It is now clear that heterogeneous reactions play an important role in controlling the concentration of ozone in the stratosphere. In this work, the loss of N2O5 on ice substrates has been studied in a flow reactor in an attempt to gain a more fundamental understanding of these reactions. The apparent reaction probability in this system was found to decrease as the substrate was exposed to N2O5. A model which corrected for the loss of surface sites was developed and although it appears to fit the data for a given experiment quite well, it is concluded that the loss of reactive sites is not the full explanation. In addition, the results of an experimental and modeling study suggest that reaction on the internal surface of the ice substrates is not a major loss mechanism for N2O5 in the current work

Kenner, Rex D.

Plumb, Ian C.

Ryan, Keith R.

English

NASA Technical Reports Server

N95-11014
LABORATORY MEASUREMENTS OF POLAR STRATOSPHERIC CLOUD RATE
PARAMETERS
Rez D. Kenner, Inn C. Plumb and Keith R. Ryan
CSIRO Division of Applied Physics,
P.O. Box 218, Lindfield NSW 2070 Australia
Abstract
It is now clear that heterogeneous reactions play an impor-
tant role in controlling the concentration of ozone in the
stratosphere. In this work, the loss of N2Os on ice sub-
strates has been studied in a flow reactor in an attempt to
gain a more fundamental understanding of these reactions.
The apparent reaction probability in this system was found
to decrease as the substrate was exposed to N2Os. A model
which corrected for the loss of surface sites was developed
and although it appears to fit the data for a given exper-
iment quite well, it is concluded that the loss of reactive
sites is not the full explanation. In addition, the results of
an experimental and modelling study suggest that reaction
on the internal surface of the ice substrates is not a major
loss mechanism for N:Os in the current work.
1 Introduction
It is now recognized that reactions which occur on the
surfaces of aerosols in the stratosphere have a significant
effect on the concentrations of ozone. Before laboratory
measurements can be used with confidence in atmospheric
models to explain and predict the effects of heterogeneous
chemistry in the stratosphere, much more detailed informa-
tion about these processes is required. In this study, the
reaction of N205 with water ice substrates has been chosen
as a sample system in order to address several fundamental
questions about heterogeneous processes as studied in tbe
laboratory.
This system has been studied by several groups [Hanson
and Ravishankara, 1991; Leu, 1988; Quinlan et al., 1990;
Tolbert, Rossi and Golden, 1988] and it was shown that the
product of the reaction is nitric acid which is retained in
the solid-phase.
N2Os(g) + H20(,) _ 2HNO3(,) (1)
where (g) and (s) refer to species in the gas and solid
phases respectively. As the reaction procceds, the surface
of the substrate is increasingly contaminated with nitric
acid which is thought to react further with water to form
nitric acid trihydrate (NAT) [Hanson and Ravishankara,
1991; Kenner, Ryan and Plumb, 1992]. Using low initial
concentrations of N2Os and short measurement times to
minimize the build-up of HNO3 product on the surface
of the ice, Hanson and Ravishankara [Hanson and Rav-
ishankara, 1991] estimated a value of _2 × 10-2 for the col-
lisional reaction probability of N205 on ice at 200 K. They
found that the apparent reaction probability decreased on
continued exposure of the ice to N2Os to a limiting value
of 6×10 -4 . They interpreted the latter value as being the
reaction probability on NAT which they reasoned would
have covered the surface completely at the doses of N2Os
used.
If the observed decrease in reaction probability is only
due to the gradual conversion of the original ice surface
into one which is covered with NAT, it should be possible
to model the experiments and obtain a value of the reaction
probability of N2Os on pure ice, "Ylc_- Then using the model
it should be possible to predict the reaction probability of
N2Os on surfaces which are intermediate between pure ice
and pure NAT. In this work the reaction probability of
N205 on ice substrates with submonolayer coverage of the
surface by HNO3 was measured and modelled.
The second question addressed in this work is the de-
termination of the correct surface area to use in converting
the observed wall-loss rate to a l'eaction probability. Most
authors have assumed that the surface of their substrates
arc smooth and non-l)orous and have used a surface area
calculated from the dimensions of their apparatus, t{e-
cently, Leu and coworkers [Key.set, Moore and Leu, 1991;
Leu, Moore and h'eyser, 1991] have observed that, for ni-
tric acid hydrate substrates, the measured wall-loss rate
increases with the thickness of the fihn in a manner which
suggests that reaction on int.ernal surfaces of those fihns is
important. They showed that corre('lion factors as large as
50 may need to be al)l)lied t,o lat)oratory data to account
for the ilwreascd area of the sul)strat(* (hi(" to its porous
structure.
In this work, lwo al)l)roa(h('s have I_(,(,i: tak_,n lo de-
t.ermine the l)ossibl( • iml)Orta]l((' (_f rca('tion (m iul(q'llal
471
https://ntrs.nasa.gov/search.jsp?R=19950004601 2020-06-16T11:34:19+00:00Z
surfacesofthesubstratein thecurrentsystem.(1)The
reactionprobabilityof N20sonicesubstrateshasbeen
measuredasafunctionofthethicknessofthesubstrate.(2)
Amodelhasbeendevelopedtofit thedatawhichincludes
diffusioni toandreactiononthesurfa_=eofidealizedpores.
2 Results and Discussion
2.1 Experiments and Models Concern-
ing the Conversion of Reactive Sites
The flow reactor and detection system used in these
studies have been described previously [Plumb and Ryan,
1981; Ryan and Plumb, 1982] and its modification and ap-
plication to the study of heterogeneous reactions at low
temperatures is described elsewhere [Kenner, Ryan and
Plumb, 1992]. N2Os was prepared by a method similar to
that described by Davidson et al. [Davidson et al., 1978]
and Cantrell et al.[Cantrell et al., 1987land was detected
at its NO + (m/e 46) fragment.
Results for two experiments measuring the loss of N2Os
on ice-coated walls of the flow reactor are shown in Figure 1
as points in semilogarithmic plots of [N205] as a function of
inlet distance. The error bars are the statistical uncertain-
ties in each point. The points in Figure 1 were obtained by
measuring [N2Os] at fixed positions 100 mm apart as the
movable inlet is moved from an inlet distance of _50 mm
to an inlet distance of 450 mm and back i.e. the points
were measured in order going round the loop in a counter-
clockwise direction. The curves shown in Figure 1 are the
results of fits using a model to be discussed below.
In all experiments in the current investigation, loops
such as those shown in Figure 1 were obtained and show
that the reaction probability of N2Os on the surface changes
during the course of an experiment. The shapes of the loops
depend on a number of experimental variables. For exam-
pie, with all other parameters held constant, the loops are
broader with a more horizontal major axis for: i) higher
[N2Os]o (compare curves a and b in Figure 1) or ii) longer
count times.
For the purposes of modelling, it is possible to represent
the overall process by a single reaction
N2Oso) + sflt20(_l ----" 2(IINO3 • _dI2())(,) (2)
For HNO3.nH20(s) = NAT, 7t = 3 and ,sf, the stoichiomet-
ric factor, would be 7 if all water molecules of hydration
are derived from the surface monolayer. A model has been
constructed to fit the current experiments based on Reac-
tion (2) in which sf and 3' are adjust able palanwters whose
values were obtained by fitting the experimental points us-
ing non-linear least-squares fitting techniques based on the
Levenberg-Marquardt method [f)r¢._s ctal.. 1!)88]. This
method includes uncertainty estimates for tho fitted pa-
rameters.
As the experiment proceeds, the ice sites are lost due to
the formation of nitric acid hydrates and the modol must
I
E
O
O
o
v
t/3
o
Z
5.0
1.0
i i i
_- - ilk'-- _
\ _[/"_ ....
\
\
\
\
I
\
b \
\
\
0.2 1 1
0 200
a
\
%
\
\
\
\
\ \
\
\ \
\
\ \
I I
4O0
Inlet Distance (mm)
Figure l: N2Os concentration as a function of inlet dis-
tance for ice-coated flow tube walls at a temperature of
175 K for two different starting concentrations of N2Os.
The points are experimental data (converted to absolute
concentrations using the experimentally determined sensi-
tivity for N2Os) and the curves are the best fit results of
the basic model described in the text.
keep track of tile surface concentration of ice and NAT sites
at each position along the flow tube as a function of time.
The reaction probability at each point is assumed to be
proportional to the number of ice sites, [tt20(s)(z,t)],
[ll_o¢,_(_,t)] (3)
_(_..t) =-,_+ [tl_O(.)]0
where [H20(.)]o is the surface concentration of ice sites on
pure ice. The rale equations are integrated to determine
the concentrations of N205 and tt20(+) as functions of posi-
tion and time. The aw+rage concentration of N2()s surviv-
ing to reach the detector is calculated for each position of
lhe morainic inlet. By setting the movabh + i,,let positions,
dwell time, tlow velocily, inlet scan Sl)eed and [N_O_,]. in
the model to mal,h those of a particular experiment, it is
possible to silnulatc that experinwnt.
The s,lrfac. _oncentration of ice sites at the I;eginning
of lhe experi,uent was assun,(.d to be lx 10 '_' on,-'. Be-
cause of the way the l,l,.hq is (u.,sl tin'to, l. Ihe .ct,lal v.lue
(tI()S('II for Ill(' Slllfiit'(' ('OllCelltl';itiOll ]Ills lIO e[[('('[ t|ll th('
vahw of y Id)l;,i,,,'d. llowevcr, I1.' v.h.' uf ...f is direclly
472
0.020
0.005
0.0
0
i i
I I
50 1O0 150
Ice Thickness (#m)
Figure 2: The values of 7,,,od from the basic model as a function of the thickness of the ice film.
proportional to the choice of the initial surface concentra-
tion of reactive sites. Based on the results of consecutive
experiments performed using the same substrate, regener-
ation of the ice sites through solid state diffusion and/or
evaporation of the HNO3 is considered to be sufficiently
slow that it can be ignored for T<200 K.
The basic model fits the data quite well as can be seen
in Figure 1, where the curves are the best fits to the data.
From the good fits to the data obtained using the basic
model, one can conclude that the main reason for the de-
crease in the reactivity during the experiments is the grad-
ual conversion of the more reactive ice sites into less reac-
tive NAT sites. However, one also finds that experiments
that differ only in the amount of exposure of the substrate
to N2Os yield different values for the reaction probability
on pure ice. This is a clear indication that the conversion
of sites is not tile full explanation.
Several additional mechanisms have been considered,
for example, higher than first-order dependence on [1-120(_)],
sites of different reactivities and involvenwnt of more than
just the top monolayer of the substrate. Models to fit the
data haw, been developed based on each of these and al-
though they each improve the fits to the data and reduce
the dependence of "y on the exlwrimental conditions, they
introduce too many I)arameters about which no informa-
tion is availabh, and no conch>ion can Iw reached as to the
corrcctlwss of any given model.
2.2 Dependence of the Reaction Proba-
bility on the Thickness of Ice and the
Porous Substrate Model
The dependence of the reaction probability of N2Os
on ice has been investigated in this work for T = 190 K,
bath gas pressure = 0.23 Tort and [N2Os]0 = 3.5x10 m
cm -3 for ice thicknesses, hi¢¢, in the range 0<h.¢_<124 pm.
The results are shown in Figure 2 where 7,,,od given by the
basic model are plotted as a function of hie,. It will be
seen that 7,,,od increases with hie, for 0_<hi_,<7 #m and is
independent of hi_, for h,_, >7 #m. The authors believe
that the lower values of 3, observed for ice thicknesses less
than 7 /,m are caused by the walls of the flow tube being
incompletely covered with ice in those experiments. This
view is supported by the large values of sf obtained for
the experiments on thin ice substrates [Kenner, Ryan and
Plumb, 1992].
The possible porosity of the ice in the current experi-
ments has been tested further by developing a model which
includes idealized pores to investigate what ranges of pore
diameters and porosities (the ratio of the vohune of internal
space to the volume of solid) are capable of explaining the
current data. By comparing the calculated loops with the
experimental data for a range of values of the mean pore
diameter and of the porosity of the substrate, it has been
found that mean pore diameters of 0.1 pm and porosities
473
of 0.03 are required to reproduce the data. These values
of mean pore diameter and porosity were tested by fitting
the same data sets used in evaluating the other models.
The deduced values of 7 differed by <20% from those ob-
tained using the basic model. The average value of X 2 and
the dependence of the calculated reaction probability on
experimental conditions were similar to those obtained us-
ing the other 'improved' models. Based on these results,
it is concluded that in the current system reaction on the
internal surface of the substrate is not a large effect hut
may explain some details of the current experiments. Any
further evaluation of this model must await experimental
measurements of the pore diameters and porosities of the
substrates.
Acknowledgement
This work was supported in part by the Boeing Commercial
Airplane Group.
References
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474


Laboratory measurements of polar stratospheric cloud rate parameters

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