A multiple charging correction algorithm for broad supersaturation scanning cloud condensation nuclei (BS2-CCN) system

High time resolution (~1 s) of aerosol hygroscopicity and CCN activity can be obtained with a Broad Supersaturation Scanning Cloud Condensation Nuclei (BS2-CCN) system. Based on a commercial DMT-CCNC, the newly designed diffusive inlet in the BS2-CCN realizes a broad supersaturation distribution in a chamber with a stable low sheath to aerosol flow ratio (SARs). In this way, a monotonic relation between activation fraction of aerosols (Fact) and critical activation supersaturation (Saerosol) can be obtained. The accuracy of the size-resolved aerosol hygroscopicity, κ, measured by the BS2-CCN system can be, however, hampered by multiply charged particles, i.e., resulting in the overestimation of κ values. As the BS2-CCN system uses multiple and continuous supersaturations in the chamber and the size-resolved Fact value is directly used to derive κ values, the multiple charging correction algorithm of the traditional CCNC where single supersaturation is applied does not work for the BS2-CCN observation. Here, we propose a new multiple charging correction algorithm to retrieve the true Fact value. Starting from the largest size bin, a new Fact value at a specific particle diameter (Dp) is updated from a measured activation spectra after removing both aerosol and CCN number concentration of multiply charged particles using a Kernel function with a given particle number size distribution. We compare the corrected activation spectra with laboratory aerosols for a calibration experiment and ambient aerosols during the 2021 Yellow-Sea Air Quality Studies (YES-AQ) campaign. The difference between corrected and measured κ values can be as large as 0.08 within the measured κ values between 0.11 and 0.37 among the selected samples, highlighting that multiple charge effect should be considered for the ambient aerosol measurement. Furthermore, we examine how particle number size distribution is linked to the deviation of activation spectra and κ values