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Abstract
Aircraft measurements of selected nonmethane hydrocarbon and halocarbon species were made in the lower troposphere of the NE Atlantic near the Azores, Portugal, during June 1992 as part of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange. In this paper, the impact of continental outflow from both Europe and North America on the study region were assessed. Four main air mass types were characterized from trajectories and trace gas concentrations: clean marine from the Atlantic, and continental air from the Iberian Peninsula, the British Isles and Northern Europe, and North America. Each classification exhibited trace gas concentrations that had been modified en route by photochemical processes and mixing. Comparison with the clean marine boundary layer (MBL) shows that the boundary layer of the predominantly continental air masses were enhanced in hydrocarbons and halocarbons by factors of approximately 2 for ethane, 5 for propane, 2–6 for ethyne and benzene, and 2–3 for C2Cl4. The same air masses also exhibited large ozone enhancements, with 2 to 3 times higher mixing ratios in the continental boundary layer air compared to the clean MBL. This indicates a primarily anthropogenic photochemical source for a significant fraction of the lower tropospheric ozone in this region. Methyl bromide exhibited on average 10–20% higher concentrations in the boundary layer affected by continental outflow than in the clean MBL, and was seen to be enhanced in individual plumes of air of continental origin. This is consistent with significant anthropogenic sources for methyl bromide. In addition, median MBL concentrations of ethene and methyl iodide showed enhancements of approximately a factor of 2 above free tropospheric values, suggesting primarily coastal/oceanic sources for these species.
Highlights
Introduction emissions [Singh and Zimmerman, 1992; Blake et al, 1994, 1995]
Methyl iodidehasan approximately5-dayatmospheric the use of these NMHC and halocarbondata in a Lagrangian lifetime againstphotolysis [Zafiriou, 1974], long enoughfor experimento determineatmospherichydroxyl,atomicchlorine oceanic emissions to be extensively mixed into the lower andvertical mixing [Wingenteret al., thisissue]
Methyl iodide and methyl bromide concentrations were enhanced in the MBL and in air with
Summary
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D2, PAGES 4501-4514, FEBRUARY 20, 1996. Nonmethane hydrocarbon and halocarbon distributions during Atlantic Stratocumulus Transition. Oxidationby HO radicalsis the primaryprocess selectedchlorofluorocarbon(sCFCs) andother halocarbonshave for removalfrom the atmospherefor most NMHCs and for the beenpublishedfor Ireland [Prinn et al, 1983; Simmondset al., halocarbons with carbon-carbon double bonds or abstractable 1993] andfor the Pacific[Elkinset al., 1993;Wang, 1993]. NMHCs are emittedfrom a variety of biogenic and exceedabout200 parts per trillion by volume (pptv), these anthropogeniscourcesandmaybe usedto distinguishfossil fuel successive reactions tend to increase the amount of ozone leakage, biomassburning, industrialactivity, and natural gas presentw, hile maintaininghigh levelsof bothHO andNOr In contrasti,n thenearabsenceof NOx, frequentlythe casein clean. European continents [e.g., Van Valin and Luria, 1988; mi (100 km) northof SantaMaria. The focusof this paperis the Koppmannet al., 1992;Parrishet al., 1992]. Methyl iodidehasan approximately5-dayatmospheric the use of these NMHC and halocarbondata in a Lagrangian lifetime againstphotolysis [Zafiriou, 1974], long enoughfor experimento determineatmospherichydroxyl,atomicchlorine oceanic emissions to be extensively mixed into the lower andvertical mixing [Wingenteret al., thisissue]
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