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Snow-Atmosphere Transfer Function for Reversibly Deposited Chemical Species in West Antarctica (OPP- 9526572)

R. Bales, U. Arizona, PI

This research involved measuring hydrogen peroxide (H2O2) and formaldehyde (HCHO) in the air and snow, firn and ice at Siple Dome in order to develop a better understanding of air-snow exchange of these two chemical species, and then use this understanding to interpret changes in concentrations of these species recorded in ice cores. Through a cooperative effort with NOAA personnel, complimentary year-round measurements of H2O2 were made at South Pole. The first major result was a physically based, mathematical model of atmosphere-to-snow-to-firn transfer of H2O2, developed as part of the Ph.D. dissertation of J. McConnell. The model describes advection of surface air through the top few meters of firn and diffusion of H2O2 into and out of snow grains in response to changing atmospheric concentrations and snowpack temperatures. Subsequent application of the model to snow-pit and ice-core data from Siple Dome and South Pole showed consistent results, despite the more than 26*deg;C difference in average temperature between the two sites. With relatively warm temperatures and low accumulation rates, data from the Siple Dome site were critical for validation of the transfer model for H2O2. It was found that the surface snow acts as an excellent proxy for atmospheric concentrations of H2O2. However, the firn continues to lose H2O2 to the atmosphere for at least 10-12 years (~3 m) after burial at South Pole. Most losses occur over about 3-5 years (~1-2 m) at Siple Dome owing to the warmer temperatures and so higher diffusion rates of H2O2 in ice. HCHO is preserved in near-surface snow and firn at Siple Dome, but concentrations are lower than for South Pole, reflecting the temperature difference between the two sites. Subsequent modeling of HCHO measurements at the two sites, using a model similar to that developed for H2O2, also gave consistent results. Concentrations of about 6 ppbw in surface snow drop to about 1 ppbw and lower below about 1-2 m depth, owing to temperature-dependent uptake and release of HCHO in response to annual temperature cycles. Concentrations from about 2 to 20 m were relatively constant with depth, and apparently reflect equilibrium with the atmosphere. Further modeling implies that annual average atmospheric HCHO levels are on the order of 10 pptv, and that degassing of HCHO from surface snow could contribute a significant fraction of the HCHO in the West Antarctic boundary layer in summer.

As a result of these findings, plus analyses and transfer modeling done on other shallow cores in West Antarctica, H2O2 and HCHO should be well preserved in the proposed deep inland core. This offers an excellent opportunity to develop a unique, high-quality record of these reactive species throughout the Holocene and back into the last glaciation. Spot analyses done on older Siple station and Byrd cores are consistent with this idea. Further, given the success with the air-snow transfer-function models at Siple Dome and South Pole, a quantitative interpretation of records in future cores is possible.