Heterogeneous Oxidation by Ozone of Naphthalene
Adsorbed at the Air-Water Interface of Micron-Size
Water Droplets
Paper # 1006
Suresh Raja and Kalliat T Valsaraj†
Gordon A. and Mary Cain Department of Chemical Engineering
Louisiana State University
Baton Rouge, LA 70803-7303
ABSTRACT
The mass transfer of naphthalene vapor to water droplets in air was studied in the
presence of ozone in the gas phase. A falling droplet reactor with water droplets of
diameters 55, 91 and 182 m was used for the study. Ozone reacted with naphthalene at
the air-water interface, thereby decreasing the mass transfer resistance and increasing the
rate of uptake of naphthalene into the droplet. A Langmuir-Hinshelwood reaction
mechanism at the air-water interface satisfactorily described the surface reaction. The
first order surface reaction rate constant, ks increased with decreasing droplet size. Three
organic intermediates were identified in the aqueous phase as a result of ozonation of
naphthalene at the surface of the droplet indicating both peroxidic and non-peroxidic
routes for ozonation. The presence of an organic carbon surrogate (Fulvic acid) increased
both the partition constant of naphthalene and the surface reaction rate of ozone.
INTRODUCTION
Organic vapors emitted to the lower troposphere are distributed horizontally by
advection and vertically by diffusion. They are redeposited on land and water surfaces by
dry and wet deposition processes. Wet deposition occurs mainly via rain, fog and snow.
Near surface deposition by fog is especially important as a scavenger of organic
molecules such as pesticides, polycyclic aromatic hydrocarbons and volatile organic
compounds resulting from anthropogenic activities. Fog is a near-surface cloud and
composed mainly of water droplets condensed on sub-micron particles. The typical sizes
of fog droplets vary from 1 to 100 m. Upon uptake of organic vapors by fog, it is
redistributed between the water and colloids in fog. The presence of a large surface area
distributed in a given volume of air makes the chemistry of fog droplets to be driven
largely by surface (heterogeneous) and multiphase processes. Uptake of organic
molecules on small droplets is dependent not only on the bulk equilibrium that exists
between air and water, but also the surface equilibrium established between the large airwater interface and air.
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the atmosphere and
result from fossil fuel combustion, biomass burning and automobile emissions. They
range in molecular size from 2-ring (naphthalene) to multi-ring compounds. PAHs
possess low aqueous solubility, low vapor pressure and are hydrophobic. Hence PAHs
1
accumulate in organic rich environments such as aerosol, sediment, soil, biota and animal
tissue. It has been shown that the air-water interfacial adsorption of these compounds is
substantial and hence conducive to transport by air bubbles in water and water droplets in
the atmosphere1,2. Several PAHs have been noted in fogwaters at concentrations much
larger than that predicted by simple gas/liquid equilibrium3,4. Previously a hypothesis has
been advanced that this is a result of increased surface area of fog droplets (due to their
small size) and the presence of surface-active organic compounds in fogwater5,6.
The overall mass transfer to a water droplet in the atmosphere is driven by a series
of steps that include (i) gas phase mass transfer of reactants, (ii) surface accommodation
and adsorption of reactants, and (iii) either a surface reaction or bulk phase diffusion
(solubilization) followed by reaction within the droplet. Figure 1 represents these
processes7,39. For compounds that do not undergo reaction, the processes of concern are
gas phase mass transfer, surface accommodation and liquid phase diffusion
(solubilization).
Figure 1. Schematic of the steps involved in the mass transfer and multiphase
reactions for a PAH vapor and oxidants at the surface of a falling water droplet in
the atmosphere. The loss of PAH via surface reaction is represented by a pseudofirst order rate constant.
Gas phase mass
transfer and
surface
accommodation
Surface reaction
Bulk phase
diffusion and reaction
Oxidants
Drop velocity, U
We conducted several single droplet laboratory experiments for PAH vapor
uptake in a falling droplet reactor in which a constant gas phase concentration was
maintained8,9. As fresh droplets traversed the reactor the PAH vapors were incorporated
into the droplets. The PAH concentration increased with droplet/gas contact time and
decreased droplet radius. Because of the millisecond droplet/gas contact, equilibrium was
not established at the reactor outlet. The kinetic data obtained were used to determine
mass transfer coefficients and mass accommodation constants for individual PAHs.
Since no other reactants were introduced into the reactor, the only fate process for the
PAH compounds was dissolution into the droplet. These experiments showed that the
partitioning of PAHs between the gas and a water droplet was larger for small droplets8,9.
This was attributed to increased partitioning of compounds to the air-water interface
through adsorption. As a consequence the equilibrium at the air-water interface required a
modified partition constant given by
2
K *WA  K WA
Equation 1
where KWA is the conventional air-water bulk phase equilibrium constant (Henry’s
constant, dimensionless molar concentration ratio). The parameter  corrects for the
enhanced partitioning due to surface adsorption,
6 K IA
Equation 2
  1

d D K WA
where KIA (m) represents the air-water interfacial adsorption constant and dD (m) is the
droplet diameter. Depending on the value of dD and KIA, the value of  can range from 1
to several orders of magnitude. Values of KIA for a variety of PAHs were measured
earlier in our laboratory using the inverse gas chromatography technique2.
There have been numerous recent reports, which have shown that chemical
reactions at the air-water interface can occur faster than homogeneous reactions in the
bulk liquid or gas phase10-17. Recently Donaldson and co-workers18,19 used laser-induced
fluorescence to provide direct evidence of the reaction of gas phase ozone with
anthracene adsorbed to a planar, thick air-water interface. They also showed that the
reaction was five times faster on an octanol-coated air-water interface. Ozone is reported
to have variable mass accommodation coefficients (10-3 to 10-2) on pure air-water
interface20,44. Similarly PAHs also show small mass accommodation coefficients8,9.
However, the presence of a PAH at the interface may increase the PAH-ozone interaction
at the air-water interface of a fog droplet19.
In the natural atmosphere the primary oxidants are the hydroxyl radical, ozone
and nitrate radical. For example, the environment in Baton Rouge, Louisiana has
predominance of ozone21 with the presence of PAHs such as Naphthalene23, where in
ozone can enter into heterogeneous reactions22. Reactions of PAH vapors with ozone can
potentially be significant in fog droplets since fog not only provides a very high surface
area but also is long lasting so that heterogeneous, multiphase reactions can transform the
compounds. It has been suggested that oxidation products of PAHs are sometimes more
toxic than the parent PAH. Most of the laboratory experiments on PAH-ozone
interactions have been on planar, thick air-water interfaces in a batch reactor19 and cannot
be directly translated to that of a highly curved surface such as a fog droplet for which a
renewable air-water interface prevails as the drop falls through the atmosphere. Whereas
a large concentration driving force for mass transfer can be maintained for a thick, planar
air-water interface, the driving force will be smaller for a water droplet in the atmosphere
due to its small size. With the above aspects in mind we undertook a laboratory study of
gas-phase ozone reaction with a typical PAH (naphthalene) vapor on micron-size water
droplets in a falling droplet reactor to elucidate the kinetics and mechanism of this
potential reaction pathway.
EXPERIMENTAL
Compounds Studied
The PAH of concern in this study was naphthalene which is a 2-ring compound and the
first in the series of PAHs. It has a high vapor pressure, low aqueous solubility and is
hydrophobic in nature. It also has a modest adsorption capacity at the air-water interface.
It was obtained at 99% purity from Aldrich Chemicals, Missouri. It is the most prevalent
3
PAH in the Baton Rouge air23. The relevant properties of the compound are given in
Table 1.
Table 1. Physico-chemical properties of naphthalene8. All values are at 298K.
Property
Value
Molecular weight
Aqueous solubility, C*
Sub-cooled liquid vapor pressure, P*
Air-water interfacial adsorption constant, KIA
Air-water bulk phase partition constant, KWA
Surface accommodation coefficient, α
Diffusivity in water, DL
Diffusivity in air, DG
Thermal velocity, Ĉ
128
0.241 mol.m-3
0.037 kPa
(27.2 ± 1.8) x 10-6 m
53 ± 4 [dimensionless molar ratio]
(2.2 ± 0.6) x 10-4
7.0 x 10-10 m2.s-1
5.7 x 10-6 m2.s-1
222 m.s-1
Experimental Apparatus
The apparatus (Figure 2) used in this experiment was a modification of our earlier
work8. In this work an aromatic hydrocarbon vapor stream along with ozone gas was fed
into the falling droplet reactor. In order to study the kinetics and uptake in the droplet
flow tube apparatus, the whole experimental apparatus was traced with heating coils and
controlled by temperature controllers obtained from Omega Engineering Inc.
The water droplets generated in this study passed through a cylindrical stainless
steel reactor (0.0245m i.d., 1.06m long). The saturated vapor stream, as generated in our
previous work8, was fed into the droplet flow tube apparatus using a perforated porous
frit sparger. Ozone gas was generated using a unit obtained from Ozone Solutions, Inc.
The ozone concentration was measured electrochemically, prior to introduction into the
droplet flow reactor, using a handheld ozone sensor obtained from Ozone Solutions Inc.
The pure ozone stream flow rate that was fed to the reactor was about 20mL/min.
Figure 2. Schematic of the falling droplet reactor where the saturated gas phase
naphthalene interacts with ozone on micron-size water droplets.
M ass Flow
Controller
Helium gas
Saturated vapor
Generator
W ires to pulse generator
O 3 Destruct Unit
Flow M eter
Inlet gas stream
Saturated
Stream
Inlet
Droplet reactor
(stainless steel)
O 3 Generator
W ater inlet
Glass nozzle assembly
for droplet generation
Ozone inlet
Gas sparger/distributor
O 3 meter
Gas stream
for GC/M S
He-Ne
Laser
Photodiode
Optical glass
Reactor outlet
gas stream
Liquid collection
chamber
W ater to GC/M S analysis
4
The droplet generation assembly comprised of a capillary glass nozzle embedded
in a piezo-ceramic tube actuated by an electric pulse generator obtained from the
University of Bremen, Germany. Droplets were generated using HPLC grade water fed to
the capillary nozzle assembly using a syringe pump (KD Scientific, Model 210). Based
on the settings in the electronic pulse generator, the desired droplet size was generated24.
For some of the experiments water droplets were generated using a commercially
available sapphire orifice assembly with different orifice sizes (20 to 100 microns). A
piezo-ceramic crystal (American Piezoceramics Inc., PA) placed above the orifice was
mechanically vibrated to generate the desired droplet size depending on the frequency of
voltage modulation8,9.
Methodology
The flow of the carrier gas into the generator column produced the saturated
aromatic hydrocarbon vapor stream. The gas phase ozone concentration obtained from
the ozone generator was split and fed to the ozone sensor periodically until a steady
ozone concentration was reached. The water droplet stream was first fed to the reactor
without ozone and collected at the bottom of the reactor. While keeping all the reactor
conditions same, the droplet stream was turned off and pure ozone was fed to the reactor
at about 20mL/min. Analysis of gas phase PAH vapors in contact with ozone gases by
GC/MS showed no significant reaction of ozone and PAH vapor phase in the absence of
the water droplets. At this point, droplet stream was restarted and collected from the
droplet collection chamber. The aqueous phase samples collected with and without the
presence of the ozone in the reactor was extracted into dichloromethane and analyzed in
the GC/MS to determine the reaction parameters and the associated reaction products.
Aliquot of the aqueous phase samples reacted with ozone was subsequently analyzed for
dissolved ozone concentration.
RESULTS AND DISCUSSION
Analysis of Reactor Data
Let us consider a single water droplet of diameter dD (m) that traverses the reactor
through a height HR (m) with a velocity U (m/s). The assumption is made that at the
droplet/air interface local equilibrium is established for PAH vapor between the bulk
water and gas phases and given by Henry’s law. In the present case we must modify that
approach by also considering the equilibrium at the air-water interface between adsorbed
PAH and vapor. The overall driving force for mass transfer has to be corrected to
account for this effect. The rate of mass transfer of PAH to the water droplet is given
by1,25-27
dC d
Equation 3
Vd
 K C A d C g  C*g
dt
where Cg* = Cd/KWA* is the equilibrium gas phase concentration, Vd = dD3/6 is the
volume of the droplet and Ad=dD2 is the surface area of the droplet. KWA* is the
modified partition constant defined in Equations (1) and (2). Note that an overall driving
force (Cg – Cg*) is used in this case and hence KC is the overall mass transfer
coefficient28. Therefore


5
dC d
C 
6 
Equation 4
 KC 
  C g  *d 
dt
dD 
K WA 
Solving the above equation with Cd=0 at t = 0, and a constant Cg, we get the following
equation for the partitioning of the PAH between the droplet and the gas phase at the
outlet of the reactor

 * 

C ()
Equation 5
K DV  d
 K *WA 1  e  


Cg


where τ is the residence time for a droplet in the reactor (=HR/U), and HR is the reactor
length. τ* is a characteristic time given by
1 dD
Equation 6
* 

 K *WA
KC 6
In the above equation, KC represents the overall mass transfer coefficient for the PAH
vapor from the gas phase to the fog droplet which involves the three steps outlined in
Figure 1. A general mass transfer expression was derived in the literature29-31 specifically
for the case in which gas phase diffusion, mass accommodation with surface reaction and
bulk solubility within the droplet controls the mass transfer.
d
1
4 1
1
Equation 7
 D 
 
K C 2 DG
C   * DL

 KW A
 k s K IA 




Each term on the RHS in the above equation represents a resistance to mass transfer. The
first term represents the gas phase diffusion resistance, the second term is the mass
accommodation term and the last term is the resistance in the liquid phase due to bulk
phase diffusion (solubility) and surface reaction. In deriving the above expression two
assumptions are made: (i) a Langmuir-Hinshelwood type reaction occurs on the surface
and (ii) bulk liquid- phase reaction is negligible. There is support for these assumptions
from recent work by Mmereki et al19 on planar water surfaces in which they showed that
gas-phase ozone reaction with PAHs occurs on the surface of water and not in the bulk.
In the above equation DG and DL (m2/s) are respectively the gas and liquid phase
diffusivity of the PAH, C (m/s) is the thermal velocity of the molecule in the gas phase,
and ks (s-1) is the overall pseudo-first order surface reaction constant for naphthalene on
the droplet. KIA (m) is the partition constant for the compound between the gas-water
interface and the gas phase as given in Equation (2). If, on the other hand, reaction is not
limiting, the PAH uptake is only limited by the solubility in the droplet and KC is
determined by the equation
d
1
4 1
1

Equation 8
 D 
  * 
K C 2 DG
C  K W A DL
Thus if uptake is solubility-limited, *sol 
limited *surf rxn 
d 
and if it is surface reaction6 DL
d 1
K *WA are the corresponding characteristic times. The Hatta
6 k s K IA
6
number Ha 
k s K IA
can be used to characterize the importance of the two domains.
D
L
K *WA

Hatta number is the ratio of the surface reaction rate to the mass transfer rate. Values of
Ha > 2 indicates the reaction at the gas-liquid interface is fast and the mass transfer is
diffusion controlled, whereas Ha < 0.02 corresponds to slow reaction on the surface that
controls mass transfer.
Ozone reaction with naphthalene vapor on water droplets
Experimentally both the droplet concentration (Cd(τ)) and the gas concentration
(Cg) at the reactor outlet are determined and hence the value of KDV() is known. Since
the droplet diameter is determined experimentally we can calculate ξ and hence KWA* is
also known. Utilizing Equation (5) one can obtain the value of the characteristic time, τ*.
Further, we can use Equation (6) to get KC. All other parameters are given in Table 1. In
the experiments the contact time,  for the water droplet was between 0.17 to 0.2 seconds.
The gas phase concentration of naphthalene varied from 50 to 150 ppmv in the
experiments. All experiments were conducted at the reactor temperature of 299±2 K and
reactor pressure of 760 Torr.
Figure 3. The vapor-to-droplet partition constant of naphthalene on different
droplet sizes with increasing gas-phase ozone concentrations. The solid lines are
predicted values based on the ks estimated from Equation 9 using kmax and C1/2
obtained from Figure 4.
Droplet-vapor partition coefficient
50
55m
40
91 m
KDV / [-]
30
20
182 m
10
0
0.0
2.0e+14
4.0e+14
6.0e+14
8.0e+14
[O3]g / molecules.cm
1.0e+15
1.2e+15
-3
Figure 3 illustrates the effect of increased gas-phase ozone concentration on the
experimental value of the droplet-vapor partition constant of naphthalene vapor from the
gas phase. Note that this partition constant is not the equilibrium value but for the
specified gas/liquid contact time used in this work. An approximate two-fold increase to
an asymptotic value of 40 for KDV was observed for the smallest droplet size (55 m), 35
7
for 91 m and 15 for the largest droplet size (182 m). The clear influence of droplet
diameter is seen in the figure. As indicated previously by us8 the partition constant is
larger for small droplet size since the surface effect becomes dominant. The overall mass
transfer coefficient for naphthalene vapor was obtained from Equations (5) and (6) as
described in the previous paragraph. Kc increased with increasing ozone concentration in
the gas phase. With increased reaction rate at the interface, the overall liquid phase
resistance decreased and the mass transfer coefficient increased. The calculated Hatta
number was greater than 2 indicating that reaction at the surface was much faster than
diffusion into the bulk of the droplet. The gas-phase resistance and the mass
accommodation resistance to mass transfer (per Equation 7) were smaller than the total
liquid-phase resistance for the small droplet (55 m). However, the gas phase resistance
became larger with droplet size and the overall resistance was larger for the 182 m
droplet. Hence, the mass transfer coefficient became smaller as the droplet size increased.
There are various lines of evidence pointing to the significance of the surface
reaction in determining the overall uptake into droplets. There is evidence indicating that
ozone concentration is ten times more at the air-water interface than in the bulk water
phase32. This is attributed to a combination of its low solubility in water (KWA = 3.2) but
significant polarizability (2.85 Å3). We observed negligible bulk phase uptake of ozone
in the droplet even at the highest gas-phase ozone concentration. Our earlier work clearly
showed that naphthalene accumulates at the air-water interface on small water droplets2
compared to the bulk phase. The non-linear dependence of the partition constant, KDV in
Figure 3 and the rate constant, ks in Figure 4 (discussed below) on the gas-phase ozone
concentration is another important evidence for the surface reaction of ozone with
naphthalene.
Figure 4. A Langmuir plot of Equation (9) with the values of kmax and
C1/2 determined from in Equation (10) and given in Table 2.
1000
800
ks / s-1
600
55 m
400
91 m
182 m
200
0
0.0
2.0e+14
4.0e+14
6.0e+14
8.0e+14
[O3] / molecules.cm
1.0e+15
1.2e+15
-3
8
Table 2. Langmuir – Hinshelwood parameters for ozone at the droplet surface for various
sizes.
Droplet kmax/ s-1
C1/2/ molecule.cm-3
N
R2
size
55 µm
435
7.21 x 1013
4
0.81
91 µm
303
1.92 x 1014
10
0.85
182 µm
169
1.31 x 1014
3
0.80
1429 (with 0.029 g/L FA)
2.8 x 1013 (with 0.029 g/L FA)
3
0.96
1400 (with 0.021 g/L FA)
4.3 x 1013 (with 0.021 g/L FA)
4
0.98
Note: N is the number of data points. R2 is the correlation coefficient for the linear fit to
Equation (10).
From the values of KC obtained above and using Equation (7) we obtained the
overall rate constant ks. This was done for different concentrations of ozone in the gas
phase and for three droplet sizes (Figure 4). The rate constant increased with decreasing
droplet size and increasing gas-phase ozone concentration and reached an asymptotic
value at high ozone concentrations. The non-linear dependence indicated that surface
reaction was the controlling process. This was also noted by Mmereki et al19 who
observed that gas-phase ozone adsorbed first on the air-water interface according to a
Langmuir adsorption isotherm and reacted with the adsorbed naphthalene via a
Langmuir-Hinshelwood mechanism. The overall observed dependence of ks on gasphase ozone concentration was given by19,40
k max [O 3 ]g
ks 
Equation 9
C1 / 2  [ O 3 ] g
where kmax is the maximum (asymptotic) value of the rate constant. It is given by k IIS0,
the product of the second-order surface reaction rate constant (kII / cm2.s-1 ) and the
surface concentration of ozone adsorption sites on the surface (S0 /cm-2). C1/2 is the
ozone concentration in the gas phase at which ks= (½) kmax and is related to the rates of
surface desorption to adsorption of ozone33. A Langmuir plot involves rearranging the
above equation in the following manner
[O 3 ] g
C1 / 2
1
 max  [O 3 ]g  max
Equation 10
ks
k
k
From the linear fit of [O3]g/ks versus [O3]g and the corresponding slopes and intercepts
the values of kmax and C½ at 298K for each droplet size can be obtained (Table 2). The
value of kmax increased and C1/2 decreased as droplet size decreased. We have reported
earlier that smaller droplets carry a higher surface concentration of naphthalene8. As a
result on smaller water droplets the surface is more conducive to the trapping of ozone.
The first order surface rate constant is related to the reaction probability of a gas-phase
ozone molecule per collision with adsorbed naphthalene on the surface through the
following equation18,40: k s 
 rxn
4
 naph C
O3
[O3 ] g , where Ωnaph is the collision cross
9
section of a naphthalene molecule (= 156.76 Å2) and CO3 is the thermal speed of an
ozone molecule (= 3.6 x 104 cm/s). The reaction probability, rxn is a ratio of the rate of
surface reaction to the total number of collisions with the surface41. For a given gas-phase
ozone concentration, the rate of ozone loss to the surface remains unchanged. However,
the number of collisions with the surface decreases as droplet diameter decreases. Hence
the reaction probability will increase with decreasing droplet diameter. As a
consequence, the overall surface rate constant will also increase. Extrapolating this
behavior to very small droplets (10 μm as in fog), we can expect a large reaction rate at
the surface. A planar interface will show a small surface reaction constant.
The properties of aerosols and fog droplets can be altered substantially by the
presence of surface-active organic carbon in them42,43. Fulvic acid (FA) is a known
surrogate that exhibits properties similar to the surface-active organic carbon in fog
droplets42. Previous work9 from our laboratory showed that the surface tension of water
decreased from 70 to 51 mN/m with 0.029 g/L of FA indicating saturation of the droplet
surface with fulvic acid. Figure 5 shows the effect of adding 0.021 and 0.029 g/L of
fulvic acid (equivalent to 12 and 17 mg of Carbon/L respectively) to the water droplet on
the naphthalene partitioning and ozone reaction for a droplet of 182 m size. The fact that
FA-covered surface is conducive to the trapping of ozone and naphthalene and decreasing
the mass transfer resistance at the surface is evident from the 2 to 3 fold increase in KDV
and the 9 to10 fold increase in ks as seen in Figure 5. Similar increases in ozone and
anthracene trapping efficiency on planar water surfaces have been noted by other
researchers19. Approximately an order of magnitude increase in both kmax and C1/2 for
ozone was observed in the presence of fulvic acid (Table 2). Thus, organic-coated
surfaces of water droplets can show much higher ozone reactivity with adsorbed PAH
molecules.
Figure 5. The effect of added fulvic acid (FA) on (a) the vapor-to-droplet partitioning
and (b) the pseudo first-order surface rate constant of naphthalene with ozone.
(b)
(a)
35
1600
with 0.029 g/L FA
with 0.029 g/L FA
1400
30
with 0.021 g/L FA
1200
with 0.021 g/L FA
25
-1
ks / s
KDV / [-]
1000
20
15
800
600
without FA
10
400
5
without FA
200
0
0
0
2e+14
4e+14
6e+14
[O3]g / molecules. cm
8e+14
-3
1e+15
0
2e+14
4e+14
6e+14
8e+14
1e+15
[O3]g / molecules. cm-3
10
As naphthalene is transferred to the droplet, its reaction with ozone at the
air/water interface will lead to several reaction products that can be identified in the
aqueous phase collected at the outlet of the reactor. Three main products were identified
by GC/MS analysis. These were 1,2-benzenedicarboxaldehyde (2), 1-naphthalenol (3)
and 1,4-naphthalenedione (4). These are also well known intermediates of ozone
oxidation of naphthalene in the bulk aqueous phase34. As shown in Scheme 1, both 3 and
4 are produced as a consequence of a peroxidic route and 2 is produced via a nonperoxidic route. Both involve attack by ozone on carbon in position number 1 of the 1,2bond in the naphthalene molecule, which has the lowest bond delocalization energy. In
the peroxidic route the elimination of oxygen molecule leads to 3 whereas the addition of
water to the peroxide or the addition of ozone molecules leads to 4. In the non-peroxidic
route the elimination of a hydrogen peroxide molecule leads to the product 2.
Figure 6. Reaction mechanism for naphthalene with gas phase ozone.
CONCLUSIONS
The presence of ozone in the gas-phase increased the rate of mass transfer of naphthalene
vapor into water droplets of diameters ranging from 55 to 182 microns in a falling droplet
reactor. The psuedo-first order rate constant for the surface reaction of ozone with
adsorbed naphthalene was modeled using the well-known Langmuir-Hinshelwood
mechanism. The surface reaction products were the same as the bulk phase ozonolysis
products. The heterogeneous reaction on water droplets was 15 times faster than the
corresponding gas-phase homogeneous oxidation in the atmosphere.
ACKNOWLEDGEMENTS
This work was supported by grants from the National Science Foundation (ATM0082836 and ATM-0355291). Any opinions, findings and conclusions, or
11
recommendations expressed in this material are those of the author and do not necessarily
reflect the views of the National Science Foundation.
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IMPLICATIONS
The heterogeneous oxidation of PAH vapor adsorbed at the air-water interface of fog
droplets by ozone can be many times higher than that in the homogeneous gas phase. As
a result, the fog droplets formed in oxidative environments can be expected to contain
transformation products of PAHs. Some of these reaction products may be potentially
more toxic than the parent PAH compound.
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