Overview

One critical issue in atmospheric chemistry is the global trend in atmospheric oxidation capacity, given primarily by the concentrations 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, carbon monoxide, and halogenated hydrocarbons involved in stratospheric ozone loss. Photochemical model calculations suggest that atmospheric OH concentrations have decreased from the last glacial maximum to the Holocene by 20-40%, and from pre-industrial times to today by up to 60%. Such changes would have caused large across-the-board perturbations to the chemistry of the atmosphere. However, the models are highly uncertain; they are based on methane and temperature records from ice cores, but must make rather arbitrary assumptions regarding past values of other variables that regulate OH levels. Ice-core archives of H₂O₂ and formaldehyde (HCHO), which are produced in the atmosphere by radical reactions closely linked to the OH budget, offer the best potential to further constrain these model estimates.
Attempts to use the H₂O₂ and HCHO records in ice cores to infer past trends in OH have assumed constant and linear transfer functions between the H₂O₂ and HCHO concentrations found in the ice (C_ice on figure) and those in the atmosphere (C_atm). However, field studies show that both deposition and subsequent air-snow exchange processes are important. The transfer functions are certainly not constant over time, and even vary between different locations.

The primary aim of our research is to determine the snow-atmosphere transfer functions for chemical species that are reversibly deposited to polar ice and snow and thus incorporated into the glacial record. Our main focus is on H₂O₂ and HCHO. A second aim is to relate changes in concentrations in the snow/firn/ice to corresponding changes in tropospheric chemistry.

Processes and steps involved in transfer function, relating concentrations in ice to those in the global atmosphere. Depth and age scales are for Greenland. Snow-to-firn transition is defined by metamorphism and grain growth; firn-to-ice transition is defined by pore closure. Adapted from Neftel, et al., NATO ARW Series I, v 30, pp 249-264, 1995.