United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park IMC 27711
Research and Development
EPA/600/S3-86/064 Mar. 1987
&EPA Project Summary
Phase Distributions of
Low Volatilty Organics in
Ambient Air
Bruce K. Cantrell, Louis J Salas, Warren B. Johnson, and James C Harper
Current strategies to control photo-
chemical air pollution rely on abating
the emission of volatile organic com-
pounds. Primarily, these compounds
exist in the vapor phase, and are those
with a carbon number of ten or less.
Recent attention has been given to low-
volatility organic compounds (LVO)
which may be partitioned between the
gas and aerosol phases, and may only
partially participate in smog formation.
A three-part study was conducted to
determine the role of such LVOs. First,
a comprehensive review was made of
the existing literature on atmospheric
LVO to evaluate existing data and the
measurement techniques used to col-
lect the data. Second, an effort was
made to develop an "estimator" for the
phase distribution of LVOs in the at-
mosphere. While exact theoretical cal-
culations of LVO adsorption on ambi-
ent surfaces are difficult, estimation
based on the phenomenology is possi-
ble. This semi-empirical calculation has
been applied to the n-alkane series with
some success.
The concurrent third part of the study
involved an attempt to develop and test
an improved and reliable collection and
analysis technique for LVO. An effective
denuder device was developed which
can be used to provide a sample of the
aerosol fraction of LVO under labora-
tory conditions. Additional research is
needed to perfect this device for the
routine determination of the aerosol/
vapor fraction in the ambient air.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back}.
Introduction
Current strategies to control photo-
chemical air pollution rely on abating
the emission of volatile organic com-
pounds. Primarily, these compounds
are those with a carbon number of ten
or less. Recent attention has been given
to low-volatility organic compounds
(LVO), which may also contribute to
smog formation. To determine the role
of such LVOs in smog formation, both
the reactivity and the phase distribution
of each compound must be known.
When these organics are emitted into
the atmosphere, they can exist in either
the gas or aerosol (solid/liquid) phase.
Under ideal conditions, the compounds
will eventually vaporize and reside
solely in the gas phase until the partial
pressure of the compound exceeds the
vapor pressure of the pure compound.
Under atmospheric conditions how-
ever, LVOs are phase distributed not
only by the pure substance's thermody-
namic properties, but also by the ad-
sorption and retention properties of the
myraid different surfaces which exist in
the atmosphere. These include both
water droplets and atmospheric aero-
sols. Most of these surfaces are, of
themselves, extremely complex and
poorly understood.
To deal with this problem, this study
was conducted in three parts. First, a
comprehensive review was made of the
existing literature on LVO. This review
covered both an evaluation of existing
-------
data on atmospheric LVOs, and the
measurement techniques used to col-
lect the data. Following this, a selection
was made of the n-alkane compounds
for the study to focus upon. These com-
pounds were selected because they rep-
resent a uniform compound structure
type covering a wide range of carbon
numbers.
In the second part of the study, an
effort was made to develop an "esti-
mator" for the phase distribution of
LVOs in the atmosphere. While exact
theoretical calculations of LVO adsorp-
tion on ambient surfaces are nearly
impossible, estimation based on the
phenomenology is possible. This semi-
empirical calculation has been applied
ttrttre n-atkartes with some success.
The third part of the study was con-
ducted concurrently with the other two.
In this portion of the study an attempt
was made to develop and test an im-
proved and reliable collection and anal-
ysis technique for determination of the
phase distribution of low volatility or-
ganics in ambient urban air. Each por-
tion of the study is discussed separately
in this report.
Conclusions
Based on the atmospheric measure-
ments of other investigators, n-alkanes
in the atmosphere are predominantly in
the vapor phase for carbon numbers
less than 18. This varies seasonally
somewhat. In the summer, atmospheric
n-alkanes up to carbon number 20 are
predominantly in the vapor phase.
The assertions in the previous para-
graph are supported by application of
the simplification of the BET theory of
adsorption. When care is taken to obtain
accurate estimates of the parameters
used in the BET theory, almost exact
correspondence with atmospheric
measurements is achieved. The critical
parameters used in estimating atmos-
pheric aerosol/vapor phase distribu-
tions are the mean temperature for the
period of interest and the atmospheric
aerosol surface concentration. The for-
mer is season-dependent while the lat-
ter is, for the most part, dependent on
the locale where the LVO aerosol/vapor
is dispersed in the atmosphere.
As a sample collection system, the
SRI sampling system performs as de-
signed. The impactor inlet provides a
clean preselection cut of the sample
aerosol fraction at 10 (xm, and the de-
nuder adsorbs the n-alkane test com-
pounds vapor while passing aerosol
with less than 2 percent loss. The ad-
sorption of compounds from the atmos-
phere, however, is not selective so that
when an attempt is made to recover the
adsorbed sample by thermal desorption!
of the denuder, extraneous compounds
(as evidenced by gas chromatographic
analysis) completely obscure the com-
pounds by interest. This occurs even
when high concentrations of test vapor
have been introduced into the atmos-
pheric sample. The extraneous com-
pounds may also cause changes in the
elution times so that transferring a
specific compound peak to a mass spec-
trometer on the basis of elution time is
doubtful. Limiting sample collection
time or dilution of the desorbed vapor
sample does not improve the sample
analysis conditions sufficiently to war-
rant using the denuder-derived sample
to analyze for LVO in the atmosphere.
Further improvements in the adhesive
material and additional tests are re-
quired before this system can be satis-
factorily employed in field studies.
Bruce K. Cantrell, Louis J. Salas, Warren B. Johnsoh, and James C. Harper
are with SRI International, Menlo Park, NC 94025.
Robert R. Arms is the EPA Project Officer (see below).
The complete report, entitled "Phase Distributions of Low Volatility Organics
in Ambient Air," (Order No. PB 87-129 540/AS; Cost: $13.95, 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 Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Onter for Environment,)! Research
Information
C'iicinn
------- |