&EFK
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-161 Jan. 1982
Project Summary
Source Resolution of Polycyclic
Aromatic Hydrocarbons in the
Los Angeles Atmosphere:
Application of a Chemical
Species Balance Method with
First Order Chemical Decay
Marc Maurice Duval and S. K. Friedlander
The chemical mass balance method
that was originally developed for
source resolution of chemical ele-
ments was extended in this study to
chemically reactive compounds in
atmospheric aerosols. The basic theo-
retical equation was formulated to
incorporate atmospheric decay
factors for reactive species. The
method was tested with selected poly-
cyclic aromatic hydrocarbons using
source emission data and atmospheric
concentrations reported in the litera-
ture. Absolute atmospheric concen-
trations of coronene, benzo(a)pyrene,
benzo(e)pyrene, benzo(g,h,i)pyrene,
and antnanthrene measured at 13
sites in the Los Angeles basin were
apportioned between automobile and
refinery emission sources.
Refinery emission patterns of poly-
cyclic aromatic hydrocarbons in the
Los Angeles basin were computed
from an analysis of auto emissions
data and ambient data from a refinery
area. Rates of atmospheric degrada-
tion of these hydrocarbons were
calculated from an analysis of auto
emission and ambient air data. Litera-
ture reports on major emissions of
polycyclic aromatic hydrocarbons
from combustion sources were
reviewed and evaluated.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Polycyclic aromatic hydrocarbons
(PAH) and a variety of other toxic organ ic
and inorganic chemical species can be
found in the aerosols commonly present
in ambient air. These aerosols have
been associated with adverse health
effects on the human populace as well
as visibility degradation and damage to
vegetation and materials. When toxic
materials are present in the atmosphere
in amounts that represent an unaccep-
table health risk or other adverse
effects, it is important to identify the
sources of these pollutants so that
appropriate control measures can be
taken. By assessing the contribution of
individual emission source categories to
pollutant concentrations in the atmos-
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phere, control strategies can be selec-
tively directed toward those sources
found to contribute significantly to the
observed environmental effects.
Control agencies have been using
dispersion modeling for many years to
estimate the impact of a particular
source on pollutant concentrations at a
receptor site. However, quantitative
accuracy of these models has been
limited by the variability and uncertain-
ties in the meteorological, emission,
and other parameters. In recent years,
receptor models of various types have
been developed as an alternative to
source resolution and apportionment of
air pollutants. In this approach, analysis
of the chemical composition of aerosol
samples from both the receptor site and
the source is used to retrospectively
generate a source apportionment of
chemical elements and species. Source
apportionment requires a chemical
mass balance (CMB) calculation that, as
.originally developed, assumes conser-
vation of mass and the emission of
characteristic patterns of chemical
elements from each class of sources.
The CMB method has been successfully
applied to the apportionment of
chemical elements in aerosols from
several parts of the United States. In this
study, the CMB method is extended to
chemically reactive organic species,
viz., PAH.
The basic equation for the CMB
method can be expressed as:
P
p,= I
where p, = mass concentration of
species i measured at the receptor site
where c,j
where p,
= mass concentration of
species i measured at
the receptor site
= mass fraction of species
i present in the mass
from source j at the
point of emission
= mass concentration of
the emissions from
source j measured at
the receptor site
For species that may react in the atmos-
phere the basic equation must be re-
formulated as follows:
P
p, = Z
j =1
where a,, = dimensionless decay
factor (fraction of
species i emitted from
source j remaining in
the aerosol at the
receptor site)
x,j = dimensionless ratio of
mass of species i to the
reference species I in
the emission from
source j
yi, = mass concentration of
reference species I
(non-reactive) from
source j at the point of
measurement
Working with an overdetermined sys-
tem of n equations with p unknowns (n
>p), the solution to the least squares
fitting is found from:
(Z) (Y) - (Q)
where (Z) = p x p matrix whose
generic term on the jth
row and mth column is:
Zjm —
n
I
i =
(Y) = p x I matric whose
term on the jth row is y,i.
(Q) = p x I matrix whose
generic term on the jth
row is:
n
q,= I
i = I
2
Of,
The decay factor, computed from the
ratio of atmospheric concentrations
found at the receptor site to the corre-
sponding concentrations measured at
the point of emission, was shown to be
related to the reaction rate coefficient,
assuming first order kinetics and condi-
tions for a continuous stirred tank
reactor.
Polycyclic aromatic hydrocarbons are
produced in combustion processes.
Because of their high boiling points,
they are readily adsorbed onto the par-
ticulate phase as they cool down.
Because of their carcinogenicity and
because they are found concentrated on
particles in the inhalable size range,
they are of particular concern in health
studies of ambient aerosols. Because
lower molecular weight PAH have suffi-
ciently high vapor pressures to warrant
concern about vaporization losses
during sampling, this study has been
confined to PAH with molecular weights
> 252 amu.
Major combustion sources emitting
PAH include coal combustion, coke
production, incineration, wood com-
bustion, open burning, and gasoline-
powered engines. Since automobile
emission characteristics depend on
many variables, survey results from a
large number of vehicles were used to
compute a representative PAH emission
profile. Although the source resolution
study was conducted in Los Angeles,
California, results from a survey of
German auto emissions were employed
because of the unavailability of such
data for American cars.
Previously published data on atmos-
pheric samples taken in Los Angeles
near a freeway junction were used to
obtain decay factors for four PAH
involved in the study (Table 1). A fifth
compound, benzo(e)pyrene, was
assumed to be a stable species. Decay
factors were obtained from a compari-
son of PAH-coronene ratios from the
German auto fleet and the Los Angeles
atmosphere. The deviation was
assumed to be a first order decay in all
species of interest. Additionally, reac-
tion rate constants were calculated
Table 1. PAH Decay Factors"
PAH
Decay Factor
Benzo(a)pyrene
Benzo(e)pyrene
Benzofluoranthenes
Benzo(g, h, i)perylene
Anthanthrene
0.48 ±0.21
1.04 ± 0.29"
0.98 ± 0.26
0.83 ±0.19
0.21 ±0.11
"Arithmetic mean within 68% confi-
dence.
*Benzo(e)pyrene was assumed to be a
stable species in the source resolution
analysis.
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assuming atmospheric residence times
equal to those for a continuous stirred
tank reactor or plug flow reactor and
compared with data reported in the
literature.
Refinery emissions of PAH were
determined from ambient air data
reported in the literature for an area in
Los Angeles downwind from a concen-
tration of petroleum refineries and
chemical plants. The area was assumed
to receive contributions from both
refineries and automobiles. Emissions
attributable to the refineries were
obtained by computing the difference
between the observed concentration for
each PAH and the corresponding calcu-
lated PAH concentration attributable to
auto emissions. The automotive contri-
bution was determined using lead
concentrations as a tracer for auto
emissions in this area together with
lead-PAH ratios determined at a Los
Angeles site assumed to be totally
dominated by auto emissions. The
calculated refinery emissions of PAH
and the measured German auto emis-
sions, corrected for their decay,
constituted the concentration matrix
that was used in computations of source
apportionment by the chemical species
balance method (Table 2).
Data from two different studies
involving 13 sites in the Los Angeles
basin were analyzed for source contri-
butions assuming that autos and
refineries were the only significant
sources for the PAH. Significant
refinery contributions were found at
four of the sites. The measured concen-
tration for each PAH was compared to
the total calculated PAH concentration
(auto plus refinery contribution) to pro-
vide a measure of the accuracy of the
results. Average deviations are within
-11 to +7%. The CMB method did not
yield- reasonable results with literature
data from a third study. Errors in the
ambient concentration vectors are
considered to be the most likely cause of
the slightly negative refinery contribu-
tions obtained in the source apportion-
ment computations.
Recommendations
The results of this study have shown
that the incorporation of decay factors
into the CMB method is a promising
approach for apportioning the concen-
trations of reactive organic species in
the atmosphere to their various emis-
sion sources. Additional studies with
more accurate data and fewer assump-
tions are needed to assess the accuracy
of this approach.
• Subsequent studies should utilize
United States'auto fleets to insure
the representatives of the emis-
sion patterns of PAH and other
species of interest.
• Improvements are needed in the
determination and validation of
decay factors.
• Studies should be extended to
other areas of the United States
with a more complex mix of emis-
sion sources.
• The combination of factor ana lysis
with CMB should be investigated
as an approach for obtaining im-
proved source resolution.
Table 2. Source Concentration Maxtrix,
PAH-Benzo(e)pyrene Ratio
PAH
Automobiles
"All PAHs were corrected for decay.
''Based on small coronene concentrations.
Refineries*
Benzofluoranthenes
Benzo(a)pYrene
Benzo(g, h. i)pyrene
Anthanthrene
Coronene
1.O8 ± O.36
0.98 ± 0.36
3. 10 ± O.87
0.57 ± O.27
1.96 ± 0.46
1.43
3.85
2.46
2.12
0.23"
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Marc Maurice Duval and S. K. Friedlander are with the Department of Chemical,
Nuclear, and Thermal Engineering, University of California, Los Angeles, CA
90024.
Stanley L. Kopczynski is the EPA Project Officer (see below).
The complete report, entitled "Source Resolution of Polycyclic Aromatic
Hydrocarbons in the Los Angeles Atmosphere: Application of a Chemical
Species Balance Method with First Order Chemical Decay," (Order No.
PB 82-121 336; Cost: $9.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
•ft U S GOVERNMENT PRINTING OFFICE, 1982 — 559-017/7438
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