Uncertainty in Predicting CCN Activity of Aged and Primary Aerosols

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 κSOA, 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.

East Asian Study of Tropospheric Aerosols and their Impact on Regional Clouds, Precipitation, and Climate (EAST‐AIRCPC)

“…To this end, numerous field experiments have been conducted in China in recent years involving measurements of CCN, aimed at better characterizing CCN and its influential factors (e.g., Li, Zhang, et al, ; Ren et al, ; Wang, Li, Zhang, et al, ; F. Zhang et al, , Zhang et al, , ). These studies provide different perspectives on the influence of aerosol particle size and composition on CCN activity.…”

Contrasting size-resolved hygroscopicity of fine particles derived by HTDMA and HR-ToF-AMS measurements between summer and winter in Beijing: the impacts of aerosol aging and local emissions

“…The aging process in the summer period is related to photochemical processing in strong solar radiation conditions. The photochemical reactions produce sulfate and secondary organic aerosol, condensing on the surface of slightly hygroscopic or nonhygroscopic primary aerosols (such as BC) (Zhang et al, 2008). To confirm such a photochemical aging effect on particle hygroscopicity, we further examine the diurnal variations of κ chem and κ gf or κ CCNc (only at the XZ site) based on observations in summer at two other sites in north China (Fig.…”

Distinct Ultrafine‐ and Accumulation‐Mode Particle Properties in Clean and Polluted Urban Environments

“…Investigating the source and chemical compositions of the ultrafine-mode (D p < 100 nm) particles is important because these ultrafine-mode particles are plentiful and can grow to large particles during pollution haze episodes (Guo et al, 2014). The hygroscopicity tandem differential mobility analyzer (H-TDMA) makes it possible to estimate aerosol chemical compositions indirectly based on the particle hygroscopicity (Swietlicki et al, 2008;Ye et al, 2011;Zhang et al, 2017). The hygroscopic growth factor (GF) is defined as the ratio of the wet diameter at a certain relative humidity (RH) and to the dry diameter.…”