SEPA
United States
Environmental Protection
Agency
Environmental Sciences Researe
Laboratory "/ _ -,
Research Triangle Park NC 27711 /1v !
Research and Development
EPA-600/S3-81-040 Dec. 1981
Project Summary
Modeling of
Oxidation in Smog
David F. Miller
Smog chamber experiments were
conducted to investigate the kinetics
of the free radical reactions of SOz in
smog and the transformation of SO2
to sulfate under simulated urban and
rural atmospheric conditions. Rate
constants were derived for three re-
actions: SO2 + HO + M - SULFATE
(60); SOz + HO2 - SULFATE (61); and
SO2 + CH3O2 - SULFATE (64); where
keo = 1600, k6i < 0.2, and k64 < 0.8
ppm'1 min 1, respectively. Oxidation
of SO2 by the unstable intermediate
HO led exclusively to particulate
sulfate. However, even under favorable
NOX conditions, particulate nitrate
was not a product. Hydrocarbon
mixtures typical of urban environments
promoted SO2 oxidation faster than
hydrocarbons selected to represent
rural conditions. Maximum SO2 oxida-
tion rates occurred during periods of
peak NO2 formation. Oxidation rates
of SOz in aged smog were 25 to 35%
of the maximum rates.
Model calculations indicated that
the SOz + HO + M - SULFATE reaction
dominated sulfate production from
SOz in polluted air. The model also
indicated that SOz oxidation is heavily
influenced by solar radiation intensity
and pollutant conditions. For clear-sky
and a variety of HC-NOX conditions,
maximum SO2 conversion rates ranged
from 3.7 to 7.4%/h.
Models used to simulate free radical
reactions in power plant plumes
showed that SOz oxidation is strongly
dependent on plume dispersion rates,
as well as on the same parameters
governing SOz oxidation in ambient
air. Although exceptions do occur for
certain HC-NO* conditions, SOz oxida-
tion rates in plumes are generally
bound by the ambient rates.
This Project Summary was devel-
oped by EPA's EnviornmentalSciences
Research Laboratory, Research Tri-
angle 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).
Introduction
The widespread distribution of sulfate
aerosols in the atmosphere and their
associated effects on visibility and
precipitation pH (acid rain) are serious
environmental concerns in the United
States. The Environmental Sciences
Research Laboratory of the U.S. Envi-
ronmental Protection Agency is working
to formulate strategies for the effective
control of sulfates. One step toward
achieving such control is the develop-
ment of models to predict regional
production of sulfate by SOz sources.
The study summarized here was con-
ducted by Battelle Columbus Laborato-
ries to broaden the information base
available on SOz reactions and trans-
formations and to provide the Environ-
mental Sciences Research Laboratory
with the predictive models necessary
for sulfate control.
Oxidation of SOz in the atmosphere is
considered the primary source of sulfate
particulates. The chemical and micro-
physical processes involved in the gas-
to-particulate transformations of oxida-
tion, however, are highly complex.
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Despite numerous research efforts to
define these processes more clearly,
many essential details pertaining to
reaction rates and mechanisms are
missing. To date, kinetic and mechanistic
studies of the elementary gas phase
reactions of SO2 in air have yielded only
that all known reactions involving
electronically excited states of S02 are
insignificantly slow in the troposphere
and that HO, H02, and R02 are the only
tropospheric species capable of oxidizing
S02 at substantial rates.
Numerous studies have presented
evidence that submicron-radii sulfates
are predominantly (NH4)xH2-xSO« with
associated water. However, although
microphysical aspects of H2S04 particle
formation are well understood, much
uncertainty surrounds defining new
particle nucleation. Some investigators
have concluded that SO2 oxidation in
the gas phase is the rate limiting step for
the formulation and growth of secondary
H2S04 particles in the atmosphere. One
objective of this study was to examine
these rate limiting reactions.
Finally, based on the new findings
and on additional smog chamber data
obtained with rural and urban hydro-
carbon systems, a photochemical smog
model was applied to simulate gas
phase S02 oxidation rates for various
atmospheric conditions.
Procedure
Experiments to investigate the kinet-
ics of the free radical reactions of SOz in
smog and to obtain data on SO2
transformation to sulfate under various
atmospheric conditions were conducted
in Battelle's 17.4 m3 smog chamber.
Rate constants were derived for the
following reactions: S02 + HO + M —
SULFATE (60); S02 + HO2 - SULFATE
(61); and SO2 + CH3O2 - SULFATE (64).
Pseudo second-order rate constants
(ppnrf1 min'1) were keo=1600, k6i<0.2,
and k64 < 0.8. The values for kei and k64
were only upper limit estimates; the
rates pertained to the overall conversion
of SOz to paniculate sulfate.
Although the mechanisms of S02
conversion were not investigated, some
work was done to determine if NO
and/or N02 form stable aerosol products
with the intermediates created after HO
addition to SO2.
Methods for Kinetic Studies
The smog chamber facility has a
surfacervolume ratio of 0.8 m"1; the
surface is polished aluminum (—80%)
and FEP Teflon™ (—20%). Irradiation
was provided by a bank of 95 fluorescent
blacklamps and 15 fluorescent sun-
lamps. Combined intensity of the lamps
yielded a N02 photodissociation rate of
approximately 0.14 min"1.
Background air for the chamber was
passed through a purification system for
dehumidification and filtering. S02 and
N02 concentrations were usually below
the detection levels of 1 ppb and 2 ppb,
respectively. Background NO concen-
trations ranged from 1 to 2 ppb.
Background total hydrocarbon ranged
between 1.9 and 2.5 ppmC (of which
most was CH4). Gas chromatographic
analyses of background air showed no
detectable olefins or aromatics in the
C2-C9 range when nonmethane hydro-
carbon was 0.1 to 0.3 ppmC. When
relatively small concentrations of
nitrous acid (a source of HO radicals and
NOX) were added to the background air
under high molecular weight hydro-
carbon contamination, irradiation re-
sulted in peak 03 concentrations in the 8
to 12 ppb range.
The reactivity of this amount of
hydrocarbon contamination in terms of
03 production corresponds to the
reactivity of about 4 to 6 ppm CO. Thus,
intentional addition of 4.5 ppmC hydro-
carbons and/or large quantities of CH4
and CO clearly overwhelmed any effects
due to background HC contaminants. In
addition, intentional introduction of
nitrous acid into the air for most of the
experiments was thought to overcome
any spurious surface contaminant
effects which might have led to the
generation of free radicals at the outset
of irradiation.
Gas phase analyses included mea-
surement of PAN, Ci-Ca hydrocarbons,
CO, 03, H2O2, HNO2, N02, NOX, NO and
SO2. Initial measurements of SO2 were
taken with a Meloy 285 flame photo-
meter. Upon irradiation, the instrument
was used to monitor sulfate aerosol.
The photometer was intermittently
operated in the total sulfur mode (S02
plus sulfate) to check on the S02
concentration.
Gas calibrations were performed
routinely. The chemiluminescense
analyzer used to monitor NO, HNO2, and
N02 was spanned before almost every
experiment. In situ 03 titrations were
conducted in the chamber to check on
the span of the 03 analyzer and the
efficiency of the NOX converters. After
an experiment (lamps off), excess NO
was injected into the chamber to more
than titrate the remaining 03.
Condensed phase analysis conce
trated on the measurement of sulf<
aerosol. The primary method for mo
toring sulfate was based on continue
sampling by a flame photometric detc
tor (FPD). Checks were made on tl
method by taking simultaneous lar
volume collections on quartz filters a
comparing the results. Using tl
tremendous difference in diffusiv
between SO2 and H2S04 aerosols,
diffusion-denuder tube was designed
remove virtually all of the S02 in the i
sample entering the FPD, while allowi:
most of the paniculate sulfur to pass
the detector. Optimization betwe<
maximum S02 removal and minimu
H2SO4 loss was determined expe
mentally by generating independent
known concentrations of S02 a:
H2SO4 aerosol in the smog chambc
The optimization work resulted in
sample flow of 200 ml/min through
35 cm lead acetate diffusion tube, wi
>99.5% removal of S02 and 35% loss
H2S04 aerosol.
Kinetic Data
The objective of the first portion of thi
project was to investigate the kinetic
(and mechanisms where feasible) c
S02 oxidation by free radicals; nameh
HO, H02, and CH302. The approac
taken to study these reactions was t
generate a variety of distribution:
between HO, H02, and CH3O2 in a smo
chamber containing S02. This wor
began with a series of smog chambe
experiments using an inorganic smo
system (HNO2/NOX/CO/SO2). By se
lecting the initial concentrations c
system components, it was possible t
affect a range of HO:HO2 ratios ani
thereby observe the relative rates of SO
oxidation attributable to the respectivi
radicals.
To obtain estimates of the free radica
concentrations, and hence to calculati
absolute rate constants, the experi
mental data were fitted with a chemica
kinetics model. Throughout the study
the S02 oxidation rates related to the
subject radicals were determined solely
in terms of sulfate aerosol product. Am
sulfur products remaining in the gas
phase were not determined. As such
the rate constants obtained represen
rate limiting kinetics for conversion o
SO2 to sulfate and not the upper limi
rates for S02 removal. In general, gooc
material balances were observed be
tween gas-phase and condensed-phase
sulfur, but once the reactions began U
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was not certain that the gas-phase
sulfur was exclusively SO2.
In the inorganic smog system, NO,
(i.e., HNOZ, NO, and N02) governed the
free radical concentrations because
their reaction rates with the radicals
were typically orders of magnitude
larger than those of the corresponding
reactions with S02. Thus, before
attempting to interpret experimental
data pertaining to S02 oxidation, the
NOx chemistry as it occurred in the
smog chamber had to be investigated.
Substantial concentrations of HN02
were used as a radical source in many of
the experiments.
Three initial experiments were con-
ducted to determine the reaction rates
for HNOz under experimental conditions
and to account for the large NO* losses
observed in most experiments. The first
experiment showed that the NO con-
centration was largely a function of
HNOz's photolysis rate and the HO + NO
recombination rate. Material balances
between the model and experimental
data were reasonably good, although
the model slightly underestimated NO,
losses.
The next experiment, conducted to
investigate further the smog chamber
reactions involving HNOz and the
formation of HN03, investigated two
routes for HNOs formation in the gas
phase: HN02 + 03 — HNOa+Ozand N205
+ H2O — 2HNOs. The results suggested
that neither reaction produced appre-
ciable amounts. The experiments also
provided no conclusive data on the
HNOa photolysis rate in the smog
chamber. However the choice of k=0.28
x I0~1min~1 seemed to fit these and
other experimental data as well as
alternative values.
The last of these three experiments
attempted to measure the reactions
involving N20s formation. Thermal
decomposition of N2O5 apparently
limited its concentrations in the tropo-
sphere. The slow rate found for homo-
genous conversion to HNO3 coupled to
the strong temperature dependence for
N20S decomposition suggested that
Nz05 persists for substantial periods in
the colder regions of the lower atmos-
phere and that clouds and perhaps
moist aerosols may be the dominant
sink for N205.
A series of experiments was also
conducted in the inorganic smog system
to study the conversion of SO2 to sulfate
via hydroxyl radicals. SOz was added to
air mixtures of HN02 and NO, in the
chamber. Sulfate aerosol formation was
observed immediately after the lamps
were turned on, and the rate of sulfate
formation was generally found to
maximize early in the irradiation period
and then gradually diminish.
The eight experiments were simulated
by kinetic modeling in order to estimate
the HO profiles and assess the overall
conversion rate for SO2, as initiated by
HO radicals.
Some investigators had suggested
that NO, could have various roles in the
transformation process and even be-
come incorporated in the condensed
phase as the reactions reached comple-
tion. To investigate these hypotheses,
various amounts of NO or NO2 were
added to some of the mixtures, in
addition to the amounts produced while
charging HN02 to the chamber.
The model gave satisfactory fits to the
experimental data, with only one
experiment in the series excepted. The
model tended to underestimate NO2
conversion and sulfate aerosol concen-
trations, particularly during the latter
half of the experimental periods. The
discrepancies rarely exceeded 20% of
the experimental values, however.
The data supported a rate constant of
1.6 x 103 ppm min~1 for the conversion
of S02 to sulfate aerosol, as initiated by
the HO radical.
The last series of experiments in the
inorganic smog system was conducted
to determine the kinetics of SO2
oxidation by hydroperoxy radicals. To
investigate the reactions in the smog
chamber, relatively large concentrations
of CO were added to mixtures of
NO,/HN02/S02. CO was found to give a
negative interference to the analyses of
SO2 and sulfate aerosols with the FPD.
This interference severely limited the
range of useful CO concentration.
Based on the data from this group of
experiments, the selection of an upper
limit for the S02 + H02 - SULFATE
reaction was arbitrary. Thus for subse-
quent modeling, 0.2 ppm"1min"1 was
chosen as a certain upper limit, although
the true rate is likely to be much lower.
In the organic smog system, experi-
menters investigated the methylperoxy-
S02 reaction beginning with the same
approach used for the H02-SO2 kinetics
studies. The approach consisted of
irradiating high concentrations of CH4
with controlled HNO2/NO,/S02 mix-
tures in the smog chamber. The reaction
of principal concern was CH302 + S02 —
CH30 + SULFATE.
Two initial experiments were under-
taken to determine the overall rate of
the above reaction based on sulfate
aerosol formation in an atmosphere of
moderate humidity. Methane was used
as the methylperoxy source in the
experiments. Although results were
inconclusive, an upper limit estimate of
0.8 pprrT1min~1 was obtained for the
conversion of S02 to sulfate via the
methylperoxy radical. The mechanism
of the conversion was not investigated.
Because of the inconclusive results
obtained with the CH4/HN02/NO,/SO2
smog system, additional experiments
were conducted using azomethane as
the methylperoxy source. However,
these experiments too were quite
limited.
Role of /VOx in SOz Oxidation
Via HO Radicals
Experiments conducted with the
HN02/NO,/SOZ smog system served to
investigate the possibility for paniculate
nitrate formation. At the conclusion of
an experiment, sample air was drawn
through acid washed and neutralized
quartz filters and analyzed for N03~,
S04*, and NH4+. To avoid HNOz or HNO3
volatilization losses, filter samples were
immediately halved; one-half was
stored in a desiccatory containing NH3,
and the other was refrigerated without
treatment.
The analytical results from nine
experiments showed no substantial
differences in either S04= or N03" for
the NH3 treated samples versus the
untreated samples. Although ammoni-
ated samples contained somewhat
higher NH4+ concentrations, nearly all
samples were deficient in NH3 when
compared to the expected stoichiometry
for (NH4)aS04 and NH4NO3.
In every case, the paniculate nitrate
concentrations were very low compared
to the sulfate concentrations. The
highest paniculate nitrate concentra-
tions were recorded for the two experi-
ments without SO2. Thus a substantial
amount of nitrate was not incorporated
in the particulate phase under the
conditions of these experiments. While
the results do not rule out the possibility
that HOS02O interacts with N02 to
produce H2S04 and HMOs, they do
indicate that nitrate (if formed via this
route) is not maintained in the aerosol
phase, even under the presumably
stabilizing influence of NH3.
Oxidation in Simulated
Urban and Rural Atmospheres
In the second portion of this study, a
series of smog chamber experiments
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was conducted to investigate some of
the factors affecting S02 oxidation
under simulated atmospheric conditions
and to develop data needed to evaluate
photochemical models of S02 oxidation
in polluted air.
Experiments were conducted using
hydrocarbon mixtures selected to
represent urban and rural conditions.
For each experiment, the initial non-
methane hydrocarbon concentration
was about 4.5 ppmC, and the initial NOX
concentration was 0.3 ppm. Apart from
the control experiments where no S02
was introduced, 0.6 ppm S02 was
injected into the chamber at either 1 or 5
h intervals after the irradiation began.
Replicate experiments were also con-
ducted in which 0.15 ppm NO was
injected, together with 0.6 ppm SO2.
This approach was designed to simulate
conditions in which S02 is emitted into
polluted air from point sources. The
experimenters attempted to determine
whether or not S02 oxidation rates
depended substantially on the interval
of the photochemical smog period, and
to what extent the presence of NO (NO is
generally emitted along with S02)
affected the oxidation rates.
Analytical methods for determining
SO2 and sulfate aerosols were identical
to those used in the kinetic experiments.
For this experimental series, two Meloy
sulfur analyzers were used to monitor
S02 and particulate sulfate simulta-
neously.
During the period of these experiments,
Battelle's large smog chamber was
lined with 5 mil FEP Teflon™ filter. The
light intensity corresponding to k was
0.17 min"1. Gas chromatographic anal-
yses for hydrocarbons were obtained
initially and at irradiation intervals of 1,
3, 5, and 7 h. PAN concentrations were
obtained hourly.
For both urban and rural hydrocarbon
mixtures, the rates of SO2 oxidation
were greatest near the periods of peak
N02 formation, and gradually diminished
thereafter. After 24 h of irradiation, the
rate of S02 oxidation was about one-
fourth as large as the maximum rate
observed with the urban hydrocarbon
mix: for the rural hydrocarbon mix, the
rate of S02 oxidation at 24 h was about
one-third the maximum rate.
The addition of NO with S02 (at
SO2:NO ratios of 4:1) at different
intervals of the irradiation period had
mixed results. When added at 5 h, NO
had small positive effects on the rates of
S02 oxidation, but when added at 1 h,
NO reduced the rates of S02 oxidation.
Kinetic Model Application
One objective of this portion of the
study was to determine if a relatively
simple model of atmospheric chemistry
could suffice to represent S02 oxidation
in polluted air. The smog chamber data
previously presented and some field
data from the Midwest Interstate Sulfur
Transformation and Transport (MISTT)
project provided the basis for applying
the model.
In most cases, the predictions for S02
conversion to sulfate were reasonably
good; within 20%forthe inorganicsmog
systems and within 25% for the HC-NO*
systems. The model tended to under-
estimate S02 conversion, and the
hydrocarbon systems when the model
estimates were only one-half the actual
rates. For the smog conditions that were
modeled, it was estimated that at least
75% and sometimes greater than 90%
of the S02 oxidation was initiated by
reactions of S02 with HO radicals.
Although several shortcomings of the
lumped model were found during its
application to these data, it was still of
interest to adapt the model to estimate
diurnal patterns of S02 oxidation in
comparison with field data. The objective
of this exercise was to estimate the rate
of S02 oxidation for power plant plumes
resident in polluted atmospheres, as a
function of solar radiation, plume
emission periods, and plume dispersion
rates.
According to the modeling results, the
production of HO and the oxidation of
SO2 depended strongly on solar radia-
tion intensity. Reduced radiation resulted
in disproportionately lower rates of S02
oxidation. For reduced radiation condi-
tions, the maximum rates of SO2
oxidation were predicted to occur near
mid-afternoon, while for clear sky
conditions, the maximum oxidation
rates occurred between 11:00 a.m. and
noon. The rates of S02 oxidation were
shown to depend on ambient HC and
NO, conditions. For the range of
conditions modeled, the maximum rate
of SO2 oxidation (clear sky) varied from
3.7 to 7.4%/h.
Plume dispersion rates affected the
induction period of NO and S02 oxida-
tion. Variations in solar radiation
intensity and thus the diurnal period of
the power plant emissions strongly
affected S02 oxidation rates. For well-
dispersed plumes and clear sky condi-
tions, maximum S02 oxidation rate
(ranging from 2.7 to 6.9%/h) wen
estimated to occur during the noon t
early afternoon periods. The patterns c
S02 oxidation predicted by the mode
agreed well with the data for thi
Labadie plume as reported in the MIST
project.
For relatively low HC:NOX ratios fo
ambient air, SO2 oxidation rates com
puted for plumes were bound by thi
ambient rates of S02 oxidation. At higl
HC:NOX ratios, the rates of SO2oxidatioi
in plumes were predicted to exceed thi
rates in ambient air. Under sucl
conditions, the model also predicted ai
excess (or bulge) in the plume 0
concentrations. These features havi
been observed in actual plume studies
Conclusions and
Recommendations
Regarding reaction 60 (S02 + OH + M
— SULFATE), it appears that all of the
S02 oxidized by this route results in the
formation of sulfate particles. According
to the analyses for particulate nitrate
they did not form stable aerosol products
with the intermediates created after HO
addition to SO2.
In the course of the experiment with
inorganic smog systems (HN02/NOX/
S02), additional kinetic information was
obtained, particularly as it pertained to
the experimental system. An upper limit
rate of 3 x 10~7 ppm"1min"1 was derived
for the homogeneous reaction between
dinitrogen pentoxide and water vapor to
yield nitric acid (N205 + H2O — HN03). It
was also determined that ozone and
nitrous acid do not react at a significant
rate to produce nitric acid. No hetero-
geneous reaction between S02 and
nitrous acid in the presence of sulfuric
acid aerosols could be detected.
Although the average reactivity for
the urban and rural hydrocarbon mix-
tures, based on rate constants ol
individual hydrocarbons with HO radicals,
was nearly equal, there were marked
differences in several important smog
parameters. With the urban hydro-
carbon mixture, the rates of NO oxidation
and xylene disappearance and the
maximum rates of SO2 oxidation were
about twice as great as for the rural
hydrocarbon mixture. The formation of
PAN and other products of N02 oxidation
were also greater for the experiments
with urban hydrocarbon.
Since SO2 oxidation is dominated by
the reaction with hydroxyl radicals, and
since this initial reaction is the rate-.
4
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limiting step in S02's conversion to
sulfate, it follows that relatively simple
kinetic models which adequately de-
scribe the variations in atmospheric HO
should suffice to estimate gas phase
SO2 transformation rates.
Experimental and theoretical simula-
tions of urban and rural atmospheres
showed that the rates of S02 oxidation
are strongly coupled to solar radiation
intensity and pollution conditions.
Although high concentrations of NO
generally suppressed or delayed HO
production and S02 oxidation, N02
inhibited SOz oxidation by scavenging
HO. Aldehydes, CO, and possibly some
hydrocarbons tended to inhibit S02
oxidation by converting HO into products
which were less efficient in oxidizing
S02. In general, reactive hydrocarbons
served to promote S02 oxidation, and
the stoichiometry or net flux of HO
affected by hydrocarbon degradation
appeared to be key to the acceleration of
SOz oxidation in polluted air.
SOz oxidation rates in moderately
polluted air or in well-aged air that was
once polluted were found for the most
part to be 60 to 80% lower than the rates
in freshly polluted air.
Modeling results indicated that S02
oxidation rates in power plant plumes
are highly dependent on plume disper-
sion rates and are coupled to the same
parameters governing SO2 oxidation in
the ambient air. For well-dispersed
plumes, most rapid SOz conversion was
predicted to occur from noon through
early afternoon. The accompaniment of
NO in plumes generally suppressed HO
and the rate of SOz oxidation. However,
under certain circumstances where
high concentrations of reactive HC were
mixed into a rapidly-dispersing plume,
the rate of SO2 oxidation was expected
to exceed the oxidation rate in ambient
air. Under such circumstances, an
excess of ozone was also predicted in
the plume.
This study pointed out the need for
further study in the area of sulfate
aerosol formation. In particular, since
the overall conversion rate for the SOz •*•
H02 — SULFATE reaction was consider-
ably less in this study than the literature
value, it is recommended that additional
high-pressure measurements of the
reaction be made. Any such effort
should include the determination of
reaction products.
David F. Miller is with Battelle-Columbus Laboratories, Columbus, OH 43201.
Joseph J. Bufalini is the EPA Project Officer (see below).
The complete report, entitled "Modeling of SO2 Oxidation in Smog," (Order No.
PB 82-101 932; Cost: $13.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telelphone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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