Introduction
Particles produced from wave breaking in marine environments, known as sea
spray aerosol (SSA), are one of the largest sources of naturally generated
aerosol to the atmosphere (Andreae and Rosenfeld, 2008; Lewis and Schwartz,
2004). SSA contributes to both direct and indirect radiative forcing on a
global scale (Lohmann and Feichter, 2005; Murphy et al., 1998). Aerosol
generation from freshwater sources, such as the Laurentian Great Lakes, has
been far less studied, with only a single ambient measurement (Slade et al.,
2010) and modeling study (Chung et al., 2011) having examined the process to
our knowledge. Slade et al. (2010) observed the production of ultrafine
(< 40 nm) aerosol, which increased in concentration as a function
of wind speed, during periods of white-capped waves over Lake Michigan.
Through regional modeling, Chung et al. (2011) found that these particles
could increase surface level aerosol number concentrations, by
∼ 20 % over the remote northern Great Lakes and by ∼ 5 %
over other parts of the Great Lakes, potentially affecting cloud nuclei (CCN)
and/or ice nuclei (IN) concentrations over the Great Lakes region. Recently,
aerosols produced from freshwater (a river) were demonstrated to have
enhanced ability to act as IN, in comparison with SSA (Moffett, 2016). Since
even a small number of IN can have large impacts on clouds and precipitation
(Ault et al., 2011; Creamean et al., 2013, 2015, 2016; DeMott et al., 2010),
including SSA (DeMott et al., 2016), this shows the potential for LSA to
impact climate.
It should be noted that the Chung et al. (2011) study was challenging because of the need
to use SSA-based parameterizations derived from bubble bursting of higher-salinity seawater due to the lack of a bubble-bursting parameterization for
lower-salinity freshwater. Due to their inherent differences from SSA, the
term lake spray aerosol (LSA) is proposed to refer to aerosol formed from
breaking waves in freshwater. Based on the intrinsic differences between SSA
and LSA, and the heterogeneous water properties between and within the Great
Lakes, methods are needed to understand aerosol production across a wide
range of ionic and organic concentrations (Chapra et al., 2012; Shuchman
et al., 2013).
Breaking waves, caused by winds that entrain air beneath the water's surface,
form bubbles that rise to the surface and burst to eject droplets into the
atmosphere (Lewis and Schwartz, 2004). Therefore, droplet
production flux is generally modeled as a function of increasing wind speed
(Lewis and Schwartz, 2004). Over freshwater higher wind speeds are
necessary to generate whitecaps, the product of bubbles formed by breaking
waves rising to the surface, in comparison to seawater
(Monahan, 1969). The minimum wind speed necessary for
freshwater whitecap production was observed by Monahan (1969)
to be 7–8 m s-1 over the Laurentian Great Lakes, compared to a
threshold wind velocity for seawater whitecap production at 3–4 m s-1
(Blanchard, 1963). However, wind speeds greater than this minimum
wind speed necessary to produce breaking waves are still observed on large
bodies of freshwater, such as the Laurentian Great Lakes
(Monahan, 1969; Slade et al., 2010), which have
a yearly mean wind speed > 6.6 m s-1 at a height of 10 m
above the lake surface for the majority of the Laurentian Great Lakes
(Doubrawa et al., 2015). In addition to differences in the wind
speed necessary for whitecap formation, the lifetime of freshwater whitecaps
is shorter than saltwater whitecaps (Monahan and Zietlow, 1969).
Combined, the higher minimum wind speed necessary for whitecap formation and
shorter whitecap lifetime in freshwater compared to seawater whitecap are
anticipated to lead to less aerosol production from bubble bursting in
freshwater than seawater.
To produce aerosols from freshwater using this mechanism, inorganic ions or
other non-volatile material must be present in the droplets to form a dry
particle after water evaporation. The Laurentian Great Lakes contain
inorganic ions (Chapra et al., 2012) and dissolved organic
carbon (DOC) (Shuchman et al., 2013), though differing in
concentration and composition from that found in the ocean. Figure 1 shows
the concentrations of a range of important ions and total organic carbon as
a function of total water conductivity (Biddanda and Cotner, 2002; Chapra
et al., 2012; Pilson, 2013; Repeta et al., 2002; Shuchman et al., 2013).
Three key aspects of Great Lakes freshwater highlight the differences from
seawater: (1) 2–5 orders of magnitude lower inorganic ions concentrations,
(2) different relative concentrations of key inorganic ions (Ca2+ > Mg2+≈ Na+≈ Cl- > SO42- > K+), and (3) total organic carbon (TOC)
concentrations on the same order of magnitude as total inorganic ion
concentrations. These differences in ion concentrations and ratios between
seawater and freshwater will lead to important differences in the properties
of bubbles from wave breaking formed in the Great Lakes and, thus, lead to
different physical and chemical properties of the resulting aerosol, in
comparison to SSA.
Concentration vs. conductivity vs. of important ions (Na+,
Mg2+, K+, Ca2+, Cl-, SO42-, and CO32-)
for freshwater (Great Lakes) and mean seawater, as well as DOC. Great Lakes
ion concentrations and conductivity are from Chapra et al. (2012), and
seawater ion concentrations and conductivity are from Pilson (2013). TOC
values for the Great Lakes are from Repeta et al. (2002), Shuchman et
al. (2013), and Biddanda and Cotner (2002), while the TOC value for seawater
is from Repeta et al. (2002). Note: K+ is fully obscured for seawater by
Ca2+.
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Previous work determined the bubble size distributions present in the water
column for freshwater and seawater during laboratory simulations of wave
breaking (Blenkinsopp and Chaplin, 2011; Carey et al., 1993; Monahan and
Zietlow, 1969; Slauenwhite and Johnson, 1999; Spiel, 1994a). An increase in
the concentration of < 1 mm bubbles in seawater compared to
freshwater primarily is thought to be due to differences in bubble
coalescence (Blenkinsopp and Chaplin, 2011; Carey et al., 1993; Monahan
and Zietlow, 1969). The higher ion concentrations in seawater inhibit bubble
coalescence, leading to a higher proportion of small bubbles. In contrast,
bubble coalescence occurs more freely in freshwater due to lower ion
concentrations, which leads to a higher proportion of large bubbles
(Lessard and Zieminski, 1971). Slauenwhite and Johnson (1999)
suggest that, in addition to coalescence, an increase in the initial break-up of bubbles in seawater vs. freshwater causes a shift to larger
diameters in the bubble size distributions for freshwater. As droplet, and
subsequent dry particle, production is, in part, dependent on the bubble
size distribution (Prather et al., 2013; Stokes et al., 2013), the
increase in smaller bubbles in seawater compared to freshwater contributes
to a different number size distribution of droplets, and therefore aerosol,
produced by bubble bursting in freshwater compared to seawater. However, the
bubble size distribution does not fully control the number size distribution
of aerosols produced by bubble bursting. The concentration and composition
of freshwater and seawater will further alter the dry particle formation by
controlling the mass that remains, and thus the size, of a dry particle
resulting from a droplet produced by bubble bursting. Droplets produced by
bubble bursting in freshwater will have lower solute concentrations, and
will form a smaller dry particle than those produced by bubble bursting in
seawater, if the initial droplet is the same size (Slade
et al., 2010).
To examine aerosol production from freshwater wave breaking, an LSA generator
was constructed based on design elements from multiple validated laboratory
SSA generators (Facchini et al., 2008; Fuentes et al., 2010; Hultin et
al., 2010; King et al., 2012; Salter et al., 2014; Sellegri et al., 2006;
Stokes et al., 2013; Zábori et al., 2012). The LSA generator can produce
aerosols from a relatively small amount of freshwater, lowering the
limitations surrounding the collection, transport, storage, and analysis of
large surface lake water samples. This increases the possible number and
variety of environmental samples that can be analyzed in a region with
heterogeneous water properties. Systematic experiments were conducted in the
LSA generator to determine the relationship between bubble size
distributions and the resulting aerosol size, concentration, and
composition. The bubble and aerosol properties were tested for simple salt
solutions (NaCl and CaCO3), simulated inorganic seawater and freshwater
solutions, and a surface water sample from Lake Michigan. This study
establishes a method to probe LSA with an interdisciplinary approach that
draws from atmospheric science (production fluxes), physical oceanography
(bubble measurements), atmospheric chemistry (aerosol physicochemical
properties), and limnology (Great Lakes water properties). This work will
broaden understanding of the effect of ion concentration and composition on
aerosol production and properties, allowing for improved parameterization of
LSA production from the Laurentian Great Lakes and other bodies of
freshwater.
Materials and methods
Materials
Synthetic seawater was produced using Instant Ocean™
(Atkinson and Bingman, 1997) prepared with 18.2 MΩ
ultrapure water. All remaining standard solutions were prepared using 18.2 MΩ ultrapure water and anhydrous analytical-grade inorganic salts
(NaCl ≥ 99 % and CaCO3 ≥ 99 %; Fisher Scientific). A
solution of 1 mmol Ca2+, 1 mmol CO32-, 0.4 mmol Mg2+,
0.4 mmol SO42-, 0.3 mmol Na+, 0.3 mmol Cl-, and 0.02 mmol K+ was prepared as synthetic freshwater based on Lake Michigan ion
concentrations reported by Chapra et al. (2012).
Freshwater was collected from the surface of Lake Michigan near Muskegon,
Michigan (43∘14′21.545 N, 86∘20′45.153 W), on 26 July 2015 in an 8 L LDPE carboy. During freshwater sampling, a multi-parameter
water quality sensor (Professional Plus, YSI, Inc.) was used to measure
freshwater properties, including temperature, pH, salinity, and dissolved
oxygen, and a handheld spectrophotometer (AquaFluor 8000) was used to
measure blue-green algae content. The freshwater was frozen after sampling
for storage and thawed prior to analysis. Frozen freshwater samples that have been thawed were analyzed by nanoparticle tracking analysis, which measures the size distribution and number concentration of insoluble residues (Axson et al., 2014). The frozen samples did not show changes in size or number concentration of insoluble components compared to unfrozen samples, indicating the sample was likely not significantly modified by freezing (Axson et al., 2016b).
The constructed lake spray aerosol generator shown as a
(a) schematic and
(b) photograph with functional components labeled. Not all components of the LSA
generator shown in the schematic are visible in the photograph.
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Aerosol generation
An LSA generator (Fig. 2) was constructed based on a design incorporating
elements from previously published laboratory SSA generators (Fuentes et al.,
2010; Hultin et al., 2010; Salter et al., 2014; Sellegri et al., 2006; Stokes
et al., 2013). The LSA generator consists of an acrylic box with a total
volume of 18 L (30 × 20 × 30 cm) and a water circulating
system controlled using a diaphragm pump (ShurFlo 2088). Water was circulated
from the tank and cycled back into the tank at a rate of 2 L min-1 as
plunging jets from four tubes (1/8 in. inner diameter) arranged in a square pattern 5 cm apart at the top
of the tank, approximately 20 cm above the water surface (depending on fill
level). Air was entrained by the plunging jets, creating a bubble plume of
approximately 5 cm in depth with 5 cm between the plume and the base of the
chamber, analogous to the wave-breaking mechanism observed in nature (Fuentes
et al., 2010). The four tubes were capped with mesh to break up the flow and
increase the surface roughness of the plunging jet before it hit the water
surface in order to obtain an accurate bubble size distribution (Stokes et
al., 2013; Zhu et al., 2000). Prior to each experiment, the LSA generator was
rinsed with 18.2 MΩ ultrapure water. Prior to and during operation,
HEPA-filtered particle-free air was pulled through the LSA generator to
prevent ambient particle contamination as flow was pulled to the instruments.
The LSA generator was maintained at positive pressure with a constant
overflow of 0.2 L min-1. All experiments were performed at room
temperature, approximately 22.0 ∘C, and the relative humidity (RH)
within the tank was maintained at ∼ 85 %, the standard RH for
ambient and laboratory SSA generation (Lewis and Schwartz, 2004).
A major advantage of the LSA generator system is that it needs a relatively
small volume of water (4–6 L) compared to other SSA generation systems
(100 L) (Salter et al., 2014; Stokes et al., 2013). However, the shallow
bubble plume generated in plunging water jet systems of reduced dimensions
such as the one discussed in this study (5 cm), and others (Fuentes et
al., 2010; Hultin et al., 2010; Sellegri et al., 2006), limits bubble plume
lifetime, as discussed in detail by Fuentes et al. (2010). Large-volume plunging jets with plume depths > 0.5 m are expected to be
representative of the lifetime of oceanic plumes (Collins et al., 2014;
Stokes et al., 2013), but those are only suitable when large amounts of
sample are available. Due to difficulties in obtaining and storing large
volume freshwater samples from multiple collection sites, these types of
large-scale aerosol generation methods are not suitable for our research. In
addition, work by Fuentes et al. (2010) demonstrated that the
shortened bubble plume lifetime does not affect the adsorption of marine
surfactant to rising bubbles in small-volume SSA generation methods and that
these systems are appropriate for studying the effects of marine organics on
SSA. Therefore, the LSA generator presented in this work, despite its
reduced dimensions, should be suitable for the study of the effect
freshwater composition on LSA production.
Bubble size distribution measurements
Digital high-speed photographs of the LSA generator plunging jet bubble
plume were collected to examine the bubble size distributions. The bubbles
were photographed using a Nikon D100 camera fitted with an AF Nikkor 24–50 mm lens and placed approximately 45 cm from the front of the tank to
capture side profiles of the bubble plume. An aperture of 4.5 was used to
achieve the narrowest depth of field possible in the resulting images. To
increase bubble clarity, two light sources (Ring 48, Neewer) were placed to
the right and left of the tank, illuminating the bubbles (Fig. 2).
Photographs were obtained at intervals > 60 s to ensure
each bubble was counted only once (Salter et al., 2014).
ImageJ was used to determine the bubble plume size distribution in each
photograph. Individual bubbles were manually identified and a circle was fit
to each bubble (Schneider et al., 2012). The bubble dimensions obtained in
pixels were converted to millimeters by a scaling factor calculated for individual
photographs in the ImageJ software from measurements of a portion of the tank
with known length visible in the photograph. The area was then converted to
diameter, reported here in millimeters, assuming the bubbles to be circular (Lewis and
Schwartz, 2004). In determining the bubble volume density, the volume of the
bubble plume was calculated from measurements of plume photographs in ImageJ.
Due to interferences of light diffraction in the LSA generator and
limitations in the camera, such as pixel size and resolution,
bubbles < 100 µm in diameter could not be distinguished
accurately from the background of the photograph and are not included in the
analysis. Another limitation inherent in this method is that it is possible
that a smaller fraction of bubbles in the focal volume were obscured by other
bubbles and not counted.
Aerosol size distribution measurements
Aerosols generated by bubble bursting exited the LSA generator and passed
through two silica gel diffusion dryers to achieve a RH of ∼ 15 %,
similar to the RH of previous measurements of aerosol size distributions of
laboratory SSA (Fuentes et al., 2010; Salter et al., 2014; Stokes et al.,
2013). After exiting the diffusion driers, the aerosol number size
distributions and total aerosol concentrations produced for each solution in
the LSA generator were measured using a scanning mobility particle sizer
(SMPS), consisting of a differential mobility analyzer (DMA; model 3082, TSI
Inc.) and condensation particle counter (CPC; model 3775, TSI Inc.), as well
as an aerodynamic particle sizer (APS; model 3321, TSI Inc.). The SMPS
operated at a sample flow rate of 0.3 L min-1 and sheath flow of
3 L min-1 and a scan rate of 5 min to obtain a size distribution for
particles with an electrical mobility diameter (dm)
between 14.1 and 736.5 nm. The APS was operated at a flow rate of
5.0 L min-1, with an aerosol and sheath flow of 1.0 and
4.0 L min-1, respectively, and a scan rate of 30 s to obtain a size
distribution for particles with an aerodynamic diameter
(da) between < 0.52 and 19.8 µm. For
each sample solution, SMPS and APS particle size distributions were collected
over a 3 h period and averaged. In order to merge the SMPS and APS size
distribution, measurements recorded in dm and
da, respectively, were converted to physical (geometric)
diameters (dp) (Khlystov et al., 2004). The relation
dm=dp
was used to convert particles sized by the SMPS, under the assumption that
the particles were spherical. Particles sized by the APS were assigned an
effective density (ρeff) of 1.2–1.6 g cm-3, a value
determined experimentally for particles produced from each individual
solution, allowing for conversion based on the following
relation:
dp=daρeffρ0,
where ρ0 is equal to unit density (1 g cm-3). The
SMPS has a tendency to undercount particle concentrations at the highest
particle diameter bins, due to the cut-off from the particle impactor, and
the APS has a tendency to undercount particle concentrations at the lower
diameter bins, due to the poor scattering efficiency of the lowest particle
diameter bins (Ault et al., 2009; Khlystov et al., 2004; Qin et al.,
2006). To compensate for these limitations, the highest and lowest particle
diameter bins of the SMPS and APS, respectively, comprising the overlapping
diameters of the two methods, were removed when stitching
(Stokes et al., 2013). All reported aerosol size
distribution modes are from fits of lognormal distributions. Aerosol blank
measurements conducted before experiments by circulating 18.2 MΩ
ultrapure water through the LSA generator showed that the background aerosol
number concentrations were < 20 cm-3, compared to an average of
350 cm-3 during freshwater samples.
Scanning electron microscopy
Particles generated from the different solutions run in the LSA generator
were impacted onto carbon type-B (Formvar film coated with carbon on copper
grid) transmission electron microscopy (TEM) grids (01910-F, Ted Pella,
Inc.) using a three-stage microanalysis particle sampler (MPS; model MPS-3,
California Measurements, Inc.). Particles were examined from the third
(smallest) stage, with a size cut of < 700 nm. Scanning electron
microscopy with energy dispersive X-ray (SEM-EDX) measurements were made at
the Michigan Center for Materials Characterization (MC)2 located at the
University of Michigan in Ann Arbor. An FEI Helios with environmental dual
focused ion beam–scanning electron microscope (FIB-SEM) was used to obtain
images of the particles. The FEI Helios was equipped with a Schottky field
emitting source operating at an accelerating voltage of 15 kV and current of
0.58 nA. Scanning transmission electron microscopy (STEM) was conducted and
a high-angle annular dark-field (HAADF) electron detector was used to
collect Z-dependent dark-field images of individual particles.
Digital images of a bubble plume generated by one plunging jet in
the LSA generator with (a) synthetic seawater,
(b) synthetic freshwater, and (c) Lake Michigan freshwater,
with brightness/contrast adjusted to increase bubble clarity.
(d) Bubble number size distributions and (e) bubble
concentrations generated by the LSA generator using synthetic seawater,
synthetic freshwater, and Lake Michigan freshwater measured by the bubble
photography method, as well as previously measured bubble size distributions
generated from synthetic seawater with a plunging waterfall
(Prather et al., 2013) and freshwater with a tipping trough
(Carey et al., 1993).
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Results and discussion
Comparison of seawater and freshwater bubble plume size
distributions
Photographs of bubble plumes generated from synthetic seawater, synthetic
freshwater, and Lake Michigan freshwater were collected to observe visual
changes in bubble plumes and to determine their respective bubble size
distributions (Fig. 3). There was an observed decrease in the concentration
of smaller bubbles in freshwater when compared to synthetic seawater, which
has been observed in previous studies (Blenkinsopp and Chaplin, 2011; Carey
et al., 1993; Monahan and Zietlow, 1969; Slauenwhite and Johnson, 1999;
Spiel, 1994b). The visual differences in the images were reflected in the
measured bubble size distributions (Fig. 3d), with the synthetic freshwater
and Lake Michigan freshwater samples producing a similar total bubble
concentration that was only 12 and 8 % (Fig. 3e), respectively, of the
total bubble concentration produced from the synthetic seawater solution.
Bubble size distributions generated from synthetic seawater showed that
bubbles were produced up to 4 mm in radius in the LSA generator (Fig. 3d),
similar to measurements of bubble size distributions for ocean waves (Bowyer,
2001; Deane, 1997; Deane and Stokes, 1999, 2002).
The production of bubbles with radii > 1 mm is important because
droplet production from bubble bursting, and the resulting dry particle size
distribution, is dependent on bubble size
(Collins et al., 2014). The bubble-bursting
process in seawater ejects two types of droplets into the atmosphere: film
and jet droplets (Blanchard and Syzdek, 1975; Blanchard and Woodcock, 1957).
Film and jet droplets typically range in size from 0.2 to 10 and
1 to 200 µm, respectively (Lewis and Schwartz, 2004). The number of
film and jet droplets produced from a single bubble in seawater is dependent
on the size of the bubble, and bubbles with radii > 1 mm produce
more film drops and bubbles < 1 mm produce jet drops in quantities
greater than 1 per bubble (Lewis and Schwartz, 2004). In addition, jet drop
size is directly correlated to bubble size (Lewis and Schwartz, 2004). If
bubbles > 1 mm are not produced by a generation method, then a
higher proportion of jet droplets will be formed, shifting the aerosol size
distribution mode and modifying the aerosol chemical composition (Collins et
al., 2014; Stokes et al., 2013). The replication of this power law decrease
in bubble concentrations at larger radii using the LSA generator is therefore
critical for the accurate reproduction of SSA (Prather et al., 2013; Stokes
et al., 2013) and LSA.
The bubble radius mode for the synthetic freshwater and Lake Michigan
freshwater bubble size distributions were observed at 280 ± 70 and
250 ± 60 µm, respectively (Fig. 3d). This is consistent with
freshwater laboratory measurements by Carey et al. (1993), which show a mode
of 300 µm and a steep drop in bubble concentration for radii below
300 µm (Fig. 3d). This bubble size mode is much larger than that
observed for seawater, for which bubble size distributions typically peak at
a radius between 40 and 80 µm (Fuentes et al., 2010; Hultin et al.,
2010; Prather et al., 2013; Sellegri et al., 2006; Stokes et al., 2013). This
means the peak mode for the synthetic seawater bubble size distribution
produced in the LSA generator was below the detectable bubble size limit of
the photographic technique used in this study. Indeed, the LSA generator
bubble size distribution for seawater in Fig. 3d has a peak mode lower than
that for freshwater and is < 100 µm. Previous work
examining seawater bubble size distributions have encountered this same
measurement limitation (Carey et al., 1993; Deane and Stokes, 2002; Hultin et
al., 2010), which was resolved by comparing the power-law-dependent decrease
in bubble concentrations at higher radii to confirm the accuracy of bubble
size distribution. Results from this comparison, presented in the
Supplement, are consistent with previous observations and
confirm that the LSA generator produces bubble plumes representative of both
oceanic and freshwater wave breaking.
However, the concentrations of bubbles produced from both freshwater and
seawater samples in the LSA generator were lower than the concentrations
representative of freshwater (Carey et al., 1993) and seawater
(Stokes et al., 2013) wave breaking previously reported
(Fig. 3d). Further, the lower concentration of bubbles compared to
previous measurements is more pronounced at larger radius (> 1 mm) bubbles. This limitation of the LSA generator is likely due to its
reduced dimensions compared to the bubble generation methods used for
comparison (Carey et al., 1993; Stokes et al., 2013). The smaller
dimensions allow for small sample volumes, but likely limit the lifetime of
the bubble plumes (Fuentes et al., 2010), as discussed in Sect. 2.2.
Aerosol size distribution characteristics obtained from lognormal
fitting for LSA generated from synthetic seawater, synthetic freshwater, and
Lake Michigan freshwater.
Solution
Mode
Diameter
Standard
Amplitude
(nm)
deviation (σ)
(cm-3)
Synthetic seawater
Primary
110 ± 4
1.52
1620
Synthetic freshwater
Primary
300 ± 40
1.00
292
Secondary
80 ± 10
0.75
206
L. Michigan freshwater
Primary
180 ± 20
0.66
794
Secondary
46 ± 6
1.42
286
Aerosol generation from seawater and freshwater
Validation of aerosol generated with synthetic seawater
To both characterize the LSA generator and compare freshwater aerosols to
those generated from seawater, aerosol size distributions generated from
synthetic seawater, synthetic freshwater, and Lake Michigan freshwater were
measured (Fig. 4). The aerosol size distribution generated for synthetic
seawater produced a total number concentration of 1195 cm-3 and
exhibited a single mode at a diameter of 110 ± 4 nm, with a geometric
standard deviation (σ) of 1.52, and an amplitude of 1620 cm-3
(Table 1). This SSA mode is in agreement with the primary diameters of SSA
modes, which ranged from 60 to 200 nm, determined using various laboratory
generation techniques (Collins et al., 2014; Fuentes et al., 2010; Hultin
et al., 2010; Prather et al., 2013; Salter et al., 2014; Sellegri et al.,
2006; Stokes et al., 2013). It was determined that the LSA generator
successfully reproduced seawater bubble and aerosol size distributions such
that the system can be used to test other applications.
(a) Average aerosol number size distributions fitted with
lognormal distributions (long dashes indicate each peak, while short dashes
represent the sum of the peaks) (b) average total aerosol number
concentration, and (c) average total aerosol number concentration
normalized by average total bubble concentration produced by the LSA
generator (particles per bubble) from synthetic seawater, synthetic freshwater,
and Lake Michigan freshwater.
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Characteristics of aerosol generation from freshwater
The synthetic freshwater and Lake Michigan freshwater produced 67 and
33 % lower total aerosol number concentrations
(dp= 0.018–18 µm), compared to the synthetic
seawater, respectively (Fig. 4b). The lower total aerosol number
concentration produced from the freshwater solutions, in comparison to the
synthetic seawater, is a reflection of the lower bubble concentrations
produced from the freshwater solutions in comparison to synthetic seawater
(Figs. 4b, 5). However, it is important to note that the Lake Michigan
freshwater produced a larger total aerosol concentration normalized by the
total bubble concentration generated than both the synthetic freshwater and
the synthetic seawater solution, which were both similar (Fig. 4c). In
contrast to the unimodal synthetic seawater aerosol size distribution, both
the synthetic freshwater and Lake Michigan freshwater aerosol size
distributions were bimodal (Fig. 4a and Table 1). The primary mode observed
for the synthetic freshwater and Lake Michigan freshwater occurred at a
diameter of 300 ± 40 and 180 ± 20 nm, respectively, which are
larger than the dominant mode observed for synthetic seawater
(110 ± 4 nm). The secondary mode was observed at a diameter of
80 ± 10 nm for the synthetic freshwater and 46 ± 6 nm for the
Lake Michigan freshwater sample. The LSA secondary mode for the Lake Michigan
freshwater is similar to previous aircraft measurements by Slade et
al. (2010), who observed a 15–40 nm particle lognormal diameter mode over
Lake Michigan. Slade et al. (2010) performed calculations of expected dry
particle diameter based on typical droplet size produced from oceanic
wave breaking and total dissolved ion content of freshwater. These
calculations indicated that the aerosol size distribution of LSA would peak
at a diameter smaller than SSA, and this would explain the measured secondary
mode generated from freshwater solutions in this study that was lower in
diameter than the primary mode of SSA (see Sect. 3.2.3). These results
indicate that wave-breaking-induced bubble bursting of freshwater in the
Great Lakes can produce aerosols through mechanisms analogous to wave
breaking on open oceans, but the size distribution of LSA has different
characteristics than that of SSA.
Aerosol vs. bubble concentrations produced by the LSA generator from
solutions of NaCl and CaCO3 of varying concentrations, Lake Michigan
freshwater, synthetic freshwater, and synthetic seawater. A best-fit line is
shown for the empirical relationship between aerosol and bubble
concentrations.
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The increased total particle concentration as well as the shift in
diameter mode to smaller sizes, for the Lake Michigan freshwater sample
compared to the synthetic freshwater, points to the possible additional
influence of organic carbon present in the Lake Michigan freshwater sample.
While the synthetic freshwater was a simplified mixture of inorganic ions
representing freshwater, the Lake Michigan freshwater contained a more
complex mixture of inorganic ions, as well as organic and biological
material present in the surface water during collection. Like the synthetic
freshwater, the synthetic seawater is a simplified mixture of inorganic ions
representing seawater. The higher total particle concentration normalized by
total bubble concentration observed for the Lake Michigan freshwater sample,
compared to the total particle concentration normalized by total bubble
concentration for the synthetic freshwater and synthetic seawater, further
demonstrates the possible influence of organic carbon present in the Lake
Michigan freshwater sample. The presence of biological material in the
freshwater sample was confirmed by spectrophotometric measurements of bulk
water at the site during sample collection, which indicated 57.2 ppb of
blue-green algae present. Given that the Lake Michigan freshwater sample was
frozen prior to analysis, it is likely that the sample did not contain
substantial living biological material when run in the LSA generator.
To further determine the influence of organic carbon between the Lake Michigan freshwater sample aerosol populations, impacted particles were analyzed by SEM to determine circularity (Fig. 6), which in seawater has been shown to increase with greater total organic carbon concentrations due to the interference of organic carbon with the crystallization process during drying on the substrate (Ault et al., 2013b). Particles generated from the Lake Michigan freshwater
sample showed median circularity values approaching unity, indicative of a
perfect circle (and thus a spherical particle in the atmosphere, as shown in
Ault et al., 2012). In comparison, particles generated from the synthetic
freshwater sample have circularity distributions peaking below 0.9 for all
size ranges measured (< 0.5 µm, 0.5–1.0 and
> 1 µm). This higher circularity is likely due to
greater organic content in the authentic Lake Michigan sample vs. the
inorganic-only synthetic freshwater sample. In addition, the complex salt
mixture in the Great Lakes, where most ion concentrations are within an order
of magnitude of each other (Ca2+ and CO32- being the largest),
is more likely to affect crystallization than for seawater, where Na+
and Cl- are present in concentrations that are an order of magnitude higher than any
other inorganic ion (Fig. 1). Future efforts will involve systematic studies
of aerosols generated from freshwater samples with a range of inorganic,
organic, and biological components.
Circularity of (a) Lake Michigan freshwater particle sample
and
(b) synthetic freshwater particles as a function of diameter from the LSA
generator, as well as example SEM images of the impacted particles used in
the analysis.
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Freshwater droplet size distribution to freshwater aerosol
size distributions
Calculations of the relationship between dry particle diameter and initial
drop diameter were explored for seawater and freshwater to determine the
effect of the initial droplet size distribution on aerosol formation. The
physical diameter of a dry (RH = 0 %) SSA particle (dp)
will typically be ∼ 4 × smaller than the diameter of the
seawater droplet (dd) it originated from (Veron, 2015).
Therefore, the dp=110 ± 4 nm aerosol mode generated
from the synthetic seawater in the LSA generator would have resulted from a
roughly dd=440 nm initial synthetic seawater droplet mode
(Table 2). In contrast, due to the lower concentration of dissolved
components in freshwater, the dp of an LSA particle is
predicted to be ∼ 20 × smaller than the dd of the
freshwater droplet it originated from (Slade et al., 2010)
(Table 2). Using this relationship Slade et al. (2010)
predicted that the size distribution of LSA shifts towards smaller,
ultrafine diameters in comparison to the size distribution of SSA. However,
these calculations were made under the assumption freshwater and seawater
bubble bursting produce the same dd size distributions,
which may not be accurate as there are differences in bubble size
distributions generated in freshwater and seawater solutions (Fig. 3d).
Fresh- and seawater droplet diameters (dd) calculated
from the mass (assuming particle density is 1.2 g mL-1) of the
dominant dry particle diameter (d0) modes produced from
synthetic seawater (SSA) and the Lake Michigan freshwater sample (LSA).
Observed dry diameter (d0)
Droplet diameter (dd)
0.110 µm SSA
0.440 µm seawater
0.046 µm LSA
0.92 µm freshwater
0.180 µm LSA
3.5 µm freshwater
Previous work, while limited, has shown differences in the size distribution
of droplets produced from freshwater bubble bursting in comparison to
droplet production from seawater bubble bursting (Resch, 1986).
Resch (1986) observed that film drops produced from freshwater are
larger than those usually reported for seawater, which for SSA can range in
d80 from 0.02 to 200 µm (Lewis and Schwartz, 2004).
Therefore, the smaller mode of the Lake Michigan freshwater aerosol size
distribution (46 ± 6 nm) observed in this study could be the result of
a freshwater film droplet mode of dd=920 nm, which is larger
than the dd≈400 nm synthetic seawater film droplet
mode (Table 2). The second mode (180 ± 20 nm) of the observed Lake Michigan
freshwater sample aerosol size distribution is likely the result of an even
larger film droplet mode at dd=3.5 µm. This second
mode is unlikely to be the result of jet drop production as bubble bursting,
in seawater, typically produces jet drops with a dd that are
10 % of the bubble diameter (dbub)
(Lewis and Schwartz, 2004), and individual bubbles in freshwater
and seawater produce jet drops at similar numbers and sizes from bubbles with
radii of 300–1500 µm (Spiel, 1994b). Therefore, even the smallest
freshwater bubble measured in this study (dbub=0.2 mm) would
likely only produce jet drops of dd=20 µm and
dp=1 µm, a far higher diameter than the
larger mode observed for the Lake Michigan sample (300 nm) (Fig. 4). Further
work is needed to determine the differences in film droplet production
between fresh and seawater bubble bursting to fully connect bubble and
aerosol size distributions observed in this study.
Aerosol and bubble generation from standard salt solutions
Bubble size and concentration from standard salts
To determine the influence of the dominant inorganic ions, and their
concentrations, in freshwater and seawater (Fig. 1) on bubble production,
bubble size distributions for NaCl (seawater proxy) and CaCO3
(freshwater proxy) solutions were determined as a function of solution
concentration (Fig. 7a, b). The radii modes of the bubble size distributions
produced from CaCO3 solutions of 0.05 and 0.15 g kg-1
(230 ± 90 µm) (Fig. 7a) were similar to the synthetic
freshwater (280 ± 70 µm) and Lake Michigan freshwater sample
(250 ± 60 µm) bubble size distributions (Fig. 3d). This
similarity in bubble size distribution radii modes is consistent with
Ca2+ and CO32- being the dominant cation and anion,
respectively,
in the calcareous Great Lakes (Chapra et al., 2012). No solutions of
CaCO3 of concentration greater than 0.15 g kg-1 could be analyzed
for bubble size distributions due to the solubility limit.
Bubble size distributions (density vs. bubble radius) generated by
the LSA generator as a function of solution concentration for
(a) CaCO3 and (b) NaCl, as well as (c) total
bubble density as a function of ion composition for CaCO3 and NaCl.
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For NaCl solution concentrations 0.05 to 35 g kg-1, total
bubble density increased with solution concentrations. The largest increase
in bubble density (2–3 orders of magnitude) primarily occurred for the
smallest bubbles (radii < 0.3 mm) (Fig. 7b), which is the same
bubble size range as the largest increase (2–3 orders of magnitude) in
bubble density between freshwater and seawater solutions that was observed
(Fig. 3d). This observed increase in bubble density from freshwater to
seawater concentration solutions is likely the result of bubble coalescence
inhibition at higher ionic concentration (Slauenwhite and Johnson,
1999), as the two electrolyte combinations tested in this study (CaCO3
and NaCl) are known to exhibit concentration-dependent bubble coalescence
effects (Craig et al., 1993a, b; Henry et al., 2007).
Typically, increasing the solution salt concentration up to 0.01 M leads to
minimal decreases in bubble coalescence relative to pure water
(Henry et al., 2007). As a result, total bubble number
concentrations increased only gradually for NaCl when solution
concentrations in the LSA generator increased from 0.05 to 1 g kg-1 NaCl (0.00086–0.017 M). However, when the solutions entered
the 0.01–0.2 M solution concentration range (1–35 g kg-1 NaCl),
where bubble coalescence is known to decrease significantly
(Sovechles and Waters, 2015), a greater rate of increase in total
bubble number concentration with increased solution concentration was
observed (Fig. 7c). These results indicate that the different ionic
concentrations affected bubble coalescence and bubble concentrations in this
study, which in turn influenced aerosol concentrations produced by bubble
bursting.
Aerosol generation from standard salts
The aerosol size distributions for the two standard salt solutions
representative of seawater (NaCl) and freshwater (CaCO3) were measured
as a function of solution concentration (Fig. 8a, b) to examine the
effect of the dominant ion present, and ionic concentration, in solution on
aerosol production. At concentrations representative of the Great Lakes,
0.05 and 0.15 g kg-1, aerosol size distributions generated from
solutions of NaCl and CaCO3 exhibited two lognormal diameter modes
(Fig. 8a, b). The primary aerosol modes produced from the 0.05–0.15 g kg-1 NaCl and CaCO3 solutions were larger in diameter than the
secondary aerosol modes (Fig. 8a, b). This is consistent with the
bimodal aerosol size distributions generated from the synthetic freshwater
(total inorganic ion content = 0.12 g kg-1) and Lake Michigan
freshwater (total inorganic ion content = 0.14 g kg-1), which also
exhibited primary aerosol modes higher in diameter than the secondary
aerosol modes (Sect. 3.2.2). At higher concentrations (0.5–35 g kg-1) more representative of
seawater total inorganic ion content (35 g kg-1), the NaCl solutions produced unimodal size distributions
(Fig. 8b), consistent with the unimodal number size distribution produced
from synthetic seawater (Fig. 4a). The bimodal aerosol number size
distribution that was observed for all freshwater concentration (0.05–0.15 g kg-1) standard salt solutions (Fig. 8a, b) and the freshwater
solutions (Fig. 4a) indicates that solution concentration is important in
determining aerosol size distribution.
Average aerosol number concentration generated by the LSA generator
as a function of solution concentration for (a) CaCO3 and
(b) NaCl, as well as (c) total aerosol number concentration
as a function of ion composition for CaCO3 and NaCl.
![]()
Solution composition, as well as concentration, was observed to affect the
aerosol size distribution (Fig. 8). The two lognormal modes of the aerosol
size distribution produced from the 0.05 g kg-1 concentration
solutions were located at a higher diameters for CaCO3 (83 ± 8; 340 ± 20 nm)
compared to NaCl (55 ± 9; 210 ± 20 nm). When CaCO3 and NaCl solution concentrations increased from 0.05 to
0.15 g kg-1, the CaCO3 modes (60 ± 10; 290 ± 10 nm)
remained at higher diameters than the NaCl modes (40 ± 6; 140 ± 10 nm), but all modes shifted to smaller diameters (Fig. 9b). The
mode diameter of the 35 g kg-1 NaCl solution (81 ± 3 nm) was smaller
than the mode of the NaCl dominant synthetic seawater solution (110 ± 4 nm), suggesting that mixtures of ions affect aerosol size distributions.
In addition, the lognormal modal diameters produced from the 0.15 g kg-1 CaCO3 solution (60 ± 10; 290 ± 10 nm) were
slightly smaller in comparison to the synthetic freshwater aerosol size
distribution modes (80 ± 10; 300 ± 40 nm), again indicating
that mixtures of ions affect aerosol size distributions. As the Great Lakes
have a wide and evolving range of inorganic ion compositions and
concentration (Fig. 1) (Chapra et al., 2012), the
dependence of aerosol size distributions on solution composition and
concentration observed in this study could significantly impact the range of
LSA size distributions in the atmosphere.
The total aerosol concentrations generated from CaCO3 and NaCl
solutions increased with solution concentration (Fig. 8) in a similar
manner to the increase in total bubble concentrations generated with
increased solution concentration (Fig. 7). The total aerosol concentration
increased slowly between solution concentrations of 0.05 and 1.0 g kg-1,
reflecting the slow increase in bubble concentrations over this
concentration range (Fig. 7). At solution concentrations greater than 1.0 g kg-1, a shift to a larger increase in total aerosol concentration
with increased solution concentration occurred. The change in relationship
between solution and aerosol concentration at solution concentrations above
1.0 g kg-1 (NaCl = 0.017 M) reflects the change in bubble
concentration above 1.0 g kg-1 (NaCl = 0.017 M) observed in this
study (Fig. 8c) and the known transition in bubble coalescence behavior
that occurs above ionic concentrations of 0.01 M (Sovechles and
Waters, 2015). Further, the direct relationship between bubble and aerosol
concentrations for the increasing standard salt solution concentrations
aligns well with the direct relationship in bubble and aerosol
concentrations for freshwater and seawater solutions (Fig. 5). These
results confirm that there is a direct relationship between solution
concentration, bubble concentration, and aerosol concentration that will
result in the production of a lower number of particles from wave breaking
in low-salt freshwater compared to wave breaking in high-salt seawater.
Conclusions
We have constructed and demonstrated the capabilities of a newly developed
LSA generator to reproduce SSA using marine salinities and to probe LSA
generation under freshwater-relevant low-salt concentrations. The LSA
generator utilizes plunging jets to entrain air and generate bubbles,
similar to other SSA generation techniques, but with modifications, such as
the addition of mesh caps on the plunging jet outlets to obtain more
accurate air entrainment by increasing surface roughness of the plunging jet
(Stokes et al., 2013; Zhu et al., 2000). The LSA generator requires lower
sample volume to generate aerosols compared to other plunging jet SSA
generators (Salter et al., 2014). The lower solution volume
requirement (4 L) allowed for generation of LSA from a variety of samples,
including a freshwater sample collected from Lake Michigan. This increases
the ease of analyzing a large number of freshwater samples, which will be
necessary to probe how the differences in composition between freshwater
locations (Chapra et al., 2012; Shuchman et al., 2013) affect aerosol
generation. Recent combined field and lake spray aerosol generator results show the composition of LSA particles at a site without a harmful algal bloom to be predominantly calcium carbonate with organic and biological components (Axson et al., 2016a).
This LSA-generator-enabled laboratory study of LSA production allowed a
direct investigation into the influence of salt concentration and
composition on aerosol production from bubble bursting in freshwater and
simplified model systems. The results show that freshwater bubble bursting,
expected during periods of high winds and high waves over freshwater
environments such as the Laurentian Great Lakes, will produce LSA. Distinct
differences in the production and properties of LSA compared to SSA from
marine environments are observed. For example, the lower concentration of
salts in freshwater compared to seawater leads to lower number
concentrations of bubbles in freshwater compared to seawater, such that a
lower number concentration of LSA is produced compared to SSA. In addition,
the differences in salt concentration between seawater and freshwater lead
to a size distribution of LSA that is bimodal compared to the unimodal SSA.
The primary and secondary lognormal modes of the aerosol size distribution
generated from the Lake Michigan freshwater sample were centered at larger
diameters (180 ± 20, 46 ± 6 nm) than the aircraft-measured
mode (15–40 nm) over Lake Michigan by Slade et al. (2010).
Lower RH aloft and the presence of other aerosol present near the modes of
the LSA size distribution in the ambient atmosphere sampled by
Slade et al. (2010) could explain the lack of agreement
with this laboratory study and reported LSA diameter modes. The larger LSA
observed in this study could better activate as CCN (Lewis and
Schwartz, 2004) than the smaller LSA observed by Slade et al. (2010) and the smaller SSA observed in this study and others; however,
further studies are needed.
While this laboratory study represents a fundamental exploration of the role
of inorganic salts in LSA production, the role of organic and biological
material present in lake water in determining LSA production and properties
is currently poorly understood. Organic, heavy metal, and biological content
of seawater is known to affect SSA production and properties (Ault et
al., 2013b; Burrows et al., 2014; Facchini et al., 2008; Guasco et al.,
2013; Lee et al., 2015; O'Dowd et al., 2008; Prather et al., 2013; Quinn et
al., 2014), and thus organic and biological components of lake water are
likely to affect LSA production, properties, and heterogeneous chemistry
(Ault et al., 2013a, 2014; Ryder et al., 2014). This study
observed the effect of organic and biological materials in lake water on LSA
through the differences in the aerosol size distributions and aerosol
circularity generated from the organic- and biological-rich Lake Michigan
freshwater sample, and the organic- and biological-free synthetic freshwater.
Lake water has a higher ratio of organic to inorganic content than seawater
(Chapra et al., 2012; Pilson, 2013), so the organic
content in lake water likely plays a larger role in LSA than the organic
content in SSA. In addition, recent increases in toxic cyanobacteria blooms
in the Great Lakes (Michalak et al., 2013) may impact air quality if
toxic components are aerosolized with LSA, as has been observed for marine
algal blooms (i.e., red tides) (Cheng et al., 2010; Woodcock, 1948).
Therefore, future studies are needed to determine the effect of the organic
and biological content in freshwater on aerosol production and resulting
properties.
The impact of LSA on radiative forcing and precipitation in the Great Lakes
region is currently uncertain (Chung et al., 2011) and more
detailed modeling based on particle mixing state is needed (Bauer et
al., 2013). For example, SSA impacts radiative forcing directly through
scattering and indirectly by acting as CCN (Collins et al., 2013), which
influences cloud properties and precipitation patterns (Wise
et al., 2009), and LSA could have a similar effect. The Great Lakes' impact
on downwind cloud cover and precipitation, known as the lake effect, is well
known, and LSA could play a role in this process (Scott and Huff,
1996). The contribution of LSA to regional aerosol concentrations may have
seasonality, with the highest production likely occurring in the fall and
late spring when wind speeds are highest and the lakes are not covered in
ice. With global climate change predicted to decrease ice extent during
winter (Wang et al., 2012) and observed increases in wind speed,
linked to warming temperatures (Desai et al., 2009), the impact of
LSA is expected to increase in the future.