AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-4257-2016A miniature Marine Aerosol Reference Tank (miniMART) as a compact breaking
wave analogueStokesM. DaleDeaneGrantCollinsDouglas B.https://orcid.org/0000-0002-6248-9644CappaChristopherhttps://orcid.org/0000-0002-3528-3368BertramTimothyhttps://orcid.org/0000-0002-3026-7588DommerAbigailSchillStevenForestieriSaraSurviloMathewMarine Physical Laboratory, Scripps Institution of
Oceanography, La Jolla, CA, USADept. of Chemistry, University of Toronto, Ontario,
CanadaDept. of Civil and Environmental Engineering, University
of California, Davis, CA, USADept. of Chemistry, University of Wisconsin, Madison, WI,
USAM. Dale Stokes (dstokes@ucsd.edu)1September2016994257426727April201619May201619July20168August2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/4257/2016/amt-9-4257-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/4257/2016/amt-9-4257-2016.pdf
In order to understand the processes governing the production of
marine aerosols, repeatable, controlled methods for their generation are
required. A new system, the miniature Marine Aerosol Reference Tank
(miniMART), has been designed after the success of the original MART system,
to approximate a small oceanic spilling breaker by producing an evolving
bubble plume and surface foam patch. The smaller tank utilizes an
intermittently plunging jet of water produced by a rotating water wheel, into
an approximately 6 L reservoir to simulate bubble plume and foam formation
and generate aerosols. This system produces bubble plumes characteristic of
small whitecaps without the large external pump inherent in the original MART
design. Without the pump it is possible to easily culture delicate planktonic
and microbial communities in the bulk water during experiments while
continuously producing aerosols for study. However, due to the reduced volume
and smaller plunging jet, the absolute numbers of particles generated are
approximately an order of magnitude less than in the original MART design.
Introduction
Sea spray aerosols (SSAs) are generated over a large portion of the Earth's
surface and form a large fraction of aerosol particulates present in the
atmosphere (e.g., Lewis and Schwartz, 2004). They are critically important
components in global biogeochemical cycles (e.g., Solomon et al., 2007) and
important modifiers of atmospheric radiative budgets. Marine aerosols are
generated primarily by processes associated with the formation of bubble
plumes and foams generated by the actions of breaking surface waves.
Breaking waves themselves play an important role in many additional
processes at the air–sea interface including mixing, current formation, heat
and momentum flux, and the bubbles entrained by breaking waves enhance gas
transport, scavenge biological surfactants, and generate ambient noise in
addition to creating aerosol particles (e.g., Woodcock, 1953; Wallace and
Duce, 1978; Rapp and Melville, 1990; Tseng et al., 1992).
Oceanic whitecaps (which are the high optical albedo footprint of a breaking
surface wave) typically form once wind speeds greater than approximately 3 ms-1 blow over a sea surface of sufficient fetch. Breaking itself
includes the impaction of the overturning wave crest with the sea surface
and subsequent entrainment and fragmentation of air into a plume of bubbles.
The plume evolves over a timescale of seconds to a few tens of seconds due
to buoyancy and turbulent flow forces acting on the entrained bubbles. The
air / water mixture of the breaking wave crest and the bubbles that reach the
sea surface after breaking form the high albedo patch characteristic of a
whitecap. Surface bubbles and the dense aggregations of bubbles that create
surface foams are the primary source of marine aerosols as the bubbles
rupture and produce a spray of jet and fluid film droplets that are ejected
into the atmosphere (Lewis and Schwartz, 2004).
In order to study marine aerosol production it is beneficial to have a
standardized method of creating them that mimics the formation processes
associated with marine foam in repeatable, controlled conditions in the
laboratory. 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). 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. Using a modification of the prior plunging water techniques, Stokes
et al. (2013) developed the Marine Aerosol Reference Tank (MART) system that
accurately reproduced the bubble plumes and marine aerosols characteristic
of an oceanic whitecap. By using an intermittent plunging sheet of water in
a larger (210 L) tank bubble plumes are formed that mimic the oceanic bubble
size distribution, including critical bubbles larger than the Hinze scale
(the transition point between bubbles stabilized by surface tension and
bubbles subject to fragmentation by turbulence at approximately 1 mm scale),
and have a temporal evolution similar to plumes measured in the ocean and in
large laboratory wave tanks.
Whitecap foam and bubble size distributions
The two primary production mechanisms of sea spray aerosols at moderate wind
speeds are the disintegration of the thin fluid films associated with
whitecap foam (film drops) and the breakup of the jet of water formed at the
base of a bubble shortly after the rupture of its film (jet drops). Both of
these mechanisms are known to be sensitive to bubble size. It follows that an
essential requirement of any laboratory system designed to produce nascent
SSA is the reproduction of the numbers and sizes of bubbles entrained by
breaking waves in the open ocean. Few bubble size distributions from natural
breaking waves have been acquired because of the difficulty of making
measurements in stormy conditions and other natural hazards (Herrero, 1985;
Melville, 1996; de Leeuw and Cohen, 2002; Stokes et al., 2002). However, some
oceanic measurements are available as well as a number of laboratory studies
(e.g., Monahan and Zeitlow, 1969; Cipiriano and Blanchard, 1981; Bezzabotnov
et al., 1986; Lamarre and Melville, 1994; Loewen et al., 1995; Leighton et
al., 1996; Deane and Stokes, 2002; de Leeuw and Cohen, 2002; de Leeuw and
Leifer, 2002; Leifer and de Leeuw, 2002, 2006; Stokes et al., 2002) and are
summarized in Fig. 1 of Stokes et al. (2013). It is now known that there is a
scale dependence to the bubble creation physics, differentiated by a length
scale known as the Hinze scale (Deane and Stokes, 2002). The Hinze scale
(aH) defines the radius of a bubble for which surface tension
forces, which tend to keep bubbles spherical, are disrupted by distorting
pressure fluctuations associated with fluid turbulence. This scale is of the
order of 1 mm in spilling and breaking waves. Bubbles smaller than the Hinze
scale are stabilized to fragmentation by fluid turbulence, whereas bubbles
larger than this scale are subject to a turbulent fragmentation cascade.
Inter-comparison of bubble size distributions from a laboratory
breaking wave, the plunging jet in the miniMART system, the original MART
system, and two distributions from sintered glass filters. The breaking wave
distribution is in absolute units; the MART and sintered glass filter bubble
distributions have been scaled as described in the text. The sloped solid
lines indicate size distribution scaling laws as measured from oceanic
bubble plumes showing the change in slope at the bubble Hinze scale (where
the lines intersect). Additional information on the size distributions can
be found in Stokes et al. (2013).
The power law dependence of the bubble size distribution as a function of
bubble radius is also different for bubbles smaller and larger than the
Hinze scale. Smaller bubbles have a somewhat variable power law scaling,
a-n with n taking values between approximately 1 to 2. The physics of
bubble fragmentation and bubble degassing drives a steeper power law
dependence for bubbles larger than the Hinze scale with n taking values
between approximately 3 to 4 (Fig. 1). Important points are that (1) breaking
oceanic whitecaps can produce large bubbles, greater than 1 mm radius and up
to 4 mm radius (Bowyer, 2001), and (2) the power law scaling of the generation
of these bubbles is controlled by fluid turbulence within the whitecap and
differentiated by the Hinze scale. In order to accurately reproduce nascent
SSA, the laboratory bubble generation mechanism needs to produce bubbles
larger than the Hinze scale and reproduce the power law dependence those
bubbles acquire through fragmentation in fluid turbulence.
The miniature Marine Aerosol Reference Tank (miniMART)
The original MART system was constructed to closely mimic the bubble plume,
foam, and aerosol generating mechanisms active during oceanic wave breaking
and to provide a portable, controllable environment in which to explore and
sample these processes (Stokes et al., 2013). The primary design of MART
included a flow-controlled closed-loop circulation system that draws water
from the tank bottom, a tank-top spillway or waterfall to produce a plunging
sheet that impacts the water surface within the tank to produce a bubble
plume, and an air-tight headspace for controlled aerosol sampling while the
system is operating. By varying the temperature of the tank contents, the
water chemistry and the characteristics of the plunging sheet (volume, angle
and distance of drop, timing of the intermittency) a wide range of
experimental conditions can be realized. The tank itself can also be used as
an incubator for the growth of planktonic organisms to investigate the
influence of biogenic exudates on SSA formation (Lee et al., 2015). A
limitation with the MART system is that it can be difficult to culture
delicate organisms in the reservoir while the external circulation pump
(1/3 HP centrifugal pump) is operational because the high flow rates (70 L min-1 within the pump casing and up to 15 L min-1 in the waterfall
flow) create high levels of fluid shear that is damaging to fragile cells.
Hence, when including cultured cells in the experimental system it is
necessary to limit pump cycling (and aerosol generation) to after the
culture has reached its exponential growth phase or reached a cell density
where losses due to pump cycling do not exceed cell creation rates.
The miniMART system (Fig. 2) described here was designed to provide a
gentle method of plunging jet generation that would minimize destructive
shear on cultured organisms and still permit the continuous generation of
aerosols for study. It was fabricated using components that are readily
available and constructed of stainless steel, Plexiglas and silicone
wherever possible to minimize chemical contaminants and facilitate cleaning.
The main tank (25 × 25 × 30 cm, 19 L total volume) was made from 1.5 cm
thick Plexiglas with an O-ring sealed, 20 mm thick Plexiglas lid to
provide airtight integrity. Separate ports are available for sampling both
the atmospheric headspace and subsurface water in the tank. Port sampling
tubes are made from stainless steel and positioned within 1 cm of the water
surface to minimize the effects of particle losses to the Plexiglas tank
walls.
Image of miniature Marine Aerosol Reference Tank (miniMART). The
primary tank (25 × 25 × 30 cm, 19 L total volume) is made from 1.5 cm thick
Plexiglas with an O-ring sealed, 20 mm thick Plexiglas lid. The
intermittent plunging jet (70 mL volume) is formed by water escaping from
alternating chambers in a rotating water wheel (20 cm diameter, 8 cm wide)
labeled (a) and powered by an external, 8 RPM motor (c) connected to the
wheel by a sealed shaft. An exit port on the wheel (indicated by the white
star) allows the water to fall approximately 10 cm from the wheel to the
water surface. The tank is filled with approximately 6 L of water to the
water fill line indicated by the arrowhead (<). A vertical
stainless steel aerosol sampling tube (b) penetrates the tank lid for
sampling near the water surface. Additional ports are located in the lid (d)
for gas input and water sampling.
Inside the tank, a 20 cm diameter, 8 cm deep, compartmentalized water wheel,
and fabricated after an ancient sakia design, is rotated at approximately 8 RPM by an externally mounted 1/15 HP motor attached to a shaft-sealed axle
that penetrates the tank rear wall. A thin silicone gasket was used to seal
the compartmentalized wheel to its removable lid, providing access to its
internal surfaces for cleaning. The contact between the silicone and the
water is minimized to a hairline gap between the lid and wheel once
assembled, to mitigate potential effects of the silicone on water surface
microlayer chemistry. The two internal chambers of the wheel provide the
intermittent release of a 70 mL water jet from approximately 10 cm above the
water surface within the tank (when filled with approximately 6 L of water)
via a hole in the chamber wall. The plunging jet sweeps across the water
surface when a chamber crosses the apex of its rotation (maximum height
above the water surface) while the opposite chamber is synchronously filling
beneath the water surface. The plunging jet impacts the water surface and
produces a bubble plume that mimics the plunging jet of water from a
breaking wave crest without the need for a powerful external pump. The small
size of the miniMART system allows it to be partially submerged (to the
water fill line) in a temperature-controlled bath allowing stabilization of
its internal temperature if necessary for plankton culture growth.
Alternatively, the miniMART can be operated in a temperature-controlled
incubator or room, a technique that has proven viable even for the much
larger MART system.
Before experimentation the miniMART system is cleaned to minimize
contamination. The internal surfaces are scrubbed with 100 % percent
isopropanol and then the entire system is filled and the sakia wheel
circulated with a 10 % isopropanol / deionized water solution for
approximately 30 min. After circulation the tank is drained and then rinsed
and filled with deionized water, and the system again circulated. Lastly,
the system is flushed with filtered freshwater or seawater for
experimentation. The system is considered clean when measurements of surface
tension from water samples are the same as those from the filtered water
supply used for experimentation (approximately 72 mNm-1 at room
temperature measured using the Wilhelmy plate method with a Krüss K3
tensiometer.)
Bubble size distribution measurements
To examine the utility of the miniMART system compared to the original MART
and as an oceanic bubble plume proxy, the size distributions of bubbles
within miniMART were compared to those produced by sintered glass filters as
well as to oceanic and laboratory wave channel distributions. The glass
filters were set at a depth of ∼ 25 cm (filter surface to
water surface), and dry nitrogen gas (0.5 L min-1) was pumped through
four filters, two 90 mm diameter type E filters and two 25 mm diameter type
A filters, similar to the setup of Keene et al. (2007). A further
description can be found in Stokes et al. (2013).
The sintered glass filter and plunging sheet bubble size distributions were
obtained utilizing methods described previously by Deane and Stokes (2002).
In brief, bubble plumes were imaged a few centimeters from the side of the
tank using a Nikon high-resolution digital camera (Fig. 3). The distribution
of bubble sizes was then obtained through computer-aided analysis of the
images. The cross-sectional areas of individual bubbles within a selected
image were determined and then transformed into equivalent spherical radii.
These data combined with an estimate of the imaging volume formed the basis of the
bubble size distributions presented in Fig. 1.
Side view of a partial bubble plume generated during miniMART
operation. The white scale bar at top of image is 1 cm. Bubbles both larger
and smaller than the Hinze scale are present. The free-fall distance between
the exit hole of the waterwheel and the water surface is approximately 10 cm
(not seen in photograph).
The reference distribution for a laboratory plunging breaking wave from
Deane and Stokes (2002) is in absolute units of bubbles m-3µm-1 radius increment, which is standard for the oceanographic
literature. The distributions for sintered glass filters and plunging water
were variable, depending on air flow, plunging sheet height and roughness,
among other factors in the MART. To facilitate comparison with the breaking
wave, the bubble size distributions for the sintered glass filters and
plunging waterfall were first converted to probability density functions
(PDFs) and then scaled by 5.6 × 106. The scaling factor was determined to
be the value that brought the miniMART, MART and breaking wave distributions
into agreement at a bubble radius of ∼ 1 mm.
Both MART and miniMART systems approximate the bubble size distribution
scaling laws found in breaking oceanic waves, including the production of
bubbles larger than aH (in this case, approximately 1.5 mm radius).
However, the number of bubbles larger than 0.1 mm radius produced by
miniMART is less than in MART by up to an order of magnitude. For bubbles
smaller than approximately 0.1 mm radius there is a greater concentration in
miniMART than in the original MART; this is attributed to the visible
turbulent suspension of these bubbles in the smaller volume of miniMART and
the buildup of greater concentration as plunging continues, whereas in the
larger volume of the MART system these small bubbles advect away from the
plunging jet and more readily degas at the water surface.
Aerosol size distributions and residence time
Particle size distributions (PSDs) were determined by a commercially
available Scanning Mobility Particle Sizer (SMPS) (Wang and Flagan, 1990) and
Aerodynamic Particle Sizer (APS) (Peters and Leith, 2003). The SMPS measures
particle mobility diameter (dm) by scanning the voltage across two
electrodes within a differential mobility analyzer (DMA) column (TSI, Inc.,
Model 3080). Sampled particles are directed past a 0.058 cm impactor to
remove particles too large for analysis and into the DMA column, which
separates particles by electrical mobility. The range of particle sizes
which can be analyzed with this method is dependent on the aerosol and
sheath flow rates, which were set at 0.6 and 3.0 L min-1, respectively,
which corresponds to particle diameters of approximately 10–600 nm.
Particles selected in the DMA are injected into a condensation particle
counter (TSI, Inc., Model 3010), which counts the particles corresponding to
the sizes selected by the DMA. Reported size distributions are corrected for
diffusive losses of particles using the SMPS processing software.
The APS (TSI Model 3321) determines the aerodynamic diameter (da) of
particles in the 0.542 to 20 µm range by measuring particle
time of flight. Particles were sampled at 5.0 L min-1 (1.0 and 4.0 L min-1, aerosol and sheath flow rates, respectively). To determine
da, particles enter the inlet of the APS and pass between two separate
paths of a continuous wave laser split with a beam splitter. From the transit
time between the laser beams, the aerodynamic diameter can be determined.
For both the SMPS and APS analysis, particles were initially passed through
silica gel diffusion dryers, where they were dried to an RH of 35 ± 3 %. The dm and da size distributions recorded were merged to
obtain an estimate of the geometric physical diameter (dp) size
distribution across the size range of both instruments. For the purposes of
merging, particles sized by the SMPS were assumed to be of a spherical
geometry, which allows for the following relation:
dm=dp.
Particles sized by the APS were assigned an effective density, ρeff, of 2.1 g cm-3, a value determined experimentally, which
allows for conversion based on the following relation:
dp=daρeffρ0
with ρ0 equal to unit density (i.e., 1 g cm-3). Both
instruments had their resolution set to 32 bins per decade for consistency
in merging. The SMPS tends to undercount particles at the high end of the
distribution due to the cut-off from the particle impactor, while the APS
can undercount particles at the low end due to poor scattering efficiency of
the smallest particles. As a result, particle bins in the overlapping size
region of the two methods were subsequently removed, excluding the largest
and smallest bins of the SMPS and APS, respectively (Fig. 4a).
(a) Number concentrations of sea spray aerosol (SSA) generated by
miniMART. The SSA particle diameter was measured at 35±3 % relative
humidity and converted to dry diameter. SSA concentrations were measured
using a TSI Scanning Mobility Particle Sizer (SMPS) for SSA < ∼ 500 nm and a TSI Aerodynamic Particle Sizer (APS) for SSA
> ∼ 600 nm. Concentrations are shown for SSA collected with the miniMART sample tube located within 2, 4, 8 and 15 cm of
the water surface as well as with a cone-shaped flared funnel (7 cm mouth
diameter) positioned approximately 1.5 cm from the water surface. Red filled
circles show number concentrations of SSA diameter from miniMART filled with
a 3.5 % NaCl solution using a SEMS (see Fig. 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 red curve is a fit to the data
assuming a single log-normal distribution (median diameter = 189 nm, width = 2.32). The vertical dashed line at 770 nm indicates the 50 %
mobility-equivalent cutoff diameter for the SEMS impactor.
Particle sampling was conducted via a 10 mm internal diameter stainless
steel tube passed through a sealed gland in the miniMART lid and positioned
with its inlet above the bubble plume. The inlet was positioned at 2, 4, 8
and 15 cm above the water surface and additional samples were taken with a
cone-shaped flared funnel (7 cm mouth diameter) attached to the end of the
sampling tube and positioned approximately 1.5 cm from the water surface.
The greatest number concentration of particles was collected when the inlet
was positioned closest (2 cm) to the water surface and the number
concentration decreased with increasing inlet height. This is most evident
in the APS data, whereas the SMPS data showed light variation attributed to
the noise in the sample signal due to the smaller number of particles
counted by the CPC in each individual size bin during an SMPS scan. The
addition of the cone to the inlet decreased the number of particles
collected, particularly in the smaller size particles (< 2 µm) perhaps due to differential deposition on the cone walls.
During miniMART operation, carrier gas (either N2 or zero air) is
supplied to the sealed tank at flow rates ranging between 1 and 10 slpm
depending on instrument sampling requirements. The carrier gas flow,
combined with particle deposition within the tank, determines the average
lifetime of a particle in the system prior to sampling. The e-folding time
with respect to mixing is set by the headspace volume (∼ 10 L)
and the carrier gas flow rate. For the three flow rates studied here (1.6,
2.6, and 3.6 slpm) the average particle lifetimes with respect to mixing are
5.6, 3.4, and 2.5 min, respectively. To assess deposition within the
tank, we arrest plunging and particle production and monitor the decay in
the size-dependent number concentration. Size-dependent decay rates are
shown in Fig. 5 as a function of carrier gas flow. The deviation in the
decay from that determined from mixing alone is a low bound on particle
deposition within the tank. Actual deposition rates are likely faster when
the water wheel is turning and the jet is plunging. As shown in Fig. 5,
particle deposition is strongly size dependent, where the observed particle
lifetimes span between approximately 1 and 4 min for a carrier gas flow
rate of 1.6 slpm.
Normalized size-dependent decay rates in particle number
concentration (cm-3) for three different dilution air flow conditions:
1.6 standard L min-1(a), 2.6 slpm (b), and 3.6 slpm (c). Particle number concentrations
are shown for size classes 0.1–0.55 µm (from the SMPS), 0.56–1.0, 1.0–3.2, and 3.4–10 µm (from the APS). The
associated e-folding lifetimes (τ) for each flow condition and size
regime, and the expected decay rates from dilution alone are discussed in
the text.
In a separate experiment, aerosol PSDs from a separate miniMART were
characterized using a Scanning Electrical Mobility Sizer (SEMS) instrument
(BMI Model 2002). The SEMS is similar to the SMPS in that particles are
characterized according to their electrical mobility diameters. However, the
SEMS DMA design allows for measurement to larger mobility diameters. Here,
the range of measured diameters was 10.3 to 946 nm. The SEMS was operated
with an impactor with a 50 % cutoff da∼ 1,150 nm at the
0.36 L min-1 sample flow rate, which corresponds to a dm∼ 770 nm, assuming ρeff=2.1 g cm3. The effective
averaging time at each size, which determines the particle counting
statistics, was either 5 or 10 s; the results from both were similar so only
the 10 s results are presented here. The measured size distributions were
corrected for diffusive losses within the SEMS assuming that the effective
length of the SEMS (consisting of the DMA column, 210Po bipolar
diffusion charger, 30 cm. Nafion dryer and other tubing) was 11 m
(Wiedensohler et al., 2012). In the experiment using the SEMS, the flow rate
of carrier gas through miniMART was 0.86 slpm, which is lower than that in
the SMPS + APS experiments discussed above. Particles were sampled from
miniMART through a silica gel diffusion dryer (RH < 20 %) and then
the flow was split to the SEMS (0.36 L min-1) and to the atmosphere (0.5 L min-1).
The tank was filled to 13 cm from the bottom of the tank with a 3.5 % NaCl
solution in Milli-Q water. The 9.5 mm OD (7.5 mm ID) stainless steel sampling
tube was positioned 2 cm above the water surface and the tube inlet was cut
at 45∘ to prevent clogging with water. A total of 16 sequential
PSD scans were measured after the system reached steady state. The average
of these 16 scans is shown in Fig. 4b. The mode peak of the SEMS PSD was
around 200 nm, similar to other results and similar to that for MART in
Stokes et al. (2013).
Although the average PSD from the miniMART measured using the SEMS peaks in
the same general size range as the SMPS, there are distinct differences. In
particular, the SEMS measurements indicate a more substantial falloff in
concentration towards smaller sizes than do the SMPS measurements. The SEMS
and APS measurements are in reasonable agreement in terms of the shape of
the distribution at larger sizes. The greater apparent falloff in the SEMS
PSD at small sizes could indicate that the internal diffusion correction
applied was too small (or too large in the SMPS) or that diffusional losses
between the miniMART and sizing instrumentation were larger in the SEMS
experiments, perhaps due to the smaller flow rate. Future experiments in
which the SMPS and SEMS are simultaneously used to characterize PSDs from
the miniMART will help to resolve this discrepancy. Regardless, the
generally good correspondence of the PSDs from miniMART with PSDs of nascent
SSA from breaking waves (Prather et al., 2013) and the MART (Stokes et al.,
2013) suggests that the miniMART can operate as a suitable SSA mimic.
Comparison of miniMART to other generation methods
As noted by Sellegri et al. (2006) and Fuentes et al. (2010), a plunging
water jet best replicates the bubble plumes generated by an oceanic
whitecap. Comparison of the bubble plume formed by the miniMART system to
those generated by air flow through sintered glass filters and to those
formed in oceanic waves and within the larger MART system (Fig. 1)
illustrates that a plunging sheet of water forms a broader spectrum of
bubble sizes than the sintered glass filters tested, including bubbles
larger than about 1 mm in radius. The slopes of the bubble density size
spectrum in the miniMART plumes are very similar to the slopes of oceanic
and laboratory breaking waves at sizes smaller and larger than the Hinze
scale (aH) as well as to the larger MART system. For comparison, the
bubble plumes generated by sintered filters have a much narrower size
spectrum and tend not to include bubbles larger than about 800 µm
radius.
The bubble plumes generated by the plunging jet within miniMART penetrate
approximately 15 cm beneath the water surface, which is not as deep as the
plumes generated by MART or by spilling breakers in the lab and ocean (Deane
and Stokes, 2002). However, the intermittent cycling of the plunging jet in
miniMART system allows the bubble plume and resulting surface foam patch to
evolve over time, creating a bubble and aerosol source that seems to be a
fairly close match to the decaying patches of foam produced by whitecaps
than that provided by constant, stationary jets. The importance of decaying
foams (as opposed to pseudo-steady-state 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.
The particle number distribution measured using the miniMART and MART system
are similar to the size distribution obtained by Fuentes et al. (2010). It is
notable that the particle number distributions obtained using the miniMART
(Fig. 4) and MART systems have less pronounced characteristics of sub-100 nm modes, with the dominant number distribution mode around 200 nm, broadly
tailing off to both larger and smaller sizes (see details for MART in Fig. 5, of Stokes et al., 2013). This result is consistent with the broad bubble
size spectrum and accurate representation of bubbles larger than 1 mm that
is achieved by both miniMART and the larger MART system. Particle number
distributions measured in both are in strong agreement with those previously
measured from breaking waves in the Scripps Institution of Oceanography
Hydraulics Laboratory (Prather et al., 2013). These measurements highlight
the importance of an accurate representation of bubble formation processes
in the creation of sea-spray aerosol in the laboratory. The primary
difference between the miniMART and MART systems is the lower particle flux
generated by the smaller and less energetic plunging jet in miniMART. For
example, the submicron- and supermicron-sized particle number, surface area and
mass concentration in MART were approximately 5000 and 345 cm-3, 1260
and 2800 µm2 cm-3, and 200 and 1735 µg m-3
(assuming a particle density of 1.8 g cm-3), respectively, at a flow
rate of 3 slpm (Stokes et al., 2013). While for the miniMART, these numbers
were approximately 90 and 60 cm-3, 160 and 900 µm2 cm-3, 50 and 125 µg m-3, respectively, at a flow rate of
2.6 slpm, necessitating longer sample integration times for some
instrumentation, like the SMPS.
(a) Twelve-hour time series of 1 Hz CPC measurements from a
miniMART containing a 500 mM solution of NaCl. (b) Allan variance plot,
calculated using the data shown in the top panel. At long integration times,
flow controller drift and temperature fluctuations likely contribute to
source fluctuations.
The reduced particle number concentrations in miniMART, in comparison to
MART, can present a challenge for particle instrumentation (e.g., size
resolved cloud condensation nuclei measurements). For instruments where the
noise is dominated by counting statistics, signal-to-noise ratios can
theoretically be improved by signal averaging. An important consideration,
with respect to miniMART, is the stability of the particle source and air
delivery as a function of instrument integration time. Allan variance can be
used to determine the timescale for which signal averaging in the miniMART
will no longer improve instrument signal-to-noise ratio (Werle et al., 1993).
Twelve continuous hours of 1 s CPC measurements from a miniMART containing a
500 mM NaCl solution are shown in Fig. 6a. The Allan variance was
calculated from these data and are shown in Fig. 6b. The analysis indicates
that improvement in signal-to-noise ratio will be achieved for averaging times up
to 100 s, after which further signal averaging will result in a decrease in
the signal-to-noise ratio. Further work is required to establish the
experimental factors that control this optimum averaging time.
Example miniMART experimental time series for a 12-day incubation
of nutrient-spiked, filtered seawater. The top panel shows continuous APS-determined aerosol size number concentration for particles from 0.6 to 3.5 µm dry diameter. The center panel shows chlorophyll a concentration
(µg L-1) and the lower panel shows colored dissolved organic matter
(cDOM, ppb) from the miniMART bulk water during the incubation. Vertical
bars indicate ±1 SD.
A primary motivation for the fabrication of miniMART was to create an SSA
analogue that allowed continuous aerosol sampling during the growth and
culturing of planktonic cells. Figure 7 shows data collected during a 12-day
miniMART incubation of sand-filtered seawater spiked with nutrients at
0 h. Aerosols were sampled continuously with an APS from the tank headspace
with a carrier gas flow of 1.9 slpm. In addition, chlorophyll a
concentrations and dissolved organic matter (cDOM) concentrations were
measured at semi-regular intervals from miniMART water drawn via a
peristaltic pump into a closed-loop analysis system (Wetlabs Ecotriplet),
and then returned, to prevent the loss of water from the system during
sampling. Exponential growth of microorganisms (primarily diatoms) peaks
around day 4 with an increase in the number density of aerosols increasing
after the initial bloom and while the chlorophyll a concentrations drop,
associated with the death of the diatoms and rapid increase in the number of
bacteria and viruses which cause cellular lysis and the increase in
dissolved organics. Similar preliminary experiments have been run showing
multiple microbial blooms and crashes during miniMART incubations for weeks
in duration. Understanding the factors that drive the variability in the
produced SSA particle concentrations that is evident in Fig. 7 is the
subject of future work.
Conclusions
In order to mimic the SSA created by oceanic whitecaps any surrogate system
must reproduce the complex two-phase flows, bubble plumes and surface foam
patches naturally generated during a breaking wave. These conditions can be
accurately replicated in large seawater breaking wave channels. However,
these facilities are not readily available, and due to their extremely large
volume it is extremely difficult to enclose them for high fidelity aerosol
sampling and difficult to carefully control the environmental conditions to
allow replicate experiments. Sintered glass filters (frits) bubbling air in
an enclosed container produce controllable plumes; however, the bubbles
produced are constrained to a narrow size spectrum much more narrow than
that observed in a natural whitecap.
When using plunging water to create bubble plumes, it is important that the
falling sheet or jet has the appropriate scale of surface roughness before
impacting the water surface in order to create the correctly sized voids along
the air–water interface (Zhu et al., 2000). The larger voids are important
for producing the correct plume bubble size distribution that includes
bubbles larger than the Hinze scale. Stationary, narrow cross-sectional area
and high velocity jets may not entrain large bubbles characteristic of
whitecaps without the correct scale of disturbances on their surface before
impacting the water.
It is apparently important that any bubble plume surrogate provide the
correct intermittency in production. 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
(Prather et al., 2013; Collins et al., 2014).
The bubble plume and resulting aerosol particle size distribution generated
within the miniMART and MART systems resembles that generated from breaking
waves within the SIO glass-walled wave channel. Confining the bubble
generation to a smaller headspace air volume (< 50 L in the MART and
∼ 10 L in miniMART) as compared to the wave channel permits a
significant increase in particle number concentrations (from 100, to
> 5000, to approximately 500 particles cm-3, for the wave
channel, MART and miniMART, respectively). As a result, the surrogate MART
and miniMART systems enable a wide variety of measurements (e.g., size
resolved hygroscopicity and heterogeneous reactivity) that are not feasible
at the low number concentrations produced in the wave channel and allow for
the controlled study of the chemistry and physics of marine bubbles, foam
and aerosols. In these systems, experiments are more easily repeatable even
while environmental variables, like the seawater and atmospheric chemistry
and the physical forcing mechanisms controlling the plume dynamics, are
manipulated. The pump-free action of miniMART allows the long-term growth
and monitoring of delicate planktonic cell cultures while continuously
producing aerosols for study, with the caveat that the flux of particles is
less than in the larger MART system, a factor which must be considered in
any particle sampling protocols. For experiments requiring the generation
and collection of large numbers of aerosol particles that do not require the
continuous presence of delicate organisms in the water, the original MART
system remains the breaking wave surrogate of choice.
Data availability
All data sets used in this
research can be provided through contact with the corresponding author. In
addition, access can be acquired via contact with the NSF Center for Aerosol
Impacts and the Environment (CAICE) at UCSD
(http://caice.ucsd.edu/index.php/research/tools/).
Acknowledgements
This research was conducted within the Center for Aerosol Impacts on Climate
and the Environment (CAICE), a National Science Foundation Center for
Chemical Innovation (CHE-1305427).
Edited by: H. Herrmann
Reviewed by: two anonymous referees
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