Iodine: Its possible role in tropospheric photochemistry

TLDR

Methyl iodide in the marine troposphere (5–10 ppt) photolyzes to produce iodine atoms. Iodine reacts with O₃, HOx, and NOx to form IO, HOI, IONO₂, and I₂, which recycle back to I and catalytically destroy O₃, increase the NO₂/NO ratio, deplete HOx radicals, convert HO₂ to OH, and are ultimately removed by heterogeneous processes. Model calculations show that iodine can strongly influence the tropospheric O₃–NOx–HOx system, but the magnitude of its effects depends on uncertain rate constants and the primary source distributions of CH₃I and other iodine compounds.

Abstract

A detailed study of the photochemistry of iodine and its oxides indicates that iodine species may play an important, but heretofore ignored, role in the tropospheric photochemical system. Methyl iodide, often observed in the marine troposphere with an average concentration of 5–10 ppt (v/v), is photolyzed and thereby produces I atoms. Chemical interactions with O 3 , H x O y , and NO x cause I to be converted to other inorganic compounds such as IO, HOI, IONO 2 , and I 2 . The production of these species and their subsequent recycling back to I can lead to the catalytic removal of tropospheric O 3 , the enhancement of the NO 2 /NO ratio, the destruction of H x O y free radicals, and the conversion of HO 2 to OH. Ultimately, tropospheric inorganic iodine (IX) is removed by heterogeneous processes. Calculations using a numerical model to simulate tropospheric photochemistry indicate that iodine may have a strong impact upon the atmospheric O 3 ‐NO x ‐H x O y system. The magnitude of these effects is dependent upon the value of several uncertain rate constants and the primary source distributions of CH 3 I and other organic and inorganic iodine compounds.