A New Method for Size‐Resolved Aerosol CCN Activity Measurement at Low Supersaturation in Pristine Atmosphere.

Understanding aerosol‐cloud interactions under pristine atmospheric conditions is crucial for meaningful global climate change predictions. However, ability of aerosol particles to act as cloud condensation nuclei (CCN) activity of aerosols in pristine atmosphere remains limited, mainly because of the difficulty of measuring it in pristine environments. Given the extremely low CCN number concentration under low supersaturations (SSs) and the complex impact of the kinetic limitation in CCN measurements, size‐resolved CCN activity measurements at low SSs cannot be achieved with traditional methods using differential mobility analyzers and a CCN counter (CCNC) because of the low sample efficiency and the impact of particles with multiple charges. To overcome this difficulty, we propose a new method for measuring size‐resolved CCN activity at low aerosol number concentrations. The new method is based on a combined system comprising an aerodynamic aerosol classifier (AAC) and a CCNC (AAC‐CCNC system). A control program was developed to achieve high‐resolution scans of the SS and particle size within a reasonable time frame. A data inversion scheme, including corrections for the transfer function and kinetic limitation, was developed to obtain accurate size‐resolved CCN activity. In July‐August 2022, the new method was used to measure the size‐resolved CCN activity on the Tibetan Plateau. This new method can advance the size‐resolved CCN measurements, thereby enhancing our understanding of aerosol‐cloud interactions in pristine atmosphere. Plain Language Summary: Cloud condensation nuclei (CCN) are the aerosols which can form cloud droplets. Assessing the ability of aerosols to serve as CCN (CCN activity) is fundamental for understanding the interactions between aerosol and clouds and their impacts on climate and precipitation. Particularly in regions with minimal human influence, such as polar areas and the Tibetan Plateau, the low aerosol concentration makes aerosol‐cloud interaction and its climate impact highly sensitive to aerosol CCN activity. However, traditional CCN activity measurement techniques are not applicable at conditions of extremely low aerosol concentrations, restricting the understanding of aerosol‐cloud interactions and their impacts in those regions. In this study, we developed a new system for CCN activity measurements at low aerosol concentration and corresponding data processing algorithm based on a particle size selection technique distinct from that used in traditional systems. The new system was successfully deployed in a field experiment conducted in Qinghai‐Tibet Plateau in summer 2022. Comparison with CCN concentration measurement indicates that the uncertainty of the CCN activity measurement is within a reasonable range. Notably, this marks the first instance of online measurement of particle size‐resolved CCN activity in the hinterland of the Qinghai‐Tibet Plateau. Key Points: A new aerodynamic aerosol classifier‐cloud condensation nuclei counter (AAC‐CCNC) system is developed for measureing size‐resolved cloud condensation nuclei (CCN) activity under low supersaturated conditions in pristine atmospheresUncertainty correction relating to the impact of kinetic limitations in the CCN counter and the transfer function of the AAC is developedThe AAC‐CCNC system is successfully deployed for size‐resolved CCN activity measurements in the Tibetan Plateau [ABSTRACT FROM AUTHOR]