ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-14107-2016Aerosol arriving on the Caribbean island of Barbados: physical properties and originWexHeikewex@tropos.dehttps://orcid.org/0000-0003-2129-9323DieckmannKatrinRobertsGreg C.ConrathThomasIzaguirreMiguel A.HartmannSusanHerenzPaulhttps://orcid.org/0000-0001-8115-8106SchäferMichaelDitasFlorianhttps://orcid.org/0000-0003-3824-9373SchmeissnerTinaHenningSilviaWehnerBirgitSiebertHolgerStratmannFrankLeibniz Institute for Tropospheric Research, Experimental Aerosol and Cloud Microphysics, Leipzig, GermanyCentre National de Recherche Scientifique, Meteo France, Toulouse, FranceScripps Institution of Oceanography, Center for Atmospheric Sciences, La Jolla, USAMeteorology and Physical Oceanography, University of Miami RSMAS, Miami, USAnow at: Eurofins GfA GmbH, Münster, Germanynow at: Leipzig Institute for Meteorology, University of Leipzig, Leipzig, Germanynow at: Max Planck Institute for Chemistry, Mainz, GermanyHeike Wex (wex@tropos.de)15November20161622141071413021March201631March201627September201618October2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/14107/2016/acp-16-14107-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/14107/2016/acp-16-14107-2016.pdf
The marine aerosol
arriving at Barbados (Ragged Point) was characterized during two 3-week long
measurement periods in November 2010 and April 2011, in the context of the
measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the
trade wInd regime over BArbados). Through a comparison between ground-based
and airborne measurements it was shown that the former are representative of
the marine boundary layer at least up to cloud base. In general, total
particle number concentrations (Ntotal) ranged from as low as 100
up to 800 cm-3, while number concentrations for cloud condensation
nuclei (NCCN) at a supersaturation of 0.26 % ranged from some
10 to 600 cm-3. Ntotal and NCCN depended on the air
mass origin. Three distinct types of air masses were found. One type showed
elevated values for both Ntotal and NCCN and could be
attributed to long-range transport from Africa, by which biomass burning
particles from the Sahel region and/or mineral dust particles from the Sahara
were advected. The second and third type both had values for NCCN
below 200 cm-3 and a clear minimum in the particle number size
distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of
these two types the accumulation mode was dominating (albeit less so than for
air masses advected from Africa), the Aitken mode dominated the other and
contributed more than 50 % of all particles. These Aitken mode particles
likely were formed by new particle formation no more than 3 days prior to the
measurements. Hygroscopicity of particles in the CCN size range was
determined from CCN measurements to be κ= 0.66 on average, which
suggests that these particles contain mainly sulfate and do not show a strong
influence from organic material, which might generally be the case for the
months during which measurements were made. The average κ could be
used to derive NCCN from measured number size distributions,
showing that this is a valid approach to obtain NCCN. Although the
total particulate mass sampled on filters was found to be dominated by Na+
and Cl-, this was found to be contributed by a small number of large
particles (> 500 nm, mostly even in the super-micron size range). Based
on a three-modal fit, a sea spray mode observed in the NSDs was found to
contribute 90 % to the total particulate mass but only 4 to 10 % to
Ntotal and up to 15 % to NCCN. This is in accordance
with finding no correlation between Ntotal and wind speed.
Introduction
Atmospheric aerosol particles can act as cloud condensation nuclei (CCN) on
which cloud droplets form, and hence there is a connection between
atmospheric aerosol particles and clouds. and examined the interactions between them as early as the 1970s and 1980s. stated that an increase in CCN leads to the formation of
more but smaller cloud droplets, assuming a constant liquid water content in
the examined clouds. This may delay the formation of precipitation, which
then could increase cloud lifetime, as proposed by .
Today, many more aspects of aerosol–cloud interactions have been described,
but we still lack understanding of the overall roles of aerosol particles,
clouds and their interactions in the climate system. This fact is manifested
in the following statement in Chapter 7 (“Clouds and Aerosols”) of the
latest report by the International Panel on Climate Change
: “Clouds and aerosols continue to contribute the
largest uncertainty to estimates and interpretations of the Earth's changing
energy budget”.
The first step in the formation of warm and mixed-phase clouds is the
activation of aerosol particles to cloud droplets. At temperatures below
0 ∘C, freezing complicates the processes ongoing in clouds. Above
0 ∘C only warm clouds exist, in which, after the activation of
aerosol particles, a number of processes influence the further development of
the cloud and its droplets, among them coagulation, condensation, and
entrainment and mixing at the cloud edges. The influence of atmospheric
aerosol particles and particularly of CCN on clouds is discussed in the context
of the droplet number concentration, droplet size, the amount of drizzle
forming from a cumulus cloud, and cloud coverage
e.g.,.
Particularly the role of aerosol particles in the formation of precipitation
remains controversial , as meteorological conditions might
be of great importance for these processes, too. To shed further light on
these issues, it is important to make atmospheric measurements under
meteorological conditions which are as stable as possible but with changing
loads of atmospheric aerosol particles. These requirements are met in the
trade wind region upwind of Barbados. Additionally, except for ship and
aircraft emissions across the Atlantic, there are no anthropogenic particle
sources upwind of Barbados for several thousands of kilometers.
Marine aerosol in general and Caribbean marine aerosol in particular has been
studied before. However, much that is known, particularly for the Caribbean,
is based on remote sensing, satellite data, modeling results or scarce data
points taken onboard of aircraft, e.g., during RICO Rain In Cumulus
over the Ocean; or BACEX Barbados Aerosol Cloud
EXperiment in 2010;. In long-term studies, filter samples have often been examined
e.g.,, but size-segregated
in situ aerosol data for marine aerosol particles in the Caribbean are still
scarce.
Aerosol particles from a marine source, often called sea spray or sea salt
particles, are generally produced in dependence of surface wind speeds over
oceans e.g.,, where the term sea
spray refers
to the possibility that also organic material might be included in these
particles. They can also be expected to be produced directly in the vicinity
of Barbados. These particles have a high hygroscopicity, and as they are
produced in moist surroundings at the lowest levels of the atmosphere, they
have comparably short lifetimes . Hygroscopic growth
measurements onboard of ships identified sea salt particles in the Pacific
and Southern oceans , in the Atlantic and Indian Ocean
, and in the western Pacific off the coast
of Asia . The occurrence of these particles was related
to wind speeds above 10 m s-1 and
larger particle sizes found at 250 and 350 nm in rare clean marine
cases but mostly only at 1000 nm;. ,
participating in the same Southern Ocean ship cruise mentioned before,
describe the presence of sea salt particles in a coarse-particle mode, saying
that they might dominate the mass size distribution but only contribute
little to number concentrations. More recently, examined
marine particle number size distributions also obtained onboard a ship off
the coast of California, US. They applied three-modal fits and attributed the
third mode, visible as a shoulder at particles sizes > 500 nm, to sea
spray aerosol. During a phase of high wind speeds of 16 m s-1, these
particles from sea spray contributed on average 16 to 28 % of all CCN at
0.3 % supersaturation, while at lower wind speeds of 12 m s-1,
this fraction decreased to 5 to 10 %. concluded that
under clean marine conditions, when the ocean surface is relatively calm,
particles from sea spray contribute very little to marine CCN number
concentrations. Comparable results were also obtained based on modeling:
retrieved steep vertical gradients in the sea salt mass
concentrations over the Atlantic and suggested that sea salt particles do not
significantly contribute to CCN. This is in agreement with a modeling study
of , where it was found that over oceans, wind speed
changes and hence aerosol particles from sea spray had a negligible effect on
marine CCN and that overall, wet removal of CCN by nucleation scavenging
played a dominant role in regulating marine CCN concentrations.
It has been assumed in the past that a few large particles produced from sea
spray, so-called giant nuclei (GN), could play an important role in the warm
rain process, i.e., in the rapid formation of precipitation in trade wind
cumuli see, e.g., reviews by. This has already been examined by and is still a topic of active
research. found that in the continental region around
the Aral Sea large salt-containing dust particles increase cloud drops to
sizes that promote precipitation. Through modeling and based on aircraft measurements done in the
Caribbean, and , respectively, found that GN have little influence on precipitation when
CCN concentrations are low. also claim that GN are
unimportant for the warm rain process in shallow, maritime clouds and suggest
in addition that this process can be modeled relatively simply based on
condensational growth of droplets formed on sub-cloud aerosol particles.
Overall, the role and importance of sea spray particles are still discussed
controversially.
Similarly, the contribution of organic compounds to marine aerosol particles
is under discussion. collected aerosol particles
< 600 nm in the marine trade wind region on Puerto Rico and found that
they contained a fraction of soluble organic material which exceeded that of
sulfates. Generally, the ocean has been assumed to be a source of organic
particulate matter, a topic on which a review can be found in
. It has therefore been speculated that the organic
fraction of marine aerosol has an influence on number concentrations of
particles and CCN and on their hygroscopicity.
On the other hand, in a modeling study by a seasonal
cycle observed for CCN concentrations at Cape Grim, Tasmania, could be
connected to a very similar seasonal cycle for atmospheric dimethyl sulfide
(DMS). claim that the observed CCN are formed by
nucleation of DMS-derived H2SO4 in the free troposphere followed by
subsequent condensational growth and coagulation, which takes several days to
over a week. When CCN are finally entrained into the marine boundary layer
(MBL), they may be hundreds or even thousands of kilometers away from the
site of the original DMS emissions. Similarly basing their conclusions on a global aerosol
microphysics model, found that in the marine boundary
layer, 55 % of all particles acting as CCN at a supersaturation of
0.2 % originate from new particle formation, with 45 % being
entrained from the free troposphere and the remaining 10 % nucleating in
the boundary layer directly. Ammonium sulfate was reported as the main
component of aerosol particles with diameters below 400 nm for marine
aerosol sampled in the Caribbean by , while larger
particles were found to mainly consist of sea salt. These results are
consistent with those of , who state that the largest
contribution to CCN originates from particles in the accumulation mode
centered at 100 to 200 nm, of which non-sea-salt sulfate was the controlling
chemical component. examined aerosol particles on Puerto
Rico, and air masses arriving from the east-northeast were found to be mostly
free of anthropogenic influences. The submicron fraction of aerosol
particles in these air masses was mainly composed of non-sea-salt sulfate,
and organic compounds did not play an important, if any, role.
Early on in atmospheric aerosol research, discovered
that Saharan dust can be transported to Barbados and the Caribbean in
general, a discovery which showed that atmospheric aerosol particles can be
transported over long distances. The dust reaches the Caribbean in lofted
layers called the Saharan air layer SAL;, but it can
also be measured at the surface throughout the year, albeit in an annual
cycle with a maximum in summer . Particles
originating from biomass burning in Africa are also frequently transported to
the Caribbean, and particularly in winter this biomass burning aerosol can
occur embedded in the dust layer
e.g.,. Using a global
aerosol model, investigated the aerosol processes
determining marine CCN concentrations and found that even in the remote
Southern Ocean, long-range transport of
continental aerosol influences day-to-day fluctuations and the highest
concentrations of CCN.
Overall, there are sources for more and less hygroscopic particles which
might contribute to the aerosol arriving in the Caribbean.
used an atmospheric chemistry model to derive global
values for CCN hygroscopicity, represented as κ.
Annual mean values at the surface of Barbados were derived to be roughly
0.55, with an annual cycle ranging from 0.3 in September to 0.7 in December.
For April and November, the months during which the measurements presented here were made, values of 0.65 and 0.5 were reported, respectively.
Here, these modeled values will be compared to those obtained from in situ
measurements during our campaigns in the framework of CARRIBA (Cloud,
Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) and
also during SALTRACE (Saharan Aerosol Long-range Transport and
Aerosol-Cloud-Interaction Experiment), another campaign which recently took
place on Barbados and in which comparably low values of 0.2
to 0.5 were found for the summer months of June and July.
To tackle open issues around warm clouds, the CARRIBA-project was initiated
by the Leibniz Institute for Tropospheric Research (TROPOS). The easternmost
island in the Caribbean, Barbados, was used as a base to examine the
interplay between undisturbed marine aerosol and warm clouds. Ground-based
and airborne measurements were performed, where the measurement platform ACTOS Airborne Cloud Turbulence
Observation System; was deployed for the latter. An overview of the measurement
activities, which took place during the two periods of CARRIBA in
November 2010 and April 2011, is given in .
In the present study, we focus on a thorough in situ characterization of the
aerosol with respect to number concentrations of particles, particularly CCN
and particle number size distributions, both on the ground and in the MBL
below cloud base, i.e., the sub-cloud layer (SCL). We will show that the
measurement station at Ragged Point, Barbados, allows for the
characterization of atmospheric aerosol representative of the SCL. We will
discuss contributions of different materials (sea salt, organics, sulfates,
mineral dust and biomass-burning) to both CCN number concentrations and
hygroscopicity. And we will show that the marine aerosol transported to the
Caribbean can be roughly categorized into three classes, where one class can
be considered clean marine (with comparably low CCN and total particle number
concentrations), one as influenced by new particle formation, and the last as
influenced by continental emissions, mostly from Africa, advected across the
Atlantic. Aerosols arriving on Barbados often have been considered clean
and/or typical marine due to the
incoming air masses being free of anthropogenic influences for thousands of
kilometers prior to arrival. But we will show that even this aerosol is not
free of continental, and hence possibly anthropogenic, influences, as already
suggested by , who claim that pristine atmospheric
aerosols today are found mainly only in the Southern Hemisphere and only very
rarely in the Northern Hemisphere.
Measurements
Measurements in the framework of CARRIBA took place during November 2010 and
April 2011 on Barbados. Barbados is the easternmost island of the Caribbean,
with prevailing easterly winds. The respective air masses arriving at the
island have been free of continental and also of anthropogenic influences
(except for ship and aircraft emissions) for several thousand
kilometers and hence for some days, and the
aerosol arriving at Barbados can be assumed to represent typical marine
aerosol. In the following, details will be given about the ground-based
measurements, followed by information about the airborne measurements done
with ACTOS.
Ground-based measurements were carried out at Ragged Point on Barbados, on
one of the easternmost tips of the island. Ragged Point is the location where
Joe Prospero and his group investigated Saharan dust transport into the
Caribbean intensively e.g.,. We used the
same measurement container,
together with its 17 m high mast towering over the 30 m high ragged cliffs.
On top of the mast, daily filter samples were taken on Whatman 41 filters,
sampling roughly 670 L min-1. Sampling and subsequent analysis was as
described in, e.g., : after sampling, the
filters were folded and stored in food-grade plastic bags under a clean hood,
and once a week the collected samples were sent to the University of Miami
where they were stored at -4∘ before they were analyzed. The
analysis was done with respect to their mass content of major water-soluble
ions (Cl-, Na+, SO42-, NO3-, K+, Ca2+) and ash.
The latter was what remained of the insoluble residual after placing the
filter in a furnace at 550∘ for 14 h, and it can be attributed to
particles from desert dust or biomass burning, distinguishing between the two
based on blackness.
An aerosol inlet had been installed at the top of the mast, i.e., at the top of a
17 m long tube. The tube had a diameter of 8 inches (≈ 0.2 m), and
a (laminar) total flow of 350 L min-1 was drawn through it. This
comparably high flow rate was used to shorten the residence time in the tube
to ≈ 90 s. The aerosol used for the measurements was sampled from
the center of this tube at its lower end and was directed into the measurement
container. A cyclone was installed where the sampling tube entered the
container. This was done to enable the inversion of the size distributions,
which could only be measured up to a dry diameter of 500 nm. The cyclone was
operated with a flow rate of 4 L min-1, where a bypass flow of
≈ 2.7 L min-1 was purged directly following the cyclone. The
50 % cutoff of the cyclone was at a dry diameter of ≈ 275 nm.
Particle losses in the cyclone for diameters up to ≈ 200 nm were
negligible, and all particles > 500 nm were removed. The efficiency of
the cyclone, the related loss of particles and related necessary corrections
are discussed in Appendix .
Measured and derived parameters and the respective instrumentation
used at Ragged Point and on ACTOS.
Symbol/Instrument/Parameterabbreviationunderlying dataMeasurement rangeRagged PointParticle number size distributionNSDDMPS system25 to 500 nmTotal particle number concentrationNtotalintegrated NSD–CCN number concentrationNCCNRP,CCNCmini-CCNC0.26 % supersaturationNCCNRP,NSDintegrated NSD–Particle hygroscopicityκCCNC withat 0.07, 0.1, 0.2, 0.4 %DMPS systemand 0.7 % supersaturationACTOSParticle number size distributionNSDSMPS system6 to 230 nmand OPC250 to 2500 nmTotal particle number concentrationNtotalCPC> 6 nmCCN number concentrationNCCNA,CCNCmini-CCNC0.26 % supersaturationNCCNA,NSDintegrated NSD–Cloud droplet size distributionsPDI1 to 180 µm
For the aerosol being fed into the instruments, different drying systems had
been installed in-line. First the aerosol was directed through two Nafion tubes.
The pressure outside the tubes was kept at 300 mbar to remove some of the
water vapor. The two dryers were followed by a diffusion dryer filled with
silica gel. The relative humidity (RH) after the passage through the
diffusion dryer was measured continuously by drawing 0.2 L min-1 of
the aerosol past a hygrometer (B+B Thermo-Technik, Hytelog RS232). Once the
RH was above 25 %, the diffusion dryer was replaced by a new one filled
with dry silica gel. The replacement had to be done roughly every 2 days.
The dried aerosol flow was fed into the instruments. An overview of the
instrumentation used at Ragged Point and on ACTOS can be seen in
Table . A Kr-85 neutralizer was installed in front of a
DMA (differential mobility analyzer, Type Vienna Hauke medium, aerosol to
sheath air flow ratio 1 : 10), and the DMA was used to select particle
sizes. The aerosol flow containing the size-selected particles was then fed
into a CPC (condensation particle counter, TSI 3010) and a CCNC cloud
condensation nucleus counter, Droplet Measurement Technologies
(DMT);. CPC and CCNC both were operated with an aerosol flow
rate of 0.5 L min-1. While this is the standard operation flow rate
for the CCNC, this is not so for the CPC, and a calibration of the detection
efficiency of the CPC at this flow rate had been done at our home laboratory
and was included in the data evaluation. At the DMA, 30 logarithmically
equidistant distributed diameters from 25 to 500 nm were adjusted, each for
1 min. Using the combination of DMA and CPC, i.e., a DMPS (differential
mobility particle sizer), particle number size distributions (NSDs) were
measured with a time resolution of 30 min. Measured size distributions were
inverted based on the charge distribution as described by
. The supersaturations set at the CCNC were changed after
each distribution and were subsequently set to 0.07, 0.1, 0.2, 0.4 and
0.7 % (in April 2011, 0.3 % was also used). A mini-CCNC
was also used to measure total (also known as polydisperse) CCN number
concentrations (NCCN). It was operated at a constant measurement
flow rate of 0.1 L min-1 while continuously scanning the temperature
difference over the activation column, set for measuring in a range of
supersaturations from 0.08 up to 0.65 % in November 2010 and up to
0.8 % in April 2011. The supersaturations of both the DMT- and the
mini-CCNC, were calibrated at the beginning and the end of the measurements
campaigns at Ragged Point.
Particle losses in the whole inlet system were either calculated, based on
tubing length and diameters or, in the case of the drying system and the
cyclone, were determined in our home laboratory (for details see
Appendices and ). The measurements
were corrected. Additionally, all particle and CCN number
concentrations reported in this study are given for standard pressure and
temperature (STP). Altogether, for the ground-based measurements, roughly
3 weeks of data were obtained for each of the 2 months.
Airborne measurements were done on ACTOS. ACTOS is known for enabling
measurements with a high spatial resolution. It is hung from a
helicopter (fixed with a 140 m long rope), which typically travels with a
true airspeed of 20 m s-1. During CARRIBA, the helicopter carrying
ACTOS was operated from a helipad located at the Grantley Adams International
Airport on Barbados. The main operational area for ACTOS flights was an area
of roughly 100 km2 upwind of the island, above the open Atlantic. Overall, 17 and
16 research flights, each lasting roughly 2 h, were done in
November 2010 and April 2011, respectively. Flights mostly started between
13:00 and 15:00 UTC (09:00 and 11:00 local time),
while a quarter of all flights were afternoon flights, going out as late as
19:30 UTC (15:30 local time). Including the time span of the
flights, measurements on ACTOS were carried out over 8 h in total, all during daylight. Due to comparably stable sea surface
temperatures which show no strong diurnal variations, the marine boundary
layer shows no strong diurnal variation either, in contrast to what is
observed for the continental boundary layer. Therefore, measurements taken
for the study presented here can be seen as representative of the marine
aerosol in the Caribbean, independent of the time of day during which they
took place.
Exemplary NSDs measured during three different ACTOS flights in the
SCL between 100 and 400 m on ACTOS (thin dotted lines) and on the ground
(thick solid lines), shown with a linear (top) and a logarithmic (bottom)
scaling on the y axis. Ground-based measurements from 25 to 500 nm were
done using a DMPS, while size distribution measurements on ACTOS were
composed from measurements of an SMPS, measuring from 6 up to 230 nm, and an
OPC covering the size range of 250 to 2500 nm. All data shown here and on
any other plot in this study are given for STP.
A detailed description of the instrumentation flown on ACTOS during CARRIBA
can be found in . In the present study, ACTOS data from
the following instruments were used (for an overview see
Table ). A CPC (TSI 3762), which was operated with a
temperature difference of 25 K between saturator and condenser, measured
total particle number concentrations (Ntotal) for particles larger
than 6 nm with a measurement frequency of approximately 1 Hz. NSDs were
measured by an SMPS scanning mobility
particle sizer; providing a full spectrum between 6 and
230 nm at a time resolution of 2 min. For larger aerosol particles an OPC
(optical particle counter 1.129 (SKY-OPC), Grimm Aerosol Technik GmbH)
measured NSDs in the range of 250 to 2500 nm at a measurement frequency of
1 Hz. This OPC was calibrated with spherical latex particles with a
refractive index of m=1.586+i0.0, and the particle number size
distributions were derived using the refractive index representative of
ammonium sulfate (m=1.53+i0.0). Also, a mini-CCNC similar to the one
operated at Ragged Point was used for measuring NCCN. All data from
the mini-CCNC on ACTOS reported in the present study were taken at a
supersaturation of 0.26 %. This mini-CCNC was calibrated prior to, during
and after the measurement campaigns, similar to the procedure applied to the
CCNCs used at Ragged Point. All aerosol instrumentation was situated in the
body of ACTOS and was fed with aerosol through a sampling line. A silica gel
diffusion dryer dried the aerosol to < 40 % RH before it was
distributed to the instruments.
For the measurement of cloud droplet size distributions, a phase doppler
interferometer (PDI) for single droplet measurements in the size range of 1
to 180 µm was used (Atrium Technologies; ).
The PDI was mounted on the outside, in front of ACTOS, to enable the
detection of cloud droplets in situ.
We end this overview of the measurements with mentioning that all data
measured on ACTOS were also loss corrected and are reported for STP conditions
throughout this work.
ResultsParticle number size distributions
Three different types of NSDs were found time and time again.
Figure exemplarily shows these three different types as
measured on the ground at Ragged Point (solid lines) and simultaneously on
ACTOS (dotted lines) in the SCL. Data were taken on day of year (DOY) 319
(black lines), 328 (green lines) and 329 (red lines) between roughly 13:00
and 15:00 UTC. For each flight all NSDs measured on ACTOS in the SCL between
100 and 400 m were averaged, and it should be noted that cloud base was
often observed at 500 m , so data included here were
selected conservatively. Similarly, all NSDs measured at Ragged Point for the
duration of the respective ACTOS flight were averaged as well. Losses were
accounted for as described in Appendix , while, for the
purpose of illustration, corrections for losses in the cyclone as described
in Appendix were not performed for the NSDs from
Ragged Point shown here.
The NSDs shown in Fig. are representative of the whole
duration of the CARRIBA campaigns. Three distinctly different particle modes
were detected in almost all cases during both months in which we measured.
The overall shape of the NSDs measured on the ground agreed with that of the
NSDs measured in the SCL up to approximately 200 to 300 nm. Measured number
concentrations for diameters below roughly 70 nm were generally slightly
larger in the ground-based data set compared to data taken on ACTOS, possibly
due to an overestimation of losses in the diffusion dryers (see
Appendix ). For diameters above approximately 200 to
300 nm, number concentrations measured at Ragged Point were below those
measured on ACTOS. This is more and more the case for increasing diameters up to 500 nm, which is the upper
diameter for which measurements at Ragged Point were made. Here, the effect
of the cyclone that was installed in the inlet system at Ragged Point becomes
visible. Generally, there is good agreement between NSDs measured on the
ground and in the SCL in the size range in which the cyclone had not yet
affected the measurements. This is a first indication of the fact that the
SCL was well mixed with respect to both heights and also regional extent and
that the inlet mast at Ragged Point did indeed sample air from the SCL
without much influence from the surf at the cliffs of Ragged Point, at least
for the size range investigated.
In the following, the three distinct types of NSDs shown in
Fig. will be discussed. NSDs resembling those shown in black
(panel a) show three modes, an Aitken, an accumulation and a sea spray
mode, which can be clearly distinguished. These NSDs will be attributed to
the “marine type” in this work. The minimum between the Aitken and
accumulation mode of the NSDs Hoppel minimum; see at
roughly 70 to 80 nm indicates the sizes above which particles had previously
been activated to cloud droplets during the history of the air mass at least
once. While passing through a cloud, soluble material is added to the
activated particles by wet phase chemistry, increasing particulate mass and
hence also the size of these particles.
At times NSDs were observed which had a very pronounced Aitken mode with a
maximum at about 30 nm, as seen in Fig. (panel b, green
curves). For these NSDs, the Hoppel minimum was clearly visible as well. The
respective NSDs will be attributed to the “Aitken type” here. The observed
small particles can be assumed to originate from new particle formation. They
could have been formed in the free troposphere long distances away, as
discussed in . Growth rates observed in the tropical
and subtropical MBL were reported to be roughly 1 to 6 nm h-1 in a
review by . Based on measurements made during the CARRIBA
campaigns, reported additional events of new particle
formation with much higher growth rates of up to several nanometers per
minute for particles with diameters of roughly 10 nm. These events occurred
just outside of clouds, in isolated air parcels with an extent of several
tens of meters. Particles of both origins, formed in the free troposphere or
just around the trade wind cumuli, may have added to the observed small
particles. Overall, the age of particles in the Aitken mode can be estimated
to be on the order of a few hours up to a maximum of 3 days. While particles
in this size range were always present, they made up more than 50 % of
all particles for Aitken type NSDs.
Finally, displayed in red in Fig. c, at times we
observed an increase in number concentrations caused by particles in the
accumulation-mode size range. During these events, NCCN increased
above 200 cm-3 (as discussed in more detail in
Sect. ) and the accumulation mode could become so large
that the Hoppel minimum was not clearly visible any more. These NSDs will be
attributed to the “accumulation type”. Later on (see
Sect. ), the three different types of NSDs will be correlated
to different air masses observed in this study.
It should be mentioned that report that NSDs
observed during ship measurements in the Southern Ocean fell into three
distinct categories, similar to those observed in this study. More recently,
based on airborne aerosol in situ measurements done in the vicinity of
Barbados, also mention that NSDs were consistent with
air mass origin, where, however, NSD measurements were only done down to
particle sizes of 100 nm, and a thorough study on these NSDs has yet to be
carried out.
The third mode that was visible in the NSDs measured on ACTOS can be
attributed to particles originating from sea spray, in accordance with
previous research e.g.,. Fitting three different
modes to the NSDs shown in Fig. reveals that particles in the
third mode contribute ≈ 90 % of the total particulate mass but
only 4 to 10 % of Ntotal and up to 15 % of NCCN
(i.e., when omitting the Aitken mode), comparable to what was found in
earlier studies .
Particle hygroscopicity and number concentrations of CCN
In the following, the particle hygroscopicity is determined, expressed as
κ, where κ can be interpreted as
representing the average particle chemistry. Data from measured NSDs and CCN
number size distributions were used to obtain activated fractions as a
function of particle size. At each diameter at which number concentrations of
particles and CCN were measured during a size scan, the activated fraction
was determined. The activated fraction curves were corrected for multiply
charged particles see, and an error function fit was
done to obtain the critical diameters for activation, i.e., the diameter at
which 50 % of all particles were activated. These critical diameters,
together with the supersaturations at which they were determined, were then
used to derive values for κ, assuming the surface tension to be that
of water. Therefore, one measured NSD results in one pair of
values for critical diameter and κ. On average, the critical diameters
were 179, 148, 81, 61 and 55 nm (with a standard deviation of
≈ 8 %) at the set supersaturations of 0.07, 0.1, 0.2, 0.3 and
0.4 %.
The retrieved κ values showed a great amount of scatter. No clear trends in κ (or
in critical diameters) could be seen when data were separated according to
different air masses, i.e., according to the different types of NSD (not shown here;
for a general discussion of the different air masses, see
Sect. ). Also no trends in κ were observed when values
obtained at different supersaturations were examined separately (as shown in
Fig. , where error bars give the standard deviation for
averaging all respective κ values). Only for a time roughly from DOY
103 to 107 during April 2011, i.e., during a very distinct dust period (see
Sect. ), was κ noticeably lower (0.56 on average) than
during all other times (0.68 on average). However, when considering the
scatter observed in κ (see the error bars in Fig. ),
these lower values still agree with the others within uncertainty. It should
be mentioned here that uncertainties in the supersaturation adjusted in the
CCNC (standard deviation of 3.3 % (relative) for supersaturation
≥ 0.1 % and 0.07 ± 0.0033 %) could at maximum contribute
only one third to this observed scatter.
κ values derived at different supersaturations for
November 2010 and April 2011 (red open squares represent the time from DOY
103 to 107 when dust was observed). Error bars represent the standard
deviation resulting from averaging all κ values obtained at the
respective supersaturations.
Overall, κ averaged 0.66. These values are comparable to results
from model calculations given in , where κ at the
surface for the region around Barbados was reported to be 0.65 and 0.5 for
April and November, respectively. furthermore reported a
yearly cycle, with lower κ of ≈ 0.4 in June and July. found lower values for κ of 0.2 to 0.5
for June and July 2013 when measuring at Ragged Point on Barbados.
Figure shows the yearly cycle of κ taken from
together with values derived in and
in this study. Within measurement uncertainty, there is good agreement, and
the yearly cycle derived in is corroborated by the
measurements.
The derived κ values suggest that the majority of the particles in
the size range between roughly 50 up to 200 nm, i.e., those from which
κ was determined, did not originate from sea spray, as κ≈1 would be expected for sea salt particles see, e.g.,. Instead, κ≈0.66
suggests the presence of sulfates see examples given in, which
generally are formed during new particle formation and wet phase chemistry.
Organic compounds generally have low values of κ on the order of 0.1
and hence were likely not present in the particulate
matter during November 2010 and April 2011. But the lower κ reported
by indicates that a larger fraction of the particulate
matter consisted of organic compounds in the summer month.
The abovementioned diameter range of 70 to 80 nm, for which the Hoppel
minimum was observed, can be used, together with the average κ of 0.66,
to obtain a rough estimate of maximum supersaturations present in trade wind
clouds along the path of the sampled air masses. Resulting values are roughly
0.2 to 0.25 %. This is slightly lower but still close to an earlier
estimate given in of 0.35 % and can be interpreted
as typical value for trade wind cumuli.
κ values reported in the literature based on modeling
and measurements (; data taken
during SALTRACE in 2013) and those derived in this study for November 2010
and April 2011 (color code for the latter is the same as that used in Fig. ).
Time series of NCCN measured with the mini-CCNC at a
supersaturation of 0.26 % both on the ground and in the air and determined
from NSDs for a cutoff diameter of 68 nm, corresponding to a κ of
0.66 at 0.26 % supersaturation.
At the supersaturation where most of the mini-CCNC data on ACTOS were taken,
at 0.26 %, the derived average κ of 0.66 corresponds to a critical
diameter of 68 nm. It is known that particle size matters more than the
chemical composition of the particles (the latter being represented by
κ here) in determining the critical supersaturation that is needed
for a particle to activate . It has been shown before for
remote continental data that NCCN can be
derived based on time-resolved measurements of NSDs and an average κ
for the examined region. To test if this was also feasible for the marine
aerosol examined in the present study, in a next step, NCCN was
calculated by integrating the NSDs beginning from the critical diameter of
68 nm onward, and was compared to NCCN measured by the mini-CCNC
at a supersaturation of 0.26 % (±0.03 %).
Scatterplot comparing the NCCN data sets shown in
Fig. . NCCNRP,NSD is compared to
NCCNRP,CCNC, NCCNA,CCNC and
NCCNA,NSD in the left, middle and right panel,
respectively. Slope and R2 for these fits is given in the panels. (For
clarity, the error bars were omitted in the left panel, where, on
average, a standard deviation of 20 % was found for
NCCNRP,CCNC.)
In this comparison, data obtained from measurements at Ragged Point and on
ACTOS were included. Therefore, the ground-based data had to be corrected
with respect to losses in the inlet system (see Appendices
and ) and losses of large particles (> 200 nm,
see Appendix ) which were missing due to the use of
a cyclone in the inlet system. Resulting NCCN can be seen in
Fig. , showing values which were derived from
measured NSDs from Ragged Point (NCCNRP,NSD) and from ACTOS
(NCCNA,NSD) and which were measured directly at Ragged
Point (NCCNRP,CCNC) and on ACTOS
(NCCNA,CCNC). (See Table for
an explanatory summary of the symbols
used here.)
There is a good correlation between NCCN measured directly and NCCN obtained from the NSDs. This can also be seen in Fig. , which shows scatterplots comparing the data sets shown in
Fig. . The most complete data set exists for
NCCNRP,NSD; therefore, this data set was used as the base
for the comparisons shown in the scatterplots. The two data sets obtained
from ground-based measurements are compared in
Fig. a. The agreement in November 2010 is excellent,
resulting in a slope of a linear fit of 0.99 and an R2 of 0.95. For the
respective data taken in April 2011 a somewhat larger scatter can be seen,
but overall, combining both data sets, a linear fit with a slope of 0.97 and
an R2 of 0.91 is obtained. When comparing NCCNRP,NSD to
data measured on ACTOS, the ground-based data were averaged for the duration
of the respective ACTOS flight. The relevant data are shown in
Fig. b. The comparison of
NCCNRP,NSD to data from the mini-CCNC on ACTOS resulted in
a linear fit with a slope of 1.00 and an R2 of 0.86; i.e., very good agreement was also found for this
comparison. Values for NCCNA,NSD are somewhat larger than NCCN derived
from the three other instruments, which can be seen exemplarily in the slope
of 1.15 and an R2 of 0.92 for the fit presented in
Fig. c. When comparing Ntotal measured on
ACTOS to Ntotal derived from the NSDs measured on ACTOS (not
shown), it was found that the latter was larger by a factor of 1.04
(±0.06), pointing towards a possible slight overestimation of number
concentrations measured for NSDs on ACTOS. But overall, as the deviation
between NCCNA,NSD and the other data on NCCN is
still close to 10 %, a value which often denotes the uncertainty in
aerosol particle counting instrumentation, we will not discuss this further
but state that all methods used to derive NCCN gave similar
results.
The analysis presented above helps to corroborate two points:
The use of an average κ, together with time-resolved NSDs, seems
to be sufficient to retrieve NCCN for the marine aerosol examined
in the present study. However, while a size-dependent trend for κ was
not found for the data presented here, which enables the claim made here,
seasonal changes in κ are to be expected. It should also be added here
that values for NCCN differ for different supersaturations. The
supersaturation of 0.26 % chosen for the work presented here is similar
to that efficiently active in the examined trade wind cumuli. Therefore, the
range of NCCN reported here represents values valid for cloud
droplet activation in the clouds.
The general agreement between NCCN derived at Ragged Point and on
ACTOS again confirms that the ground-based data measured for the present study
can be assumed to be representative of the SCL and that the SCL generally
was well mixed. This is also corroborated by the excellent agreement between
wind speed measured at Ragged Point and on ACTOS as shown in
Fig. .
Because of these two points mentioned here,
Ntotal was derived from all NSDs measured at Ragged Point, and, as
presented in the following, this rather extensive data set was then used to
discriminate between different air masses arriving at Ragged Point.
Wind speed as measured at Ragged Point (30 min average) and on
ACTOS (averaged for each research flight).
Overview over the three observed distinct aerosol types. Typical
NSDs are shown in Fig. .
Time series. (Upper panels) mass of the residuals from filter
samples remaining after heat treatment (ash) – grey striped areas indicate
days for which no data was available. (Lower panels) NCCN (same as
in Fig. ) and Ntotal, both derived from
NSDs measured at Ragged Point. For the color coding of Ntotal see
the legend (based on the definitions of different air masses given in
Table ). The background is colored light grey or in orange
stripes when trajectories indicated that air masses were advected from Africa
(see the two boxes in Fig. ).
Particle number concentrations, aerosol types and their originParticle number concentrations and corresponding different aerosol types
NSDs measured at Ragged Point which were used to derive NCCN in the
previous section were also used to derive Ntotal. The derived
values for Ntotal represent particle number concentrations in the
size range ≥ 25 nm. The lower panels in Fig. show
time series for Ntotal, depicted in either black, green or red (an
explanation for the color coding follows below). For comparison,
NCCN is shown again, as a grey line (using data derived from ground-based NSDs, i.e. NCCNRP,NSD).
Now we will relate observed values of Ntotal and NCCN to
the three different types of NSDs (see Fig. 1) that were observed in our campaigns; i.e., a discrimination
between phases with different aerosol types is made based on measurements
made at Ragged Point, meaning on altogether 6 weeks of data with a time
resolution of roughly 30 min. Table summarizes the
characteristics of the resulting three different aerosol types.
During both months, there were long periods when Ntotal and
NCCN showed the same trends. At times, however, variations in
Ntotal were observed which were not seen for NCCN (see,
e.g., Fig. for DOY 324.5 to 329.5 and 107.5 to 111).
During these times, Ntotal was often at least twice as large as
NCCN. These increases in Ntotal were caused by an
increase in number concentrations for particles with sizes in the Aitken
mode, i.e., in a size range where the particles were still too small to act
as CCN at the examined supersaturation. Corresponding NSDs typically were of
the Aitken type, as shown in Fig. b. The curve depicting
Ntotal in Fig. was colored green for phases
where these Aitken type NSDs were present, or more precisely the curve was
colored green when Ntotal rose above 375 cm-3 while
Ntotal≥2⋅NCCN. During these phases,
NCCN can be seen to be mostly below 200 cm-3.
For times of simultaneous trends in Ntotal and NCCN and
values for Ntotal above 375 cm-3, NSDs were generally of the
accumulation type (see Fig. c). The respective curve depicting
Ntotal was colored red. During these times Ntotal<2⋅NCCN, and NCCN was above 200 cm-3 in general.
For the remaining times, i.e., when the curve of Ntotal in
Fig. is shown in black, values for Ntotal were
below 375 cm-3. NCCN mostly was below 200 cm-3 during
these times, and corresponding NSDs were generally of the marine type (see
Fig. a).
Ten-day back trajectories arriving at Ragged Point at a height of
500 m. Trajectories were calculated every 4 h (each calculation is shown as
a separate dot, which is separately visible when air masses moved fast). The
trajectories shown here are those arriving at Ragged Point during the
measurement periods in November 2010 (left panel) and April 2011 (right
panel). The map in the background was taken from the NASA FIRMS Web Fire
Mapper. The orange and grey boxes represent those regions in the Sahara and
Sahel, respectively, which a trajectory had to cross to be characterized as
being influenced by either of them. More details are given in the text.
Origin of the different aerosol types
In the following, we will discuss the influence of the origin of different
air masses on the observed aerosol type. The connection between both air mass
origin and aerosol type will be examined, considering to what extent the
origin determines the aerosol type and also to what extent the aerosol type
is generally related to the air mass origin. We mainly focus on the extent to
which aerosol arriving on Barbados was influenced by continental emissions
from Africa and, vice versa, how often elevated values of NCCN can
be traced back to continental emissions from Africa. In this context, aerosol
of the Aitken type can be viewed as a subgroup of the marine type aerosol,
and therefore, for the inspection presented here, these two types were mostly
summarized.
The background of Fig. was colored based on an analysis
of 10-day back trajectories arriving at Ragged Point at a height of 500 m,
i.e., close to cloud base heights . This height was
chosen as being representative of the SCL, but also as trajectories
calculated for 100 m (next lower elevation available) are more prone to
uncertainties. Trajectories were calculated every 4 h with FLEXTRA (FLEXible
TRAjectory model) driven by operational ECMWF
analysis/forecast fields with a horizontal resolution of 1∘ by
1∘ and 91 vertical levels. Whenever such a trajectory crosses the
Sahel or the Sahara in Africa, the background of Fig. is
colored grey or shows orange stripes, respectively. More precisely, coloring
of the background was applied when the respective trajectory had crossed
either one or both of the two following areas while being at a height of less
than 3000 m at least once during the passage of these
areas: 15 to 0∘ W in all
cases, 7 to 15.5∘ N representing the Sahel region, and 15.5 to
25∘ N representing the Sahara (as indicated by the grey and orange
boxes in Fig. ). The grey box coincides with a region in the
Sahel from which biomass burning emissions can be expected. The maps in the
background of Fig. were taken from the NASA FIRMS Web Fire
Mapper,
https://firms.modaps.eosdis.nasa.gov/firemap/
indicating locations of fires for all of November 2010 (left panel) and
April 2011 (right panel), where small squares in white, light red or dark red
indicate the presence of fire, larger numbers of fire events being indicated
by darker coloring. In the Sahel region (roughly indicated by the grey box),
fire was observed on every single day during both months, with an extension
further eastward in November 2010. The orange box in Fig.
represents a region with dust emissions, including the western Sahara and
Adrar Mountains for which frequent dust mobilization was observed in the past
. Note that the 10 days for which back trajectories were
obtained do not allow us to determine whether air masses had passed over the
region east of the chosen areas. The height requirement of 3000 m was, in
any case, at the lower end of the heights up to which dust or biomass-burning
emissions can occur ; i.e., it
was chosen conservatively.
Statistics on trajectories. (Percentages in brackets indicate the proportion of the total, given before the brackets, for each type.)
Number of trajectoriesFrom AfricaaNot from Africab201011838 % (64 % accumulation type)62 % (73 % Aitken or marine type)20111077 % (88 % accumulation type)93 % (93 % Aitken or marine type)Both years22524 % (68 % accumulation type)76 % (84 % Aitken or marine type)
a Trajectory crossed one or
both boxes representing the Sahara and the Sahel. b Trajectory did
not cross either of the two boxes representing the Sahara and the Sahel.
Additionally, trajectories shown in Fig. are colored in green,
red or black if they arrived at Ragged Point during a time when the aerosol
was of the Aitken, accumulation or marine type.
Looking at Fig. , it can be seen that the red colored
phases marked by accumulation type aerosol often coincide with times when the
background was colored. This shows that air masses advected from Africa often
lead to an increase in Ntotal due to an increased particle number
in the accumulation mode, i.e., in the CCN size range. Of all trajectories
(225 for the duration of both campaigns), those crossing one or both of the
boxes shown in Fig. make up 24 %, and for 68 % of
these, the aerosol was of the accumulation type when arriving at Ragged
Point. Overall, 76 % of all 225 trajectories did not cross any of the two boxes
shown in Fig. . Of these, 84 % did not show an increase in
particle number concentrations in the accumulation mode.
Table summarizes these numbers and shows additional
details.
As a side note, it should be added that there was a short time of roughly a
day at the beginning of the campaign in 2010 (during DOY 312) during which
trajectories took a rather more southerly route and approached South America.
Looking at Fig. , these trajectories can be seen colored in red
with starting points in the Atlantic, close to the equator. Some of the
observed large numbers in Ntotal during that time might also be
attributed to particles originating from the South American continent; however, related air masses might also have had contact with Africa more than
10 days prior to arrival at Ragged Point.
Statistics on NSDs. (Percentages in brackets indicate, how many of
those mentioned before the bracket had trajectories originating from the
respective region.)
Number of NSDsMarine and Aitken typecAccumulation typec201081744 and 16 % (75 % not from Africa)b40 % (58 % from Africa)a201197967 and 20 % (99 % not from Africa)b13 % (53 % from Africa)aBoth years179657 and 18 % (90 % not from Africa)b25 % (57 % from Africa)a
a Trajectory arriving at the time
of the NSD measurement crossed one or both boxes representing the Sahara and
the Sahel. b Trajectory arriving at the time of the NSD measurement
did not cross either of the two boxes representing the Sahara and the Sahel.
c According to the characteristics given in Table .
So far, it has been discussed how the origin of a trajectory from a certain region influenced the observed aerosol type arriving at Ragged Point. The following paragraph
discusses how the presence of the different aerosol types can be traced back
to the air mass origin.
Heights of the 10-day back trajectories prior to arriving at Ragged
Point.
Overall, for the three different aerosol types defined earlier, 1976 separate
NSD measurements (i.e., also 1976 values for Ntotal and for
NCCN) exist. For 75 % of these separate NSDs, the aerosol
showed Aitken or marine type characteristics, and for 90 % of all those,
the corresponding trajectories had not crossed the two marked regions in
Africa (see Table ). For 57 % of the measurements made
during phases with accumulation type aerosol, the trajectories had crossed
one or both of the two marked regions in Africa. NSDs with a pronounced
Aitken mode were always connected to air masses that did not come from
Africa, and they made up ∼ 18 % of all NSDs. All related
information is given in Table .
Additionally, Fig. shows the history of the altitudes
of air parcels arriving at Ragged Point as determined by the trajectories.
Generally, elevations below 100 m were not observed, and most air parcels
originated at heights well above 1000 m 10 days prior to arrival at Ragged
Point. Also, for all aerosol types, almost all air parcels were found below
1000 m during the day prior to arrival. showed that
Ntotal was similar in the SCL and in the cloud layer (up to a
height of 2000 m) during the CARRIBA campaigns. The cumulus clouds observed
frequently during CARRIBA form in air lifted from the SCL, and this air is
replaced from above, explaining this observed similarity of the aerosol in
these two different layers. Altogether, this and our observations of
agreement between aerosol parameters as observed on ACTOS and at Ragged Point
allow the conclusion that the SCL is well mixed down to the heights at
which aerosol was sampled at Ragged Point.
Discussion of the different aerosol types
Aerosol of the accumulation type often occurred during times when increased
amounts of insoluble material were analyzed on the daily filter samples taken
at Ragged Point (see top panel of Fig. , denoted as ash).
The corresponding increase in Ntotal and NCCN can hence
be expected to originate from biomass burning or desert dust particles, i.e.,
from particles of continental origin. This is in agreement with the above
discussed origin of the respective air masses in the Sahara or Sahel region
in Africa. While particles advected from Africa are known to generally occur
in layers (SAL), particles from these layers descend and increase particle
number concentrations at ground level and in the SCL. Our measurements indicate
that they are distributed comparably homogeneously, as otherwise the
agreement between ground-based and airborne measurements described above
should not have been seen so clearly.
Particularly high amounts of ash were detected during April 2011 on DOY 100
and 102 to 106. The high value of ash on DOY 100 was only observed for this
filter sample and might be due to contamination. Roughly during DOY 103 to
107 (i.e., the end of DOY 106), Ntotal and NCCN also
indicate accumulation type aerosol, however with lesser intensity than could
be expected from the amount of ash on the filter.
stated that particularly strong Saharan dust layers were observed during
this phase at heights of up to 3 km; this information was obtained from
lidar measurements that were done on the east coast of Barbados by
the Max Planck Institute for Meteorology in Hamburg, Germany. The
corresponding NSDs measured on ACTOS show a strikingly strong particle mode,
extending from ∼ 250 nm to above 2 µm and appearing as a
clearly visible shoulder. With values of dN/dlogDp of
450 to 520 cm-3 (and 280 cm-3) at 300 nm for measurements on
DOY 104 and 105 (and 106), respectively, these shoulders are much more
pronounced than they are for the NSD shown as typical for the
accumulation type in Fig. . Such large particle number
concentrations in this size range were not observed at other times. This is
indicative of mineral dust particles from the Sahara with sizes
being present during this time.
The time from DOY 107 till the end of 110 was characterized by aerosol of the
Aitken type based on Ntotal, NCCN and NSDs. However, an
increased mass concentration of ash was observed around DOY 109 and 110.
During these times, comparably low wind speeds prevailed. They were around or
below 3 m s-1, while otherwise they were often above 5 m s-1 and sometimes even above
10 m s-1 (see Fig. ). Winds recorded at Ragged
Point usually came from the east to the southeast (108∘
(±24∘) and 104∘ (±20∘) during November 2010
and April 2011, respectively), but during DOY 109 to 110 they came from a
northeasterly to north-northeasterly directions. At the same time, several
small fires were observed in the north of Barbados. These conditions might
have led to an influence of local pollution on the ground-based measurements
at Ragged Point, which has already been mentioned briefly in
. Hence, some of the elevated values observed for
Ntotal, which indeed originated in particles in the Aitken-mode
size range, might not have been due to new particle formation but to large
amounts of small particles from fresh biomass burning aerosol; this might
explain the increased mass concentrations of ash during this time. Except for
the local influence on the aerosol observed at Ragged Point and discussed
here, a general correlation between wind direction and Ntotal was
not observed.
Times with aerosol of the Aitken type generally are marked by an increase in
small particles, and it has already been described in Sect.
that these were recently nucleated, less than ≈ 3 days prior to
arriving at Ragged Point. The respective trajectories, colored in green in
Fig. , show that the related air masses came from northern
directions and had spent some days over the Atlantic. Curves and bends in
them might imply that they were included in deep-pressure systems, related
to precipitation; this may be lowering the amount of available particles of
larger sizes, hence lowering the particle surface area (i.e., the
condensational sink) available for chemical reactions, and increasing the
likelihood for new particle formation. SO2, a gaseous precursor generally
assumed to play a crucial role in new particle formation, might have been
emitted into the air masses by oceanic plankton in the form of DMS (dimethyl
sulfide). Otherwise it might have been picked up over the North American
continent, where a number of the 10-day back-trajectories originated.
Unfortunately, the hygroscopicity analysis presented in
Sect. does not allow us to draw conclusions on the
chemical nature of the Aitken mode particles observed in this study, as the
smallest particle diameters for which κ was obtained were around
50 nm. But based on the modeling results by and
, it can be assumed that the recently nucleated particles
likely originated in the free troposphere, at least to a large extent.
Alternatively, they were formed in the vicinity of trade wind cumuli, as
suggested by . According to , these
small particles will eventually grow sufficiently so that they add to keeping
up NCCN, to which they contribute roughly half of all particles in
the marine aerosol .
Summarizing, we found that the marine aerosol observed on Ragged Point (and
likely in large marine areas upwind of Barbados and the Caribbean in general)
was largely dominated by the origin of the air masses. This is likely true
not only for the 2 months during which measurements were made but in
general. When air masses were advected from Africa, in roughly two thirds of all
cases an increase in Ntotal above 375 cm-3 was observed,
caused by particles in the accumulation-mode size range which might have
consisted of rather insoluble material (biomass burning or mineral dust).
These particles were, however, observed to act as CCN at supersaturations
above 0.26 %. This is in line with , who also found
that dust arriving at the Cape Verde Islands had acted as CCN in trade wind
cumuli. It also agrees with , who show that mineral dust
particles contribute significantly to CCN number concentrations, particularly
in outflow regions of deserts, which, as shown here, might
extend at least as far as Barbados for the Sahara.
Comparison of horizontal wind speed to Ntotal.
Not all air masses originating from Africa carried continental particles in
such large numbers. However, when air masses had not originated in Africa,
increases in Ntotal above 375 cm-3 mostly were caused by
small particles recently formed over the Atlantic and NCCN
generally was below 200 cm-3.
Particles originating from sea spray
In the following section, it will briefly be discussed whether sea spray
particles contributed noticeably to Ntotal. Following this brief
excursion, we will examine the correlation between particles observed by the
PDI during ACTOS flights in the SCL in the size range from 500 nm up to
cloud droplet sizes and particulate mass from sea spray aerosol as determined
from filter samples taken at Ragged Point.
Regarding the first issue, Fig. shows the
relation between Ntotal to the horizontal wind speed as measured at the top of the mast at Ragged Point. Wind speed data had been measured with a
time resolution of 1 min, and 30 min averages were used for this
comparison. No correlation between wind speed and Ntotal was found.
This indicates that the majority of the particles observed at Ragged Point,
and hence in the examined SCL, did not originate from sea spray. This is in
agreement with the earlier reported fraction of only 4 to 10 % that
particles in the observed sea spray mode contribute to Ntotal (see
Sect. ). It further corroborates a statement made in
Sect. , namely that the derived values for κ
indicate the presence of sulfates in the examined CCN size range up to
200 nm rather than that of salts from sea spray.
Concerning the super-micron size range, Fig. shows the
mass concentrations of Na++ Cl- derived from the filter samples
taken at the top of the mast at Ragged Point, in relation to different other
parameters. On the filters, which sampled particles up into the super-micron
size range, there was usually some amount of Cl- exceeding the
stoichiometric relation between Cl- and Na+. This is not surprising as
NaCl is not the only salt present in seawater. Cl- and Na+ are by far
the most abundant ions. On the filters, they accounted for
≈ 80 % of the total dissolved mass and around 60 % of the
total mass on average. In seawater, also Mg+, SO42-, Ca2+ and
K+ are present, to name only the next most-abundant ones. For CARRIBA,
there were no data available on the amount of Mg+. SO42- made up
close to 10 % of the total mass; the sum of Ca2+ and K+ found on
the filters was about 3 % (±1 %) of the total sampled mass.
SO42- can be attributed either to particulate matter originating from
seawater or to non-sea-salt sulfate. Hence we will take the sum of Na+ and
Cl- as a proxy for particulate matter originating from sea spray. If we
had used the stoichiometric amount of NaCl for the following comparisons,
instead, the results we obtained would have been similar to those presented
below.
Mass concentrations of Na++ Cl- from filter samples taken
at Ragged Point in comparison to wind speed (upper panel) and to number and
mass concentrations (middle and lower panel, respectively) of particles
detected with the PDI in the SCL.
In panel a of Fig. , the mass concentration of
Na++ Cl- is related to the daily averaged wind speed (averaged
over the time during which a filter was taken; filters were changed
daily around sunrise on Barbados at 09:30 UTC). Grey and red data points
represent data from November 2010 and April 2011, respectively; open symbols
denote days during which data from PDI are available. Overall, there might be
a slight tendency for larger amounts of Na++ Cl- to be present for
times with greater wind speeds, but this is not a clear trend. This might be
attributed to comparably low wind speeds prevailing during both CARRIBA
campaigns, as earlier studies reported that sea spray particles were only
observed for wind speeds exceeding 10 m s-1, increasing in numbers with increasing wind
speed .
In panels b and c of Fig. , the mass concentration of
Na++ Cl- is related to number and mass concentrations of large
atmospheric aerosol particles which were observed with the PDI on ACTOS. The
PDI was applied to measure droplet size distributions in clouds. However, it
often also detected signals outside of clouds and in the SCL (where clouds were certainly not present), albeit less frequently than in clouds. These signals
were interpreted as originating from hygroscopically grown particles in the
super-micron size range. When comparing the mass concentration of
Na++ Cl- derived from the filter samples to the number
concentration of particles detected by the PDI in the SCL
(Fig. c), a good correlation is seen. However, it should
be mentioned here that PDI flights were only made for a small range of
different atmospheric conditions. Diameters detected by the PDI were obtained
at atmospheric conditions, and the respective dry particle sizes were derived
as follows: we assumed that the particles originated from sea spray,
therefore attributing to them a κ of 1, based on values derived in
. From this, hygroscopic growth factors
were calculated for ambient RHs. This was done separately for each second
during which particles were detected, and the measured sizes were converted to
dry sizes using these growth factors. The corresponding dry particle
diameters were on average around 3.5 µm for all flights. Submicron
particles with diameters down to around 500 nm were occasionally detected,
but the majority of the counts detected with the PDI were found in the
diameter range > 1.5 µm. For the further data evaluation, dry
particle diameters together with the respective number concentrations were
used to calculate the average particulate mass concentration detected by the
PDI in the SCL for each flight, assuming a density of 2.16 g cm-3
(i.e., that of NaCl). Results are shown in Fig. c,
together with a linear fit (forced through the origin), where the fit has a
slope of 1.60 and R2 is 0.42. Hence, particulate mass concentration
derived from the PDI measurements is on average a factor of 1.60 above that
of Na++ Cl- sampled on the filters.
A direct comparison of the mass concentrations sampled on the filters with
those derived from PDI measurements is, however, difficult for several
reasons.
As briefly touched upon above, we only used Na++ Cl- from the
filter samples in this comparison, and due to neglecting other ions, the mass
concentration related to particles from sea spray derived from the filter
samples might be up to 20 % too low.
Values for κ and the density, which were needed to derive mass
concentrations from the PDI measurements were only estimates. The value for
κ was chosen slightly on the larger side of likely values (a range
from roughly 0.85 to 1.1 is given in for the most
hygroscopic mode in marine aerosol). Changing κ to lower or larger
values would, respectively, increase or decrease mass concentrations derived for the PDI. An increase in κ to that of NaCl (1.3) would lower the
mass concentrations derived from PDI such that they still exceed the filter
values, but only by a factor of 1.3. However, such a large value of κ
for sea salt particles, which consist of a mixture of different salts, is not
to be expected. In summary, using different, but still justifiable, values
for κ and the density would not erase the discrepancy.
While filter data were daily averages, data from PDI were taken for the
duration of an ACTOS flight, i.e. for 2 h at most. But meteorological
conditions are overall comparably stable, and the mass derived from PDI
measurements exceeds that determined from the filter samples in all cases,
i.e., a constant bias is observed. This contradicts a randomized sampling
error; therefore, the differing sampling times can also not explain the
discrepancy well, unless a daily cycle in particulate mass would be assumed
to exist, of which there is no indication.
The PDI has a lower cutoff in the size measurement of 1 µm (at
ambient RH), while no clear lower cutoff for sizes of particles from sea
spray can be given. Considering that sea spray particles form the observed
third particle mode (see Sect. ), the use of PDI data for the
analysis presented here should result in values that are too low; however,
those particles with sizes < 500 nm that were neglected will only
contribute the smaller fraction to the overall particulate mass due to their
smaller sizes. Nevertheless, this argument would also support the idea that the mass
concentrations of Na++ Cl- derived from the filters should exceed
those derived from PDI measurements, as the PDI misses small particles, an
observation which is not made.
Larger mineral dust particles could have been detected by the PDI, too.
However, this is unlikely, as those days during which PDI data were taken
(November 2010, from DOY 324 on) belong to phases with marine or
Aitken type aerosol, except for one.
Possibly a few large particles with sizes well in the super-micron
size range were detected by the PDI but missed by the filter sampling, which
would lead to mass concentrations from the PDI measurements exceeding those
from Na++ Cl- sampled on the filters.
Although there are these issues concerning the comparison presented here, it
might still be said that the correlation between the mass concentrations of
Na++ Cl- derived from filter samples with those derived from PDI
measurements shown in Fig. c indicates that sea spray
particles were present on Barbados and were found predominantly for larger
sizes (> 500 nm) with a large contribution to sea-spray-related
particulate mass from super-micron particles, as supposed already earlier in
this study. As particles < 500 nm were not considered here, number
concentrations of particles from sea spray will be larger than those
presented in Fig. b, making up a certain percentage of
Ntotal (see Sect. ). But based on the PDI
measurements outside of clouds it was found that a small number of large
particles (well up into the super-micron size range) was always present
during the measurements, possibly contributing most of the sea salt aerosol
mass derived from filter measurements. These few large sea spray particles
might act as giant nuclei and hence could influence the formation of
precipitation. But this topic is beyond the scope of the work presented here and will not be discussed further.
Summary and conclusions
In summary, the data set presented here consists of airborne measurements
and of 6 weeks (3 weeks each during November 2010 and April 2011) of
continuous in situ aerosol data sampled in the Caribbean. Comparison of
ground-based and airborne in situ aerosol measurements showed that ground-based measurements at Ragged Point were representative of the marine aerosol
in the sub-cloud layer arriving at Barbados. We deduce this from the
similarity in NSDs measured on ACTOS and at Ragged Point, and also from the
similar values of NCCN in the data sets obtained on the ground and
in the air.
Based on these results, the continuous ground-based measurements were used
for a more thorough analysis of the aerosol and its origin. Three distinct
types of air masses were observed, discriminated based on Ntotal,
NCCN and the shape of the NSDs (see Fig. and
Table ). Accumulation type aerosol was connected to long-range transport from the Sahara and Sahel region, where either mineral dust
from the desert and/or biomass-burning particles from the Sahel were involved in raising the particle number concentrations in the accumulation mode.
Although particles from both biomass burning and mineral dust are sometimes
assumed to be of rather insoluble nature, they have been shown to contribute
to CCN , and an increase in
NCCN due to their presence was also observed in the present study.
However, air masses from Africa did not always contain increased particle
loadings. Roughly one third of the times when air masses were advected from
Africa, the aerosol was characterized as marine type. In total, 75 % of
the time the aerosols arriving at Ragged Point were characterized as Aitken and marine type, and these aerosols mostly could be attributed to air masses
which had not originated in Africa (to 90 %). It is evident that
sometimes new particle formation occurred in these aerosols during transport,
an observation which was made ∼ 18 % of the time. Particularly due
to these periods, Ntotal is not a good proxy for NCCN, as
then the former was larger than the latter by more than a factor of 2.
It can be speculated that there is a background of NCCN of some 10
to 200 cm-3 constantly fed by particles formed by new particle
formation; this background grew sufficiently large by condensational growth
to become activated after a few days. This background NCCN level is
higher when continental particles from Africa (and possibly rarely also from
South America) are transported across the Atlantic. It should be mentioned
that NCCN depends on the supersaturation at which it was
determined. But values of NCCN discussed in this study were taken
at a constant supersaturation of 0.26 %, and the Hoppel minimum was found
to correspond to a supersaturation of 0.2 to 0.25 %. Therefore, on
average, NCCN as reported here can be taken to be relevant for
droplet activation processes observed in the atmosphere in the marine
environment of the Caribbean and possibly beyond. A companion paper (in
preparation by ) examines in more detail, which conditions
were found in separate clouds, concerning maximum supersaturation and number
of activated particles.
Filter samples were taken, however, without any size segregation. They
showed that the majority of the total particulate mass was comprised of
Na+ and Cl-. A PDI measured small droplets in the lower
super-micron size range outside of clouds, which can be interpreted as haze
particles formed on giant nuclei. Mass concentrations of these droplets were
correlated with mass concentrations of Na++ Cl- determined from
the filter samples, and a weak correlation with the wind speed was also seen.
A correlation of Ntotal with wind speed was, however, not observed,
which is in agreement with the fact that particles originating from sea spray
were found to only contribute a few percent to Ntotal in accordance
also with results by . The data set for the giant nuclei
obtained from PDI measurements is small, but it might still be assumed that
most of the mass of Na++ Cl- found on the filters originated from
sea spray particles in the super-micron size range. This is in agreement with, e.g., , , ,
and ; i.e., sea spray particles
might contribute much to particulate mass but do not contribute significantly
to number concentrations of CCN, at least not for wind speeds like those
prevailing during the CARRIBA campaigns, i.e., below
≈ 10 m s-1.
An average value for particle hygroscopicity of κ=0.66 was found
based on size-segregated CCN measurements (0.56 during a phase of
intense mineral dust advection, 0.68
otherwise). modeled κ for surface locations in
the Caribbean, obtaining values of 0.65 and 0.5 for April and November,
respectively, however, with a seasonal cycle showing lower values during
summer months. Our values and those of agree within
measurement uncertainty. A κ of 0.66 suggests that sulfates were a
major component of particles in the size range from ≈ 50 to 200 nm,
for which κ was determined, and it points to no or only a small
fraction of organic compounds. This is in agreement with results reported by
and . However, measurements made
at the same location on Barbados by in June and
July 2013 derived much lower values for κ of 0.2 to 0.5, again in
agreement with , indicating a possibly larger organic
fraction in the particulate matter at that time of the year. Coming back to
the filter measurements, it has to be argued that an estimation of the
particle hygroscopicity κ from the chemical composition found on the
filters would yield values that are too large due to the predominance of
particulate mass contributed by a few large and very hygroscopic particles.
It is worth noting that also and
found that κ derived based on chemical composition exceeded that
derived from CCN measurements for remote continental and Arctic aerosol,
respectively.
The average κ derived from CCN measurements could be used, together
with measured NSDs, to calculate NCCN with high accuracy. In
, it was also possible to mode NCCN well for a time series
of 17 months for the alpine site of Jungfraujoch in Switzerland, based on one
average value for κ together with time- and size-resolved NSD
measurements. Our observations together with these from literature, and the
fact that κ from filters has been observed to exceed that derived from
CCN measurements, might indicate that a derivation of κ based on CCN
measurements could be advantageous compared to using the chemical composition
from filter samples when aiming at the determination of NCCN for
further use in modeling.
In summary, the results presented here corroborated and extended earlier
knowledge about marine aerosol in the Caribbean. Although the aerosol on
Barbados is often considered clean marine, continental aerosol advected
mainly from Africa can still influence Ntotal, NCCN and
the shape of the NSDs, carrying comparably high loadings of mineral dust or
biomass burning particles. Therefore, a direct anthropogenic influence on the
marine aerosol upwind of and in the Caribbean, brought on by man-made fires,
cannot be completely excluded. This is in line with results reported in
, where it is stated that pristine aerosols (i.e., those
free of anthropogenic influence) on Earth are largely only found in the
Southern Hemisphere, while these pristine environments are only found
temporarily and with a spatially patchy distribution in the Northern
Hemisphere. Information on particle composition derived from our data
suggests that the majority of particles in the CCN size is made up of
sulfates, and these particles mostly originated from new particle formation.
An influence of organic substances on particle hygroscopicity in the CCN size
range could not be seen for the month during which measurements were made but
might be present in summer . A few large sea salt particles
contribute the major fraction of total particulate mass but only contribute a
small amount to the total particle number concentrations. Studies like the
present one can help complete the picture concerning atmospheric aerosol
properties, can be used as a base for model studies or can help corroborate
results obtained in model studies, strengthening our trust in the respective
models.
Data availability
All data for this article are available upon request to the corresponding
author. The time series of NCCN from the mini-CCNC measured at
Ragged Point (see Fig. 4) have been provided to GASSP (the Global Aerosol
Synthesis and Science Project), for which a publication is currently under
review ().
Accounting for particle lossesCorrections due to the reduced flow rate used for CPC measurements and due to inlet losses
At Ragged Point, particle number concentrations were measured with a CPC
which was fed with an aerosol flow rate of 0.5 L min-1 instead of its
regular flow rate of 1 L min-1, which already necessitated that all
measured values be multiplied by a factor of 2. It was determined at our home
laboratory that the reduced flow rate furthermore resulted in a reduction of
the counting efficiency, such that measured values additionally had to be
corrected by a factor of 1.2.
Losses in all inlet tubes, from the top of the Ragged Point tower down to the
inlet of the different instruments, were calculated based on length and
diameter of the inlet tubes and the flow passing through them. This resulted
in a size-dependent correction factor ranging from around 1.135 for number
concentrations for the smallest measured particle size (25 nm) up to below
1.005 for the largest measured sizes at 500 nm.
Size-dependent particle losses in the two Nafion and the one diffusion dryer were all determined through measurements in the laboratory. Particularly
large losses were found for particles with smaller sizes in the diffusion
dryer (e.g., correction factors of 1.03, 1.46 and 2.11 had to be applied at
500, 100 and 50 nm, respectively).
All of the corrections described here were obtained based on calibration
measurements in the laboratory and on the calculation of size-dependent
losses in tubes, which can be calculated precisely, too. The size-dependent
correction factors increase with decreasing particle size, so that the
overall correction factor from the losses described here increases from 2.5
to 3 over the size range of 500 to 200 nm, while it is, e.g., above a
factor of 4 below 70 nm and up to a factor of 7 for the lowest measured
sizes. Hence, the uncertainty for the correction factors are larger for
smaller particle sizes. This might contribute to an overestimation of the
particle number concentrations obtained in the size range below the Hoppel
minimum, as observed when NSDs measured at Ragged Point were compared to
those measured on ACTOS (see Fig. and
Appendix ).
Nevertheless, applying the corrections was mandatory, and all correction
factors described here were accounted for before NSDs measured at Ragged
Point were used for further data evaluation.
Correcting data from the mini-CCNC with respect to inlet losses
The mini-CCNC operated at Ragged Point also had to be corrected for particle
losses; however, as it did not detect the smaller particles (roughly those below
the Hoppel minimum), the correction factor had to be determined separately.
These total losses originating from the inlet tubes and the dryers in the
particle size range detected by the mini-CCNC were determined as follows.
All NSDs measured at Ragged Point during November 2010 and April 2011 were
considered. The integrated particle number concentrations in the size range
of a diameter (Dp) of 68 nm (Hoppel minimum) up to 500 nm were
determined twice for each NSD: once based on data corrected following
Appendix ((dNdlogDp)corr) and once based on data which had not yet been
corrected for losses in the inlet tubes and in the dryers
((dNdlogDp)uncorr). The overall
average factor Fi (where i stands for “losses in the inlet
system”) between these two sets of particle number concentrations was found
to be 1.405 (±0.030). The following equation demonstrates the approach described above:
Fi⋅∫68nm500nmdNdlogDpuncorrdlogDp=∫68nm500nmdNdlogDpcorrdlogDp.
This correction factor Fi was applied to all values measured for
NCCN at 0.26 % supersaturation at Ragged Point before
considering them in the comparisons presented in the above study.
Corrections necessary due to the use of a cyclone
As described in Sect. , a cyclone was installed in the inlet
system at Ragged Point. Due to this cyclone, particle losses occurred for
sizes > 200 nm, and particles > 500 nm were completely removed. Here
it will be described how data measured on ACTOS were used to determine the
fraction of large particles that was not detected by the Ragged Point
data set.
In a first step, a comparison was made between all NSDs measured on ACTOS and
the respective NSDs measured at Ragged Point. From the ACTOS data set, for
each flight one average NSD from all measurements in the SCL was used. From
the ground-based data set, similarly all NSDs measured during the duration of
each flight were averaged (all these NSDs had been corrected as discussed in
Appendix ). To enable the comparison, measured particle
number concentrations from both Ragged Point (NRP) and ACTOS
(NACTOS) were interpolated so that data from both data sets were available at the same diameters. The comparison resulted in the curves shown
in Fig. . NSDs measured at Ragged Point overestimate
particle number concentrations for diameters below ∼ 70 nm by more
than 10 %, an observation for which a possible explanation was already
given in Appendix . For particle sizes above
∼ 200 nm, the effect of the cyclone starts to be visible, and at
300 nm, already 50 % of all particles are captured in the cyclone. The
kink at about 300 nm likely originates in the fact that ACTOS measurements
for smaller sizes were done with the SMPS while those above this size were
made with the OPC; i.e., the transition from one instrument to an other
caused a slight jump in the ACTOS data.
Bin-wise comparison of NSDs measured at Ragged Point and on ACTOS
(i.e., NRP/NACTOS).
Figure shows the efficiency of the cyclone as determined
in our home laboratory (grey dots; a grey line representing the corresponding
fit), showing a cutoff diameter of the cyclone (i.e., where 50 % of all
particles are removed) of 525 nm. This diameter relates to the humidified
aerosol, e.g., as it enters the measurement container at Ragged Point. When
assuming κ=0.66 and relative humidities of 70, 80 and 90 %
(corresponding to growth factors of 1.43, 1.6 or 1.945, respectively), the
red lines in Fig. are obtained as the efficiency curves
for the respective dried aerosol. Also shown in Fig. is
the black curve from the upper panel (here now depicted as small black squares), i.e., the average
ratio of number concentrations measured at Ragged Point to those measured at
ACTOS (NRP/NACTOS). These data fit the thickest red line
well, albeit being slightly broader. The relative humidities at Barbados were
around 70 to 80 % at temperatures of about 27 ∘C in
November 2010 and 65 to 75 % at temperatures of about 25.5 ∘C in
April 2011. The temperature inside the measurement container was kept at
around 23 to 24 ∘C, which indeed would lead to relative humidities
from around 80 % to above 90 % for the aerosol in the tubing inside
of the container. The corresponding dry cutoff diameter of the cyclone was
≈ 275 nm. In general, the agreement for particle sizes
≥ 180 nm between the impactor efficiency curve determined in the
laboratory and values obtained for NRP/NACTOS justifies
the assumption that the latter describes the impaction efficiency well, and
hence this curve can be used to
correct NSDs measured at Ragged Point for losses occurring in the cyclone
(for particle sizes ≥ 180 nm). However, this correction alone is not
able to account for particle losses in size ranges where no particles passed
the cyclone (≥ 500 nm).
Impaction efficiency of the cyclone, i.e., the size-dependent
fraction of particles passing the cyclone (fpassing). The grey dots
result from calibration measurements done in our home laboratory with dry
size-selected ammonium sulfate particles; the grey line is the respective
fit. The red lines originate from this fit when it is assumed that particles
were hygroscopically grown while passing the cyclone and were dried only
afterwards. Data shown in black are the same as in Fig. ,
i.e., NRP/NACTOS.
To also correct for losses occurring for particle sizes in the CCN size range
≥ 500 nm, a procedure comparable to the one introduced in
Appendix was carried out next. NSDs from ACTOS were
artificially lowered by multiplying them with the impaction efficiency or,
more precisely, with NRP/NACTOS in the size range
≥ 180 nm and with 0 for ≥ 500 nm. Then, integrated particle
number concentrations were made from 68 nm upwards for both the original NSDs
and those artificially lowered. The factor between these values then yields
the fraction of larger particles (i.e., those missing due to the cyclone but
additionally also those > 500 nm which were not detected at all at Ragged
Point), compared to all particles.
The following equation again demonstrates the approach described above, i.e.,
how the factor Fl (where l stands for “missing larger particles”)
was obtained:
Fl⋅∫68nm2500nmdNdlogDpNRPNACTOSdlogDp=∫68nm2500nmdNdlogDpdlogDp.Fl was very similar for most cases: it was determined to be 1.15
(±0.05) on average for the measurements made in November 2010 and also
for those in April 2011, with the exception of the time span from DOY 103 to
107, where the average Fl was 1.26 (±0.01). This particular
time span coincides with the time when slightly lower κ values
were also observed and during which a strong dust event occurred. The difference
in Fl during and outside of this dust phase was larger than the
uncertainty in this value. Hence, a value of Fl of 1.15 was used
for all times except for the time span from DOY 103 to 107, for which 1.26 was
used.
Fl is now the factor by which the integrated particle number
concentrations in the accumulation mode (or, more precisely, in the size
range ≥ 68 nm) have to be multiplied in order to correct for the
abovementioned missing large particles in the Ragged Point data set. This
factor was used to correct NCCN obtained
from the mini-CCNC at Ragged Point at a supersaturation of 0.26 %, and
the corrected values of NCCN can then be expected to be valid for
the atmosphere. Fl was also used to correct the integrated particle
number concentrations in the size range ≥ 68 nm when NCCN or
Ntotal were derived from NSDs measured at Ragged Point (without
further correction for particles in the size range < 68 nm). The values
thus obtained for NCCN and Ntotal as determined from
Ragged Point measurements can then also be expected to be representative of
the atmosphere.
Acknowledgements
We thank Joe Prospero for access to his measurement station at Ragged Point.
We are grateful to Stephan Henne, EMPA (Swiss Federal Laboratories for
Materials Science and Technology), for kindly providing the 10-day back
trajectories (to be found at http://lagrange.empa.ch/FLEXTRA_browser).
We acknowledge the use of FIRMS data and imagery from the Land Atmosphere
Near-real time Capability for EOS (LANCE) system operated by the
NASA/GSFC/Earth Science Data and Information System (ESDIS) with funding
provided by NASA/HQ. This project was partly funded by the DFG (SI 1534/3-1)
and the European FP7 project BACCHUS (Impact of Biogenic vs. Anthropogenic
emissions on Clouds and Climate: towards a Holistic UnderStanding, grant
agreement no. 49 603445). Thanks also to David Farrel from the Caribbean
Institute for Meteorology and Hydrology (CIMH) for logistical support. And
last but not least thanks to Gisela and Dagobert Wex, at whose home I,
Heike Wex, found quiet and where I was well taken care during several week-long periods during which
quite a bit of the data evaluation and text presented here came into
being. Edited by: B. Weinzierl
Reviewed by: three anonymous referees
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