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A large number of hydrocarbons, halocarbons, and organic nitrates were quantified in whole air samples acquired for the NASA-sponsored GTE missions PEM-Tropics A and B. The samples were collected in electro-polished stainless steel canisters from two aircraft while flying over the Pacific Basin. Two nominally identical multicolumn multidetector gas chromatographic analytical systems were employed. Whole air samples were also used as working and calibrated standards and were collected specifically for this purpose. This paper describes the analytical procedure employed during PEM-Tropics B. Minor differences in the PEM-Tropics A system will also be discussed. More than 3,900 samples were analyzed for 34 gases during PEM-Tropics A, over 4,500 samples were analyzed for 58 gases during PEM-Tropics B. An overview is presented of the collection, analysis, and quantification of whole air samples during the PEM-Tropics missions, along with an analysis of the analytical precision achieved during these missions.

The molecules CF2Cl2 and CFCl3 are released into the atmosphere following their extensive use as propellants for aerosol spray cans and in refrigeration. Since they are chemically inert and have low solubility in water, these chlorofluoromethanes have very long atmospheric residence times and can be detected throughout the troposphere in amounts roughly corresponding to the integrated world industrial production to date. The most important sink for atmospheric CFCl3 and CF2Cl2 appears to be photolytic dissociation in the stratosphere by ultraviolet radiation around 2000 Å. Upon photolysis the chlorofluoromethanes release chlorine atoms, which initiate an extensive catalytic chain reaction Cl + O3 → ClO + O2 ClO + O → Cl + O2 leading to the net destruction of O3 in the stratosphere. This chain reaction can be diverted through reaction of ClO with NO, which interconnects the NOx and ClOx catalytic cycles. The Cl‐ClO chain is interrupted by the reaction of Cl with methane or other hydrogenous species to form HCl, and it is renewed by reaction of OH with HCl. One‐dimensional diffusion calculations show that present O3 depletion levels resulting from the presence of the chlorofluoromethanes are of the order of 1%. This depletion would increase up to 15 or 20% if the chlorofluoromethane injection were to continue indefinitely at present rates. Furthermore, the calculations show that the full stratospheric effect of the photodissociation of CF2Cl2 and CFCl3 is not immediately felt after their introduction at ground level because of the delay required for upward diffusion to the 25‐ to 30‐km level. If the atmospheric injection of these compounds were to terminate only after causing an observable depletion of stratospheric ozone, the depletion would intensify for sometime thereafter and would remain significant for a period of a century or more.

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 I x (I x =I + IO + HI + HOI + 21202 +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 03 destruction estimated. Total column 03 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. IntroductionOf the trace gases in the troposphere, ozone, together with the free radicals generated by its photolysis, is most responsible for defining the oxidizing capacity of the troposphere. Within the troposphere, the mixing ratio of this trace gas is influenced by both transport and photochemical processes [e.g., Fabian and Pruchniewicz, 1977; Mahlman et al., 1980; Chameides and Walker, 1973; Fishman and Crutzen, 1977; Liu et al.,1980]. Conventional thinking suggests that it is the reaction of peroxy radicals, (e.g. HO 2 CH302, and RO2, where "R" is any organic grouping) with NO to produce the product species NO 2 that forms the basis of photochemical 03 formation. Photolysis of NO 2 leads to the release of an O atom which, via reaction with 02 , results in the formation of one net 03 molecule. Photochemical destruction occurs when the 03 photolysis product O(•D) reacts with H20 to produce two hydroxyl radicals, OH, or when hydroperoxyl HO 2...