PNNL

Abstract

Aerosol introduces the largest uncertainties in model-based estimates of anthropogenic sources on the Earth's climate. A better representation of aerosol in climate models can be expected from an individual processing of aerosol type and new aerosol modules have been developed, that distinguish among at least five aerosol types: sulfate, organic carbon, black carbon, sea-salt and dust. In this study intermediate results of aerosol mass and aerosol optical depth of new aerosol modules from seven global models are evaluated. Among models, differences in predicted mass-fields are expected with differences to initialization and processing. Nonetheless, unusual discrepancies in source strength and in removal rates for particular aerosol types were identified. With simultaneous data for mass and optical depth, type conversion factors were compared. Differences among the tested models cover a factor of 2 for each, even hydrophobic, aerosol type. This is alarming and suggests that efforts of good mass-simulations could be wasted or that conversions are misused to cover for poor mass-simulations. An individual assessment, however, is difficult, as only part of the conversion determining factors (size assumption, permitted humidification and prescribed ambient relative humidity) were revealed. These differences need to be understood and minimized, if conclusions on aerosol processing in models can be drawn from comparisons to aerosol optical depth measurements.