvent extraction
followed by combustion (1), (ii) continuous temperature
ramping (2, 3), (iii) two temperatures in oxidizing and/or
non-oxidizing atmospheres (4-16), (iv) thermal/optical
reflectance (TOR) (17-20), and (v) thermal/optical transmittance (TOT) (21-29). Thermal methods heat a portion of the
sample in a continuous or stepwise manner under different
atmospheres, with subsequent detection of the volatilized
or oxidized carbon that leaves the sample. With consistent
standardization, these methods provide equivalent measures
of total carbon (TC, the sum of OC and EC). They define the
split between OC and EC based on combinations of combustion temperatures, residence time at each temperature,
the composition of the atmosphere surrounding the sample,
and light reflected from or transmitted through the filter.
Atmospheric EC, bearing little resemblance to mineral
forms such as graphite or diamond, is the dark-colored, lowvolatile carbon fraction that does not appreciably evolve
without oxidants at temperatures below ∼700 °C. However,
pyrolyzing and charring of OC on and within the filter as the
sample is heated may add to the original EC that was collected
from the atmosphere, artificially inflating its concentration.
Red light from a laser monitors the darkening of the particle
deposit on the filter due to OC charring. When oxygen (O2)
is added to the analysis atmosphere (i.e., carrier gas) at a
sufficiently high temperature (e.g., >350 °C) (11), this black
char combusts along with the original EC on the filter and
the filter becomes whiter. This darkening and lightening is
measured by light reflected from the filter surface and
transmitted through the entire filter. When the reflected or
transmitted light attains its original intensity, the pyrolyzed
and charred OC (POC) is considered to have been removed,
and all of the remaining carbon is associated with the EC
that was originally on the filter. Therefore, a partitioning can
be made by assigning carbon evolved before this split point
to OC and after this point to EC. Johnson et al. (17) and Yang
and Yu (28) pointed out that this partitioning assumes that
(i) charred OC evolves before original EC in the thermal
analysis and (ii) charred OC and original EC equally attenuate
reflectance (R) and transmittance (T).
TOR and TOT methods have been compared with each
other and with other OC and EC methods in several published
inter-method and inter-laboratory studies (25, 27, 30-35)
with inconclusive results. Methodological differences between OC and EC vary depending on the nature of the samples
analyzed, the analysis protocols, and the instrumentation
applied. Analysis protocols and instrumentation differ with
respect to (i) temperature plateaus, (ii) residence time at
each plateau, (iii) charring corrections by light reflectance or
transmittance, (iv) optical monitoring configuration and
wavelength, (v) combustion atmospheres, (vi) temperature
ramping rates, (vii) carrier gas flow through or across the
sample, (viii) oven flush, (ix) sample aliquot and size, (x)
location of the temperature monitor relative to the sample,
(xi) oxidation and reduction catalysts, (xii) evolved carbon
detection method, and (xiii) calibration standards. Different
samples may react differently with various analysis protocols,
thereby giving different results. Potential sample biases
include (i) nonuniform particle deposits on the filter; (ii)
particle deposits that are too light or too dark, making TOR
and TOT charring corrections uncertain; (iii) organic vapor
filter adsorption and its charring during heating; (iv) catalytic
10.1021/es034936u CCC: $27.50
© 2004 American Chemical Society
Published on Web 07/20/2004
FIGURE 1. Experimental configuration for thermal/optical reflectance (TOR) and thermal/optical transmission (TOT) carbon analysis (DRI
Model 2001 thermal/optical carbon analyzer, Atmoslytic Inc., Calabasas, CA). The filter punch holder is open on top and bottom to minimize
transmittance attenuation. The carrier gas flows above and below [not through] the sample. A small thermocouple is located below the
filter holder to minimize difference between sample and thermocouple temperatures.
and oxidation interactions between OC, EC, and noncarbonaceous material in the sampled particles; and (v)
changes in optical properties of the particles during thermal
evolution.
Each of these variables needs to be studied systematically
to quantify their contributions to differences among the many
thermal methods applied throughout the world. The objective
of this work is to evaluate differences in the OC/EC split due
to two variables: temperature protocol and detection of
charring by R or T. This is done by comparing the EC
measured with two temperature protocols that bracket the
conditions applied in U.S. PM2.5 (particles with aerodynamic
diameters less than 2.5 µm) networks and simultaneously
monitoring R and T for different aerosol mixtures. All other
variables remain constant by performing these analyses on
the same instrument with the same carrier gases. This
experimental approach is complemented by a modeling effort
that reinforces its findings (35). The experimental approach
is described below, followed by the results and a discussion
of the observations.
Experimental Approach
Ambient PM2.5 samples on quartz-fiber filters (2500 QATUP, Pall Life Sciences, Ann Arbor, MI) from the nonurban
IMPROVE (Interagency Monitoring of Protected Visual
Environments) visibility network (36) and the urban Fresno,
CA, supersite (37) were used for these experiments. IMPROVE
samples are acquired on 25 mm diameter filters from which
only three 0.506 cm2 punches can be taken, one of which is
used for carbon analysis (20). Thirty IMPROVE samples were
chosen for this study representing a variety of compositions
to generalize the findings from the single-site Fresno samples.
To perform several experiments requiring numerous
separate punches, large samples were taken on prebaked
406 cm2 quartz-fiber filters (tissue quartz, Pall Life Sciences,
Ann Arbor, MI) at the Fresno supersite. These filters were
mounted in a high-volume (1130 L min-1) sampler with a
PM2.5 size-selective inlet (Andersen Instruments, Smyrna, GA).
Samples were taken from 0000 to 2400 PST approximately
one to two times per week from August 24, 2002, through
April 18, 2003. PM2.5 carbon at Fresno includes contributions
from vegetative burning, cooking, diesel exhaust, gasoline
exhaust, and secondary organic aerosol that vary throughout
the year (38-43). The high-volume filters were not preceded
by organic vapor denuders because previous comparisons
with and without denuders found approximately equivalent
positive and negative sampling artifacts for Fresno aerosol
(42), and real OC was much larger than artifact OC after
dynamic blank subtraction for most 24-h samples. Fifty-eight
of these Fresno samples were used in this study.
Figure 1 illustrates the analysis configuration that is
capable of implementing a variety of thermal and optical
carbon analysis protocols. One low-temperature (LowT)
protocol (20) and one high-temperature (HighT) protocol
(26) were adopted to represent extremes for the magnitude
of temperature plateaus and the residence time at each
temperature. As documented in Figure 2a, the LowT protocol
achieves plateaus of 120, 250, 450, and 550 °C in an ultrahighpurity helium (He) atmosphere (He >99.999%, followed by
a moisture/hydrocarbon/O2 trap), remaining at each plateau
until a well-defined carbon peak has evolved. In general, R
and T begin to decrease at temperatures >250 °C, indicating
that some of the remaining OC on and within the sample is
charring. After 2% O2 in 98% He is added at 550 °C, additional
carbon evolves, most of which consists of EC and POC, as
indicated by the rapid increase of both R and T. These optical
signals achieve their original values (horizontal dotted lines
in Figure 2a) at different times during the analysis, and the
difference in the charring correction between the two
methods (i.e., TOR or TOT split) is identified in Figure 2a.
The LowT protocol continues to increase temperatures from
550 to 700 °C and then to 800 °C, with the residence time
defined by the flattening of carbon signals. R usually returns
to its initial value before T; therefore, EC determined with
the TOR charring correction is usually higher than with the
TOT correction.
The HighT protocol (Figure 2b) implemented here heats
the sample to 310, 480, 615, and 900 °C over set periods of
60-90 s in the same pure He atmosphere used for the LowT
protocol. The sample is then cooled to 600 °C, at which time
the 2% O2 in 98% He atmosphere is introduced. The final
temperature plateaus are 600, 675, 750, 825, and 920 °C for
45-120 s. In Figure 2b, R achieves its original value before
the addition of O2, indicating the removal of EC, POC, or
other light-absorbing material in the pure He environment.
Chow et al. (25) attributed the early evolution of EC to
increased oxidation or catalysis at high temperatures (>700
°C) by minerals (e.g., MnO2, Fe2O3, SiO2, Al2O3) that coexist
with the carbon particles in the atmosphere or are part of
the filter. Lin and Friedlander (44) demonstrate how catalysts,
such as NaCl, lower the EC decomposition temperature, while
Fung (13) quantifies how the reaction kinetics between pure
graphite and MnO2 increase with temperature. Most of the
Fresno samples retain a reddish tinge rather than the white
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FIGURE 2. Example thermograms for a medium-loaded Fresno sample (TC ) 24.5 µg/cm2 or 6.9 µg/m3) for (a) the low-temperature (LowT)
protocol and (b) the high-temperature (HighT) protocol implemented in the Figure 1 configuration. Reflectance (R) and transmittance (T)
are in relative units. The nominal LowT temperature plateaus in pure He are 120 °C (OC1), 250 °C (OC2), 450 °C (OC3), and 550 °C (OC4).
Temperatures are increased when the flame ionization detector (FID) response returns to baseline or remains at a constant value.
Temperature plateaus in the 2% O2 in 98% He atmosphere are 550 °C (EC1), 700 °C (EC2), and 800 °C (EC3). Pyrolyzed/charred organic carbon
(POC) is defined as the carbon that is measured after the introduction of the He/O2 atmosphere at 550 °C but before R and T return to their
initial values. POC is reported as a negative value when R or T attain their initial values prior to O2 introduction. OC is defined as OC1
+ OC2 + OC3 + OC4 + POC, and EC is defined as EC1 + EC2 + EC3 - POC. The HighT temperature plateaus in He are 310 °C (60 s),
480 °C (60 s), 615 °C (60 s), and 900 °C (90s). Temperature is reduced to ∼600 °C before addition of 2% O2 in He followed by temperature
plateaus at 600, 675, 750, and 825 °C, each for 45 s, and 920 °C for 120 s. POC for both TOR and TOT corrections are reported as positive
or negative relative to the time at which O2 is added. The early dip in reflectance in the HighT analysis (at ∼100 s) likely results from
the rapid volatilization of nitrate.
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FIGURE 3. Equivalence of total carbon (TC) for Fresno samples
measured with the LowT and HighT protocols. (The dashed line
indicates a 1:1 correspondence. TC ranges from 6 to 145 µg/cm2,
corresponding to ambient concentrations of 1.7 to 40.9 µg/m3.)
FIGURE 4. Equivalence of TOR- and TOT-corrected EC for Fresno
samples using the LowT and HighT protocols. (The dashed line
indicates the 1:1 correspondence.)
color of a clean filterafter completion of analysis, which
indicates that mineral oxides are present with the carbon.
For both LowT and HighT protocols, EC is defined as carbon
evolved after the filter reflectance or transmittance returns
to its initial value (laser split), regardless of when the O2 is
introduced. A laser split point occurring before the addition
of O2 is referred to as an “early split”, which occurred in
∼83% of the Fresno samples using the HighT_TOR protocol
and in ∼40% of the samples using the HighT_TOT protocol.
Only one Fresno sample had an early split for LowT_TOR,
and none had an early split for LowT_TOT. Examination of
all HighT_TOR/TOT thermograms shows a wide range on R
and T increases during the 900 °C/He step, from negligible
to large amounts. This is consistent with the variation of
sample compositions.
Four samples from Fresno and two from IMPROVE were
so heavily loaded that an initial transmittance could not be
detected, thereby decreasing the absolute precision of the
TOT correction. Initial reflectance was detected on all
samples.
HighT_TOR and HighT_TOT in Figure 4 (0.29 ( 0.02). Figure
6 also shows the discrepancy between TOR and TOT
corrections for the IMPROVE samples. A good agreement
between TOR EC from the two temperature protocols for the
IMPROVE samples (Figure 6) demonstrates that the Figure
5 results are not caused by some peculiarity of the Fresno
aerosol.
In summary, within a given temperature protocol, TOR
and TOT corrections give different EC results, with their
differences amplified in the HighT protocol as the initial and
maximum temperatures in a non-oxidizing atmosphere
increase and residence times at each plateau decrease.
Regardless of the operational definitions for EC, the results
for these samples show that the TOR charring correction
gives equivalent EC estimates for large differences in thermal
evolution temperatures. This finding is consistent across a
range of sample locations and source contributions.
Results
Figure 3 demonstrates that LowT and HighT temperature
protocols yield the same TC for the 58 Fresno samples. A
similar equivalence was demonstrated for the 30 IMPROVE
samples (slope ) 0.99 ( 0.005, intercept ) 0.1 ( 0.2 µg/cm2,
R2 ) 0.999). These TC comparisons indicate that the sample
deposit is homogeneous and that the carbon analysis
precision is within (2 µg/cm2 or (5% for the average Fresno
carbon concentration of 46 µg/cm2. It also shows that the
maximum temperature applied in the HighT protocol (120
°C higher than the maximum in the LowT protocol) does not
release additional carbon, which might not be the case if
some carbonaceous material (possibly carbonates) evolved
at temperatures >800 °C (45). The maximum TC for Fresno
samples is ∼145 µg/cm2, corresponding to an ambient
concentration of ∼40.9 µg/m3.
On average, TOT resulted in ∼30% lower EC than TOR for
the LowT protocol and ∼70% lower EC for the HighT protocol,
as shown in Figure 4. Figure 5a shows that EC with the TOR
correction is the same regardless of the temperature protocol
for the Fresno samples. On the other hand, LowT_TOT EC
is about twice the HighT_TOT EC (Figure 5b). Therefore,
LowT_TOR EC and HighT_TOT EC differ the most, with a
slope of 0.29 ( 0.03 (Figure 5c), very similar to that of
Discussion
Potential Causes of TOR and TOT Differences. To elucidate
how optical monitoring affects the charring correction,
multiple punches from the same filter were analyzed by both
the LowT and HighT temperature protocols for several Fresno
samples. The analyses were stopped at different temperatures, the samples were cooled rapidly to a temperature of
<40 °C in the most recent analysis atmosphere (pure He or
2% O2 in 98% He), and the punches were removed for
immediate visual examination. Photographs were taken of
the front and back of the retrieved punches. They were then
cut in half, sandwiched between two microscope slides, and
photographed through an optical microscope to evaluate
the visual appearance of the filter cross-section. Results for
one of the samples are shown in Figure 7 to illustrate what
was found for all of the tested samples. Darkening of both
sides of the filter, as previously reported by Huntzicker et al.
(18), Chow et al. (20), and Yang and Yu (28), is evident in
Figure 7. The cross-sections show continued darkening
throughout the filter as temperatures increase in the He
environment, even though most of initial light absorbing
material, representing the ambient PM2.5 deposit, appears to
be near the surface. This is consistent with POC forming
within and throughout the filter thickness.
Figure 7a shows an example of a heavily loaded Fresno
sample during the LowT protocol while Figure 7b shows a
punch from the same sample during the HighT protocol. R
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FIGURE 6. TOR-corrected EC from the LowT protocol and both TOR
and TOT-corrected EC from the HighT protocol for the IMPROVE
network samples. The IMPROVE comparison includes 30 samples
from 16 IMPROVE network sites (SAGU1, SIPS1, STAR1, UPBU1,
WHIT1, SEN1, ACAD1, MELA1, MOMO1, PUSO1, HECA1, REDW1,
WASH1, GUMO1, SEQU1, and YOSE1) that represent a variety of
carbon source contributions (fires, secondary organic aerosol,
vehicle exhaust, carbonates). (The dashed line indicates the 1:1
correspondence.)
FIGURE 5. Comparisons between: (a) TOR-corrected EC from the
LowT and HighT protocols for Fresno samples, (b) TOT-corrected
EC from the LowT and HighT protocols, and (c) TOR-corrected EC
from the LowT protocol and TOT-corrected EC from the HighT protocol
for Fresno samples.
and T signals (quantified indicated by the reflection and
transmission photodetectors, respectively) are also presented
to allow a more quantitative estimate of the filter darkening.
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The initial T signal (mV) is low for this sample due to a high
EC deposit (LowT/HighT_TOR EC ∼25 µg cm-2), which
appears to reside at or near the surface of the filter according
to the cross-section picture. This agrees with the calculations
by Chen et al. (35) that show >80% of the aerosol deposition
is located on the top one-third to one-half of this type of
quartz-fiber ambient filter sample. Darkening occurs throughout the filter as heating continues in the pure He environment.
T reduces to zero as the filter becomes opaque due to
blackening, but changes in R are still detectable. The R
signal reaches a minimum of 516 mV at 549 °C for the LowT
protocol and 365 mV at 897 °C for the HighT protocol in the
He atmosphere, consistent with more charring during the
HighT protocol. As shown in Figure 7, the backside and crosssection appear to be darker for the HighT protocol compared
to the LowT protocol at the respective minimum R.
The additional charring occurring at higher temperature
steps (>550 °C) in He suggests that certain organic compounds have not completely pyrolyzed below 550 °C in the
HighT protocol. This charring could be reduced if a longer
residence time were implemented at lower temperature steps
to allow more of these compounds to leave the filter prior
to the onset of charring. Kirchstetter et al. (45) showed that
much of the adsorbed organic gases on backup filters evolve
at <250 °C within a sufficient time. The LowT protocol
includes a longer residence time (∼10 min) at lower initial
temperatures (120 and 250 °C) than the HighT protocol.
However, since the LowT protocol never reaches temperatures higher than 550 °C in He, the question of whether this
longer residence time produces significantly less charring
warrants further investigation. At 677 °C (He/O2) during the
HighT protocol (Figure 7b), the filter transmittance nearly
attains its initial values, but the reflectance at this moment
is substantially higher than its initial value. This is consistent
with the observation that POC within the filter remains when
a large fraction of the surface deposit has been removed.
Two processes might contribute to a uniform distribution
of POC in the filter: (i) charring of volatile or semi-volatile
organic compounds that adsorb onto quartz fibers during
FIGURE 7. Visual and microscopic investigations of the sample (TC ) 122 µg/cm2 or 34.4 µg/m3) at different temperature stages during
(a) LowT and (b) HighT protocols. Reflectance (R) and transmittance (T) are in relative units of mV. He and He/O2 are the combustion
atmospheres when the sample was retrieved from the analyzer. The fourth column from the left in each sequence shows the temperature
and reflectance or transmittance at the point of maximum darkening of the filter. The HighT reflectance (365 mV at 897 °C) is much lower
than the LowT reflectance (516 mV at 549 °C), indicating a larger amount of charring for the higher HighT temperatures. (The punch size
is 0.506 cm2, the filter thickness is ∼0.43 mm, and the areal density is ∼5.8 mg/cm2.)
sampling or (ii) diffusion of vaporized organic matter from
the filter surface downward as the sample is heated. Organic
vapor adsorption is a well-known artifact in ambient carbonaceous aerosol sampling (45-47), and charring of these
adsorbed organic materials may account for a substantial
fraction of POC. Similar charring is seen on IMPROVE backup
filter punches (not shown here) with no particle deposition,
but for these backup filters the TOR and TOT corrections are
equivalent and yield negligible EC.
These results are consistent with an extended KubelkaMunk radiative transfer model that relates R and T for
different distributions within a filter of materials having
different light absorption and scattering properties (35). This
model is consistent with the convergence of TOR and TOT
corrections in the case of only a shallow surface deposit of
EC or only a uniformly distributed POC through the filter.
It suggests divergence between TOR and TOT corrections
when EC and POC exist concurrently at the surface and are
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distributed throughout the filter, respectively, especially when
the surface EC evolves prior to the POC. Therefore, the
difference between TOR and TOT partly depends on the POC/
EC ratio in the sample. This theory explains why samples
dominated by diesel exhaust and graphite powder with a
dark surface deposit often yield the same EC values for TOR
and TOT corrections, while complex ambient mixtures yield
different EC values (35). It can also explain why backup filters
used to evaluate organic vapor adsorption, which have no
surface deposit and experience uniform charring throughout
their cross-sections, show similar TOR and TOT corrections.
EC and POC Apportionment. For quantifying EC, an
alternative to the thermal/optical analysis with TOR or TOT
charring correction is the estimation of light absorption of
an aerosol deposit compared to that of a blank filter. While
the absorption of EC within quartz-fiber filters is difficult to
measure absolutely, reflectance and transmittance of light
have long been used to estimate relative EC on filters (4850). For example, incremental light attenuation of a filter
due to deposited aerosol is widely used in the aethalometer
(51, 52) and particle soot absorption photometer (PSAP) (53)
to estimate EC concentrations based on a linear relationship
between incremental light attenuation and EC mass. Lindberg
et al. (54) reevaluated this empirical relationship and
confirmed its validity for common atmospheric particulate
samples. Incremental light attenuation primarily due to EC
absorption (τATN,EC) is quantified by
τATN,EC ) -ln
()
Ti
Tf
(1)
where Ti and Tf are the filter transmittance at the beginning
and end (i.e., blank) of thermal analysis, respectively. The
EC mass absorption efficiency within the filter (Ea,EC) is
defined as
Ea,EC )
τATN,EC
[EC]
(2)
where [EC] indicates the EC areal mass concentration
(µg/cm2) on the filter. Ea,EC (usually in m2/g) may be higher
than its actual value in the atmosphere due to multiple
scattering effects from the filter medium. The incremental
light attenuation due to POC (τATN,POC) can be defined in
analogy to eq 1
τ(t)ATN,POC ) -ln
( )
T(t)
Ti
(3)
where T(t) is defined as the transmittance at any instant before
EC and POC evolve (i.e., before R and T start to increase)
during thermal analysis. The POC mass absorption efficiency
(Ea,POC) becomes
Ea,POC )
τATN,POC
[POC]
(4)
where [POC] is the areal mass concentration (µg/cm2) of
POC. τATN,EC is assumed to be constant before oxidation
occurs. Ea,EC, Ea,POC, [EC], and [POC] are unknown for each
sample. Parameters being measured every second include
R, T, and evolved carbon mass (see Figure 2a); τATN,EC and
τATN,POC before O2 introduction can therefore be determined.
The 1-s areal carbon concentration determined from the
flame ionization detector (FID) does not have a one-to-one
correspondence to the optical measurements owing to peak
broadening and a tailing effect caused by transit through
the C f CO2 and CO2 f CH4 conversion ovens. Comparison
of optical and carbon measurements at the end of each
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temperature plateau, however, is meaningful for the LowT
protocol in which the FID signal levels off before the
temperature advances to next plateau. Through the LowT
protocol, carbon evolved at each temperature plateau is
completely measured (see Figure 2a). It is further assumed
that carbon left on the filter at the end of the 550 °C plateau
before the addition of O2 contains only EC and fully formed
POC. R and T nearly always attain their minima for the
entire analysis at 550 °C in He during the LowT protocol,
consistent with no EC or POC being burned. The following
analysis will not apply to the HighT protocol because the
carbon peaks are not completely associated with specific
temperature plateaus owing to the short and fixed residence
time at each plateau.
With these relationships and assumptions, T(t) in eq 3 is
chosen at the end of the 550 °C plateau before the addition
of O2 to calculate τATN,POC. The following relationship applies
for each sample:
[carbon]after O2 ) [EC] + [POC] )
τATN,EC τATN,POC
+
Ea,EC
Ea,POC
(5)
Replicate analysis has indicated the precision of τATN,EC
and τATN,POC, generally within 10%. If Ea,EC and Ea,POC are
reasonably constant from sample to sample, a linear least
squares optimization (linear regression) of the measured
[carbon]after O2 on τATN,EC and τATN,POC yields estimates of the
inverse absorption efficiencies for EC and POC. Estimates
for EC and POC can then be made for each sample via
eqs 2 and 4. Additional conditions are that the minimum T
at maximum charring must be detectable, and there are
no early OC/EC splits. These conditions were met for 31
of the 58 Fresno samples. For these samples, the zerointercept correlation (R2) between [carbon]after O2 and τATN,EC
is 0.78. R2 between [carbon]after O2 and τATN,POC is only 0.37. R2
improves to 0.89 for the multiple linear regression model
of eq 5. The inverse of the regression coefficients Ea,EC and
Ea,POC are 21.6 ( 1.9 and 52.8 ( 10.6 m2/g of C, respectively.
Ea,EC of ∼20 m2/g of C is at the upper limit of reported
values for the EC absorption efficiency (e.g., refs 55 and 56)
of atmospheric aerosols, but it is typical of transmission
measurements on quartz-fiber filters (57). Ea,POC is ∼2.5
times higher than Ea,EC, implying that POC for the same
amount as EC causes a stronger attenuation than the
atmospheric EC for the Fresno samples. Conny et al. (58)
observed very different characteristics between EC and POC
in terms of their surface-to-volume ratio with a scanning
electron microscope. Multiple scattering within such optically
dense quartz-fiber filters could enhance the absorption of
EC and POC by different degrees.
The partitioning of EC and POC in [carbon]after O2 can also
be calculated from eq 5 using a Ea,POC/Ea,EC ratio of 2.5 (i.e.,
∼52.8/21.6). Figure 8 shows the partitioning for the Fresno
samples and compares them with the OC/EC splits determined by TOR and TOT with the LowT protocol. TOR EC
agrees better with the linear regression EC. For most of the
lightly loaded samples, the difference is within (10%. TOT
EC, however, is generally 30-40% less than the calculated
EC from regression and TOR EC.
The regression model was also applied to 68 randomly
selected samples from the IMPROVE network with carbon
concentration ranges (TC: 16-107 µg/cm2) similar to those
of the Fresno samples. Results were similar (R2 ) 0.85; Ea,EC
) 18.5 ( 2.3 m2/g of C, Ea,POC ) 48.5 ( 3.9 m2/g of C). Figure
9 compares the maximum attenuation (τATN,EC + τATN,POC)
with [carbon]after O2 for the Fresno and IMPROVE samples
(see eq 5 for their relationship). Generally, the absorption
efficiency of each EC and POC mixture falls within the Ea,EC
and Ea,POC estimates. These analyses suggest the utility of
FIGURE 8. Partitions of original EC and POC in carbon evolved after 2% O 2 is added to the He atmosphere for the LowT protocol. The
partitioning is based on incremental absorbance regression method for 31 Fresno samples with a detectable minimum transmittance. EC
determined from reflectance correction (TOR EC) and transmittance correction (TOT EC) is also shown. The error bars indicate the precision
of the TOR and TOT EC measurements.
Acknowledgments
This work was sponsored, in part, by the National Park Service
IMPROVE Carbon Analysis Contract C2350000894, the U.S.
Environmental Protection Agency STAR Grant RD-831086010, and Fresno Supersite EPA Cooperative Agreement
R-82805701. The conclusions are those of the authors and
do not necessarily reflect the views of the sponsoring agencies.
Mention of commercially available products and supplies
does not constitute an endorsement of these products and
supplies. Field samples were provided by the IMPROVE
network and by the Fresno Supersite. The authors thank Mr.
Scott Scheller and Mr. Peter Ouchida of the California Air
Resources Board for the Fresno site operation, Mr. Dale Crow
and Mr. Steve Kohl of the Desert Research Institute (DRI) for
the laboratory operation, Dr. Peter Barber of DRI for reviewing
the manuscript and supplying valuable comments, and Mr.
Norman Mankim and Mr. Eric Dieterle for assembling and
editing the manuscript.
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FIGURE 9. Comparison of carbon measured after the addition of O 2,
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protocol analysis. The 68 IMPROVE samples were selected from 25
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Received for review August 26, 2003. Revised manuscript
received January 2, 2004. Accepted May 18, 2004.
ES034936U