LAUSR

We present a case study (September 20 – October 13, 2015) of synergistic, multi-instrument observations of aerosols, clouds and the marine boundary layer (MBL) at the Eastern North Atlantic (ENA) ARM site centered on a period of exceptionally low (20 – 50 cm-3) surface accumulation mode (0.1 – 1 μm) aerosol particle number concentrations. We divide the case study into three regimes (high, clean and ultra-clean) based on daily median number concentrations, and compare finer resolution (hourly or less) observations between these regimes. The analysis focuses on the possibility of using these ultra-clean events to study pristine conditions in the remote MBL, as well as examining evidence for a recently proposed conceptual model for the large-scale depletion of CCN-sized particles in post-frontal air masses. Relative to the high and clean regimes, the ultra-clean regime tends to exhibit significantly fewer particles between 0.1 and 0.4 μm in diameter and a relatively increased prevalence of larger accumulation mode particles. In addition, supermicron particles tend to dominate total scattering in the ultra-clean regime, and there is little evidence for absorbing aerosol. These observations are more in line with a heavily scavenged but natural marine aerosol population and minimal contribution from continental sources such as anthropogenic pollution, biomass burning or dust. The air masses with the consistently lowest accumulation mode aerosol number concentrations are largely dominated by heavily drizzling clouds with high liquid water path (LWP) cores, deep decoupled boundary layers, open cellular organization and notable surface forcing of sub-cloud turbulence, even at night.