First global evaluation of the representation of water uptake within ten earth system models

Aerosol optical properties, such as particle light scattering, depend on the particle size and chemical composition, which in turn are affected by the particle’s ability to take up water. Thus, particle hygroscopic growth will have an impact on the optical properties and in turn will affect the aerosol-radiation interaction and the calculations of the Earth’s radiative balance. The dependence of particle light scattering on relative humidity (RH) can be described by the scattering enhancement factor f(RH), defined as the ratio between the particle light scattering coefficient at a given RH divided by its dry value.

In our previous work (Burgos et al., 2019), we carried out a standardized analysis of scattering in-situ measurements at 26 sites around the globe, creating a benchmark dataset (open access via EBAS, http://ebas.nilu.no/). The project continues with the present work, which is part of the AeroCom phase III INSITU project: Evaluation of hygroscopicity of aerosol optical properties. Here, we present a comprehensive model-measurement evaluation of f(RH) for ten different earth system models. Modelled and measured scattering enhancement factors are compared for 22 sites, representative of Arctic, marine, rural, mountain, urban and desert aerosols.

Overall, a large variability and diversity in the magnitude of predicted f(RH) amongst the models is found and the modelled f(RH) tends to be overestimated relative to the measurement values. This difference cannot be explained by the aerosol type. Agreement between models and measurements was strongly influenced by the choice of RH_ref. Models show a significantly larger discrepancy with the observations if model dry conditions are set to RH=0% instead of RH=40%. Model parameterizations of aerosol hygroscopicity and mixing state may be driving the observed diversity among models as well as the discrepancy with measurements. Measurement conditions have to be considered in this type of evaluation, specifically the fact that “dry” measurements may not be “dry” in model terms.

This work has been submitted to ACPD.

Burgos, M., Andrews, E., Titos, G., Alados-Arboledas, L., Baltensperger, U., Day, D., Jefferson, A., Kalivitis, N., Mihalopoulos, N., Sherman, J., Sun, J., Weingartner, E., and Zieger, P.: A global view on the effect of water uptake on aerosol particle light scattering, Scientific Data, 6, https://doi.org/10.1038/s41597-019-0158-7, 2019.