A multiple charging correction algorithm for broad supersaturation scanning cloud condensation nuclei (BS2-CCN) system
Abstract. 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.
Najin Kim et al.
Najin Kim et al.
Najin Kim et al.
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The manuscript presents a novel approach for reducing the multiple charging effects from the CCN activity measurements using a BS2-CCN system. Kim et al. (2022) discusses the application of a new probability density function for reducing multiple charging effects from measurements obtained using the BS2-CCN. A prominent advantage of the method explained in Kim et al. (2022) is that applied the charge correction is applied to number concentrations measured at each particle size. Therefore, the effect on larger multiply charged particles on the size-resolved number concentrations becomes easier to detect and eliminate.
Overall, the work is new and the method to apply multiple-charging corrections for the BS2-CCN instrumentation has not been done before. The work extends on previously published works (e.g., the kernel function is added to the Kim et al 2021 multiple chare correction) with the BS2-CCN data. In general, the main concern is that multiple factors affect CCN measurement (aerosol shape, aerosol aggregation, viscosity, volatility, solubility, surface activity) and these effects are confounded; it is difficult to isolate the effects of multiple charging alone. However, the data collected from the BS2-CCN counter and subsequent analysis will be important for understanding CCN spectra of atmospheric aerosol and thus the work warrants publication. The following questions and comments address ideas that maybe unclear to the reader in the manuscript.