A multiple-charging correction algorithm for a 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 CCNC (CCN counter), 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 (SAR). In this way, a monotonic relation between the activation fraction of aerosols (F-act) and critical activation supersaturation (S-aerosol) can be obtained. The accuracy of the size-resolved aerosol hygroscopicity, kappa, measured by the BS2-CCN system can be, however, hampered by multiply charged particles, i.e., resulting in the overestimation of kappa values. As the BS2-CCN system uses multiple and continuous supersaturations in the chamber and the size-resolved F-act value is directly used to derive kappa 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 F-act value. Starting from the largest size bin, a new F-act value at a specific particle diameter (D-p) is updated from a measured activation spectrum after removing both aerosol and the CCN number concentration of multiply charged particles using a kernel function with a given particle number size distribution. We compare the corrected activation spectrum with laboratory aerosols for a calibration experiment and ambient aerosols during the 2021 Yellow-Sea Air Quality Studies (YES-AQ) campaign. It is noted that this algorithm is only applied to the monomodal particle distribution. The difference between corrected and measured kappa values can be as large as 0.08 within the measured kappa values that range between 0.11 and 0.37 among the selected samples, highlighting that the 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 kappa values.

Automated summary

A high time resolution of aerosol hygroscopicity and CCN activity can be achieved using the BS2-CCN system. However, the accuracy of the size-resolved aerosol hygroscopicity measurement can be affected by multiply charged particles, resulting in the overestimation of kappa values. A new multiple-charging correction algorithm is proposed to retrieve the true activation fraction value.