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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L07804, doi:10.1029/2008GL033462, 2008
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Marine aerosol chemistry gradients: Elucidating primary and
secondary processes and fluxes
Darius Ceburnis,1 Colin D. O’Dowd,1 Gerard S. Jennings,1 Maria Cristina Facchini,2
Lorenza Emblico,2 Stefano Decesari,2 Sandro Fuzzi,2 and Jonas Sakalys3
Received 1 February 2008; accepted 22 February 2008; published 2 April 2008.
[1] Production mechanisms of aerosol chemical species, in
terms of primary and secondary processes, were studied
using vertical concentration gradient measurements at the
coastal research station in Mace Head, Ireland. Total
gravimetric PM1.0 mass, sea salt and water insoluble
organic carbon (WIOC) concentration profiles showed a net
production at the surface (i.e. primary production), while
nssSO 4 and water soluble organic carbon (WSOC)
concentration profiles showed a net removal at the
surface. These observations indicate that WSOC was
predominantly of secondary origin and that WIOC was
predominantly of primary origin. Derived PM1 mass
fluxes compared reasonably well with those previously
obtained from an eddy covariance (EC) technique
following a power law relationship with the wind speed
(FPM1 = 0.000096*U 4.23). For cases with clear primary
organic mass fluxes in the flux footprint WIOM mass
fluxes ranged between 0.16 and 1.02 ng m2 s1 and
WIOM/sea salt mass ratio was 0.34 – 3.6, in good
agreement with previous measurements at Mace Head.
Citation: Ceburnis, D., C. D. O’Dowd, G. S. Jennings, M. C.
Facchini, L. Emblico, S. Decesari, S. Fuzzi, and J. Sakalys
(2008), Marine aerosol chemistry gradients: Elucidating primary
and secondary processes and fluxes, Geophys. Res. Lett., 35,
L07804, doi:10.1029/2008GL033462.
1. Introduction
[2] Marine aerosols, despite their low concentrations,
contribute significantly to the global aerosol budget considering 70% Earth’s coverage by oceans. Significant impact of
marine aerosols on global climate was first demonstrated by
Charlson et al. [1987]. Sulphate aerosols produced from
plankton sulphur emissions were considered a dominant
species in marine atmosphere. Later O’Dowd and Smith
[1993] argued that significant amount of submicron sea salt
particles can be found in marine atmosphere and reported
the first wind-speed relationship for accumulation mode sea
salt. Also O’Dowd et al. [1999] have shown that sea salt
aerosol may significantly affect the properties of marine
CNN. More recently, O’Dowd et al. [2004] have suggested
that CCN carbon concentration in marine aerosols can be
linked to primary production at the ocean surface. The
1
School of Physics and Centre for Climate and Air Pollution Studies,
Environmental Change Institute, National University of Ireland, Galway,
Galway, Ireland.
2
Istituto di Scienze dell’Atmosfera e del Clima - CNR, Bologna, Italy.
3
Institute of Physics, Savanoriu, Vilnius, Lithuania.
Copyright 2008 by the American Geophysical Union.
0094-8276/08/2008GL033462$05.00
detailed study by Yoon et al. [2007] revealed strong seasonal cycle of physico-chemical properties of marine aerosol and linked it to the observed seasonal cycle of
chlorophyll-a concentration in ocean surface waters. However, hitherto, definitive proof, linking the organic matter
enrichment in marine aerosol to primary production processes, has been lacking.
[3] Previous flux measurements in North Atlantic marine
air using an eddy covariance (EC) technique [Geever et al.,
2005] have found a strong dependency of primary sea-spray
flux on the wind speed (Flux > 106 m2 s1 at wind speeds
of > 10 ms1) and that approximately 50% of the number
flux was attributed to the Aitken mode (10–100 nm) and
50% to the accumulation mode (100 nm – 500 nm). These
measurements represented the first pseudo-size-segregated
aerosol flux measurements in marine environment indicating a significant contribution of primary marine particles to
the submicron Aitken and accumulation modes. However,
the eddy covariance technique, including its modifications
(relaxed eddy accumulation or disjunct eddy covariance)
with near real-time measurement techniques, do not produce
unambiguous results for aerosol chemical fluxes.
[4] Given that off-line chemical analysis requires relatively long sampling times (of several days) to obtain
sufficient particulate matter in clean marine air, alternative
approaches to micrometeorological flux techniques are
required. In this study we adapt an experimental set-up
used by Valiulis et al. [2002] to determine near-surface
chemical gradients and resultant fluxes of aerosols in NE
Atlantic marine air.
2. Experimental Methods
[5] Vertical concentration profiles of sub-micron aerosol
chemistry were taken in clean marine air at the Mace Head
NE Atlantic research station. The sampling was conducted
on a 22m tower approximately 80 m from the shoreline at 3,
10 an 30 m heights (the highest sampling point was installed
on an extended pole) relative to the ground which was
approximately 5 m above MSL. The tower is located
upwind of the station buildings relative to the ocean. The
tower location relative to the sampling sector ensured
minimal if any contamination from building induced turbulence [de Leeuw et al., 2002].
[6] PM1.0 size selective inlets (PMX inlet, Sven Leckel
Ingenieurburo GmbH, Germany) were used to collect
aerosols on quartz filters. An automated clean sector
sampling system ensured sampling only of air (1) reaching
the site from a controlled sector between 190°– 300° and
(2) with a total particle concentration (d50% = 14 nm)
below 700 cm3. The system monitored both parameters
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that surf zone emissions at a range of 80– 180 m from the
tower could have contributed up to 20% of the difference in
concentration between the 3 m and 10 m levels and <5%
between 10 m and 30 m level. The difference in concentration (or gradient) between 3 and 10 m reaches 90% of it’s
value at 1170 m from the coast line, while the difference
between 10 and 30 meters reaches 90% of its value at
4840 m (Figure 1). Emissions from greater distances have
minimal contribution to the flux footprint and gradient
profile in terms of upward fluxes, but clearly, such emissions can influence the absolute concentration which in turn
influences the deposition flux magnitude. In other words,
the gradient flux footprint was limited to within 5 km region
of open water adjacent to the coast line for detecting upward
fluxes while the concentration footprint is typically 10–
100 times greater distance upwind.
Figure 1. Dependence of the relative concentration
difference between 3 and 10 m, and 10 and 30 m, on the
distance from the measurement point (coast line).
at a time and activated sampling when both conditions
were met. The sampling criteria ensured that air mass was
of clean marine origin. Post analysis also confirmed air
mass cleanliness with black carbon concentrations of less
than 50 ng m3 and a transit time of backward trajectories
of 4 – 5 days over the ocean [O’Dowd et al., 1993; Cooke
et al., 1997; Cavalli et al., 2004]. Eighteen 7-day samples
were obtained, with the actual accumulated sampling times
ranging from 20 to 98 hours (10 to 58% of the time)
depending on the above conditions. Quality control
resulted in 50% of the sample sets being discarded due
to too short sampling periods and 9 profile sets retained
for this analysis.
[7] Samples were collected from April to October 2005,
which covered the period of high biological activity over the
Northeast Atlantic. Samples were analysed for gravimetric
mass, Na+, SO2
4 , water soluble (WSOC) and insoluble
carbon (WIOC). Analytical details can be found in Cavalli
et al. [2004].
3. Results and Discussion
3.1. Source Region and Flux Footprint
[8] Previous flux measurements in marine air at Mace
Head [Geever et al., 2005] demonstrated that the flux
measured at the 22 m height had a flux footprint peak at
approximately 600 –1000 m from the coastline with significant contributions at distances of up to 10 km from the
coastline. In this study three levels were used for the
measurements, therefore, a slightly different approach to
estimate the flux footprint was necessitated. The extent of
the source region (flux footprint area) contributing to the
concentration gradient and the potential influence of surfzone emissions to the difference in concentration between 3
to 10 and 10 to 30 meters is estimated using the methodological approach used by Valiulis et al. [2002]. In this
analysis, we use the micrometeorological data set from the
Geever et al. [2005] study as that study corresponds to a
range of similar meteorological conditions at the same
location. The analysis of the flux footprint and source
regions influencing the different measurement heights found
3.2. Aerosol Chemical Gradients
[9] The average percentage contribution of aerosol chemical components to PM1 mass at 10 m level was as follows:
sea-salt - 24%, nssSO4 - 32%, WSOM - 25% and WIOM 19%. The WIOM/sea salt mass ratio varied from 0.34 to
3.6, in good agreement with previous measurements at
Mace Head [Cavalli et al., 2004; Yoon et al., 2007]. In
order to deal with the wide range of concentrations encountered in each profile, concentrations were normalised to the
sum of the total gradient concentration of individual chemical species mass. Such normalisation gives equal weight to
each individual profile. After normalisation, the profiles of
each mass category were averaged, resulting in statisticallymeaningful variances around the mean value. Figure 2a
shows the averaged normalised concentration profiles of
different species for the whole sampling period. Differences
between concentrations at all three heights were generally
statistically significant. PM1 mass, sea salt and WIOC
exhibited a decreasing concentration profile with height,
and WSOC showed the opposite pattern.
while nssSO2
4
The decreasing profile represents an upward mass flux and,
therefore, respective chemical species are produced at the
sea surface, while species exhibiting an increasing vertical
profile (with height) point to a source either produced
homogeneously in the boundary layer or from aloft (for
example cloud processing or entrainment from the free
troposphere). Thus, it can be concluded that sea salt and
WIOC are produced at the ocean surface via primary or
bubble-mediated production mechanism while nssSO2
4 and
WSOC must be produced via secondary (or gas-to-particle)
aerosol formation processes. It should be noted that the
removal of certain species at the surface is caused by dry
deposition only, as wet scavenging below the cloud evenly
removes aerosol particles.
[10] While it is well known that sea salt is a primary
aerosol and nssSO2
4 is a secondary aerosol, the distinct new
finding in this study is that WSOC and WIOC species
exhibited contrasting profiles and thus different formation
mechanisms. The WSOC showed consistently increasing
concentration profiles, while the individual WIOC profiles
showed variable gradients which warrant further exploration. In fact, the WIOC gradients were found to fit into three
distinct classifications as shown in Figure 2b. The first
classification exhibited a clear and strong negative gradient
corresponding to a significant primary surface source (net
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Figure 2. (a) Normalised vertical concentration profiles of PM1 mass, sea salt, nss SO42, water soluble (WSOC) and
insoluble carbon (WISOC) at Mace Head. (b) Normalised vertical concentration profiles of water insoluble carbon
(WISOC) depicting three different classification types (1 – 3). Each profile is an average of 2 – 4 individual profiles
representing same gradient classification type. Error bars represent standard deviation of the normalised concentration
average.
production) within the flux footprint; the second exhibits net
removal through a positive gradient between 3 –10 m and
almost zero gradient between 10 – 30 m; and the third
classification exhibits a deposition flux gradient (positive)
between 3 – 10 m and a source flux gradient (negative)
between 10 – 30 m. Given that enrichment of organic
component in primary aerosol is related to enrichment of
organic matter at the ocean surface, this range of behaviour
can be interpreted in terms of the location of biologically
active region relative to the flux footprint. If this region is
within the flux footprint (5 km from the coastline), a strong
negative gradient as in classification 1 will result. However,
if the oceanic organic matter or primary organic aerosol
production region is significantly and exclusively upwind of
the flux footprint (say 20 km), the aerosol will have become
well mixed throughout the boundary layer and a clear
deposition (positive) gradient will be observed as in classification 2. Classification 3 can result from a combination of
both scenarios. The classifications above are consistent with
satellite analysis of the spatial and temporal development of
chlorophyll-a distributions [Yoon et al., 2007].
[11] These results are the first experimental evidence of
different production mechanisms of the water soluble and
water insoluble organic carbon in marine aerosol, indicating
that WSOC is predominantly of secondary origin while
WIOC is predominantly of primary origin.
3.3. Mass Flux of Chemical Species and Relationship
to Wind Speed
[12] The averaged profiles presented in Figure 2a effectively show the mean concentration gradient and, hence, the
direction of the flux over a period of a few months. While
the direction of the flux was the main motivation for the
study, an attempt was made to calculate the mass flux from
individual gradients over each of the sampling periods using
detailed meteorological measurements performed by Geever
et al. [2005] in similar marine conditions. First-order
closure turbulent flux parameterisation, often known as a
gradient transport theory or K-theory, can be expressed
according to Stull [1988] as following:
F ¼ Kz
dc dz z
ð1Þ
where F is the flux, Kz is the turbulent-transfer coefficient;
dc/dz is the concentration gradient. Therefore, having Kz
and the measured concentration gradient, it is possible to
calculate mass flux of any chemical species. The approach,
however, does not allow distinguishing between upward
and downward fluxes, but rather enables estimation of the
net fluxes. The uncertainty of the estimated flux would
largely depend on the gradient function fitted to the
measured profile and the variability of the turbulent-transfer
coefficient at any given wind speed.
[13] The turbulent-transfer coefficient was derived from
its relationship with the horizontal wind speed using the
Geever et al. [2005] data set obtained under similar marine
conditions and same ensemble range. The relationship
followed a power law (Kz = 0.00117 U 2.067, r2 = 0.62,
P 0.01). The turbulent-transfer coefficient representing
each individual sampling period of this study was derived
from 30 min averages of the horizontal wind speed measured
during that period. Then the Kz values were averaged to obtain
the resultant turbulent-transfer coefficient corresponding to
each sampling period of this study. We admit that such an
approach introduced a certain degree of variability, but
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Figure 3. (a) Concentration gradient fitting procedure using linear and power functions. (b) PM1 flux relationship with
the wind speed at 22 m level. The calculated fluxes obtained from Geever et al. [2005] using the accumulation mode
diameter 0.140 mm and the density of 1.56 g cm3.
long sampling time required for off-line chemical analysis
necessitated averaging of environmental conditions. We
believe that the magnitude of the flux error would largely
depend on the variability of Kz value at any given wind
speed.
[14] The mean flux (and range of fluxes encountered) of
nine 7-day samples for individual chemical species at the
3 m level were: PM1 = 18 ng m2 s1 [4– 106]; sea salt =
48 ng m2 s1 [11 – 400]; nssSO4 = 14 ng m2 s1 [51–
(1.2)]; WSOM = 11 ng m2 s1[1040]; WIOM =
4.9 ng m2 s1 [1.3 – 35]. The mean fluxes were calculated
using equation 1 and the best fit to the data points.
[15] It was mentioned above that the uncertainty of the
calculated flux will depend on the gradient fitting procedure. The concentration profiles in Figure 2a suggest a
power function. However, due to the potential contribution
of surf-zone emissions to the magnitude of the concentration at 3 meter level and hence, on the shape of the gradient,
a linear gradient profile was considered too, ignoring the
lowest 3 m-level concentration. Three different fitting
exercises are presented in Figure 3a: using linear and two
different power functions: f = a + bx, f = a + bxc and f = axb.
PM1 mass concentration profiles were fitted with three
different functions seeking the best correlation with the
measurements and the fluxes calculated at 22 m level using
equation 1. It has been shown, that the measurements
performed at 22 m level are free from surf-zone influence
[Geever et al., 2005]. The ‘‘linear gradient’’ approach
yielded constant gradient. The gradient fitting procedure
can be considered as a sensitivity analysis of the calculated
flux. Then the calculated flux data were fitted with the power
function to reveal the flux relationship to the wind speed and
presented in Figure 3b. The impact of the fitting procedure
is small up to wind speed values of about 9 m s1, however,
the curves start to diverge at higher wind speeds. The area
between the broken curves can be considered as the uncertainty range of the calculated flux. Calculated fluxes of
WIOM for cases with clear primary production ranged from
0.16 to 1.02 ng m2 s1 at a wind speed of 8.4 and 7.9 m s1,
respectively. Note that at similar wind speeds WISOM fluxes
can differ by six times, which indicates the importance of
seasonal biological activity in the flux footprint area.
[16] The presented mass flux calculation method involved averaging up of individual samples and needs an
independent evaluation by comparing it with the flux
estimated using eddy covariance method by Geever et al.
[2005]. In order to compare the mass flux with the number
flux, a particle diameter and density had to be set. The
number flux can be converted into mass flux assuming
modal diameter and density of accumulation mode particles,
because contribution of Aitken mode particles to the PM1
mass is negligible. The density of the particles was set at
1.56 g cm3 and the diameter of the accumulation mode
was set at 140 nm based on long term observations of
aerosol physical and chemical properties at Mace Head
[Cavalli et al., 2004; Yoon et al., 2007]. The mass flux
estimated by eddy covariance method compared best with
the gradient flux, using the linear gradient fitting procedure
(FPM1 = 0.000096*U 4.23). The more important result
though was that regardless of the fitting procedure, the flux
relationship to the wind speed always followed a power law
relationship (F = a*Ub, r2 = 0.76 0.90). It can be argued
that the uncertainty of the flux was not caused by the fitting
procedure, but rather due to the lack of more high wind
cases, which would have certainly helped to constrain a
more reliable relationship. A generally good agreement
between the two methods demonstrates that the number
flux measurements using eddy covariance method can be
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successfully combined with off-line chemical measurements
in the development of a combined organic-inorganic sea
spray source function [O’Dowd et al., 2008].
4. Conclusions
[17] Production mechanisms and gradient fluxes of aerosol
mass and inorganic and organic components in marine air
over the Northeast Atlantic were studied using vertical
concentration profiles measured at Mace Head. Total gravimetric mass, sea salt and WIOC exhibited a net upward flux,
pointing to the primary source at the sea surface. NssSO2
4
and WSOC showed an opposite profile indicating a net
downward flux, pointing to the removal of the species, which
also indicates that it is produced via secondary processes. The
difference in WSOC and WIOC concentration gradients in
clean marine air masses is the first experimental evidence of
the different formation mechanisms of these species, namely,
secondary and primary, respectively. Calculated fluxes
depend significantly on the gradient fitting procedure, but
consistently demonstrated power law relationship with the
wind speed. The estimated gradient fluxes are the first
experimental estimates of the primary production rates for
WIOC in the marine atmosphere. The total submicron aerosol
mass flux relationship as a function of wind speed, described
by a power lay (FPM1 = 0.000096*U 4.23), agreed well with
previous eddy-covariance measurements performed under
similar conditions.
[18] Acknowledgments. This work was supported by EPA Ireland
grant 2003-FS-CD-LS-12-M1, Lithuanian State Science & Study Foundation grant C-23/2005 and EU Framework 6 project MAP. The present
research study was also supported by the strategic FISR Programme
‘‘Sustainable Development and Climate Changes’’ sponsored by the Italian
Ministry of the University and Scientific Research (MIUR) and developed
in the frame of the cooperative project between CNR and MIUR ‘‘Study of
the direct and indirect effects of aerosols and clouds on climate (AEROCLOUDS)’’. The authors also wish to thank anonymous reviewers for their
constructive and helpful comments.
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