Atmospheric aerosols are tiny particles suspended in the air that represent one of the greatest uncertainties in our knowledge about climate change. Earth system models are the scientists’ best tool to forecast future scenarios of climate change but they require accurate information to produce reliable predictions. Now a new study by researchers at the Department of Environmental Science and colleagues from Europe, Asia and the USA shows large discrepancies across different climate models with respect to how aerosol particles take up water molecules in the atmosphere. The study, which was funded by the US Department of Energy, was published recently in Atmospheric Chemistry and Physics.
Aerosol particles influence several atmospheric processes and feedbacks that appear to be highly important for climate. The influence of aerosol particles is controlled in part by the relative humidity of the atmosphere, which modifies their size and chemical composition as the particles take up water, a property known as aerosol hygroscopocity. One of the most pressing needs related to this is to simulate how aerosol hygroscopicity affects the optical properties of aerosols, i.e. how they interact with light. Interestingly, while some aerosol-related variables such as the total aerosol optical depth (AOD) seem to be well estimated by models, others, such as the contribution of water to AOD, vary across different models.
In the current study, the team were interested in how well Earth system models predict the water uptake of aerosol particles and its influence on aerosol optical properties. “In our project, we assessed the capacity of ten different Earth system models to estimate the water uptake of atmospheric aerosol particles by evaluating their predictions against a novel dataset of in-situ measurements,” says Maria Burgos, Researcher at the Department of Environmental Science and lead-author of the study. “We found a large diversity among the different models with a tendency to overestimate light scattering at elevated relative humidity.”
According to Elisabeth Andrews, Senior Researcher at the University of Colorado and principle investigator of the project, this is the first study to evaluate a wide variety of models with measurements of aerosol hygroscopicity and on a global scale. “Our study highlights potential improvements in the hygroscopicity parameterization scheme for some models, as well as ideas for future experiments,” she says.
Paul Zieger Assistant Professor at the Department of Environmental Science and co-principal investigator of the project hopes that this work will help improve current modelling efforts to understand aerosol behaviour and their potential effects on climate. “Our work may help to identify key issues and shortcomings within specific aerosol parametrisations in Earth system models. We have provided new and important findings on how to improve the modelling of aerosol hygroscopicity in the complex world of climate models” he adds.
The study is part of AeroCom, an international consortium to advance the understanding of global aerosol particles and their representation in Earth system models with the aim to improve climate predictions.