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

• Main Authors: Zheng, Guangjie, Wang, Yang, For full list of authors, see publisher's website.
• Format: Text
• Language: unknown
• Published: Scholars' Mine 2018
• Subjects: Aerosol; Aerosol property; Albedo; Boundary layer; Cloud; Cloud condensation nucleus; Concentration (composition); Marine atmosphere; Mixing ratio; Particle size; Seasonal variation; Size distribution; Atlantic Ocean; Atlantic Ocean (North); Azores; Graciosa [Azores]; Portugal; Environmental Engineering; Aitken; North Atlantic
• Online Access: https://scholarsmine.mst.edu/civarc_enveng_facwork/1601; https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=2602&context=civarc_enveng_facwork

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 are derived 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, during a 3- year period from 2015 to 2017. Measurements impacted by local pollution on Graciosa Island and during occasional intense biomass burning and dust events are excluded from this study. Submicron aerosol size distribution typically consists of three modes: Aitken (At, diameter Dp < ∼ 100 nm), accumulation (Ac, Dp within ∼ 100 to ∼ 300 nm), and larger accumulation (LA, Dp >∼ 300 nm) modes, with average number concentrations (denoted as NAt, NAc, and NLA below) of 330, 114, and 14 cm-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, in-cloud 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 Acmode particles include entrainment of FT aerosols and condensation growth of Aitken-mode particles inside the MBL, while in-cloud 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 ...