TABLE 4. Calculated Henry’s Law Coefficient at 283 K and of the Wet Deposition Rate Coefficient at 283 Ka
H (291 K)
H (283 K)
fx
fx
kwd
kOH
3 -1
estimated lifetime
without washout (days)
-
1
-1
-1
(cm mol s-1
compound
(M atm
)
(M atm
)
(291 K)
(283 K)
(s
)
)
3
3
4
4
4
4
4
4
4
4
4
4
4
4
-6
-6
-6
-6
-6
-6
-6
-12
-12
-12
-12
-12
-12
-12
2
1
2
1
1
2
1
OH1C4
OH2C4
OH4C4
OH3C4
OH4C4
OH5C5
OH4C5
9.00 × 10
8.90 × 10
1.36 × 10
1.40 × 10
2.90 × 10
3.67 × 10
2.05 × 10
2.27 × 10
2.24 × 10
3.43 × 10
3.53 × 10
7.31 × 10
9.25 × 10
5.17 × 10
0.08
0.08
0.12
0.12
0.22
0.26
0.17
0.18
0.18
0.25
0.26
0.42
0.47
0.33
1.7 × 10
1.7 × 10
2.4 × 10
2.4 × 10
4.0 × 10
4.5 × 10
3.2 × 10
3.0 × 10
1.3 × 10
8.5 × 10
4.5 × 10
5.8 × 10
9.1 × 10
5.7 × 10
4.6
7.0
2.2
3.6
3.0
2.1
3.0
a
Assum ing an annual rainfall of 1 m /year. Also shown are calculated rate coefficients for the reaction with the OH radical based on the estim ation
m ethod of Kwok and Atkinson (45). The loss rate due to reaction with OH is calculated assum ing a daytim e OH concentration of 10 cm . The
photolysis rate coefficients are assum ed to be 1 × 10
6
-3
-6
-1
s .
primary and secondary H atom abstraction (10). In the
reaction of 2-propyl nitrate with OH, the contribution of the
tertiary H atom becomes more significant at lower temper-
atures, leading to a smaller temperature dependence. The
recent room-temperature measurements as well as the
temperature-dependent rate coefficients (10, 42, 43) are
substantially different than those measured previously (44).
The hydroxy nitrates studied here contain more than two
different hydrogen atoms. In contrast to alkyl nitrates, where
as rate coefficients, absorption cross sections, quantum
yields) is necessary to calculate the atmospheric lifetime of
these species. The lifetime estimated here (>2 days) implies
x
that these compounds may be important NO reservoir
species and can survive long range transport in the tropo-
sphere, especially in dry environments.
Although it is expected that the ambient concentrations
of hydroxy nitrates will decrease during rainy periods, hydroxy
nitrates have still been detected during rain events (28). This
observation has led to the suggestion that their concentration
is not greatly influenced be wet deposition (28). Alternatively,
it may be an indication of the high flux of hydroxy nitrates
into the atmosphere, especially in urban areas, which can
sustain detectable amounts even in wet environments. This
observation is challenging and requires additional field
measurements under different conditions.
2
the O-NO group slows the H atom abstraction, the hydroxy
group is expected to accelerate it. Therefore, it is difficult to
estimate the rate coefficients for reactions of hydroxy nitrates
with the OH radical based solely on alkyl nitrates reactivity.
Using the structure-reactivity estimation method of Kwok
and Atkinson (45), the rate coefficients of hydroxy nitrates
with the OH radical can also be estimated and are shown in
6
Table 4. Assuming a daytime OH concentration of 1 × 10
-
3
cm , the lifetimes of the compounds range from 2.5 to 14
days (see Table 4). The presence of the OH group, and the
larger distance from the nitrooxy group especially in γ- and
δ-hydroxy nitrates, accelerates the reaction with OH com-
pared with the alkyl nitrates. These estimates suggest that
the structure of hydroxy nitrates will have a large effect on
its reactivity and therefore on the lifetime of a given species.
Acknowledgments
This work was partially funded by the U.S.-Israel binational
science foundation (U.S.-Israel BSF) and the Sussman Family
Center for Environmental Studies, Weizmann Institute. Y.R.
is incumbent of the William Z. and Eda Bess Novick career
development chair.
Absorption cross-sections for alkyl nitrates have been
measured in a few studies (46-50). To the best of our
knowledge, only one measurement exists for â-hydroxy
nitrate, that of (nitrooxy)ethanol (47). Roberts and Fajar (47)
found that (nitrooxy)ethanol has a cross-section approxi-
mately a factor of 3 lower than that of methyl nitrate at 300
nm. Assuming that saturated hydroxy nitrates have lower
photolysis rate coefficients than the alkyl nitrates (J ≈ 1 ×
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9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 7, 2000