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Abstract:
Pulsed laser methods for OH generation and de-
tection were used to study atmospheric degradation reac-
tions for three important biogenic gases: OH
+
isoprene
(Reaction R1), OH
+
α
-pinene (Reaction R2) and OH +
1
-
3-carene (Reaction R3). Gas-phase rate coefficients were
characterized by non-Arrhenius kinetics for all three
reactions. For (R1),
k
1
(241–356 K)
=
(
1
.
93
±
0
.
08
)
×
10
−
11
exp
{
(
466
±
12
)/T
}
cm
3
molecule
−
1
s
−
1
was deter-
mined, with a room temperature value of
k
1
(297 K)
=
(
9
.
3
±
0
.
4
)
×
10
−
11
cm
3
molecule
−
1
s
−
1
, independent of bath-gas
pressure (5–200 Torr) and composition (M
=
N
2
or air). Ac-
curacy and precision were enhanced by online optical mon-
itoring of isoprene, with absolute concentrations obtained
via an absorption cross section,
σ
isoprene
=
(
1
.
28
±
0
.
06
)
×
10
−
17
cm
2
molecule
−
1
at
λ
=
184
.
95 nm, determined in this
work. These results indicate that significant discrepancies be-
tween previous absolute and relative-rate determinations of
k
1
result in part from
σ
values used to derive the isoprene
concentration in high-precision absolute determinations.
Similar methods were used to determine rate coefficients
(in 10
−
11
cm
3
molecule
−
1
s
−
1
)
for (R2)–(R3):
k
2
(238–
357 K)
=
(
1
.
83
±
0
.
04
)
×
exp
{
(
330
±
6
)/T
}
and
k
3
(235–
357 K)
=
(
2
.
48
±
0
.
14
)
×
exp
{
(
357
±
17
)/T
}
. This is the first
temperature-dependent dataset for (R3) and enables the
calculation of reliable atmospheric lifetimes with respect
to OH removal for e.g. boreal forest springtime condi-
tions. Room temperature values of
k
2
(296 K)
=
(
5
.
4
±
0
.
2
)
×
10
−
11
cm
3
molecule
−
1
s
−
1
and
k
3
(297 K)
=
(
8
.
1
±
0
.
3
)
×
10
−
11
cm
3
molecule
−
1
s
−
1
were independent of bath-
gas pressure (7–200 Torr, N
2
or air) and in good agree-
ment with previously reported values. In the course of this
work, 184.95 nm absorption cross sections were determined:
σ
=
(
1
.
54
±
0
.
08
)
×
10
−
17
cm
2
molecule
−
1
for
α
-pinene and
(
2
.
40
±
0
.
12
)
×
10
−
17
cm
2
molecule
−
1
for
1
-3-carene.
