Elevated ozone in the boundary layer at South Pole - Doug Davis

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ARTICLE IN PRESS D. Helmig et al. / Atmospheric Environment 42 (2008) 2788–2803 2801 Solomon, 2002). Lower surface winds and temperatures were observed at SP, following a long-term trend towards increased inversion strength in the 1990s (Neff, 1999), a period when the AAO was in its positive index state. Thus, increases in the AAO as reported by Thompson and Solomon (2002), if they continue, should lead to more frequent episodes of light winds and stagnation in the SP region. Our data show the strong dependency of ozone production on boundary layer stability. It is noteworthy that the increasing surface ozone trend during 1990–2004 has exclusively resulted from an increase in ozone during November–January (Oltmans et al., 2006; Helmig et al., 2007a), when surface layer photochemical ozone production chemistry is expected to be most important. Therefore, we hypothesize that a stronger AAO, by fostering more stable boundary layer conditions, may have influenced ozone production in the surface layer and has contributed to the observed recent increases in the SP surface ozone record. 3.9. Comparison of SP with other polar sites The ozone enhancements in the SP surface layer are unique compared to other polar research sites. For instance, at Summit, Greenland, ozone chemistry has been noted to be much different. Summit is at similar elevation and with similar year-round snowpack. However, being at 721N Summit experiences significant diurnal radiation cycles. The snowpack remains at sub-freezing temperatures year-round, although is some 10–151 warmer during the summer than at SP, with daytime snow surface temperatures regularly warming up to 10 to 5 1C (Helmig et al., 2007c). Episodes with increased ozone at Summit are related to transport events with a frequent occurrence of transport from the higher troposphere/lower stratosphere as well as occasional upslope flow with polluted air from lower latitudes (Helmig et al., 2007e). Our ANTCI data and earlier studies (Oltmans and Komhyr, 1976; Crawford et al., 2001) have shown that high ozone at SP originates near the surface and is not transported from higher altitudes. Furthermore, there is no indication for polluted, anthropogenically influenced air reaching SP. Summit, in contrast to SP, during summer is subject to substantial diurnal radiation and temperature cycles and the MBL is much more dynamic; e.g. stability regimes change frequently and are inhomogeneous with altitude (Cohen et al., 2007). Snowpack temperatures at Summit are higher and surface heating during sunny daytime conditions results in convective heating, which contributes to boundary layer growth and increased vertical mixing. Stable atmospheric conditions at Summit mostly occur during night, when there is very little sunlight to drive photochemistry. Air reaching Summit is mostly representative of NH, lower tropospheric composition, rather than being transported upslope over the Greenland glacial ice shield. Consequently, the effective footprint and residence time of air in contact with the snow surface on average is much shorter and sustained residence of air in a shallow surface layer, as at SP, is not encountered at Summit. Under these conditions, NO concentrations and ozone production in the surface layer do not build up to the high levels observed at SP (Davis et al., 2004). 4. Conclusions Enhanced ozone concentrations are a frequent phenomenon in the summertime surface and lower boundary layer at SP. Ozone is predominantly produced and transported from the high altitude Antarctic plateau in the area surrounding SP from N to SE. Ozone production occurs by photochemical processes in a shallow surface layer, during stable, light wind, strongly stratified boundary layer conditions. These experiments show that strong vertical ozone gradients, which result from a buildup of ozone in the surface layer, are a common, summertime condition at SP. Our data further illustrate that even between the surface and the 17 m-high inlet of the ARO observatory up to 5 ppbv ozone gradients can be encountered. Hence, ozone mixing ratios will depend on the sampling height and consideration of the inlet location will be of importance in comparing the SP ozone record with data from other sites. Previously reported upwards ozone fluxes out of snow in other environments may have resulted from similar conditions of photochemical ozone production in a shallow atmospheric layer above the snow surface. These new observations solidify the previous analyses and estimates of summertime ozone production chemistry at SP. Our measurements point towards the occurrences of ozone production rates that are in the upper range of previous calculations. These data provide new evidence that polar surface ozone concentrations are tied to
2802 ARTICLE IN PRESS D. Helmig et al. / Atmospheric Environment 42 (2008) 2788–2803 photochemical processes in the sunlit snowpack, chemical reactions in the atmospheric surface layer and boundary layer dynamics. These comparisons denote the remarkable conditions at SP. In contrast to the pre-ISCAT understanding, it is likely that the lower boundary layer of large areas of Antarctica should be considered a spring–summertime source of surface ozone as has been shown in the 3D modeling results of Wang et al. (2007). The Antarctic plateau represents a unique situation on this planet, where the combination of snowpack emissions, sustained stable boundary layer regimes, and the presence of 24-h unmodulated sunlight can be found. The significant ozone production chemistry above the snowpack that results from these conditions has hitherto not been reported from any other polar or clean-air environment on Earth. Natural ozone production in the lower troposphere has been known to occur mostly in air affected by biomass burning plumes and in areas that are subjected to high NO x levels from lightning. The chemistry occurring in the boundary layer at SP represents another situation with significant ozone production in an environment that is virtually devoid of human impacts. Acknowledgments This research was supported through the United States National Science Foundation (Office of Polar Programs, Grant #0230046). A. Drexler, J. Seiffert and M. Warshawsky helped with the balloon experiment at SP and I. Brown and T. Morse assisted in the data analysis and preparation of some of the color figures. We thank Raytheon Polar Services and the US 109th Air National Guard for providing excellent logistical support and the South Pole staff for an extraordinary effort in accommodating the tethered balloon experiment. References Anbach, W., 1974. The influence of cloudiness on the net radiation balance of a snow surface with high albedo. Journal of Glaciology 13, 73–84. Bocquet, F., Helmig, D., Oltmans, S.J., 2007. Ozone in the interstitial air of the mid-latitude snowpack at Niwot Ridge, Colorado. Journal of Alpine, Arctic and Antarctic Research, in press. Chen, G., Davis, D., Crawford, J., Hutterli, L.M., Huey, L.G., Slusher, D., Mauldin, L., Eisele, F., Tanner, D., Dibb, J., Buhr, M., McConnell, J., Lefer, B., Shetter, R., Blake, D., Song, C.H., Lombardi, K., Arnoldy, J., 2004. A reassessment of HO x South Pole chemistry based on observations recorded during ISCAT 2000. Atmospheric Environment 38, 5451–5461. Cohen, L., Helmig, D., Neff, W., Grachev, A., Fairall, C., 2007. Boundary layer dynamics and its influence on atmospheric chemistry at Summit, Greenland. Atmospheric Environment, in press, doi:10.1016/j.atmosenv.2006.06.068. Crawford, J.H., Davis, D.D., Chen, G., Buhr, M., Oltmans, S., Weller, R., Mauldin, L., Eisele, F., Shetter, R., Lefer, B., Arimoto, R., Hogan, A., 2001. Evidence for photochemical production of ozone at the South Pole surface. Geophysical Research Letters 28, 3641–3644. Davis, D., Nowak, J.B., Chen, G., Buhr, M., Arimoto, R., Hogan, A., Eisele, F., Mauldin, L., Tanner, D., Shetter, R., Lefer, B., McMurry, P., 2001. Unexpected high levels of NO observed at South Pole. Geophysical Research Letters 28, 3625–3628. Davis, D., Chen, G., Buhr, M., Crawford, J., Lenshow, D., Lefer, B., Shetter, R., Eisele, F., Mauldin, L., Hogan, A., 2004. South Pole NO x chemistry: an assessment of factors controlling variability and absolute levels. Atmospheric Environment 38, 5375–5388. Davis, D.D., Huey, G., Crawford, J., Chen, G., Wang, Y., Buhr, M., Helmig, D., Neff, W., Blake, D., Arimoto, R., Eisele, F., 2007. A reassessment of Antarctic Plateau reactive nitrogen and its impact on the oxidizing properties of the near surface atmosphere. Atmospheric Environment, submitted for publication. Dibb, J.E., Talbot, R.W., Munger, J.W., Jacob, D.J., Fan, S.-M., 1998. Air-snow exchange of HNO 3 and NO y at Summit Greenland. Journal of Geophysical Research 103, 3475–3486. Dibb, J.E., Arsenault, M., Peterson, M.C., Honrath, R.E., 2002. Fast nitrogen oxide photochemistry in Summit, Greenland snow. Atmospheric Environment 36, 2501–2511. Eisele, F., Davis, D.D., Helmig, D., Oltmans, S.J., Neff, W., Huey, G., Tanner, D., Chen, G., Crawford, J., Arimoto, R., Buhr, M., Mauldin, L., Hutterli, M., Dibb, J.E., Blake, D., Brooks, S.B., Johnson, B., Roberts, J.M., Wang, Y., Tan, D., Flocke, F., 2007. ANTCI Overview Paper. Atmospheric Environment, submitted for publication. Galbally, I., Allison, I., 1972. Ozone fluxes over snow surfaces. Journal of Geophysical Research 77, 3946–3949. Harris, J.M., Draxler, R.R., Oltmans, S.J., 2005. Trajectory model sensitivity to differences in input data and vertical transport method. Journal of Geophysical Research 110, D14109, doi:10.1029/2004JD005750. Helmig, D., Boulter, J., David, D., Birks, J.W., Cullen, N.J., Steffen, K., Johnson, B.J., Oltmans, S.J., 2002. Ozone and meteorological boundary-layer conditions at Summit, Greenland during June 3–21, 2000. Atmospheric Environment 36, 2595–2608. Helmig, D., Oltmans, S.J., Carlson, D., Lamarque, J.F., Jones, A., Labuschagne, C., Anlauf, K., Hayden, K., 2007a. A review of surface ozone in the polar regions. Atmospheric Environment, in press, doi:10.1016/j.atmosenv.2006.09.053. Helmig, D., Johnson, B., Warshawsky, M., Morse, T., Neff, W., Eisele, F., Davis, D.D., 2007b. Nitric oxide in the boundarylayer at South Pole during the Antarctic Tropospheric Chemistry Investigation (ANTCI). Atmospheric Environment, in press, doi:10.1016/j.atmosenv.2007.03.061. Helmig, D., Bocquet, F., Cohen, L., Oltmans, S.J., 2007c. Ozone uptake to the polar snowpack at Summit, Greenland.
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