Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 16, 15593-15604, 2016
http://www.atmos-chem-phys.net/16/15593/2016/
doi:10.5194/acp-16-15593-2016
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
19 Dec 2016
Nighttime atmospheric chemistry of iodine
Alfonso Saiz-Lopez1, John M. C. Plane2, Carlos A. Cuevas1, Anoop S. Mahajan3, Jean-François Lamarque4, and Douglas E. Kinnison4 1Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain
2School of Chemistry, University of Leeds, Leeds, UK
3Indian Institute of Tropical Meteorology, Pune, India
4Atmospheric Chemistry Observations & Modeling Laboratory, National Center for Atmospheric Research, Boulder, 10 Colorado, USA
Abstract. Little attention has so far been paid to the nighttime atmospheric chemistry of iodine species. Current atmospheric models predict a buildup of HOI and I2 during the night that leads to a spike of IO at sunrise, which is not observed by measurements. In this work, electronic structure calculations are used to survey possible reactions that HOI and I2 could undergo at night in the lower troposphere, and hence reduce their nighttime accumulation. The new reaction NO3+ HOI  →  IO + HNO3 is proposed, with a rate coefficient calculated from statistical rate theory over the temperature range 260–300 K and at a pressure of 1000 hPa to be k(T)  =  2.7  ×  10−12 (300 K/T)2.66 cm3 molecule−1 s−1. This reaction is included in two atmospheric models, along with the known reaction between I2 and NO3, to explore a new nocturnal iodine radical activation mechanism. The results show that this iodine scheme leads to a considerable reduction of nighttime HOI and I2, which results in the enhancement of more than 25 % of nighttime ocean emissions of HOI + I2 and the removal of the anomalous spike of IO at sunrise. We suggest that active nighttime iodine can also have a considerable, so far unrecognized, impact on the reduction of the NO3 radical levels in the marine boundary layer (MBL) and hence upon the nocturnal oxidizing capacity of the marine atmosphere. The effect of this is exemplified by the indirect effect on dimethyl sulfide (DMS) oxidation.

Citation: Saiz-Lopez, A., Plane, J. M. C., Cuevas, C. A., Mahajan, A. S., Lamarque, J.-F., and Kinnison, D. E.: Nighttime atmospheric chemistry of iodine, Atmos. Chem. Phys., 16, 15593-15604, doi:10.5194/acp-16-15593-2016, 2016.
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Short summary
Electronic structure calculations are used to survey possible reactions that HOI and I2 could undergo at night in the lower troposphere, and hence reconcile measurements and models. The reactions NO3 + HOI and I2 + NO3 are included in two models to explore a new nocturnal iodine radical activation mechanism, leading to a reduction of nighttime HOI and I2. This chemistry can have a large impact on NO3 levels in the MBL, and hence upon the nocturnal oxidizing capacity of the marine atmosphere.
Electronic structure calculations are used to survey possible reactions that HOI and I2 could...
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