Observations and analysis of low-CCN events on Graciosa Island, Azores

Description

Cloud condensation nuclei (CCN) influence the radiative budget of the earth by increasing cloud droplet concentration and reducing droplet size (the first aerosol indirect effect). Because of this, many studies have focused upon high-CCN regions or events in an effort to demonstrate this effect. However, the theory of Twomey clearly shows that the strength of the first indirect effect is dependent not only upon the nature of the CCN and clouds in the perturbed cloud, but also depends upon these properties in the unperturbed state. As such, there is a need to explore and understand the factors controlling the concentration of CCN in the clean marine boundary layer. We use CCN data from the 20 month CAP-MBL field deployment of the AMF to the Azores to explore low CCN events. The data reveal that extended periods with very low CCN concentrations frequently occur at the Azores. Further analysis of the AMF dataset provides a comprehensive view of these events through the use of multiple datasets. We use a combination of AMF data together with back-trajectory and large-scale meteorological analysis to understand the airmass origins for the low CCN events, and find that many of the low CCN events have a common meteorological basis that involves southerly low level flow and rather low wind speeds. There is no clear signature indicating that local precipitation events are depleting the aerosols, suggesting that airmass evolution over a longer period is necessary. Further analysis will be used to investigate the processes involved in depleting the air mass of CCN and the implications for weather and climate.