PNNL

Abstract

Abstract. The uptake of water by atmospheric aerosols has a pronounced effect on the particle light scattering properties which in turn are strongly dependent on the ambient relative humidity (RH). Earth system models need to account for the aerosol water uptake and its influence on light scattering in order to properly capture the overall radiative effects of aerosols. Here we present a comprehensive model-measurement evaluation of the particle light scattering enhancement factor f (RH), defined 5 as the particle light scattering coefficient at elevated RH (here set to 85 %) divided by its dry value. The comparison uses simulations from 10 global aerosol models and a global dataset of surface-based in situ measurements. In general, we find a large diversity in the magnitude of predicted f (RH) amongst the different models which can not be explained by the site types. There is strong indication that differences in the model parameterizations of hygroscopicity are driving at least some of the observed diversity in simulated f (RH). An important finding is that the models show a significantly larger discrepancy with the 10 observations if RHref =0 % is chosen as the model reference RH compared to RHref =40 %. The multi-site average ratio between model outputs and measurements is 1.64 in the former case and 1.16 in the latter. The disagreement is believed to originate from the hygroscopicity parameterisations at the lower RH range which may not implement all phenomenon taking place (i.e. not fully dried particles and hysteresis effects). Our results emphasize the need to consider the measurement conditions in such comparisons and recognize that measurements referred to as ’dry’ may not be dry in model terms.