Abstract
Abstract. Controlled laboratory studies of the physical and chemical properties of sea spray aerosol (SSA) must be under-pinned by a physically and chemically accurate representation of the bubble-mediated production of nascent SSA particles. Bubble bursting is sensitive to the physico-chemical properties of seawater. For a sample of seawater, any important differences in the SSA production mechanism are projected into the composition of the aerosol particles produced. Using direct chemical measurements of SSA at the single-particle level, this study presents an intercomparison of three laboratory-based, bubble-mediated SSA production schemes: gas forced through submerged sintered glass filters ("frits"), a pulsed plunging-waterfall apparatus, and breaking waves in a wave channel filled with natural seawater. The size-resolved chemical composition of SSA particles produced by breaking waves is more similar to particles produced by the plunging waterfall than those produced by sintered glass filters. Aerosol generated by disintegrating foam produced by sintered glass filters contained a larger fraction of organic-enriched particles and a different size-resolved elemental composition, especially in the 0.8–2 μm dry diameter range. Interestingly, chemical differences between the methods only emerged when the particles were chemically analyzed at the single-particle level as a function of size; averaging the elemental composition of all particles across all sizes masked the differences between the SSA samples. When dried, SSA generated by the sintered glass filters had the highest fraction of particles with spherical morphology compared to the more cubic structure expected for pure NaCl particles produced when the particle contains relatively little organic carbon. In addition to an intercomparison of three SSA production methods, the role of the episodic or "pulsed" nature of the waterfall method on SSA composition was under-taken. In organic-enriched seawater, the continuous operation of the plunging waterfall resulted in the accumulation of surface foam and an over-expression of organic matter in SSA particles compared to those produced by a pulsed plunging waterfall. Throughout this set of experiments, comparative differences in the SSA number size distribution were coincident with differences in aerosol particle composition, indicating that the production mechanism of SSA exerts important controls on both the physical and chemical properties of the resulting aerosol with respect to both the internal and external mixing state of particles. This study provides insight into the inextricable physicochemical differences between each of the bubble-mediated SSA generation mechanisms tested and the aerosol particles that they produce, and also serves as a guideline for future laboratory studies of SSA particles.
Highlights
The physicochemical properties of Sea spray aerosol (SSA) particles have been associated with the physical characteristics of bubbles in many prior publications (e.g., Blanchard and Woodcock, 1957; Lewis and Schwartz, 2004; Sellegri et al, 2006; Fuentes et al, 2010b; de Leeuw et al, 2011; King et al, 2012); direct comparison with SSA produced by a breaking wave in the laboratory, assumed to be a good proxy for nascent sea spray, has only recently been realized (Prather et al, 2013)
The differences in size distribution shape and modal diameter suggest that the dominant SSA production mechanism of SSA from the sintered glass filters could be different from SSA produced by wave breaking and the plunging waterfall
Due to uncertainties in the projections of global climate that stem from natural aerosol sources, detailed studies of SSA in controlled environments approximating preindustrial conditions are of great importance (Menon et al, 2002; Ghan et al, 2013; Tsigaridis et al, 2013)
Summary
Understanding the production and characteristics of natural atmospheric aerosol particles is critical for constraining their influence on our global climate (e.g., Charlson et al, 1992; Ramanathan et al, 2001; Menon et al, 2002; Lohmann and Feichter, 2005; Carslaw et al, 2013; Ghan et al, 2013; Tsigaridis et al, 2013) and for the accurate prediction of chemical processes in the atmosphere (Andreae and Crutzen, 1997; Brown and Stutz, 2012). Laboratory studies are quite commonly conducted to generate and study nascent SSA that is uncontaminated by particles found in the marine boundary layer that are produced by other sources These studies produce SSA from disintegrating foam in natural seawater or proxy materials mainly by means of sintered glass bubblers or plunging water jets. Since it is well known that SSA is produced by the bursting of air bubbles at the sea surface (e.g., Blanchard and Woodcock, 1957; Lewis and Schwartz, 2004; de Leeuw et al, 2011), the differences between generation methods for SSA in the laboratory differ primarily by the method of bubble production. Aerosol generation by means of the wave-breaking method provides the closest proxy to natural SSA currently available in a controlled environment
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