Abstract
Abstract. In order to accurately assess the influence of fatty acids on the hygroscopic and other physicochemical properties of sea salt aerosols, hexanoic, octanoic or lauric acid together with sodium halide salts (NaCl, NaBr and NaI) have been chosen to be investigated in this study. The hygroscopic properties of sodium halide sub-micrometre particles covered with organic acids have been examined by Fourier-transform infrared spectroscopy in an aerosol flow cell. Covered particles were generated by flowing atomized sodium halide particles (either dry or aqueous) through a heated oven containing the gaseous acid. The obtained results indicate that gaseous organic acids easily nucleate onto dry and aqueous sodium halide particles. On the other hand, scanning electron microscopy (SEM) images indicate that lauric acid coating on NaCl particles makes them to aggregate in small clusters. The hygroscopic behaviour of covered sodium halide particles in deliquescence mode shows different features with the exchange of the halide ion, whereas the organic surfactant has little effect in NaBr particles, NaCl and NaI covered particles experience appreciable shifts in their deliquescence relative humidities, with different trends observed for each of the acids studied. In efflorescence mode, the overall effect of the organic covering is to retard the loss of water in the particles. It has been observed that the presence of gaseous water in heterogeneously nucleated particles tends to displace the cover of hexanoic acid to energetically stabilize the system.
Highlights
Marine aerosol is one of the most abundant types of natural particulate matter in the Earth’s troposphere
This work studied the effect of a surfactant layer of hexanoic, octanoic and lauric acid, which are present in the Earth’s troposphere, on the hygroscopic properties of sodium halide sub-micrometre particles, which are constituents of sea salt aerosol
It was found that the hygroscopic properties of sodium halide particles covered with hexanoic, octanoic or lauric acid change both with the nature of the inorganic salt and the carboxylic acid
Summary
Marine aerosol is one of the most abundant types of natural particulate matter in the Earth’s troposphere. Sea salt particles play an active role in the Earth’s radiative balance, influence mass transfer of gaseous compounds and cloud-precipitation mechanisms, contribute to the formation of cloud condensation nuclei and have highly reactive surfaces that take part in heterogeneous and multiphase chemical reactions (Andreae and Rosenfeld, 2008; Carslaw et al, 2010; O’Dowd and De Leeuw, 2007; Finlayson-Pitts, 2003; Lewis and Schwartz, 2004; Quinn and Bates, 2011; Rossi, 2003) They can take up significant amounts of water, exhibiting deliquescence and efflorescence properties under atmospheric conditions (Freney et al, 2009; Martin, 2000; Metzger and Lelieveld, 2007; Mikhailov et al, 2013; Wise et al, 2012) that can change the particles’ phase and size, together with other interrelated physicochemical properties: for example, water uptake increases particle size, favouring their sedimentation.
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