EXPERIMENTAL METHODS

1
THE COMPARISON BETWEEN THERMAL OPTICAL
TRANSMITTANCE ELEMENTAL CARBON AND AETHALOMETER
BLACK CARBON MEASURED AT THE MULTIPLE MONITORING
SITES
Cheol-Heon Jeong1, Doh-Won Lee1, Eugene Kim1,
Philip K. Hopke1, Robert Gelein2
Clarkson University1, University of Rochester2
INTRODUCTION
Carbonaceous aerosol may be considered as a
complex mixture and one of the most abundant
component in urban PM2.5. The carbonaceous
compound is composed of two main fractions,
organic carbon (OC) which is volatile and
elemental carbon (EC) which is a polymerized and
dark fraction of aerosol. EC is formed by
thermal process of OC, combustion processes, and
it is chemically inert and essentially
nonvolatile at ambient temperature. There are
little measurements of high time resolution OC
and EC using thermal optical transmittance (TOT)
in various monitoring sites. The data acquisition
of continuous OC and EC that may provide higher
time resolution than filter-based measurements is
one of the important steps applied to regional
aerosol characterization studies and source
apportionment studies. Comparing optically
difference among carbonaceous compounds, black
carbon (BC) is most polymerized fraction of the
carbonaceous compounds and may be considered
primarily as strong light absorbencies. Although
both EC and BC were known to have similar
fraction in carbonaceous compounds and they were
often considered as same compounds, BC aerosols
have somewhat different thermal, optical, and
chemical characteristics due to the variable
absorption coefficient. The study reports the
results of the intercomparison study conducted to
determine temporal variations of carbonaceous
aerosols and to evaluate the difference between
EC and BC measurements during various atmospheric
conditions in two urban sites.
EXPERIMENTAL METHODS The monitoring site in
Rochester was adjacent to a moderate traffic road
and located 5 kilometer away from downtown
Rochester. In Philadelphia, the monitoring site
was located 500 m away from a light traffic road
and 1 kilometer away from a highway I-95. OC and
EC can determine by using thermal optical
protocol, the National Institute of Occupational
Safety and Health (NIOSH) method 5040 with
correction by the thermal-optical transmission
(TOT) employed high temperature profile and fixed
residence time. Figure 1 shows the thermogram of
the semi-continuous OC/EC analyzer. Aerosol BC
was measured using the Aethalometer (AE-20, Magee
Scientific) with the optical attenuation method
since BC can strongly absorb light over a broad
region of infrared red (? 880 nm ), and BC mass
was obtained by determining the attenuation of
light transmitted through a sampled filter.
Figure 1. Thermogram of the Sunset Lab
semi-continuous OC/EC analyzer.
RESULTS AND DISCUSSIONS Diurnal variations of
semi-continuous OC and EC during weekdays and
weekends in Philadelphia are shown in Figure 2.
As expected, EC concentration during weekdays
peaked during morning rush-hour and much higher
than EC concentrations measured during weekends.
It strongly suggests that sources of EC were
closely related with human activities. In
contrast to the diurnal trend of EC, although OC
obtained during weekdays were somewhat higher
than the concentrations during weekends, the
overall difference between weekdays and weekends
was negligible during afternoon. It might be
expected that sources of OC were hardly related
with traffic sources and they would be linked
with a variety of sources such as stationary
source combustion. The diurnal variations of EC
and OC measured in Rochester are also shown in
Figure 3. The result strongly supports the
diurnal patterns of carbonaceous aerosols, which
means clear morning peaks of EC occurred during
rush-hour were found, whereas there was no
significant temporal variation of OC. In
Rochester, BC concentrations during weekdays
peaked around 9 a.m., and broad peak was found
around 3 to 5 p.m. However, during weekends there
was no peak as a function of time of day. In
Philadelphia, a peak was observed around early
morning rush-hour and the highest concentration
occurred in 6 a.m. during weekdays, whereas there
was no significant diurnal variation of BC during
weekends. Thus, the variations of BC were
strongly correlated with motor vehicles combined
with low mixing height in summer. Two-hour
integrated EC obtained in Rochester from June 7
to June 19, 2000 and in Philadelphia from July 10
to August 2, 2002 were compared with BC (Figure
5). Aethalometer BC were higher than EC in both
sites while they were closely correlated.
However, the slope was 3.3 in Rochester and the
slope was 1.4 in Philadelphia. The difference of
the slopes in both sites suggests that the
absorption coefficient of BC is variable with
time and location and probably depend on the
physical and chemical characteristics of black
carbons. It, also, presents that the actual
absorption coefficients of BC in Rochester and
Philadelphia were more likely to be larger than
the manufacturers absorption coefficient, 16.6
m2 g-1. In Rochester, the sampling site was
located in 3 m from a local road, thus the very
high absorption coefficient might be due to the
geographical reason of the site. The somewhat
poor correlation between EC and BC in
Philadelphia might have been due to the variable
aerosol characteristics during several haze
events.
Figure 4. Comparison between weekdays and
weekends diurnal variations of black carbon.
Figure 2. Comparison of diurnal variations of
elemental carbon and organic carbon measured
from July 10 to August 2,2002 in Philadelphia,
PA.
Figure 3. Diurnal variations of elemental carbon
and organic carbon measured from June 6 to June
18, 2002, in Rochester, NY.
Figure 5. Comparison between elemental carbon
and black carbon during the summer of 2002.
Figure 6 shows the differences between
semi-continuous EC and Aethalometer BC during the
haze episodes in Philadelphia. During severe
haze events, the differences between BC and EC
concentrations tended to be increased while BC
sharply peaked. Thus, BC concentrations in
Philadelphia might be overestimated due to the
hygroscopic behavior of BC during the summer haze
events.
Acknowledgement This work was, also, sponsored in
part by the Pennsylvania Department of
Environmental Protection and the US
Environmental Protection Agency though
Cooperative Agreement CR 827591.
R827354
Figure 6. Semi continuous elemental carbon and
continuous black carbon for Philadelphia,PA,
during the summer of 2002.