A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer

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Title: A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer
Author(s): Carpenter, LJ
Andrews, SJ
Lidster, RT
Saiz-Lopez, A
Fernandez-Sanchez, M
Bloss, WJ
Ouyang, B
Jones, RL
Item Type: Journal Article
Abstract: Ocean emissions of inorganic and organic iodine compounds drive the biogeochemical cycle of iodine and produce reactive ozone-destroying iodine radicals that influence the oxidizing capacity of the atmosphere. Di-iodomethane (CH2I2) and chloro-iodomethane (CH2ICl) are the two most important organic iodine precursors in the marine boundary layer. Ship-borne measurements made during the TORERO (Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOC) field campaign in the east tropical Pacific Ocean in January/February 2012 revealed strong diurnal cycles of CH2I2 and CH2ICl in air and of CH2I2 in seawater. Both compounds are known to undergo rapid photolysis during the day, but models assume no night-time atmospheric losses. Surprisingly, the diurnal cycle of CH2I2 was lower in amplitude than that of CH2ICl, despite its faster photolysis rate. We speculate that night-time loss of CH2I2 occurs due to reaction with NO3 radicals. Indirect results from a laboratory study under ambient atmospheric boundary layer conditions indicate a k CH2I2+NO3 of ≤4 × 10−13 cm3 molecule−1 s−1; a previous kinetic study carried out at ≤100 Torr found k CH2I2+NO3 of 4 × 10−13 cm3 molecule−1 s−1. Using the 1-dimensional atmospheric THAMO model driven by sea-air fluxes calculated from the seawater and air measurements (averaging 1.8 +/− 0.8 nmol m−2 d−1 for CH2I2 and 3.7 +/− 0.8 nmol m−2 d−1 for CH2ICl), we show that the model overestimates night-time CH2I2 by >60 % but reaches good agreement with the measurements when the CH2I2 + NO3 reaction is included at 2–4 × 10−13 cm3 molecule−1 s−1. We conclude that the reaction has a significant effect on CH2I2 and helps reconcile observed and modeled concentrations. We recommend further direct measurements of this reaction under atmospheric conditions, including of product branching ratios.
Publication Date: 5-Oct-2015
Date of Acceptance: 25-Sep-2015
URI: http://hdl.handle.net/10044/1/56741
DOI: https://dx.doi.org/10.1007/s10874-015-9320-6
ISSN: 0167-7764
Publisher: Springer Verlag
Start Page: 144
End Page: 156
Journal / Book Title: Journal of Atmospheric Chemistry
Volume: 74
Issue: 2
Copyright Statement: ©The Author(s) 2015. This article is published with open access at Springerlink.com
Keywords: Science & Technology
Life Sciences & Biomedicine
Physical Sciences
Environmental Sciences
Meteorology & Atmospheric Sciences
Environmental Sciences & Ecology
di-iodomethane
NO3 radical
Atmosphere
Ocean
Iodine
ABSORPTION CROSS-SECTIONS
CRIEGEE INTERMEDIATE CH2OO
TROPICAL ATLANTIC-OCEAN
PHOTOCHEMICAL DATA
IODINE CHEMISTRY
INORGANIC IODINE
EASTERN PACIFIC
ALKYL IODIDES
PHOTOLYSIS
GAS
Science & Technology
Life Sciences & Biomedicine
Physical Sciences
Environmental Sciences
Meteorology & Atmospheric Sciences
Environmental Sciences & Ecology
di-iodomethane
NO3 radical
Atmosphere
Ocean
Iodine
ABSORPTION CROSS-SECTIONS
CRIEGEE INTERMEDIATE CH2OO
TROPICAL ATLANTIC-OCEAN
PHOTOCHEMICAL DATA
IODINE CHEMISTRY
INORGANIC IODINE
EASTERN PACIFIC
ALKYL IODIDES
PHOTOLYSIS
GAS
0306 Physical Chemistry (Incl. Structural)
0399 Other Chemical Sciences
Meteorology & Atmospheric Sciences
Publication Status: Published
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