United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/036 J u ne 1985
Project Summary
Chemical Transformation
Modules for Eulerian Acid
Deposition Models
Volume II. The Aqueous-Phase
Chemistry
Michael R. Hoffmann and Jack G. Calvert
This study was carried out as part of
the research effort of the National
Center for Atmospheric Research in the
development of a Eulerian acid deposi-
tion model. It focuses upon the review
and evaluation of mechanistic and ki-
netic data for aqueous-phase reactions
that lead to the production of acidic
substances in the environment. The in-
tent of this research is to provide a
framework that can be used to develop
a state-of-the-art aqueous-phase mech-
anism for use in the Regional Acid
Deposition Model under development
at NCAR. In the research effort de-
scribed in this report, 31 tables of kinetic •
data and 27 tables of thermodynamic
data were compiled and evaluated. A
set of recommended rate expressions
and kinetic data was then prepared for
the predominant aqueous-phase path-
ways leading to acid generation in the
troposphere.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search 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
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'
expected to occur in years ahead. Govern-
ment leaders of many nations are attempt-
ing to evaluate alternative control strate-
gies for acid deposition that can alleviate
the existing and potential future prob-
lems.
The understanding of the nature and
importance of the various chemical path-
ways to acid generation within the tropo-
sphere is one of the several prerequisites
to the development of scientifically sound
strategy for the control of acid deposition.
The present study was initiated 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
complex chemical processes that control
acid generation. Existing models often
involve the use of only fixed rates of
transformation of S02 and NO, to sulfuric
acid and to nitric acid, respectively.
Uncertainties in the source-receptor re-
lationships that these models provide
arise from many factors; among others,
they are very sensitive to the rates of
chemical transformation of the precursors
to the acids. This sensitivity arises largely
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from the fact that the precursors of the
acids and the acids themselves are not
removed from the atmosphere 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 deposition 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 precursors, NO and
NO2. 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 mechan-
isms that describe acid generation in
terms of known rate laws and chemical
theory. The intent of the effort described
in this Project Report was to review and
evaluate available mechanistic and kinet-
ic data for aqueous-phase reactions that
lead to acid generation in the troposphere.
The data compiled in this study will
provide a framework for developing a
chemical module for the NCAR regional
acid deposition model.
Approach
An extensive review of the literature
was undertaken to identify all possible
chemical reactions that might be relevant
to aqueous-phase cloud chemistry. Re-
actions were subdivided into reversible
and irreversible categories. Reversible
reactions were quantitatively described
in terms of the appropriate mass action
expressions, stoichiometric coefficients,
equilibrium constants, and reaction en-
thalpies. Irreversible reactions were de-
scribed likewise in terms of their stoichi-
ometries, empirical rate laws or, in some
special cases, fundamental rate expres-
sions, rate constants, and activation
parameters.
Based on this literature review, the
data were compiled into two sets of tables.
One set of tables contains kinetic data for
the reactions of interest; the other set
contains thermodynamic data for the
aqueous-phase reactions of relevance to
acid formation. After the tables were
compiled, the major pathways for
aqueous-phase acid formation were iden-
tified and the kinetic data for these
reactions were critically evaluated. As a
result of this evaluation, we prepared a
set of recommended rate expressions and
kinetic data for use in acid deposition
modeling.
Results
Thirty-one tables of kinetic data were
compiled in this study. Twenty-five of
these tables contain data for aqueous-
phase reactions involving free radicals.
The majority of these tables focus on
hydroxyl radicals as the principal oxidant.
The other tables focus on the kinetics of
S(IV) oxidation by 03, H202, 02 (catalyzed
by Fe+3 and Mn*2), organic species such
as PAN and nitrogen-containing species.
Kinetic data for the oxidation of N(lll) are
also presented.
Twenty-seven tables of thermodynamic
data were compiled. ' Special attention
was devoted to processes involving gas-
liquid, metal-ligand, acid-base, precipita-
tion-dissolution and aldehyde - S(IV)
adduct equilibria.
A set of recommended rate expressions
and kinetic data was prepared for the
predominant pathways leading to S(IV)
oxidation. These pathways include reac-
tion of S(IV) with H202, 03, 02 (Fe+3 and
Mn+2 catalyzed), OH, HONO, CH3OOH,
CH3C03H, PAN, H02, HCHO, and soot.
Special attention was devoted to a
critical evaluation of rate data for the
S(IV)-03 reaction. Kinetic data obtained
by five different investigators were ana-
lyzed. It was found that the data of most of
the investigators could be fit by a multi-
term rate law of the following form:
+ k2a2)[S(IV)](03]
Conclusions
In this study we formulated a reason-
ably complete set of aqueous-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
conserve computer time and allow effi-
cient 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 conduct-
ing this simplification, however, we do
not want to sacrifice the ability of the
model to predict the rates of acid genera-
tion with reasonable accuracy. We have
concluded that the only way one can test
adequately the scientific accuracy of any
highly abbreviated chemical reaction
scheme is scheme that includes all
relevant acid-forming chemical proces-
ses. This is the approach that we have
adopted in the study.
.. M. R. Hoffmann is with the California Institute of Technology, Pasadena. CA
91125; J. G. Calvert is with the Ntaional Center for Atmospheric Research,
Boulder, CO 80307.
Mar da C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Chemical Transformation Modules for Eulerian
A cidDeposition Models: Volume II. The A queous-Phase Chemistry." (Order No.
PB 85-198 653/AS; Cost: $16.00, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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: 1985-559416/27076
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