Fluxes of sea spray aerosols were measured with the eddy
covariance technique from the Penlee Point Atmospheric Observatory (PPAO) on
the southwest coast of the United Kingdom over several months from 2015 to
2017. Two different fast-responding aerosol instruments were employed: an
ultra-fine condensation particle counter (CPC) that detects aerosols with
a radius above ca. 1.5 nm and a compact lightweight aerosol spectrometer
probe (CLASP) that provides a size distribution between ca. 0.1 and 6 µm. The measured sea spray emission fluxes essentially all originated from
the shallow waters upwind, rather than from the surf zone/shore break.
Fluxes from the CPC and from the CLASP (integrated over all sizes) were
generally comparable, implying a reasonable closure in the aerosol number
flux. Compared to most previous observations over the open ocean, at the
same wind speed the mean sea spray number fluxes at PPAO are much greater.
Significant wave height and wave Reynolds numbers explain more variability in
sea spray fluxes than wind speed does, implying that enhanced wave breaking
resulting from shoaling in shallow coastal waters is a dominant control on
sea spray emission. Comparisons between two different wind sectors (open
water vs. fetch-limited Plymouth Sound) and between two sets of sea states
(growing vs. falling seas) further confirm the importance of wave
characteristics on sea spray fluxes. These results suggest that spatial
variability in wave characteristics need to be taken into account in
predictions of coastal sea spray productions and also aerosol loading.
Introduction
Sea spray aerosols formed from wave breaking impacting the Earth's radiative
balance both directly by scattering light (Haywood et al., 1999; Lewis and
Schwartz, 2004) and indirectly by affecting marine cloud formation (Clarke
et al., 2006). At high wind speeds sea spray also has the potential to
influence the air–sea transfer of heat and gases (e.g. Andreas et al., 1995;
Jeong et al., 2012). For atmospheric chemistry, sea spray
droplets provide an important medium for heterogeneous reactions (Sievering
et al., 1991; Kim et al., 2014). In coastal regions where roughly half of the
world's population resides, sea spray constitutes a major component of
particulate matter in the marine atmospheric boundary layer. Particulate
matter is directly relevant for human health and is subject to air quality
regulations.
Breaking waves entrain air into water, resulting in plumes of bubbles within
the top metres of the ocean and the appearance of whitecaps (Thorpe, 1992).
Sea spray aerosols are primarily formed from the bursting of those bubbles.
Two modes of sea spray aerosols are commonly observed: the film drop mode
(predominantly submicron) arising from disintegration of relatively large
bubbles, and the jet drop mode (≥ 500 nm to a few microns in radius)
from relatively small bubbles (e.g. Blanchard, 1963). Tearing of the wave
crest under conditions of very high winds can also lead to large spume
droplets (tens of microns to millimetres; Monahan et al., 1983), which may
contribute significantly to the total sea spray mass flux but not to the
number flux. The causal relationship between bubbles and sea spray has led
to the application of the “whitecap” method for the estimation of sea
spray flux over the ocean, in which the spray production flux from unit area
of a foam surface is prescribed based on other measurements and scaled up by
the whitecap fraction (e.g. Monahan et al., 1986; Mårtensson et al., 2003; Clarke et al., 2006). However, the parameterization of whitecap fraction as a function of wind speed has an uncertainty of about an order of magnitude in
moderate winds (e.g. Anguelova and Webster, 2006).
Recent works suggest that the whitecap fraction is also sensitive to
parameters such as sea state (Scanlon and Ward, 2016; Brumer et al., 2017a)
and water temperature (Salisbury et al., 2014; Salter et al., 2015). In
shallow waters, wave breaking (and so whitecap fraction) depends not only on
wind forcing, but also on the interactions between wind waves and swell with
the bottom topography (e.g. de Leeuw et al., 2000; Resio et al., 2002). The
bubble number concentrations within the surf zone can be about 2 orders of
magnitude higher than over the open ocean (e.g. Brooks et al., 2009). van
Eijk et al. (2011) estimated surf zone aerosol flux on sandy beaches to be
about an order of magnitude higher than for the open ocean under similar
wind conditions.
There are only a few datasets of direct measurements of sea spray fluxes by
the eddy covariance (EC) method. Based on data from an Arctic cruise,
Nilsson et al. (2001) published the first EC measurements of aerosol number
fluxes, which correlated strongly with wind speed. Their data suggest that
sea spray source flux consists of a film drop mode centred at a
∼ 50 nm radius and a jet-drop-mode centre at a 500 nm radius.
Geever et al. (2005) measured submicron aerosol fluxes at a radius
> 5 and > 50 nm at the coastal station of Mace Head
on the west coast of Ireland. They found comparable aerosol number fluxes in
the Aitken mode (5–50 nm) and in the accumulation mode (50–500 nm).
Norris et al. (2008) measured size-distributed aerosol fluxes between 0.15
and 3.5 µm radius at the Duck Pier on the east coast of the United States. They showed that sea spray flux increases with the local wind speed up to a
radius of 1 µm. More recently Norris et al. (2012, 2013b) measured size-distributed sea spray fluxes (0.18 < radius < 6.61 µm)
over the open ocean, and they explored the wind speed and wave Reynolds number
dependences of the flux. The wave Reynolds number (primarily a function of
wind stress and significant wave height) was found to explain about twice as
much variance in the open ocean sea spray fluxes than wind speed alone
(Norris et al., 2013b).
Total and size-distributed aerosol number fluxes have seldom been measured
simultaneously, precluding an assessment of the sea spray flux closure and
distribution. Here we report concurrent measurements of effectively
“total” (radius > 1.5 nm) and size-distributed sea spray fluxes
(about 0.1 < radius < 6 µm radius) from a coast site
over several months from February 2015 to February 2017. We examine how sea
spray fluxes vary with wind speed (1–21 m s-1), significant wave
height (0.2–3.1 m), wave Reynolds number (7×103–2×106) and other surface ocean parameters. We also examine
the size distribution and closure in aerosol number fluxes at different sea
states.
Experiment
The Penlee Point Atmospheric Observatory (PPAO, http://www.westernchannelobservatory.org.uk/penlee/, last access: 13 December 2019) on the southwest
coastal of the United Kingdom has proven to be a suitable site for eddy
covariance measurements of air–sea transfer (Yang et al., 2016a, b,
2019). PPAO sits about 11 m above mean sea level and a few tens of metres away
from the water's edge. The eddy covariance (EC) system, including the fast
aerosol sensors and a sonic anemometer (R3, Gill), is mounted on a mast at
about 18 m above mean sea level. An ultra-fine condensation particle counter
detecting aerosols > 1.5 nm radius (condensation particle counter (CPC) 3025A, TSI) and a compact
lightweight aerosol spectrometer probe (CLASP; Hill et al., 2008) providing
size spectra for radii between ca. 0.1 and 6 µm at ambient humidity were
employed at PPAO. The CPC was used for flux measurements between 24 February
and 3 June 2015. The CLASP unit deployed between 17 February and 1 March 2015 had a size range of 0.11 to 6 µm. This encompasses 1 week of
overlap with the CPC, which we primarily use to study the closure of aerosol
number fluxes. A slightly different CLASP unit was deployed between 21 December 2016 and 16 February 2017, which had a size range of 0.15 to 6 µm. Because of the narrower size range of this second unit and thus more
undetected film drop aerosols, data from the 2016–2017 period were only used
to contrast between open water and fetch-limited conditions. In this paper,
we operationally use the word “total” to refer to aerosol source flux
(i.e. corrected for deposition; see below) either from the CPC or integrated
over the entire CLASP size range, unless otherwise specified.
Previous EC observations of momentum, sensible heat, CO2, and CH4 fluxes at PPAO show that two wind sectors are representative of air–water
transfer: the southwest sector for which airflow is from the open ocean and
the northeast sector with airflow from the fetch-limited Plymouth Sound
(Yang et al., 2016a, b, 2019). A flux footprint model for spatially
homogeneous conditions (Kljun et al., 2004) predicts that under typical
southwesterly winds, the majority of the turbulent flux at a sensor height
of 18 m above mean sea level comes from waters several hundred metres upwind
of the site with a mean water depth of ∼ 20 m. When winds are
from the northeast, the flux footprint is over the Plymouth Sound and does
not overlap with land on the opposite side of the sound (5–6 km away)
except possibly under strongly stable conditions.
In the eddy covariance method, aerosol number concentration (C) measured at
high frequency (here 10 Hz) is correlated with the vertical wind velocity
component and averaged over time to yield the net aerosol flux
(=C′w′‾, where primed quantities are perturbations from the
mean and the overbar is the averaging operator). The measured net flux is
the sum of the source (upwards, positive) and deposition (downwards,
negative) flux components (Nilsson et al., 2001; Geever et al., 2005; Norris
et al., 2013b). The sea spray source flux must be derived from the net flux
by subtracting a deposition flux (=C⋅‾Vd). Here
Vd, negative in sign, is the aerosol deposition velocity from Slinn and Slinn (1980) (their Eq. 4), which accounts for gravitational settling
and is computed assuming the density of sea salt. Both the CLASP and the CPC
fluxes are initially computed as 10 min averages and then filtered for
non-stationary turbulence conditions following Yang et al. (2016a). Valid
fluxes can be averaged (e.g. to hourly intervals) to reduce noise.
For the CLASP, a small fraction of the aerosols are lost to the short inlet
(∼ 25 cm) during the 0.1 s of transit time. The loss is size
dependent, and predicted inlet efficiency varies from essentially unity for
aerosols smaller than 2 µm radius (film droplets and most of the jet
droplets) to ∼ 0.5 at 5 µm and to ∼ 0.1
at 8 µm (Pui et al., 1987). Spume aerosols are likely too large to be
efficiently sampled by the CLASP. We corrected for aerosol loss in the CLASP
inlet prior to the flux calculations. Following Norris et al. (2012), the
CLASP measurements are converted from net fluxes to source fluxes using the
size-resolved Vd. Integrated over all CLASP sizes, this deposition
correction amounts to -25 cm-2 s-1 in the mean (up to
∼-200 cm-2 s-1) when winds were from the southwest.
This represents 14 % of the net flux on average (up to ∼ 50 %). For submicron aerosols only (i.e. radius of 0.1–0.5 µm), the
deposition correction is -7 cm-2 s-1 in the mean. Humidity flux
induces a bias in the CLASP aerosol flux (Fairall et al., 1984). This is
because the particles are sized at ambient humidity, but grow or shrink with
local relative humidity (RH). We correct for this bias using the modified
bulk correction scheme described by Sproson et al. (2013). The final CLASP
fluxes are presented at a constant relative humidity of 80 % following the
aerosol growth rate for sea salt reported by Gerber (1985). Together these
corrections amount to typically 20 %–30 % of the total CLASP source number
flux at PPAO. A robust humidity observation was unavailable from December 2016 to February 2017. Thus the CLASP measurements during that phase could
not be fully corrected for humidity effects and were thus more uncertain.
The ultra-fine CPC sub-sampled from a Teflon tube (18 m long, 0.64 cm ID,
flow rate of ∼ 15 L min-1). The use of Teflon tube
is generally not recommended for aerosol measurements, as its non-conductive
nature can lead to significant aerosol losses; it was employed here out of
convenience because the CPC was sub-sampling from an existing inlet tube for
CO2 flux measurements (Yang et al., 2016b). A brief test of CPC
measurements at PPAO between using the long Teflon inlet tube and a
∼ 5 m, 0.32 cm ID stainless steel tube did not show any
obvious difference in the aerosol number concentration. The CPC cospectra
also do not suggest severe aerosol losses at high frequencies. The better-than-expected transmission through the Teflon sampling tube may be because
the tube had been used under high flow rate at PPAO for nearly a year prior
to the CPC measurements. It was well coated with sea salt, which probably
increased its conductivity and thus reduced electrostatic aerosol losses.
The long sampling tube resulted in a delay time of ∼ 3 s in
the CPC signal relative to the turbulent wind measurements, as determined by
the maximum covariance method between aerosol concentration and vertical
wind velocity. This delay is close to the expected time based on inlet
length, diameter, and flow rate in the main tube (2.3 s). The CPC inlet
efficiency, accounting for its length, a 90∘ turn and three bends
(not including any electrostatic losses), is predicted to be essentially
unity for 0.3 < radius < 2 µm (Pui et al., 1987). For
radius < 0.3 µm, the mean inlet efficiency is 0.96 (< 0.8 for nucleation-mode aerosols). For radius > 3 µm, the
efficiency drops to ∼ 0.7. Previous observations in the marine
atmosphere show that total aerosol number is usually dominated by particles
below a radius of about 0.3 µm (e.g. Hoppel et al., 1990), which
suggests only a minor inlet loss for the CPC (≤ 4 %). We choose not
to apply any inlet efficiency correction to the CPC flux since the
corrections are probably small, and the size distribution of aerosol flux
below 0.1 µm (lower cut-off of CLASP) is not known. We correct our CPC
fluxes for high-frequency flux attenuation due to the finite instrument
response time (1 s for 95 % change) using an empirical filter function
approach analogous to that for gas flux measurements (e.g. Yang et al., 2013). Increasing with wind speed, this correction amounts to
∼ 10 % of the CPC number flux for the average condition at
PPAO.
The deposition correction for the CPC flux is only approximate due to both
uncertainties in Vd and the lack of knowledge of the fine-/Aitken-mode aerosol size distribution. The deposition velocity for submicron aerosols
over water, dependent on aerosol size and environmental conditions, is on
the order of approximately -0.01 to -0.1 cm s-1 (Slinn and Slinn 1980;
Duce et al., 1991). The aerosol size distribution below 100 nm radius was not
measured at PPAO but previous maritime observations of submicron aerosols
generally suggest peak number concentrations at radii of ∼ 25
and 100 nm (e.g. Hoppel et al., 1990). We compute the wind-speed-dependent
Vd at these two aerosol sizes using the Slinn and Slinn (1980)
parameterization, which amounts to -0.034 and -0.010 cm s-1 for the
mean conditions at PPAO. For simplicity, we take the average of the two
Vd datasets and multiply it by the CPC number concentration to estimate
the deposition flux. When winds were from the southwest, the deposition flux
amounts to 33 cm-2 s-1 in the mean (19 % of the net flux).
Frequency-weighted cospectra of total CLASP and CPC net aerosol fluxes
averaged to wind speed bins are shown in Figs. 1 and 2 for the open water
wind sector. In low to moderate winds, the aerosol cospectra are broadly
consistent with the theoretical spectral shape for turbulent transfer
(Kaimal et al., 1972) and with previously observed gas cospectra at PPAO
(Yang et al., 2016a). With increasing wind speeds the magnitudes of the
cospectra increase, reflecting greater sea spray fluxes. The CPC cospectra
are much noisier than those of the CLASP. This may be because most of the
aerosols detected by the CLASP arise from sea spray emission. In contrast,
only a very small fraction of the aerosols detected by the CPC participate
in rapid air–sea exchange (i.e. sea spray emission); the vast majority comes
from other sources (e.g. pollution) and increases the random measurement
noise in the CPC flux (see analogous discussion on methane flux by Yang et
al., 2016a).
Averaged CLASP frequency-weighted total (net) aerosol number
cospectra in wind speed bins for the southwest (open water) wind direction.
Results and discussionsSea spray flux closure and wind speed dependence
Aerosol fluxes were almost always positive (i.e. from sea to air),
indicating sea spray emission. Figure 3 shows the time series of total
aerosol number concentrations and source fluxes from the CPC and the CLASP
(during the 1 week of overlap), relative humidity and the CLASP deposition
correction term, significant wave height and tide height above the reference height, and wind speed and wave Reynolds number. Winds were coming from the
southwest for the majority of this week with peak speed > 16 m s-1. The CPC detected baseline concentrations of a few hundred aerosols
per cubic centimetre, typical for a marine atmosphere. Many short and sharp spikes (of
the order of thousands of reciprocal cubic centimetres) are apparent in the CPC time series. These
spikes are often coincident with spikes in sulfur dioxide and carbon dioxide
and are likely from ship exhaust emissions. CPC fluxes during such brief
periods of excessive variability (i.e. relative standard deviation over
50 %) are removed on the basis of non-stationarity.
Averaged CPC frequency-weighted (net) aerosol number cospectra in
wind speed bins for the southwest (open water) wind direction.
Aerosol number concentration integrated over all sizes is much lower and
more constant from the CLASP. For the southwest wind sector, the median
supermicron and submicron aerosol concentrations detected by the CLASP were
11 and 24 cm-3 respectively. The magnitude of the former is consistent
with the typical sea spray aerosol number concentration in the marine
boundary layer (e.g. O'Dowd and Smith, 1993). The CLASP did not detect ship
plumes as ship-emitted particles tend to be small (< 30 nm radius)
and below the measurement size cut-off (e.g. Hobbs et al., 2000; Petzold et
al., 2008). Despite the large differences in total number concentrations
between the CPC and CLASP, total aerosol fluxes from the two instruments
were generally comparable in magnitude. The CPC fluxes were slightly higher
than the integrated CLASP number fluxes by a mean (median) difference of 45
(40) cm-2 s-1 during this week. As discussed in Sect. 3.3, this
difference is likely due to the part of the film drop mode not completely
captured by the CLASP as a result of its lower size cut-off (∼ 0.1 µm).
Times series of (a) CPC and integrated CLASP aerosol number
concentration, (b) total aerosol number source flux from the CPC and CLASP,
(c) relative humidity and the integrated CLASP deposition correction, (d) significant wave height and tide height, and (e) wind speed (colour-coded by
wind direction) and wave Reynolds number.
Hourly total aerosol number source fluxes from the CPC and CLASP
vs. wind speed (data from 2015). Error bars correspond to standard
deviations within the bins. Also shown are source flux relationships derived
by Geever et al. (2005; > 5 nm radius) from the coastal site of
Mace Head and Nilsson et al. (2001; > 5 radius nm) from the open
ocean of the Arctic.
Sea spray flux peaked during periods of high winds and large waves. At low
tide, the distance between the water's edge and the flux sensors increases,
shifting the centre of the flux footprint closer to the shoreline.
Previously observed transfer rates of momentum, sensible heat and gases
(CO2 and CH4) did not vary with the tidal height (Yang et al., 2016a, b), suggesting that the narrow surf zone (width of a few metres)
beyond the rocky shoreline in front of PPAO has a negligible influence on
the measured fluxes. The same appears to be true for the aerosols, as
periods of low tide do not consistently result in large sea spray fluxes
(Fig. 3). Thus unlike Geever et al. (2005), we do not need to filter out
data for low-tide conditions. On 27 February 2015, during light, northerly
winds from over the land, aerosol fluxes from both instruments were near
zero, as expected.
Figure 4 shows the wind speed dependence in aerosol source flux from the CPC
(February to June 2015) and from the CLASP (February to March 2015). Here we
have restricted data to the southwest wind sector (open water) only. Across
most of the wind speed range, total aerosol source fluxes from the CPC and
the CLASP show a similar relationship with wind speed. Sea spray source flux
from PPAO amounts to about 200 cm-2 s-1 in the mean (median of
∼ 150 cm-2 s-1), increasing non-linearly with wind
speed up to ∼ 1000 cm-2 s-1 at a wind speed of 20 m s-1. These fluxes are in reasonable agreement with the source flux
relationship found by Geever et al. (2005; > 5 nm radius) for a
coastal site at Mace Head during high tide, when wave breaking at the shore
did not unduly influence their measurements. In comparison, the relationship
found by Nilsson et al. (2001; > 5 nm radius) from the Arctic
Ocean is significantly higher than the PPAO measurements at wind speeds
above ∼ 12 m s-1. Bin averages of total aerosol number
flux measurements at PPAO are plotted on a log scale against wind speed in
Fig. 5 along with the parameterizations from Nilsson et al. (2001) and
Geever et al. (2005) for both net and source fluxes. Measured net fluxes at
PPAO agree very well with the net fluxes from Geever et al. (2005). The
source fluxes show greater discrepancy due to the different deposition
correction schemes applied.
Non-wind-speed controlling factors on sea spray fluxes
Sea spray flux is more strongly associated with significant wave height
(Hs, derived from the full wave spectrum) than with wind speed (Fig. 6). Wave data when winds were from the southwest are taken from a Waverider
buoy from Looe Bay, about 16 km west of PPAO (http://www.channelcoast.org/data_management/real_time_data/charts/?chart=98, last access: 13 December 2019). The stronger dependence on Hs is
reflected in the hourly CPC data (n=452) by both higher R2 (0.65)
and Spearman's rank correlation coefficient (R=0.71) compared
to the dependence on wind speed (R2=0.47 and Spearman's R=0.51). When Hs was below 0.5 m, the mean CPC source flux averaged to
about zero. Such a strong dependence of sea spray flux on wave height is
typically not observed over the open ocean (e.g. Norris et al., 2013b) and
is likely due to coastal wave breaking. Equilibrium wind waves at wind
speeds > 10 m s-1 as well as swell have wavelengths that are
longer than twice the water depth within the PPAO flux footprint (Pierson
and Moskowitz, 1964) and should shoal due to interactions with the sea
floor. We thus expect coastal waves to break more frequently and generate
more sea spray compared to open ocean waves at the same wind speeds.
Bin-averaged total aerosol number fluxes (net and source) from the
CPC and the CLASP (in log scale) vs. 10 m wind speed (data from 2015). Also
shown are net and source flux relationships derived by Geever et al. (2005;
> 5 nm radius) from the coastal site of Mace Head and Nilsson et
al. (2001; > 5 radius nm) from the open ocean of the Arctic.
Error bars (2× standard error) are smaller than the marker size and thus
not displayed.
Following Zhao and Toba (2001), we computed the wave Reynolds number as
RHw=u*Hs/ν, where u* is the friction
velocity from eddy covariance and ν is the kinematic viscosity of
seawater. Sea spray flux increases approximately linearly with the wave
Reynolds number (RHw, Fig. 7). The source flux dependence on
RHw is slightly weaker than on Hs but stronger than on wind speed
(R2=0.62 and Spearman's R=0.63) for the open water sector.
Norris et al. (2013b) found a linear dependence between sea spray flux and
RHw. They also argued that below a critical RHw of 7.2×104, wave
breaking does not occur and sea spray source flux should be zero, in general
agreement with observations here. The linear dependence of sea spray flux at
PPAO on RHw is qualitatively consistent with their open ocean results.
The RHw parameterization from Norris et al. (2013b) integrated over all
CLASP size bins is shown in Fig. 7. We see that sea spray fluxes measured
at PPAO exceed those open ocean observations by about an order of magnitude
(but are smaller than estimates over a surf zone by Clarke et al., 2006, as
shown in Sect. 3.3).
Hourly total aerosol number source fluxes from the CPC and CLASP
vs. significant wave height (data from 2015).
The influence of waves on coastal sea spray generation is further
illustrated in the comparison between open water and fetch-limited
conditions (Fig. 8). We use data from the second CLASP deployment
(December 2016 to February 2017) for this analysis, as winds were seldom
from the northeast during the first CLASP deployment (February–March 2015).
Here we separate the integrated sea spray fluxes measured by the CLASP into
two different wind sectors: the southwest (open water) and the northeast
(facing the Plymouth Sound with a fetch over water of ∼ 5 km).
At a given wind speed, sea spray fluxes were generally greater for the open
water sector than for the fetch-limited sector. Wind speed was an even
poorer predictor of sea spray flux from the open water during this period,
as some high sea spray fluxes were observed at low wind speeds due to the
presence of large swell. Fluxes from both wind sectors show better
correlations against Hs, though with different trends; higher fluxes are
observed for the fetch-limited conditions than for open water at a given
wave height. Here Hs in the Plymouth Sound is predicted using a
parameterization for fetch-limited waters (Resio et al., 2002) as a function
of fetch and friction velocity since a direct wave measurement was not
available. The different functional dependencies of the sea spray flux on
wind speed and wave state for open water and fetch-limited conditions are
reconciled when their joint influence is accounted for by the wave Reynolds
number. When plotting against RHw sea spray fluxes from both wind
sectors fall closely on the same curve (Fig. 8c). This clearly illustrates
the importance of jointly accounting for the influence of both wind and
waves on air–sea exchange. This result is consistent with the recent
findings of Brumer et al. (2017a, b) for whitecaps and gas transfer and from
Norris et al. (2013b) for sea spray fluxes.
Hourly total aerosol number source fluxes from the CPC and CLASP
vs. wave Reynolds number (log scale; data from 2015). Also shown are a
linear fit to the CPC fluxes and the Reynolds number parameterization from
Norris et al. (2013b, integrated over all CLASP size bins).
Total aerosol number net flux from the CLASP (December
2016–February 2017) vs. wind speed (a), significant wave height (b), and
wave Reynolds number (c). Data are separated into two distinct wind sectors: the southwest sector that faces the open water and the northeast sector that faces the Plymouth Sound (fetch of ∼ 5 km). Power fits of
aerosol fluxes to Hs and linear fits to RHw are also shown.
Total aerosol number source flux from the CPC for the southwest,
open water wind sector versus wind speed (a) and versus significant wave height (b). Data on both plots are separated into periods of increasing wind
speed (red crosses) and decreasing wind speed (blue dashes; see Sect. 3.2
for details).
There is an observable sea-state dependence in the sea spray flux for the
open water sector. Here we separate the CPC flux data into two groups of sea
states: increasing wind speed (i.e. hourly increase by more than 1 m s-1) and decreasing wind speed (i.e. hourly decrease by more than 1 m s-1). These correspond approximately to younger–growing seas and
older–more developed seas, respectively. As shown in Fig. 9, at
intermediate wind speeds (∼ 10 m s-1) the aerosol flux is,
in the mean, about twice as high during periods of decreasing wind speed than
during periods of increasing wind speed, qualitatively consistent with
Norris et al. (2013b) for sea spray fluxes and Callaghan et al. (2008) for
whitecap fraction. Hs is also larger during periods of decreasing wind
speed. Waves shoal and are be likely to break near the coast
regardless of their state of development. Thus larger waves from more
developed seas tend to lead to greater sea spray fluxes in these kinds of
coastal environments.
Size-distributed aerosol number from the CLASP, bin-averaged
according to wind speed (a) and significant wave height (b). Data are
limited to the southwest (open water) wind sector only. Error bars indicate
standard errors. Radius adjusted from ambient humidity to a relative
humidity of 80 %.
Size-distributed source number flux from the CLASP, bin-averaged
according to wind speed (a) and significant wave height (b). Data are
limited to the southwest (open water) wind sector only. The open ocean
source flux parameterizations from Clarke et al. (2006) and Norris et al. (2013b) are approximated at a wind speed of 10 m s-1, while the
measured surf zone flux from Clarke et al. (2006) is more than an order of
magnitude higher. Error bars indicate standard errors. Radius adjusted from
ambient humidity to a relative humidity of 80 %.
It is interesting that there is a large discrepancy in magnitude in the
source flux vs. RHw relationship between the open ocean observations
from Norris et al. (2013b) and those at PPAO (Fig. 7) – if RHw is
such a good predictor of air–sea fluxes, why does it fail to reconcile these
two datasets? We suggest two possible reasons. First, shoaling may result
in more frequent and intense wave breaking near the coast compared to the
open ocean as the waves steepen upon approaching the shore (e.g. Elgar et
al., 1997). Second is a potential difference in aerosol production per unit
area whitecap between a coastal region with shoaling waves and the open
ocean as a result of the different wave breaking and bubble generation
processes (Deane and Stokes, 1999; Lewis and Schwartz, 2004; de Leeuw et al., 2011). Bubble populations near the sea surface were generally found to be
higher close to the coast than in open water (Johnson and Cooke, 1979;
Brooks et al., 2009). Different void fractions have also been observed
beneath plunging and spilling breaking waves (Rojas and Loewen, 2010).
Testing of these hypotheses requires observations of the relationships
between whitecaps, bubbles and RHw in the nearshore region.
Laboratory measurements suggest dependence of the sea spray flux on water
temperature and salinity (e.g. Mårtensson et al., 2003; Salter et al., 2014),
while Tyree et al. (2007) showed that the addition of natural organic matter
increased the submicron aerosol flux by 50 %. Previous observations of
aerosol composition in marine environments imply that a significant fraction
of sea spray is made up of organic materials (e.g. O'Dowd et al., 2004). We
investigate these dependencies by comparing our sea spray flux data to
surface ocean parameters from the marine station L4 (6 km south of PPAO) of
the Western Channel Observatory (http://westernchannelobservatory.org.uk/, last access: 13 December 2019). From February to June 2015, sea
surface temperature and salinity varied between 9.2 and 12.5 ∘C
and between 35.1 and 35.3, respectively. Chlorophyll a increased from 0.8 to
3.1 mg m-3 during the spring phytoplankton bloom, while coloured
dissolved organic matter (from the E1 buoy 18 km offshore) also varied by
about a factor of 4. We test the importance of these surface ocean
parameters by examining their correlations vs. FSSA′, where
FSSA′ is the total sea spray source flux (FSSA) minus a polynomial
fit to FSSA (as a function of wind speed or Hs). No significant
correlation was found. This suggests that within the range of conditions
observed at PPAO during this deployment, waves and wind are the 1st-order
drivers for sea spray formation, while the other parameters appear to be of
little importance.
Size-distributed aerosol number concentrations and fluxes
Figure 10 shows number distribution (dN/dR80) vs. radius at a humidity
of 80 % (R80) averaged in wind speed as well as significant wave
height bins. Data are taken from February to March 2015 and for the southwest
(open water) wind sector only. The overall distribution in concentration is
fairly typical of the marine atmosphere, with most aerosols in the submicron
mode. The number distributions are more clearly segregated by significant
wave height than by wind speed for radius up to about 2 µm. Above this cut-off, the number distribution seems to be largely independent of Hs or
wind speed.
In accordance with the total aerosol number fluxes, size-distributed fluxes
from the PPAO (dF/dR80) are significantly higher than measurements over
the open ocean (Fig. 11). For example, in moderate seas at PPAO the
measured source flux at R80 of 1 µm exceeds 106 m-2 s-1µm-1, compared to approximately 105 m-2 s-1µm-1 from Norris et al. (2013b). Sea spray flux from the
coastal seas near PPAO is lower than estimates from the surf zone by Clarke
et al. (2006). Readers interested in other previous measurements and
parameterizations of size-distributed sea spray source fluxes are referred
to reviews by O'Dowd and de Leeuw (2007) and de Leeuw et al. (2011).
Ratio of size-distributed source number flux in film drop mode
(here R80=100 nm) to that in the jet drop mode (here R80=500 nm) as well as normalized dF/dR80 at these sizes as a function of significant wave height.
Size-distributed aerosol number concentrations as well as fluxes peak at the
lowest size bin of CLASP (0.1 µm radius at ∼ 80 % RH),
near the typical mode centre for film droplets (Mårtensson et al., 2003;
Tyree et al., 2007). We can crudely estimate the contribution of film drop
fluxes below the detection cut-off for the CLASP by fitting a log-normal
distribution to the observed dF/dR80. Here we assume that the observed
dF/dR80 at a R80 of 0.1 µm represents the peak in the film
drop mode. Integrating the log-normal fit from 1.5 nm to 0.1 µm yields
the “missing” film drop flux (median value of ∼ 40 cm-2 s-1), which amounts to about 25 % of the total measured CLASP number
flux. This is consistent with the finding that the total aerosol number flux
from the CLASP was lower than that from the CPC by a mean (median) of 45
(40) cm-2 s-1. Future observations of the fine aerosol size
distribution (e.g. by a scanning mobility particle sizer) at PPAO should
provide more information on the robustness of our assumption above.
About 70 % of the sea spray number fluxes measured by CLASP are submicron
(at RH of 80 %), with the vast majority of the aerosol number flux
residing between radius 0.1 and 1.1 µm (Fig. 11). Aerosols with
a radius greater than 2 µm make up only ∼ 1 % of the
integrated CLASP number flux. The distribution of source number flux below a
radius of about 2 µm is more clearly segregated by Hs than by wind speed, as with the aerosol number size distribution. Above 2 µm radius, size-distributed fluxes no longer seem to depend on Hs or wind speed.
There is a subtle decreasing trend in the ratio between film drop mode
(radius of ∼ 100 nm at 80 % RH) and jet drop mode (radius of
∼ 500 nm at 80 % RH) with increasing significant wave height
(Fig. 12). An examination of normalized dF/dR80 (by the respective
mean flux) shows that with greater wave height, the jet drop mode appears to
increase more steeply than the film drop mode. A similar shift in the shape
of size-distributed aerosol flux with growing seas has been observed
previously by Norris et al. (2013a). They showed that the bubble spectra
change with wind speed, with the concentrations of small bubbles (which lead
to jet-mode aerosols) increasing more rapidly than the large bubbles (which
lead to film-drop-mode aerosols).
Concluding remarks
Eddy covariance measurements of sea spray fluxes originating from the
shallow waters upwind of PPAO show that about 70 % of the total detected
number fluxes were submicron. A reasonable closure is found between the
aerosol number flux from the CPC (> 1.5 nm in radius) and the
total number flux from the CLASP (0.1–6 µm in radius) after
considering the incomplete detection of film-mode aerosols by the latter.
Sea spray fluxes from the open water wind sector at PPAO increase with wind
speed with a dependence that is similar to previous coastal sea spray flux
measurements at Mace Head (Geever et al., 2005). Our observed fluxes are
greater in magnitude than most previous open ocean measurements except those
reported by Nilsson et al. (2001), but are lower in magnitude than previous
surf zone estimates (e.g. Clarke et al., 2006).
Sea spray formation in this coastal environment is strongly dependent on sea
state. Both significant wave height (Hs) and wave Reynolds number
(RHw) are better predictors of sea spray fluxes than local wind speed.
The importance of waves is further confirmed by comparing sea spray fluxes
measured from the open water sector to fluxes from the fetch-limited
Plymouth Sound, where waves were much smaller at a given wind speed. For
both wind sectors, sea spray fluxes correlate with Hs more strongly than
with wind speed, but with two very different relationships. The wave
Reynolds number (RHw) reconciles the fluxes from the two sectors. This finding is consistent with those of Brumer et al. (2017a, b), who found
RHw to be a much better predictor than wind speed of both the whitecap fraction and gas transfer velocity in different wind–wave regimes.
Sea spray fluxes measured at PPAO (open water sector) are likely only
representative of the nearest few kilometres from shore. The median supermicron
and submicron sea spray fluxes from the CLASP during February–March 2015
were about 60 and 100 cm-2 s-1, respectively. The residence times
of supermicron and submicron aerosols in a 500 m deep marine atmospheric
boundary layer (MABL) against dry deposition are of the order of 0.6 and 6 d (at a respective deposition velocity of -1 and -0.1 cm s-1; Slinn and Slinn, 1980). The residence time for submicron aerosols is likely
further reduced by wet deposition (∼ 3 d; Lewis and
Schwartz, 2004). At these timescales, the expected steady-state supermicron
and submicron aerosol number concentrations for a well-mixed MABL based on
our measured fluxes would be on the order of 60 and 300 cm-3,
respectively. These are significantly higher than the observed median
supermicron and submicron aerosol concentrations from the CLASP of 11 and 24 cm-3 for the open water wind sector. Clearly aerosol concentrations and fluxes are not in steady state in this coastal environment, consistent with previous findings by Andreas et al. (2010) and Freire et al. (2016). The
fluxes are enhanced near the coast due to increased wave breaking resulting
from shoaling of waves in shallow water, while the concentrations reflect
both sea spray generated within the flux footprint and the aerosol
sources and sinks further upwind. To map out the spatial distributions of
sea spray fluxes, measurement techniques such as eddy covariance from a ship
and aerial imaging of whitecap fraction (e.g. from an unmanned aerial
vehicle) are needed.
Data availability
The data collection for this work was unfunded, and thus the data will not be submitted to a central depository. Please contact the corresponding author for the data.
Author contributions
MY carried out the majority of the measurements. IB provided the CLASP and
CPC instruments. IB and SN helped with instrument installation and data
analysis. TB helped with maintenance of the instruments at PPAO. MY prepared
the paper with contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Trinity House owns the Penlee site and has kindly agreed to rent the
building to PML so that instrumentation can be protected from the elements.
We are able to access the site thanks to the cooperation of Mount Edgcumbe
House. We thank John Prytherch and Matt Salter (Stockholm University) for
useful discussions; Frances Hopkins, Timothy Smyth and Phil Nightingale (Plymouth Marine
Laboratory, PML) for support; Robin Pascal and Margaret Yelland (National
Oceanography Centre, Southampton) for the loan of the R3 sonic anemometer;
and Ben Carlton (PML) for setting up data communication. The wave data from
Looe Bay are provided by the Channel Coastal Observatory. The Penlee site is
part of the Western Channel Observatory, which is funded by NERC's National
Capability programme. This work is contribution number 6 from the Penlee Point Atmospheric Observatory
(since the grant number is listed below under financial support).
Financial support
This research has been supported by the Natural Environment Research Council (grant no. NE/N018044/1).
Review statement
This paper was edited by Lynn M. Russell and reviewed by Sebastian Landwehr and Christopher Fairall.
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