The atmospheric chemistry in the Last Glacial Maximum : A model study

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Laepple, T. , Schultz, M. , Emmons, L. , Madronich, S. and Lohmann, G. (2006): The atmospheric chemistry in the Last Glacial Maximum : A model study , oral presentation, European Geosciences Union General Assembly 2006, Vienna, Austria 07 April 2006. .
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The oxidation capacity of the atmosphere is ultimately controlling the lifetime andtherefore the burden of many trace gases including the greenhouse gas methane.Studying the atmospheric chemistry during the Last Glacial Maximum (LGM) combinedwith trace gas measurements in ice cores will help to validate our understandingof the earth system, to understand the interglacial changes in methane concentrations,and to interpret the trace gas concentration data found in ice cores.We modeled the tropospheric chemistry with a state-of-the-art three-dimensionalchemical transport model (CTM), MOZART, driven by the meteorology output froman atmospheric circulation model, ECHAM. Using a dynamical vegetation model, includinga fire module, we estimated the vegetation as well as the resulting biogenicand biomass burning emissions such as hydrocarbons, carbon monoxide (CO), andnitrogen oxides (NOx), all of which are known to affect atmospheric chemistry. Wealso took into account the effects of changed vegetation on chemistry due to depositionfluxes, surface reflectivity (albedo), and dust source regions. Model runs usingpresent day conditions, preindustrial conditions as well as LGM conditions were performed.Among other differences in the chemical composition, the results indicate aslight enhancement of the oxidation capacity which is mainly driven by the reducedbiogenic VOC emissions and the reduced methane emissions. In general the troposphericchemistry is relatively stable due to counteracting effects of different emissionsource changes and climate changes. Further we used our model simulations to calculatethe nitrate deposition flux and estimated the NOx source contributions to nitrate concentrations measured in icecores. In contrast to literature our results suggest thatlightning is the most important NOx source controlling the nitrate concentration intropical ice cores.

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