Abstract
Seven years of data collected at the Atmospheric Radiation Measurement
(ARM)’s Eastern North Atlantic (ENA) site are analyzed to understand the
controls of Cloud Condensation Nuclei (CCN) concentrations in the
region. Day-night differences in the aerosol data as segregated by wind
direction demonstrated the aerosol observations to be impacted by local
emissions when the wind direction (wdir) is between 90° and 310°
(measured clockwise from the North where air is coming from). Data
collected during marine conditions (wdir<90° or
wdir>310°) showed the CCN concentrations to be higher in
the summer months as compared to the winter months. CCN budget analysis
revealed advection and precipitation scavenging being primarily
responsible for modulating the CCN concentrations at the site on monthly
timescales, with rain rates driving the precipitation scavenging term.
High (greater than 75th percentile) and low (lower than 25th percentile)
CCN events were identified for each month to characterize the
sub-monthly variability of CCN concentrations. Low CCN events had deeper
clouds, stronger rain rates, and lower free-tropospheric aerosol mass at
the ENA site as compared to the high CCN events. Analysis of satellite
data of air-parcels 48 hours prior to their arrival at the ENA site
demonstrated the air parcels during low CCN events to encounter higher
cloudiness, stronger rain rates, and higher cloud top heights as
compared to the high CCN events. The results presented herein provide
key constraints for model evaluation studies and climatological studies
conducted at the ENA site.