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 O3, HxOy, and NOx cause I to be converted to other inorganic compounds such as IO, HOI, IONO2, and I2. The production of these species and their subsequent recycling back to I can lead to the catalytic removal of tropospheric O3, the enhancement of the NO2/NO ratio, the destruction of HxOy free radicals, and the conversion of HO2 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 O3‐NOx‐HxOy system. The magnitude of these effects is dependent upon the value of several uncertain rate constants and the primary source distributions of CH3I and other organic and inorganic iodine compounds.

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...

Abstract. Reported here are the first Austral summer measurements of NO at South Pole (SP). They are unique in that the levels are one to two orders of magnitude higher (i.e., median, 225 pptv) than measured at other polar sites. The available evidence suggests that these elevated levels are the result of photodenitrification of the snowpack, in conjunction with a very thin atmospheric mixing depth. Important chemical consequences included finding the atmospheric oxidizing power at SP to be an order of magnitude higher than expected. Measurement Techniques and Model DescriptionNO was measured using a modified chemiluminescence instrument. The instrument was operated in a 50% duty cycle mode, switching between measurements and zero every 5 minutes. Two standard addition calibrations and zero air (artifact) tests were performed each day. The NO calibration gas was intercompared with other NIST traceable standards both before and after the field deployment and was found to be within the manufacturers tolerance (+2 %). The 2cs detection limit for the NO system used in this study was estimated at 6 pptv. After the field deployment, the instrument's calibration was further evaluated using an NO calibration system developed at Georgia

New particle formation in a tropical marine boundary layer setting was characterized during NASA's Pacific Exploratory Mission-Tropics A program. It represents the clearest demonstration to date of aerosol nucleation and growth being linked to the natural marine sulfur cycle. This conclusion was based on real-time observations of dimethylsulfide, sulfur dioxide, sulfuric acid (gas), hydroxide, ozone, temperature, relative humidity, aerosol size and number distribution, and total aerosol surface area. Classic binary nucleation theory predicts no nucleation under the observed marine boundary layer conditions.

Abstract. A sulfur field study (SCATE) at Palmer Station Antarctica (January 18 to February 25) has revealed several major new findings concerning (dimethyl sulfide) DMS oxidation chemistry and the cycling of sulfur within the Antarctic environment. Significant evidence was found supporting the notion that the OH/DMS addition reaction is a major source of dimethyl sulfoxide (DMSO). Methane sulfonic acid (MSA(g)) levels were also found to be consistent with an OH/DMS addition mechanism involving the sequential oxidation of the products DMSO and methane sulfinic acid (MSIA). Evidence supporting the hypothesis that the OH/DMS addition reaction, as well as follow-on reactions involving OH/DMSO, are a major source of SO2 was significant, but not conclusive. No evidence could be found supporting the notion that reactive intermediates (i.e., SO3) other than SO2 were an important source of H2SO 4. Quite clearly, one of the major findings of SCATE was the recognition that a large fraction of the Antarctic oxidative cycle for DMS (near Palmer Station) took place above the boundary layer (BL) in what we have labeled here as the atmospheric buffer layer (BuL). Although still speculative in places, the overall picture emerging from the SCATE field/modeling results is one involving major coupling between chemistry and dynamics in the Antarctic. At Palmer the evidence points to frequent episodes of rapid vertical transport from a very shallow marine BL into the overlying BuL. Due to the combination of a long photochemical lifetime for DMS and the frequency of shallow convective events, a large fraction of ocean released DMS is transported into the BuL while still in its unoxidized state. There, in the presence of elevated OH and low aerosol scavenging, high levels of oxidized sulfur accumulate. Parcels of this BuL air are then episodically entrained back into the BL, thereby providing a controlling influence on BL SO2, DMSO, and DMSO2. Additionally, because SO2 and DMSO are major precursors to H2SO 4 and MSA, BuL chemistry, in conjunction with vertical transport, also act to control BL levels of the latter species. Although many uncertainties remain in our understanding of Antarctic DMS chemistry, the above picture already suggests that previous chemical interpretations of Antarctic field data may need to be altered.

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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 HO2 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 HO2 radicals are found to be converted to H2O2 by aqueous‐phase chemical reactions at a rate that suggests that this mechanism could produce a significant fraction of the H2O2 found in cloud droplets. The rapid oxidation of sulfur species dissolved in cloudwater by this free‐radical‐produced H2O2 as well as by aqueous‐phase OH radicals could conceivably have a significant impact upon the chemical composition of rain.

Results from the two campaigns clearly quantify, from a trace gas perspective, the seasonal differences in the continental outflow that were qualitatively anticipated based upon meteorological considerations, and show the impact of major meteorological features within the region on the quality of tropospheric air over