Dye Laser Photolysis of C3-C7 Aldehydes in the 280-330 nm
Region
Paper # (1064)
Lei Zhu,1 Yongxin Tang,1 Yunqing Chen,2 and Thomas Cronin3
1
Wadsworth Center, New York State Department of Health, Department of Environmental
Health and Toxicology, State University of New York, Albany, New York 12201-0509;
2
Department of Physics, Rensselaer Polytechnic Institute, Troy, NY 12180; 3Department of
Natural Sciences, State University of New York, Cobleskill, NY 12043.
ABSTRACT
We have investigated the photolysis of propionaldehyde (C2H5CHO), n-butyraldehyde
(CH3(CH2)2CHO), n-pentanal (CH3(CH2)3CHO), n-hexanal (CH3(CH2)4CHO), and n-heptanal
(CH3(CH2)5CHO) at 5 nm intervals in the 280-330 nm region by using dye laser photolysis in
conjunction with cavity ring-down spectroscopy. Their absorption cross sections have been
obtained at each wavelength studied. The HCO radical is a photodissociation product of all
aldehydes. The HCO radical quantum yields have been determined as a function of photolysis
wavelength (), aldehyde pressure, and nitrogen buffer gas pressure. The peak HCO radical
yields are 0.980.11, 0.840.04, 0.200.06, 0.150.02, and 0.150.06 for propionaldehyde, nbutyraldehyde, n-pentanal, n-hexanal, and n-heptanal at 310 nm/315nm, respectively, where the
uncertainty (1) represents experimental scatter. The near unity HCO quantum yield from
propionaldehyde photolysis at 310 nm indicating that C2H5CHO+h  C2H5+HCO is the
dominant photolysis pathway. The difference in the peak HCO quantum yields when going from
propionaldehyde to n-butyraldehyde has been attributed to the opening of the Norrish II channel
from the photolysis of n-butyraldehyde. The peak HCO radical yields decrease with increasing
alkyl chain length when going from n-butyrladehyde to n-pentanal, but this decreasing trend does
not continue for aldehydes with chain length longer than or equal to five carbon atoms. The
shape of the HCO quantum yields versus wavelength curve and the magnitude of the HCO
radical yields are similar for n-pentanal, n-hexanal and n-heptanal. The dependence of the HCO
quantum yield on nitrogen buffer gas pressure was examined between 6 and 387 Torr; no
dependence was observed. The end-products from the closed-cell photolysis of C3-C7 aldehydes
at selective wavelengths were analyzed by mass spectrometry, HPLC, and FTIR, and their yields
have been estimated. The occurrence of the Norrish II channel for n-butyraldehyde, n-pentanal,
n-hexanal, and n-heptanal and the photocyclization channels for n-hexanal and n-heptanal have
been confirmed. In the carbon bond model, the photochemistry of acetaldehyde has been used to
represent the photochemistry of >C1 aldehydes. Possible scheme to regroup aldehydes according
to similarity in photochemistry has been suggested.
1
INTRODUCTION
Aliphatic aldehydes are introduced into the atmosphere through biogenic and anthropogenic
emissions or through photo-oxidation of tropospheric organic compounds. Their atmospheric
chemistry has been studied extensively as a result of their central role in the formation of
photochemical smog, peroxyacetyl nitrate (PAN), and ground-level ozone.1,2 The major gasphase removal pathways for saturated aliphatic aldehydes in the atmosphere are reactions with
OH radicals and unimolecular photodissociation. Rate constants for OH radical reactions with
C1-C7 aldehydes have been reported previously.3-11 Photodissociation of aldehydes is an
important source of free radicals in the atmosphere.12 The photolysis of formaldehyde (CH2O)
and acetaldehyde (CH3CHO) has been studied extensively.13-16 Determination of the wavelengthdependent photolysis quantum yields of propionaldehyde (C2H5CHO), n-butyraldehyde
(CH3(CH2)2CHO), n-pentanal (CH3(CH2)3CHO), n-hexanal (CH3(CH2)4CHO), and n-heptanal
(CH3(CH2)5CHO) is necessary in order to determine their atmospheric fates and to establish the
relationship between product quantum yields and aldehyde alkyl chain length. It also permits an
estimation of their tropospheric radical formation rate constants.
Aliphatic aldehydes exhibit a weak UV absorption band in the 240-360 nm region as a result of
an electric dipole-forbidden but vibronically allowed n* transition.17 The thermodynamically
allowed dissociation pathways following their excitation in the UV region are as follows:
RCHO  h  R  HCO (  339 nm)
 RH  CO (all )
 R’H  [CH2=CHOH]  R’H  CH3CHO
(  978 nm for R’=C2H3, C3H5, C4H7, C5H9)
 RCO  H (  306 nm)
 2-methyl-cyclo-R’’-OH (1.3 m for R’’=C5H8 and C6H10)
 2-ethyl-cyclo-RIII-OH (1.3 m for RIII = C4H6 and C5H8)
(1)
(2)
(3)
(4)
(5)
(6)
where threshold wavelengths were calculated from the corresponding enthalpy changes.
Photolysis channels 1, 2, and 3 are radical formation, molecular elimination, and Norrish II
channels. Channel 4 is another radical-formation channel but was found to be minor for aliphatic
aldehydes.18 Channels (5) and (6) are photocyclization channels and they are available for C5
aldehydes.
We present in this paper results obtained from an experimental study of the photolysis of
propionaldehyde, n-butyraldehyde, n-pentanal, n-hexanal, and n-heptanal at 5 nm intervals in the
280-330 nm region using dye laser photolysis combined with cavity ring-down technique.19,20
Their absorption cross sections have been obtained at each wavelength studied. The quantum
yields of HCO from the photolysis of these aldehydes and their dependences on photolysis
wavelength, aldehyde pressure, and nitrogen buffer gas pressure have been acquired. Absolute
HCO radical concentration was calibrated either relative to the formaldehyde photolysis,
HCHOh  HHCO, for which the recommended HCO radical yield at each photolysis
2
wavelength is available21 or to the Cl+H2CO  HCl+HCO reaction. The end products from the
closed-cell photolysis of n-butyraldehyde, n-pentanal, n-hexanal, and n-heptanal at selective
wavelengths have been identified with mass spectrometry, HPLC, and FTIR, and their
corresponding yields have been estimated. Radical formation rate constants from the photolysis
of C3-C7 aldehydes have been estimated as a function of zenith angle for cloudless conditions at
sea level and at 760 Torr nitrogen pressure.
EXPERIMENTAL METHODS
The experimental setup has been described in detail elsewhere.22-25 Two laser systems were used
in the experiments. One was used to photolyze aldehydes and the other was used to probe the
HCO radical product. The second-harmonic output of a tunable dye laser pumped by a 308 nm
XeCl excimer laser (200 mJ/pulse) was used as photolysis radiation. Laser dyes used included
coumarin 153, rhodamine 6G, rhodamine B, rhodamine 101, sulforhodamine 101, and DCM. The
photolysis laser beam entered the reaction cell at a 15o angle with the main cell axis through a
sidearm. The probe laser beam (613-617 nm; 0.01 nm laser bandwidth) from a nitrogen-pumped
dye laser was directed along the main optical axis of the cell. The pump and the probe laser
beams overlapped one another at the center of the reaction cell, which was vacuum-sealed with a
pair of high-reflectance cavity mirrors. The base path length between the two cavity mirrors was
50 cm. A fraction of the probe laser pulse was injected into the cavity through the front mirror
and its intensity decayed as the light bounced back and forth inside the cell. The light-intensity
decay inside the cavity was measured by monitoring the weak transmission of light through the
rear mirror with a photomultiplier tube (PMT). The PMT output was amplified, digitized and
sent to a computer. The decay curve was fitted to a single-exponential decay function. The ringdown time constant and the total loss per optical pass were calculated. By measuring the cavity
losses with and without a photolysis pulse, the HCO absorption from the photolysis of C3-C7
aldehydes was obtained. A pulse/delay generator was used to vary the delay time between the
firing of the photolysis and the probe lasers. The photolysis laser pulse energy was measured with
a calibrated joulemeter. Gas pressure was measured at the center of the reaction cell with a
capacitance manometer. Quantum yield measurements were made at a laser repetition rate of 0.1
Hz to ensure the replenishment of the gas samples between successive laser pulses. The spectrum
scan was performed at a laser repetition rate of 1 Hz. All experiments were carried out at an
ambient temperature of 293  2 K.
The mass spectra of the stable end-products from the photolysis of C4-C7 aldehydes were taken
with a residual gas analyzer mass spectrometer connected online with the photolysis cell. A
Fourier transform infrared spectrometer was also used to characterize photolysis end products.
The highest purity propionaldehyde, n-butyraldehyde, n-pentanal, n-hexanal, and n-heptanal were
purchased from Aldrich and were purified by repeated freeze-pump-thaw cycles at 77 K before
each experiment to remove volatile impurities. Formaldehyde was generated by pyrolysis of
paraformaldehyde (95% purity) at 110 oC. Nitrogen (99.999% purity) and chlorine (99.5%
purity) were used without further purification.
3
RESULTS AND DISCUSSION
Absorption Cross Sections of C3-C7 Aldehydes in the 280-330 nm Region.
The room temperature absorption cross sections of propionaldehyde, n-butyraldehyde, npentanal, n-hexanal, and n-heptanal have been determined at 5 nm intervals in the 280-330 nm
region. They are shown in Figure 1 and listed in Table 1. These saturated aliphatic aldehydes
display a broad absorption band in the near UV region resulting from a vibronically allowed
n* transition. The absorption cross section at each wavelength was obtained by monitoring
the transmitted photolysis photon intensity as a function of aldehyde pressure in the cell and by
applying Beer's law to the experimental data. The error bars quoted (1) are the estimated
precision of cross section determinations which include the standard deviation for each
measurement plus the standard deviation about the mean of at least four repeated experimental
runs. As seen from Figure 1, the absorption cross sections initially increase with chain length
when going from propionaldehyde to n-butyraldehyde in the 295-330 nm region while the cross
section values of n-pentanal, n-hexanal, and n-heptanal are almost the same.
Figure 1: Absorption Cross Sections of C3-C7 Aldehydes
Cross Section (10-20 cm2)
8
6
4
2
0
270
propionaldehyde
n-butyraldehyde
n-pentanal
n-hexanal
n-heptanal
280
290
300
310
320
330
340
Wavelength (nm)
Table 1. Absorption Cross Sections (10-20 cm2, base e) of C3-C7 Aldehydes vs. Wavelength
 (nm)
 (prop)
 (n-butyr)
 (n-penta)
 (n-hexan)
 (n-hepta)
280
5.680.11
4.960.06
5.480.15
5.450.13
5.440.33
285
7.050.22
5.850.11
6.230.09
6.130.19
6.390.82
290
6.020.16
5.880.06
6.090.15
6.040.19
5.990.41
295
6.090.07
6.140.04
6.560.17
6.450.18
6.620.43
300
5.460.21
5.820.11
5.800.16
5.940.25
5.610.19
305
4.460.19
5.680.08
5.430.09
5.610.24
5.290.41
310
3.770.06
4.170.02
4.410.07
4.380.18
4.720.34
315
3.050.17
3.380.07
3.740.09
3.750.27
3.990.29
320
2.010.08
2.280.14
2.360.10
2.510.13
2.940.28
325
1.360.13
1.860.13
1.810.07
1.960.23
1.850.31
330
0.750.01
1.110.15
1.030.15
1.230.16
1.060.21
4
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Time-Resolved Studies of the Photolysis of C3-C7 Aldehydes in the 280-330
nm Region.
Shown in Figure 2 is a cavity ring-down absorption spectrum of the product after 290 nm
photolysis of propionaldehyde. The similarity of the product absorption spectrum to the
previously reported absorption spectrum26 of HCO in the same wavelength region indicates that
the HCO radical is a photolysis product of propionaldehyde. Similar product absorption spectra
were also obtained from the photolysis of n-butyraldehyde, n-pentanal, n-hexanal, and nheptanal. The cavity ring-down spectrometer was tuned to the HCO X2A''(0,0,0)  A2A'(0,9,0)
R bandhead at 613.8 nm, and the HCO concentration was followed as a function of time.
Presented in Figure 3 is a temporal profile of HCO from the 290 nm photolysis of 3 Torr
propionaldehyde along with a fit of the HCO decay profile using the following kinetic scheme:
HCO+HCO  H2CO+CO
HCO+C2H5  products
C2H5+C2H5  C4H10
 C2H4+C2H6
HCO+C2H5CHO  products
(5)
(6)
(7a)
(7b)
(8)
This modeling scheme assumes that HCO+C2H5 is the only important radical formation channel
from the photolysis of propionaldehyde at 290 nm, an assumption supported by the near unity
HCO quantum yield ( = 0.950.06) from the photolysis of propionaldehyde at 290 nm
described later in this paper. Time-resolved HCO decay profiles from the photolysis of 3, 6, and
9 Torr propionaldehyde were compared with those calculated by the ACUCHEM simulation
program27. The following input parameters were used: rate constants for the HCO+HCO,
HCO+C2H5, C2H5+C2H5, and HCO+C2H5CHO reactions (kHCO+HCO, kHCO+C2H5, kC2H5+C2H5, and
kHCO+C2H5CHO) and the initial HCO concentration ([HCO]0). Literature kC2H5+C2H5 value of
1.910-11 cm3molecule-1s-1 was used in the fitting.28 Initial values of kHCO+HCO, kHCO+C2H5, and
kHCO+C2H5CHO were given to the program, and the simulated HCO profiles were compared with
the experimental results. After several iterations and adjusting of kHCO+HCO, kHCO+C2H5, and
kHCO+C2H5CHO values, optimum fits of the experimental profiles were accomplished. kHCO+HCO,
kHCO+C2H5, and kHCO+C2H5CHO thus extracted are (6.01.5)10-11, (6.51.5)10-11, and
(1.50.2)10-14 cm3molecule-1s-1, respectively, where uncertainty (1) represents experimental
5
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scatter only. The HCO decay profiles at all three propionaldehyde pressures are well fitted by the
extracted kHCO+HCO, kHCO+C2H5, and kHCO+C2H5CHO. The accuracy of the kHCO+HCO and kHCO+C2H5
measurements is affected by accuracy in the determination of the HCO absorption cross section
(HCO) and its initial concentration ([HCO]0) and by the time resolution of the cavity ring-down
spectroscopy (17-21 s around 613 nm). The initial HCO concentration was in the range of
4.81013 to 8.51013 cm-3 for propionaldehyde pressures between 3 and 9 Torr. The overall
uncertainty for the extracted kHCO+HCO and kHCO+C2H5 is about 50%. Values of kHCO+HCO and
kHCO+C2H5 thus obtained agree well with the recommended rate constant21 for the HCO+HCO
reaction (k=2.510-11-10.010-11 cm3molecule-1s-1 at 300 K) and the previously reported rate
constant29 for the C2H5+HCO reaction ((7.21.6)10-11 cm3molecule-1s-1). The influence of the
numerical value of kHCO+C2H5CHO on the HCO decay profiles becomes important at time scale on
the order of hundreds of microseconds. Because the HCO decay profiles were measured at
several propionaldehyde pressures under the condition that [HCO]0 << [C2H5CHO]0, the overall
uncertainty in the value of kHCO+C2H5CHO is about 20%. We have also measured the HCO+HCO,
HCO+R, and HCO+RCHO (R = n-C4H9 and n-C5H11) rate constants from the photolysis of npentanal and n-hexanal.
HCO Radical Quantum Yields from the Photolysis of C3-C7 Aldehydes in the
280-330 nm Region.
The HCO radical quantum yields from the photolysis of propionaldehyde, n-butyraldehyde, npentanal, n-hexanal, and n-heptanal were determined from the ratio of the HCO concentration
produced in the pump/probe laser overlapping region to the absorbed photon density in the same
region. The overlapping region could be viewed as a rectangular solid with center overlapped
with that of the cell, with width and height defined by those of the photolysis beam, and length of
the rectangular solid defined by (beam width)(tan15°)-1, where 15° is the crossing angle between
the pump and the probe laser beams. The photolysis beam is absorbed by aldehydes over the
entire level arm through which it travels. The absorbed photolysis photon density in the
pump/probe laser overlapping region could be derived from the difference in the transmitted
photolysis beam energy entering (Ein) and leaving (Eout) that region, the individual photon energy
(hc/) at the photolysis wavelength (), and the volume (v) of the overlapping region by using
the following equations:
6
Absorbed photon density 
E in  E out
c
h v

v = beam width  beam height  length of the rectangular solid
= beam width  beam height  (beam width)(tan15°)-1
The photolysis beam energy entering or leaving the pump/probe laser overlapping region can be
calculated from the incident photolysis beam energy entering the cell (E0), the absorption cross
section () and the density (n) of aldehyde in the cell, and the absorbing path length by applying
Beer’s law:
Ein = E0exp(-nl1)
Eout = E0exp(-nl2)
where l1 is the distance between the photolysis beam entrance and the beginning of the
pump/probe laser overlapping region, and l2 is the distance between the photolysis beam entrance
and the end of the pump/probe laser overlapping region. The incident photolysis beam energy
was measured by a calibrated Joulemeter placed in front of the cell. The incident beam energy
inside the cell was corrected for transmission loss at the front cell window and for reflection of
the photolysis beam from the rear cell window. The HCO concentration after the photolysis was
acquired by measuring its absorption at 613.80 nm at a photolysis and a probe laser delay of 15
s. To convert HCO absorption into absolute concentration, we determined the absorption cross
section of HCO at the probe laser wavelength relative to that from the formaldehyde photolysis,
H2CO+h  HCO+H, for which the recommended HCO quantum yield at each photolysis
wavelength is available21 or from the Cl+H2CO  HCl+HCO reaction.
The dependence of the HCO radical quantum yields on C3-C7 aldehyde pressure was examined.
Displayed in Figure 4 are plots of the HCO quantum yields (HCO) as a function of
propionaldehyde pressure at photolysis wavelengths of 290 and 320 nm. The HCO radical yields
decrease with increasing propionaldehyde pressure, possibly due to the quenching of the excited
precursor to dissociation by the ground-state propionaldehyde molecules and the increasing
HCO+HCO, C2H5+HCO, and HCO+C2H5CHO reactions at higher propionaldehyde pressures.
In order to separate the contribution of HCO radical reactions from the quenching process, we
have corrected the HCO radical yields for HCO radical reactions at 15 s and have included in
Figure 4 both the uncorrected and the corrected HCO radical yields. As seen from Figure 4, there
was no propionaldehyde pressure quenching effect when photolysis study was conducted at 290
nm. The corrected HCO radical yields still decrease with increasing propionaldehyde pressure at
320 nm suggesting an aldehyde pressure quenching effect at longer photolysis wavelengths.
Since 320 nm is close to the photodissociation threshold of propionaldehyde, increasing
propionaldehyde pressure quenched the excited molecule to below its dissociation limit. The
corrected reciprocal HCO quantum yields were plotted against propionaldehyde concentration
([C2H5CHO]) according to the Stern-Volmer equation:
1/HCO=1/HCO° +(kQC2H5CHO/kDC2H5CHO)•[C2H5CHO]
7
(9)
Figure 4: The HCO Radical Yield as a Function of Propionaldehyde Pressure
1.2
photolysis = 290 nm
HCO
1.0
0.8
0.6
0.4
Circles: uncorrected HCO yields
Diamonds: corrected HCO yields
0.2
0.0
0
2
4
PC
1.2
6
2H5CHO
8
10
12
(Torr)
photolysis = 320 nm
1.0
HCO
0.8
0.6
0.4
Circles: uncorrected HCO yields
Diamonds: corrected HCO yields
0.2
0.0
0
2
4
6
8
10
12
PC2H5CHO (Torr)
where HCO° is the HCO quantum yield extrapolated to zero propionaldehyde pressure and
kQC2H5CHO/kDC2H5CHO is the ratio of quenching to unimolecular decay rate constant of the excited
propionaldehyde. For those photolysis wavelengths where the corrected HCO yields are
independent of propionaldehyde pressure, kQC2H5CHO/kDC2H5CHO is equal to zero. Illustrated in
Figure 5 is a plot of 1/HCO vs. [C2H5CHO] at 320 nm photodissociation wavelength; note that it
is linear. Experimentally derived HCO° from the photolysis of propionaldehyde, nbutyraldehyde, n-pentanal, n-hexanal, and n-heptanal as a function of the photolysis wavelength
are tabulated in Table 2 and plotted in Figure 6.
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As seen from Figure 6, the zero pressure HCO quantum yields from the photolysis of
propionaldehyde, n-butyraldehyde, n-pentanal, n-hexanal and n-heptanal exhibit significant
wavelength dependencies and decrease at both the longer and the shorter wavelength ends. The
decrease in HCO quantum yields with decreasing wavelength at the shorter wavelength end can
possibly be attributed to the opening of an additional photolysis pathway, such as the formation
of RHCO, at higher photon energies. The reduced HCO quantum yields at the longer
wavelength end may be the result of dissociation at near-threshold wavelengths. The peak HCO
quantum yields from the photolysis of propionaldehyde, n-butyraldehyde, n-pentanal, n-hexanal,
and n-heptanal are 0.980.11, 0.840.04, 0.200.06, 0.150.02, and 0.150.06 at 310 nm/315
nm, respectively, where the uncertainty (1) represents experimental scatter. The near unity
HCO quantum yield from propionaldehyde photolysis at 310 nm indicating that C2H5CHO+h
8
 C2H5+HCO is the dominant photolysis pathway. The difference in the peak HCO quantum
yields when going from propionaldehyde to n-butyraldehyde has been attributed to the opening of
the Norrish II channel from the photolysis of n-butyraldehyde. The peak HCO radical yields
decrease with increasing alkyl chain length when going from n-butyraldehyde to n-pentanal, but
this decreasing trend does not continue for aldehydes with chain length longer than or equal to
five carbon atoms. The shape of the HCO quantum yields versus wavelength curve and the
magnitude of the HCO radical yields are similar for n-pentanal, n-hexanal and n-heptanal. Since
n-pentanal, n-hexanal, and n-heptanal have -hydrogen atoms or -hydrogen atoms which can be
readily abstracted intramolecularly by the O atom to form Norrish type II photoelimination
products or to form photocyclization products, similar yields of intramolecular isomerization
products can be expected from the photolysis of these longer chain aldehydes. As a result,
similar HCO radical yields can be expected from the photolysis of aldehydes with chain length of
five carbon atoms or longer. On the other hand, n-butyraldehyde only has primary –hydrogen
atoms which are less reactive than the secondary –hydrogen atoms. Therefore, radical formation
is the preferred process from n-butyaldehyde photolysis.
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The dependence of the HCO radical quantum yields on total pressure was examined by keeping a
constant aldehyde pressure and changing the nitrogen buffer-gas pressure. The HCO radical
quantum yields were found to be independent of total pressure (6–387 Torr) to within the
experimental error limit in the 280-330 nm region. For the atmospheric purpose, we feel it is a
good approximation to set the zero pressure HCO quantum yields from the photolysis of C3-C7
aldehydes equal to those in the presence of 760 Torr of nitrogen since nitrogen is an extremely
poor quencher of vibronically excited aldehyde.
End-Product Studies with Mass Spectrometry, HPLC, and FTIR.
n-Butyraldehyde, n-pentanal, n-hexanal, and n-heptanal were photolyzed in a closed cell by an
excimer laser or by a dye laser. The photolysis end-products were analyzed by a residual gas
analyzer mass spectrometer, an HPLC, or an FTIR. A summary of the product yields obtained
from the end-product study is given in Table 3 and Table 4:
9
Table 2. Values of 0HCO from the Photolysis of C3-C7 Aldehydes.

(nm)
0HCO
0HCO
Propionaldehyde
280
285
290
295
300
305
310
315
320
325
330
0.850.06
1.010.07
0.950.06
0.980.06
0.920.06
0.950.08
0.980.11
0.910.05
1.080.07
1.070.14
0.840.08
0HCO
0HCO
0HCO
n-Butyraldehyde
n-Pentanal
n-Hexanal
n-Heptanal
0.220.02
0.240.02
0.280.04
0.300.04
0.310.04
0.520.05
0.740.08
0.840.04
0.770.08
0.740.06
0.590.10
0.0580.006
0.0950.009
0.100.02
0.140.01
0.100.02
0.150.02
0.140.02
0.200.06
0.140.02
0.0850.034
0.0870.015
0.0730.007
0.0910.017
0.0890.011
0.100.01
0.0880.006
0.120.01
0.150.02
0.140.02
0.100.01
0.0950.018
0.0790.024
0.0810.013
0.0900.011
0.0980.016
0.120.03
0.110.03
0.140.04
0.150.06
0.100.01
0.110.02
0.100.02
0.0980.026
Table 3. End-Product Yields from the Photolysis of n-Hexanal and n-Heptanal at 308 nm.
Reactant
Photolysis Channel
n-Hexanal
CH3CHO + C4H8
Quantum Yield
Mass spectrometry
0.280.04
CO + C5H12
FTIR
0.310.03
0.200.07
0.340.07
HCO + C5H11
2-methyl-cyclopentanol 0.120.02
0.420.11
2-ethyl-cyclobutanol
n-Heptanal
CH3CHO + C5H10
0.260.02
CO + C6H14
0.270.03
0.230.09
HCO + C6H13
2-methyl-cyclohexanol
0.400.04
2-ethyl-cyclopentanol
10
0.380.07
Table 4. End-Product Yields from the Photolysis of n-Butyraldehyde and n-Pentanal.
Reactant
 (nm)
n-Butyraldehyde 290
310
330
n-Pentanal
290
310
330
Photolysis Channel
C2H4+CH3CHO
C3H6+CH3CHO
(mass)
~0.55
~0.52
~0.56
(HPLC)
~0.84
~0.91
~0.68
Photodissociation Rate Constants to Form HCO Radicals in the Atmosphere.
The atmospheric radical formation rate constants (krad) from the photolysis of C3-C7 aldehydes
were calculated using the actinic solar flux (J()) reported by Demerjian and coworkers,30 the
aldehyde absorption cross sections (()), and the HCO radical quantum yields at 760 Torr
nitrogen pressure (HCO()) using the relationship:
krad = ()·HCO()·J()
(10)
Radical-formation rate constants from the photodissociation of C3-C7 aldehydes were estimated
as a function of the zenith angle under cloudless conditions at sea level and for best-estimate
albedo (5% in the 290-330 nm region31). The results are shown in Fig. 7.
Figure 7: Radical Formation Rate Constant as a Function of Zenith Angle
50
propionaldehyde (square)
n-butyraldehyde (diamond)
n-pentanal (hexagon)
n-hexanal (circles)
n-heptanal (triangles)
-6 -1
krad (10 s )
40
30
20
10
0
0
20
40
60
80
100
Zenith Angle
Our estimated HOx radical-production rate constants for zenith angles in the 30-60° range are
3.8×10-5 to 1.6x10-5 s-1 for propionaldehyde, 3.5×10-5 to 1.5x10-5 s-1 for n-butyraldehyde,
6.710-6 to 2.710-6 s-1 for n-pentanal, 5.810-6 to 2.410-6 s-1 for n-hexanal, and 5.910-6 to
2.510-6 s-1 for n-heptanal. The OH radical production rate constants from ozone photolysis
followed by O(1D)/H2O reaction are about 6.6x10-5 to 1.5x10-5 s-1 for zenith angles in the 30-60°
range. Photodissociation of aldehydes can be a significant source of HOx radicals in the polluted,
VOC rich atmospheric environments. Assuming a photolysis quantum yield of unity in the
actinic UV region for propionaldehyde, n-butyraldehyde, n-pentanl, n-hexanal, and n-heptanal,
their corresponding atmospheric photodissociation lifetimes are on the order of 6.0-17.4 h, 4.911
13.7 h, 4.8-13.4 h, 4.5-12.6 h, and 4.4-12.3 h for zenith angles in the 0-60o range. Rate
constants3,5,7-11 for OH radical reactions with propionaldehyde, n-butyraldehyde, n-pentanal, nhexanal, and n-heptanal are on the order of 1.710-11, 2.410-11, 2.510-11, 2.610-11, and
3.010-11 cm3molecule-1s-1, respectively, which correspond to OH radical reaction lifetimes of
8.2, 5.8, 5.6, 5.3, and 4.6 h for an average noontime OH concentration32 of 2106 cm-3.
Therefore, both photolysis and OH radical reactions are important removal pathways for C3-C7
aldehydes in the atmosphere.
Implications on Atmospheric Modeling
In the carbon bond model, aldehydes have been grouped together while acetaldehyde has been
used as a surrogate for aldehydes >C1. Results from our aldehyde photolysis study indicate that
the photolysis pathways and quantum yields of large aldehydes (C4) are very different from
those of acetaldehyde due to the opening of the Norrish II channel and the photocyclization
channels. As a result, acetaldehyde photolysis is not representative of the photolysis of large
aldehydes. Based upon the results of the photolysis product channel and quantum yield
measurements of C3-C7 aldehydes, we recommend to divide >C1 aldehydes into three groups
when modeling the photochemistry of saturated aliphatic aldehydes. The first group consists of
acetaldehyde and propionaldehyde. The second group consists of n-butyraldehyde. The third
group consists of C5 aldehydes.
ACKNOWLEDGMENTS
We are grateful for the support provided by the National Science Foundation under grant Nos.
ATM-9610285, ATM-0000252, and ATM-0300294.
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Keywords
Photolysis
Aldehydes
Absorption cross sections
Radical quantum yields
Norrish II process
Photocyclization
Cavity ring-down spectroscopy
Photodissociation rate constant and lifetime
14