United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/015 Apr. 1985
Project Summary
Chemical Transformation
Modules for Eulerian Acid
Deposition Models: Volume I.
The Gas-Phase Chemistry
J. Alistair Kerr and Jack G. Calvert
The study focuses on the review
and evaluation of mechanistic and
kinetic data for the gas-phase reac-
tions that lead to the production of
acidic substances in the environment.
A master mechanism was designed
that treats the chemistry of nitrogen
oxides, sulfur dioxide, ozone,
hydrogen peroxide, ammonia, the
simple amines (methyl, dimethyl,
trimethyl, and ethyl amines), chlorine,
hydrogen sulfide, dimethyl sulfide, the
hydrocarbons (methane, ethane, pro-
pane, butane, 2,3-dimethylbutane, the
C,-C, alkanes, ethylene, propylene,
trans-2-butene, isobutene, benzene,
toluene, m-xylene, isoprene, alpha-
pinene), and the variety of oxidation
products of these species including
the transient free radicals, aldehydes,
ketones, hydroperoxides and other
molecules. Reaction mechanisms and
rate constants were identified for
those chemical transformations for
which major uncertainties remain and
for which additional experimental and
theoretical work is needed.
This Project Summary was
developed by EPA's Atmospheric
Sciences Research Laboratory,
Research Triangle Park, NC, to an-
nounce 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
Early signs of ecological damage have
been observed in certain sensitive areas of
the world that are deficient in soils with a
good acid buffering capacity and that are
recipients of a large input of acids
through "acid rain" and/or dry deposi-
tion. Scientists throughout the world are
actively working to assess the extent of
damage that has occurred and can be ex-
pected to occur in years ahead. Govern-
ment leaders of many nations are
attempting to evaluate alternative control
strategies for acid deposition that can
alleviate the existing and potential future
problems.
The understanding of the nature and
importance of the various chemical
pathways to acid generation within the
troposphere is one of the several pre-
requisites to the development of scientifi-
cally sound strategy for the control of
acid deposition. The present study was in-
itiated as part of the research effort at the
National Center for Atmospheric Research
to develop a regional, Eulerian acid
deposition model.
In most existing acid deposition models
that have been employed in control
strategy development, no attempt has
been made to incorporate the many com-
plex chemical processes that control acid
generation. Existing models often involve
the use of only fixed rates of transforma-
tion of S02 and NOX to sulfuric acid and
to nitric acid, respectively. Uncertainties
in the source-receptor relationships that
these models provide arise from many
factors; among others, they are very sen-
sitive to the rates of chemical transforma-
tion of the precursors to the acids. This
sensitivity arises largely from the fact that
the precursors of the acids and the acids
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themselves are not removed from the at-
mosphere with equal facility. Once sulfur
dioxide is oxidized to sulfuric acid aerosol,
it is dry deposited much less rapidly than
is S02. If aerosols composed of sulfuric
acid and its salts (ammonium bisulfate,
ammonium sulfate, etc.) are incorporated
into precipitating clouds, then the deposi-
tion of these species can be faster than
that of gaseous S02. On the other hand,
the nitric acid formed in the troposphere
is much more rapidly deposited on the
surface of the earth than are its precur-
sors, NO and N02. Thus, the amount of
acid and the chemical nature of the acids
deposited at sites many kilometers from
the source of the precursors are sensitive
functions of the rates with which the
acids are formed as well as the rates with
which these acids are transported by the
motion of the air mass in which they are
contained. It follows that the development
of chemical modules for use in acid
deposition models should be based upon
chemical mechanisms that describe acid
generation in terms of known rate laws
and chemical theory.
In creating chemical mechanisms for
modeling acid generation in the
troposphere, there is no satisfactory
analogue to the methods that were used
to develop highly simplified, oxidant-
generating mechanisms. These
mechanisms depended heavily on the
simulations of ozone generation in "smog
chamber" experiments. The very products
that are most important for models of
acid development (sulfuric acid, nitric
acid, hydrogen peroxide, etc.) are very
short-lived in most chambers because of
their removal at the walls of the chamber.
No knowledge of their rates of generation
or their concentrations in the free at-
mosphere can be derived from such ex-
periments. Any reaction scheme,
however, must eventually be tested or
verified through simulation of actual rate
data derived from both simple and com-
plex reaction mixtures in the laboratory
under conditions that simulate those of
the troposphere or, preferably, in direct
tropospheric measurements of the ap-
propriate reactants and products.
Although many aspects of the mechanism
can be tested through use of laboratory
and field data, verification of key aspects
of the mechanism is not possible from the
data available today.
In this work, a reasonably complete
gas-phase mechanism has been formu-
lated in an attempt to identify all of the
potentially significant acid precursors and
their chemistry. Simplification and
parameterization of the known chemistry
can be made following suitable sensitivity
tests on the complete chemical
mechanism.
Approach
As a starting point in our evaluation,
we compared the kinetic data (reaction
mechanisms and rate constants) for over
200 reactions that have been used by
various researchers to model the forma-
tion of photochemical smog. Based on
these comparisons, we formulated a set
of recommended rate constants for the
reactions. Although the majority of the
selected rate constants are in good accord
with those recommended by others, there
are some significant differences. Most of
these differences are in the following
areas: NOX chemistry where recent exten-
sive studies have been made; rate con-
stants for the reactions of organic peroxy
and alkoxy radicals; reactions of Criegee
intermediates; and the photochemistry of
the ketone, aldehyde, and other various
oxidation products of the hydrocarbons.
Some of the differences that exist in the
hydrocarbon oxidation mechanisms result
from differences in the interpretation of
the existing, very limited data, including
the effect of structure on the rate con-
stants for the reactions of the peroxy and
oxy radicals that result from the oxidation
of the various hydrocarbons.
Based on mechanistic considerations,
we formulated a more complete
mechanism that is designated as the gas-
phase "master mechanism" of acid
generation. This mechanism includes the
relevant inorganic chemistry involving NO,
N02, N03, N205, S02, H2S, O3, H2O2,
NH3, CI2, HON02, H2S04, MONO,
H02N02, CH3SCH3, CH3NH2, (CH3)2NH,
(CH3)3N, and C2H6NH2. The tropospheric
chemistry of several hydrocarbons is con-
sidered: methane, ethane, propane,
n-butane, isobutane, 2,3-dimethylbutane,
the C6-CB alkanes, ethylene, propylene,
trans-2-butene, isobutene, benzene,
toluene, m-xylene, isoprene, and alpha-
pinene. The chemistry of hydrocarbon ox-
idation products (aldehydes, ketones,
peroxides, etc.) is also considered in some
detail. The master mechanism includes
the chemical reactions that lead to the
development of the organic acids (formic
acid, acetic acid, etc.). Although the
master mechanism contains over one
thousand elementary reactions, these
reactions represent only a small fraction
of the total tropospheric chemistry.
In the next stage of the planned effort,
sensitivity studies will be made using the
master mechanism, and a simplified but
scientifically realistic reaction scheme will
be derived. Because of the immediate
need for a working first-phase chemical
transformation model, in separate parallel
studies at NCAR, a highly simplified gas-
phase and liquid-phase mechanism has
been created using a more empirical ap-
proach that was less time consuming and
consistent with the schedule for the
phase-one model development. Suitable
modification and replacement of elements
of the preliminary mechanism will be
made in accordance with the findings of
this more detailed mechanism study.
After suitable simplification of the gas-
phase chemistry mechanism has been
achieved, the resulting reaction scheme
will be coupled to a liquid-phase chemical
mechanism. A sensitivity study of this
mechanism is planned as well in order to
develop the necessary simplified, com-
bined gas-phase and liquid-phase
chemistry module that is to be used in the
final version of the Eulerian acid deposi-
tion model.
Results
A number of major uncertainties in the
gas-phase chemistry mechanism was
revealed during this study. Detailed sen-
sitivity studies, however, will be required
to determine which of these uncertainties
induce significant changes in the rates of
acid generation. Nevertheless, at this time
it is possible to make a qualitative assess-
ment of the master mechanism and to
identify the key reactions for which pre-
sent kinetic data are inadequate.
There are significant uncertainties that
remain in the chemistry of the nitrogen
compounds. N206 and N03 can be very
important sources of nitric acid during the
nighttime hours. Current literature shows
a large divergence among the different ex-
perimental estimates of the equilibrium
constant for the reaction, N2O5 — N03 +
N02. The calculated concentration of the
reactive N03 radical and the significance
of nitric acid formation are directly de-
pendent on the value chosen for this
equilibrium constant.
Several important mechanistic details
that bear directly or indirectly on the
generation of acids in the tropospheric ox-
idation of hydrocarbons, remain uncer-
tain. In the case of alkanes, the major
areas that require further study are: (a)
the extent of nitrate formation from the
reaction of peroxy alkyl radicals with nitric
oxide, and (b) the relative rates of alkoxy
radical decomposition, rearrangement,
and disproportionation. Knowledge of the
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mechanisms of tropospheric oxidation of
alkenes is incomplete as well. The areas
of greatest uncertainty include: (a) the
rates of the beta-hydroxy-alkoxy radical
decomposition and reaction with oxygen,
and (b) the rate constants for the reac-
tions of the Criegee intermediates with
NO, N02, H20, and SO2. The least well
understood of the hydrocarbon oxidation
mechanisms is that for the aromatic
hydrocarbons. The present chemistry is
not well established experimentally and re-
quires further study before confidence can
be placed in the mechanism and the in-
fluence of these species on HO, H02, and
R02 radical concentrations and acid pro-
duction. Product mass balances are also
poor for the oxidation of the natural
hydrocarbons. Further elucidation of the
chemistry of these systems will be
necessary to determine the influence of
these species on acid generation in the
troposphere.
One of the most neglected areas of
research is the chemistry of the carbonyl
compounds. Photochemical quantum
yield data for tropospheric conditions are
not now available for most of the car-
bonyl species that form in the at-
mosphere. Experimental estimates of both
the quantum efficiencies and the nature
v of the primary processes that generate
free radicals are needed. It is also impor-
tant to establish the extent of radical
generation from aldehydes and ketones.
These products are expected to have a
significant influence on the chemistry that
produces oxidants such as ozone and
hydrogen peroxide.
Organic hydroperoxides can oxidize
bisulfite ion readily to form sulfuric acid in
cloud water. For tropospheric conditions,
the most likely sources of these com-
pounds are the reaction between HO2
radicals and peroxy organic radicals. Only
two of the many hundreds of these reac-
tions that can occur in the troposphere
have been studied kinetically. The gas-
phase chemistry of the hydroperoxides is
equally poorly defined. Quantitative infor-
mation on both the photochemical pro-
cesses and the reactions of HO radicals
with the hydroperoxides are required.
The simplest of the peroxyacylnitrates
has been shown to be a good oxidant of
bisulfite ion in water solutions, and the
higher homologues are probably effective
as well. In addition, the decomposition of
these compounds to form acylperoxy
radicals and nitrogen dioxide can act as a
source of nighttime acid generation. Rate
data related to only two of the many
peroxyacylnitrates now exist. Further
studies are needed to determine the role
of these species in acid generation.
There are very few experimental data
that allow an adequate test of the com-
plex gas-phase mechanisms proposed to
explain acid generation in the
troposphere. The compounds that are
most important in any evaluation (sulfuric
acid, nitric acid, hydrogen peroxide,
organic peroxides, hydrocarbons and their
oxidation products) are usually not
simultaneously measured in simulated at-
mospheric studies or in field studies.
Such data are critical to any meaningful
test of a mechanism. New experimental
measurements of the important acids and
acid precursors are required in both
simulated and actual tropospheric air
masses in order to test current chemistry
mechanisms. The development of im-
aginative new methods for laboratory
studies (other than typical smog chamber
studies) is encouraged to minimize the
wall removal of acids and acid precursors.
Furthermore, it is recommended that field
studies include measurements of reactive
hydrocarbons, aldehydes, ketones,
organic acids, hydrogen peroxide, methyl
hydroperoxide, as well as the commonly
measured compounds, NOX, S02, 03,
H2S04, HIM03, and the salts of these
acids. Vertical profiles of the concentra-
tions should also be made at a variety of
locations in the eastern United States.
Without such measurements, acid deposi-
tion models have an undesirable flexibility
in the choice of initial reactant concentra-
tions, and it is impossible to judge the ac-
curacy of a mechanism from the limited
experimental data.
Conclusions
In this study we formulated a
reasonably complete set of gas-phase
reactions in an attempt to identify all of
the potentially significant acid-forming
processes. The chemistry module that is
to be used in our acid deposition model
must be highly simplified in order to con-
serve computer time and allow efficient
operation of the model. Because of this,
considerable simplification of the reaction
scheme presented in this study will be
needed before it is suitable for use in our
acid deposition model. In conducting this
simplification, however, we do not want
to sacrifice the ability of the model to
predict the rates of acid generation with
reasonable accuracy. We have concluded
that the only way one can test adequately
the scientific accuracy of any highly ab-
breviated chemical reaction scheme is to
start from a scientifically sound reaction
scheme that includes all relevant acid-
forming chemical processes. This is the
approach that was adopted in our study.
J. A. Kerr is with the University of Birmingham, Birmingham. England and J. G.
Calvert is with the National Center for Atmospheric Research, Boulder, CO,
Marcia C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Chemical Transformation Modules for Eulerian
Acid Deposition Models: Volume I. The Gas Phase Chemistry," (Order No. PB
85-173 714/AS; Cost: $22.00, 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 Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
* U S. GOVERNMENT PRINTING OFFICE: 1985-559-016/27033
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