Chemistry of sea-salt particles in the summer Antarctic atmosphere

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Abstract

The chemistry of sea-salt particles was investigated in summer Antarctica at a site about 150 km from the open ocean. Aerosol samples were collected using a low-pressure impactor which divides particles into 12 size fractions over the aerodynamic particle diameter range 0.045–15 μm. Measured sea-salt particle concentrations were clearly lower than concentrations typically observed at coastal Antarctica. The mass size distribution of sea salt was tri-modal with a submicron mode centering at 0.5–1 μm and two supermicron modes centering slightly below 2 μm and somewhere between 2 and 10 μm, respectively. On average more than 70% of sea salt was found in the supermicron size range, the lower supermicron mode being usually the dominant. Sea-salt particles displayed a large chloride loss with respect to the bulk sea water. The average loss percentage was more than 90% for submicron particles and decreased to about 50% for particles larger than 3 μm in diameter. The primary ions causing the chloride loss were sulfate, nitrate, and methanesulfonate (MSA). The aerosol MSA to non-sea-salt sulfate weight ratio seemed to have been increased by the presence of sea-salt particles. Particulate nitrate was associated strongly with sea salt, the most likely formation pathway being the interaction of nitric acid or some other gaseous nitrogen compounds with these particles in the Antarctic atmosphere.

Introduction

Sea salt is a major contributor to total particulate matter in the Antarctic atmosphere, being usually the most abundant aerosol component at coastal Antarctic sites (Hall and Wolff, 1998). The typically low concentrations of ammonia and crustal particles make sea salt the most important alkaline counterpart for acidic compounds over most of the Antarctica (Wagenbach et al., 1998a). Detailed understanding of Antarctic sea-salt particle chemistry is thereby essential when investigating the cycling and radiative impacts of various sulfur and nitrogen compounds over the Antarctica, as well as in glacio-chemical studies related to these compounds.

At most Antarctic sites the concentration of sea-salt particles is somewhat larger in winter than in summer (Tuncel et al., 1989; Savoie et al., 1993; Wagenbach et al., 1998a). Sea-salt particles collected during the winter display frequently a sulfur deficit with respect to bulk sea water, which has recently been ascribed to the production and fractionation of these particles on newly formed ice surfaces at air temperatures below about −8°C (Hall and Wolff, 1998). During the summer when the fractionation is expected to be minor, sea-salt particles provide a potentially important medium for various trace gases to react with (Minikin et al., 1998; Wagenbach et al., 1998a, Wagenbach et al., 1998b). As a result of these reactions the chemical composition of both the gas and the particulate phase may be significantly affected.

Studies concerning Antarctic sea-salt chemistry rely mainly on filter sampling with low size resolution for collected particles. While suitable for investigating the seasonal cycles and correlations between different aerosol components, filter samples cannot provide detailed information on different interactions between sea-salt particles and specific compounds. In this paper the chemistry of sea-salt particles is investigated based on size-segregated aerosol measurements carried out at a continental Antarctic site during the summer. Our principal aim is to enhance the general understanding of sea-salt particle chemistry occurring in the summer Antarctic atmosphere. The topics we investigate include the distribution of sea salt and associated compounds over the particle size spectrum, the replacement of chloride with acidic compounds in sea-salt particles, and the importance of sea-salt particles in the cycling of various gaseous and particulate compounds over Antarctica.

Section snippets

Experimental

During December 97–February 98, a field campaign was conducted at the Finnish station Aboa (73°03′S, 13°25′W) in western Queen Maud Land, Antarctica. The measurement station stands on a nunatak Basen about 500 m above the sea level, and at a distance of about 150 km from the open ocean (Weddell sea). The measurement site was located 200 m away and slightly upward from the station and the generator building next to it. Possible contamination sources included the power generator and the occasional

Sampling conditions and data quality

During the aerosol sampling, the ambient 48 h average temperature varied between −8.4 and −1.4°C, and the corresponding relative humidity between 47 and 74%. The average wind speed was relatively high, ranging from 8 to 25 m s−1. Even larger wind speeds were encountered during a storm event which interrupted the measurements for 3 days (9–11 January). The average particle number concentration given by the CPC, after removal of the obvious contamination spikes from the data, varied between 235 and

Summary and conclusions

During January–February, 1998, field campaign to investigate aerosol chemistry was conducted at the Finnish station Aboa (73S, 13W) located in Antarctica about 150 km from the open ocean. Aerosol samples were collected using a small deposit area impactor (SDI) which divides particles into 12 size fractions over the size range 0.045–15 μm of particle aerodynamic diameter. Of each 48 h sample, major ions were determined using ion chromatography.

Measured sea-salt particle concentrations varied

Acknowledgements

This work was funded by the Ministry of Trade and Industry, by the Finnish Institute of Marine Research, by Maj and Tor Nessling foundation, and by the Academy of Finland (Contract no. 46906). All personnel in the FINNARP97 expedition are gratefully acknowledged.

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