Exploring the Potential of long-term Aerosol Size Distribution Measurements as Proxies for Cloud Condensation Nuclei in Welgegund, South Africa 

Abstract

Cloud condensation nuclei (CCN) play a central role in regulating cloud microphysical properties and, consequently, aerosol-cloud interactions, which are the largest source of uncertainty of radiative forcing (RF) in global climate models. For an aerosol to be able to act as CCN at certain atmospheric conditions, high enough aerosol hygroscopicity (κ) and particle size (dp) are required. These properties can be directly characterized using cloud condensation nuclei counters (CCNc) and, when measured over extended periods, provide valuable constraints for the representation of aerosol–cloud interactions in climate models. However, long-term CCN observations remain sparse in the Southern Hemisphere, limiting the ability to evaluate and improve model performance using observational constraints. In this study we focus on measurements from the atmospheric measurement station in Welgegund, South Africa. The station is strategically located to capture air masses influenced by pristine grassland background as well as major anthropogenic source regions. The site is situated approximately 100 km southwest of the Johannesburg–Pretoria conurbation at approximately 1480 m above mean sea level. The coexistence of pristine and polluted air mass influences provides an opportunity to compare the CCN-activity of natural and anthropogenic aerosol in southern Africa, where other measurement stations are sparce. The aim is to evaluate the potential of using long-term measurements of particle number size distributions (PNSD) produced since 2010 at Welgegund station as a proxy for CCN concentrations. This is done by utilizing an unpublished dataset from a measurement campaign with a CCNc counter during January 2017-April 2017. The analysis will evaluate the commonly used approximation of particles larger than 100 nm as a proxy for CCN during the campaign and then utilize this information to the 15-year PNSD data set. Applicable conditions are first determined by analyzing the characteristics of CCN (κ, critical diameter at given supersaturations dcrit), potential diurnal cycle of CCN concentrations, the correlation to other relevant meteorological parameters and sources of the air masses during the campaign. In the future, the presented dataset is expected to contribute to the development of a standardized long-term CCN observational framework to support the integration of southern African measurements into global CCN databases. Thereby helping to address the current geographical imbalance of observations, that can be used to reduce climate model RF-uncertainty associated with aerosol–cloud interactions.