THE CHEMICAL-COMPOSITION OF ANCIENT ATMOSPHERES - A MODEL STUDY CONSTRAINED BY ICE CORE DATA

A coupled chemistry radiation transport two-dimensional model of the l ower and middle atmosphere was adapted to study the chemical compositi on of the atmosphere at preindustrial time and last glacial maximum (L GM). The model was constrained by trace gas concentrations (CO2, CH4, and N2O) inferred from polar ice core records. The formulation of trop ospheric dynamics and chemistry was improved in order to more accurate ly simulate the transport and the oxidation processes below the tropop ause. Our objectives are to infer the changes in middle-atmosphere tem perature, ozone layer, and oxidation capacity of the atmosphere (e.g., methane lifetime) over the last 18,000 years. A middle-atmosphere coo ling was obtained between LGM and preindustrial Holocene (PIH) as well as between PIH and present time. This is mainly due to changes in the CO2 and chlorofluorocarbon (CFC) concentrations, respectively. CFCs a re also the main contributors to the middle-atmosphere ozone decrease since PIH. Between LGM and PM the compensating effects of CO2 and N2O lead to little variation in stratospheric ozone. A 17% decrease in tro pospheric OH was obtained between LGM and PIH, whereas the model provi des a 6% OH increase since PM. The corresponding changes in the methan e sink are too small to have played a dominant role in the past methan e concentration changes. Our model derived methane emissions for LGM, PIH, and present time are in good agreement with methane sources evalu ated during these three periods.