Direct aerosol chemical composition measurements to evaluate the physicochemical differences between controlled sea spray aerosol generation schemes

Abstract

Abstract. Controlled laboratory studies of the physical and chemical properties of sea spray aerosol (SSA) must be underpinned by a physically and chemically accurate representation of the bubble-mediated production of nascent SSA particles. Bubble bursting is sensitive to the physicochemical 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 undertaken. 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, andPublished by Copernicus Publications on behalf of the European Geosciences Union.

A miniature Marine Aerosol Reference Tank (miniMART) as a compact breaking wave analogue

“…Natural whitecap plumes and the resulting surface foam evolve over a timescale of seconds to tens of seconds, whereas continuous water jets impacting the surface at a fixed location create subsurface flow fields unlike breaking events. Continuous sparging of air through frits and nozzles or air entrainment by continuous jets can also create three-dimensional surface foams that do not evolve and dissipate like those within oceanic whitecaps, and these can bias physical and chemical attributes of the aerosols created when the bubbles rupture Collins et al, 2014).…”

“…The importance of decaying foams (as opposed to pseudo-steadystate foams, for which decay rates are matched by bubble entrainment rates) remains an open question, but may be important. For example, the jet drop production mechanism may be somewhat suppressed in steady-state foams if they are more than a single bubble layer thick because the top layer of foam film can absorb jet drop aerosols produced at the air-water interface (Collins et al, 2014). Foams allowed to decay, even if they are initially three-dimensional in structure, will eventually devolve into two-dimensional rafts of bubbles which will not suppress jet drops.…”

“…4b). Blue filled circles show an example of a SSA number concentration in unfiltered, natural seawater in a MART system from Collins et al (2014). (b) The average number-weighted size distribution (black line) and the ±1σ band (gray region) measured by the SEMS.…”

“…The detailed investigations by Sellegri et al (2006) and Fuentes et al (2010) have shown that the best method for the generation of proxy marine aerosols is by creating a bubble plume from a plunging jet of water. In addition Collins et al (2014) have shown that the method of bubble production influences the chemical composition of laboratory-generated sea spray aerosol, with a plunging water method showing better agreement with aerosol produced from laboratory breaking waves than did aerosol generated via the sintered glass filter method. The plunging jet apparatus used by Fuentes et al (2010) used a relatively small volume of water (6 L) in an 11 L tank filled to a depth of 11 cm.…”

Temperature-dependent formation of NaCl dihydrate in levitated NaCl and sea salt aerosol particles

“…1 The phase state of sea spray particles controls their physical and chemical properties, for example, in interaction with ozone 2 or in heterogeneous nucleation of ice. [3][4][5] Although SSA has a complex composition, 6 the phase transition of NaCl can play an important role in the final state and morphology of ambient aerosol particles. NaCl aerosol can exist either as aqueous solution droplets or in one of the two crystalline forms: anhydrous NaCl and NaCl dihydrate (NaCl·2H 2 O).…”