Abstract
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 HO2/OH. This analysis is based on the first ever observations of CH3I 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 CH3I 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 Ix (Ix = I + IO + HI + HOI + 2I2O2 + INOx) 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 O3 destruction estimated. Total column O3 destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv Ix 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.
Highlights
Introduction leadsto the releaseof anO atom which, via reactionwithOf thetracegasesin thetroposphereo, zone,togetherwith 02, results in the formation of one net 03 molecule.the free radicals generated by its photolysis, is most responsiblefor defining the oxidizing capacity of the Photochemicadl estructionoccurswhen the 03 photolysis producOt (D) reactswith H20 to producetwohydroxyl troposphereW. ithin the tropospheret,he mixing ratio of this radicals,OH, or whenhydroperoxyHl O2 andOH radicals tracegasis influencedby bothtransportandphotochemical reactwith 03
The free radicals generated by its photolysis, is most responsiblefor defining the oxidizing capacity of the Photochemicadl estructionoccurswhen the 03 photolysis producOt (D) reactswith H20 to producetwohydroxyl troposphereW. ithin the tropospheret,he mixing ratio of this radicals,OH, or whenhydroperoxyHl O2 andOH radicals tracegasis influencedby bothtransportandphotochemical reactwith 03
In contrastto the tropics,the Pacific rim datareveal thanthatat low altitudes.,uponreflectingon the the presenceof many elevatedCH3I observationsat high fact that frequentlythere was a high degree of natural altitude,with valuesat 10-12 km reaching >1 pptv
Summary
As previouslyreviewedby ChameidesandDavis [1980], thereis ampleevidence showingthatthe methylhalide CH3I is a metabolicbyproductof many speciesof marine algae. In contrastto the tropics,the Pacific rim datareveal thanthatat low altitudes.,uponreflectingon the the presenceof many elevatedCH3I observationsat high fact that frequentlythere was a high degree of natural altitude,with valuesat 10-12 km reaching >1 pptv These variabilityin thelevelsof bothspeciesandthatthesampling high-altitudevalues reflect the influence of observations integrationtimesdifferedsignificantly(e.g., DMS, 3-4 min; recordedduringPEM-West A flight 9. The strong vertical pumping action from this evidencesupportingthisconclusioncanbe foundin the level typhoonproducedenhancedhigh-altitudelevelsof bothCH3I of correspondencbeetweentheDMS andCH3Imeasurements as well as DMS [Newell et al, this issue; Thorntonet al., shownin Figure 1 This figureshowsa scatterploot f these this issue]Median valuesof CH3I are seento rangefrom two speciesunderconditionsfor whichtheir lifetimeswould highsof 0.6 to 1 pptv in the marineBL to lows of 0.1 to be somewhatsimilar, i.e., data recordedfor altitudesabove 0.15 at middle and high-altitudes.The Pacific rim regionis 4 km.
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