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Absolute Absorption Cross-Section and Photolysis Rate of I2 A Absolute absorption cross-section and photolysis rate of I2 A. Saiz-Lopez, R. W. Saunders, D. M. Joseph, S. H. Ashworth, J. M. C. Plane To cite this version: A. Saiz-Lopez, R. W. Saunders, D. M. Joseph, S. H. Ashworth, J. M. C. Plane. Absolute absorption cross-section and photolysis rate of I2. Atmospheric Chemistry and Physics, European Geosciences Union, 2004, 4 (5), pp.1443-1450. hal-00295493 HAL Id: hal-00295493 https://hal.archives-ouvertes.fr/hal-00295493 Submitted on 1 Sep 2004 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Atmos. Chem. Phys., 4, 1443–1450, 2004 www.atmos-chem-phys.org/acp/4/1443/ Atmospheric SRef-ID: 1680-7324/acp/2004-4-1443 Chemistry and Physics Absolute absorption cross-section and photolysis rate of I2 A. Saiz-Lopez1, R. W. Saunders1, D. M. Joseph1, S. H. Ashworth2, and J. M. C. Plane1 1School of Environmental Sciences, University of East Anglia, Norwich, UK 2School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, UK Received: 23 February 2004 – Published in Atmos. Chem. Phys. Discuss.: 5 May 2004 Revised: 27 July 2004 – Accepted: 20 August 2004 – Published: 1 September 2004 Abstract. Following recent observations of molecular io- Chatfield and Crutzen, 1990; Davis et al., 1996; McFiggans dine (I2) in the coastal marine boundary layer (MBL) (Saiz- et al., 2000). Furthermore, catalytic cycles involving chlo- Lopez and Plane, 2004), it has become important to deter- rine and bromine appear not to be sufficient to explain O3 mine the absolute absorption cross-section of I2 at reasonably depletion in the lower stratosphere at low latitudes, leading high resolution, and also to evaluate the rate of photolysis of to speculation that iodine also plays a role in stratospheric the molecule in the lower atmosphere. The absolute absorp- chemistry (Solomon et al., 1994; Bosch¨ et al., 2003). Iodine tion cross-section (σ ) of gaseous I2 at room temperature and may also be central to other atmospheric phenomena, includ- pressure (295 K, 760 Torr) was therefore measured between ing new particle formation in marine environments (O’Dowd 182 and 750 nm using a Fourier Transform spectrometer at a et al., 2002a, 2002b; Makel¨ a¨ et al., 2002) and the enhance- resolution of 4 cm−1 (0.1 nm at λ=500 nm). The maximum ment of halogen/inter-halogen release via uptake on sea-salt absorption cross-section in the visible region was observed at aerosol (Vogt et al., 1999; McFiggans et al., 2002). The pri- λ=533.0 nm to be σ =(4.24±0.50)×10−18 cm2 molecule−1. mary source of iodine in the atmosphere has been postulated The spectrum is available as supplementary material accom- to be the evasion of biogenic iodocarbons from the sea sur- panying this paper. The photo-dissociation rate constant (J ) face (Carpenter, 2003; Carpenter et al., 2003). CH3I, CH2I2, of gaseous I2 was also measured directly in a solar sim- CH2IBr and CH2ICl have all been detected in marine envi- −1 ulator, yielding J (I2)=0.12±0.03 s for the lower tropo- ronments, with coastal emissions showing a direct correla- sphere. This is in excellent agreement with the value of tion with tidal height and high solar irradiance (Carpenter et 0.12±0.015 s−1 calculated using the measured absorption al., 1999, 2001). In terms of the open ocean, evaporation of cross-section, terrestrial solar flux for clear sky conditions molecular iodine (I2) from the water surface, via oxidation and assuming a photo-dissociation yield of unity. A two- of iodide ions in seawater under solar irradiation (Miyake stream radiation transfer model was then used to determine and Tsunogai, 1963) or by reaction with atmospheric ozone the variation in photolysis rate with solar zenith angle (SZA), (Garland and Curtis, 1981) have also been suggested. from which an analytic expression is derived for use in atmo- The recent detection of molecular iodine at Mace spheric models. Photolysis appears to be the dominant loss Head, Ireland by differential optical absorption spectroscopy process for I2 during daytime, and hence an important source (DOAS), suggests that I2 may also make a significant contri- of iodine atoms in the lower atmosphere. bution to the overall iodine budget in the atmosphere (Saiz- Lopez and Plane, 2004). I2 was found at parts per trillion (ppt) levels in the coastal marine boundary layer (MBL), with a marked increase in concentration around low tide. This cor- 1 Introduction relation between I2 production and tidal height suggests an oxidative stress mechanism within exposed macro-algal beds Over the past two decades, there has been a growing inter- as the probable source of I in the coastal MBL (McFiggans est in the role that iodine chemistry plays in a number of 2 et al., 2004). In the troposphere, I atom production takes atmospheric processes, in particular the iodine-catalysed de- 0 place through photo-dissociation of iodocarbons (RI/R I2) struction of O3 in the troposphere (e.g. Jenkin et al., 1985; RI+hν −→ R + I or R0I + hν −→ R0I + I (R1) Correspondence to: J. M. C. Plane 2 ([email protected]) © European Geosciences Union 2004 1444 A. Saiz-Lopez et al.: Absolute absorption cross-section and photolysis rate of I2 The subsequent reaction of I atoms with atmospheric O3 DOAS retrievals, and a measured rate of I2 photolysis (ap- leads to the formation of iodine monoxide (IO), which regen- propriate for atmospheric modelling), do not appear to be erates I atoms on photolysis, available in the literature. In this paper we will describe a measurement of the absolute absorption cross-section of I2 I + O3 −→ IO + O2 (R2) from the vacuum ultra-violet to the near infra-red. The pho- tolysis rate of I , determined directly in a solar simulator, IO + hν −→ I + O (R3) 2 will then be compared with values calculated using the mea- sured absorption spectrum and single and two-stream radia- or participates in iodine-catalysed O destruction cycles 3 tion models. through reactions with HO2 (forming HOI), NO2 (forming IONO2) or halogen oxides (Jenkin et al., 1985; Davis et al., 1996; McFiggans et al., 2000). The IO self-reaction has also 2 Experimental been identified as an important process leading to the ho- mogeneous nucleation of MBL aerosol. New particle for- 2.1 Cross-section determination mation is thought to occur via the polymeric build-up of io- dine oxide aerosol formed by reactions of the iodine dioxide The I spectrum was recorded using a Fourier Transform (OIO) radical, one of the products from the self-reaction of 2 (FT) spectrometer (Bruker, Model IFS/66). Iodine crystals IO (Bloss et al., 2001; Hoffmann et al., 2001; Jimenez et al., (Lancaster, 99.5%) were placed in an optical cell and allowed 2003). The visible-infrared spectrum of I has been the sub- 2 to equilibrate at room temperature (295 K) and pressure ject of numerous previous studies (Rabinowitch and Wood, (760 Torr of air) before each measurement was made. The 1936a; Goy and Pritchard, 1964; Ogryzlo and Thomas, 1965; spectrum was recorded in three overlapping regions: 182– Brewer and Tellinghuisen, 1972; Tellinghuisen, 1973). I has 2 500 nm, GaP diode detector, deuterium light source; 260– a well-defined absorption structure in the visible region of 555 nm, GaP diode detector, tungsten lamp; 500–1100 nm, the spectrum corresponding to the B←X electronic transition Si diode, Tungsten lamp. A CaF beamsplitter was used in (Calvert and Pitts, 1966; Tellinghuisen, 1973). Photolysis in 2 each case. All measurements were made at a resolution of the visible region can proceed directly via three contributing 4 cm−1 (0.1 nm at λ=500 nm). The high resolution spectrum transitions (Tellinghuisen, 1973; Gray et al., 2001): measured by the FT spectrometer was scaled in the contin- 3 + 1 + uum region to the average of a series of spectra recorded us- I2 + hν −→ I + I (A 5(1u ) ←− X 6g ) (R4a) ing a grating spectrometer (Acton SpectraPro SP-556-I, grat- − ∗ 3 + 1 + ing 1200 grooves mm 1, resolution 0.2 nm), under carefully I2 + hν −→ I + I (B 5(0 ) ←− X 6 ) (R4b) u g controlled conditions (295 K, 1 atmosphere of air). 1 1 + In order to calculate the absolute cross-section of I2 from I2 + hν −→ I + I (C 5u ←− X 6 ) (R4c) g its absorption spectrum, an accurate measurement of the 2 vapour pressure of I2 is required. However, available data where I represents a ground state atom I( P3/2) and I* de- 2 is sparse (see below), and so a new measurement was per- notes a meta-stable I( P1/2) atom. Subsequent relaxation 00 from the meta-stable state to the ground state occurs through formed. Iodine crystals were placed in a 1/4 stainless steel quenching processes or radiative decay (τ∼50 ms, Okabe, U-tube, which was sealed at one end by a pair of Baratron 1978): pressure transducers, with 0–1000 Torr and 0–10 Torr ranges. The other end of the tube was attached to a vacuum line in- ∗ 2 2 I ( P1/2) −→ I( P3/2) (R5) corporating a diffusion pump, from which the tube could be isolated by a valve.
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