References for GEISA-2015 Thermal and Near-InfraRed absorption cross-sections sub-database Update

A: GEISA-2015 THERMAL INFRARED (TIR) ABSORPTION CROSS-SECTIONS

Thirty-nine molecules were represented by cross sections in GEISA-2011. The GEISA-2015 compilation has been updated with additional cross-section datasets, consisting of multiple temperature–pressure combinations, for new and existing molecules. This represents more than about 50% increase in information volume, with the introduction of 41 new molecular species. This update stems mainly from two sources:

1. CROSS-SECTION SET FROM THE University of Oslo [1,2] AND THE UNIVERSITY OF CASTILLA-LA MANCHA [ 3,4,5].

These compounds (halocarbons, bromocarbons, bromofluorocarbons, bromochlorofluorocarbons, halogenated alcohols, halogenated, fluorinated ethers and perfluorinated compounds) contribute to the global warming;

Updated IR absorption cross-sections for CFC-12 and 31 fluorinated compounds, included in a recent review of Hodnebrog et al. [1] were added to GEISA-2015 and are listed in Table 1, which provides for each compounds: its chemical formula, common name, spectral information, i.e.: spectral range, gas bath, foreign broadening pressure (Pa), and references. These compounds (halocarbons, bromocarbons, bromofluorocarbons, bromochlorofluorocarbons, halogenated alcohols, halogenated, fluorinated ethers and perfluorinated compounds) contribute to the global warming.

References

[1] Ø. Hodnebrog, M. Etminan, J.S. Fuglestvedt, G. Marston, G. Myhre, C.J. Nielsen, K.P. Shine, T.J. Wallington, Global warming potentials and radiative efficiencies of halocarbons and related compounds: A comprehensive review, Reviews of Geophysics. 51 (2013) 300-378.

[2] C.J. Nielsen, Private communication, 2014

[3] E. Jiménez, M. Antinolo, B. Ballesteros, E. Martinez, J. Albaladejo, Atmospheric Lifetimes and Global Warming Potentials of CF₃CH₂CH₂OH and CF₃(CH ₂)₂CH₂OH, Phys. Chem. Chem. Phys. 11 (2010) 4079-4087.

[4] M. Antiñolo, S. González, B. Ballesteros, J. Albaladejo, E. Jiménez, Laboratory Studies of CHF₂CF₂CH ₂OH and CF₃CF₂CH₂OH: UV and IR Absorption Cross Sections and OH Rate Coefficients between 263 and 358 K, J. Phys. Chem. A. 116 (2012) 6041-6050.

[5] M. Antiñolo, Fluoroalcohols and Fluoroaldehydes in the Troposphere: Kinetics and Photochemistry in the Gas Phase sutdied by Pulsed Laser Techniques, PhD Thesis, University of Castilla-La Mancha. 2011.

[6] G. Myhre, F. Stordal, I. Gausemel, C.J. Nielsen, E. Mahieu, Line-by-line calculations of thermal infrared radiation representative for global condition: CFC-12 as an example, J. Quant. Spectrosc. Radiat. Transfer 97 (2006) 317-331.

[7] G. Acerboni, J.A. Beukes, N.R. Jensen, J. Hjorth, G. Myhre, C.J. Nielsen, J.K. Sundet, Atmospheric degradation and global warming potentials of three perfluoroalkenes, Atmos. Environ. 35 (2001) 4113-4123.

[8] G. Myhre, C.J. Nielsen, D.L. Powell, F. Stordal, Infrared absorption cross section, radiative forcing, and GWP of four hydrofluoro(poly)ethers, Atmos. Environ. 93 (1999) 4447-4458.

[9] S.M. Ryan, C.J. Nielsen, Global Warming Potential of Inhaled Anesthetics: Application to Clinical Use, Anesth. Analg. 111 (2010) 92-98.

[10] B. D'Anna, S.R. Sellevag, K. Wirtz, C.J. Nielsen, Photolysis study of perfluoro-2-methyl-3-pentanone under natural sunlight conditions, Environ. Sci. Technol. 39 (2005) 8708-8711.

[11] N. Oyaro, S.R. Sellevag, C.J. Nielsen, Study of the OH and Cl-initiated oxidation, IR absorption cross-section, radiative forcing, and global warming potential of four C-4-hydrofluoroethers, Environ. Sci. Technol. 38 (2004) 5567-5576.

[12] N. Oyaro, S.R. Sellevag, C.J. Nielsen, Atmospheric chemistry of hydrofluoroethers: Reaction of a series of hydrofluoro ethers with OH radicals and Cl atoms, atmospheric lifetimes, and global warming potentials, J. Phys. Chem. A. 109 (2005) 337-346.

[13] S.R. Sellevag, C.J. Nielsen, O.A. Sovde, G. Myhre, J.K. Sundet, F. Stordal, I.S.A. Isaksen, Atmospheric gas-phase degradation and global warming potentials of 2-fluoro ethanol, 2,2-difluoroethanol, and 2,2,2-trifluoroethanol, Atmos. Environ. 38 (2004) 6725-6735.

[14] S.R. Sellevåg, B. D'Anna, C.J. Nielsen, Infrared Absorption Cross-Sections and Estimated Global Warming Potentials of CF₃CH₂CH₂OH, CHF₂CF₂CH₂OH, CF₃CF₂CH₂OH, CF₃CHFCF₂CH₂OH, and CF₃CF₂CF₂CH₂OH, Asian Chemistry Letters. (2007) 33-40.

[15] S.R. Sellevåg, T. Kelly, H. Sidebottom, C.J. Nielsen, A study of the IR and UV-Vis absorption cross-sections, photolysis and OH-initiated oxidation of CF₃CHO and CF₃CH ₂CHO, Phys. Chem. Chem. Phys. 6 (2004) 1243-1252.

Table 1: Summary of GEISA-2015 infrared absorption cross-sections update. Data from University of Oslo [1,2] and the University of Castilla-La Mancha [3,4,5] at room temperature and spectral resolution of 1 cm^-1.

Molecule	Common or Chemical Name	Spectral range (cm^-1)	Bath gas	Foreign broadening pressure (Pa)	Refs ^.a
CCl₂F₂	CFC-12	800–1300	N₂	101325.72	[6]
CCl₂F₂	CFC-12	800–1300	Pure	–	[6]
CF₂=CF₂	PFC-1114	100–2600	Pure	–	[7]
CF₃CF=CF₂	PFC-1216	100–2600	Pure	–	[7]
CF₂=CFCF=CF₂	Perfluorobut-2-ene	100–2600	Pure	–	[7]
CHF₂OCF₂OCHF₂	HFE-235ca12	25–3250	Pure	–	[8]
CHF₂OCF₂CF₂OCHF₂	HFE-338pcc13	25–3250	Pure	–	[8]
(CF₃)₂CHOCH₂F	HFE-347mmz1 (Sevoflurane)	400–4000	Pure	–	[9]
CHF₂OCHClCF₃	HCFE-235da2 (Isoflurane)	400–4000	Pure	–	[9]
(CF₃)₂CFC(O)CF₂CF₃	Perfluoro(2-methyl-3-pentanone)	450–2000	Pure	–	[10]
CHF₂CF₂CH₂OCH₃	HFE-374pcf	450–3200	Pure	–	[11]
CF₃CF₂CH₂OCH₃	HFE-365mcf	450–3200	Pure	–	[11]
CF₃CH₂OCH₂CF₃	HFE-356mf-f	450–3200	Pure	–	[11]
(CF₃)₂CHOCH₃	356mmzEβγ	450–3200	Pure	–	[11]
CHF₂CHFOCF₃	1,1,2-Trifluoro-2-(trifluoromethoxy)-ethane	440–3200	Pure	–	[12]
CF₃CHFOCF₃	HFE-227ea	440–3200	Pure	–	[12]
CHF₂OCHFCF₃	HFE-236 (Desflurane)	440–3200	Pure	–	[12]
CF₃CHFCF₂OCH₂CH₃	1-Ethoxy-1,1,2,3,3,3-hexafluoropropane	440–3200	Pure	–	[12]
CF₃CF₂CF₂OCHFCF₃	1,1,1,2,2,3,3-Heptafluoro-3-(1,2,2,2-tetrafluoroethoxy)-propane	440–3200	Pure	–	[12]
CHF₂OCH₂CF₃	HFE-245fa2	440–3200	Pure	–	[12]
CF₃CH₂OCH₃	HFE-263fb2	440–3200	Pure	–	[12]
CF₃CFHCF₂OCF₂H		500-3500	Pure	–	[12]
CH₂FCH₂OH	2-fluoroethanol	80–4800	Pure	–	[13]
CHF₂CH₂OH	2,2-difluoroethanol	70–4800	Pure	–	[13]
CF₃CH₂OH	2,2,2-trifluoroethanol	70–4800	Pure	–	[13]
CF₃CF₂CH₂OH	2,2,3,3,3-pentafluoropropan-1-ol	400–4000	Pure	–	[14]
		500–4000	He	773.3-13065.6 ^b	[4]
		500–4000	Pure	129.3-533.3 ^b	[4]
CHF₂CF₂CH₂OH	2,2,3,3-tetrafluoro-1-propanol	400–4000	Pure	–	[14]
		500–4000	He	746.6-12798.9 ^b	[4]
		500–4000	Pure	41.3-200.0 ^b	[4]
CF₃CF₂CF₂CH₂OH	2,2,3,3,4,4,4-Heptafluoro-1-butanol	400–4000	Pure	–	[14]
CF₃CHFCF₂CH₂OH	2,2,3,4,4,4-Hexafluoro-1-butanol	400–4000	Pure	–	[14]
CF₃CH₂CH₂OH	3,3,3-trifluoropropan-1-ol	400–4000	Pure	–	[14]
CF₃CH₂CH₂OH	3,3,3-trifluoropropan-1-ol	500–4000	He	666.6-20797.9	[3]
CF₃(CH₂)₂CH₂OH	4,4,4-trifluoro-1-butanol	500–4000	He	666.6-21864.5	[3]
CF₃CHO	trifluoroethanol	400–2500	Pure	–	[15]
CF₃CH₂CHO	3,3,3-trifluoropropanal	400–3500	Pure	–	[15]
CF₃CH₂CHO	3,3,3-trifluoropropanal	500–4000	He	626.6-7999.2	[5]
CF₃(CH₂)₂CHO	4,4,4-trifluorobutanal	500–4000	He	493.3-9065.7	[5]

^a Each reference corresponds to a single T/P dataset.

^b Experiments performed with diluted compound in He provide the same infrared absorption cross-sections.

2. CROSS-SECTIONS SET FROM THE University of York [1,2].

This set includes complementary data for 4 molecular species already present in GEISA-2011, i.e.: C₂H₆, C₃H₈, CH ₃CN, C₃H₆O, as well as added 3 new added molecular species, i.e.: methanol: (CH₃OH) , trifluoromethane (CHF₃) and acetaldehyde (CH₃CHO).

References

[1] J.J. Harrison, P.F. Bernath, Mid- and long-wave infrared absorption cross sections for acetonitrile, J. Quant. Spectrosc. Radiat. Transfer 113 (2012) 221-225.

[2] J.J. Harrison, Private communication, 2013.

--> Complementary data for species already implemented in GEISA-2011

Ethane ( C₂H₆)

Infrared absorption cross sections for ethane over the spectral range 2545–3315 cm^-1 [1] have been included in GEISA-2015. These cross sections provide a higher degree of accuracy for tropospheric sounding than can currently be obtained using the line list. Spectra of ethane / dry synthetic air mixtures inside a 26-cm cell were recorded at fourteen pressure–temperature combinations using a high-resolution FTIR spectrometer (Bruker IFS 125 HR) at 0.015 cm^-1 resolution (using the Bruker definition of 0.9/MOPD).

Reference

[1] J.J. Harrison, N.D.C Allen, P.F. Bernath, Infrared absorption cross sections for ethane (C₂H₆) in the 3 mm region, J. Quant. Spectrosc. Radiat. Transfer 111 (2010) 357-363.

Propane CH₃CH₂CH₃ (C₃H₈)

Absorption cross sections have been included in GEISA-2015 for the first time over the spectral range 2540–3300 cm^-1 [1]. They cover the spectral region where propane has its strongest-intensity absorbance features (C-H stretch). Spectra of propane / dry synthetic air mixtures inside a 26-cm cell were recorded at twelve pressure–temperature combinations using a high-resolution FTIR spectrometer (Bruker IFS 125 HR) at 0.015 cm^-1 resolution (=0.9/MOPD).

Reference

[1] J.J. Harrison, P.F. Bernath, Infrared absorption cross sections for propane (C₃H₈) in the 3 mm region, J. Quant. Spectrosc. Radiat. Transfer 111 (2010) 1282-1288.

Acetone ((CH₃)₂CO)

Two new datasets have been added in GEISA-2015; these cover the spectral ranges 830–1950 cm^-1 [1] and 2615–3300 cm ^-1 [2]; the new mid-IR cross sections have been combined with a renormalized subset of those in GEISA-2011 to create a more optimised dataset for this spectral region. For the new measurements, spectra of acetone / dry synthetic air at a number of pressure–temperature combinations were recorded by a high spectral resolution FTIR spectrometer (Bruker IFS 125 HR) at 0.015 cm ^-1 resolution (=0.9/MOPD) using a cooleable White cell with a maximum path length of 19.32 m.

References

[1] J.J. Harrison, N. Humpage, N.D.C. Allen, A.M. Waterfall, P.F. Bernath, J.J. Remedios, Mid-infrared absorption cross sections for acetone (propanone), J. Quant. Spectrosc. Radiat. Transfer 112 (2011a) 457-464.

[2] J.J. Harrison, N.D.C. Allen, P.F. Bernath, Infrared absorption cross sections for acetone (propanone) in the 3 mm region, J. Quant. Spectrosc. Radiat. Transfer 112 (2011b) 53-58.

Acetonitrile (CH₃CN)

GEISA-2015 contains additional new infrared absorption cross sections, covering the spectral ranges 880–1700 cm^-1 [1] and 2550–3300 cm^-1 [2]. Spectra of acetonitrile / dry synthetic air at a number of pressure–temperature combinations were recorded by a high-resolution FTIR spectrometer (Bruker IFS 125 HR) at 0.015 cm ^-1 resolution (=0.9/MOPD) using a cooleable White cell with a maximum path length of 19.32 m.

References

[1] J.J. Harrison, P.F. Bernath, Mid- and long-wave infrared absorption cross sections for acetonitrile, J. Quant. Spectrosc. Radiat. Transfer 113 (2012) 221-225.

[2] N.D.C. Allen, J.J. Harrison, P.F. Bernath, Acetonitrile (CH ₃CN) infrared absorption cross sections in the 3 μm region, J. Quant. Spectrosc. Radiat. Transfer 112 (2011) 1961-1966.

--> Molecular species added since the GEISA-2011 edition

Methanol (CH₃OH )

Two new infrared absorption cross section datasets have been added to the database, covering the spectral ranges 877-1167 cm^-1 and 2600-3250 cm^-1 [1].

Spectra of methanol/dry synthetic air at a number of pressure–temperature combinations were recorded by a high-resolution FTIR spectrometer (Bruker IFS 125 HR) at 0.015 cm^-1 resolution (=0.9/MOPD) using a cooleable White cell with a maximum path length of 19.32 m.

Reference

[1] J.J. Harrison, N.D.C. Allen, P.F. Bernath, Infrared absorption cross sections for methanol, J. Quant Spectrosc Radiat Transfer 113 (2012) 2189-2196.

Trifluoromethane ( CHF₃, HFC-23)

New infrared absorption cross sections for trifluoromethane over the spectral range 950–1500 cm^-1 [1] have recently been made available; these are included in GEISA for the first time. Spectra of trifluoromethane / dry synthetic air mixtures inside a 26-cm cell were recorded at twenty-seven pressure–temperature combinations using a high-resolution FTIR spectrometer (Bruker IFS 125 HR) at 0.015 cm ^-1 resolution (=0.9/MOPD).

Reference

[1] J.J. Harrison, Infrared absorption cross sections for trifluoromethane, J. Quant. Spectrosc. Radiat. Transfer 130 (2013) 359-364.

Acetaldehyde (CH₃CHO)

Acetaldehyde, a trace molecular species, found in the Earth’s atmosphere, plays an important role as a source of ozone (O₃ ), PAN and HOx radicals.

Infrared absorption cross-sections have been measured by Tereszchuk et al. [1] in the 3 µm region (2400–3400 cm ^-1) from spectra obtained using a FTIR spectrometer at a resolution of 0.005 cm^-1. See Ref. [1] for details.

Reference

[1] K.A. Tereszchuk, P.F. Bernath, Infrared absorption cross-sections for acetaldehyde (CH₃CHO) in the 3 µm region, J. Quant. Spectrosc. Radiat. Transfer 112 (2011) 990-993

The absorption cross section (in units of cm² molecule ^-1) provided by York University for GEISA-2015 update are summarized in Table 2.

Table 2: Absorption cross-sections provided by the University of York for GEISA-2015 update. For each molecular species listed are given: the temperature (T) range (K), the pressure (P) range (Torr), the number of T/P sets, and the spectral range (cm ^-1).

In green: updated molecular species, already implemented in GEISA-11

In blue: newly implemented molecular species in GEISA-2015

Molecule	Temperature range (K)	Pressure range (Torr)	Number of T/P sets	Spectral range (cm^-1)
Ethane C₂H₆	194-297	49-763	14	2545-3315
Propane CH₃CH₂CH₃(C₃H₈)	195-296	40-763	12	2540-3300
Acetone (CH₃)₂CO	194-298	50-700	19	830-1950
Acetone (CH₃)₂CO	195-296	49-759	12	2615-3300
Acetonitrile CH₃CN	203-297	50-760	12	880-1700
Acetonitrile CH₃CN	208-296	50-760	11	2550-3300
Methanol CH₃OH	204-295	50-761	12	877-1167
Methanol CH₃OH	204-296	51-761	12	2600-3250
Trifluoromethane CHF₃	188-294	23-762	27	950-1500
Acetaldehyde CH₃CHO	200-297	50-762	16	2400-3400

B: GEISA-2015 NEAR INFRARED (NIR) ABSORPTION CROSS-SECTIONS

The 1-3 µm near-infrared spectral region is of great interest for atmospheric remote-sensing and planetary science.

The sections below present and document a first set of seven high-resolution absorption cross-sections as reference data in the near-infrared region, for molecules and bands where no theoretical prediction is available.

An overview of its contents is given in Table 3. The common names of the molecules and their formula are listed in the two first columns. The spectral regions covered (cm^-1), the spectral resolution (cm^-1), the maximal uncertainties of the spectral position (cm ^-1) and absorption cross-sections (%), are given in columns two to five, and references in the final column.

Table 3: Summary of the molecules whose experimental absorption cross-sections in the NIR are newly implemented provided in GEISA-2015 database.

Molecule		NIR range (cm^-1)	Spectral resolution (cm^-1)	Uncertainty		Refs.
Molecule		NIR range (cm^-1)	Spectral resolution (cm^-1)	position (cm^-1)	cross-section (%)
Acetonitrile	CH₃CN	6814 – 7067	0.001	0.01	15	[1]
Methyl iodide	CH₃I	7473 – 7497	0.001	0.01	10	[2]
Methyldioxidanyl	CH₃O₂	7474 – 7497	0.025	0.01	30	[2]
Formaldehyde	H₂CO	6547 – 7051	0.001	0.005	20	[3,4]
Hydroperoxy radical	HO₂	6604 – 6696	0.003	0.01	15	[5,6]
Nitrous acid	HONO	6624 – 6645	0.005	0.01	40	[8]
Ammonia	NH₃	6880 – 6997	0.001	0.005	20	[9]

Acetonitrile (CH₃CN)

Absorption cross-sections of Acetonitrile between 6814 and 7067 cm^-1were measured, by O’Leary et al. [1 ], with off-axis CW-CEAS at 5 mbar with a resolution of about 0.001 cm^-1. There are about 4630 absorption lines in this spectrum. Absorption features of H₂O in this region have been removed from the spectrum. Approximately 200 individual overlapping spectral segments have been concatenated to cover the entire spectral range.

Reference

[1] D.M. O’Leary, A.A. Ruth, S. Dixneuf, J. Orphal, R. Varma, The near infrared cavity-enhanced absorption spectrum of methylcyanide, J. Quant. Spectrosc. Radiat. Transfer 113 (2012) 1138-1147.

Methyl iodide , also called iodomethane (CH₃I)

Measurements, by Farag¢ et al. [2], of methyl iodide were made using CW-CRDS in the wavenumber range 7473–7497 cm^-1at a total pressure of 50 Torr and a resolution of 0.001 cm^-1. CH ₃I was prepared as a diluted mixture in helium, and its concentration was determined from calibrated flowmeters. The spectrum was measured in several small portions in order to minimize a shift in the baseline between measurement of the spectrum with and without CH ₃I. There are small gaps in the spectrum at wavelength ranges corresponding to absorption lines of water where data have been erased because water was present in the cell.

Reference

[2] E.P. Faragó, B. Viskolcz, C. Schoemaecker, C. Fittschen, Measurement of the absorption spectrum and of absolute absorption cross-sections of CH ₃O₂ Radicals and CH₃I in the near IR Region, J. Phys. Chem. A 117 (2013) 12802-12811.

Methyldioxidanyl (CH₃O₂)

Measurements, by Farag¢ et al. [2], of methyldioxidanyl were made using CW-CRDS coupled to laser photolysis. The wavenumber range 7474–7493 cm^-1 was scanned at a resolution of 0.025 cm^-1. CH₃O₂ was generated by pulsed photolysis of CH₃I in the presence of O₂. Absolute CH₃O ₂ concentrations have been deduced by measuring the time-resolved absorbance following the photolysis pulse and adjusting the decay rate to the well-known rate constant of the self-reaction of CH₃O₂ radicals [2]. Calibration of CH₃O ₂ concentration was obtained by measuring the time-resolved evolution of the CH₃O₂ concentration and fitting the kinetic decay traces of CH₃O₂ to a bimolecular reaction. Using the well-known rate constant of the self-reaction allows retrieval of the initial CH₃O₂ concentrations. A generally broad absorption spectrum was obtained containing three striking absorption features located at 7748.18, 7489.16 and 7493.33 cm^-1 . For these three characteristic lines absolute absorption cross-sections of 3.41x10^-20, 3.40x10^-20and 2.11x10^-20 cm² were established, respectively. The remainder of the broad spectrum was scaled according to these cross-sections. Within the error limit of the measurement the cross-sections were not affected by changes of the pressure between 50 and 100 Torr. The error is estimated to be 30%, mostly due to uncertainty in the rate constant for the self-reaction.

Reference

Formaldehyde (H₂CO)

Absorption cross-sections for formaldehyde were measured, by Staak et al. [3], with CW-CEAS at 2 mbar in the range 6547–7051 cm^-1 with a resolution of about 0.001 cm^-1. The absorption cross-sections were evaluated by comparison with the known measured line-intensities of CO ₂and H₂O. H₂CO was prepared by pyrolysis of paraformaldehyde under vacuum. The gaseous H₂CO was ﬁrst passed through a cooling trap below 200 K to remove water vapor and polymerization products of H₂CO. The monomeric H₂CO was trapped and stored at 77 K under vacuum. H₂CO gas was introduced into the cavity by slowly heating the solid H₂CO from the cooling trap; the temperature of the system was 291 ± 2 K. It was found that the absorption cross-sections from Staak et al. [3] were systematically a factor of 2 too large. A summary is given in Ruth et al. [4] together with new absorption data on H₂CO between 6804 and 7051 cm^-1. GEISA-2015 contains the data from Ruth et al. [4] and those of Staak et al. [3] corrected by a factor of 2 in the spectral range 6547–7051 cm^-1.

References

[3] M. Staak, E.W. Gash, D.S. Venables, A.A. Ruth, The rotationally-resolved absorption spectrum of formaldehyde from 6547 to 6804 cm^-1, J. Molec. Spectrosc. 229 (2005) 115-121.

[4] A.A. Ruth, U. Heitmann, E. Heinecke, C. Fittschen, The rotationally-resolved absorption spectrum of formaldehyde from 6550 to 7050 cm^–1, Z. Phys. Chem. 229 (2015) 1609-1624.

Hydroperoxy radical (HO₂)

Measurements of the HO₂radicalwere made by Thiebaud et al. [5] and Ibrahim et al. [6] at a total pressure of 50 Torr. HO₂ radicals were generated by reaction of Cl-atoms with CH ₃OH in the presence of O₂. Cl-atoms were generated either by photolysis of SOCl₂ at 248 nm or by photolysis of Cl₂ at 351 nm. The spectrum was measured in the 6604-6696 cm^-1 wavenumber range with a resolution of better than 0.003 cm ^-1. A few selected lines were calibrated by determining the absolute, initial HO₂ concentration in the same way as CH₃O ₂: time resolved HO₂ absorption decays were measured following their pulsed photolysis. As the decay is governed by a bimolecular reaction, the initial HO₂ radical concentration can be deduced from the shape of the decay . More details on the pressure broadening in HO₂ can be found in [7].

References

[5] J. Thiebaud, S. Crunaire, C. Fittschen, Measurements of line strengths in the 2n₁ band of the HO₂ radical using laser photolysis/continuous wave cavity ring-down spectroscopy (cw-CRDS), J. Phys. Chem. A 111 (2007) 6959-6966.

[6] N. Ibrahim, J. Thiebaud, J. Orphal, C. Fittschen, Air-broadening coefficients of the HO₂ radical in the 2n₁ band measured using cw-CRDS, J. Mol. Spectrosc. 242 (2007) 64-69.

[7] H. Bouzidi, M. Djehiche, T. Gierczak, P. Morajkar, C. Fittschen, P. Coddeville, A. Tomas, Low pressure photolysis of 2,3-pentanedione: quantum yields and reaction mechanism, J. Phys. Chem. A 119 (2015) 12781-12789.

Nitrous Acid (HONO)

Measurements of nitrous acid were made, by Jain et al. [8], in the range 6623.6–6645.6 cm^-1with a resolution of 0.005 cm ^-1, using CW-CRDS coupled to laser photolysis. HONO was generated in situ by photolysis of H₂O₂ in the presence of NO. Calibration of the HONO concentration (and hence the cross-sections) was achieved through modelling the kinetics of the time resolved concentrations of the OH and HO₂ radicals, which are generated in the H₂O₂ photolysis.

Reference

[8] C. Jain, P. Morajkar, C. Schoemaecker, B. Viskolcz, C. Fittschen, Measurement of absolute absorption cross-sections for nitrous acid (HONO) in the near-infrared region by the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique coupled to laser photolysis, J. Phys. Chem. A 115 (2011) 10720-10728.

Ammonia (NH₃)

Absorption cross-sections, as a function of wavelength for NH₃ were measured by O'Leary et al. [9], with off-axis CW-CEAS at 0.2 mbar (6883–6997 cm^-1) and at 11.5 mbar (6850–6997 cm^-1). A total of 1117 NH₃ lines are contained in the spectrum.

It should be noted that 262 lines of the NH₃ line-by-line sub-database are present in this cross-sections spectral region. Among these lines, only 49 have been assigned. We retain the unassigned lines, which are useful for many purposes, and we have implemented the cross-sections, in addition, to provide the total absorption in that region.

Reference

[9] D.M. O'Leary, J. Orphal, A.A. Ruth, U. Heitmann, P. Chelin, C.E. Fellows, The cavity-enhanced absorption spectrum of NH₃ in the near-infrared region between 6850 and 7000 cm^-1, J. Quant. Spectrosc. Radiat. Transfer 109 (2008) 1004-1015.