Infrared absorption cross-sections for peroxyacyl nitrates (nPANs)

Article abstract of DOI:10.1016/j.cplett.2008.10.020

Absorption spectra of peroxyacyl nitrates (nPANs) at room temperature (298 ± 3 K) have been measured in the mid-infrared range (700-2050 cm^-1) using an environmental chamber/FTIR absorption system. nPANs are generated in situ as main products in the NO₃ radical reactions with aldehydes. Both peak absorption cross-sections and integrated band intensities for the principal absorption bands in the peroxypropionyl nitrate (PPN), peroxy-n-butyryl nitrate (PnBN), peroxy-n-valeryl nitrate (PnVN), peroxyacryloyl nitrate (APAN) and peroxycrotonyl nitrate (CPAN) spectra have been derived from quantitative laboratory measurements. Results are compared with literature data when available and in those cases, a good agreement is observed.

Full text of DOI:10.1016/j.cplett.2008.10.020

Chemical Physics Letters 465 (2008) 207–211
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Infrared absorption cross-sections for peroxyacyl nitrates (nPANs)
a
a,*
a
a
b
a
E. Monedero , M.S. Salgado , F. Villanueva , P. Martín , I. Barnes , B. Cabañas
a
Departamento de Química Física, Facultad de Ciencias Químicas, Universidad de Castilla La Mancha, Avda Camilo José Cela 10, 13071 Ciudad Real, Spain
Physikalische Chemie/FB C, 9, Bergische Universität, Gaubstrabe 20, D-42097 Wuppertal, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Absorption spectra of peroxyacyl nitrates (nPANs) at room temperature (298 ± 3 K) have been measured
in the mid-infrared range (700–2050 cm ) using an environmental chamber/FTIR absorption system.
nPANs are generated in situ as main products in the NO radical reactions with aldehydes. Both peak
3
absorption cross-sections and integrated band intensities for the principal absorption bands in the per-
oxypropionyl nitrate (PPN), peroxy-n-butyryl nitrate (PnBN), peroxy-n-valeryl nitrate (PnVN), peroxyac-
ryloyl nitrate (APAN) and peroxycrotonyl nitrate (CPAN) spectra have been derived from quantitative
laboratory measurements. Results are compared with literature data when available and in those cases,
a good agreement is observed.
Received 25 March 2008
In final form 8 October 2008
Available online 11 October 2008
À1
Ó 2008 Elsevier B.V. All rights reserved.
1
. Introduction
lated to the photochemical ozone formation and on the oxidising
potential of the atmosphere [1,4]. Moreover, the long thermal life-
Peroxyacyl nitrates [RC(O)OONO
2
] play an important role in ur-
time of nPANs in the cold upper troposphere allow them to trans-
port NO over wide areas, taking NO from polluted regions and
ban and regional air quality and in tropospheric chemistry. They
are usually referred to as nPANs and they have no known direct
emission sources and are formed in situ by photochemical reac-
tions involving volatile organic compounds (VOCs), such as alde-
hydes, and oxides of nitrogen. Thus, aldehydes are eliminated in
the atmosphere mainly by photolysis or by the reaction with OH
2
2
releasing it in remote locations where active nitrogen chemistry
can influence the production of surface ozone [5]. Peroxyacyl ni-
trates have also received regular attention as eye irritants, muta-
gens, possible agents of skin cancer and phytotoxins [6–11]. But
despite its importance, nPANs are not included in urban air quality
monitoring networks and long-term trends can only be examined
from data obtained in studies carried out in recent years [12,13].
Measurements of nPAN and its homologues have most often
been accomplished by gas chromatography with electron capture
detection (GC-ECD) [12–18]. However, the infrared technique of
measuring trace gases in the air has been evolving with rapid pro-
gress during last years [19]. IR spectroscopy could provide an alter-
native to remotely detect trace organic species such as nPANs by
virtue of their characteristic infrared signature and to determine
global concentrations. It could be an alternative measurement
method to the electron capture gas chromatography avoiding the
problem of the thermal decomposition of nPANs [20].
and NO
and NO
3
radicals. Both atmospheric oxidants (OH at day-time
radicals at night-time) are expected to react preferentially
3
with aliphatic aldehydes through the abstraction of the aldehydic
H-atom. These processes will give acyl radicals, RC(O), which com-
bine with O
rapidly react with NO to give NO
to ozone formation or react with NO
nitrate (RC(O)O NO ) [1].
2
to give acylperoxy radicals RC(O)O
which is then photolyzed leading
to form a stable peroxyacyl
2
. These radicals
2
2
2
2
The reaction with OH is very slow and constitutes a negligible
loss of PAN throughout the troposphere [2]. The rate constant for
thermal dissociation, however, has very strong temperature
dependence that control atmospheric lifetimes of PANs in the low-
er troposphere. At NO/NO
lifetime of PAN varies from about 40 min at temperatures of
00 K to several months at temperatures characteristic of the
upper troposphere. Thus, PANs are quite stable in the mid-upper
troposphere and they can be transported over long distances from
polluted continental regions into the remote troposphere such over
the Pacific Ocean [3].
2
ratios typical for the troposphere, the
In this Letter, using an environmental chamber/FTIR absorption
system, we identify the night-time processes of generation of some
3
nPANs [RC(O)OONO
peroxy-n-butyryl nitrate (PnBN, R = CH
nitrate (PnVN, R = CH (CH –), peroxyacryloyl nitrate (APAN,
R = CH @CH–) and peroxycrotonyl nitrate (CPAN, R = CH
CH @CH–) in the atmosphere due to the reaction of NO radical
2
]: peroxypropionyl nitrate (PPN, R = CH
3 2
CH –),
3
(CH –), peroxy-n-valeryl
2 2
)
3
2 3
)
2
3
–
2
3
Therefore, nPANs species are important in the atmosphere for
several reasons. They act as reservoirs of NO and are closely re-
x
with aldehydes (propionaldehyde, n-butyraldehyde, n-valeralde-
hyde, acrolein (prop-2-en-1-al) and crotonaldehyde (but-2-en-1-
al)) respectively. Using the infrared spectra obtained for the
considered compounds, peak absorption cross-sections and inte-
grated band intensities of the peroxyacyl nitrates formed have been
*
Corresponding author. Fax: +34 926295318.
E-mail address: Sagrario.salgado@uclm.es (M.S. Salgado).
0
009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.10.020

2
08
E. Monedero et al. / Chemical Physics Letters 465 (2008) 207–211
determined. These data will provide a useful spectral reference
dataset for investigations of the potential for retrieval of concentra-
tion information for these compounds from IR spectra of the Earth’s
atmosphere.
The analysis of the complex spectra and the quantification of
reactant and products were carried out by comparison with Quan-
titative reference spectra of the organic compounds, N
2 5
O , NO,
NO , HNO , HCHO and CO, which were taken from a calibrated
2
3
infrared spectra data bank archived by the laboratory. The known
concentration of the reference spectra and the subtraction factors
allowed the concentration of each identified compound to be
determined.
2
. Experimental method
All of the experiments were carried out in a 405 L Pyrex cylin-
In the case of peroxyacyl nitrates, their spectra were obtained
from the final product spectrum of the reaction after the absorp-
tion bands of the identified products and reactants were sub-
tracted. nPANs were identified by comparison with reference
spectra previously published when available [22–25]. This residual
spectrum (Fig. 1 in the case of PPN compared with reference one)
drical glass reactor (1.5 m length and 60 cm inner diameter) with
Teflon coated metal end flanges. This reactor has previously been
described in detail [21]. In brief, a white mirror system (base path
length 1.4 m) mounted inside the reactor is coupled, by an external
mirror system, to a Fourier transform-spectrometer (Nicolet Mag-
na 550) and this arrangement enables the in situ monitoring of
shows several intense absorptions at around 796, 1037, 1300,
both reactants and products by long-path infrared absorption
À1
À1
1
741 and 1834 cm , which are characteristic of the NO
2
scissors,
using a total pathlength of 50.4 m and a resolution of 1 cm
.
NO
2
symmetric stretch, NO
2
asymmetric stretch, and CO stretching
The spectrometer was directly controlled by OMNIC software pro-
vided by Nicolet and running on a personal computer, which was
also used to store raw data.
modes, respectively, in peroxyacyl nitrates.
Chemicals used in this study and their sources and purities
2
were as follows: synthetic air 99.99%, NO 98% (in volume), NO
Nitrate radicals were generated by thermal decomposition of
9
9.5%, O 99.95%, supplied by Messer-Griesheim. Organics: propi-
2
N
2
O
5
. The N
O
2 5
was synthesized in a separate setup by the reaction
onaldehyde 99%, n-butyraldehyde 99%, n-valeraldehyde 97%, acro-
lein 99% and crotonaldehyde 99% were supplied by the Aldrich
Chemical Co. and purified by successive trap-to-trap distillations.
of excess O
3
with NO and trapped and stored at 195 K.
2
First, to obtain the wall losses, the organic compound was intro-
duced alone into the reaction chamber under reduced pressure and
the wall loss rate (k
of plots of ln([C ]/[C
tration of the compound at t = 0 and C
For the studied compounds, wall deposition losses were estimated
3
) for the compound was derived from the slope
]) against time, where C is the initial concen-
its concentration at time t.
0
t
0
3. Results and discussion
t
3.1. Peak absorption cross-sections
À5
À6 À1
to be negligible (10 –10 ) s , although this was corrected in the
experiments. After this determination, N was flushed into the
chamber through a Teflon line by evaporating solid N . In total,
the measurement time period for one experiment was about
O
2 5
The peak cross-sections calculation for PPN, PnBN y PnVN is
similar, and we will explain as example the procedure in the case
of PPN.
2 5
O
3
2
0 min, with 128 scans recorded per spectrum over a period of
min and 15 such spectra collected. Once the N has been con-
PPN is clearly the sole significant product obtained in the reac-
tion of propionaldehyde with NO . Assuming yields of 100% for this
3
2
O
5
sumed, possible wall deposition of the products formed was stud-
ied as described above and once again the losses were negligible.
The experiments were conducted at total pressure of
product and neglecting all potential losses, it is reasonable to
determine PPN concentrations using propionaldehyde consump-
tion during the experiment. The intensities of the absorption fea-
tures observed in the product spectra at 796, 1050, 1304, 1738
7
60 ± 10 Torr (synthetic air) and at a temperature of 298 ± 3. The
1
4
À1
initial aldehydes concentration range was (0.3–8.9) Â 10 mole-
and 1835 cm grew proportionally (i.e. their relative intensities
À3
cule cm
6
and the initial N
2
O
5
concentration range was (1.4–
remained constant) and they are all due to PPN. Therefore, the peak
absorption cross-section at each wavelength was obtained as the
slope of plots of the infrared absorbance as a function of the prod-
1
3
À3
2
.3.1) Â 10 molecule cm . Initial NO concentration range was
1
3
À3
(
0.5–7.6) Â 10 molecule cm
.
Fig. 1. Left trace: infrared residual absorption spectra observed of the reaction of NO
3
with propionaldehyde (A, assigned to PPN), n-butyraldehyde (B, assigned to PnBN), n-
valeraldehyde (C, assigned to PnVN), acrolein (D, assigned to APAN) and crotonaldehyde (E, assigned to CPAN), after subtraction process. Right trace: literature spectra of PAN,
PPN, APAN and CPAN for comparison [24,26]. Some small contaminants can be seen but it was impossible to remove completely. That is one of the reasons that we assumed
1
5% error in subtraction process. Complete spectra data base is available in Ref. [31].

E. Monedero et al. / Chemical Physics Letters 465 (2008) 207–211
209
2
1
1
0
8
6
1
760-830 cm-1
a 0.9
1835 cm-1
1738 cm-1
b
1
1
010-1080 cm-1
250-1340 cm-1
1
1
7
301 cm-1
044 cm-1
96 cm-1
1700-1777 cm-1
835-1845 cm-1
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
0
1
14
1
1
2
0
8
6
4
2
0
0
2E+17
4E+17
6E+17
PPN] x l/ cm-2 molecule
8E+17
1E+18
1.2E+18
0
2E+17
4E+17
6E+17
[PPN] x l/ cm-2molecule
8E+17
1E+18
1.2E+18
[
Fig. 2. (a) Infrared absorbance taken at the quoted band centre as a function of PPN concentration  pathlength. (b) Integrated band area vs [PPN]  pathlength, for each of
the wavenumber interval due to PPN.
Table 1
Wavenumbers
PPN
PnBN
PnVN
APAN
CPAN
À1
(
cm
)
7
7
9
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
93
96
50
(10.8 ± 3)a
(13.1 ± 0.2)a
(9.04 ± 0.09)a
(2.76 ± 0.09)a
(5,40 ± 0.09)a
(2.59 ± 0.05)a
(7.31 ± 0.14)a
(6.95 ± 0.3) 6.5b
a
037
044
050
070
193
205
300
301
304
738
738
740
741
748
810
818
832
834
835
(4,.4 ± 0.03)a
(15.6 ± 0.7) 18b
a
(2.14 ± 0.1) 1.8b
a
a
b
(2.88 ± 0.2) 3.4
(11.2 ± 0.3) 13
(5.76 ± 0.12)a
a
b
(12.5 ± 0.3) 11b
a
(10.1 ± 0.2)a
(20.6 ± 0.2)a
(7.84 ± 0.2)a
(1.98 ± 0.30)
a
(28,5 ± 0.4) 32b
a
(14.5 ± 0.2)a
(24.2 ± 1) 28b
a
(6.84 ± 0.1)a6b
(7.78 ± 0.5) 8.0b
a
(4.66 ± 0.04)a
(3.63 ± 0.06)a
(5.97 ± 0.07)a
a
À19
À1
2
Peak absorpBUGH Wuppertal, Wuppertaltion cross-section (in units 10
Values previously determined for APAN and CPAN [26].
molecule cm ), for PPN, PnBN, PnVN, APAN and CPAN determined in this work.
b
uct of PPN concentration with the optical pathlength according to
Lambert–Beer’s law. Fig. 2a shows these plots for each of the wave-
number due to PPN. Similar plots could be obtained for the calcu-
lations of the peak absorption cross-sections of PnBN and PnVN.
Values determined are shown in Table 1. The errors quoted repre-
of CO and HCHO could be calculated using calibrated spectra for
these compounds and, therefore, from the difference with acrolein
consumed we can ascertain the concentration of APAN. The same
procedure was followed in the case of the reactions of crotonalde-
3
hyde with NO where crotonylperoxy nitrate (CPAN) and CO were
sent the two standard deviation errors (2
r
) calculated from Beer’s
detected as products [25]. APAN and CPAN were identified by com-
parison with a reference spectrum [26]. Values of peak absorption
law fits at the bands centres. Because wall loss deposition rates
were already considering in the calculations, the main systematic
uncertainty arose from uncertainties in the FTIR subtraction proce-
dure and is considered about 15%. No other values for peak absorp-
tion cross-sections for these compounds have previously been
published.
Table 2
Peroxypropionyl nitrate integrated band intensities (IBI, cm2 molecule cm
À1
À1
)
compared with bibliographic references.
Band
Integrated band
Gaffney
et al. [24]
Le Crane
et al. [30]
This work
For APAN and CPAN peak cross-sections evaluation there are
À1
(
cm
)
strength
several differences. Thus, in the NO
3
radical-initiated oxidation of
À1
(
cm
)
acrolein, formaldehyde, CO and acryloylperoxy nitrate (APAN)
were the main reaction products detected [25]. In order to deter-
mine the concentration of APAN we estimated that all acrolein
consumed was used to form HCHO, CO and APAN (because other
products are not observed in a significant way). The concentration
À17
À17
À17
À17
À17
À17
À17
7
96
760–830
1.97 Â 10
1.23 Â 10
2.86 Â 10
5.03 Â 10
2.45 Â 10
2.00 Â 10
1.17 Â 10
2.88 Â 10
5.16 Â 10
(1.83 ± 0.7) Â 10
(1.04 ± 0.5) Â 10
(2.56 ± 0.1) Â 10
(4.40 ± 0.9) Â 10
(2.29 ± 0.5) Â 10
À17
À17
À17
À17
À17
À17
À17
1044
1010–1080
1250–1340
1700–1777
1777–1880
1301
1738
À17
1835
2.6 Â 10

2
10
E. Monedero et al. / Chemical Physics Letters 465 (2008) 207–211
Table 3
Integrated band intensities (IBI) calculated in this work for PnBN and PnVN and spectral ranges used for calculations.
PnBN
Band
PnVN
Band
Integrated band strength
IBI/cm2
(molecule cm
Integrated band strength
IBI/cm2
(molecule cm )
À1
À1
À1
À1
À1
À1
À1
À1
(
cm
)
(cm
)
)
(cm
)
(cm
)
À17
À17
À17
À17
À17
À17
À17
7
96
763–849
980–1185
1223–1340
1777–1691
1777–1880
(2.56 ± 0.05) Â 10
(3.52 ± 0.20) Â 10
796
1050
1304
1738
1832
764–825
1010–1187
1270–1350
1705–1773
1773–1873
(1.20 ± 0.10) Â 10
(1.45 ± 0.15) Â 10
(1.82 ± 0.14) Â 10
(3.16 ± 0.11) Â 10
(1.21 ± 0.17) Â 10
1037
1300
1741
1834
À17
(3.62 ± 0.2) Â 10
(5.80 ± 0.2) Â 10
(2.33 ± 0.05) Â 10
À17
À17
Table 4
Integrated band intensities (IBI) calculated in this work for APAN and CPAN and spectral ranges used for calculations.
APAN
CPAN
Band
Band
Integrated band strength
IBI/cm2
(molecule cm
Integrated band strength
IBI/cm2
(molecule cm
À1
À1
À1
À1
À1
À1
À1
À1
(
cm
)
(cm
)
)
(cm
)
(cm
)
)
À17
À17
À18
À17
7
93
769–818
1047–1104
1180–1220
1265–1332
1734–1763
1792–1843
(2.42 ± 0.07) Â 10
(2.84 ± 0.07) Â 10
(4.49 ± 0.02) Â 10
793
950
1193
1300
1740
1810
765–812
930–986
1175–1215
1270–1328
1730–1760
1780–1847
(1.91 ± 0.08) Â 10
À19
1070
1205
1300
1748
1818
(8.12 ± 0.3) Â 10
(4.90 ± 0.2) Â 10
(2.95 ± 0.1) Â 10
(2.68 ± 0.1) Â 10
À18
À17
À17
À17
(2.53 ± 0.1) Â 10
(5.20 ± 0.4) Â 10
(1.40 ± 0.05) Â 10
À18
À17
À17
(1.71 ± 0.07) Â 10
cross-section determined here for APAN and CPAN are also shown
in Table 1. Values obtained are in good agreement with previously
determined for these compounds (also presented in the table) by
Orlando and Tyndall [26] with deviation always lower than 15%.
Additionally, we used the absorption cross-sections previously cal-
culated by Orlando and Tyndall [26] for calculation of APAN and
CPAN concentrations in our experiments, and similar results have
been obtained, confirming the reliability of the calculation method
of concentrations.
previously determined by Orlando and Tyndall [26], we confirm
again the reliability of the IBI values determined.
The infrared absorption cross-sections determined here will in-
crease the available database required for calculation of atmo-
spheric nPANs concentrations, indicating that IR spectroscopy
could provide an alternative technique to remotely detect trace or-
ganic species by virtue of their characteristic infrared signature
and then to determine global concentrations. For nPANs evalua-
tion, it is important to consider, when quantitative determinations
were made, the overlapping bands observed in the spectra re-
ported here.
3.2. Integrated band intensities (IBI) calculation
Integrated band areas at room temperature of the five principal
Acknowledgments
absorption bands obtained for peroxyacyl nitrates were deter-
mined by integrating over the wavenumber covered by the absorp-
tion band of interest. Once the integrated band area was
determined, the plot of these values versus ([peroxyacyl
nitrate] Â pathlength) allowed us to obtain the integrated band
intensities (IBI) as the slopes of the least squares linear regression
fits. Fig. 2b shows this plot in the case of PPN. Similar plots could be
obtained for the calculations of the integrated band intensities of
PnBN, PnVN, APAN and CPAN that have similar absorption bands
in the IR region.
Esperanza Monedero and Florentina Villanueva thanks to the
‘Ministerio de Educación y Cultura’ and Junta de Comunidades de
Castilla La Mancha respectively, for personal grants. This work
was supported by the project BQU-2001-1574 granted by the
C.I.C.Y.T (‘Comisión Interministerial de Ciencia y Tecnología’). The
authors also thank the cooperation of the Wuppertal University,
especially the Physikalische Chemie for the possibility of develop-
ing these experiments.
There are several studies concerning to IBI calculations for per-
oxyacetyl nitrate PAN and for peroxypropionyl nitrate PPN [24,26–
References
[
[
1] R. Atkinson, Atmos. Environ. 34 (2000) 2063.
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3
0]. Reference IBI values for PPN are compared with the ones ob-
tained in our experimental study in Table 2. As it can be seen in
the table, within the stated errors, our data are in good agreement
with other calculations (deviation always lower than 15%). Table 3
summarizes obtained results for PnBN and PnVN although, to date,
no other calculations of the integrated band intensities of these
compounds are available in literature. Reasonability, we expect
that the values of Integrated Band Intensities calculated for PnBN
and PnVN are well determined.
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achieved. Due to the good agreement (deviation always lower than
[
[
1
5%) of peak absorption cross-sections for these compounds with

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