Investigating the Applicability of a Global Average Calibration Line for Ambient Size-Resolved Cloud Condensation Nuclei (CCN) Measurements: A Technical Note

Aerosol-cloud-precipitation interaction represents the largest uncertainty in climate change's current and future understanding. Therefore, aerosol properties affecting the cloud and precipitation formation and their accurate estimation is a first step in developing improved parameterizations for the prognostic climate models. Over the last couple of decades, a commercially available Cloud Condensation Nuclei Counter (CCNC) has been deployed in the field and laboratory for characterizing CCN properties of ambient or atmospherically relevant laboratory-generated aerosols. How-ever, most of the CCN measurements performed in the field are often compounded with the erroneous estimation of CCN concentration and other parameters due to a lack of robust and accurate CCNC calibration. CCNC is not a plug-and-play instrument and requires prudent calibration and operation, to avoid erroneous data and added parameterization uncertain-ties. In this work, we propose and demonstrate the usability of a global calibration equation derived from CCNC calibra-tion experiments from 8 contrasting global environments. Significant correlation was observed between the global calibration and each of the 16 individual experiments. A significant improvement in the correlation was observed when the calibration experiments were separated for high-altitude measurements. Using these equations, we further derived the effective hygroscopicity parameter and found lower relative uncertainty in the hygroscopicity parameter at higher effective supersaturation. Our results signify that altitude-based pressure change could be of importance for accurate calibration at high-altitude locations. Our results are consistent with previous studies emphasizing the criticality of the accurate CCN calibration for the lower supersaturations.

Automated summary

Aerosol-cloud-precipitation interaction is a major source of uncertainty in understanding climate change. Accurately estimating the aerosol properties that affect cloud and precipitation formation is crucial for improving climate models. However, inaccurate calibration of the Cloud Condensation Nuclei Counter (CCNC) has resulted in erroneous data and parameterization uncertainties. In this study, a global calibration equation derived from CCNC calibration experiments in different environments was proposed and demonstrated to improve the accuracy of CCN measurements, especially at high-altitude locations.