Ultra-clean Conditions over the Eastern North Atlantic ARM Site

Wood, R., University of Washington

Cloud-Aerosol-Precipitation Interactions

Warm Boundary Layer Processes, Aerosol Processes

Pennypacker S and R Wood. 2017. "A Case Study in Low Aerosol Number Concentrations Over the Eastern North Atlantic: Implications for Pristine Conditions in the Remote Marine Boundary Layer." Journal of Geophysical Research – Atmospheres, 122(22), doi:10.1002/2017JD027493.


We observed minimal continental/anthropogenic influence on the aerosol population during an extended case study of "ultra-clean" conditions, characterized by the extreme depletion of cloud condensation nuclei-sized particles, over the Eastern North Atlantic (ENA) Atmospheric Radiation Measurement (ARM) Climate Research Facility site on Graciosa Island, Azores. Our observations also show that these depletions are most likely driven by vigorous scavenging (i.e., wash out) in heavily drizzling, post-frontal, open-cellular, boundary-layer clouds.


Since the ultra-clean regime features relatively minimal continental/anthropogenic influence, it presents a new natural laboratory for exploring questions related to processes and variability in the pre-industrial remote marine environment. Our observations during these conditions also motivate further consideration of aerosol-cloud-precipitation-turbulence interactions in the post-frontal boundary layer.


Our extended (September 20-October 13, 2015) case study from the ENA ARM site centers around a period of extremely low aerosol particle (sizes 0.1-1 micron) concentrations. We explore aerosol size distributions and optical properties, as well as trace gas concentrations, within and between three regimes of aerosol number variability during this case study. Combined with meteorological reanalysis and back trajectory analysis, this helps us determine the degree to which each regime may represent the absence of anthropogenic influence in the remote marine boundary layer. We also use the extensive remote-sensing capabilities of the ENA ARM site to characterize cloud and boundary-layer structure during ultra-clean conditions.