Abstract

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 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 Ac-mode 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 steady-state concentration estimated from major sources and sinks. NAt is mainly controlled by entrainment of FT aerosol, coagulation loss, and growth of Aitken-mode particles into the Ac-mode size range. Our calculation suggests that besides the direct contribution from entrained FT Ac-mode particles, growth of entrained FT Aitken-mode particles in the MBL also represent a substantial source of cloud condensation nuclei (CCN), with the highest contribution potentially reaching 60 % during summer. The growth of Aitken-mode particles to CCN size is an expected result of the condensation of sulfuric acid, a product from dimethyl sulfide oxidation, suggesting that ocean ecosystems may have a substantial influence on MBL CCN populations in the ENA.

Highlights

  • Low clouds, especially stratocumulus, are the dominant cloud type in terms of spatial coverage of the Earth’s surface, and are of vital importance to the Earth’s climate (Wood, 2012)

  • We examine the long-term variation of aerosol properties, trace gas mixing ratios, and meteorological parameters measured at the Atmospheric Radiation Measurement (ARM) eastern North Atlantic (ENA) site from 2015 to 2017

  • One major source of marine boundary layer (MBL) aerosol in the ENA is the entrainment of free troposphere (FT) air, which contains both particles from longrange transport of continental pollution and those formed through new particle formation (NPF) in the FT (Quinn and Bates, 2011; Sanchez et al, 2018)

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Summary

Introduction

Especially stratocumulus, are the dominant cloud type in terms of spatial coverage of the Earth’s surface, and are of vital importance to the Earth’s climate (Wood, 2012). Several field campaigns, including the North Atlantic Regional Experiment (NARE) campaign during 1991 to 2001 (Parrish et al, 1998), the Atlantic Stratocumulus Transition Experiment (ASTEX) during June 1992, the second Aerosol Characterization Experiment (ACE-2) during summer 1997, and the Clouds, Aerosol, and Precipitation in the Marine Boundary Layer (CAP-MBL) campaign (Wood et al, 2015) from May 2009 to December 2010 took place in the ENA They are either more focused on other subjects (e.g., ozone chemistry for NARE (Parrish et al, 1998) and cloud properties for CAP-MBL (Wood et al, 2015) or are short-term studies (e.g., ACE-2; Raes et al, 2000, ASTEX; Albrecht et al, 1995). We present an overall picture of the processes that drive MBL aerosol properties in the ENA, and the implications are discussed (Sect. 7)

Measurement overview
Optical properties
Cloud and MBL properties
Air mass origin
Absorbing aerosols
Modes of aerosol size distributions
Seasonal variations of each mode
Key aerosol sources and sinks
Estimated rate of the potential key processes
Controlling processes of the larger accumulation mode
Contributions of SSA to Aitken and accumulation modes
Controlling processes of the accumulation mode
Controlling processes of the Aitken mode
Findings
Conclusions
Full Text
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