Potential contribution of coastal sources to aerosol properties at Mace Head.
Given that the aerosol measurements presented in this Letter are taken at a coastal
station, the potential contribution of coastal sources and coastal biota to the measured
signal is examined. Also, the representativeness of the coastal aerosol properties are
compared with similar off-shore measurements.
To address these issues we have compiled the main results from previous off-shore
studies of marine aerosol and compare these to what we have observed at Mace Head.
In doing so, we attempt to build a series of arguments in support illustrating that, for
the particle sizes in question, there are no significant differences between what would
be observed off-shore and that which is observed at Mace Head. There can be two
definitions of coastal zone effects. One could relate to the very local surf zone and the
other could relate to a coastal zone of the order of 100 km in extent. With respect to
the latter, the question arises: is biological activity biased towards the coastal zone
and if so, can the aerosol properties sampled at Mace Head be dominated by the
coastal biota.
1. Large scale coastal effect. To deal with the 100 km coastal zone effect, we can
only illustrate that during plankton blooms, the biological activity over the open ocean
is similar, if not higher, than that observed in the 100 km coastal zone.
Bloom offshore
Figure 1. Sea-Wifs imagery of plankton activity over NE Atlantic.
Figure 1 displays Sea-Wifs imagery of plankton activity over the NE Atlantic. From
the imagery, a clear plankton bloom is visible about 500 km off the coast. This is well
off the continental shelf and illustrates more plantkton activity over the open ocean as
opposed to the coastal region. Therefore, air arriving at Mace Head is likely to have
passed over more extensive regions of biological activity over the open ocean
compared to the coastal region. Similarly, for chlorophyll, as shown in the
manuscript, it is seen to be as abundant off-shore as it is in the coastal region.
2. Small scale coastal effects. The small scale coastal zone effect corresponds, more
or less, to the surf zone upwind (in westerlies) of Mace Head. If there was an effect
from the surf zone, which is approximately 50 m in transect dimension, to negate our
results being representative of the open ocean, it would have to be demonstrated that
60-80% (i.e. the organic matter fraction) of the accumulation mode aerosol number
and/or mass is produced as the air advects over the surf zone. Taking a conservative
wind speed of 5 m s-1, the advection time over the surf zone is 10 seconds. In the
following subsections, we will demonstrate that such a surf zone effect is unlikely.
2.1. Accumulation mode properties.
We examine the general facts of the HBA-period accumulation mode properties
observed at Mace Head and explore what the open ocean characteristics would be like
of we assume that the organic fraction is produced in the surf zone.
Accumulation Mode concentration is 100 cm-3
Mode Diamater is 0.16 m
65% of accumulation mode mass is organic
10% is sea-salt
25% is nss-sulpahte
Facts:
Hypothesis
1.
Aerosol is internally mixed and is coastal
If the aerosol is internally mixed and the organic fraction comes predominantly from
the coast region, then the assumption is that the open ocean aerosol size distribution is
equal the Mace Head Distribution minus the Organic Fraction.
Result: Open ocean mode radius must be 0.1-0 m.
During three flights in Spring/Summer, O’Dowd et al., 1999 (O'Dowd, C.D., J. Lowe,
M.H. Smith and A.D. Kaye, The relative importance of sea-salt and nss-sulphate
aerosol to the marine CCN population: An improved multi-component aerosoldroplet parameterisation. Q. J. Roy. Met. Soc., 125, 1295-1313. 1999) measured
aerosol and cloud properties a few hundred km to the west of Mace Head and found
more diameters very consistent with what we report in this study. In the O’Dowd et
al, 1999, study, volatility measurements were made coupled to an optical particle
counter. O’Dowd et al attributed the volatile fraction of the aerosol at temperatures up
to 300C to be nss-sulphate, although in light of the current results, it seems to be more
likely that a large fraction of this volatile mass could have been organic matter.
If we assume that sea-salt would be produced also in the surf zone, this would require
off shore mode diameters during periods of biological activity to be significantly less
that - 0.1 m. We are not aware of any measurements during spring, summer or
autumn time that support such a conclusion.
Hypothesis 2.
Aerosol is externally mixed and coastal
If the aerosol is externally mixed, then 65% of the particles are sourced from the
immediate coastal zone. Result: Open Ocean accumulation mode concentration,
including both sulphate and sea-salt particles is 35 cm-3. These concentrations rarely
have been reported over the North Atlantic (perhaps under extreme precipitation). If
we attribute some of the sea-salt fraction to being formed in the surf zone, then the
open ocean accumulation mode concentration would be of the order of 20-25 cm-3.
2.2 Principal Component Analysis.
The size of the particle distribution which would be expected to be most influenced by
the surf zone would be the coarse mode, measured by the TSI APS (Aerodynamic
Particle Sizer). A Principal Component Analysis was conducted on the APS data at
Mace Head, however, no correlation was found with tidal height.
Detail: In order to find out whether clean sector aerosol spectra were influenced by
certain meteorological or local conditions, factor analysis or more specifically
principal component analysis (PCA) was performed. Factor analysis is a statistical
technique used to identify a relatively small number of factors that can be used to
represent relationships among sets of many interrelated variables. The goal of factor
analysis is to identify the non-directly-observable factors based on a set of observable
variables. PCA is a special case of factor analysis which transforms the original set of
intercorrelated variables into a set of uncorrelated variables, that are linear
combinations of the original variables. The first principal component is the linear
combination of the variables which accounts for a maximum of the total variability of
the data set. The second principal component explains a maximum of the variability
not accounted for by the first component, and so on. The objective is to find a
minimum number of principal components that explain most of the variance in the
data set. The principal components are statistically independent and, typically, the
first few components explain almost all the variability of the whole data set.
A data set was constructed using meteorological parameters (wind speed, wind
direction, temperature, relative humidity), aerosol spectra obtained by aerodynamic
particle sizer (APS) and electrical low pressure impactor (ELPI)) and a tidal height (a
measure of the distance from the surf zone) as independent variables. Aerodynamic
particle counter covered 0.5 – 10 μm range while ELPI covered 0.030 – 10 μm range.
Time series of 10min averages (1300 values) were used in the data set. Factor
loadings lower than 0.4 generally were not considered. There were number of
different data sets (including all or selected variables) used to examine sensitivity of
PCA, especially the sensitivity to the tidal height.
PCA revealed few obvious principal components (PC) other than
temperature/relative humidity. There were two aerosol modes (sub and super micron)
exhibited as PCs. However, it was found that tidal height was very independent
variable associated with these modes, with low or insignificant loadings overall (0.39
or less). Therefore it can be concluded that based on PCA analysis there was no
indication of coastal effects on aerosol number population.
2.3 Primary Aerosol Flux measurements at Mace Head.
A parallel eddy-correlation flux measurement programme was running at Mace Head
where total particle fluxes, at sizes larger than 10 nm and 100 nm were conducted
(Geever, M., R.J. Flannagan, C.D. O'Dowd, J.S., Van Ekeran, E.M. Martensson and
E.D. Nilsson, Measurements of primary marine aerosol J. Aer. Sci., S637-638, 2003.)
Careful analysis of the micro-meteorological data indicated that at high tide, the footprint was representative of the open ocean while at low tide and low wind speeds, the
flux was influenced from the tidal zone. No correlation between wind speed and flux
was observed for low tide while a good correlation was observed for high tide. The
maximum flux observed at low tide was 107 m-2 s-1. To examine the contribution of
the surf zone to the measured background concentration of 100 cm-3 for sizes larger
than 100 nm, we assume
(1)
(2)
(3)
(4)
that the SMPS measurement point is always in the surf-zone plume.
The surf zone is 50 m wide
Wind speed is 5 m s-1
The measurement point is 50 m from the surf and the surf plume mixes up to
10 m.
We take a 1 m3 parcel of air advecting over the surf zone. The transit time is 10 s. As
it advects for 10 s, there is a net input of particles is 108 m-3. This is diluted to 107 m-3
as the plume mixes to 10 m height. This corresponds to an input in terms of
concentration of 10 cm-3 as a result of passage over the surf zone. Approximately 50%
of this flux is attributed to accumulation mode sizes which leaves a concentration
input of 5 cm-3 (5%) to the accumulation mode concentration of 100 cm-3. This is
considered as an upper limit since wind speed is typically greater than 5 m s-1.
In the following section, for comparison, the input of primary marine aerosol during
transit over the North Atlantic is estimated based on the aforementioned flux
measurements.
2.4 Input of primary marine aerosol during transit over the North Atlantic.
In the manuscript, we state that the primary marine aerosol flux can account for an
input of approximately 700 cm-3. How this input is calculated is outlined in the
following text. Back-of-the-envelop conditions are: boundary layer height = 100 m.
Transit time = 4 days. Mean wind speed= 6-10 m s-1.
Input=Source_Flux*time/Boundary Layer Height=106 m-2 s-1 x 3.5 105 s = 3.5 x 1011
m-2 mixed into 1000 m height results in 3.5 x 108 particles m-3 = 3.5 x 102 cm-3 =350
cm-3. The actual flux at 6 m s-1 is 2 x 106 m-2 s-1 and is 3 x 106 m-2 s-1 at 10 ms-1 so
the net input is between 700-1050 cm-3. Allowing for removal processes, an open
ocean source can explain the observed accumulation mode and Aitken mode
concentrations.
2.5 Gradient Measurements at Mace Head.
During the summer of 2002, the UK NAMLEX field campaign was conducted at
Mace Head. During this campaign, the UMIST (University of Manchester Institute of
Science & Technology) group deployed an AeroDyne Aerosol Mass Spectrometer
(AMS) to determine the physical and chemical properties of marine aerosol at Mace
Head. During these studies, they sampled, alternatively every hour, from the 7 m and
22 m level. Even though micro meteorological measurements indicated the
development of a internal boundary layer as the air advected over the land, they
concluded that
“..measurements of observable submicron sulphate and organic mass made by an
Aerodyne aerosol mass spectrometer (technique described in Allan et al.,
2003), in general, show no discernable difference between 7 and 22 m sample
heights in all wind directions (J. Allan, UMIST, pers. comm. 2004)....
Thus, it has to be concluded that if there is no descernable difference between physics
and chemistry of accumulation mode aerosol at 7 m and 22 m, and that the 22 m level
is representative of the open ocean, then the measurements presented in this study are
representative of the open ocean and not significantly, in terms of our main
conclusions, influenced by surf zone of coastal effects.