Investigation of the Transfer Function Between Atmosphere and Snow Concentrations of Hydrogen Peroxide at South Pole
Joseph R. McConnell and Roger C. Bales
Department of Hydrology and Water Resources, University of Arizona, Tucson, 85721
A critical issue in atmospheric chemistry today is to assess the global trend in the concentration of the hydroxyl radical (OH) and hydrogen peroxide (H2O2). Oxidation by OH represents the main sink for a number of environmentally important atmospheric gases including methane (CH4), carbon monoxide (CO) and the halogenated hydrocarbons involved in stratospheric ozone loss. Extensive efforts have been made over the past few years to use photochemical models to estimate past and future atmospheric concentrations of OH and hydrogen peroxide (H2O2) [Thompson et al., 1993; Stafflebach, 1990]. However, these model studies of past atmospheres lack sufficient data for validation. Of the primary oxidants, only H2O2 is preserved in the ice, but the correspondence between atmospheric and ice core hydrogen peroxide is not well understood. To be useful for oxidant modeling, the transfer processes that relate atmosphere to snow to ice concentrations must be well defined.
Our research is an investigation of the atmosphere-ice transfer function for reversibly deposited chemical species, primarily hydrogen peroxide and formaldehyde, through laboratory, field, and computer modeling studies. Reversible implies that some fraction of the deposited mass of these species cycles between the atmosphere and snow as conditions such as atmospheric concentration and temperature change. The objective is to develop the capability to describe concentrations of reversibly deposited species 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.
Building on similar field studies at Summit, Greenland, and Siple Dome, Antarctica, we have made measurements of H2O2 concentrations in the atmosphere and in the surface, near-surface and deep snow, and firn at the South Pole. What sets South Pole apart as a site for atmosphere-snow transfer studies is (a) the long-term instrumental record of meteorology and atmospheric chemistry, (b) the ongoing, year-round presence of qualified staff to make chemical and meteorological measurements, (c) the lack of a diel temperature cycle, and (d) the distance from open water and anthropogenic activity. As part of our cooperative agreement with the Climate Monitoring and Diagnostics Laboratory (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 that time and a number of surface snow and shallow snow pit samples were collected and analyzed. 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. For firn analyses, the samples are melted and the peroxide measurement made directly.
Since December 1994 the CMDL winterover staff have collected surface and near-surface snow samples for this project. Samples are collected approximately once a week throughout the year. Each sampling event consists of collecting approximately five surface snow and ten near-surface samples. These are returned each summer to our laboratory in Tucson for analyses of H2O2 and major ion concentrations and oxygen isotopes.
The measured surface snow concentrations for the samples analyzed to date are shown in Figure 1. Using a physically based model for the atmosphere-snow transfer processes, and assuming rapid ventilation at the snow surface, we inverted a portion of this surface snow record to an estimate of the atmospheric concentration throughout the year (Figure 2) [McConnell et al., 1997a]. The model is based on the advection-dispersion equation with the rate of uptake and release of H2O2 in the snow pack controlled by spherical diffusion within individual snow grains. The estimated atmospheric concentration compares favorably with both photochemical model results and with short-duration atmospheric measurements made in November-December 1994. Note that measurements made in January 1996 do not compare as favorably, perhaps, because of local pollution from construction at the new clean air facility that may have affected photochemistry.
Fig. 1. (a) H2O2 concentrations and (b) sample depth in surface snow at the South Pole. Error bars show one standard deviation for replicated (typically six) surface snow samples [after McConnell et al., 1998].
Fig. 2. (a) Measured H2O2 in surface snow at South Pole, (b) surface temperature, and (c) Estimated atmospheric concentration of H2O2 derived through inversion of surface snow samples and from photochemical modeling. Spot direct atmospheric measurements are shown as circles [after McConnell et al., 1997b].
We have extended this physically based modeling into the firn at South Pole. The physically based model was used to simulate the H2O2 concentration measured in snow pits where an independent estimate of accumulation history was available from snow accumulation studies [McConnell, 1997; McConnell et al., 1997b; 1998]. The results (Figure 3) indicate that snow, which is strongly over-saturated with H2O2 at formation, remains generally over-saturated when buried because it is cut off from the atmosphere before it can release excess peroxide. The model results suggest that temperature and accumulation rate, along with snows physical properties, like grain size and permeability, are the primary factors that determine preservation of H2O2 in the snow pack.
Fig. 3. Measured (horizontal bars) and simulated (solid lines) concentration profiles for four snow pits at South Pole. The pits were sampled in January 1996 and are located in a long-term snow accumulation array about 500 m from South Pole Station [after McConnell et al., 1998].
The snow pack model has also been used to simulate snow pit profiles from Siple Dome, Antarctica. The results (Figure 4), while preliminary, indicate that the physically based model has wide applicability since it correctly simulates the 4-and 40-fold decreases in H2O2 concentration from the surface to 1-m depth observed at South Pole and Siple Dome, respectively.
Fig. 4. Observed snow pit (horizontal bars), firn cored (dashed), and physically based model simulations (solid) of H2O2 concentrations in snow and firm at (a) South Pole and (b) Siple Dome, Antarctica. Annual average snow accumulation was used in the simulations so individual summer peaks and winter troughs will not coincide with observations [after McConnell, 1997].
Our understanding of the transfer function that relates atmosphere-to-snow-to-firn concentration of reversibly deposited chemical species such as H2O2 has improved dramatically over the past 3-4 years. This improvement would not have been possible without the year-round surface snow and snow pit samples and atmospheric data that we have obtained through our cooperative project with CMDL.
McConnell, J.R., Investigation of the atmosphere-snow transfer process for hydrogen peroxide, Ph.D. Dissertation, Department of Hydrology and Water Resources, Univ. Arizona, 1997.
McConnell, J.R., R.C. Bales, R.W. Stewart, A.M Thompson, M.R. Albert, and R. Ramos, Physically based modeling of atmosphere-to-snow-to-firn transfer of H2O2 at South Pole, J. Geophys. Res., in press, 1998.
McConnell, J.R., J.R. Winterle, R.C. Bales, A. M. Thompson, and R. W. Stewart, Physically based inversion of surface snow concentrations of H2O2 to atmospheric concentrations at South Pole, Geophys. Res. Lett., 24(4):441-444, 1997a.
McConnell, J.R., R.C. Bales, and D.R. Davis, Recent intra-annual snow accumulation at South Pole: Implications for ice core interpretation, J. Geophys. Res., 102(D18): 21,947-21,954, 1997b.
Staffelbach, T., Formaldehyd- und ammonium-messungen an schnee-, firn- und eisproben aus polaren und alpinen regionen, PhD thesis, Physikalisches Institut, Universitat Bern, 1990.
Thompson, A. M., J. A. Chappellaz, I. Y. Fung, and T. L. Kucsera, The atmospheric CH4 increase since the last glacial maximum, 2, Interaction with oxidants, Tellus, 45B, 242-257, 1993.
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