Characterization of the WELAS and WHOPS
In the following, the performance of the WELAS and the WHOPS is carefully
assessed, including detailed uncertainty analyses. Table
summarizes the uncertainties of all relevant measured and derived quantities,
including references to the corresponding sections and figures, where they
are discussed.
Counting efficiency
The size-resolved counting efficiency of the WELAS2
was tested against a condensation particle counter (CPC 3025a, TSI Inc.) in
order to verify the number concentration measurements and to determine the
lower cut-off diameter (scattering cross section). The measured counting
efficiency expressed as a function of the raw voltage signal εCE, meas(Vraw) is defined as
εCE, meas(Vraw)=nWelas, raw(Vraw)nCPC,
where nWelas, raw(Vraw) is the particle number
concentration measured by the WELAS for quasi-monodisperse ammonium sulfate
particles size selected by a DMA that produces a certain raw voltage signal
Vraw, and nCPC is the corresponding value reported by
the CPC. The raw voltage signal is chosen as a reference axis for the
counting efficiency because Vraw, rather than particle size or
scattering cross section, determines the detection probability. Note that
Vraw is inferred from the instrument output as described in
Sect. . This calculation involves the WELAS-internal voltage
calibration factor CV (Eq. ), which underlines that
the knowledge of this factor is essential.
The plateau value of the measured WELAS counting efficiency reached for
Vraw>8 mV is at ∼132 % (Fig. ). This
means that the WELAS reports significantly higher number concentration
readings than the CPC, which has an accuracy of ±10 %. Such a bias
was already observed by Heim et al. (2008), who explained it by the fact that
the measuring volume in the WELAS used in the firmware calculations is
incorrect, thus leading to a bias (fixed factor) in the concentration
readings. As a consequence, the number concentration readings of the WELAS
have to be corrected. The measurements shown in Fig. were
performed with several Xe lamps of the same type but different number of
hours of operation. The counting efficiency as a function of Vraw
was equal within experimental uncertainty for all lamps, as expected.
Consequently the efficiency curve presented in Fig. is valid
for all measurements carried out with this lamp type, as long as the
transmission of the optical fiber cables remains constant.
Comparison of measured and theoretical Mie curves calculated
for m=1.59. The square markers, PSL measurements 1–4, show
results from four different experiments and the black line, PSL mean,
shows the mean value over the four experiments. In (a)
a constant scattering cross section calibration factor of
Cσ=3.71×10-15 m2mV-1 was
applied to analyze raw signals of the PSL measurements, while in
(b) the voltage-dependent calibration function Cσ(Vcal) described in Eq. () and Fig. was
applied.
The decrease of the efficiency for small particles (small Vraw),
as seen in Fig. , can be explained as follows: the intensity of
the light scattered by a particle has to be above a certain threshold to be
detected by the scattering detector. Fluctuations of the ratio between
scattering cross section and the raw voltage signal due to random noise are
responsible for the fact that only a certain percentage of particles with a
scattering cross section slightly above or below the lower detection limit
will be measured or not. The finite width of the DMA transfer function has
little influence here, as it is much smaller than the observed width of the
lower cutoff of the WELAS. The raw voltages at 50, 90 and
99 % of the plateau value of the CE are 3.7, 5.2 and 7.7 mV,
respectively (Fig. ).
In the range of the lower detection limit, the CE for particles with a
certain scattering cross section is expected to depend on the incident light
intensity, as Vraw is proportional to it (Eq. ).
However, the voltage calibration factor CV varied less than
10 %, thus indicating a relatively low variability in lamp intensity.
This makes an unambiguous assignment of optical diameters to the primary
Vraw axis possible (see additional abscissa scales in
Fig. ), using the mean value of CV(2.23). The
equivalent optical diameters for 50, 90 and 99 % of the
plateau value are ∼280, ∼325 and ∼395nm for PSLs and ∼330, ∼390 and
∼430nm for ammonium sulfate. This lower detection limit is
significantly higher than specified by the manufacturer (200 nm for
PSLs). The largest tested particle size corresponds to a PSL equivalent
optical diameter of approximately 600 nm. The counting efficiency is
expected to remain constant at the observed plateau value for larger
diameters, at least in the submicron size range, where potential impaction
losses are unimportant. In the following, all data were corrected with the
counting efficiency curve shown in Fig. , in order to correct
for the concentration bias as well as the decreasing detection efficiency in
the range of the lower detection limit.
Calibration of the scattering cross section measurement
The data analysis and calibration approach to retrieve quantitative
scattering cross section values and optical diameters from the raw
pulse height signals of the WELAS is described in detail in
Sect. . The calibration factor Cσ for the scattering
cross section is obtained by measuring PSL spheres with a certified
size (see Eq. ). Cσ is expected to be independent
of the raw pulse height, i.e., independent of the size of the PSL
spheres used to determine it. In order to test this, measurements with
PSL spheres of nine different diameters between 260 nm and
1 µm were performed, thereby covering the range of
interest for the WHOPS measurements.
Figure shows the comparison of the scattering cross section
measured for the PSL particles with the theoretical Mie curve calculated for
the WELAS specifications as described in Sect. 2.2. The squares in different
colors depict four different experiments (distributed over the course of 5
months) and the abscissa denotes the PSL diameters. For Fig. a,
a constant scattering cross section calibration factor (Cσ=3.71×10-15 m2mV-1) was applied in Eq. () to
all experimental data (Vcal). Cσ was chosen such to
minimize the “χ-square value” between the single points and
theoretical curve. Figure a reveals that the main
features of the theoretical Mie curve, like the shoulder between
300–400 nm, are well reproduced by the WELAS measurement. However,
PSL particles with diameters above 800 nm seem to scatter about
10 % less than predicted by Mie theory, and small particles with diameters
<400 nm scatter approximately 10 % more than expected.
Sensitivity studies with variations of the input parameters in the Mie
calculations showed that this systematic size-dependent bias cannot be
significantly reduced by changing the scattering angles, the light spectrum,
the index of refraction or assuming a narrow width of the PSL size
distribution rather than a perfectly monodisperse aerosol. The exact reason
for this minor disagreement is currently unknown. A possible explanation
could be a non-linear behavior of the detector or electronic parts amplifying
and processing these signals.
Actual ratio (Cσ) between the calculated scattering
cross section of PSL spheres and the calibrated voltage
(Vcal) measured by the light scattering detector plotted
against Vcal. A linear fit (red line) through the data
points is used as scattering cross section calibration function,
Cσ(Vcal), for the WELAS data
analysis. Additionally, the shaded area represents ±15 %
deviation around the fit. Approximately 85 % of all data
points fall within this uncertainty. The black dashed line depicts
the constant calibration factor Cσ=3.71×10-15 m2mV-1 applied for Fig. a.
The systematic deviations observed in Fig. a imply that
Cσ is not a constant, contrary to expectations. Therefore,
Cσ was separately determined for each PSL data point, using
Eq. (), and plotted against the calibrated voltage (Fig. ).
This analysis reveals a systematic increase of the calibration factor
Cσ with increasing voltage signal. Therefore, we decided to
use a voltage-dependent calibration function, Cσ(Vcal), for the quantitative analysis of the WELAS signals
(see Eq. ). A linear fit, Cσ(Vcal)m2mV=1.25×10-17m2(mV)2Vcal[mV]+3.25×10-15m2mV, was selected for this
calibration function. 85 % of the measurement points lie within
a 15 % deviation band around the linear fit, indicating that the
scattering cross section can be measured within an accuracy of around
15 % (1 SD) when using this calibration function. Figure b
shows the PSL measurements analyzed with this calibration function. The fact
that the systematic deviations presented in Fig. a disappeared
in Fig. b justifies the use of this calibration function for
all further data analysis.
Uncertainty of the optical sizing of dry particles
To retrieve the accuracy of the optical diameter measurement, the 15 %
accuracy of the measured scattering cross section is used as input for error
propagation calculations. This uncertainty is present despite the calibration
with Caldust. Nevertheless, repeated measurements of monodisperse
ammonium sulfate particles revealed that the random variability, which
corresponds to the precision of the scattering cross section measurement,
amounts to less than 2 % (1 SD). Figure depicts the
resulting estimated relative precision and accuracy of the dry optical
diameters for four different indices of refraction: 1.59, 1.50, 1.40 and
1.33, representing PSL, the range often found for atmospheric aerosol and
pure water, respectively. The uncertainty for optical diameters, resulting
from a fixed uncertainty in the scattering cross section, depends on the
local gradient of the Mie curve and thus on the particle size and index of
refraction. This explains why the relative sizing precision and accuracy
shown in Fig. strongly depends on diameter and index of
refraction. It is clearly visible that the precision of the optical sizing is
much better than the accuracy. On average, the relative sizing accuracy is
9 % for these indices of refraction in the range between 300 nm
and 1 µm, with minimal and maximal values of 4 and
22 %, respectively, for m=1.50 at 0.469 µm and m=1.333 at
0.300 µm. The precision amounts on average to ∼2 % with
minimal and maximal values of 0.1 % and 5 % (dashed lines in
Fig. ) and therefore plays a minor role (for measurements with
sufficient counting statistics). The error analysis presented in
Fig. is only valid if particles have a constant index of
refraction, e.g., for the optical sizing of a dry aerosol sample.
Relative uncertainty of the optical
sizing by the WELAS as a function of optical diameter and for
dry particles with different indices of refraction. The accuracy estimate (solid lines) is based
on error propagation calculations for a fixed accuracy of
±15 % for the measured scattering cross sections (note: the y axis shows the absolute
value of the larger relative size change when disturbing the scattering cross section by ±
its uncertainty). Additionally, the precision of the optical sizing (dashed lines) is illustrated.
Uncertainty of hygroscopic growth factors
Optical sizing of solution droplets. Panels (a) and
(b) show theoretically calculated scattering cross sections versus
growth factor (solid green lines) for the example of particles with a dry
index of refraction of 1.50 and dry diameters of 300 and 500 nm,
respectively. The solid red and blue lines are theoretical curves for
particles with a constant index of refraction, and the green lines indicate
the effect of a calibration bias (see text for details). Panels (c)
and (d) display the sizing uncertainties due to different causes for
the example particles from panels (a) and (b),
respectively.
The Mie curves (green lines in Fig. ) for solution droplets
that grow by absorption of water are less steep than those for a constant
index of refraction (red and blue curves in Fig. ). Hence, for
particles growing by water uptake the sizing accuracy has to be calculated
with respect to the green lines. The red and green solid lines in
Fig. a–b are equivalent to those in Fig. but for
using a dry index of refraction of 1.50. The Mie curves for solution droplets
with a dry size of 300 nm, solid green lines in Fig. a,
have a local maximum at GF=1 and a local minimum at GF=1.2.
This results in sizing ambiguities for solution droplets with a scattering
cross section value in between those of the local extremes, i.e., for the
growth factor range GF<1.4. This “ambiguity uncertainty” is shown
as red shading in Fig. c. For particles with a true GF of
1.0<GF<1.2 (range between the positions of the local extremes), the
ambiguity can potentially result in an over- or undersizing, while for
particles with GF between 1.2 and 1.4 the ambiguity results in potential
undersizing. The associated relative uncertainty is larger than
±20%. Therefore we only report data for GF>1.4 in the case
of 300 nm particles. A low ambiguity uncertainty of ∼6%
also occurs for GF>4.8, which is only relevant for very hygroscopic
particles at very high relative humidity. For the 500 nm particles, an
ambiguity uncertainty of mostly <∼7% occurs in the range
GF≥2.8 for particles with mdry=1.50
(Fig. d). In case of higher mdry, ambiguity also
becomes an issue in the range GF<∼1.15 (see dashed green line in
Fig. a).
The precision of calibrated scattering cross section measurements with the
WELAS is ∼±2% (1 SD; see Sect. ). The associated
precision of inferred GFs (violet curves in Fig. c–d) is
obtained by changing the scattering cross section σs by
±2% (note that, in order to separate the uncertainties in GF due to
different causes, the Mie curves were smoothed in the upper GF range before
inferring the GF dependence of the precision and accuracy). The precision
typically causes less than 3 % GF uncertainty, except for
Ddry=500nm and GF<1.4, where this uncertainty
increases by up to 6 % due to the lower gradient of the Mie curve. The
accuracy of the scattering cross section measurement by the WELAS (1 SD =
±15 %; see Fig. and discussion above) is more relevant
than the precision for the uncertainty of GFs determined with the WHOPS. An
absolute scattering cross section calibration bias is partially accounted for
by adjusting the index of refraction of the dry particles according to the
dry-mode WHOPS measurement. This ensures that no GF bias occurs at
GF=1 (cf. blue curve in Fig. d). However, the Mie
curves for the adjusted index of refraction (dashed and dash-dotted green
lines in Fig. a-b for mdry=1.53 and 1.47 and
mdry=1.53 and 1.46, respectively), have a slightly different
slope than the “true” Mie curve for mdry=1.50, such that a
scattering cross section calibration bias still causes a GF uncertainty for
GF>1. Comparing all sources of uncertainty in the GF retrieval
reveals that the accuracy of the scattering cross section calibration gives
the dominant contribution except for GF<∼1.4 and
Ddry=300nm (any mdry), where the ambiguity
uncertainty dominates. Overall, the accuracy of the retrieved GF is around
5–12 % in the ranges without substantial sizing ambiguity, depending on dry
size, GF and effective index of refraction (uncertainty analyses for indices
of refraction are presented in the Supplement). However, it is
important to note that the accuracy of the RH measurement (±2 %)
propagates to a corresponding relative GF uncertainty of 12–17 % (at
RH=95 %
and κ values between 0.1 and 0.5).
Retrieval of the index of refraction of PSL and ammonium sulfate
The dry effective index of refraction (mdry) of an aerosol
sample can be retrieved from the dry-mode WHOPS measurement, as
described in Sect. . To verify this procedure, tests were
performed in the laboratory with PSL spheres and size-selected (by the
DMA in the WHOPS) ammonium sulfate ((NH4)2SO4)
particles, which were prepared as described in Sect. 2.3. Note that
the size-selected ammonium sulfate aerosol sample also contains
multiply charged particles which are larger than the singly charged
particles. However, they appear as a well-separated mode in the WELAS
size distribution measurement and are neglected for this analysis.
Figure illustrates the retrieved effective indices of refraction for
particle diameters between 290 and 1000 nm. The
four different experiments with certified PSL spheres are the same as
those already shown in Figs. and . The retrieved index of
refraction for PSLs was found to be m=1.60±0.04 (mean ±1 SD), which agrees well with the literature value of m≈1.59 (see Sect. ). These four PSL experiments were used to
determine the scattering cross section calibration function
Cσ(Vcal) for the WELAS (see
Sect. /Eq. and Sect. /Fig. ). Therefore, the
retrieval of the index of refraction for the very same PSL experiments
is an internal consistency check of the instrument calibration and
data analysis approaches. Besides this, the scatter of the PSL data
points in Fig. reveals the precision of the retrieved index of
refraction (1 SD ≈±0.04 absolute), which is given by
the random noise of the individual PSL data points around the mean
calibration function (see Fig. ).
The dry-mode WHOPS measurements of the size-selected ammonium sulfate
particles provide an independent test of the index of refraction retrieval
for an aerosol sample that has different optical properties and that was not
used for calibration purposes (except for testing the counting efficiency,
which is independent of the optical sizing). The retrieved index of
refraction for dry ammonium sulfate particles is found to be m=1.49±0.02 (mean ±1 SD), which is ∼2.3 % lower than the mean
literature value of m=1.53 (Toon et al., 1976) found by
weighing the stated values for the wavelength range of interest. This
systematic difference is small given the fact that the selected mobility
diameter is tainted with some uncertainty (accuracy is assumed to be < 5 %)
and that part of it can likely be explained by the slight non-sphericity of dry
ammonium sulfate particles produced by nebulizing a solution and subsequent
drying. Zelenyuk et al. (2006) reported a decreasing effective density with
increasing mobility diameter for ammonium sulfate particles. This is
qualitatively consistent with the observed increasingly negative bias in the
retrieved index of refraction, though non-sphericity may also have some
influence on the light scattering cross sections.
Retrieved refractive indices for dry, nebulized ammonium
sulfate, (NH4)2SO4 (reddish colors; plotted against
mobility diameter) and different polystyrene latex spheres, PSL (bluish colors;
plotted against certified diameter) measurements.
In summary, the retrieval of the effective index of refraction of the
dry particles gives a good approximation, given the fact that the main
use of it is to ensure that meaningful growth factors are obtained by
relating the inferred optical diameters to the selected mobility
diameters, as described in Sect. .
Measured hygroscopicity of ammonium sulfate particles
(a) Shows a typical example of the hygroscopic
growth factors (red markers) measured by the WHOPS for pure ammonium
sulfate particles of different dry sizes along with the relevant
relative humidity (blue markers) simultaneously measured in the
WHOPS. In (a) the error bars around the measured GFs indicate the
accuracy of 6–10 % as described in Sect. . The
theoretical growth factors (ADDEM Model; Topping et al., 2005)
corresponding to the measured RH are shown in black, where the error
bars indicate the propagated uncertainty corresponding to an
experimental accuracy of the measured relative humidity of ∼±2 % (absolute). The markers and error bars shown in
(b) indicate the statistics (mean and standard deviation
from eight independent experiments) of the relative difference between
measured and theoretical growth factor of ammonium sulfate particles
at RHs between 89 and 97 % for different dry
sizes. The right y axis shows the hypothetical RH bias at RH = 95 %, which would
explain the corresponding observed GF bias shown on the left
y axis.
The accuracy of hygroscopic growth factors measured by the WHOPS in the wet
mode was verified against Köhler theory using pure ammonium sulfate
particles which were generated as described in Sect. 2.3. The WHOPS
measurements were analyzed using the approach presented in
Sect. and Fig. , i.e., accounting for the
effect of absorbed water on the index of refraction of the solution droplets.
Figure a shows a typical example of the GFs measured by the
WHOPS for different dry particles diameters (red markers) along with the
relevant relative humidity (blue markers) simultaneously measured in the
WHOPS. The error bars on the measured GFs reflect the accuracy of 6–10 % as
discussed in Sect. (see also Fig. ). The
corresponding theoretical growth factors, calculated for the measured RH
using Köhler theory (ADDEM Model; Topping et al., 2005), are shown with
black markers. As explained in Sect. 2.1, the RH sensors applied in the WHOPS
have an absolute accuracy of ∼±2 % RH. The error bars around the
theoretical GFs depict the propagated uncertainty corresponding to this RH
accuracy (note that the propagated uncertainty at RH ∼ 96 % is larger than
that at RH = 95 %, which is listed in Table ). The agreement
between the measured and theoretical GF is very good for the example shown in
Fig. a. In order to determine the long-term performance of the
WHOPS, results of eight independent ammonium sulfate experiments distributed over
7 months are shown in Fig. b. The crosses depict the mean
relative deviation of the measured GF from the theoretical value at the same
RH (between 89–97 %), while the error bars show the standard deviation.
The right y axis in Fig. b explains what absolute RH bias
would be necessary to explain the corresponding GF bias shown on the left
y axis at an RH of 95%. The mean relative deviation of measured GFs from
theory is almost negligible with values between 0 and 4.5 % for
different dry diameters, which corresponds to an absolute RH bias between
0 and 0.7 %. The fact that also all error bars fall between
-2 and +11 % GF bias, which corresponds to -0.4 and
+1.4 % absolute RH bias, indicates that all individual data points fall
within the expected uncertainty taking into account that the accuracy
of the RH measurement, which is likely the main limiting factor of growth
factor measurements by the WHOPS, is estimated to be as much as ±2 %
(absolute).
Figure shows that it is possible to accurately measure the GF of
pure ammonium sulfate particles between 300 and
531 nm. The measurements for a dry diameter as small as
300 nm are only possible since the ammonium sulfate solution
droplets grow clearly above the lower WELAS detection limit, which is
at ∼390 nm for dry ammonium sulfate particles
(Sect. and Fig. ). Furthermore, the GF of 300 nm ammonium
sulfate particles is also above the threshold, which makes unambiguous
optical sizing possible even at this dry diameter (Sect. and
Fig. ). However, it is not possible to measure the complete growth
factor distribution of aerosol samples that contain non- or only
slightly hygroscopic particles for dry sizes below ∼400 nm, as they would remain undetected by the WELAS, thereby
biasing the measured GF distribution.
First field measurements
Here we present first results from an airborne application of the
WHOPS on board the Zeppelin NT during one long-distance flight in the
Netherlands which took place in the afternoon on 22 May 2012. The
Zeppelin started at the international airport of Rotterdam The
Hague, then reached the Cabauw Experimental Site for Atmospheric
Research (CESAR) ∼40 km east of Rotterdam, where
it turned back westwards, passed Rotterdam again, flew over the North
Sea until ∼20 km from the coast, before turning back
eastwards and finally landed at the airport (see Fig. a and b). The
average altitude flown was 200 m above ground, except for the
part over the sea, marked with a red circle in Fig. a, where a short
height profile was performed reaching up to 700 m above
ground, before flying back to the airport at an altitude of
200 m above ground.
κ values derived from different measurements at the CESAR
site in Cabauw in this and previous studies plus simulated global
values.
Compound
κlow
κup
κmedian
Instrument
Dry diameter [nm]
Cabauw (ambient)
0.25
0.75
0.38
WHOPS (this study)
500
Cabauw (ambient)
0.14a
0.53a
0.29a
Wet-Neph
mean of size distribution
Cabauw (ambient)
0.11b
0.44b
0.26b
HTDMA
165
Global continental
0.06c
0.48c
EMAC Model
10–1000
Global remote marine
0.90c
1.00c
EMAC Model
10–1000
Global mixed marine/continental
0.40c
0.60c
EMAC Model
10–1000
κmean
κstd
Continental (ambient)
0.3d
0.1d
review – various methods
< 300
Marine (ambient)
0.7d
0.2d
review – various methods
< 300
a Zieger
et al. (2011), κ calculated from GF retrieved from
Wet-Neph data from their Fig. 6.b Zieger et al. (2011), κ calculated from HTDMA data from their Fig. 6.c Pringle et al. (2010), range of κ from Table 1 simulated
at Earth's surface level from ECHAM/MESSy Atmospheric Chemistry (EMAC) model
to simulate global fields of the effective hygroscopicity parameter κ
which approximately describes the influence of chemical composition on the
cloud condensation nucleus activity of aerosol particles.d Andreae and Rosenfeld (2008), summary for κ values from
Sect. 2.1.3 from several HTDMA and cloud condensation nuclei counter
measurements.
Figure b displays the mean GFs at 95 % RH (color coded,
averaged over one measurement cycle of 200 s) measured by the WHOPS along
the flight track for 500 nm particles. The measured GFs were not
recalculated for small deviations between the target RH of 95 % and the
actual RH measured in the WHOPS (corrected for equilibration time effects),
because the latter was always within 95±0.6 %. For the 500 nm
particles, mean GFs were found to be between 1.79 and 2.43, with a median
value of 2.02. In order to compare the aerosol hygroscopicity measurements
made in this study with literature data, the hygroscopicity parameter
κ was calculated from the measured mean GF according to Eqs.
() and () in Sect. 1. The WHOPS-derived κ values are
listed in Table , along with literature data for similar
aerosol types. The WHOPS values reach from κ=0.25 to 0.75, with
a median κ of 0.38 for Ddry=500 nm. This matches
well with the range of κ values of 0.3±0.1 (mean ± SD) and
0.7±0.2 that are considered to be representative for continental and
marine aerosols, respectively, based on a review of cloud condensation nuclei
measurements (Andreae and Rosenfeld, 2008). Pringle et al. (2010) performed
global model simulations and reported a κ value of 0.27±0.21 for
continental aerosol. Higher κ values of 0.4–0.6 and 0.9–1.0 were
reported for mixed marine/continental aerosols and remote marine aerosols,
respectively. These values cover the range from in situ measurements well.
The time series of the measured mean GFs and κ values for
500 nm particles is shown in Fig. e (black curve).
The error bars indicate the accuracy of ±10 % (1 SD; see
Sect. ). Higher κ values (median =0.55) were found
during the first part of the flight, when the Zeppelin headed inland towards
Cabauw and back to Rotterdam (from now on referred to as “Part 1”), while
lower κ values (median =0.31) were found during the second part of
the flight, which was directed towards the sea and back to Rotterdam (from now
on referred to as “Part 2”). The prevailing local wind direction (arrows
in Fig. c), measured on board the Zeppelin NT, differed
clearly between these two regions (the change in local wind direction is
marked by the dashed gray line): during “Part 1” northwesterly winds
were dominant, while during “Part 2” northeasterly winds prevailed.
Besides, the wind speed is illustrated in Fig. c, showing
generally low values (2–4ms-1). The number concentration of
particles with D>300nm obtained from the polydisperse WELAS
measurement (turquoise line in Fig. d) increases sharply just
after the local wind direction changes. This gives clear evidence that the
Zeppelin NT entered a different air mass during the second part of the
flight.
Back trajectory analysis (not shown) was performed for different places and time periods and revealed
that the probed air masses had maritime and/or continental influences. However, the whole flight was
dominated by low wind speeds and changes in the local wind direction were not captured by the HYSPLIT
model such that the model output was not reliable.
Example of airborne WHOPS measurements on board the Zeppelin NT
from 22 May 2012 in the Netherlands; (a) flight track color coded by
UTC time; (b) mean GFs (color coded) at RH = 95 % for
Ddry = 500 nm particles; (c) time series of the local
wind speed (red curve) and wind direction (red arrows) measured on board
the Zeppelin NT; (d) time series of the flight altitude (gray line)
and number concentration of particles with D>300nm measured by the
polydisperse WELAS1 (turquoise line); (e) time series of the
measured mean GF (left axis) and corresponding κ values (right axis)
for the selected particle size of 500 nm; additionally the mean GF of all
particles with GF > 1.5 (κ > 0.12; lower detection limit when
selecting 300 nm particles) is shown for the 300 nm and 500 nm particles;
the gray dashed line across panels (c), (d) and
(e) marks the point in time when the local wind direction changed
between the first and second part of the flight; (f) GF-PDF for
humidified 500 nm particles separately averaged over “Part 1” and “Part
2”; (g) equivalent to (f) but showing results for 300 nm
particles; the gray area covers the GF range that cannot be reliably detected
for this particle size.
Figure f illustrates the averaged GF distribution (GF-PDF) for a dry
diameter of 500 nm, where the turquoise line and the solid
blue line illustrate the results for “Part 1” and “Part 2”, respectively.
Particles covering the whole GF range between approximately 1 and 3
were observed. This does not necessarily imply external mixture at a certain
time. However, the fact that particles with GFs substantially smaller and
larger than the observed minimal and maximal mean GF occurred indicates that
the aerosol was at least sometimes externally mixed (a time-resolved analysis
of the mixing state is not possible due to the limited counting statistics).
On average, about 15 % of 500 nm dry particles had GF <1.1, which
could be explained with externally mixed dust (e.g., Herich et al., 2009),
fresh soot (e.g., Tritscher et al., 2011) or biological particles (e.g.,
Després et al., 2012). Mineral dust and possibly biological material are
more likely, as the size distribution of such particles is known to extend
down to the submicron size range (Després et al., 2012; Mahowald et al.,
2014), while fresh soot particles typically show up at smaller sizes
(∼100 nm; Rose et al., 2006). Around 74 % of the 500 nm particles
had GF >1.5 at the dry size of 500 nm. These particles must have
contained a high fraction of inorganic salts, which are the most hygroscopic
aerosol components (e.g., Topping et al., 2005). The remaining 11 % of the
500 nm particles had GFs between 1.1 and 1.5, most likely an internal mixture
of organic/inorganic compounds (e.g., Gysel et al., 2004; Meyer et al., 2009).
The dominance of more hygroscopic particles explains the relatively large
mean and median GFs and κ values presented above. One clear
distinction between the two air masses is the shift of the more hygroscopic
fraction to smaller GFs during “Part 2” compared to “Part 1”, while the
non- and less-hygroscopic fractions remained essentially unchanged. During
“Part 1”, the main, more hygroscopic mode peaked at a GF of ∼2.5
(κ∼0.8). This is more hygroscopic than, e.g., pure ammonium sulfate
or ammonium nitrate particles (e.g., Petters and Kreidenweis, 2007; Topping
et al., 2005) and can only be explained with very hygroscopic sea-salt
particles (pure NaCl, NaNO3 and Na2SO4 particles have
κ values at RH = 95 % of 1.32, 0.86 and 0.67, respectively).
Sea-salt particles are frequently observed in the accumulation mode aerosol
of marine environments (Swietlicki et al., 2008). During “Part 2”, the
main, more hygroscopic mode shifted towards smaller GFs peaking at a GF of
∼2 (κ∼0.4), which is typical for a continental background
aerosol (Swietlicki et al., 2008) although aged sea-salt particles with
a substantial organic fraction would also be possible.
For the 300 nm particles, shown in Fig. g, the GF-PDF could only be
measured by the WHOPS for the range GF > 1.5 due to the lower detection
limit of the WELAS (see Sect. ). This leads to incomplete GF-PDFs and
thus also to a high bias in the inferred mean GFs. Nevertheless, these
measurements still provide some useful insight. The GF-PDF shown in Fig. g
also reveals a similar trend with the air mass change as seen for the
500 nm particles diameter: the more hygroscopic mode shifts from very
high GFs during “Part 1” towards slightly lower GFs during “Part 2”.
Figure g further reveals that the averaged GF-PDF in the range GF >1.5
is quite similar for the 300 nm and 500 nm particles,
although the 300 nm particles seem to be shifted towards just
slightly larger growth factors. The lower detection limit of the WELAS made
it impossible to determine whether the number fraction of particles
with GF <1.5 is also similar for the two investigated dry sizes.
For a spatially resolved comparison of the size dependence of particle
hygroscopicity, Fig. e additionally contains the mean GF of
those particles with GF >1.5 for the dry diameters 300 nm (green
symbols) and 500 nm (red symbols). This comparison reveals a similar
spatial pattern with two distinct air mass types for both sizes. The mean GF
values of the more hygroscopic particle fraction are just slightly larger at
300 nm dry diameter compared to 500 nm, as already seen from
the GF-PDFs shown in Fig. f and g. However, this subtle
difference is within the experimental uncertainty of the WHOPS (see error
bars in Fig. e). The almost constant difference between the
black and red lines in Fig. e further confirms that the change
in the mean GF of the 500 nm particles between the two parts of the
flight can mostly be attributed to a shift of the more hygroscopic mode with
GF >1.5, while the number fraction and GF of the particles in the range
GF <1.5 stayed quite constant over the whole flight, in agreement with
Fig. 12f and g.
Zieger et al. (2011) reported aerosol hygroscopicity data from previous
measurements at the CESAR site in Cabauw, at which the Zeppelin passed by in
this study, determined with two different experimental techniques:
monodisperse HTDMA measurements at RH =90 % and polydisperse
humidified nephelometer (Wet-Neph) measurements. These two approaches differ
from the WHOPS technique with regard to the selected/representative dry
diameters as well as the set RH (see Table for more details).
The κ values determined by Zieger et al. (2011) using the Wet-Neph and
size distribution measurements were between 0.14 and 0.53 (median =0.29)
and those derived from the HTDMA measurement lay between 0.11 and 0.44
(median =0.26), which compares well to the WHOPS data of this study, even
though these measurements were taken at different times and dry sizes. The
lower κ values of the HTDMA were explained by the not fully detected
contribution of sea salt above 165 nm (maximum dry diameter set in
the HTDMA). Zieger et al. (2011) could further classify the observations into
different air mass types with distinct aerosol hygroscopic properties,
including marine, polluted marine and continental influence. Within this
study, it was further observed that the particle hygroscopicity was
increased during periods with marine air masses. This further supports the
above interpretation of the two air mass types encountered in this study
during the first and second parts of the flight.
Using the approach described in Sect. , it was also possible for
the 500 nm particles to retrieve the effective index of refraction of
the dry particles which was found to be 1.42±0.01
(mean ± 1 SD; the estimated accuracy is
±0.04) for either part of the flight. This compares well with
literature values for the index of refraction in the visible
wavelength range of 1.44–1.52 for marine aerosol (Stock et al.,
2011), while it is somewhat lower than values of 1.5–1.57 reported
for continental aerosol (Stock et al., 2011; Ebert et al., 2002).