Foam droplets generated from natural and artificial seawaters

Abstract

[1] Submicrometer sea salt aerosol (SSA) particles are routinely observed in the remote marine boundary layer (MBL); these aerosols include cloud condensation nuclei and so affect the earth's radiative balance. Here foams designed to mimic oceanic whitecaps were generated in the laboratory using a range of bubbling flow rates and aqueous media: unfiltered seawater, filtered seawater, artificial seawater, and mixtures of filtered and artificial seawater. The number and sizes of dried foam droplets in the particle diameter, D p , range 15-673 nm were measured. Particle size distributions for natural and artificial seawaters were unimodal with a dN/d logD p mode at D p % 100 nm (%200 nm at 80% RH). The foam droplet mode falls within the range of reported mode diameters (D p = 40-200 nm) for submicrometer SSA particles observed in the remote MBL. The present laboratory results were scaled up to estimate submicrometer SSA particle fluxes; this extrapolation supports the hypothesis that foam droplets are the most important source of SSA particles by number. The foam droplet flux from the oceans was estimated to be 980 cm À2 s À1 for a fractional white cap coverage, W, of 0.2%. These results compared well with foam droplet fluxes reported elsewhere. The origins of variability in foam droplet fluxes were also evaluated. Natural organic matter affected foam droplet flux by a factor of 1.5; this was less than (1) the effect of bubbling flow rate on foam droplet flux (factor of 5) and (2) the uncertainty in W (factor of 3-7).

Effect of soluble surfactant on bubble persistence and bubble‐produced aerosol particles

“…Garrett [1968] found that particle production from foams (created by a fritted tip) increased by up to a factor of 3 when insoluble, monolayer-forming surfactants were added to a bubbling solution. Similar increases in aerosol production from foams have been reported more recently with the addition of oleic acid [Tyree et al, 2007] or diatomaceous bioexudate [Fuentes et al, 2010b] to seawater. In addition, Fuentes et al observed a corresponding shift in the submicrometer size distribution of the generated particles to smaller sizes.…”

“…We do not believe that the deficit can be accounted for by the absence in this work of any particular type of particle potentially found in natural sea spray aerosols, such as small organic particles or microgels, since source fluxes measured above artificial seawaters are comparable to those measured above natural seawaters under similar bubbling conditions (OC = 0 mM curve in Figure 7) [Fuentes et al, 2010a;Martensson et al, 2003;Tyree et al, 2007].…”

“…[5] However, the theory fails to account for the even smaller sea spray particles observed in many recent laboratory and ambient studies [Clarke et al, 2006;Fuentes et al, 2010a;Hultin et al, 2010;Keene et al, 2007;Martensson et al, 2003;Modini et al, 2010;Russell and Singh, 2006;Sellegri et al, 2006;Tyree et al, 2007]. Specifically, there is little explanation for the dominant SSA accumulation mode centered at~0.1-0.2 mm dry particle diameter.…”

“…It is more difficult to compare our results directly with those from group bubble experiments, for which data have only been reported semiquantitatively or not on a per-bubble basis. In general, such studies have found enhancements [Fuentes et al, 2010b;Garrett, 1968;Tyree et al, 2007] or no change [Blanchard, 1963] in particle concentrations or fluxes with increasing surfactant concentration. However, it is not clear if these observations resulted from changes in the aerosol producing potential of individual bubbles or from the surface bubble size distribution dynamics and foam structure.…”

A Marine Aerosol Reference Tank system as a breaking wave analogue for the production of foam and sea-spray aerosols

“…Several different methods have been used to generate surrogate marine aerosols within enclosed tanks including pressurized atomizers (Svenningsson et al, 2006;Riziq et al, 2007;Saul et al, 2006;McNeill et al, 2006;Braban et al, 2007;Niedermeier et al, 2008;Taketani et al, 2009), forcing air through glass filters or sintered materials (Cloke et al, 1991;Martensson et al, 2003;Sellegri et al, 2006;Keene et al, 2007;Tyree et al, 2007;Wise et al, 2009;Hultin et al, 2010;Fuentes et al, 2010) and by a plunging water jet (Cipriano and Blanchard, 1981;Sellegri et al, 2006;Facchini et al, 2008;Fuentes et al, 2010).…”