JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, XXXXXX, doi:10.1029/2006JD007729, 2007       Click       Here         for      ...
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Foam Droplets Generated from Natural and Artificial Seawaters
Foam Droplets Generated from Natural and Artificial Seawaters
Foam Droplets Generated from Natural and Artificial Seawaters
Foam Droplets Generated from Natural and Artificial Seawaters
Foam Droplets Generated from Natural and Artificial Seawaters
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Foam Droplets Generated from Natural and Artificial Seawaters


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Foam Droplets Generated from Natural and Artificial Seawaters

  1. 1. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, XXXXXX, doi:10.1029/2006JD007729, 2007 Click Here for Full Article2 Foam droplets generated from natural and artificial3 seawaters4 Corey A. Tyree,1 Virginie M. Hellion,1,2 Olga A. Alexandrova,3 and Jonathan O. Allen1,35 Received 30 June 2006; revised 2 February 2007; accepted 2 March 2007; published XX Month 2007.6 [1] Submicrometer sea salt aerosol (SSA) particles are routinely observed in the remote7 marine boundary layer (MBL); these aerosols include cloud condensation nuclei and8 so affect the earth’s radiative balance. Here foams designed to mimic oceanic whitecaps9 were generated in the laboratory using a range of bubbling flow rates and aqueous media:10 unfiltered seawater, filtered seawater, artificial seawater, and mixtures of filtered and11 artificial seawater. The number and sizes of dried foam droplets in the particle diameter,12 Dp, range 15–673 nm were measured. Particle size distributions for natural and artificial13 seawaters were unimodal with a dN/d logDp mode at Dp % 100 nm (%200 nm at14 80% RH). The foam droplet mode falls within the range of reported mean diameters15 (Dp = 40–200 nm) for submicrometer SSA particles observed in the remote MBL. The16 present laboratory results were scaled up to estimate submicrome ter SSA particle17 fluxes; this extrapolation supports the hypothesis that foam droplets are the most important18 source of SSA particles by number. The foam droplet flux from the oceans was estimated19 to be 980 cmÀ2 sÀ1 for a fractional white cap coverage, W of 0.2%. These results20 compared well with foam droplet fluxes reported elsewhere. The origins of variability in21 foam droplet fluxes were also evaluated. Natural organic matter affected foam droplet22 flux by a factor of 1.5; this was less than (1) the effect of bubbling flow rate on foam23 droplet flux (factor of 5) and (2) the uncertainty in W (factor of 3–7).24 Citation: Tyree, C. A., V. M. Hellion, O. A. Alexandrova, and J. O. Allen (2007), Foam droplets generated from natural and artificial25 seawaters, J. Geophys. Res., 112, XXXXXX, doi:10.1029/2006JD007729.27 1. Introduction are the main source of sea salt aerosol (SSA) particles 45 [Paterson and Spillane, 1969; Blanchard, 1983; Lewis and 4628 [2] In the remote marine boundary layer (MBL), aerosol Schwartz, 2004]. Here we describe laboratory experiments 4729 particles with diameters at ambient relative humidity (RH) examining the effect of seawater composition on the diam- 4830 less than 1 mm are an abundant and climatologically impor- eter and flux of SSA particles generated from foams like 4931 tant class of particles. Submicrometer particles affect the those at sea. These experiments are designed to supplement 5032 radiative balance in the remote marine atmosphere directly particle measurements in the MBL, from which the effects of 5133 by scattering light [Schwartz, 1996; Murphy et al., 1998; seawater composition on particle generation cannot be 5234 Quinn et al., 1998], and indirectly by acting as cloud determined directly. 5335 condensation nuclei (CCN) [O’Dowd et al., 1997; Murphy [3] SSA particles in the MBL usually exist as liquid drops 5436 et al., 1998; Mason, 2001; Pierce and Adams, 2006]. Greater whose diameters depend on the water content, and so on 5537 than 90% of particles in the MBL are submicrometer in ambient RH. The diameter of a particle at ambient RH is 5638 diameter [Fitzgerald, 1991; Jaenicke, 1993] and their chem- Dp,1. RH is typically 80% in the MBL. Since the mole 5739 ical composition indicate two main sources, sea salt fraction of water in salinity 33 seawater is 0.98, the relative 5840 [O’Dowd and Smith, 1993; Bates et al., 1998] and non- humidity in equilibrium with a freshly formed SSA particle 5941 seasalt (nss) sulfate produced from the oxidation of gas- is 98% [Lewis and Schwartz, 2004, p. 53]. The diameters of 6042 phase sulfur species [Nguyen et al., 1983; Charlson et al., SSA particles at RH = 80 and 98%, Dp80 and Dpo, respec- 6143 1987; Sievering et al., 1992; Capaldo et al., 1999]. Wind tively, are related to their dry particle diameter by Dp % 1 2 6244 stress on the ocean surface produces whitecap foams, which Dp80 % 1 Dpo [Fitzgerald, 1975; Lewis and Schwartz, 2004, 4 63 p. 53]. Submicrometer marine particles are usually grouped 64 1 by dry diameter in the Aitken (Dp = 25– 85 nm), accumu- 65 Chemical Engineering Department, Arizona State University, Tempe, Arizona, USA. lation (Dp = 85– 250 nm), and coarse (Dp ! 250 nm) modes 66 2 Now at Ecole Nationale Sup’erieure d’Electrochimie et d’Electro- [Bates et al., 1998; Heintzenberg et al., 2004]. An ultrafine 67 metallurgie, Institut National Polytechnique de Grenoble, Grenoble, France. mode (Dp = 5 – 25 nm), consisting of freshly nucleated 68 3 Civil and Environmental Engineering Department, Arizona State particles, is also sometimes observed [Bates et al., 1998; 69 University, Tempe, Arizona, USA. O’Dowd et al., 2001; Heintzenberg et al., 2004]. Number 70 Copyright 2007 by the American Geophysical Union. concentrations are often dominated by the ultrafine and 71 0148-0227/07/2006JD007729$09.00 Aitken size modes [Bates et al., 1998; Heintzenberg et al., 72 XXXXXX 1 of 17
  2. 2. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX73 2004]. MBL aerosol particles with a dry diameter, Dp 5 of the bubble cavity, which breaks apart to form ‘‘jet drops’’ 13474 80 nm are generally considered to be CCN [O’Dowd et al., [Blanchard, 1989]. 13575 1997; Murphy et al., 1998; Chuang et al., 2000; Pierce and [7] Jet drops are a likely source of SSA particles with 13676 Adams, 2006]. Because ultrafine and Aitken mode particles Dp > 1 mm. The number of jet drops produced from single 13777 are often too small to be CCN, accumulation mode particles bubble bursting decreases with increased bubble diameter. 13878 constitute a significant fraction of marine CCN [O’Dowd and Bubbles larger than 3 mm produced only one jet drop while 13979 Smith, 1993; Chuang et al., 2000; Yoon and Brimblecombe, 300 mm bubbles produced five to six jet drops [Cipriano 14080 2002]. and Blanchard, 1981]. Jet drop diameters at formation were 14181 [4] Submicrometer sea salt has been observed in the approximately one-tenth the diameter of their parent bubble 14282 Aitken [Clarke et al., 2003], accumulation [O’Dowd and diameter [Blanchard and Woodcock, 1957]. Submicrometer 14383 Smith, 1993; Quinn et al., 1998; Bates et al., 1998], and jet drops would have to be produced from bubbles smaller 14484 coarse modes [Bates et al., 1998; Huebert et al., 1998; than 20 mm. However, bubbles of this diameter will not 14585 Campuzano-Jost et al., 2003]. New nss-sulfate particles can surface in large numbers since they are expected to dissolve 14686 form when the gas phase oxidation products of dimethyl before reaching the surface. For example, bubbles smaller 14787 sulfide (DMS) or SO2 nucleate to form ultrafine particles than 100 mm rising in 0.3 m of seawater dissolve completely 14888 [Clarke et al., 1998]. Initially, in the ultrafine mode, these prior to reaching the surface [Blanchard and Woodcock, 14989 nss-sulfate particles can grow via heterogeneous processes 1980]. Though measurements of jet drop diameter generally 15090 to form Aitken [Fitzgerald, 1991; Clarke et al., 1998] and cover only the supermicrometer size range [Lewis and 15191 accumulation mode particles [Ayers et al., 1991; Hegg and Schwartz, 2004, p. 193], jet drops are an unlikely source 15292 Hobbs, 1992]. Mixtures of sea salt and nss-sulfate can form of submicrometer SSA particles. 15393 when gas phase sulfur-containing species condense on [8] Film droplets can also be a source of SSA particles 15494 existing sea salt particles and cloud droplets [Hoppel et al., with Dp > 1 mm [Lewis and Schwartz, 2004, p. 204]. The 15595 1989; Fitzgerald, 1991; Sievering et al., 1992; Clarke and number of film droplets formed per bubble is dependent on 15696 Porter, 1993; O’Dowd et al., 1997]. Because SSA particles bubble diameter [Blanchard and Woodcock, 1957; Resch 15797 are originally alkaline, nss-sulfate uptake and formation are and Afeti, 1991; Spiel, 1998]. Bubbles smaller than % 2 mm 15898 enhanced on sea salt particles [O’Dowd and Smith, 1993; produced no film droplets [Resch and Afeti, 1991; Spiel, 15999 McInnes et al., 1994; Katoshevski et al., 1999; Laskin et al., 1998] while bubbles larger than %2 mm produced 100 to 160100 2003]. SSA 65 particles therefore play a dual role in the 200 film droplets [Spiel, 1998]. Although many authors 161101 marine atmosphere; they are a large direct source of sub- have suggested that film droplets can be submicrometer in 162102 micrometer particles [O’Dowd and Smith, 1993] and also diameter [Mason, 1957; Cipriano et al., 1983; Blanchard 163103 affect the formation of nss-sulfate [Sievering et al., 1992; and Syzdek, 1988], film droplets in the range Dp 1 mm are 164104 Clarke and Porter, 1993]. largely uncharacterized. 165105 [5] SSA particles can be identified by their Na content [9] Even if single bubble bursting produced SSA particles, 166106 since Na is conserved in atmospheric particles [McInnes the contribution of single bubble bursting to SSA particles is 167107 et al., 1996; Clark et al., 2001; Campuzano-Jost et al., expected to scale with the frequency of these events. An 168108 2003]. Na+ comprised approximately 30% of the ionic alternative source of submicrometer SSA particles is foam 169109 species mass of submicrometer particles near Cape Grim; bubble bursting. At sea, bubble coalescence following air 170110 since Na+ is 31% of sea salt mass, in this case sea salt made entrainment leads to whitecap foam formation. Foam forma- 171111 up nearly all of the submicrometer particle ionic mass tion is always observed following wave breaking according 172112 [Huebert et al., 1998]. Sea salt comprised approximately to Blanchard [1983], who estimated foam bubbles out- 173113 80% of the ionic mass in submicrometer particles in the number single bubbles by a factor of 10. In addition to 174114 remote Southern Ocean MBL [Quinn et al., 1998]. Measure- outnumbering single bubbles, foam bubble bursting also 175115 ments of marine particle ionic composition are consistent produces many more droplets than does an equivalent num- 176116 with thermal volatility measurements that show a SSA ber of single bubbles [Paterson and Spillane, 1969]. On the 177117 particle mode at Dp % 200 nm [O’Dowd and Smith, 1993]; basis of the relatively high SSA particle production from 178118 for example, sea salt comprised 58% of the accumulation foam bubbles and their ubiquity at sea, Paterson and Spillane 179119 mode ionic mass (number geometric mean dry diameter, [1969] concluded that ‘‘the overwhelming contributor to the 180120 Dp;g = 110 nm) and greater than 99% of coarse mode ionic sea salt aerosol population is believed to be the rafts of foam 181121 mass (Dp;g = 540 nm) collected over the Southern Ocean produced by breaking waves.’’ 182122 region [Bates et al., 1998]. [10] A few investigators have studied the effect of foam 183123 [6] Particles produced from single bubble bursting have formation on SSA particle production [Garrett, 1968; 184124 been extensively studied in the laboratory [Blanchard, Cipriano and Blanchard, 1981; Monahan et al., 1982; 185125 1963; Day, 1964; Resch and Afeti, 1991; Spiel, 1998]. Woolf et al., 1987; Martensson et al., 2003; Sellegri et al., ˚ 186126 Particles are formed when single bubbles rise to the surface; 2007]. The particles produced from foam bubble bursting 187127 the bubble film drains until the film thickness is on the order are referred to here as ‘‘foam droplets.’’ Woolf et al. [1987] 188128 of 1 mm [Spiel, 1998] and then the bubble film shatters. A simulated whitecap formation by colliding two parcels of 189129 toroid composed of film material breaks apart to form water and measured foam droplet size distributions over the 190130 primary ‘‘film droplets’’ [Spiel, 1998]. Secondary film range Dp = 0.25– 10 mm. The smallest particle size bin, Dp = 191131 droplets appear to be produced from impaction of the larger, 250– 750 nm, contained the largest number of foam drop- 192132 downward moving primary film droplets. Milliseconds after lets. Hoppel et al. [1989] generated foam droplets by 193133 the bubble bursts, an unstable jet can form from the collapse bubbling through a frit in seawater and measured foam 194 droplet size distributions over the range Dp = 12– 800 nm. 195 2 of 17
  3. 3. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX 1992]. Under conditions that whitecap foams form, the 225 bubble upwelling region causes an outflow of water and 226 removal of the surface microlayer above the rising bubble 227 plume [Blanchard, 1983]. Thus it is the seawater organic 228 matter in the bubble entrainment zone that is most likely to 229 affect foam droplet production. 230 [12] Laboratory studies of the effect of organic matter on 231 droplet production have been inconclusive. In single bubble 232 experiments, organic matter has been shown to increase 233 droplet production [Blanchard, 1963], decrease droplet pro- 234 duction [Day, 1964; Paterson and Spillane, 1969], or have no 235 effect at all [Morelli et al., 1974]. Single bubble experiments 236 have shown that organic matter in natural seawater can 237 suppress film droplet production [Day, 1964; Paterson and 238 Spillane, 1969]. Garrett [1968] showed that bubbling 239 through fresh seawater resulted in a 25% decrease in foam 240 droplet production relative to a 30% NaCl solution. Attempts 241 to characterize the effect of organic matter on SSA particle 242 production also have been made by adding surface-active 243 organic species to artificial seawater. Oleic acid is often 244 selected as a model surface-active compound in laboratory 245 SSA particle experiments because it is a component of both 246 DOM and POM [Blanchard, 1963; Garrett, 1968; Morelli 247 et al., 1974; Monahan et al., 1980]. Garrett [1968] showed 248 Figure 1. Schematic diagram of foam droplet apparatus. that the addition of oleic acid suppressed foam formation and 249 Compressed air was filtered through a high efficiency increased particle production. 250 particulate air (HEPA) filter and metered using a rotameter [13] In this work, we measure the number and size of 251 (F), and then passed through a diffuser to generate bubbles. foam droplets produced in a controlled laboratory setting 252 Aerosol was sampled through a diffusion drier. Particle size in order to study the effect of seawater composition and 253 distributions were measured using a Scanning Mobility bubbling conditions on foam droplet production. The effect 254 Particle Sizer (SMPS). of salinity was studied by generating foam droplets from 255 artificial seawaters with salinities in the range 0 – 70%. The 256 effect of organic content was studied by generating foam 257196 Foam droplets had a dN/dDp mode at Dp % 70– 110 nm. droplets from seawater containing natural DOM and POM 258197 Martensson et al. [2003] generated foam droplets by bub- ˚ (unfiltered seawater), natural DOM (filtered seawater), and 259198 bling through a frit in salinity 33% artificial seawater and artificial seawater containing surrogate DOM (oleic acid). 260199 measured foam droplet size distributions over the range The laboratory results are compared with remote, marine 261200 Dp = 0.02– 5 mm. At a seawater temperature of 25°C, foam particle size distributions and used to estimate the production 262201 droplets had a bimodal number size distribution with dN/d of submicrometer SSA particles from oceanic foams. 263202 logDp modes in the ranges Dp = 80– 120 nm and Dp = 1.5–203 2.5 mm. At 15°C, foam droplets had a bimodal number size204 distribution with dN/d logDp modes in the ranges Dp = 50– 2. Experimental Methods 264205 60 nm and Dp = 1.5– 2.5 mm. These submicrometer foam [14] Foam droplets were generated by bubbling air through 265206 droplet dN/d logDp modes fall within the range of mean dry aqueous media (see Figure 1). Compressed air was filtered 266207 diameters reported for submicrometer SSA particle modes, through a high efficiency particulate air (HEPA) filter and 267208 Dp = 40– 200 nm [O’Dowd and Smith, 1993; Murphy et al., metered using a calibrated rotameter with a precision of 268209 1998; Nilsson et al., 2001; Clarke et al., 2003]. 0.1 l minÀ1. Air then was passed through a stainless steel 269210 [11] It has been hypothesized that variability in submi- tube to a fine-pore diffuser (Aquatic Eco-Systems, Model 270211 crometer SSA particle flux estimates could be explained in ALR80SS, 3 Â 1.5 inches, 80 mm mean pore size, 271212 part by spatial variation in the amount and nature of Apopka, FL). In experiments focused on the effects of 272213 seawater organic matter [Reid et al., 2001; Geever et al., subsurface bubble diameter, the fine-pore diffuser was 273214 2005]. Seawater organic matter consists of a complex exchanged for a medium-pore diffuser (Aquatic Eco-Systems, 274215 mixture of dissolved and particulate species, including Model ALR8, 3 Â 1.5 inches, 140 mm mean pore size, 275216 lipids, amino acids, amino sugars, cells, and cell fragments. Apopka, FL). The height and diameter of the glass column 276217 Dissolved organic matter (DOM) and particulate organic were 60 and 15 cm, respectively. The glass column was capped 277218 matter (POM) concentrations vary greatly with depth, time, by a cone-shaped piece of aluminum foil. The cap was formed 278219 and location [Millero and Sohn, 1992]. Dissolved organic to the glass surface of the column and the aerosol particle- 279220 carbon (DOC) concentrations in the surface mixed layer, sampling inlet. The cap was not sealed. Excess air was vented 280221 which extends to a depth of about 100 m, range from about from the headspace so that background aerosol was not 281222 1 mg ClÀ1 [Benner et al., 1992] in the open ocean to 10 mg sampled. The separation between the water level and the apex 282223 ClÀ1 in coastal areas [Sharp et al., 1993]. POM concen- of the cone-shaped cap was 25–30 cm over the range of 283224 trations are about one-fifth that of DOC [Millero and Sohn, bubbling flow rates studied; note that jet drops are ejected to a 284 3 of 17
  4. 4. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX285 maximum height of 20 cm [Spiel, 1997]. The displacement washes with deionized (DI) water. The column was then 347286 height, and so the void fraction in the column, was measured rinsed with three sequential washes of 50 mL of isopro- 348287 for each bubbling flow rate. panol (Burdick and Jackson, High Purity Solvent, 99.9+%) 349288 [15] The diffuser was placed approximately 1 cm above the and three sequential washes of 50 mL of dichloromethane 350289 bottom of the glass column so that bubble flow was unaf- (DCM) (Burdick and Jackson, GC/GC-MS grade, 351290 fected by the column bottom. The column was filled with 99.9+%); the column was allowed to dry in a fume hood. 352291 aqueous media to a height of 40.6 cm (±0.2 cm), which The diffuser stone was rinsed with DI water, 50 mL of 353292 corresponds to a volume of 7.2 l. The bubble rise distance in isopropanol, and DI water again. The diffuser was then 354293 the column was then 32– 39.5 cm, which approximates the sonicated in a precleaned beaker containing DI water for 355294 natural circulation depth of oceanic bubbles [Lamarre and 10 min and rinsed with DI water. For the final cleaning 356295 Melville, 1992; Woolf, 1997]. Entrained air bubbles can reach step, the diffuser was placed in a precleaned beaker 357296 depths of several meters [Thorpe, 1986], but the majority of containing DI water, where air was bubbled through the 358297 entrained air is located within %50 cm of the sea surface diffuser for at least 10 min. The DI water was changed and 359298 [Lamarre and Melville, 1992]. Note that the circulation depth the process repeated twice. Stainless steel tubing and tube 360299 utilized here is an order of magnitude greater than that used in fittings were cleaned with 5 mL of detergent solution, rinsed 361300 prior experiments [Garrett, 1968; Martensson et al., 2003; ˚ with DI water, and rinsed with three sequential 5 mL washes 362301 Sellegri et al., 2007]. of isopropanol and then three sequential 5-mL washes of 363302 [16] Any effect of organic matter on SSA particle pro- DCM. The interior surface of the aluminum cap was rinsed 364303 duction is likely due to organic matter which partitions to with three sequential 5-mL washes of isopropanol and then 365304 the air-water interface of the bubble during ascent. Some three sequential 5-mL washes of DCM. 366305 time is necessary for this interface to reach steady state. The [20] The particle size distributions of foam droplets were 367306 bubble residence time for this apparatus was %5 s. Similar measured for a range of aqueous media and bubbling 368307 to the characteristic bubble residence times of oceanic superficial velocities (see Table 1). Artificial seawater was 369308 bubble plumes, which evolve rapidly beneath breaking prepared from analytical grade salts and DI water. The salt 370309 waves, is on the order of 1– 5 s [Graham et al., 2004]. mixture contained, by mass, 87.9% NaCl (Fluka, ACS 371310 This residence time is significantly shorter than that esti- reagent grade, 99.5+%), 11.7% MgSO4 Á 7H2O (Sigma- 372311 mated using a Stokes terminal velocity; a plume of bubbles Aldrich, ACS reagent grade, 98+%), and 0.4% NaHCO3 373312 causes upwelling flow which in turn causes bubbles in a (Sigma-Aldrich, ACS reagent grade, 99.7+%). This salt 374313 plume to rise faster than single bubbles in still water. mixture includes Na+, ClÀ, Mg2+, and SO2À, which together 4 375314 [17] The aerosol sampling inlet was a 0.64 cm i.d. copper account for 97% of the inorganic salt mass in seawater 376315 tube placed approximately 15 cm above the top of the foam [Horne, 1969]. NaHCO3 served to buffer the pH of the 377316 layer. The aerosol was sampled at 300 cm3 minÀ1, corre- artificial seawater [Pilson, 1998]. The solution pH was 7.8, 378317 sponding to laminar flow in the sampling tube. Foam which is comparable to the typical surface seawater pH of 379318 droplets were dried using a diffusion dryer, which had a 8.2. The artificial sea salt mixture was prepared so that the 380319 length of 30 cm, an annular diameter of 2.5 cm, and a ionic mass ratios of Na+, ClÀ, Mg2+, and SO2À were 4 381320 residence time of approximately 30 s. In the dryer, aerosol comparable to that in seawater. The mass ratios (and their 382321 flow was surrounded by CaSO4 dessicant separated by a corresponding mass ratios in seawater) were ClÀ = Na+ = 383322 polypropylene mesh tube. The desiccant was replaced after 1.54 (1.80), SO2À =Mg2+ = 3.95 (2.11), SO2À/ClÀ = 0.09 4 4 384323 approximately 2 hours of use. The total residence time of the (0.14), and Mg2+=Na+ = 0.033 (0.12). Artificial seawaters 385324 foam droplets in the transport line was approximately 35 s. were prepared with the following salinities (±0.05%): 386325 The calculated RH at the exit was 20% [Tyree and Allen, 70, 33, 20, 10, 1, and 0%; these media were designated 387326 2004], which is below the crystallization point, 45%, for AS70, 245 AS33, AS20, AS10, AS1, and AS0 (pure DI 388327 NaCl [Biskos et al., 2006]. water), respectively. 389328 [18] Sizes of the dried particles were measured using a [21] Natural seawater was collected off the Scripps Insti- 390329 TSI (St. Paul, MN) Model 3080 Scanning Mobility Particle tute of Oceanography (SIO) Pier (32 ± 520N, 1170160W) on 391330 Sizer (SMPS) equipped with a Model 3081 Long Differen- 9 June 2004 and 20 February 2006. Water was siphoned 392331 tial Mobility Analyzer and a Model 3025A Condensation through a tube suspended approximately 20 cm beneath a 393332 Particle Counter. The SMPS system recorded the concen- surface float to avoid sampling of the surface microlayer. 394333 tration of particles with mobility diameters in the range Seawater was stored in precleaned brown glass bottles 395334 14.9 – 673 nm. Four 2.5-min measurements (one scan per maintained at 0°C. A portion of each seawater sample 396335 measurement) were recorded for most experiments; only was filtered using 0.7 mm pore Whatman GF/F glass fiber 397336 two 2.5-min measurements were recorded for artificial filters in a separatory funnel to remove POM. Prior to 398337 seawater experiments using salinities of 1, 10, and 20%. filtration, the filters were cleaned using sequential isopro- 399338 Following a change in airflow or aqueous media, the panol and DCM washings aided by sonication for 30 min. 400339 apparatus was run for at least 5 min in order to purge the Because foam droplet production may be dependent on 401340 system before acquiring data. water temperature [Martensson et al., 2003], seawater was ˚ 402341 [19] The apparatus was thoroughly cleaned in order to warmed to at least 20°C before experimentation. The 403342 minimize contaminants including naturally occurring organic seawater temperature was maintained at 20°– 25°C for 404343 species and detergents. The glass column was first rinsed each experiment. All natural seawater experiments were 405344 with water in order to remove visible salt residues. The completed within 48 hours of seawater collection. 406345 column was cleaned with two sequential washes of 0.5 l of [22] For the June 2004 seawater sample, concurrent in 407346 Alkanox detergent solution, followed by two sequential situ temperature and salinity measurements were provided 408 4 of 17
  5. 5. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXXt1.1 Table 1. Summary of Foam Droplet Size Distribution Results for a Range of Superficial Bubbling Velocities (vg) and Aqueous Mediaat1.2 vg = 0.090 cm sÀ1 vg = 0.33 cm sÀ1 vg = 1.2 cm sÀ1 vg = 1.6 cm sÀ1 Ntotal, Dp;g , Ntotal, Dp;g , Ntotal, Dp;g , Ntotal, Dp;g ,t1.3 Aqueous Media cmÀ3 nm sg cmÀ3 nm sg cmÀ3 nm sg cmÀ3 nm sgt1.4 AS70: 70% artificial seawater – – – – – – 24,000 98.8 1.77 – – –t1.5 AS33: 33% artificial seawater (Oct 2004) 7,070 86.0 1.61 7,100 91.0 1.60 17,000 97.8 1.70 23,300 99.7 1.72t1.6 AS33: 33% artificial seawater (Jan 2006) 6,830 89.9 1.57 6,450 97.3 1.62 16,800 105 1.80 22,200 114 1.82t1.7 AS33-MP: 33% artificial seawater 3,390 83.4 1.70 4,000 89.2 1.75 5,610 89.4 1.77 6,190 89.4 1.80 medium-pore diffusert1.8 AS20: 20% artificial seawater – – – – – – 12,100 90.1 1.67 – – –t1.9 AS10: 10% artificial seawater – – – – – – 9,980 86.1 1.73 – – –t1.10 AS1: 1% artificial seawater – – – – – – 2,650 62.5 1.75 – – –t1.11 AS0: DI water 40.3 58.5 2.04 – – – 0 NA NA – –t1.12 OA10: 10 mg lÀ1 oleic acid in AS33 12,500 94.5 1.66 – – – 31,100 84.0 1.82 – – –t1.13 OA1: 1 mg lÀ1 oleic acid in AS33 12,100 94.1 1.66 – – – 31,700 85.0 1.73 – – –t1.14 OA0.1: 0.1 mg lÀ1 oleic acid in AS33 10,500 92.3 1.70 – – – 22,400 89.4 1.71 – – –t1.15 FS100: 100% filtered seawater collected 4,770 86.9 1.64 7,510 96.3 1.54 17,500 102 1.66 23,200 105 1.69 Jun 2004, 3.07 mg ClÀ1t1.16 FS100: 100% filtered seawater collected 6,730 89.4 1.61 9,060 96.3 1.66 17,700 102 1.75 24,000 102 1.77 Feb 2006, 2.28 mg ClÀ1t1.17 FS10: 10% (vol) filtered seawater; 90% AS33 – – – – – – 16,800 96.8 1.69 – – –t1.18 FS1: 1% (vol) filtered seawater; 99% AS33 – – – – – – 16,900 96.8 1.68 – – –t1.19 UFS100: 100% unfiltered seawater 6,390 89.0 1.67 10,400 93.8 1.58 18,700 99.3 1.65 26,700 103 1.71 collected Jun 2004 a t1.20 Ntotal is total number concentration, Dp;g is dried number geometric mean diameter, and sg is geometric standard deviation. 409 by the Network for Environmental Observations of the and February 2006 seawater samples. All other natural 447 410 Coastal Ocean (NEOCO). The NEOCO instrumentation seawater media (i.e., FS1, FS10, and UFS100) used only 448 411 includes a Sea-Bird Seacat CTD instrument located 2 m the June 2004 seawater sample. 449 412 below the mean low water level. The temperature and [25] In order to isolate the effect of a surrogate DOM 450 413 salinity of the near surface were 20.3°C and 33.47%. compound on foam droplet production, foam droplets were 451 414 Concurrent chlorophyll fluorometric measurements at the generated from aqueous media with a range of oleic acid 452 415 SIO Pier were provided by the University of California concentrations (Fisher Chemical, NF/FCC grade). Solu- 453 416 Coastal Environmental Quality Initiative. The seawater tions were made from salinity 33% artificial seawater with 454 417 had a chlorophyll concentration of 1.8 mgmÀ3. For the oleic acid concentrations of 0.1 mg lÀ1 (0.08 mg ClÀ1, OA0.1), 455 418 February 2006 seawater sample, concurrent measurements 1 mg lÀ1 (0.8 mg ClÀ1, OA1), and 10 mg lÀ1 (8 mg ClÀ1, 456 419 were provided by the Southern California Coastal Ocean OA10). The oleic acid was added to AS33 in the column and 457 420 Observing System (SCCOOS). The instrumentation is part mixed using the bubbling airflow for at least 5 min prior to 458 421 of the SCCOOS automated shore station at the SIO pier, measurement. 459 422 which includes a suite of automated sensors located at the [26] In order to compare our results with those from other 460 423 water surface. The temperature, salinity, and chlorophyll experiments, it is convenient to express the bubbling flow 461 424 concentration were 14.5°C, 33.31%, and 0.10 mg mÀ3. rate as a superficial gas velocity, vg, which is equal to the 462 425 [23] Filtered seawater and an artificial seawater blank _ bubbling flow rate (V air ) divided by the column area (A). A 463 426 were analyzed for DOC. These samples were acidified with standard bubbling superficial velocity, vg = 1.2 cm sÀ1, was 464 427 quartz-distilled hydrochloric acid to pH 2 and the dissolved used for all aqueous media and diffusers. Foam droplets were 465 428 inorganic carbon (DIC) removed by sparging with nitrogen also generated from the AS33, AS70, FS100, and UFS100 466 429 (ultra-high purity grade) for 5 min [Sharp, 2002]. Samples using vg = 0.090, 0.33, and 1.6 cm sÀ1. These velocities were 467 430 were stored at 0°C and the DOC content was determined selected because observations showed foam morphologies 468 431 within 48 hour of seawater collection using a Shimadzu were different for each of these vg. 469 432 TOC-V Total Organic Carbon Analyzer. 271 The DOC [27] In order to isolate the effect of subsurface bubble 470 433 concentrations were 3.07 and 2.28 mg ClÀ1 for the June diameter on foam droplet diameter and production, foam 471 434 2004 and February 2006 samples, respectively. The DOC droplets were generated from AS33 using a fine-pore and 472 435 concentration in AS33 was 0.05 mg ClÀ1. medium-pore diffuser stone. Photographs of the bubble 473 436 [24] In order to study the effect of natural organic matter plume produced by the fine-pore and medium-pore diffusers 474 437 on foam droplet production, foam droplets were generated at a water depth of 5 – 15 cm were analyzed to determine the 475 438 from aqueous media with a range of organic content. These diameter of bubbles produced. The diameters of 200 bubbles 476 439 aqueous media included 100% filtered seawater (FS100) were determined with a measurement precision of ±50 mm by 477 440 and 100% unfiltered seawater (UFS100). Solutions with comparing the bubble width to the known width of a 478 441 lower concentrations of natural DOM were made by dilut- submerged stainless steel tube. 479 442 ing filtered seawater with salinity 33% artificial seawater; 443 these were composed of 10% filtered seawater and 90% 3. Results 480 444 artificial seawater by volume (FS10) and 1% filtered sea- 3.1. Subsurface Bubbles 481 445 water and 99% artificial seawater (FS1). Foam droplet 446 experiments using FS100 were conducted with June 2004 [28] Subsurface bubbles rise through the water column 482 and coalesce into a foam at the surface. The connection is 483 5 of 17
  6. 6. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX Figure 2. Bubble size distributions in salinity 33% artificial seawater (AS33) produced by fine-pore and medium-pore diffusers using a bubbling superficial velocity (vg) of 0.33 cm sÀ1. The shaded band encompasses the majority of the oceanic bubble size distributions reported for depths 1.5 m and wind speeds 10 m above the sea surface (U10) greater than 10 msÀ1 [Lewis and Schwartz, 2004, p. 241].484 therefore indirect between the diameter, Db, and concentra- compare the subsurface Nb measured here with those in 519485 tion, Nb, of subsurface bubbles and the nature of the foam the ocean, the laboratory Nb should be multiplied by 520486 from which particles are produced. Nevertheless, the appa- fractional whitecap coverage, W. Since the average W is 521487 ratus used here was designed to generate subsurface bubbles on the order of 0.01 [Monahan, 1986], the number of 522488 comparable in Db and Nb to those observed under breaking bubbles generated from the fine-pore diffuser for vg = 523489 waves. Db was measured optically using two different 0.33 cm sÀ1 is comparable to the high end (top of shaded 524490 diffusers at two bubbling superficial velocities, vg, in band in figure) of the estimated range for oceanic bubble 525491 AS33 (salinity 33% artificial seawater). The fine-pore dif- concentrations. Also note that since the bubble diameter is 526492 fuser produced bubbles with mean Db, Db = 230 mm independent of vg, Nb for vg = 1.6 cm sÀ1 is approximately 527493 (standard deviation, s = 80 mm) for vg = 0.33 cm s-1 and 5 times higher than for vg = 0.33 cm sÀ1. 528494 Db = 250 mm (s = 95 mm) for vg = 1.6 cm sÀ1 (see Figure 2). [31] The Nb and Db generated from the fine-pore diffuser in 529495 The medium-pore diffuser produced bubbles with Db = media containing natural organic matter were similar to those 530 À1 496 1000 mm (s = 310 mm) for vg = 0.33 cm s and Db = measured for AS33. In contrast, the addition of oleic acid to 531497 950 mm (s = 330 mm) for vg = 1.6 cm sÀ1. Thus bubble artificial seawater caused a dramatic increase in Db which 532498 diameters were approximately independent of vg. was dependent on oleic acid concentration. Many of the 533499 [29] Nb was determined for vg = 0.33 cm sÀ1 using Db bubbles in OA0.1 (salinity 33% artificial seawater with 534500 and the void fraction measurements; these bubble size 0.1 mg lÀ1 oleic acid added) had Db % 250 mm, compa- 535501 distributions were compared to oceanic bubble size distri- rable to the bubbles present in AS33; however, some of 536502 butions summarized by Lewis and Schwartz [2004, p.239] the bubbles coalesced near the diffuser to form bubbles 537503 [Kolovayev, 1976; Walsh and Mulhearn, 1987; Bowyer, with Db 0.2 cm. In OA10 nearly all bubbles coalesced to 538504 2001; de Leeuw and Cohen, 2002] (see Figure 2). These form bubbles with Db % 0.5 cm. Though the effect of 539505 measurements have shown that the oceanic bubble size salinity on Db was not quantified, observations indicated 540506 distribution spans the range Db = 50– 800 mm. But as noted that Db decreased slightly with increased artificial seawa- 541507 previously, the bubbles in the range Db = 50– 100 mm are ter salinity from 1 to 70%. Bubbles in AS0 were much 542508 expected to dissolve as they circulate beneath a breaking larger, Db % 0.5 cm. 543509 wave [Blanchard and Woodcock, 1980]. Therefore bubbles510 in the range Db = 100 – 800 mm are the most relevant to SSA 3.2. Foam Characterization 544511 particle production. The fine-pore diffuser produced bubbles [32] Bubbling through liquids with varying salinity, organic 545512 in the range Db = 60– 800 mm. The medium-pore diffuser content, and vg produced varying 341 foam morphologies, 546513 produced bubbles in the range Db = 550– 2000 mm. Since the from surfaces with no foam to those with continuous foam 547514 Db range produced by the fine-pore diffuser more closely greater than 1 cm thick. Foam was formed in every experiment 548515 matches that found at sea, the fine-pore diffuser was used for except the AS0 (DI water) and OA10 experiments. Since pure 549516 all other experiments. liquids do not foam [Bikerman, 1973], the absence of foam 550517 [30] Note that the oceanic Nb shown in Figure 2 generally when bubbling through AS0 suggested that the apparatus was 551518 included periods with no wave breaking. In order to free of contaminants. Foam formation was observed upon the 552 6 of 17
  7. 7. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX553 addition of small amounts of sea salt (AS1). Increased salinity those for HEPA384 filtered air. These results suggest that 614554 from 1 to 70% caused only small increases in foam thickness for vg = 1.2 cm sÀ1 sufficient excess airflow excluded 615555 and foam bubble diameter. ambient aerosol from the headspace while some background 616556 [33] Foam thickness in AS33 was dependent on vg. For vg = aerosol was sampled for vg = 0.090 cm sÀ1. Note that the 617557 0.090 cm sÀ1, the surface was approximately 90% covered by concentration of background aerosol particles was at least 618558 rafts of foam bubbles. In most cases, the foam bubble two orders of magnitude less than those observed for media 619559 diameters were approximately 0.5 cm, indicating that bubble other than AS0. 620560 coalescence occurred at the surface. The coverage and [38] Particle size distribution measurements were made for 621561 thickness of the foam was enhanced as vg was increased to many combinations of aqueous media and vg (see Table 1). In 622562 0.33 cm sÀ1. For vg = 0.33 cm sÀ1, the surface was covered by most cases, four particle size distributions were measured 623563 aged foam and an active upwelling region; this upwelling during a 10-min sampling period. The two replicate experi- 624564 region appeared intermittently where a stream of bubbles ments for AS33 and vg = 1.2 cm sÀ1 were representative. 625565 breached the surface. For vg = 1.2 and 1.6 cm sÀ1, thicker Within each experiment, the size distributions for each of four 626566 foam layers formed and the active upwelling region appeared measurements were similar; each distribution had one dis- 627567 less frequently. tinct mode at Dp % 100 nm and Ntotal of each distribution were 628568 [34] The presence of organic matter in the aqueous media within 1% (see Figure 3). This indicates that the particle size 629569 produced noticeable changes in foam morphology. The distributions were stable over the 10-min sampling period. 630570 appearance of foams formed from artificial seawater can Because particle size distributions were stable and unimodal, 631571 be described as ‘‘soda pop like,’’ since these contained the dried number geometric mean diameter, Dp;g , geometric 632572 bubbles with relatively short surface lifetimes and nearly standard deviation, sg, and Ntotal were determined from the 633573 uniform size. In contrast, the appearance of natural seawater mean of the four measurements. The particle size distribu- 634574 foams can be described as ‘‘beer-like,’’ since these tions were represented by dN/d logDp; references to a peak or 635575 contained bubbles with relatively long surface lifetimes mode in these distributions are based on this representation. 636576 and heterogeneous sizes. The presence of organic matter [39] We determined whether experimental results were 637577 in the aqueous media also produced changes in foam equivalent using a two-sample t test on Ntotal. Sample 638578 thickness. For vg = 0.090 and 0.33 cm sÀ1, FS100 (100% standard deviations, si, for each experiment (usually four 639579 filtered seawater) and UFS100 (100% unfiltered seawater) runs) were comparable and in the range 67– 1300 cmÀ3, 640580 foams were similar in thickness to those of AS33. However, with the exception of OA10 and vg = 1.6 cm sÀ1 which had 641581 FS100 and UFS100 produced coarser foams with the largest an si of 3700 cmÀ3. Excluding this experiment, values of si 642582 bubbles having diameters on the order of 1 cm. For vg = 1.2 were not correlated with Ntotal or any experimental param- 643583 and 1.6 cm sÀ1, FS100 and UFS100 foams were noticeably eter; therefore the underlying distributions were assumed to 644584 thicker than AS33 foams. A foam layer was maintained across have the same standard deviation. We then calculated 645585 the entire liquid surface during experiments with media con- DNtotal as the difference between Ntotal for every run and 646586 taining natural organic material and vg ! 0.33 cm sÀ1. UFS100 the mean for the runs in an experiment excluding the OA10 647587 and FS100 foams were similar in morphology. June 2004 and vg = 1.6 cm sÀ1 experiment. The ensemble sample 648588 FS100 and February 2006 FS100 foams were also similar in standard deviation of DNtotal for all runs across all experi- 649589 morphology. The dilution of dissolved organic matter content ments, s0, was 440 cmÀ3. Below, we refer to experimental 650590 had no noticeable effect on foam morphology and thickness, as results as ‘‘indistinguishable’’ if the Ntotal were indistin- 651591 foams produced from FS100, FS10, and FS1 were similar. guishable at 95% confidence based on a two-sample t test 652592 [35] The effect of oleic acid on foam formation was using s0 = 440 cmÀ3. For Dp;g, the s and relative standard 653593 concentration dependent. Foams formed on OA0.1 and deviation among the runs were 1.1 nm and 1%. Comparing 654594 OA1 contained a mixture of bubble sizes and were slightly the two AS33 experiments, Dp;g values differ by as much as 655595 thinner than those on AS33. Foams did not form on OA10; 15 nm for the same vg. Results were not quantitatively 656596 instead the large bubbles burst independently at the water compared based on Dp;g since lognormal curve fitting 657597 surface. appears to introduce artificial variability. 658598 3.3. Foam Droplet Experiments 3.4. Effect of Salinity 659599 [36] Preliminary experiments were conducted to investi- [40] The effect of salinity on foam droplet production was 660600 gate the presence of extraneous particles in the apparatus. examined using artificial seawaters with a range of salinities 661601 These particles may have had three sources: (1) back- and vg = 1.2 cm sÀ1 (see Table 1). Ntotal of foam droplets 662602 ground particles from laboratory air, (2) particles in the increased from 2650 to 24,000 cmÀ3 for salinities of 1 to 663603 bubbling air, and (3) particles from aqueous media impu- 70%, respectively (see Figure 4). Dp;g of the particles also 664604 rities. Particle number concentrations with no bubbling increased from 62.5 to 98.8 nm over the same range of 665605 flow averaged 551 cmÀ3; this is the same as the concentration salinity. The geometric standard deviations were in the range 666606 in ambient laboratory air and is due to flow of laboratory air 1.67– 1.80 and were comparable for all artificial seawater 667607 into the head space of the apparatus. Number concentrations solutions. 668608 were also measured while bubbling air through AS0 in order [41] The effect of vg on Ntotal generated from AS33 was 669609 to detect extraneous particles from either the bubbling air or examined by generating foam droplets using vg = 0.090, 0.33, 670610 aqueous media impurities. 1.2, and 1.6 cm sÀ1. In both AS33 experiments, Ntotal values 671611 [37] Total number concentrations, Ntotal, were 40.3 cmÀ3 were indistinguishable for vg = 0.090 and 0.33 cm sÀ1. Ntotal 672612 for vg = 0.090 cm sÀ1 and 0 cmÀ3 for vg = 1.2 cm sÀ1. linearly increased with vg over the range vg = 0.33–1.6 cm sÀ1 673613 Particle counts recorded for vg = 1.2 cm sÀ1 were equal to (see Figure 5). 674 7 of 17
  8. 8. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX Figure 3. Particle size distributions of dried foam droplets generated using salinity 33% artificial seawater (AS33). Bubbles were generated using a fine-pore diffuser and a bubbling superficial velocity (vg) of 1.2 cm sÀ1. Lognormal distributions from two experiments shown as lines; four runs were made for each experiment, these data are shown as symbols.675 [42] The effect of subsurface bubble diameter on foam [43] We investigated the effect of salinity on foam droplet 685676 droplet production was examined by generating foam size at formation (i.e., wet droplet diameter), which can be 686677 droplets using a fine- and medium-pore diffuser with calculated from Dp;g as678 AS33. Ntotal generated using the medium-pore diffuser was less than that generated using the fine-pore diffuser 679 r mo 1=3680 for each vg (see Figures 5 and 6). Ntotal generated from the Dpo;g ¼ Dp;g ss ð1Þ rsw md681 medium-pore diffuser increased linearly with vg. Larger682 bubbles from the medium-pore diffuser tended to produce where Dp;g is the number geometric mean diameter at 689683 foam droplets slightly smaller than those from the fine-pore formation (98% RH), rss is the density of sea salt (2.2 g cmÀ3) 690684 diffuser. [Lewis and Schwartz, 2004, p. 49], rsw is the density of 691 Figure 4. Particle size distributions of dried foam droplets generated from artificial seawater with salinities in the range 0 – 70%. Bubbles were generated using a fine-pore diffuser and a bubbling superficial velocity (vg) of 1.2 cm sÀ1. 8 of 17
  9. 9. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX Figure 5. Total particle number concentration (Ntotal) as a function of bubbling superficial velocity (vg) for different diffusers and aqueous media: salinity 33% artificial seawater (AS33), 100% filtered seawater (FS100), and 100% unfiltered seawater (UFS100). Unless specified a fine-pore diffuser was used.692 seawater (1.03 g cmÀ3 for salinity 33% seawater), and mo/md mass for 80% RH. The diameters at formation and 80% 700693 is the ratio of seawater to solute mass at formation (for RH conditions were calculated assuming volume additivity 701694 example, mo/md = 1000/33 for salinity 33%) [Lewis and and that the salinity of the wet droplet was that of the 702695 Schwartz, 2004, p. 54]. The geometric mean diameter of aqueous media from which the droplets originated. Foam 703696 foam droplets at 80% RH can also be calculated from Dp;g as droplets formed from low salinity media were significantly 704 larger at formation than those from high salinity media (see 705 Figure 7). r m80 1=3 706 Dp80;g ¼ Dp;g ss ð2Þ [44] The number and sizes of foam droplets were mea- 707 rsw md sured over the salinity range 1 – 70%. The salinity of surface 708698 where Dp80;g is the number geometric mean diameter at seawater typically varies over the much narrower range 33– 709699 80% RH and m80/md is the ratio of seawater mass to solute 37% [Millero and Sohn, 1992]. Using a linear interpolation 710 Figure 6. Particle size distributions of dried foam droplets generated using fine-pore and medium-pore diffusers and a bubbling superficial velocity (vg) of 1.2 cm sÀ1. 9 of 17
  10. 10. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX Figure 7. Geometric mean diameters for particles generated from artificial seawater with sea salt mass fractions of 0.001 – 0.07 (salinities 1 – 70%). Particles were dried before measurement; particle diameters at 80% RH and formation (wet, 98% RH) were calculated from dried diameters. Bubbles were generated using a fine-pore diffuser and a bubbling superficial velocity (vg) of 1.2 cm sÀ1.711 of the present laboratory results for salinities 10– 70%, it is artificial seawater (FS10) by volume, and 1% filtered seawater 747712 estimated that Ntotal would vary by less than 5% over the and 99% artificial seawater (FS1). Foam droplets were gener- 748713 oceanic salinity range. AS1 was excluded from the interpo- ated from FS10 and FS1 using vg = 1.2 cm sÀ1. Ntotal from 749714 lation since the size distributions of bubbles in such low FS100, FS10, and FS1 were similar (see Figure 8); these were 750715 salinity waters appears to differ qualitatively from that with indistinguishable from each other, except for FS100 and 751716 salinities greater than %10% [Lewis and Schwartz, 2004, 460 FS10 for which Ntotal differed by less than 5%. 752717 p. 249]. Foam droplet size would also not differ significantly [47] Foam droplets were also generated from FS100 753718 over the range 33– 37%. In some instances, surface seawater collected from the SIO Pier in February 2006. Ntotal for 754719 salinity can be much lower (for example, the Black Sea has a February 2006 FS100 was approximately 20–40% higher than 755720 salinity of 17%) or slightly higher (for example, the Red Sea that from June 2004 FS100 for vg = 0.090 and 0.33 cm sÀ1. 756721 has a salinity of 40%) than the typical range described. Ntotal Ntotal increased linearly with vg for both FS100 media (see 757722 of foam droplets produced from salinity 17% seawater Figure 5). Ntotal from the two FS100 media were indistin- 758723 would be 30% lower than that produced from salinity guishable for vg = 1.2 cm sÀ1 and differed only slightly (3%) 759724 33% seawater. for vg = 1.6 cm sÀ1. The seawater had a slightly lower DOC 760 concentration, 2.28 mg Cl-1 than that collected in June 2004, 761725 3.5. Effect of Organic Matter 3.07 mg ClÀ1. The summer seawater (June 2004) had a 762726 [45] The effects of natural DOM and POM on foam droplet chlorophyll concentration, 1.8 mgmÀ3, one order of magni- 763727 production were examined using filtered and unfiltered tude greater than in the winter seawater (February 2006), 764728 seawater. The seawater collected from the SIO Pier in June 0.10 mgmÀ3. The seasonal differences in DOC composition 765729 2004 had a DOC concentration of 3.07 mg Cl-1, characteristic may have affected foam droplet production for low vg. 766730 of coastal seawater. A comparison of foam droplet production [48] Foam droplets from seawater solutions with organic 767731 from 100% unfiltered seawater (UFS100) and filtered sea- matter can be compared to those from AS33 which had no 768732 water (FS100) collected in June 2004 demonstrated that the DOM. For vg ! 1.2 cm sÀ1, foam droplets generated from 769733 presence of POM led to an approximately 35– 40% increase AS33 and FS100 (June 2004 and February 2006) were 770734 in Ntotal relative to FS100 for vg = 0.090 and 0.33 cm sÀ1 (see indistinguishable. For lower bubbling superficial velocities, 771735 Table 1). The enhanced droplet production for UFS100 vg = 0.090 and 0.33 cm sÀ1, N total from AS33 was 772736 relative to FS100 was smaller (%10%), but significant for approximately constant, while the presence of DOM and 773737 vg = 1.2 and 1.6 cm sÀ1 (see Figures 5 and 8). Ntotal generated POM leads to Ntotal either significantly greater or less than 774738 from UFS100 and FS100 increased linearly with vg (see that from AS33. 775739 Figures 5 and 9). The presence of POM did not appear to [49] Droplets were generated from artificial seawater to 776740 affect the particle size distribution, which was unimodal and which oleic acid, which is sometimes used as a surrogate 777741 centered at Dp % 100 nm (dN/dDp mode was centered at Dp % for natural DOM, was added in concentrations from 0.1 to 778742 80 nm). 10 mg lÀ1. For vg = 0.090 and 1.2 cm sÀ1, the addition of 779743 [46] The effect of natural DOM on foam droplet production 0.1 mg lÀ1 oleic acid (OA0.1), 1 mg lÀ1 (OA1), and 10 mg lÀ1 780744 was also examined using mixtures of salinity 33% artificial (OA10) caused significant increases in Ntotal relative to 781745 seawater and filtered seawater collected in June 2004. Mix- FS100 (see Figure 10). Ntotal was dependent on oleic acid 782746 tures were prepared containing 10% filtered seawater and 90% concentration up to 1 mg lÀ1; Ntotal for OA1 and OA10 were 783 10 of 17
  11. 11. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX Figure 8. Particle size distributions of dried foam droplets generated from the following solutions: 100% unfiltered seawater (UFS100), 100% filtered seawater (FS100), 10% filtered seawater and 90% artificial seawater (FS10), 1% filtered seawater and 99% artificial seawater (FS1), and salinity 33% artificial seawater (AS33). Bubbles were generated using a fine-pore diffuser and a bubbling superficial velocity (vg) of 1.2 cm sÀ1.784 indistinguishable. Though Ntotal was affected markedly by [50] The nature of organic matter appears to affect foam 794785 oleic acid, the particle size distributions produced from each droplet production, especially for low vg. June 2004 FS100, 795786 oleic acid media were unimodal and centered at Dp % 90 nm. February 2006 FS100, and OA1 have DOC concentrations 796787 The magnitude of the oleic acid effect on foam droplet in the range 1 – 3 mg ClÀ1. Ntotal produced from these three 797788 concentration varied with particle diameter. For example, the media differed by up to 150%. The addition of oleic acid 798789 concentrations of 30, 50, and 90 nm particles generated from suppressed foam formation and led to increase in subsurface 799790 OA10 using vg = 1.2 cm sÀ1 were factors of 3.1, 1.9, and 1.4, bubble diameters relative to FS100. The morphologies of 800791 respectively, higher than that generated from AS33. This size foams formed from June 2004 FS100 and UFS100, and 801792 dependent effect of oleic acid on foam droplet concentration February 2006 FS100, were qualitatively identical; how- 802793 was not observed for media containing natural organic matter. ever, Ntotal from these samples differed by up to 40% for 803 Figure 9. Particle size distributions of dried foam droplets generated from June 2004 filtered seawater (FS100) using a fine-pore diffuser and bubbling superficial velocities (vg) in the range 0 to 1.6 cm sÀ1. 11 of 17
  12. 12. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX Figure 10. Particle size distributions of dried foam droplets generated from the following solutions: salinity 33% artificial seawater containing 0 (AS33), 0.1 (OA0.1), 1.0 (OA1), and 10 (OA10) mg lÀ1 oleic acid. Bubbles were generated using a fine-pore diffuser and a bubbling superficial velocity (vg) of 1.2 cm sÀ1.804 vg 0.33 cm sÀ1. Below we use the variability in Ntotal [1968] is not available but is estimated to be in the range 839805 which results from differences in natural organic matter 0.1– 0.5 cm sÀ1. Here Ntotal from OA10 was 1.8 times 840806 to bound estimates of foam droplet flux over the world’s greater than that from UFS100 for vg = 0.090 and 1.2 cm sÀ1. 841807 oceans. This agrees well with Garrett [1968], who found that the 842 addition of 12.5 mg lÀ1 oleic acid to a 30% NaCl solution led 843809 4. Discussion to a factor of 1.6 increase in Ntotal relative to unfiltered 844 seawater. Note that in both of these experiments, oleic acid 845810 4.1. Comparison With Other Measurements was added in concentrations comparable to that of total 846811 [51] The present size distributions compare well with natural DOC. However, oleic acid makes up approximately 847812 other foam droplet laboratory experiments. Laboratory 10À5 of natural DOC mass [Marty et al., 1979]. Thus it is not 848813 foams generated by wave breaking simulation [Woolf et surprising that droplet production from saline solutions with 849814 al., 1987], bubbling through frits [Hoppel et al., 1989; oleic acid was substantially different from droplet production 850815 Martensson et al., 2003; Sellegri et al., 2007], splashing ˚ from natural seawater. 851816 with a weir [Sellegri et al., 2007], and bubbling through a [53] Thermal volatility, ionic composition, and particle 852817 diffuser frit (this paper) all produce dried droplets with flux measurements indicate a submicrometer SSA particle 853818 peak number concentrations in the submicrometer size mode can be present in the range Dp = 40– 200 nm [O’Dowd 854819 range. This paper demonstrates a dN/d logDp mode at and Smith, 1993; Murphy et al., 1998; Kreidenweis et al., 855820 Dp %100 nm (dN/dDp mode at Dp %80 nm), which falls 1998; Nilsson et al., 2001; Clarke et al., 2003, 2006]. The 856821 within the range of reported values, Dp = 50– 110 nm, for present results, that sea foams produce large numbers of SSA 857822 the location of the submicrometer SSA mode [Hoppel et particles with a mode at Dp % 100 nm, are consistent with 858823 al., 1989; Martensson et al., 2003; Sellegri et al., 2007]. ˚ these measurements. Other investigators have found low 859824 The variation in foam droplet diameter with water tempera- concentrations of accumulation mode particles that contain 860825 ture may explain variations in size distributions observed in Na [Meszaros and Vissy, 1974; Gras and Ayers, 1983; ´ ´ 861826 the MBL [Martensson et al., 2003; Sellegri et al., 2007]. ˚ Jennings and O’Dowd, 1990; Bigg et al., 1995]. Since the 862827 Martensson et al. [2003] also reported a coarse dN/d logDp ˚ accumulation mode is almost always observed in the MBL 863828 mode for foam droplets at Dp %2.5 mm. Supermicrometer [Heintzenberg et al., 2004], these measurements indicate 864829 particles could not be detected in the present experiments. that a second source, such as growth of Aitken mode 865830 [52] Laboratory experiments have shown that the pres- particles that do not contain Na [Hoppel and Frick, 866831 ence of oleic acid affects droplet production from single 1990], can also contribute to accumulation mode aerosol 867832 bubbles [Blanchard, 1963; Morelli et al., 1974] and foams number concentrations. 868833 [Garrett, 1968]. In these experiments, oleic acid was either834 added to seawater or NaCl solutions in order to approximate 4.2. Estimation of Oceanic Foam Droplet Flux 869835 the effect of natural organic matter [Blanchard, 1963; [54] SSA particle production is commonly estimated from 870836 Garrett, 1968; Morelli et al., 1974]. In the work of Garrett wind speed at an elevation of 10 m, U10 [Andreas et al., 871837 [1968] and the present work, foams were suppressed upon 1995]. These parameterizations are motivated by the rela- 872838 addition of oleic acid. The vg for the experiments of Garrett tion between wind speed and whitecap coverage. Estimates 873 12 of 17
  13. 13. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX874 of supermicrometer SSA particle flux vary by more than an [57] Estimates of dFw/d logDp80 were bounded by a low 926875 order of magnitude even for similar wind speeds [Andreas and high estimate, which correspond to the dN/d logDp80 927876 et al., 1995; Lewis and Schwartz, 2004]. Although labora- distributions obtained from June 2004 FS100 for vg = 928877 tory experiments have found that bursting bubbles produce 0.090 cm sÀ1 and UFS100 for vg = 1.6 cm sÀ1, respectively 929878 submicrometer SSA particles [Mason, 1957; Cipriano et al., (see Figure 11). The foam droplet fluxes from these experi- 930879 1983; Martensson et al., 2003; Sellegri et al., 2007], few ˚ ments cover the range of fluxes from all the experiments with 931880 parameterizations of submicrometer SSA particle flux are AS33, FS1, FS10, FS100, and UFS100 at all values of vg. The 932881 available. It has been hypothesized that variations in sub- variability due to vg is 5 which is the maximum variability of 933882 micrometer SSA particle concentration in the MBL could be Ntotal over vg = 0.090 – 1.6 cm sÀ1 for any single media. The B 934883 explained in part by variations in the concentration and symbol denotes a multiplicative variability. The variability 935884 nature of seawater organic matter thought to affect SSA due to organic matter is estimated to be B1.5 which is the 936885 particle production [Reid et al., 2001; Geever et al., 2005]. maximum variability of Ntotal between media for any single 937886 Seawater organic matter is known to affect whitecap foam vg. We assume that these sources of variability are indepen- 938887 formation and decay [Garrett, 1967] and was shown in the dent, so that the total variability in extrapolating the labora- 939888 laboratory to affect SSA particle production from bubble tory measurements to estimate dFw/d logDp80 from sea foams 940889 bursting [Garrett, 1968; Paterson and Spillane, 1969]. is B7.5. 941890 [55] Parameterizations of SSA flux can be developed [58] The variability in dFo/d logDp80 is also subject to 942891 from the number concentrations measured here by first uncertainty in the parameterization of W as a function of U10 943892 estimating the size-dependent flux per whitecap foam area, [see equation (5)]. Uncertainty in W has been estimated as 944893 dFw/d logDp80, by the flux per foam area in the laboratory B7 for U10 = 8.5 msÀ1 and B5 for U10 = 15 msÀ1 [Lewis 945 and Schwartz, 2004, p. 266]. The total uncertainty in dFo/d 946 dFw dN logDp80 including variability in dFw/d logDp80 is then B53 947 ¼ vg ð3Þ for U10 = 8.5 msÀ1 and B38 for U10 = 15 msÀ1. 948 d log Dp80 d log Dp80 [59] The present estimates of foam droplet flux were 949 compared to the supermicrometer SSA particle flux esti- 950895 dFw/d logDp80 can then be scaled to the size-dependent flux mates of Monahan [1986], applicable for Dp80 = 1.6– 20 mm, 951896 per ocean area, dFo/d logDp80 and those of Smith et al. [1993] (Dp80 = 2 – 50 mm) (see 952 Figure 11). Comparisons were made for selected values of 953 dFo dFw U10, 6 msÀ1 (W = 0.002) and 15 msÀ1 (W = 0.04). The 954 ¼W ð4Þ d log Dp80 d log Dp80 parameterization of Monahan [1986] is based on laboratory 955 whitecap measurements. The parameterization of Smith et al. 956898 where W is the fraction of the sea surface covered by whitecap [1993] is based on concentration measurements at 10-m 957899 foams, which has been parameterized by Monahan [1986] as height. For the purposes of comparison, each parameteriza- 958 tion was converted to dFo/d logDp80 and particle diameter at 959 80% RH, Dp80, using the relationship Dp80 = 2Dp. At W = 960 W ¼ 3:84 Â 10À6 U10 3:41 ð5Þ 0.002, the total foam droplet flux per ocean area, Fo, for 961 particles in the accumulation mode size range (Dp80 = 962902 [56] In order to estimate dFw/d logDp80 from these experi- 170– 500 nm, Dp = 85– 250 nm), is approximately 10 and 963903 ments, one must estimate the range of laboratory vg that are 100 times Fo for supermicrometer SSA particles according 964904 representative of oceanic whitecap foams. At sea, a dense to the parameterizations of Monahan [1986] (Dp80 = 1.6– 965905 subsurface bubble plume forms just after wave breaking 20 mm) and Smith et al. [1993] (Dp80 = 2 – 50 mm), 966906 [Monahan and Lu, 1990]. As the wave stops spilling, a respectively. 967907 residual, decaying foam patch is present. The characteristic [60] Submicrometer SSA particle flux estimates made 968908 time for whitecap foam decay is approximately 5 s [Monahan here can be compared with those of Ma ˚rtensson et al. 969909 et al., 1980]. In contrast, the foams generated in the present [2003], applicable for a seawater temperature of 20°C and 970910 experiments were approximately constant in thickness and Dp80 = 0.04 – 5.6 mm; and with those of Clarke et al. [2006] 971911 constrained by glass walls. It is expected that the character- (Dp80 = 0.02 – 16 mm). The parameterization of Martensson ˚ 972912 istic lifetime of foam bubbles in the laboratory apparatus is et al. [2003] is based on laboratory foam droplet measure- 973913 more uniform than that on the oceans. Bubbling through ments. The parameterization of Clarke et al. [2006] is based 974914 100% unfiltered seawater (UFS100) using vg = 0.090, 0.33, on number distributions of nonvolatile particles measured 975915 1.2, and 1.6 cm sÀ1 produced foam thicknesses of 0.2, 0.8, 2, above a surf zone. The parameterizations of Martensson et ˚ 976916 and 3 cm, respectively; all of these are similar to oceanic al. [2003] and Clarke et al. [2006] fall within the range of 977917 whitecap foam thickness, %1 cm [Wilheit, 1979]. The range oceanic submicrometer SSA particle fluxes estimated from 978918 of vg tested here appears to produce foam morphologies the present results in the size range Dp80 = 60 –800 nm (see 979919 which span whitecap foams at all stages. Therefore we Figure 11). Martensson et al. [2003] used salinity 33% ˚ 980920 estimate oceanic flux of submicrometer foam droplets and artificial seawater; vg was not reported but was estimated to 981921 its variability using the laboratory measurements over the be vg 0.06 cm sÀ1. Direct comparisons between the 982922 range vg = 0.090 – 1.6 cm sÀ1. The total flux from whitecap parameterization of Martensson et al. [2003] can be made ˚ 983923 foams is expected to be within the fluxes estimated here for with the present results for AS33 and vg = 0.090 cm sÀ1; the 984924 extreme values of vg because foam droplets are produced over flux of all particles with Dp80 = 200 nm was more than an 985925 all stages of whitecap evolution. order of magnitude lower in the present results. The present 986 results include a range of bubbling conditions and compo- 987 13 of 17
  14. 14. XXXXXX TYREE ET AL.: SEAWATER FOAM DROPLETS XXXXXX Figure 11. SSA particle flux estimates for two fractional whitecap coverages, W, of 0.002 (light grey), and 0.04 (dark grey), characteristic of wind speeds at 10 m height, U10, equal to 6 and 15 msÀ1, respectively. Foam droplet flux estimates, shown as shaded regions, were extrapolated from the present laboratory results. The width of the shaded regions corresponds to variability in foam droplet production observed due to superficial velocity (vg) and organic matter composition. The black region depicts where the estimates of foam droplet flux for two values of W overlap. The best estimates of foam droplet flux are shown as dashed lines.988 sitions at the ocean, our best estimate of submicrometer containing sodium can also comprise a significant fraction 1020989 SSA particle flux was calculated as the mean of the UFS100 of accumulation mode particles by number [O’Dowd and 1021990 experiments conducted at four different vg (see Figure 11). Smith, 1993; Murphy et al., 1998]. It has been hypothesized 1022991 Because the best estimate of the total particle flux per that the sources of Na in MBL accumulation mode aerosol are 1023992 whitecap foam area, Fw, is larger than that for the AS33 (1) foam bubble bursting [Clarke et al., 2003; Martensson˚ 1024993 and vg = 0.090 cm sÀ1 experiments, the flux estimates et al., 2003; Sellegri et al., 2007], (2) the tail of coarse mode 1025994 of Martensson et al. [2003] were comparable to our best ˚ SSA particles [Bates et al., 1998], and (3) the growth of 1026995 estimate. This best estimate of total flux, Fo = 980 B Aitken mode particles that contain at least some Na [Murphy 1027996 7.5 cmÀ2 sÀ1 for W = 0.002, was comparable to the estimates et al., 1998; Pierce and Adams, 2006]. The present experi- 1028997 of Martensson et al. [2003] (Fo = 1300 cmÀ2 sÀ1) and that of ˚ ments support the hypothesis that the direct emission of SSA 1029998 Clarke et al. [2006], Fo = 700 cmÀ2 sÀ1. particles from foams is also an important source of Na in the 1030 ‘‘accumulation’’ mode. Natural organic matter did not affect 10311000 5. Conclusions foam droplet diameter, and therefore does not explain the 1032 variability in reported mean diameters for submicrometer 10331001 [61] The present experiment was designed to study foam SSA particles. 10341002 and foam droplet production at sea. Foams were formed by [63] Foam droplet flux was affected by up to 40% due to 10351003 streams of bubbles which had subsurface diameters smaller natural DOM and POM. This effect was determined from 10361004 than 500 mm. Foam droplets produced from the resulting two seawater samples collected from the same location, but 10371005 foams had unimodal size distributions with a dN/d logDp in different seasons. The effect of natural organic matter on 10381006 mode at a dry diameter of %100 nm. These results agree foam droplet flux was complex; natural organic matter was 10391007 with prior laboratory foam droplet experiments [Martensson ˚ shown to increase or decrease foam droplet flux, an effect 10401008 et al., 2003; Sellegri et al., 2007] and were consistent with which depended on (1) POM, (2) bubbling superficial 10411009 the submicrometer SSA particle modes commonly observed velocity, and (3) seasonal variations in the composition of 10421010 in the remote MBL [O’Dowd and Smith, 1993; Murphy DOM. The effects of bubbling flow rate and natural organic 10431011 et al., 1998; Bates et al., 1998; Nilsson et al., 2001]. The matter on foam droplet flux variability were estimated to be 10441012 approach used here is a convenient method to generate B5 and B1.5, respectively. 10451013 surrogate SSA particles in the laboratory. [64] Droplet production was also affected by surrogate 10461014 [62] The presence of Na and Cl in accumulation mode organic matter. The addition of 1.0 and 10 mg lÀ1 of oleic 10471015 (Dp = 85– 250 nm) particles indicates sea salt makes up a acid to saline solutions increased droplet production relative 10481016 substantial fraction of these particles in the MBL [Huebert to natural seawater by approximately a factor of 2. This 10491017 et al., 1998; Bates et al., 1998; Murphy et al., 1998]. direct correlation between droplet production and oleic acid 10501018 Sodium can comprise a significant fraction of the accumu- was not observed for natural organic matter. Furthermore, 10511019 lation mode particle mass [Bates et al., 1998]. Particles foams formed readily on natural seawater but were sup- 1052 14 of 17