Douglas D. Davis

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.

Calculations are presented that simulate the free radical chemistries of the gas phase and aqueous phase within a warm cloud during midday. It is demonstrated that in the presence of midday solar fluxes the heterogeneous scavenging of OH and HO 2 from the gas phase by cloud droplets can represent a major source of free radicals to cloud water, provided the accommodation or sticking coefficient for these species impinging upon water droplets is ≥10 −4 . The aqueous‐phase HO 2 radicals are found to be converted to H 2 O 2 by aqueous‐phase chemical reactions at a rate that suggests that this mechanism could produce a significant fraction of the H 2 O 2 found in cloud droplets. The rapid oxidation of sulfur species dissolved in cloudwater by this free‐radical‐produced H 2 O 2 as well as by aqueous‐phase OH radicals could conceivably have a significant impact upon the chemical composition of rain.

A new analysis of tropospheric iodine chemistry suggests that under certain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests that modest shifts could result in the critical radical ratio HO 2 /OH. This analysis is based on the first ever observations of CH 3 I in the middle and upper free troposphere as recorded during the NASA Pacific Exploratory Mission in the western Pacific. Improved evaluations of several critical gas kinetic and photochemical rate coefficients have also been used. Three iodine source scenarios were explored in arriving at the above conclusions. These include: (1) the assumption that the release of CH 3 I from the marine environment was the only iodine source with boundary layer levels reflecting a low‐productivity source region, (2) same as scenario 1 but with an additional marine iodine source in the form of higher molecular weight iodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurring in a region of high biological productivity. Based on one‐dimensional model simulations, these three source scenarios resulted in estimated I x (I x = I + IO + HI + HOI + 2I 2 O 2 + INO x ) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningful enhancements in O 3 destruction estimated. Total column O 3 destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv I x source scenario to be more typical of the tropical marine environment; however, for specific regions of the Pacific (i.e., marine upwelling regions) and for specific seasons of the year, much higher levels might be experienced. Even so, significant uncertainties still remain in the proposed iodine chemistry. In particular, much uncertainty remains in the magnitude of the marine iodine source. In addition, several rate coefficients for gas phase processes need further investigating, as does the efficiency for removal of iodine due to aerosol scavenging processes.