Introduction
The influence of the number concentration of available cloud condensation
nuclei (CCN) on the number concentration of cloud droplets was shown for the first time in
. They demonstrated that smoke from sugar
cane fires leads to a significant increase in the number of cloud droplets.
Furthermore, a change in CCN followed by a change in the cloud droplet number
size distribution leads to a change in the shortwave albedo of clouds
. Since these first studies the influence of the
availability of aerosol particles on the microphysical properties of clouds
has been investigated many times. On the other hand, clouds strongly
influence the incoming solar radiation, and thus they may also influence the
formation and distribution of aerosol particles, e.g., by new particle
formation NPF;.
Identifying the origin of ultrafine aerosol particles (smaller than 20 nm)
in the remote marine atmosphere has been an objective since the 1990s. NPF
requires not only low particle surface area concentration but also a
sufficient concentration of potential precursor gases.
observed an increase in the number concentration of small particles
accompanied by a decrease in the surface area concentration in the marine
boundary layer. concludes that, for clean environments, the
role of clouds has to be taken into account due to the photochemical
production of SO42- as well as the cleansing effect
that lowers the aerosol particle surface
concentration.
Several studies from the 1990s show increased particle concentrations near
the top and outflow of individual marine cumulus clouds
e.g.,. found
regions with high relative humidity around cumulus clouds and increased
particle concentrations therein. These regions coincide with regions of
increased turbulence. Results from suggest that this
phenomenon happens not only in the marine environment but also at tops of
continental clouds, where they found peaks of ultrafine particles at
stratocumulus cloud tops over Germany. observed a high
number concentration of aerosol particles < 100 nm at tops of stratiform
clouds in the Arctic. observed an increase in CCN
concentration within such clouds, which demonstrates the influence of clouds
on the aerosol distribution. A model study done by
demonstrated that bimolecular nucleation within clouds is a viable mechanism
for the particle production. observed regions of increased
ultrafine particle number concentrations near a marine frontal cloud in a
height of approximately 6 km. Here, the nucleation was observed in 60 km wide
bands with enhanced intensity of upwelling shortwave radiation.
The results of these previous studies were obtained by aircraft measurements
equipped with commercial condensation particle counters (CPCs). The observed
events had a horizontal extent of approximately 1 km or more, and shorter
events were not discussed due to limitations in the experimental setup. One
commonly used aircraft, the C-130, was operated at an average speed of about
100 m s-1. Typical CPCs with a temporal resolution of approximately
1 s therefore allow for detection of events with a length of ∼ 200 m.
This study presents results from the CARRIBA (Cloud, Aerosol, Radiation and
tuRbulence in the trade wInd regime over BArbados) campaign using two
helicopter-borne measurement platforms that simultaneously sampled basic
meteorological parameters, in situ cloud microphysical and aerosol variables,
and cloud-reflected spectral radiation. The advantage of these
helicopter-borne measurements is the low true air speed of 20 m s-1
combined with a high temporal resolution. The observed data from 28
measurement flights were obtained in the marine boundary layer and the cloud
layer, where 91 individual cases of NPF were found. Besides providing an
investigation in a new geographic environment, these measurements were made
with very high spatial resolution (approximately 20 m) and with a lower size
limit of 6 nm for aerosol detection. This allows for investigation of
statistical properties of the NPF events and their correlation with
meteorological, microphysical, and radiation variables. An introduction to
the CARRIBA project and the scientific goal are given in .
Instrumentation
The ACTOS measurement platform
The helicopter-borne measurement payload ACTOS was used to perform
measurements with high temporal and spatial resolution within the marine
boundary layer up to a height of ≈3000 m above sea level. ACTOS is
an autonomous system that is attached to a helicopter and flown with a true
airspeed of about 20 m s-1 to ensure measurements are safely
outside of the helicopter's downwash . The payload is
equipped with fast sensors for measuring the three-dimensional wind vector,
temperature, static pressure, and humidity. A navigation unit provides
attitude angles, position, and velocity vector components to transfer the
wind measurements into an Earth-fixed coordinate system. In addition to the
meteorological standard parameters, cloud microphysical properties such as
liquid water content (LWC), and cloud droplet size distribution are
measured during cloudy conditions.
A real-time data acquisition system and independent power unit complete
ACTOS. A telemetry link to the helicopter ensures online monitoring of basic
parameters during the flight. One scientist is onboard to fine-tune the
flight pattern with regard to the observed cloud situation. In this study all
time series are given in seconds of day (SOD) in UTC.
Aerosol measurements on ACTOS
Additional instrumentation to measure aerosol particle number size
distributions (NSDs) between 6 nm and 2.5 µm, as well as the total
particle number concentration (N), was installed on ACTOS during CARRIBA. A
common inlet was used for all aerosol measurements, leading the sample flow
through a diffusion dryer to ensure dry measurement conditions (< 50 %
relative humidity). Afterwards, a flow splitter divides the flow line into
four lines for the individual instruments. These instruments include a total
condensation particle counter (CPC; model 3762A, TSI Inc., St. Paul, MN,
USA), a scanning mobility particle sizer (SMPS), and an optical particle
counter (OPC; model 1.129, Grimm Aerosol Technik, Ainring, Germany). The SMPS
system was built at TROPOS and mainly consists of a Kr-85 neutralizer (model
3077A, TSI Inc.), a Hauke-type differential mobility analyzer (short version), and a CPC (model 3762A,
TSI Inc.). This system was optimized in terms of weight and power consumption
for operation on ACTOS. The SMPS measures aerosol particle number size
distributions from 6 to 250 nm with a temporal resolution of 120 s. The OPC
measures particle number size distributions from 250 nm to 2.5 µm with a
temporal resolution of 1 s. Therefore the combination of both instruments
provides aerosol number size distributions from 6 nm to 2.5 µm using the
inversion described by . SMPS measurements have been
corrected for variations in the volume flow due to pressure changes during
the flight and also for diffusion losses within the inlet line. The CPC
measures the total particle number concentration (NCPC) with
particle diameter Dp>6 nm and a temporal resolution of 1 s. This
instrumentation was introduced in . The measured
aerosol parameters were all corrected for standard temperature (T0=288K) and pressure (p0=1013.15 hPa).
In addition, the recently developed Fast CPC (FCPC) has been
implemented on ACTOS and was used to measure the total particle number
concentration (NFCPC) of aerosol particles >7 nm with a temporal
resolution of ≈ 10 Hz. The FCPC has its own inlet on ACTOS, which is
operated with a higher flow to minimize the residence time between inlet and
FCPC.
While the FCPC has been tested intensively on the ground ,
the CARRIBA campaigns were the first application for airborne measurements.
The TSI CPC was used as a reference during periods with relatively constant
N to compare the FCPC in terms of long-term stability. During the two
campaigns no systematic deviation between the two instruments was
observed. During two flights of the first campaign in November 2010, technical
problems occurred; those data have been excluded from this study. In
preparation for the second campaign in 2011, the FCPC was improved and a
second one was additionally installed on ACTOS. Thus, for the flights in
April 2011, data from two FCPCs are available. Here, both instruments worked
well and observed particle number concentrations from the two instruments
agree within an uncertainty of ±10 %.
Time series of the two FCPC (FCPC1 and FCPC2) and the TSI CPC measured during a flight section on 22 April 2011.
Figure shows a time series of NFCPC and NCPC on 22 April 2011 at cloud level. The background concentration during
this flight varied between 150 and 250 cm-3 but individual
events show particle number concentrations of more than 8000 cm-3. These
events are detected by all three instruments, but the TSI CPC, with its lower
time resolution, is not able to capture the maximum concentration measured by
the FCPC. Moreover, the small-scale structure is resolved by the two FCPCs
only. The time series demonstrates nicely how well both FCPCs agree, and in
the following figures only data from FCPC1 will be shown.
Radiation measurements using SMART-HELIOS
Instrumentation to sample upward spectral irradiances
Fλ↑ and cloud-reflected spectral radiances
Iλ↑ was installed on SMART-HELIOS. Measurements
performed with SMART-HELIOS are reported in and
. Optical fibers connect each of the two optical inlets to two
respective grating spectrometers. These cover the wavelength λ range
between 180 and 1000 nm in the visible to
near-infrared spectral wavelength range (VNIR) and between 900 and 2200 nm in the shortwave-infrared spectral wavelength range
(SWIR). In the VNIR the spectrometers have a spectral resolution, defined by
full width at half maximum, of 2–3 nm; in the SWIR this is 8–9 nm. The temporal resolution of sampled Fλ↑
and Iλ↑ is 0.1–0.3 s. The
field of view of the radiance inlet is 2∘, resulting in a footprint
2.5m in the cross-track and 4.2–8.5 m in the
along-track direction (depending on the flight level above cloud top and the
integration time of the sample). The optical inlets and spectrometers are
calibrated with certified calibration standards traceable to the National
Institute of Standards and Technology (NIST). The uncertainties in the
Fλ↑ and Iλ↑ signals arise from
uncertainties introduced during the radiometric calibration, as well as
uncertainties in the calibration standards and the spectrometer signal, and
are estimated to be ±6% in the VNIR and ±10% in the SWIR
.
Results
For the CARRIBA campaigns, the measured particle number concentration in the
marine boundary and cloud layer, excluding NPF events, was usually
≈ 100–300 cm-3 for the analyzed flights. During several flights,
short events of increased particle number concentrations with maximum values
of up to 17 000 cm-3 were observed. In this study, events with a
maximum number concentration of more than 1000 cm-3 were considered
to be NPF events. This threshold was arbitrarily chosen but exceeds the
background value by more than a factor of 3. Furthermore, subsequent
maxima were considered to be the same event if the concentration decreased to
the background value for less than 2 s (or 40 m); otherwise they were counted
as individual events. Using these criteria, 91 events were detected in the
cloud layer during measurements flights with valid FCPC data. The hypothesis
is that these particles were formed by nucleation and grew to detectable
sizes within the lifetime of the individual cloud sampled. Thus, these events
will be called NPF events. Interestingly, almost all of the NPF events were
observed in the vicinity of clouds. We did not find any dependence on
orientation, flight direction, or upwind versus downwind direction relative
to the cloud. Theoretically, such particles can also be artifacts, created by
droplet fragmentation at the inlet . In our case
with a true airspeed of 20 m s-1, aerodynamic breakup will not occur, and
impaction breakup would occur only for cloud droplets larger than
approximately 26 µm in diameter. Most of the cloud droplets observed
during CARRIBA were smaller , and thus breakup would not
occur. Another aspect excluding such artifacts is that most of the events
were observed out of clouds. Furthermore, two different inlets were used for
the different CPCs, but all events were observed at both CPCs at the same
time.
New particle formation events: examples
NPF events occurred at different locations relative to surrounding clouds.
Figure shows examples of observed NPF to illustrate the
variety of cases. Some instances were found near cloud edges (at both upwind
and downwind sides) or between clouds, where the decrease in LWC was closely
connected with an increase in N (see Fig. a and b).
Increased particle concentrations were observed during entering and leaving
clouds, which rules out the possibility of artificial particle production.
Furthermore, increased values in N were found in the entrainment zone,
i.e., in regions at the cloud edge where the LWC was nonzero but
fluctuating (see Fig. c). Other cases like the one shown
in Fig. d seem to occur without the presence of a nearby
cloud, although from the in situ measurements alone the possibility that
these observations were performed above cloud top cannot be excluded. These
four cases will be discussed later in more detail.
Time series of NCPC, NFCPC, and LWC over 40 s each during an event of increased particle number
concentration (a) at a cloud edge, (b) between clouds, (c) in the entrainment
region, and (d) without clouds nearby.
The examples shown in Fig. a–d also demonstrate the
different resolution of the two CPCs: the laminar-flow-type CPC, with its time
resolution of 1 s, cannot resolve the smallest structures that can be measured
by the FCPC. Thus, the maximum concentration also cannot be resolved by the
CPC because the variations appear smoother in this instrument. Some of the
events last only a few seconds, which is close to the detection limit of the
CPC.
The events have been identified by a significant increase in the total
particle number concentration; however, the events are too short to obtain a
complete particle number size distribution for the respective period. Thus,
we have to speculate about size and origin of these particles. There are no
anthropogenic particle sources because the measured concentration of CO2 does not follow N in any case. Since there are no other sources, the
observed particles are very likely produced by NPF – i.e.,
they were formed by homogeneous nucleation from one or more condensable
species and subsequently grew into detectable sizes.
Because the SMPS measures individual particle diameter ranges sequentially
within a scan, even with limited time resolution these measurements can give
insight into whether these particles occur mainly in the ultrafine particle
size range. During some events the observed diameter from the SMPS was in the
ultrafine particle range (< 20 nm), and the number size distribution showed
a maximum around this value. This was the case for the NPF event in
Fig. c. The SMPS started a scan from 6 nm at SOD 52 924, which is shown in Fig. together
with the following number size distribution without NPF. Around 20 s later,
between SOD 52 940 and 52 950, the event was observed in NFCPC,
corresponding to a selected diameter in the SMPS between 10 and 20 nm. One hundred and twenty seconds
later, the SMPS reaches the upper end and measures at 230 nm. Similar cases
were observed a few more times, but never at larger diameters in the
NSD. This finding confirms our hypothesis that the event of increased
particle number concentrations is caused by the formation of ultrafine
particles and that these cases are indeed NPF events. However, the presented
NSD is only representative of one NPF event and different events might
exhibit a different distribution. Furthermore, the maximum of the NSD does not
show the real maximum of the NPF event, because the SMPS was measuring in a
certain diameter range, defined by the measurement program.
Number size distributions (NSD) measured on 23 April 2011, starting at SOD 52 924 (cf. Fig. c).
Statistics
FCPC data from 28 flights are available. During 4 of those flights no NPF events were
detected, and during 24 flights between 1 and 13 individual events were
counted. This means that NPF events
were observed during 83 % of the investigated flights. Figure shows the distribution of
the horizontal extent of each individual event, assuming a constant flight
speed of 20 m s-1. More than 50 % of all events were observed to have a
horizontal extent of less than 100 m. This explains why they have rarely been
observed during previous aircraft experiments. Such aircraft have a flight
speed of at least 50–70 m s-1 (www.eufar.net), and most commercial CPCs
in the past had a time resolution of ≥ 1 s. Thus, events of increased
number concentrations that cover a distance of 100 m or less could not be
resolved with these systems.
Frequency distribution of the horizontal extent of the observed 91 new particle formation events.
Figure shows the distribution of maximum particle
concentration for each individual event. To exclude the influence of
individual outliers, the 95th percentile was used as a proxy for the maximum
concentration. The maximum observed concentration during CARRIBA was
17 000 cm-3, the most frequent maximum concentration occurred between 1000
and 2000 cm-3, and the frequency decreased with increasing number
concentration.
Maximum concentration (95th percentile) of the observed 91 new particle formation events.
Correlation of NPF with other variables
The 91 NPF events occurred at various locations within the cloud layer,
varying in length, maximum concentration, shape, and the correlation with
other parameters. An interpretation of the nucleation and growth processes
inherent in the observed NPF events is complicated for two reasons:
(i) the FCPC measures particles > 7 nm, i.e., the nucleation process has
occurred some time before the measurement and conditions may have changed,
and (ii) there are no measurements of potential precursor gases. However,
ACTOS and SMART-HELIOS provide meteorological, microphysical, and upward
radiation measurements that can be compared to NPF events for suggestions of
correlation. NPF events have been observed at heights between 600 and 2200 m, always within the cloud layer. Interestingly, NPF of a specific
measurement flight occurred in a narrow height range, i.e., within 200–300 m. These height ranges were often connected to a change in the number
concentration in the vertical profile. Thus, mixing with cleaner air may play
a role for the NPF event.
First, we consider correlations of NPF and existence of nearby cloud. During
the second CARRIBA campaign in 2011 measurements of the upward spectral
irradiance Fλ↑ and cloud-reflected spectral radiance
Iλ↑ by SMART-HELIOS are available for all flights. The
Iλ↑ data yield the benefit of giving information about
the cloud field directly around and below ACTOS. As shown by
, comparing spectra sampled over cumuli with those sampled
over the ocean surface or the island allows for a clear differentiation
between these surfaces via radiance ratios R=Iλ1↑Iλ2↑. In this work
λ1=720nm and λ2=644nm, resulting
in a range indicating cloudy data between R=0.6 and 0.75, depending
on the cloud optical thickness and solar zenith angle. Samples over the ocean
show R<0.6, while R>1 for measurements over the
island due to the sharp increase in reflectivity of vegetation in the near
infrared. The cloud ratio R yields a reliable estimate of the
position of each NPF event relative to the respective cloud field. This is in
contrast to in situ-measured LWC data, because, due to the inhomogeneous
cloud structure, ACTOS sometimes dipped in and out of cloud tops or entered
(left) the cloud later (earlier) than the cloud edge at lower levels. In
2011, 38 of the 44 NPF events were connected to clouds: at the cloud edge,
only a few seconds away from a cumulus or above a cloud. These findings
emphasize the strong connection between NPF and cloud boundaries.
Correlations with other measured variables can also be explored. In the
following, three representative examples of time series including NPF events
are given to illustrate the differences between individual events and
potential correlations with other parameters. Presented variables are w
(vertical wind speed), r (water vapor mixing ratio), N (total particle
number concentration > 7 nm measured by CPC and FCPC), LWC (liquid water
content) to show areas where clouds were present, Fλ↑
(spectral upward irradiance with λ=333 nm), and R (cloud
ratio).
Section of the measurement flight on 14 April
2011 through a cumulus cloud. Presented variables are w (vertical wind
speed), Fλ↑ (spectral upward irradiance at 333 nm), r
(water vapor mixing ratio), NCPC and NFCPC (total particle
number concentration measured by CPC and FCPC), LWC (liquid water content),
and R (cloud ratio).
Figure shows a NPF case at the edge of a cumulus cloud
indicated by LWC >0 adjacent to increased values in N. Furthermore,
R shows values between 0.6 and 0.7 around this cloud, indicating
that ACTOS was measuring above a cloudy area when at the location of the NPF
event. Before and after the cloud area, R increased to values
above 1, indicating measurements over the island without cloud coverage. The
total particle number concentration increases from ≈ 300 to 3500 cm-3 at the cloud edge. The irradiance
increases above the cloud by a factor of 2 compared to the region further
away from the cloud. The reason is the higher reflectivity of the cloud
surface compared to the darker ocean e.g.,. From the cloud
ratio it can be concluded that the NPF event occurs above a part of the
cloud, i.e., still in the region with enhanced irradiance. Interestingly, the
NPF event also corresponds to a downdraft region relative to the cloud.
During the NPF, a correlation between N and r is observed.
Section of the measurement flight on 22 April 2011; same variables as in Fig. .
Figure shows another case of NPF where the enhanced number
concentration occurred between two cloud traverses by ACTOS. The 22 April
2014 flight was characterized by the lowest particle number concentrations in the
cloud-free background during the whole campaign. In the selected section
shown in Fig. , N varied between 100 and 150 cm-3 and increased to more than 12 500 cm-3 during the NPF
event. The cloud ratio R indicates that there was continuous
cloud coverage below the measurement height, indicating that the three
individual regions of enhanced LWC (sampled in situ by ACTOS) belong to the
same cloud. This can occur when ACTOS briefly leaves the cloud top and
reenters the cloud a little later. The irradiance is enhanced over the whole
region by a factor of 2 compared to the cloud-free background. Here, the NPF
occurs in a region where r is still increased compared to background
values.
Figure shows a third NPF case observed at the cloud edge
as well as within the cloud (entrainment region), where increased particle
number concentrations are correlated with a decrease in LWC. The background
concentration in the cloud-free regions was around 200 cm-3, and the
maximum during the event was around 14 000 cm-3. The cloud ratio
R indicates that ACTOS was still above a cloud at ≈ SOD 52 950, i.e., the NPF event occurred above the cloud. Around the observed
cloud, R is fluctuating – i.e., there were some patches of clouds below
ACTOS. Fλ↑ has its maximum at the same side of the cloud
as N, which is also the side reflecting the sun during that flight.
Fλ↑ is enhanced by a factor of 1.8 above the background
value in the cloud-free environment.
The short NPF event within the cloud is related to a minimum in LWC and
occurs in the so-called entrainment region at the cloud edge. This region is
characterized by strong small-scale mixing processes leading to evaporation of cloud droplets and therefore a release of potential precursor
gases. In combination with turbulent mixing and an increased irradiance, this
region provides preferable conditions for NPF. The eddy-like structures in
LWC and N illustrate the mixing at the cloud edge nicely and have also
been observed in a few more cases. This is in good agreement with results
from , who identified NPF in the outflow
regions of cumulus clouds driven by enhanced gas concentrations in connection
with photochemical activity.
Section of the measurement flight on 23 April 2011; same variables as in Fig. .
From these examples a connection between NPF events and clouds as regions
with increased UV radiation is obvious. During this campaign, 38 out of 44
cases were directly connected to a cloud and an enhanced irradiance was
measured. These 38 NPF events in the vicinity of clouds can be additionally
divided into two categories, depending on the relative position of ACTOS to
the respective cloud and the solar azimuth angle of the sun. Cases with ACTOS
between cloud and sun, such as ACTOS approaching and leaving the illuminated
side of an individual cumulus, are characterized by increased reflectivity in
the UV spectrum (Vant-Hull, 2007). Cases where ACTOS is in a position above a
cloud are included in this category, which comprises 32 of the observed 38
NPF events associated with clouds. In the remaining six NPF events the cloud is
positioned between ACTOS and the sun, suggesting that ACTOS and SMART-HELIOS
were probing in a cloud shadow. Thus, these cases are associated with
increased UV radiation, which cannot be explained with the flight direction
relative to the cloud–sun axis alone. Here, 3-D effects possibly occur – i.e., the
enhanced irradiance is caused by reflections at another part of the cloud.
Together with the fact that 38 out of 44 cloud cases are directly connected
to an increase in Fλ↑, the UV irradiance seems to be an
important factor for new particle production in cloudy regions.
Figure shows the increase in measured irradiance
compared to a background value without cloud for the observed cases from
April 2011. The increase varies between 1.2 and 3.3, with a median of 1.78.
This is in good agreement with the study of , who published
model simulations to demonstrate that a UV enhancement of a factor of 2
produces reasonable values in H2SO4 compared with their observations.
From ACTOS measurements, no measurements of chemical precursors are
available, but H2SO4 is very likely involved in the nucleation process.
Probability density function of the
relative increase in measured irradiance at 333 nm above the individual cloud
compared to a background value without cloud for all NPF events during April
2011.
Estimated age of particle bursts and implications for particle growth rate
The observations of NPF events are often characterized by
short, burst-like spikes with remarkably sharp edges, implying that they have
not become well mixed and diluted by turbulence. At the same time, NPF events
are only observable when they have diameters above 7 nm. In this section we
use these two observations to estimate a lower bound on the particle growth
rate. In order to make a quantitative estimate we select one burst, shown in
Fig. , which was chosen because it is representative of a
typical NPF event.
Penetration of a cloud field with a burst of increased number
concentrations N of ultrafine particles between two cloud penetrations. The
clouds are marked by the blue areas, which represent LWC in arbitrary units.
Additionally, the dots show the local energy dissipation rate ε
averaged over a relative flight path of roughly 20 m.
The total horizontal extent of the burst is approximately 50 m, and we take
this as the characteristic dimension of the puff of newly formed particles. A
question we can ask is, what is the time required for a spatially localized
population of particles to reach this size through turbulent diffusion? The
concept of Richardson pair dispersion describes the relative separation of
two fluid elements in a turbulent flow for length scales within the inertial
subrange, or ∼1 mm to ∼100 m. The mean-square separation
distance scales as |δx|2=gεt3, where
ε is the turbulent kinetic energy dissipation rate and g is the
Richardson constant (typically thought to have a value of 0.1 to ∼ 5)
. Recent laboratory and direct numerical
simulation results suggest g≈0.5, so while we will
consider the full range of possible values, we take this as the most likely
value. We do not have any information about the size of the initial burst,
but to be conservative in estimates of growth rate we assume that it is at
the bottom of the turbulence inertial subrange, e.g., ∼1 mm. In that
case, we can obtain an approximate age of the burst as T=L2/gε1/3, where L=|δx|21/2 is the characteristic size of the burst.
The time series of ε is also shown in Fig. ,
and we use the mean value over the length of the burst, ε≈2×10-3 m2 s-3. Using L≈50 m, we
estimate T≈136 s. Assuming the particles must reach d=7 nm (starting at a cluster size below 1 nm) within this time, we can
estimate a growth rate of 3 nm min-1. Accounting for the range of
possible g results in growth rates of 2 to 7 nm min-1. For this
growth rate, ≈1–4×109 cm-3 vapor molecules are
needed see. In practice it is very probable that, in
addition to sulfuric acid, extremely low volatility organic compounds
(ELVOCs) are also involved in the process see,
because the saturation vapor pressure of condensing vapors should be very
small for effective molecular flux into growing newly formed particles, and
ELVOCs fulfill these criteria. ELVOC precursors could live inside clouds and
convection could transport ELVOC precursors as well as SO2 as a sulfuric
acid precursor to cloud edges. The solar radiation outside the cloud and
precursors coming from evaporating cloud are able to make photochemical
reaction efficient enough to produce ELVOCs. These kinds of growth rates have
actually previously been observed at coastal sites like Mace Head
.
Conclusions
This study presents meteorological, aerosol, cloud, and radiation measurements
from the CARRIBA campaign that was performed close to the Caribbean island
Barbados, in the trade-wind cumulus environment. Ninety-one cases of strongly
increased particle number concentration caused by NPF were
detected in the cloud layer and analyzed to consider event properties and
correlations. No NPF events have been observed in the
always cloud-free sub-cloud layer, while cumulus clouds were present during
all flights in the cloud layer. The available database does not allow for
process studies to be performed, but we have sought to identify conditions favoring NPF. Most of the NPF cases were connected to clouds
representing an environment with increased irradiance in the ultraviolet (UV)
spectral wavelength range. NPF requires photochemical activity for the
production of precursor gases. In our case no measurements of gases are
available, but the increased UV irradiance at cloud boundaries provides a
perfect region for the production of precursors. This could be intensified by
turbulent mixing, which is typical for the entrainment regions and therefore
for cloud edges. This was also observed for some cases in this study by high
fluctuations of thermodynamic variables in the region where NPF occurs. Such
small-scale mixing processes may enhance the formation rate of new particles
because of strong nonlinearities in the system . From
vertical profiles we found variations in particle number concentrations in
the height where NPF occurred for the majority of cases; thus mixing with
cleaner air may also be one aspect forcing nucleation and growth. Most likely,
the connection of the different issues is required: enhanced UV radiation to
produce the precursor gases, cloud edges as a region with increased
turbulence, and mixing with cleaner air reducing the condensational sink.
observed increased particle number concentrations in the
surrounding region of cumulus clouds, which was also characterized by high
relative humidities. During CARRIBA, regions with high relative
humidities around clouds were also detected by . In this
study, a few cases with correlations between NPF and humidity were also
observed, but for other cases no relationship was found.
The presented results agree with earlier publications suggesting that marine
clouds play an important role not only as particle sinks due to activation
and the following effects on the radiation budget but also as a source of
aerosol particles. Cloud edges or cloud tops have been identified as
preferred regions for NPF in earlier studies, but these were limited to a few
individual cases only. Here, we detected 91 cases demonstrating the relevance
for the marine environment. In combination with results from earlier
publications e.g.,, this leads to the conclusion that this phenomenon is not only of
regional interest, since it was found in completely different regions of the
world. The results are also consistent in general with a modeling study by
and earlier observations from , where
Aitken particles were also connected to NPF near clouds.
This was explained by a reduction in the condensation sink due to clouds and
precipitation. In addition, cloud updrafts loft dimethyl sulfide from the
ocean to the cleaner cloud layer, where it is oxidized to SO2 and
H2SO4. This step requires enhanced radiation and provides precursor
gases for NPF.
Another interesting result from this study is the estimate of the lifetime of
particle bursts. The mean length of the observed NPF events was 100 m,
resulting in a lifetime of less than 300 s. Aerosol instrumentation on ACTOS
is restricted to the measurement of particles larger than 7 nm only; thus these
particles grew within this lifetime to detectable sizes, i.e., 7 nm. This
implies growth rates of several nanometers per minute, which is in contrast to typical
growth rates mainly from ground-based measurements reported in the literature
(1–7 nm h-1; ) and is possibly caused by the
additional effect of turbulent mixing. However, reported
growth rates up to 200 nm h-1 (3.3 nm min-1) from a coastal
site in Ireland; thus the values deduced here are realistic. However, these
growth rates are not possible due to sulfuric acid alone, in particular under
clean marine conditions, and it is very probable that extremely low volatility
organic compounds (ELVOCs) are involved here .