Abstract
Size-resolved Particle Activation Ratio (SPAR), defined as the size-dependence of CCN activity of aerosol particles, is generally parameterized by a scheme that neglects the contribution of nearly-hydrophobic particles. In this paper, it was found that nearly-hydrophobic particles can contribute up to 20% to the CCN activity for large particles and the estimation of particle hygroscopicity would be significantly biased using the original parameterization scheme. An improved parameterization scheme considering the contribution of nearly-hydrophobic particles was proposed and applied on measurements conducted at a rural site in the North China Plain during summer 2013. For hygroscopic particles in range of 50–70 nm diameter, which might be dominated by organic compounds, their number fraction (fn) and midpoint activation diameter (Da) fitted with the improved scheme were up to 20% lower than the parameters fitted with the original scheme. Both parameterization schemes of SPAR were further applied in a comparison study of aerosol hygroscopicity measurements between Cloud Condensation Nuclei (CCN) Counter coupled with Differential Mobility Analyzer (DMA-CCNC) and Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) during the Xianghe Campaign. Applying the improved scheme, the correlations of derived fn for hygroscopic particles between measurements of DMA-CCNC and HTDMA were improved (R increased by up to 0.2 with the uncertainty of the regression to be about 10%). In addition, for particles smaller than 100 nm, the size-dependence of hygroscopicity parameter and the diurnal cycle of fn derived using the improved scheme, based on the DMA-CCNC measurements were more consistent with those derived with the HTDMA measurements. These results highlight the importance of considering nearly-hydrophobic particles in SPAR curves fitting and in the calculation of aerosol hygroscopicity based on DMA-CCNC measurements.
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