10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

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Marine Boundary Layer Aerosol in Eastern North Atlantic: Seasonal Variations and the Key Controlling Processes

GUANGJIE ZHENG, Tamara Pinterich, Allison Aiken, Robert Bullard, Edward Luke, Pavlos Kollias, Chongai Kuang, Stephen Springston, Janek Uin, Thomas Watson, Rob Wood, Jian Wang, Brookhaven National Laboratory

     Abstract Number: 788
     Working Group: Remote/Regional Atmospheric Aerosol

Abstract
The response of marine low cloud systems to changes in aerosols represents one of the largest uncertainties in climate simulations. Major contributions to this uncertainty derive from poor understanding of aerosol under natural conditions and the perturbation by anthropogenic emissions. The Eastern North Atlantic Ocean (ENA) is a region of persistent but diverse marine boundary layer (MBL) clouds, and whose albedo and precipitation are highly susceptible to perturbation in aerosol properties. In this study, we examine aerosol properties, trace gas concentrations, and meteorological parameters measured at the ARM ENA site on Graciosa Island since 2014. Aerosol size distribution from 55 nm to 1 μm typically consists of three modes: Aitken (At), Accumulation (Ac) and Sea Spray Aerosol (SSA) modes. Observed CCN number concentrations are in general agreement with the sum of Ac and SSA mode concentrations. The number concentration of SSA mode, mainly driven by sea spray aerosol (SSA) production and precipitation scavenging, is higher in winter and lower during summer. The entrainment of free tropospheric aerosols, including long range transported continental pollution, represents a major source of At and Ac mode particles. In comparison, SSA often contributes to a minor fraction of At and Ac mode number concentrations, with estimated mean annual contribution around 4.5% and 21%, respectively. For the combined population of At and Ac mode particles, coalescence scavenging represents the major sink, while coagulation of At mode particles with Ac and SSA mode particles and activated droplets inside clouds can also play a role. Higher contribution of entrained free tropospheric aerosol is coincident with stronger precipitation during spring and winter seasons. As a result, At and Ac mode number concentrations exhibit weak seasonal variations, with slightly higher levels during the spring and summer seasons.