7974 J. Phys. Chem. A, Vol. 102, No. 41, 1998
Platz et al.
isomers have a molecular weight of 186 mass units. The bottom
panel in Figure 13 shows the contribution of ions of mass 186
to the total ion chromatogram. As seen from Figure 13 ions of
mass 186 make a major contribution to the total ion chromato-
gram. The second largest peak in panel A, which elutes at 27
min, 7 s has a mass of 220 and a fragmentation pattern consistent
with its identification as a monochlorinated product (C12H9-
ClO2). We believe it likely that chlorination occurs as the
sample is condensed in the cold trap. The fragmentation pattern
of each 186 mass ion showed a loss of the following three
fragments: a 17 mass fragment (OH), a 28 mass fragment (CO),
and a 29 mass fragment (CHO). These fragments are expected
from phenolic compounds28 and are consistent with the forma-
tion of the (C6H5O)2 dimer. Authentic reference samples of
2,2′-biphenol, 4,4′-biphenol, and 4-phenoxyphenol were avail-
able, and the response of the GC-MS system was tested for
these compounds. From its retention time and fragmentation
pattern the largest 186 mass product peak (see Figure) was
identified as 4-phenoxy phenol. The other three peaks were
not identified. The results obtained using the GC-MS system
are entirely consistent with the conclusions based on the analysis
above that the major fate of phenoxy radicals in our experiments
is self-reaction.
chamber experiments.29 Further work is needed to establish
the products of the reaction of phenoxy radicals with NOx and
to investigate whether other species (for example, ozone)
compete with NOx for the phenoxy radicals.
Acknowledgment. We thank Robert Lesclaux (Universite´
Bordeaux) for preprints of refs 5 and 6. The work conducted at
Risø was funded by the European Union and Ford Forschung-
szentrum Aachen.
References and Notes
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4. Conclusions
We present here a large body of data concerning the
atmospheric fate of phenoxy radicals. The reaction of phenoxy
radicals with O2 is extremely slow, and an upper limit of k2 <
5 × 10-21 cm3 molecule-1 s-1 at 296 K is reported. This result
is consistent with the upper limit of k2 < 2 × 10-18 cm3
molecule-1 s-1 at temperatures up to 500 K reported by Berho
and Lesclaux.5 In contrast to their behavior toward O2, phenoxy
radicals react rapidly with NO and NO2 with rate constants of
(1.88 ( 0.16) × 10-12 and (2.08 ( 0.15) × 10-12 cm3
molecule-1 s-1 at 296 K. Our result for k3 is in good agreement
with the recent measurement by Berho et al.6 of k3 ) (1.7 (
0.1) × 10-12 cm3 molecule-1 s-1 at 293 K. In moderately
polluted urban atmospheres the concentration of NOx (NO +
NO2) is typically 1-10 ppb, i.e., 108-107 times less than that
of O2. From the present work we conclude that phenoxy
radicals react at least 4 × 108 times more rapidly with NOx
than with O2. It then follows that under conditions typical of
moderately polluted urban air, reaction of phenoxy radicals with
O2 is not important. It seems likely that the fate of phenoxy
radicals in such environments will be reaction with NOx, which
will presumably lead to the formation of nitroso- and nitrophe-
nols, thereby sequestering NOx and slowing down the reactions
responsible for ozone formation. This conclusion is consistent
with the large negative incremental reactivity (ozone-forming
potential) of benzaldehyde, which has been observed in smog
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