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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