Uncertainty in Predicting CCN Activity of Aged and Primary Aerosols. Issue 21 (4th November 2017) - Zhang, Fang
Abstract: Understanding particle CCN activity in diverse atmospheres is crucial when evaluating aerosol indirect effects. Here aerosols measured at three sites in China were categorized as different types for attributing uncertainties in CCN prediction in terms of a comprehensive data set including size‐resolved CCN activity, size‐resolved hygroscopic growth factor, and chemical composition. We show that CCN activity for aged aerosols is unexpectedly underestimated ~22% at a supersaturation ( S ) of 0.2% when using κ‐ Kohler theory with an assumption of an internal mixture with measured bulk composition that has typically resulted in an overestimate of the CCN activity in previous studies. We conclude that the underestimation stems from neglect of the effect of aging/coating on particle hygroscopicity, which is not considered properly in most current models. This effect enhanced the hygroscopicity parameter ( κ ) by between ~11% (polluted conditions) and 30% (clean days), as indicated in diurnal cycles of κ based on measurements by different instruments. In the urban Beijing atmosphere heavily influenced by fresh emissions, the CCN activity was overestimated by 45% at S = 0.2%, likely because of inaccurate assumptions of particle mixing state and because of variability of chemical composition over the particle size range. For both fresh and aged aerosols, CCN prediction exhibits very limited sensitivity to κ SOA, implying a critical role of other factors like mixing of aerosol components within and between particles in regulating CCN activity. Our findings could help improving CCN parameterization in climate models. Key Points: CCN activity for aged aerosols is significantly underestimated, but it is overestimated for urban aerosols with large concentrations of primary particles The particle hygroscopicity parameter ( κ ) was enhanced by between ~11% (polluted conditions) and 30% (clean days) due to the aging/coating effect CCN prediction exhibits very limited sensitivity to κ SOA, implying a critical role of other factors in regulating CCN activity