POTENTIAL IMPACT OF IODINE ON TROPOSPHERIC LEVELS OF OZONE AND OTHER CRITICAL OXIDANTS

A new analysis of tropospheric iodine chemistry suggests that under ce rtain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests t hat modest shifts could result in the critical radical ratio HO2/OH. T his analysis is based on the first ever observations of CH3I in the mi ddle and upper free troposphere as recorded during the NASA Pacific Ex ploratory Mission in the western Pacific. Improved evaluations of seve ral 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 rel ease of CH3I from the marine environment was the only iodine source wi th 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 scenar io 2 but with the release of all iodocarbons occurring in a region of high biological productivity, Based on one-dimensional model simulatio ns, these three source scenarios resulted in estimated I-x (I-x =I + I O + HI + HOI + 2I(2)O(2) +INOx) yields for the upper troposphere of 0. 5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of th ese, only at the 1.5 and 7 pptv level were meaningful enhancements in O-3 destruction estimated. Total column O-3 destruction for these case s 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 environ ment; however, for specific regions of the Pacific (i.e., marine upwel ling 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 rem ains in the magnitude of the marine iodine source, In addition, severa l rate coefficients for gas phase processes need further investigating , as does the efficiency for removal of iodine due to aerosol scavengi ng processes.