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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-574
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Jul 2018

Research article | 04 Jul 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Marine boundary layer aerosol in Eastern North Atlantic: seasonal variations and key controlling processes

Guangjie Zheng1, Yang Wang1, Allison C. Aiken2, Francesca Gallo2, Mike Jensen1, Pavlos Kollias1,3, Chongai Kuang1, Edward Luke1, Stephen Springston1, Janek Uin1, Robert Wood4, and Jian Wang1 Guangjie Zheng et al.
  • 1Environmental and Climate Science Department, Brookhaven National Laboratory, Upton, New York, USA
  • 2Earth System Observations, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
  • 3School of Marine and Atmospheric Sciences, Stony Brook University, State University of New York, Stony Brook, New York, USA
  • 4Department of Atmospheric Science, University of Washington, Seattle, USA

Abstract. The response of marine low cloud systems to changes in aerosol concentration 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 (ENA) is a region of persistent but diverse marine boundary layer (MBL) clouds, whose albedo and precipitation are highly susceptible to perturbations in aerosol properties. In this study, we examine MBL aerosol properties, trace gas mixing ratios, and meteorological parameters measured at the Atmospheric Radiation Measurement Climate Research Facility’s ENA site on Graciosa Island, Azores, Portugal from 2015 to 2017. Measurements impacted by local pollutions on Graciosa Island and during occasional intense biomass burning and dust events are excluded from this analysis. Submicron aerosol size distribution typically consists of three modes: Aitken (At), Accumulation (Ac), and Larger Accumulation (LA) modes, with average number concentrations (denoted as NAt, NAc and NLA below) of 330, 114, and 14cm−3, respectively. NAt, NAc and NLA show contrasting seasonal variations, suggesting different sources and removal processes. NLA is dominated by sea spray aerosol (SSA), and is higher in winter and lower in summer. This is due to the seasonal variations of SSA production, coalescence scavenging, and dilution by entrained free troposphere (FT) air. In comparison, SSA typically contributes a relatively minor fraction to NAt (10%) and NAc (21%) on an annual basis. In addition to SSA, sources of Ac mode particles include entrainment of FT aerosols and condensation growth of At mode particles inside MBL, while coalescence scavenging is the major sink of NAc. The observed seasonal variation of NAc, being higher in summer and lower in winter, generally agrees with the estimate based on the major sources and sink. NAt is mainly controlled by entrainment of FT aerosol, coagulation loss, and growth of At mode particles into Ac mode size range. Our calculation suggests besides the direct contribution from entrained FT Ac mode particles, growth of entrained FT At mode particles in the MBL also represent a substantial source of cloud condensation nuclei (CCN), with the highest contribution potentially reaching nearly 60% during summer. The growth of At mode particles to CCN size is expected a result of the condensation of sulfuric acid from dimethyl sulfide oxidation, suggesting that ocean ecosystems may have a substantial influence on MBL CCN populations in ENA.

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