EPA/600/A-94/195
HETEROGENEOUS DEGRADATION OF OXYGENATED INTERMEDIATES
E.0. Edney
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
1 INTRODUCTION
Although environmental chemistry studies have tended to focus on
homogeneous gas and aqueous phase processes, it has become increasingly
clear over the past decade that heterogeneous mechanisms play key roles
in many environmental processes among them acid deposition, formation of
respirable and visibility reducing particulates, and damage to crops,
vegetation, and materials. Mounting evidence also suggests that the
atmospheric fates of certain chemicals, once thought to be governed by
purely gas phase mechanisms, are affected by heterogeneous processes.
For example, heterogeneous reactions of halogenated species on polar
stratospheric clouds and sulfate aerosols can enhance stratospheric 0,
depletion. Now questions have even been raised concerning the possibility
that heterogeneous reactions of oxygenated intermediates influence oxidant
formation in the troposphere.
2 TROPOSPHERIC HETEROGENEOUS CHEMISTRY OF OXYGENATED INTERMEDIATES
In general, tropospheric oxygenated intermediates are formed from
reactions of volatile organic compounds with OH, 0,, and NO,. Examples
of such compounds include Oj and free radicals, carbonyl compounds, NO,
compounds, peroxides, alcohols, and organic acids. Each of these
oxygenates may undergo further gas phase reactions, but each can react
heterogeneously. Oxygenates can participate in nucleation processes that
form new particles. They can deposit on particulates, land and vegetated
surfaces, and structures. Furthermore, many of the oxygenates are water
soluble and are susceptible to absorption and reaction in tropospheric
aqueous media including cloudwater, fogwater, liquid precipitation, dew,
hygroscopic aerosols and large aquatic bodies including lakes, rivers,
streams, and oceans. Examples of oxidation products that have been
detected in the troposphere are shown in Table 1. The issue that
ultimately must be addressed is whether heterogeneous reactions of these
types of compounds significantly affect the chemistry of the troposphere.
The uptake of gas compounds into aqueous droplets has been addressed
by a number of atmospheric chemists."'" The uptake of a water soluble
reactive compound into an aqueous droplet of radius r and dimensionless
liquid water content L has been described by the following steps: (1)
transfer of the gas from the bulk gas phase to the droplet interface; (2)
transfer across the gas-liquid interface; (3) volatilization of dissolved
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gases; (4) dispersion of dissolved gases throughout the droplet; (5)
attainment of aqueous equilibria; and (6) aqueous-phase reactions.*
TABLE 1 Oxygenated Intermediates
Compound Class
Examples
Oj and Free Radicals
Saturated Carbonyls
Saturated Dicarbonyls
Unsaturated Carbonyls
Peroxides
Organic Acids
NO, Compounds
0,, OH, H02, NO,
HCHO, CHjCHO
CHOCHO, CHjCOCHO
CHj-CHCOCHj, CHj-CCHjCHO
HOOH, CHjOOH
HCOOH, CHjCOOH
NOj, N305, HNOj, CH,C(0)00N0,
It is convenient to discuss the overall uptake process in terms of a
number of time constants. rm, the gas phase mass transfer time, is
defined by
where rJ/3Dt is the gas phase diffusion time and 4r/3va is the interfacial
gas transfer time. D, is the gas phase diffusion coefficient; v is the
average gas phase molecular velocity; and a is the mass accommodation
coefficient. Other relevant time constants include rM the aqueous phase
diffusion time, r„ the aqueous phase reaction time, and rn the gas phase
reaction time. The gas-aqueous equilibrium time is defined by r„- - HRTr«,
where H is the Henrys' law constant, R is the universal gas constant, and
T is the* temperature.
The following system of coupled first order differential equations
describes the time evolutions of the gas phase number densities n and the
aqueous concentrations [A]
where N,» is Avogadro's number. The homogeneous chemical production rates
are represented by Q, and Q. and the corresponding destruction rates are
S, and S.. The constant f, is included to take into account, perhaps in a
crude way, that the aqueous media is present only some fraction of the
time.
In general, a numerical solution of these equations with f» - 1 and a
complete parameterization of the moisture cycle is required to accurately
determine the gas and aqueous phase concentrations. However, some
information on how time constants affect removal of gas phase oxygenates
can be obtained by solving the coupled equations for a compound that has
no homogeneous gas or aqueous phase sources, but undergoes first order
reactions in both phases. Assuming further that the dissolved gas
concentration is at steady state, the equations for the gas and aqueous
phase concentrations can be combined and rearranged to yield the following
loss equation
- rJ/3Dg + 4r/3va,
dn/dt - -(f.L/r«)(n - (A]H~/HRT) + Q, - S,
and
d(A]/dt - (f./r,)(n/lC - [Aj/HRT) + f.(Q. - S.)
dn/dt - - (l/r„ + l/f„)n,
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where the heterogenous removal time rh is
U - rc (1 + r),
with rc, the heterogeneous collision time, defined by
rc - T.t/fX.
The expression for rh takes certain limiting forms based on the relative
sizes of Tr. and r„. If rr, is much greater than r.,, then
t„ - rri/f»LHRT. Under these conditions, the equilibration process is
sufficiently rapid so that the dissolved gas remains in equilibrium with
its gas phase component and the heterogeneous loss is controlled by the
rate of reaction in the aqueous phase. At the other extreme, where r„
is much less than Th - re and the uptake is controlled by gas phase
mass transfer.
A simple numerical example comparing heterogeneous removal times of
hypothetical cloudwater and hygroscopic aerosols serves as a useful tool
for further clarifying the processes that control the uptake of gas phase
compounds to aqueous droplets. rh values were calculated for T - 285 °K
for a cloud droplet of radius 10 pm and liquid water content of 10'' that
was present 5X of the time and a persistent .10 urn aerosol with a liquid
water content of 10". The gas and aqueous phase diffusion coefficients
were 0.1 and 1 x 10's cmJ s'1, respectively. The Henrys' law constant was
1000 mol L"1 atm"1. The mass accommodation coefficient was 0.1 and the gas
phase molecular velocity was 2 x 10* cm s"1.
The calculated gas phase mass transfer times for the cloud droplet and
aerosol were 4.0 x 10"' and 7.0 x 10"* s, respectively. The equilibration
time for the cloudwater was 9.4 x 10"' s and the corresponding value for
the aerosol was 1,6 x 10"* s. As expected, the gas phase mass transfer and
equilibration times were much shorter for the aerosol than for the larger
cloud droplet. The effective collision times were similar; the value for
the aerosol was 700 s and the cloudwater value was 800 s. Figure 1 shows
the heterogeneous removal time as a function of the aqueous phase reaction
time.
Under some circumstances such heterogeneous reactions may be better
sinks for oxygenates than homogeneous gas phase reactions. For example,
gas phase reaction times for carbonyl compounds are on the order of days.
Uptake to the hypothetical cloudwater starts to compete with the loss rate
corresponding to rt| - 1 day when r„ is less than about 100 s. Published
modeling studies suggest that there are situations where bimolecular
reactions of dissolved oxygenates in cloudwater are fast enough to satisfy
this type of condition. The aqueous reaction of HCH0 with OH is one such
example.4 On the other hand, for the hypothetical aerosol with the much
lower water content, rtm must be less than about .10 s to begin to compete
with the gas phase reaction. This may require reactions in aerosols where
the dissolved gas hydrolyses very rapidly or possibly reacts with highly
concentrated nonvolatile species. Such aerosols might include those that
are highly acidic or alkaline or contain high levels of transition metals.
o
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10"
10"
10'°
t?
I 108
«
H 10«
104
10*
10-6 1(H 1CH 1 102 10" 106
x„ (sec)
FIGURE 1 Heterogeneous removal time as a function of aqueous reaction
time
3 LITERATURE RESULTS
3.1 Effect of Cloudwater -on Global Photochemistry
Lelieveld and Crutzen conducted a modeling study to assess the impact
of cloudwater on global photochemistry/4,1' The global gas phase
photochemistry was based on the CH*-C0-NQ,-HQ« system where HO, represents
both HO, and OH radicals. The aqueous reactions of the chloride,
bicarbonate, and aqueous formaldehyde, HO,, oxidant, and hydrogen peroxide
system made up the cloudwater chemistry model. Gas phase concentrations
were calculated for the northern and southern hemispheres and the equator,
for both winter and summer seasons. The calculations were performed for
a single cloudwater droplet radius of 10 ftm with liquid water contents
ranging from 2-4 x 10"'. A typical wetness cycle, consisted of 3 h with
clouds followed by 18 cloud free hours. Aerosol affects were not
included.
Numerical solution of the mass balance equations showed significant
coupling between the gas . and aqueous systems, with the presence of
cloudwater decreasing HOj, OH, HCHO, 0,, and NO, levels. A number of
multiphase reaction schemes that contributed to these decreases were
identified. HCHO, HOj, and to a far lesser extent 0S are water soluble.
Upon dissolution in cloudwater, HCHO forms the hydrated form of HCHO,
CHj(0H)j, and HOj will function as a weak acid, undergoing some dissociation
forming H* and. the reactive species Or. CHj(0H)j reacts with aqueous OH
radicals generating HCOOH, which along with its ionization product HCOO ,
can react with OH to form HOj and COj.
OH + CHj(OH)} + 0, ---> HCOOH + HOj + HjO
OH + HCOOH + 0, > COj + HOj + H,0
OH + HCOO" + 0, ---> CO, + HOj + OH
This aqueous reaction scheme conserves HO, but it is an overall HO, sink
because it removes gas phase HCHO that otherwise could photolyze to form
two HOj radicals.
XX
.cr
cr
Aerosol
Cloud Water
.cr
-------
HCHO + hv + 20j > 2H0j + CO
The aqueous reaction between Oj and 0," is an important component of the
coupling scheme because it is a sink for gas phase 03 and in effect cycles
HOj back to reactive OH.
0, + 0,- + HjO ---> OH + 20, + OH"
Dissolution of HOj also interferes with Oj production by separating the
highly soluble H02 radical from slightly soluble NO. This reduces the
conversion of NO to NO, by HO,. Irreversible absorptions of NO, radicals
and NjOs by cloudwater, important nighttime processes, remove reactive NO,
compounds from the gas phase.
The modeling results showed that the presence of cloudwater reduced
NO,, HCHO, and HO, concentrations by 20 to 30% in the summer and 65 to 80%
in the winter. On the average, for each 03 molecule consumed by the gas
phase reaction sequence
Oj + hi/ > 0(lD) + Oj
0('D) + H,0 ---> 20H
approximately 10 to 15 05 molecules were destroyed by aqueous reaction in
the winter and 1 to 2 in the summer. The model simulations also showed
that net 0) destruction in NO, poor regions increased by factors of 2 to
4. In NO, rich areas net 0S formation rates decreased about 40%.
3.2 NjOj and NO, Reactions in Cloudwater and Tropospheric Aerosol
Dentener and Crutzen conducted a modeling study to investigate the
impact of heterogeneous removal of reactive NO, compounds by uptake to
cloudwater and tropospheric aerosols on global photochemistry and
springtime smog formation in the northeastern United States.* The global
gas phase chemistry was again based on the CH» oxidation mechanism. Iso-
butane served as the surrogate compound for biogenic and anthropogenic
hydrocarbons for the smog simulations. Heterogeneous removal involving
cloudwater and aerosols was limited to that due to NA and NO,. Gas phase
mass transfer controlled uptake to aerosols and the cloudwater cycling
time controlled removal by cloudwater.
The modeling results showed that cloudwater and hygroscopic aerosols
decreased global NO, concentrations by 50% and 0, and OH concentrations
by 9%. The major removal process in the northern hemisphere was uptake
to hygroscopic sulfate aerosols. In the tropics and southern hemisphere,
where there are lower aerosol levels, liquid cloudwater served as an
additional sink. Furthermore, heterogeneous reactions may also affect
traditional smog formation. Heterogeneous reactions of NOj and NjOj on
sulfate aerosols reduced springtime 0> levels in the northeast United
States by 15%. However, this 0S reduction was not very sensitive to the
aerosol concentration, but rather was controlled mainly by the reaction
of NO, with Oj that forms NOj.
3.3 Sea Salt Aerosol Reactions in the Marine Boundary Layer
Chameides and Stelson investigated heterogeneous reactions in sea salt
aerosols in the marine boundary layer.' The purpose of the study was (1)
to try to explain the 3-4 neq m"1 of non-sea salt S0»5' found in sea salt
aerosols in the marine boundary layer and (2) to assess whether chloride
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deficits found in these aerosols could be due to reactions of chloride
with aqueous 0, or OH that form volatile Cl2. The size of the Cl deficit
is sufficient to produce Cl2 levels that could alter atmospheric lifetimes
of gas phase compounds.
The calculations were conducted for a constant composition clean air
photochemical mixture in the presence of a distribution of alkaline sea
salt aerosols. Aqueous reactions of the chloride, bromide, sulfur oxide,
bicarbonate and aqueous HO,, oxidant, and hydrogen peroxide system made up
the aerosol chemistry model. Model simulations were conducted for a 1.2
im aerosol with a liquid water content of 1.5 x 10*10 at 961 relative
humidity and for a 0.75 nm aerosol with a liquid water content of 3.7 x
10" at 80% relative humidity. The non-ideal nature of the aerosols was
taken into account by including activity coefficients.
The proposed reaction schemes did not explain the chloride deficits.
Reaction of dissolved 0, with aerosol chloride is too slow to affect the
budgets of chloride or 0j. Reactions of Cl" ions with aqueous OH can form
Cl radicals. However, the Cl radicals will react with Cl" to form Cl2" that
then reacts mainly with 0/ and H0j to form two Cl". The overall reaction
simply conserves the chloride ion concentration. On the other hand, S02
oxidation by Oj can generate about 0.75 neq m"3 of non-sea salt SO*1". This
removes 1-4 x 10" mol of S02 per year from the marine boundary layer, a
significant quantity. The alkalinity of the aerosol accelerates the
reaction between 0, and S(IV) compounds, with the buffering capacity
essentially controlling the S02 removal.
3.4 Transition Metal Catalyzed Reactions of HO} Radicals on Tropospheric
Aerosols
Ross and Noone carried out a modeling study to determine if Cu
catalyzed reactions of H0j radicals on tropospheric hygroscopic ammonium
bisulfate aerosols were sufficiently fast so that under ambient conditions
the heterogeneous reaction is likely to be compete with the homogeneous
gas phase HO, recombination reaction.1 In their analysis, the
heterogeneous removal time for the Cu catalyzed reaction was compared with
the reaction time for the gas phase H0» recombination reaction. The
chemical and physical characteristics of the aerosols were based on field
data from a site in central Sweden. The calculations were performed for
a range of humidities.
The model calculations showed that for humidities less than 902, the
Cu concentrations exceeded .01 mol L'1 and the catalyzed aqueous
recombination reaction was sufficiently rapid so that the uptake rate was
controlled by gas phase mass transfer. Only at higher relative humidities
did the aqueous rate of reaction influence r*. Comparison of u with rrg
for the gas phase H02 recombination reaction showed that for an H02
concentration of 10 pptv removal by aerosols was faster than the gas phase
reaction when the relative humidity exceeded 851. In addition to copper,
iron and manganese, transition metals commonly present in atmospheric
aerosols, may catalyze removal of Hj02 on atmospheric aerosols. However,
these metals as well as copper could be tied up in complexes that prevent
their serving as effective catalysts and thus questions remain in terms
of the atmospheric implications of this study.
/
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4 LABORATORY INVESTIGATION OF HETEROGENEOUS REMOVAL OF OXYGENATED
INTERMEDIATES
Recently we carried out a laboratory screening study (1) to determine
the extent that oxygenated intermediates of an irradiated o-xylene/NoySO-
mixture are taken up into aqueous media and (2) to establish their fate
upon evaporation of the aqueous media. Here the preliminary results of
the study are presented.
4.1 Experimental
The experiment was conducted using the fully instrumented dew point
controlled continuous stirred tank reactor shown in Figure 2 to irradiate
o-xylene/N0,/S02/air mixtures. The resulting steady state smog mixtures
were exposed to aqueous films on stainless steel panels, located in an
external exposure chamber that contained no lights. Each of the seven
port positions in the exposure chamber was equipped with a chiller back
plate to maintain panel temperatures below the air dew point, thus
minimizing evaporation. The air flow through the exposure chamber was
turbulent, with wind speeds of 3 m s'1. At the end of the exposure, the
films were collected and analyzed for their ionic and carbonyl contents.
Carbonyl concentrations were determined using the 2,4 di-nitro-phenol-
hydrazine method. Details concerning the operation of the reactor and
exposure chamber and the gas and aqueous phase chemical analyses are
reported elsewhere.*
Smog Chamber
Vol = 11.3 m3
F = 100 Lymin
Inlet
Mani-
fold
Clean
Air
.System
Exhaust
Blower
Exposure Chamber Vol = 261
V„ = 3mis Re = 30,000
Sample
Ports
Gas
Monitor
Chiller
Dew Point
Control
System
Humidification
System
FIGURE 2 Continuous stirred tank reactor and exposure chamber
The second phase of the experiment consisted of placing one ml aliquots
of exposed aqueous film samples in a Teflon tube and passing clean dry air
across the aliquot to enhance evaporation. Two bubblers containing
deionized (DI) water were placed in series at the end of the tube to
collect volatile water soluble evaporation products. Once the aliquot
-------
evaporated, the tube was rinsed twice with DI water and the rinse and the
contents of the bubbler were chemically analyzed.
Three separate five hour exposure experiments were conducted where
aqueous films, approximately 0.5 mm in thickness, initially containing
47.2 nmol cm"2 of HjOj were exposed to irradiated o-xylene/N0,/S02/air
mixtures. HjO? was introduced into the films to control the SO, deposition.
The average chemical composition of the irradiated steady state mixtures
is shown in Table 2.
TABLE 2 Average Chamber Gas Concentrations
Compound
Concentrati
o-xylene
1170
NO,
285
NO
15
HNO,
7
0,
31
S0j
91
HCHO
25
CHjCHO
4
CHOCHO
6
CHjCOCHO
57
(CHjCO)j
16
2-CHjCtH»CH0
7
HCOOH
23
CHjCOOH
38
4.2 Results and Discussion
The ions CHjCOO", HCOO", NO,", SO,' , SO*', and H* were detected in the
exposed acidic aqueous films. The carbonyl analysis showed the presence
of HCHO, CHOCHO, and CHjCOCHO. The aqueous concentrations were converted
into fluxes J that were used to calculate the effective deposition
velocity vd using the equation J - v«n. The results, that are based on 21
measurements, are shown in Table 3.
I^BLE 3 Effective Deposition Velocities
Compound v«(cm s"1)
SO, . 0.72 ± 0.03
HNO, 0.88 ± 0.10
HCOOH 0.71 ± 0.11
CHjCOOH ' 0.41 ± 0.05
HCHO , 0.33 ± 0.02
CHOCHO 2.00 ± 0.40
CHjCOCHO 0.90 ± 0.10
S03, HNOj, HCOOH, CH,C0OH and the carbonyl compounds HCHO, CHjCOCHO, and
CHOCHO deposited to the films. Gas phase mass transfer controlled uptake
of highly soluble HNO,. Results of previous experiments suggest that under
these conditions S02 uptake is limited by the amount of H202 available to
oxidize HS03".* The deposition data were not sufficient to unravel the
factors controlling the uptakes of the remaining carbon bearing oxidation
-------
products. Gas phase transfer, saturation effects, and possibly aqueous
reactions could have affected the uptakes.
The evaporation results shown in Table 4 were obtained from chemical
analyses of the bubbler contents and tube rinses. Another important
experimental result is the observation that a residue formed during
evaporation that did not dissolve in the DI rinses. The quantity in the
table labelled collected is the sum of the amounts detected in the bubbler
samples and tube rinses. 2 volatile is the percentage of this total that
was found in the bubblers.
TABLE 4 Evaporation Results
Compound
Applied
Collected
% Volatile
nmol
nmol
CHjCOO"
158
143
85
HCOO"
111
112
80
NO,"
49
22
100
S0,a"
412
429
0
H*
692
756
13
HCHO
58
46
100
CHOCHO
72
0
-
CHjCOCHO
357
63
100
Most of the CH5COO' and HCOO* was recovered as gas phase CH,COOH and
HCOOH. As expected, all the SO,8' was collected in the tube rinse, most
of it probably as H2S04. The HCHO data are consistent with significant
volatilization of HCHO. There were major losses of KfOj', CHOCHO, and
CHjCOCHO. Only 45% of the NOj" was recovered, all as volatile gas phase
species, most likely HNOi. No CHOCHO was detected in the tube rinses or
the bubblers. Only 18% of the CHjCOCHO was accounted for, all of it in the
gas phase.
These preliminary laboratory results suggest that CHOCHO and CH,COCHO
are subject to absorption into aqueous media and thus could possibly
undergo aqueous reactions with OH radicals. The evaporation data suggest
that HCHO, and HCOOH and CHjCOOH can volatilize out of evaporating acidic
solutions. The absence of mass balances for CHOCHO, CHjCOCHO and N0S" and
the presence of a water insoluble residue raises the interesting
possibility that uptakes of these dicarbonyl compounds and HNOj to aqueous
media are not always completely reversible and they could contribute to
aerosol formation. However, the laboratory data are also consistent with
the formation of volatile evaporation products that were not detected.
Further investigations are required to define the fate processes and to
assess whether these results have atmospheric implications.
5 CONCLUSIONS
Modeling studies suggest heterogeneous reactions of oxygenated
intermediates can in principle affect the chemistry of the troposphere.
There appear to be situations where reactions of dissolved oxygenated
intermediates are. sufficiently fast so that they can compete with
homogeneous gas phase reactions and by doing so affect 0,, HCHO, free
radical and NO, concentrations. In addition, there is some evidence that
highly reactive aerosols may serve as sinks for oxygenates.
However, these studies were based by and large on complex multiphase
mechanisms that although representing the best available information, lack
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experimental confirmation. Furthermore, field verification of the model
predictions is, in general, lacking. In the case of 03 formation,
uncertainties in homogeneous gas phase mechanisms and hydrocarbon and NO,
emission inventories may mask the importance of heterogeneous processes.
Thus the key question of whether heterogeneous processes significantly
affect tropospheric pollution levels such as 03 concentrations remains
unanswered.
Additional modeling and laboratory studies are needed to assess the
impact of heterogeneous reactions. The multiphase chemical model should
include the types of processes discussed here, but also other important
processes including the adsorption and absorption of low volatility
oxidation products, inorganic aerosol chemistry, and photo-induced radical
production in aqueous media.10 Laboratory studies should be conducted (1)
to measure uptake coefficients of oxygenated intermediates as functions
of the chemical composition of the aqueous media and the photolytic
condition; (2) to refute or confirm multiphase reaction schemes; (3) to
further refine the underpinnings of the chemistry of low volatility
oxidation products; and (4) to investigate reactions in aerosols and in
evaporating media.
6 REFERENCES
1. Schwartz, S.E. in NATO, Adv. Sci. Ser. (ed. Jaeschke, W.), Vol. G6,
415-471, 1986.
2. Chameides, W.L. J. Geophys. Res., 89, 4739-4755, 1984.
3. Jacob, D.J. J. Geophys. Res., 91, 9807-9826, 1986.
4. Lelieveld, J. and P.J. Crutzen. J. Atmos. Chem., 12, 229-267,
1991.
5. Lelieveld, J. and P.J. Crutzen. Nature, 343, 227-233, 1990.
6. Dentener, F.J and P.J. Crutzen. J. Geophys. Res., 98, 7149-7163,
1993.
7. Chameides, W.L. and A.W. Stelson. J. Geophys. Res., 97, 20565-
20580, 1992.
8. Ross, H.B. and K.J. Noone. J. Atmos. Chem., 12, 121-136, 1991.
9. Edney, E.O. D.J. Driscoll, E.W. Corse, and F.T. Blanchard. Atmos.
Environ., 28, 1189-1196, 1994.
10. Faust, B.C. and J.M. Allen. J. Geophys. Res., 97, 12913-12926,
1992.
DISCLAIMER: This paper has been reviewed in accordance with the U.S.
Environmental Protection Agency's peer and administrative review polices
and approved for presentation and publication. Mention of trade names or
commercial products does not constitute endorsement or recommendation for
use.
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TECHNICAL REPORT DATA
—
1. REPORT NO.
EPA/600/A-94/195
2.
3.RECIE
4, TITLE AND SUBTITLE
Heterogeneous Degradation of Oxygenated
Intermediates
5.REPORT DATE
6.PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
E.0.Edney
8.PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Atmospheric Research and Exposure Assessment
Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
10.PROGRAM ELEMENT NO.
CClAlE
11. CONTRACT/GRANT NO.
InHouse
12. SPONSORING AGENCY NAME AND ADDRESS
Atmospheric Research and Exposure Assessment
Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13.TYPE OF REPORT AND PERIOD COVERED
Symposium Proceedings
9/93-9/94
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Issues surrounding the importance of including heterogeneous processes when
determining the fate of oxygenated intermediates in the troposphere are discussed.
Results of recent investigations are reviewed and preliminary data from a
laboratory study are presented. In the laboratory study heterogeneous processes
that determine the fate of oxidation products of irradiated o-xylene/N0X/S02/air
mixtures were investigated. The review includes summaries of (1) the effect of
oxidant reactions in cloud water on global photochemistry; (2) the impact of N205
and N03 reactions in cloud water on global photochemistry; (3) the effect of sea
salt aerosol reactions in cloud water and tropospheric aerosols; and (4) the impact
of transition metal catalyzed reactions of H02 radicals on tropospheric aerosols.
17. KEY WORDS AND DOCUMENT ANALYSIS
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b.IDENTIFIERS/ OPEN ENDED TERMS
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