disappearance of the OH radical followed a simple exponential
rate law:
(ii) O3 rate constant measurements
The outdoor European Photo-Reactor (EUPHORE) facility
located in Valencia, Spain was used. EUPHORE is composed
of two half spherical chambers each of 200 m3 volume. They
are made of 0.13 mm thick FPE foil with more than 80%
light transmission in the UV-Visible region (280–640) nm.
Purified air and reactants were introduced into the chambers
via ports located on the floor and homogeneous gas mixtures
were obtained by the use of powerful mixing fans. The
chambers differ in the analytical instruments connected to
each of themand in the optical path-length for the in situ
FTIR analysis (326.8 or 553 m). Additional analysis of the
reactants and products was provided by NOx , O3 and CO
analysers, gas chromatography, HPLC and GC-MS. Details
on this facility can be found elsewhere [e.g. ref. 7 and 8]. The
chamber used for this study is the one equipped with 326.8 m
optical path-length for in situ FTIR measurements. The IR
0
½OH ¼ ½OH0eÀk t; where k0 ¼ k1½EVE þ k0
t
0
k1 is the rate constant for the reaction of OH with EVE. The
decay rate k’ is the first-order OH disappearance rate in the
presence of EVE and k0’ is the first-order rate constant for OH
removal in absence of EVE (attributed to the diffusion of OH
radicals out of the detection zone and to their reaction with
H2O2). Typically, EVE and H2O2 concentrations were in the
ranges (1 À 35) Â 1013 and (1.5 À 15) Â 1013 molecule cmÀ3
,
respectively. k0’ and k’-k0’ were in the ranges 75–430 sÀ1 and
745–25000 sÀ1, respectively. Experiments were conducted in the
temperature and pressure ranges, 230–372 K and 30–320 Torr of
helium, respectively. In all conditions, the OH decays were
found to be exponential over at least three lifetimes.
The experimental conditions employed allowed us to per-
form all measurements with a high level of accuracy. The high
[EVE]=[OH]0 ratios and low OH concentrations made negli-
gible the contribution fromsecondary reactions involving the
products of reactions to the measured rate constants. Neither
the variation of the gas flow rates through the reactor nor the
change in the total pressure of the systemshowed measurable
effects on the measured values of k1 . The contribution of the
reaction of OH with photofragments of EVE was also negli-
gible since this unsaturated ether is not photolysed at 248 nm,
the wavelength used to generate OH radicals. Its absorption
cross section was measured in this work at 254 nm using a pen
ray Hg lamp coupled to a 100 cm long cell and was found to be
sꢀ 2 Â 10À22 cm2 moleculeÀ1 at 298 K. As expected, variation
in the photolysis fluence (2–15 mJ cmÀ2) had no effect on the
determined rate constants. EVE was purified to better than
99% and hence loss of OH radicals by reaction with impurities
in the gas mixtures is expected to be insignificant.
spectra were recorded every
interferograms with a resolution of 1 cmÀ1. EVE concentration
was monitored at its absorption band centred at 3000 cmÀ1
5 min by co-adding 280
.
Ozone was measured in real time by an ozone analyzer with a
detection limit of 1 ppb. Known amounts of EVE and O3 were
introduced into the chamber along with SF6 (a stable
compound used to measure the dilution rate in the photo-
reactor resulting fromleaks in the chamber). The rate constant
of the reaction of O3 with EVE was derived fromthe best
fitting of the reaction time-concentration of O3 and EVE
profiles.
(iii) OH-initiated oxidation mechanism study
Two different chambers were used, a 100 L Teflon bag sur-
rounded by six lamps (Sylvania, G30W) with irradiation at 254
nmat LCSR-Orle ans (the same as that used for the kinetic
measurements described above) and at EUPHORE with sun-
light irradiation. At LCSR, the reactant and its oxidation
products were monitored using gas chromatography with a
flame ionisation detector (GC-FID) under similar conditions
as for the kinetic measurements. At EUPHORE, the two
chambers were used.
The obtained values of k1 and the experimental conditions
are listed in Table 1. The roomtemperature rate constant,
taken as the average of all values obtained at 298 K is:
k1 ¼ (6.8 0.7) Â 10À11 cm3 moleculeÀ1
s
À1. The quoted error
for k1 includes 2s fromthe least-squares analysis and the
systematic error (5%, due to uncertainties in measured con-
centrations). The measured values of k1 shown in Table 1 are
plotted in the Arrhenius formin Fig. 1. An un-weighted least
squares analysis of the ln k1 vs. 1=T yields the following
(iv) O3-initiated oxidation mechanism study
The experimental conditions were the same as for the kinetic
study. Fromthese experiments, we were able to extract the
reaction rate constant of O3 with EVE as well as the reaction
products distribution.
Table 1 Reaction OH + C2H5OCH=CH2 (EVE): Experimental con-
ditions and measured rate constants
P=
Torr
[EVE]=1014
k’Àk’0=
(k1 2s) Â 10À11
=
Materials
T=K
molecule cmÀ3
sÀ1
cm3 moleculeÀ1 sÀ1
The purity of the chemicals was as follows: He (UHP certified
to > 99.9995% (Alphagaz));N2–O2 (80–20) was certified to
> 99.995% (Alphagaz). Ethyl vinyl ether (> 99%) was from
Aldrich. The 50 wt.% H2O2 solution, obtained fromProlabo,
was concentrated by bubbling heliumthrough the solution to
remove water for several days prior to use and constantly
during the course of the experiments. For the ozonolysis
experiments, ozone was produced using an ozone generator.
372
372
348
323
298
298
298
298
298
298
298
272
272
272
272
252
252
240
233
230
100
100
100
100
285
30
100
100
100
100
100
30
320
100
100
100
100
100
100
100
0.12–1.41
0.20–1.91
0.26–1.87
0.21–1.99
0.76–3.37
0.21–1.13
0.45–3.52
0.31–1.70
0.20–2.21
0.34–2.24
0.26–2.33
0.16–1.36
0.32–2.25
0.19–2.14
0.34–2.53
0.14–1.44
0.35–2.75
0.17–1.81
0.19–2.36
0.17–1.67
745–7056
5.05 0.29
5.06 0.31
5.73 0.13
6.23 0.17
6.88 0.26
7.13 0.23
6.98 0.15
7.14 0.49
6.66 0.25
6.53 0.45
6.84 0.37
8.49 0.23
7.77 0.26
8.03 0.21
7.64 0.51
9.06 0.34
8.61 0.37
9.68 0.25
10.78 0.24
10.97 0.19
1052–9290
1683–10708
1454–12942
5256–23675
2031–8474
3302–24602
2124–12133
1294–14542
2220–14969
1431–16466
1589–11765
2677–17162
1706–17032
2257–19430
1426–12586
2521–24091
1665–17675
2187–24850
2137–18684
3. Results and discussion
OH rate constant measurements
Absolute measurements. Kinetic experiments were performed
under pseudo-first-order conditions with the concentration of
ethyl vinyl ether in large excess over the OH concentration
([EVE]0> 100 Â [OH]0). Typically, the initial OH concentration,
[OH]0 , was around 2 Â 1011 molecule cmÀ3. The rate of
614
Phys. Chem. Chem. Phys., 2002, 4, 613–619