Relationship between the cloud condensation nuclei concentration and aerosol optical properties and their impact factors

Description

Cloud condensation nuclei (CCN) concentration is one of the most important properties for understanding the effects of aerosols on climate forcing. However, global measurements of CCN concentrations are scanty. To assess the remote sensing of CCN concentration, this study investigates the relationship between the aerosol optical properties and CCN concentration, as well as the effects of meteorological factors (e.g., relative humidity, wind, hygroscopicity, etc.) on their relationship, based on the observations at the Atmospheric Radiation Measurement (ARM) Climate Research Facility’s Southern Great Plains (SGP) site and observations from ARM Mobile Facility (AMF) deployments— the Graciosa Island (GRW) site in the Azores, the Black Forest region (FKB) site in Germany, the Ganges Valley region (PGH) site in India, and the Niamey (NIM) site in Niger.

The relationship between CCN concentration at 0.4% supersaturation and aerosol optical depth (AOD) at 500 nm is fitted by a power law equation, which showed both good correlation with R2=0.88 at the SGP site and poor correlation with R2=0.19 at the GRW site. Use of the AOD and Angstrom exponent (α) can improve their relationship at both sites, with R2 increasing to 0.95 and 0.56 at the two sites, respectively. Since the aerosol scattering/absorption coefficients were measured at the same altitude as the CCN measurements, the relationship between the aerosol scattering/absorption coefficients and CCN concentration is better than that between the AOD and CCN concentration. The ambient relative humidity (RH) plays an important role in the relationship. Aerosol optical loading (e.g., AOD) measured at lower RH or dry condition is a better proxy of CCN than under high RH or wet conditions. The hygroscopic growth factor affects the relationship between CCN and AOD, which may be accounted for by virtue of the swelling factor. For the relatively strong absorbency aerosol with single-scattering albedo (SSA) <0.8, the relationship between the combination of aerosol extinction coefficients and α and CCN concentration is better than that between the aerosol extinction coefficient alone and CCN concentration. However, for relatively strong scattering aerosol with SSA>0.9, the result is the reverse. This indicates that the aerosol compositions also significantly influence the ability of remote sensing CCN concentration and must be considered in order to improve this relationship.