Absolute absorption cross-section and photolysis rate of I<sub>2</sub>

Abstract

Abstract. Following recent observations of molecular iodine (I2) in the coastal marine boundary layer (MBL) (Saiz-Lopez and Plane, 2004), it has become important to determine the absolute absorption cross-section of I2 at reasonably high resolution, and also to evaluate the rate of photolysis of the molecule in the lower atmosphere. The absolute absorption cross-section (σ) of gaseous I2 at room temperature and pressure (295K, 760Torr) was therefore measured between 182 and 750nm using a Fourier Transform spectrometer at a resolution of 4cm-1 (0.1nm at λ=500nm). The maximum absorption cross-section in the visible region was observed at λ=533.0nm to be σ=(4.24±0.50)x10-18cm2molecule-1. The spectrum is available as supplementary material accompanying this paper. The photo-dissociation rate constant (J) of gaseous I2 was also measured directly in a solar simulator, yielding J(I2)=0.12±0.03s-1 for the lower troposphere. This is in excellent agreement with the value of 0.12±0.015s-1 calculated using the measured absorption cross-section, terrestrial solar flux for clear sky conditions and assuming a photo-dissociation yield of unity. A two-stream radiation transfer model was then used to determine the variation in photolysis rate with solar zenith angle (SZA), from which an analytic expression is derived for use in atmospheric models. Photolysis appears to be the dominant loss process for I2 during daytime, and hence an important source of iodine atoms in the lower atmosphere.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume III – gas phase reactions of inorganic halogens

“…The three most recent studies (Tellinghuisen, 1973;Bauer et al, 1998;Saiz-Lopez et al, 2004) report absorption cross sections in the pressure-independent continuum (λ≤500 nm) which are in excellent agreement, differing by less than 5% at 500 nm. The present recommendation therefore adopts a reference value of σ =2.24×10 −18 cm 2 molecule −1 at 500 nm, which is the average of the values reported in the three investigations.…”

“…essentially in the continuum), based on a number of studies (Rabinowitch and Wood, 1936a;Tellinghuisen, 1973;Bauer et al, 1998;Saiz-Lopez et al, 2004), is well determined. Measurements in the structured region of the spectrum have been found to be very sensitive to experimental conditions, depending on the absolute concentration of I 2 and on pressure of bath gas, at the spectral resolutions typically employed.…”

“…The visible-infrared absorption spectrum of I 2 has been the subject of a number of studies and discussions (Rabinowitch and Wood, 1936a;Tellinghuisen, 1973;Okabe, 1978;Bauer et al, 1998;Saiz-Lopez et al, 2004). The spectrum is dominated by the transition from the X 1 ground state into the bound B 3 upper state, which results in rovibrational structure in the wavelength range 650-500 nm, with the dissociative continuum at ≤499 nm.…”

“…Cross sections averaged over a wavelength interval in this region should tend towards the true value when partial saturation of rotational lines is avoided though use of very low I 2 concentrations or at higher bath gas pressures when the lines become sufficiently broadened. The recent measurements of Saiz-Lopez et al (2004), performed at high resolution and with high bath gas pressures, are therefore used to define the shape of the spectrum, with the preferred values based on their data averaged over 5 nm intervals.…”

Iodine monoxide at a clean marine coastal site: observations of high frequency variations and inhomogeneous distributions

“…IO was first observed at Mace Head Atmospheric Research Station (MHD), located on the west coast of Ireland, in 1997 by Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) (Alicke et al, 1999) and has since been measured at a number of mid-latitude sites such as Lilia, France (Peters et al, 2005), Dagebüll, Germany (Peters et al, 2005) and Roscoff, France (Mahajan et al, 2009), with varying IO mixing ratios observed, up to a maximum of 29 ± 8.8 pptv observed by Seitz et al (2010) at Mweenish Bay just south of Mace Head on the west coast of Ireland. These previous spatially-averaged observations of IO at MHD have provided important information about the role of macroalgae in initiating iodine chemistry through the emission of molecular iodine (e.g., Saiz-Lopez and Plane, 2004) and, to a lesser extent, iodocarbons (e.g., Carpenter et al, 2001). These studies also linked iodine species to new particle formation and initiated chamber studies into the role of specific macroalgae in the emission of iodine (Bale et al, 2008;Dixneuf et al, 2009;Ball et al, 2010).…”