Collision induced broadening of ν1 band and ground state spectral lines of sulfur dioxide perturbed by N2 and O2

https://doi.org/10.1016/j.jqsrt.2017.05.013Get rights and content

Highlights

  • SO2 spectroscopic parameters for remote sensing of Earth atmosphere.

  • New determination of N2-, O2- and air-broadening coefficients.

  • IR TDL spectroscopy within the atmospheric region around 8.8 µm.

  • mm-/sub-mm wave spectroscopy in the 104 GHz–1.1 THz spectral range.

  • K′′a and J″ dependence of SO2-N2 and SO2-O2 broadening parameters.

Abstract

To monitor the constituents and trace pollutants of Earth atmosphere and understand its evolution, accurate spectroscopic parameters are fundamental information. SO2 is produced by both natural and anthropogenic sources and it is one of the principal causes of acid rains as well as an important component of fine aerosol particles, once oxidized to sulfate. The present work aims at determining SO2 broadening parameters using N2 and O2 as atmospherically relevant damping gases. Measurements are carried out in the infrared (IR) and mm-/sub-mm wave regions, around 8.8 µm and in the 104 GHz–1.1 THz interval, respectively. IR ro-vibrational transitions are recorded by using a tunable diode laser spectrometer, whereas the microwave spectra are recorded by using a frequency-modulated millimeter-/submillimeter-wave spectrometer. SO2-N2 and SO2-O2 collisional cross sections are retrieved for several ν1 band ro-vibrational transitions of 32S16O2, for some transitions belonging to either ν1 + ν2  ν2 of 32S16O2 or ν1 of 34S16O2 as well as for about 20 pure rotational transitions in the vibrational ground state of the main isotopic species. From N2- and O2- broadening coefficients the broadening parameters of SO2 in air are derived. The work is completed with the study of the dependence of foreign broadening coefficients on the rotational quantum numbers.

Introduction

Nowadays, remote sensing techniques allow monitoring and accurately retrieving the concentration profiles of atmospheric constituents and trace pollutants, since these molecules have strong rotation and vibration-rotation absorptions in the microwave (MW) and infrared (IR) spectral domains, respectively [1], [2], [3], [4]. The satellites used for sounding the terrestrial atmosphere embark spectrometers that provide a large amount of spectral information at ever increasing quality in terms of spectral coverage, resolution and signal-to-noise ratio [1]. For these reasons, spectroscopic parameters are of fundamental importance for exploiting remote sensing applications in atmospheric and climate research, environmental monitoring and gas-phase analysis. As a matter of fact, only their accurate knowledge allows an retrieval of concentrations and distributions of the gas phase molecular species in the atmosphere. The relevant spectroscopic parameters include line-by-line parameters, i.e., transition frequencies and their intensities, pressure broadening coefficients and pressure induced shifts, and their temperature dependence. The existing spectroscopic data are then collected into a number of different databases, among which the most important ones are HITRAN [5], GEISA [6], JPL [7], and the Cologne database [8]. Within this framework, the aim of laboratory spectroscopy is to provide spectroscopic parameters for a wide variety of species of atmospheric, astrophysical and industrial relevance. Furthermore the study of collisional broadening and shifting coefficients, these being related to the intermolecular potential, can shed light on the driving forces ruling the scattering events in the gas phase (see e.g. [9], [10], [11], [12], [13] and references therein).

Sulfur dioxide (SO2) is an important molecule for Earth atmosphere since it actively enters in the sulfur cycle and it is one of the causes of acid rains. SO2 has a lifetime in the atmosphere of about a day, it is oxidized quickly, thus leading to aerosol formation and acid deposition [14]. As sulfate, it is an important component of fine aerosol particles (PM10 and PM2.5). Furthermore, sulfate aerosol also affects Earth's radiation balance either thorough direct scattering of sunlight or indirectly via modification of cloud albedoes. On a global scale, the majority of natural SO2 is produced by volcanoes [15], [16], [17] and by the oxidation of sulfur gases produced by the decomposition of plants. Therefore, natural emissions usually occur at high altitude or far from city centers, and hence the SO2 background concentration in clear air is about 1 ppb [18]. On the other hand, a sizeable amount of SO2 is of anthropogenic origin, being produced by combustion of fossil fuels as well as by non-ferrous smelting processes for the conversion of ores to free metals [18]. In addition, this compound is largely employed in food-preserving and wine making industries. It is thus not surprising that SO2 has received a great deal of attention from the scientific community and it is still the subject of a number of spectroscopic studies.

The fundamental ν1 and ν3 bands of 32S16O2 isotopologue have been thoroughly analyzed in 1980s by Guelachvili and co-workers [19], [20] and subsequently reinvestigated by Flaud et al. together with the ν2 fundamental and the 2ν2  ν2 hot band [21]. In 2005 Müller and Brünker have accurately re-determined the rotational parameters for the ground and v2 = 1 states [22]. Some ro-vibrational transitions, belonging to ν1 and ν3 bands, have been object of study by Sumpf with the aim of determining line-by-line transition intensities [23], while accurate intensity determinations have been performed within the spectral region 940–1400 cm−1 region by Fourier transform IR spectroscopy [24]. Since 1992 different studies have focused on the determination of SO2 self- [23], [25], [26], [27], [28], [29], [30], [31], [32] and foreign-broadening coefficients [26], [27], [33], [34], [35], [36], [37]. A few years ago, a complete listing of sulfur dioxide self-broadening coefficients has been compiled by combining the results obtained from IR and MW spectroscopy with semiclassical calculations [38]. In addition, the homodimer of SO2 has been investigated by Tasinato and Grimme, theoretically using dispersion-corrected density functional theory (DFT-D3) as well as experimentally by means of tunable diode laser (TDL) IR spectroscopy [39]. In that work, the dissociation energies of (SO2)2 and (CH2F2)2 have been determined experimentally from the broadening of the ro-vibrational transitions of the corresponding monomers collisionally perturbed by a range of damping gases [39]. Recently, SO2 has been included in an HITRAN-like database of line parameters for molecules of planetary interest perturbed by H2, He or CO2 [40]. Very recently, Ceselin et al. exploited IR and mm-/sub-mm wave spectroscopy to retrieve new line-by-line pressure broadening parameters of SO2 perturbed by He, H2 and CO2 [41].

In the present contribution, our work aiming at the determination of SO2 broadening parameters for atmospheric and astrochemical applications is extended by considering the atmospherically relevant N2 and O2 buffer gases.

Section snippets

Experimental details

The broadening parameters of SO2 perturbed by N2 and O2 as buffer gases have been determined both in the infrared (IR) and millimeter/sub-millimeter (mm-/sub-mm) wave region. IR measurements have been carried out at the Laboratory of Molecular Spectroscopy of Venice (LMS-Ve), whereas mm-/sub-mm ones have been performed in the Laboratory of mm-/sub-mm wave Spectroscopy of Bologna (LMS-Bo).

For what concerns IR measurements, SO2 high resolution spectra were recorded by using a TDL spectrometer

Results and discussion

The radiating species investigated in the present work is an asymmetric near-prolate rotor belonging to the C2v symmetry point group. It has three vibrational normal modes: ν1 and ν3 correspond to the Odouble bondSdouble bondO symmetric and asymmetric stretching, respectively, while ν2 represents the bending motion. The ν1 fundamental belongs to the A1 symmetry species and it gives rise to a B-type band located at about 1151.7 cm−1. The presence of two identical oxygen nuclei (zero nuclear spin) allows, for all

Conclusions

In the present work, a line-by-line list of SO2 foreign-broadening coefficients has been retrieved by the analysis of several ro-vibrational transitions belonging to the 32S16O2 ν1 band (in the 8.8 µm region) as well as a number of pure rotational transitions of the vibrational ground state (in the mm-/sub-mm wave range), with O2 and N2 as damping gases. In addition, 10 transitions of the ν1 + ν2  ν2 32S16O2 hot band and 6 lines belonging to ν1 of 34S16O2 have also been analyzed in the IR region.

Acknowledgments

The present work has been financially supported by MIUR (PRIN 2012 funds for project "STAR: Spectroscopic and computational Techniques for Astrophysical and atmospheric Research"), Università Ca' Foscari Venezia (ADiR funds), University of Bologna (RFO funds) and Scuola Normale Superiore (funds for project COSMO: "Combined experimental and computational spectroscopic modeling for astrochemical applications"). GC thanks Università Ca' Foscari Venezia for her research fellowship.

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      Up to now, a large number of experimental and theoretical studies have been devoted to determining its accurate spectroscopic parameters like line positions [5–6], line intensities [7–9] and broadening coefficients [10–28]. The quantum number and temperature dependence of broadening coefficients of SO2 have been studied on self-broadened [10–17] or perturbed by N2 [10–11,19–22], O2 [20–22], H2 [21,23–25], He [19–21,24–25], CO2 [24–27] and Ne, Ar, Kr and Xe [23,28]. Theoretical calculations on the line-broadening coefficients were usually based on the semi-classical theory [20].

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