Abstract
Abstract. Cloud optical properties in the trade winds over the eastern Caribbean Sea have been shown to be sensitive to cloud condensation nuclei (CCN) concentrations. The objective of the current study was to investigate the CCN properties in the marine boundary layer (MBL) in the tropical western North Atlantic, in order to assess the respective roles of inorganic sulfate, organic species, long-range transported mineral dust and sea-salt particles. Measurements were carried out in June–July 2013, on the east coast of Barbados, and included CCN number concentrations, particle number size distributions and offline analysis of sampled particulate matter (PM) and sampled accumulation mode particles for an investigation of composition and mixing state with transmission electron microscopy (TEM) in combination with energy-dispersive X-ray spectroscopy (EDX). During most of the campaign, significant mass concentrations of long-range transported mineral dust was present in the PM, and influence from local island sources can be ruled out. The CCN and particle number concentrations were similar to what can be expected in pristine marine environments. The hygroscopicity parameter κ was inferred, and values in the range 0.2–0.5 were found during most of the campaign, with similar values for the Aitken and the accumulation mode. The accumulation mode particles studied with TEM were dominated by non-refractory material, and concentrations of mineral dust, sea salt and soot were too small to influence the CCN properties. It is highly likely that the CCN were dominated by a mixture of sulfate species and organic compounds.
Highlights
Clouds play an important role with respect to the radiation budget and climate on Earth
The objective of the current study is to investigate to what extent organic species, nss inorganic species, sea salt and longrange transported Saharan mineral dust may influence the cloud condensation nuclei (CCN) properties in the marine boundary layer (MBL) in that region
Wind direction, relative humidity (RH) and precipitation are shown in Fig. 1 for the time period from 14 June to 15 July 2013, corresponding to the day of year (DOY) range from 165 to 196
Summary
Clouds play an important role with respect to the radiation budget and climate on Earth. Significant uncertainties are related to the radiative forcing of aerosol–cloud interactions in the climate system (Boucher et al, 2013). Global climate is in particular sensitive to aerosol–cloud interactions over the oceans due to the large surface of the oceans and the significant difference in albedo between clouds and the open ocean. There are significant climate-relevant uncertainties related to the CCN budget in the marine environment (Pierce and Adams, 2006), and in general it is very challenging to incorporate marine boundary layer (MBL) clouds correctly in climate models (Bony and Dufresne, 2005). Submicrometer aerosol particles in the pristine North Atlantic marine environment are comprised of sea salt as well as sulfate and organic species dominating during periods with high marine biological activity (O’Dowd et al, 2004). Mineral dust particles from Northern Africa are transported by the trade winds across the Atlantic Ocean during the summer season in the Northern Hemisphere (Prospero and Lamb, 2003). Twohy et al (2009) reported that a large fraction (79 %) of cloud droplet residual particles were comprised of crustal material in the Published by Copernicus Publications on behalf of the European Geosciences Union
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