Atmospheric Chemistry of Selected Hydroxycarbonyls
J. Phys. Chem. A, Vol. 104, No. 17, 2000 3999
to the reactant mixtures to suppress the formation of O3 and
hence of NO3 radicals. The initial reactant concentrations (in
The initial concentrations of the hydroxycarbonyls, cyclo-
15
13
15
13
hexane, and O3 were ∼2.4 × 10 , 3.5 × 10 , and 3.44 × 10
-
3
14
-3
molecules cm units) were CH3ONO, (2.1-2.4) × 10 ; NO,
molecules cm , respectively, and the reactions were monitored
for up to 3.8 h. The concentrations of the hydroxycarbonyls
were measured by GC-FID as described above. Ozone concen-
trations were measured by ultraviolet absorption using a Dasibi
1003-AH ozone analyzer.
1
4
(
1.8-2.2) × 10 ; and hydroxycarbonyl and reference com-
13
pound, ∼2.4 × 10 each. n-Octane was used as the reference
compound for the OH radical rate constant determinations, and
irradiations were carried out for 6-45 min. No additions were
made to the chamber during the OH radical reactions, and hence
Dt ) 0 for these experiments. To assess the importance of
photolysis of the hydroxycarbonyls during the OH radical rate
Product Studies. Products were identified and quantified
from the reactions of the OH radical with 3-hydroxy-2-butanone
and 4-hydroxy-3-hexanone, from both the kinetic experiments
1
3
constant determinations, the hydroxycarbonyls (∼2.4 × 10
(see above) and irradiated CH ONO-NO-3-hydroxy-2-bu-
3
-
3
molecules cm each) were photolyzed in air in the presence
tanone (or 4-hydroxy-3-hexanone)-air mixtures by GC-FID and
1
5
-3
of 7.1 × 10 molecules cm of cyclohexane (to scavenge any
OH radicals formed during the irradiation) for up to 60 min at
the same light intensity as used in the kinetic experiments.
For the measurement of the rate constants for the reactions
of the NO3 radical with the hydroxycarbonyls, NO3 radicals were
by combined gas chromatography-mass spectrometry (GC-
MS). The initial reactant concentrations and GC-FID analysis
procedures were similar to those employed in the kinetic
experiments described above. Gas samples were collected onto
Tenax-TA solid adsorbent for the GC-MS analyses, with
thermal desorption onto a 60 m DB-5 fused silica capillary
column in a HP 5890 GC interfaced to a HP 5970 Mass
Selective Detector and operated in the scanning mode.
16,17
generated in the dark by the thermal decomposition of N2O5,
and 1-butene, crotonaldehyde [CH3CHdCHCHO], or methac-
rolein [CH2dC(CH3)CHO] were used as the reference com-
-
3
pounds. The initial reactant concentrations (in molecules cm
Chemicals. The chemicals used, and their stated purities, were
cyclohexane (high-purity solvent grade), American Burdick and
Jackson; 2,3-butanedione (99%), crotonaldehyde (99+%), 3,4-
hexanedione (95%), 1-hydroxy-2-butanone (95%), 3-hydroxy-
2-butanone, 1-hydroxy-2-methyl-3-butanone (65%), methac-
rolein (95%), and n-octane (99+%), Aldrich Chemical Co.;
1-hydroxy-3-butanone (95+%), 4-hydroxy-3-hexanone (95+%),
and 3-hydroxy-3-methyl-2-butanone (90+%), TCI America; and
NO (g99.0%) and 1-butene (g99.0%), Matheson Gas Products.
Methyl nitrite and N O were prepared and stored as described
13
units) were hydroxycarbonyl, ∼2.4 × 10 ; 1-butene, crotonal-
13
13
dehyde, or methacrolein, ∼2.4 × 10 ; NO2, (2.4-4.8) × 10 ;
and one to four additions of N2O5 (each addition corresponding
to an initial N2O5 concentration in the chamber of (1.0-6.8) ×
13
-3
1
0
molecules cm ) were made to the chamber during an
experiment. The factor Dt to take into account dilution was Dt
)
0.0012 per N2O5 addition to the chamber.
The concentrations of the hydroxycarbonyls and the reference
compounds were measured by gas chromatography with flame
ionization detection (GC-FID) during the experiments. For the
analysis of the hydroxycarbonyls, n-octane, crotonaldehyde and
2
5
previously,15,16 and NO was prepared just prior to use by
2
reacting NO with an excess of O . O in O diluent was prepared
2
3
2
3
methacrolein, 100 cm volume gas samples were collected from
as needed using a Welsbach T-408 ozone generator.
the chamber onto Tenax-TA solid adsorbent, with subsequent
thermal desorption at ∼225 °C onto a 30 m DB-1701 megabore
column held at 0 °C and then temperature programmed to 200
Results
-
1
Photolysis. Photolysis of the hydroxycarbonyls in air at the
same light intensity as used in the OH radical rate constant
determinations for up to 60 min showed <2% loss of any
hydroxycarbonyl. Hence photolysis of the hydroxycarbonyls
studied was of no importance during the CH3ONO-NO-
hydroxycarbonyl-n-octane-air irradiations employed for the
determination of the OH radical reaction rate constants (which
involved irradiation for e45 min). Furthermore, over the 5-h
period of the photolysis experiment the concentrations of the
hydroxycarbonyls changed by <2%, showing that dark decays
of the hydroxycarbonyls were also negligible.
OH Radical Rate Constants. A series of CH ONO-NO-
hydroxycarbonyl-n-octane-air irradiations were carried out,
and the data obtained are plotted in accordance with eq I in
Figures 1 and 2. Good straight line plots are observed, and the
rate constant ratios k1/k2 obtained from least-squares analyses
of the data are given in Table 1. These rate constant ratios are
°
C at 8 °C min . For the analysis of 1-butene, gas samples
3
were collected from the chamber in 100 cm all-glass, gastight
syringes and transferred via a 1 cm stainless steel loop and
3
gas sampling valve onto a 30 m DB-5 megabore column held
at -25 °C and then temperature programmed to 200 °C at 8 °C
-
1
min . Based on replicate analyses in the dark, the GC-FID
measurement uncertainties for the hydroxycarbonyls were
typically <2%. GC-FID response factors for the hydroxycar-
bonyls, the reference compounds, and selected products (see
below) were determined by introducing measured amounts of
the chemicals into the 7900 L chamber and conducting several
replicate GC-FID analyses.18 NO and initial NO2 concentrations
were measured using a Thermo Environmental Instruments, Inc.,
Model 42 chemiluminescent NO-NOx analyzer.
3
Rate constants, or upper limits thereof, for the reactions of
the hydroxycarbonyls with O3 were determined in the dark by
measuring the decay rates of the hydroxycarbonyls in the
13,19
placed on an absolute basis by use of a rate constant k for the
presence of measured concentrations of O3.
Cyclohexane
2
reactions of the OH radical with n-octane at 296 K of 8.67 ×
was added to the reactant mixtures to scavenge any OH radicals
formed in the reaction systems. Providing that any measured
loss of the hydroxycarbonyls was due only to reaction with O3,
then
-12
3
-1 -1
5
10
cm molecule
s
((20%). The resulting rate constants
k1 are also given in Table 1.
OH Radical Reaction Products. GC-FID analyses of ir-
radiated CH3ONO-NO-hydroxycarbonyl-n-octane-air mix-
tures showed the formation of products from the 3-hydroxy-2-
butanone and 4-hydroxy-3-hexanone reactions (but not from the
other hydroxycarbonyls). Matching of GC retention times and
mass spectra with those of authentic standards showed that the
products are 2,3-butanedione (biacetyl) from 3-hydroxy-2-
butanone and 3,4-hexanedione from 4-hydroxy-3-hexanone.
ln([hydroxycarbonyl] /[hydroxycarbonyl] ) ) k [O ](t - t )
t
t
3
3
0
0
(II)
where k3 is the rate constant for the reaction
O + hydroxycarbonyl f products
(3)
3