United States
[Environmental Protection
Agency
Ftesearch and Development
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
EPA/600/S3-91/002 Mar. 1991
\yEPA Project Summary
Atmospheric Transport and
Deposition of Polychlorinated
Dibenzo-p-Dioxins and
Dibenzofurans
Ronald A. Hites
Polychlorinated dibenzo-p-dloxins
(PCDD) and polych lorinated
dibenzofurans (PCDF) are toxic com-
pounds which are dispersed through
the environment by atmospheric trans-
port and deposition. It has been previ-
ously shown that there is a varying
mixture of these compounds produced
by combustion (the source of these
compounds). Conversely, sedimentary
sinks have been shown to have a con-
stant pattern. Therefore, the goal of this
project was to study PCDD/F In ambi-
ent air and rain samples (the transport
media linking source to sink) in an ef-
fort to enhance our understanding of
the physical/chemical parameters con-
trolling the transformations which take
place during atmospheric transport and
deposition. Analytical methods included
high-volume air sampling, wet-only rain
sampling, column chromatographic
cleanup, and an electron capture,
negative ionization form of gas chro-
matographic mass spectrometry. Air
arid rain samples were divided into va-
por-phase and particle-bound or dis-
solved and particle-bound fractions,
respectively. The results showed that
total PCDD/F concentrations in
Bloomington, Indiana, had a geometric
standard deviation range from 1.4 to
4.4 pg/m3 in air and from 63 to 220 pg/L
in rainwater. The vapor-to-particle (V/P)
ratio for individual congeners ranged
from 0.01 to 30. This ratio is controlled
by the individual congener's vapor
pressure and the ambient air tempera-
ture. Estimates of washout and the
Henry's law constants were obtained
using average ambient air and rain data.
Total washout ranged from 10,000 to
90,000 for the tetra- through octa-ho-
mologues; Henry's law constants
ranged from 1.3 x 10-5 to 8.9 x 1041
atm*3/mole. Atmospheric concentra-
tions were shown to be consistent with
previously observed sediment fluxes.
Principal component analysis was used
to compare the observed air and rain
data with literature data for sources
and sinks. This analysis indicates that
the observed homologue profiles bridge
the gap between source and sink pro-
files. The underlying characteristics of
individual PCDD/F congeners (such as
vapor pressure and Henry's law con-
stant) show trends with level of chlori-
nation. When transport and depositional
processes act on those trends, the
varying mixture in the sources becomes
the consistent pattern in the sinks.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search Triangle 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).
Printed on Recycled Paper
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Introduction
Polychlorinated dibenzo-p-dioxins and
polychlorinated dibenzofurans (PCDD/F)
are ubiquitous in the environment and are
of concern because of the high toxicity of
certain congeners. PCDD/F are discharged
daily into the environment in low concen-
trations by industrial and municipal waste
incineration, car exhaust, and pulp and
paper mill effluents. Chlorophenol-based
herbicides and insecticides contain trace
amounts of PCDD/F. Once PCDD/F are
released into the environment, they be-
come distributed throughout various envi-
ronmental compartments, including the
atmosphere, which transports PCDD/F
long distances.
While PCDD/F are transported through
the atmosphere, transformations may oc-
cur. The differences between homologue
profiles of typical PCDD/F sources and
sinks suggest that transformations of
PCDD/F do, in fact, occur. For example, a
typical homologue profile of PCDD/F pro-
duced by incineration, the major source of
PCDD/F, shows a random distribution of
PCDD/F. However, sediments, the ultimate
sinks for PCDD/F, have a PCDD/F homo-
logue profile that is enriched in
octachlorodioxin. Also, the PCDD/F ho-
mologue profiles of air and rainwater show
an enhancement of octachlorodioxin.
Therefore, we conclude that PCDD/F un-
dergo transformations while they reside in
the atmosphere.
To understand the environmental trans-
port and fate of PCDD/F, it was important
to study the factors that control their at-
mospheric transport and deposition and
to examine possible degradational path-
ways for PCDD/F. Specifically, the ques-
tions we addressed included: (a) What
are PCDD/F concentrations in ambient air?
(b) How are atmospheric PCDD/F parti-
tioned between the vapor and particle
phases? (c) Do concentrations of PCDD/F
show seasonal variations? (d) Do PCDD/F
concentrations change from rural to urban
areas? (e) How are PCDD/F deposited
from the atmosphere? To answer these
questions, we performed a detailed, 3-
year study of PCDD/F in the ambient air
of Bloomington, Indiana.
Procedure
To accomplish our study goals, both air
and rain samples were collected. Air
samples were collected using a high-vol-
ume air sampler equipped with a glass
fiber filter; vapors were collected on poly-
urethane foam plugs. A total of 95 air
samples was taken over a 3-year period.
Subcooled-liquid vapor pressures for
PCDD/F were measured using a gas
chromatographic retention time correlation
method; these vapor pressures were used
in analyzing the air data. Fourteen rain
samples were collected using a wet-only
rain sampler. Particles in the rain sample
were separated from dissolved PCDD/F
using a glass fiber filter.
All samples were either Soxhlet- or liq-
uid-liquid extracted and subjected to silica
and alumina column chromatographic
cleanup. All analyses were performed by
electron capture, negative ionization, low
resolution mass spectrometry. All PCDD/F
were quantitated as congener classes.
Results and Discussion
Typical PCDD/F concentrations mea-
sured in Bloomington air ranged from 1-4
pg/m3. All air samples showed a predomi-
nance of octachlorodioxin. Total PCDD/F
concentrations showed no seasonal
variations, but the vapor-to-particle ratios
changed These ratios varied from 0.01 to
30 and were dependent on the
compound's subcooled-liquid vapor pres-
sure and the ambient temperature. Heats
of adsorption for PCDD/F were calculated
and ranged from 11-32 kcal/mole. Heats
of adsorption increased with increasing
level of chlorination, indicating that the
vapor-to-particle ratio for more chlorinated
PCDD/F has a greater temperature de-
pendence than for the lesser chlorinated
compounds. Total PCDD/F concentrations
in Indianapolis, Indiana, an urban location,
were found to be higher (tens of pg/m3)
than total PCDD/F concentrations at Trout
Lake, Wisconsin, a rural location (tenths
of pg/m3}. In addition, the homologue pro-
file of PCDD/F in rural air was determined
to be closer to that of sediments than was
the homologue profile of urban air.
The homologue profiles of PCDD/F
inrain were similar to those of sediments.
Total concentrations of PCDD/F in rain
were 10-84 pg/L in the dissolved-phase
and 37-320 pg/L in the particle-phase.
Using the accumulated air data, vapor
scavenging ratios were calculated and
ranged from 7.7 x 103 to 2.7 x 106; particle
scavenging ratios ranged from 1.2 x 104
to 7.2 x 104. Particulate scavenging is the
major wet removal process affecting
PCDD/F, and total overall scavenging effi-
ciency of PCDD/F increases with increas-
ing level of chlorination. Depositional fluxes
of PCDD/F were calculated, and dry
deposition was determined to be five times
more efficient than wet deposition.
Conclusions and
Recommendations
Our data support the following scenario.
A broad range of PCDD/F are introduced
into the atmosphere by numerous sources,
forming a uniform, urban, ambient air mix-
ture. As the air mass moves away from
the sources, it is diluted with cleaner air
and starts to "age." Less chlorinated con-
geners are found to a greater extent in
the vapor phase, and thus, they may un-
dergo vapor-phase photodegradation;
particle-phase photodegradation probably
does not occur. This enhances the rela-
tive concentrations of the more chlorinated
congeners. The particles, with their en-
hanced load of the more chlorinated con-
geners, are deposited by both wet and
dry processes. Although the dry process
dominates, the efficiency of the wet method
improves for the more chlorinated conge-
ners. Once in the water column, Henry's
law constants predict greater vaporization
of the less chlorinated congeners. This
further enhances the relative proportion of
the more chlorinated congeners that pass
through the water column to the sedi-
ment. All of these processes favor a ho-
mologue profile enriched in the more
chlorinated congeners Therefore,
octachlorodioxin is most predominant in
the sediments. Our data suggest that only
the most chlorinated PCDD/F congeners
are environmentally persistent. This find-
ing may be of interest to policymakers
because these congeners tend to be the
least toxic.
•&U. S. GOVERNMENT PRINTING OFFICE: 1991/548-028/20177
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Ronald A. Hites is with Indiana University, Bloomington, IN 47405.
Robert L Harless is the EPA Project Officer (see below).
The complete report, entitled "Atmospheric Transport and Deposition of Polychbrinated
Dibenzo-p-dioxins and Dibenzofurans, "(Order No. PB91- 144 667/AS; Cost: $23.00,
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:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
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Penalty for Private Use $300
EPA/600/S3-91/002
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