JOSEPH R. MC CONNELL AND ROGER C. BALES
Department of Hydrology and Water Resources, University of Arizona, Tucson, 85721
INTRODUCTION
A key to understanding past and future climate lies in understanding the oxidizing capacity of the atmosphere through time. Extensive efforts were made over the past few years to use photochemical models to estimate past and future atmospheric concentrations of hydroxyl radical, ozone, and hydrogen peroxide which are the primary oxidants in the atmosphere [Thompson et al., 1995]. However, these model studies of past atmospheres lack sufficient data for validation. Ice cores provide temperatures and CH4 concentrations, but two of the oxidants, hydroxyl radical and ozone, are not preserved in the ice, and the correspondence between atmospheric and ice core hydrogen peroxide is not well understood [Neftel et al., 1995]. A glacial ice record of atmospheric formaldehyde, an intermediate species in the oxidation of CH4 by hydroxyl radical attack, could also help to lead to a reconstruction of the local hydroxyl radical concentration in the paleo-atmosphere. As with hydrogen peroxide, however, the correspondence of atmospheric and ice-core formaldehyde concentration is not known. Therefore, if the formaldehyde and hydrogen peroxide records in the polar ice cores are to be useful for oxidant modeling of both past and future climates, the transfer functions that relate atmosphere to snow to ice concentrations must be well defined.
Our research is an investigation of the atmospheric-ice transfer function for the reversibly deposited chemical species formaldehyde and hydrogen peroxide through laboratory, field, and computer modeling studies. Reversible implies that the species can be released from the snow and firn back to the atmosphere as conditions change and can be deposited back to the snow in response to further changes. The objective is to develop the capability to describe concentrations of formaldehyde and hydrogen peroxide in snow and ice, as functions of depth and time, given concentrations and conditions in the local atmosphere and properties of the snow and ice. This modeling capability will then be inverted and used to infer possible past atmospheric concentrations from observations of what is trapped in the ice, providing additional data constraints on tropospheric modeling.
PROCEDURES
As part of our cooperative agreement with CMDL, we deployed a detector for measuring H2O2 at the South Pole for a 3-week period in late November and early December 1994 and for a 2-week period in January 1996. Atmospheric measurements were made almost continuously during these time periods. When used for atmospheric analyses, our custom built H2O2 detector makes atmospheric measurements at approximately 10-minute intervals using a glass coil scrubber to transfer the peroxide from the continuous stream of air to water and then a chemical fluorescence method to determine the concentration in the water [Bales et al., 1995]. When making snow and firn analyses, the samples are melted and the peroxide measurement made directly. A number of surface snow and shallow snowpit samples were collected and analyzed. In 1994, systematically and stratigraphically placed firn samples were collected from a 2.2 m backlit snowpit and a 1.0 m snowpit, both located in the clean air sector. Four snowpits, located adjacent to the snow stake accumulation field near the South Pole, were excavated and sampled at 1 cm resolution in 1996.
The CMDL winterover staff collect approximately 12 surface-snow samples each week throughout the year. This sampling started in December 1994 and continues currently. The samples collected prior to mid-January 1996 were returned to our laboratory in Tucson and were analyzed for H2O2 and, on a subset of the samples for 18O, and some ionic species.
RESULTS
Average atmospheric H2O2 concentrations for the 3-week period in spring 1994 were approximately 320 pptv and, as expected, no diel cycle was observed [Fuhrer et al., 1996]. Occasional rapid changes in atmospheric H2O2 were observed and a qualitative comparison with air parcel trajectories suggest a correlation of atmospheric H2O2 concentration with a source area, although there is also a correlation with local meteorological conditions. This is the subject of ongoing atmospheric chemistry modeling [Thompson, 1995]. Concentrations in summer 1996 were lower at approximately 100 pptv. Construction at the new clean air facility at the South Pole may have degraded these measurements somewhat.
Snowpit H2O2 concentrations from a pit collected in January 1996 are shown in Figure 1. Note the very obvious annual cycles in H2O2. Along with the other three snowpits from 1995 [McConnell et al., 1995], these data will be used to parameterize and validate a snowpack model that we have developed and applied at Summit, Greenland, to investigate the atmosphere to snow transfer process [McConnell, et al., 1996a]. Some revisions to the model are required because snow accumulation at the South Pole is much more sporadic [McConnell, et al., 1996b].
Fig. 1. Variations in hydrogen peroxide concentrations with depth at South
Pole.
The H2O2 concentrations in the surface snow samples are shown in Figure 2. Along with complimentary laboratory data [Conklin, et al., 1992], these surface snow samples have been used to estimate the annual cycle in atmospheric H2O2 concentration [McConnell et al., 1996c]. The results indicate that the surface snow provides a good proxy of the atmospheric H2O2 concentration throughout the year at the South Pole. Evaluation of the surface snow data continues.
Fig. 2. Estimated atmospheric concentration of hydrogen peroxide at South Pole from a physically based inversion of surface snow concentrations.
CONCLUSION
A record of the oxidizing capacity of the atmosphere would improve our understanding of interactions between atmosphere and climate. This information could be used to better verify and parameterize global and local atmospheric circulation and photochemical models and so aid in predicting the impact of anthropogenic increases in greenhouse gases. However, a required step in reconstructing the oxidizing capacity of the atmosphere from polar ice cores is to fully understand the transfer relationship between atmospheric and snow/ice concentration for reversibly deposited species such as hydrogen peroxide and formaldehyde.
Based on year-round surface snow samples collected for us by CMDL staff, we conclude that the surface snow is acting as an effective archive of the atmospheric loading of hydrogen peroxide during the year. Whether or not this archive is preserved and under what conditions is the focus of ongoing work. Photochemical modeling is underway to better understand the atmospheric concentration time series measured in 1994 and 1996, especially in the context of the annual cycle inferred from the surface snow. Physical and chemical modeling of the near-surface snow and firn continues as we investigate the atmosphere-surface snow relationship and the differences in the near-surface firn profiles at Summit, Greenland, and the South Pole. The cooperation of the NOAA personnel in collecting surface snow samples and making atmospheric measurements have proven invaluable in our research into the atmosphere-to-snow transfer process, both for H2O2 and for reversibly deposited chemical species in general.
REFERENCES
Bales, R.C., M.V. Losleben, J.R. McConnell, K. Fuhrer, and A. Neftel, H2O2 in snow, air, and open pore space in firn at Summit, Greenland, Geophys. Res. Lett., 22(10), 1261-1264, 1995.
Conklin, M.H., A. Sigg, A. Neftel, and R.C. Bales, Atmosphere-snow transfer function for hydrogen peroxide: microphysical considerations, J. Geophys. Res., 98(D10), 18,367-18,376, 1992.
Fuhrer, K., M. Hutterli, and J.R. McConnell, Overview of the recent field experiments for the study of the air-snow transfer of H2O2 and HCHO, in Chemical Exchange Between the Atmosphere and Polar Snow, edited by E. Wolff and R. Bales, NATA ASI Series I, 675 pp., Springer-Verlag, 1996.
McConnell, J.R., M. Conklin, and R. Bales, Hydrogen peroxide trends in South Pole firn, poster paper presented at the NATA ARW: Process of Chemical Exchange Between the Atmosphere and Polar Snow, II Ciocco, Italy, 1995.
McConnell, J.R., R.C. Bales, J.R. Winterle, H. Kuhns, and C.R. Stearns, A lumped parameter model for the atmosphere-to-snow transfer function for hydrogen peroxide, J. Geophys. Res., in press, 1996a.
McConnell, J.R., R.C. Bales, D.R. Davis, Recent intra-annual snow accumulation at South Pole: Implications for ice core interpretation, J. Geophys. Res., in press, 1996b.
McConnell, J.R., A.M. Thompson, and R.C. Bales, Surface snow as a proxy for atmospheric hydrogen peroxide at South Pole, EOS, Trans. AGU, 5156, 1996c.
Neftel, A., R.C. Bales, and D.J. Jacob, Hydrogen peroxide and formaldehyde in polar snow and their relation to atmospheric chemistry, in Ice Core Studies of Global Biogeochemical Cycles, edited by R. Delmas, NATO ASI Series I, Vol. 30, pp. 249-264, 1995.
Thompson, A.M., Photochemical modeling of chemical cycles: issues related to
the interpretation of ice core data, in Ice Core Studies of Global Biogeochemical
Cycles, edited by R. Delmas, NATO ASI Series I, Vol. 30, pp. 265-297, 1995.
