Introduction
The importance of measuring the concentration and size distribution of
aerosols in the lower atmosphere has been highlighted by various studies.
For instance, their presence in ambient air can have direct effects on human
health (e.g. Zemp et al., 1999; Brunekreef and Holgate, 2002), and their
interaction with solar radiation and clouds are affecting regional and
global climate (Hansen et al., 1992; Ramanathan et al., 2001; Ammann et al.,
2003; Diner et al., 2004; Kanakidou et al., 2005; Quaas et al., 2008). When
very high concentrations of volcanic ashes are present, they can affect the
atmospheric visibility, the radiative budget, and the air traffic (e.g.
Chazette et al., 2012). In the middle atmosphere, aerosols play a
significant role in ozone stratospheric chemistry, including the formation
of polar stratospheric clouds through heterogeneous reactions with nitrogen
and halogen species (e.g. Hanson et al., 1994, 1996). The concentration and
size of the particles are highly variable due to the large variety of
aerosol sources and properties, both of natural and man-made origin, and
because of their altitude-depending residence time. To understand and
predict aerosol impacts, it is important to develop observation and
monitoring systems allowing for their full characterization.
Instruments have been developed for routine measurements or for dedicated
campaigns. Observations can be conducted from the ground, from unmanned
aerial vehicles (UAVs), from aircrafts, from balloons, and from satellites.
To retrieve the physical properties of the aerosols, it is necessary to
combine the information obtained with different instruments. In situ
mass spectrometers (Murphy et al., 2007) and aerosol-collecting instruments
(Brownlee, 1985; Blake and Kato, 1995; Allan et al., 2003; Bahreini et al.,
2003; Ciucci et al., 2011) provide their composition. Optical instruments
performing remote sensing measurements from the ground or from space with
photometric, lidar, and extinction techniques (e.g. Shaw et al., 1973;
Dubovik and King, 2000; Bitar et al., 2010; Winker et al., 2010; Salazar et
al., 2013) provide indications on the size distribution and on the nature of
the particles (liquid, carbon, minerals, ice, ...), generally
assuming a priori hypotheses in the retrieval process. Complementarily, in situ optical
measurements with optical particle counters can provide more accurate
information on the local size distributions of the particles (Deshler et al.,
2003).
The present study deals with optical aerosol particles counters (OPCs). The
corresponding measurement principle relies on the properties of light
scattered by particles injected in an optical chamber and crossing a light
beam (e. g. Grimm and Eatough, 2009). The measurements are usually conducted
at “large” scattering angles, typically around 90∘ with
collecting angle of a few tens of degrees. At such angles, the light
scattered is depending both on the size of the particles and on their
refractive index. Conventional counters are calibrated using latex and glass
beads and are post-calibrated using Mie calculations (Mie, 1908) for liquid
aerosols (the refractive index of latex beads and liquid aerosols is well
known, assuming no imaginary part of the index, i.e. non-absorbing aerosols).
Some instruments can also be post-calibrated for the observation of specific
particles, as desert dust or urban pollutants, assuming a given value of
their refractive index and some assumption on their shape.
The refractive index dependence can be partially determined by performing
measurements at different scattering angles, since the variation of the
scattered intensity with scattering angles is strongly dependent on the
refractive index of the particles (Volten et al., 2006; Francis et al.,
2011). Thus, performing simultaneous measurements at different angles can
provide an indication of the nature of the particles. Such an approach was
used by Eidhammer et al. (2008) at angles of 40 and
74∘ mainly for the identification of mineral particles, and by
Gayet et al. (1997) with a ring of detectors covering the whole scattering
angle range for the identification of cloud droplets and icy particles.
Scanning electron microscope image of ambient air aerosols
(courtesy Jose Vanderlei Martins, Institute of Physics of the University of
Sao Paulo, Brazil).
Another approach was proposed by Renard et al. (2010a); in this case,
measurements are conducted at small scattering angles, below 20∘,
where the light scattered is less sensitive to the refractive index of the
particles. In this angular region, the scattered light is dominated by
diffraction (which is not sensitive to the refractive index), at least for
irregular grains such as those found in the atmosphere (Fig. 1). Such
low-dependence of the refractive index was confirmed by measurements
conducted at a scattering angle around 15∘ for different types of
irregular grains (Lurton et al., 2014). In this case, the light scattered is
mainly dependent on the size of the particles, allowing a better
determination of the corresponding size distribution. However, the main
problem of measurement at small angles is stray-light contamination. Thus a
real-time correction of this signal offset due to the stray light, which can
vary with time, must be developed (as explained in Sect. 2.1 and in Renard et
al., 2010a).
Aerosol particles counters are often used on the ground; some of them are
used in the free atmosphere on-board aircraft or large balloons during
dedicated campaigns, for example for the studies of desert dust events or
volcanic aerosols (Bukowiecki et al., 2011; Jégou et al., 2013; Ryder et
al., 2013) or for stratospheric studies (Rosen, 1964; Ovarlez and Ovarlez,
1995; Deshler et al., 2003; Renard et al., 2008, Renard et al., 2010b). We
propose here a new optical particle counter concept called LOAC (Light
Optical Aerosol Counter) that is light and compact enough to perform
measurements on the ground and under all kinds of balloons in the
troposphere and in the stratosphere, including meteorological balloons. LOAC
uses a new approach combining measurements at two scattering angles. The
first one is around 12∘, an angle for which scattering is weakly
sensitive to the imaginary part of the refractive index of the aerosols,
allowing the retrieval of the particle size distribution. The second one is
around 60∘, where the light scattered is strongly sensitive to the
refractive index of the particles, and thus can be used to evaluate their
typology (liquid droplets are transparent, minerals are semi-transparent, and
carbonaceous particles are strongly absorbing).
In this first paper, we will present the principle of measurements and
calibration, and cross-comparison exercises with different instruments that
detect atmospheric aerosols. In the companion paper, we illustrate first
scientific results from airborne observations on-board balloons and unmanned
aircraft.
Principle of measurements
Instrument concept
LOAC is a modular instrument, for which some parts can be changed depending
on the measurement conditions. For measurements under the balloon or on the
ground in low-wind conditions, the aerosols are collected by a metal
profiled inlet designed to optimize the sampling conditions when oriented in
the wind direction. The particles are drawn up to the optical chamber
through an isostatic tube and then to the injector that focusses the particle
flux inside the laser beam. LOAC uses a small vane-type pump (having a
life-time of 3 weeks in continuous operation) working at ∼ 2 L min-1. The pump is connected to the exit of the optical chamber by a
flexible plastic tube. In-flight tests under sounding balloons have shown
that the rotation speed of the pump is not affected by pressure variations.
For measurements in windy and rainy conditions, the inlet can be replaced by
a total suspended particulate or TSP inlet rejecting rain droplets and
particles greater than 100 µm. For long-duration measurements, the
small pump can be replaced by a robust pump; to maintain the aerosol
detection efficiency, the pump flow must be in the range 1.3–2.7 L min-1.
To minimize its weight, the optical chamber is in plastic
Delrin®. The weight, including the pump, is 300 g. The
electric consumption is 340 mA under 8 V (which corresponds to a power of
3 W). The optical chamber and the pump can fit in a rectangle box of about
20 × 10 × 5 cm3.
The LOAC instrument; upper panel: principle of measurement; lower
panel: picture of the instrument (the inlet tube is not presented here).
LOAC is mainly designed for the detection of irregular grains, such as those
present in ambient air (Fig. l). It uses a statistical approach for the
size and concentration retrievals, as is done for the laboratory PROGRA2
instruments dedicated to the study of optical properties of irregular
levitating grains (Renard et al., 2002). Because of their shape, their
orientation and their rotation in the air flow, the scattering properties of
an individual grain vary with time at a given scattering angle (this
variation could be more than a factor of 2, as shown during laboratory tests by
photodiodes and imagery measurements with PROGRA2). This must be taken into
account for the calibration and data analysis. Thus, we propose here a
calibration approach that can differ from the one used for other optical
counters.
The sampled air crosses a laser beam of 25 mW working at the wavelength of
650 nm. The stability of the laser is within ±5 %; the laser is
always operated in its nominal temperature range, even during stratospheric
flights. The homogeneity of the beam is ±20 %. The scattered
light is recorded by two photodiodes at scattering angles, respectively, in
the 11–16∘ channel (hereafter called the 12∘ channel) and 55–65∘ channel (hereafter called the
60∘ channel), as shown on Fig. 2. Instead of using lenses to
collect the light, the photons travel directly to the photodiodes through
pipes, providing fields of view of a few degrees. The collecting area of the
photodiodes is larger than the diameter of the pipes. This system prevents
optical misalignment problems in case of vibrations and strong temperature
variations like those encountered during atmospheric balloon flights. Such a
concept of scattering measurements without collecting lenses was previously
tested and validated by Daugeron et al. (2007).
The electronic sampling is at 40 kHz and the transit time of particles
inside the laser beam is equal or lower than 700 µs. As said before, a
real-time correction is needed for the high stray-light contamination at
small scattering angles. For this reason, the stray-light correction method
presented in Renard et al. (2010a) was applied to the LOAC measurements. The
stray light acts as a kind of continuum, which can slightly vary over time
due to changes in the temperature and pressure conditions and possible dust
contamination in the optical chamber. The light scattered by the particles
is superimposed on this continuum, which can be assumed as a continuous
base-line over a short time interval. This baseline is determined before and
after the intensity pulse produced by the particles that cross the laser
beam.
Example of the output voltage recorded by the 12∘ channel photodiode for ambient air particles crossing the laser beam. The
red line corresponds to the threshold for the peak detection. When a
particle is detected, the signal must return back below the threshold to
allow the detection of the next one.
The maximum of the intensity pulse is obtained after subtracting the
stray-light contamination. Figure 3 presents an example of real ambient air
measurements of the time evolution of the intensity scattered by a 5 µm
particle and by few submicronic particles. The pulse is slightly asymmetric,
because the particles decelerate when crossing the optical chamber. This
deceleration occurs because the diameter of the optical chamber is larger
than the diameter of the inlet, and the particles encounter pressure
relaxation. Some secondary intensity maxima may be present in the pulse and
can be attributed to the rotation of irregular shaped particles in the air
flow. The search for a new intensity peak is inhibited until the output
voltage recorded by the photodiode decreases to a given threshold,
represented in Fig. 3 by the red line. This procedure prevents multiple
counting of the same particle (of irregular shape) that exhibits secondary
intensity maxima. The threshold, or detection limit, corresponds to the
output voltage level on which particles can be detected even if some
electronic noise is present. The electronic noise can change with time
because of the sensitivity of the electronic components to atmospheric
temperature variations. The instrument performs a check of its noise level
after 15 min of measurements. If the noise differs by more than 50 % from
the previous check, an electronic re-calibration is automatically performed
to estimate the offset variation and to adjust the calibration. A processing
software is applied after the experiment to check the offset time evolution
during the 15 min periods and to correct the raw measurements.
Calibration
The calibration of an optical counter is not an easy task, especially for
the detection of irregular particles (Whitby and Vomela, 1967; Gebhart,
1991; Hering and McMurry, 1991; Belosi et al., 2013). A first presentation
of the calibration procedure for measurements at small scattering angles
using a LOAC optical chamber can be found in Lurton et al. (2014).
The calibration procedure is conducted for the 12∘ channel, which
is almost insensitive to the refractive index of the particles. The
60∘ channel will be used as a comparison to the 12∘ channel measurements to determine the typology of the aerosol, as explained
in the Sect. 2.4. To conduct such determination, the 60∘ channel must have the same output voltage thresholds as
the 12∘ channel, to perform direct comparison of the counting detected by the two
channels.
Monodisperse latex beads, which are perfect transparent spheres, have been
used for diameter calibration in the 0.2–4.8 µm range; glass beads
have been used at 5 µm (see Figs. 2 and 3 of the Lurton et al., 2014
paper for the LOAC response to monodisperse beads). In fact, Mie
calculations show that the scattered intensity encounters strong
oscillations linked to small changes both in diameter and in scattering
angle. Conventional aerosol counters use large field of view, typically a
few tens of degrees, to average these oscillations. On the opposite, the
LOAC 12∘ and 60∘ channels have a field of view only of
few degrees and use no lens. The detected scattered intensity at the
12∘ channel is then very sensitive to the position of the
individual bead inside the laser beam, and thus to its scattering angle.
Taking into account this constraint, we considered here only the highest
intensity scattered by each size class of monodisperse beads.
The electronic noise is lower than 20 mV at ambient temperature and lower
than 10 mV when the electronics is exposed to negative temperatures.
Statistically speaking, the noise is divided by the root mean square of the
number of identical measurements (here the number of events detected in a
given size class). To reach a 1 mV accuracy in the case of 20 mV noise, which is
necessary to be able to discriminate the smaller size classes and to
establish accurately the size distribution, at least 20 × 20 (= 400)
particles must be detected for each size class.
During laboratory calibration, it is easy to reach such concentration levels
using monodisperse beads. During real measurements in the atmosphere, we
must ensure that such particle concentrations are indeed present for the
LOAC size classes below 1 µm. The LOAC has an integration time of
10 s, with a pumping flow of about 2 L min-1. Even in low polluted
ambient air at ground (“background conditions”), typical counting
measurements available in the literature have shown that concentrations are
greater than 1 particle cm-3 for size classes smaller than 0.5 µm (e.g. Ketzel et al., 2004), which corresponds to
2000/6 = more
than 300 particles during the 10 s LOAC integration time. For particles in the
0.5–1 µm size classes, concentrations are greater than 0.1 particle cm-3, giving more than 30 particles. Thus, 2 min of measurements
will provide good statistics for the LOAC data analysis.
For all the cross-comparison exercises presented below, the measurements
were integrated from 2 to 15 min. For the 2 min integration time, the
number of particles given above must be multiplied by 12, giving at least
3000 for the three first size classes and 300 for the other ones. For a 15 min integration time, these numbers must be multiplied again by 7.5.
Thus, the LOAC class identification can be conducted with the expected
accuracy in the ambient air. Obviously, in the case of polluted air, all of these
values could be also 2 to 3 orders of magnitude higher (1000 particles per cm3 between 0.2 and 0.3 µm is often encountered).
In the case of very low particle concentrations, such as those that can be
encountered during flights in the stratosphere with typically less than 1 particle cm-3 greater than 0.2 µm, the size attribution will be
less accurate. Thus, the retrieved size distributions and the time evolution
of the concentration will be more scattered and need to be averaged in
altitude.
For the calibration in the 7–45 µm size range, different natures of
irregular grains have been used: carbon particles, dust sand of various
types, ashes and salts (see for example Fig. 4 of the Lurton et al., 2014
paper). The size selection was obtained using sieves. For diameters at
∼ 90 µm, calibrated silicon carbide grains were used, the
size being characterized by the provider. The diameter presented here
corresponds to an equivalent (or optical) diameter, which can differ
significantly from the aerodynamic diameter or from the electric mobility
diameter used by non-optical instruments for ambient air measurements.
At least 30 grains are necessary to ensure a mean random orientation, to
be able to derive a mean equivalent diameter. The relation between the
output voltage recorded by the detector and the particle size was derived by
considering the diameter where the concentration of detected particles is at
its maximum. The measurements with different nature of grains confirm that
no significant dependence on the particle type exists for the variation of
the scattered intensity with their diameter, consistent with the Fig. 8 of
Renard et al. (2010a) and Fig. 5 of Lurton et al. (2014).
Taking into account the laser departure from homogeneity, the electronic
noise, and the statistical approach, the uncertainty in size calibration is ±0.025 µm for particles smaller than 0.6 µm, 5 % for
particles in the 0.7–2 µm range, and of 10 % for particles greater
than 2 µm. Figure 4 presents the calibration curve for the
12∘ channel, with the particle size vs. the photodiode output
voltage above the detection limit (updated from Lurton et al., 2014).
Mie theoretical calculations were conducted taking into account the LOAC
field of view (12–16∘). In fact, the LOAC detection
of particles smaller than 0.6 µm is conducted for output voltage levels
where the electronic noise might be not negligible; thus the Mie theoretical
calculations must be convoluted with the LOAC noise function to be compared
to real measurements.
Calibration curve of the output voltage recorded by the
12∘ channel photodiode as a function of particle diameter. Beads
were used in the 0.2–5.0 µm range; irregular grains selected by
sifters were used for the largest size. The Mie calculations were conducted
for the LOAC field of view, and were convoluted by the LOAC electronic noise
for particles smaller than 0.6 µm. The difference between the Mie
scattering calculations and LOAC measurements for diameters greater than 5 µm is due to the small aperture of the field of view coupled with the
roughness of the particle shapes; the measurement curve is fitted by a power
law.
The calibration with the latex beads captures well the large-amplitude Mie
oscillations up to 5 µm in diameter. In particular, the amplitude of
the oscillations at 1, 2 and 5 µm are well reproduced. For the larger
sizes, calibrated with irregular grains, the evolution of the scattered
intensity (or output voltage) with size is lower than the one expected from
the Mie calculation. Lurton et al. (2014), on a paper dedicated to the
light scattered at small angles below ∼ 20∘, have
shown that, for irregular grains and for a field of view of a few degrees,
the scattered intensity could derive almost only from diffraction. The
authors have introduced in the Mie calculation a roughness parameter ρ, calculated from the standard deviation of the particle shapes from a
perfect sphere; ρ is sensitive to the shape of the particles but also
to their surface roughness. When ρ is greater than 0.01, the light
scattered is dominated by diffraction. Microscopy images of real atmosphere
particles greater than a few µm has shown that ρ is always
greater than 0.01; as a comparison ρ∼ 0.005 for
spherical beads. A good illustration of the light scattering properties of
such irregular grains can be found in Weiss-Wrana (1983).
Monte-Carlo modelling for the response of the counting system for
particles larger than 1 µm. The response is almost linear up to
10 particles cm-3, and decreases for large concentrations.
In ambient air, the micronic and submicronic (sub-µm) solid particles
have also an irregular shape (e.g. Xiong and Friedlander, 2001; McDonald and
Biswas, 2004). The Mie oscillations that are present for perfect spherical
particles will disappear, being strongly smoothed. In the case of liquid
particles measurements, the droplets are sufficiently deformed by the
characteristics of the air flow passing through a small tube and relaxing
afterwards in the LOAC optical chamber. The droplets significantly depart
from the spherical shape, thus the Mie oscillations also disappear. As a
consequence, the scattered intensity will increase continuously with
increasing size.
The output voltage evolution for particles with diameters larger than a
few µm can be fitted using a power law. The best fit is obtained using
a power law where D is the particle diameter. This fit crosses also the
middle of the Mie oscillations for the sub-µm sizes, as shown on
Fig. 4. It seems reasonable to use this fit for all the particles in the
0.2–100 µm size range, to establish a one-to-one relation between
diameters and detector output voltages. Such fit prevents multiple solutions
in the diameter determination for a given output voltage. Thus, the
calibration for the size class threshold will be calculated for this
D1.0 fit. This calibration approach must be validated by comparison
with other instruments and techniques of measurements providing size
distribution, which is the purpose of Sect. 3 of the paper.
Based on this analysis, the LOAC detection size range is between 0.2 and
∼ 100 µm. LOAC, with its present calibration procedure,
is operated for the detection of irregular grains and droplets, but not for
perfect spherical solid grains, such as latex or metal beads for which
uncertainties arise from the smoothing of Mie oscillations by the
calibration curve (in this case, the total concentration is correct but the
size attribution can be erroneous).
Overall, a total of 19 size classes are defined for diameters between 0.2
and 100 µm (Table 1). The upper limit can be lower, however, depending
on the sampling collection cut-off of the inlet. The size classes are chosen
as a good compromise between the instrument sensitivity and the expected
size distribution of ambient air aerosols.
The 19 size classes of LOAC for concentration measurements.
Diameter range (µm)
0.2–0.3
0.3–0.4
0.4–0.5
0.5–0.6
0.6–0.7
0.7–0.9
0.9–1.1
1.1–3.0
3.0–5.0
5.0–7.5
7.5–10.0
10.0–12.5
12.5–15.0
15.0–17.5
17.5–20.0
20.0–22.0
22.0–30.0
30.0–40.0
40.0–100.0
Concentration measurements
Counting is conducted while the particles cross the laser beam one by one
and are classified in size classes corresponding to the scattered
intensities. The measurements integrated over a time span of 10 s are converted
to number densities or particles cm-3. The detectors of the two
channels (12 and 60∘) work asynchronously.
This discrete detection works well for large particles greater than 2 µm, with uncertainty in size attribution of 10 %. For smaller particles,
the size determination is within the calibration errors bars (±0.025 µm for particles smaller than 0.6 µm,
5 % in the 0.7–1 µm
range) if more than 400 particles are detected for each size classes.
The counting uncertainty could be derived from the Poisson counting
statistics. This uncertainty, defined as the relative standard deviation, is
60 % for aerosol concentrations of 10-2 cm-3, 20 % for
10-1 cm-3, and 6 % for concentrations higher than 1 cm-3.
Nevertheless, such calculation does not take into account the real
instrumental uncertainties dominated by the electronic noise and the inlet
sampling efficiency, as explained in Sect. 2.5. In addition, key aspects
concerning the counting of small particles and of large particles at high
concentration are discussed below.
The optical and electronic response of the system has been modelled by a
numerical Monte-Carlo method, taking into account the shape of the laser
beam, the speed of the particles inside the laser beam and the instrument
noise. To ensure a good statistical approach, 104 particles were
randomly injected for each size class. The ratio of the number of detected
particles over the number of injected particles provides the detection
efficiency for each size class. For the smaller particles, the photodiodes
cannot detect the whole transit of the particles inside the laser beam. Just
the brighter part of the pulse of the scattered intensity is observable and
the observed pulse duration in the laser beam is reduced (four of
such small pulses are present in Fig. 3). The signal of the output voltage
is close to or less than the noise and for this reason some particles cannot
be detected. As the diameter of the particles increases to yield greater
scattered intensity and longer pulse duration, the detection efficiency
increases and reaches 100 % for particles larger than 1 µm. The
concentrations of submicron size particles are then corrected by the on
board LOAC data-processing using these detection efficiency coefficients. As
the observed pulse duration of submicron particles in the laser beam is
short, the effective acquisition time can be reduced down to 35 µs
instead of around 700 µs for the largest particles. This enables a
greater number of small particles to be detected. Also taking into account the detection efficiency for the smaller particles, up to 3000 particles cm-3 can be (statistically) detected.
For particles larger than 1 µm, the observed pulse duration in the
laser beam is at its maximum (∼ 700 µs) and the counting
efficiency is 100 %; the expected maximum detectable concentration is about 15 particles cm-3 because of the pump flux, the width of the
laser beam and the observed scattering volume.
Nevertheless, higher concentrations of total particles above 1 µm
size could be estimated using a statistical approach. Another Monte-Carlo
numerical modelling was conducted to establish the relationship between the
number of particles > 1 µm detected and the number of
particles injected in the laser beam (Fig. 5). In the simulations,
particles were randomly injected in time, with concentrations increasing
from 0 to 500 particles cm-3 by step of 1 particle cm-3. The
higher the concentration, the lower the probability that the scattered
intensity peak decreases below the threshold to start a new count. The
response is almost linear up to 10 particles cm-3, reaching a kind of
saturation in counting values at around 15 particles cm-3. When the
mean time between the transit of two particles in the laser beam is smaller
than the transit time of one particle in the beam, the detected
concentrations became smaller than the real ones, and an inverse
proportionality between real and detected concentrations appears. It is
obvious that such a corrective procedure must be used only in dense aerosol
media (more than 10 particles cm-3 greater than 1 µm), such as fog
or clouds, i.e. in conditions which must be confirmed by independent
measurements. At present, this procedure is applied only when large droplets
are detected by LOAC using the typology procedure presented below. In this
case, up to 200 large particles cm-3 can be detected. This procedure
increases the concentration uncertainties by about 30 %.
Related to this, the LOAC measurements of submicronic particles could become
inaccurate in the case of concentration of particles > 3 µm
exceeding a few particles cm-3. The high probability of the presence of
large particles crossing the laser beam will mask the simultaneous presence
of the smaller particles; also the response time of the electronics can be
increased by a strong illumination of the detectors. These two phenomena
will lead to a significant underestimation of the concentrations of
particles < 1 µm. This effect is present in particular in
clouds and in fog measurements. For concentrations of particles > 30 µm exceeding 1 particle cm-3, as found in cirrus, LOAC
underestimates the concentrations of particles smaller than 5 µm. Thus,
concentration measurements of the smallest size classes in such fog/cloud
media must be used cautiously.
Principle of the determination of the “speciation index” D2/D1
(the example presented here uses real measurements).
Aerosol typology
The scattered light recorded at a scattering angle around 60∘ is
very sensitive to the refractive index of the particles and thus to their
nature (as said before this phenomenon appears at scattering angles greater
than ∼ 20∘). The more absorbing the particles, the
lower the light scattered. Thus we use the dependence in the refractive index
of the 60∘ channel response as a diagnostic for the nature of the
particles. This channel uses the same output voltage thresholds (in mV) as
the 12∘ channel, in order to perform a direct comparison of the
counting detected by two channels. For a given size class and for a given
particle concentration recorded in the 12∘ channel, the
concentration detected by the 60∘ channel decreases when the
imaginary part of the refractive index increases. This increase of the
imaginary part leads also to an underestimation of the real size of the
particles, and thus produces a diameter bias in the size distribution
(diameter vs. concentration) for the 60∘ channel with respect to
the 12∘ channel. An example of the procedure used to determine
this effect is presented in Fig. 6, where the size distributions of the
two channels are presented. For a given particle size of the 12∘ channel (noted D1), we consider the concentration value of the
60∘ channel. Then we search for the same concentration value on
the 12∘ channel (a linear interpolation is used if needed). The
corresponding diameter is then determined (D2). Finally, we define a
so-called “speciation index” as the ratio D2/D1. The more absorbing the
particles, the higher this ratio. This procedure is conducted for each size
class.
This procedure works well for irregular particles, but not for solid
symmetrical particles; in this latter case, the Mie oscillations produce
strong fluctuations in the evolution of the speciation index with size (we
have indeed observed this effect inside some cirrus clouds). Also, this
procedure must be used only for a large enough number of detected particles
per size class, because of the irregular shape of the particles. Laboratory
tests have shown that about 20 particles in a size class are sufficient to
be able to indicate the aerosol typology. In its nominal operating mode,
LOAC provides the speciation index every 1 min. For the analysis of
continuous ground-based measurements presented below, we have conducted the
typology detection with an integration time of 15 min (assuming that the
aerosols are stationary).
Different types of particles have been tested in the laboratory to assess
the amplitude of the speciation index throughout the measurement size range:
organic carbon, black carbon, desert dust or sand from different origins
(excluding black sand), volcanic ashes, plaster, salt (NaCl), water
droplets, droplets of mixture of water and sulphuric acid. They can be
classified in four families: carbonaceous particles, minerals, salts and liquid
droplets. Figure 7 presents the curves obtained in laboratory for the
various samples. Then, “speciation zones” charts (speciation index vs.
real diameter) are defined by the minimum and maximum speciation index
values reached by each family, taking into account the measurement
uncertainties. Among solid particles, carbonaceous particles produce the
higher speciation index and salt the lower, mineral particles being in
between. Detailed analysis has shown that most of the carbon particles are
in the lower part of the carbon speciation zone, while some strongly
absorbing particles, perhaps black carbon (with its fractal shape), are in the
middle and upper part of the carbon speciation zone. For all solid
particles, the global trend is a decrease of the speciation index with
increasing size. On the contrary, the liquid droplets speciation index
exhibits an increase with increasing diameter.
The case presented in Fig. 6 has D1 = 0.35 µm and D2 = 0.51 µm,
leading to a ratio of 1.46, which is in the carbon speciation zone.
The speciation indices obtained from LOAC observations in the atmosphere are
compared to these reference charts obtained in the laboratory. The position
of the data points in the various speciation zones provides the main
typology of the particles. In principle, this procedure can be conducted for
each size class. In fact, due to the statistical dispersion of the results,
it is better to consider several consecutive size classes to better conduct
the identification. This is in particular necessary for the identification
of droplets, whose speciation zone crosses all the speciation zones of the
solid particles.
It is obvious that the identification of the typology of the particles works
well only in the case of a homogenous medium, when the speciation indices are
not scattered through the various speciation zones.
At present, the speciation zones are established for particles expected to
be found in the troposphere and stratosphere, but the database is still
evolving. Additional laboratory measurements can be conducted to retrieve
the speciation zones for specific particles in the case of measurements in new
specific environments.
Uncertainties measurements and reproducibility
The instrument is industrially produced by Environnement-SA
(http://www.environnement-sa.com); more than 110 copies were produced by the
end of 2015. We must first evaluate the measurements uncertainty of one LOAC
copy, and then the reproducibility of measurements from different copies of
LOAC in the same ambient air.
Tests have been conducted for the different parts of the instrument: diode,
pump, photodiode and electronics, to assess the measurements uncertainty to
be added to the Poisson counting statistics. The stability of the pump flow
over 1 hour is about ±5 %, which induces a ±5 %
concentration uncertainty. The pump was tested at low temperature and low
pressure in balloon flights in the stratosphere and no obvious instability
nor loss of performance have been detected. As said before, the laser
stability is within ±5 %. Finally, optical tests have been
conducted to evaluate the variability of the response of the photodiodes at
given intensity levels. Overall, the detectors response provides an
uncertainty of less than ±5 %. Taking into
account all of these uncertainties, we can expect an uncertainty for total
concentration measurements better than ±20 % for one copy of LOAC.
It is necessary to evaluate the reproducibility of the measurements from
different copies of LOAC. In general, the variability of the pump flow was
less than ±0.2 L from one pump to another but, since the value of the
flow is an input parameter in the post-processing software, it is
recommended to monitor the flow rate by a flow metre before a balloon flight
or during ground based measurements. Tests have been conducted with eight
copies of LOAC in a “pollution test room” at LPC2E laboratory (Orléans, France).
Various types of solid particles have been injected in the chamber. For an
integration time of at least 10 min, a standard deviation of ±15 % (1σ) from the mean concentrations has been obtained between
the different instruments for particles smaller than 10 µm and for the
two channels. The standard deviation increases up to ±30 % for
particles larger than 10 µm, due to the low concentrations of such
particles.
Evolution of the speciation index with diameters for various
families of samples; measurements were conducted in laboratory with LOAC
using pure samples.
The total concentrations uncertainties evaluated for one copy of LOAC and
the standard deviation obtained for eight copies are similar. Thus, we can
evaluate that the uncertainty for total concentrations measurements is ±20 % when concentrations are higher than 1 cm-3 (for a 10 min integration time). For lower concentrations, the uncertainty is
dominated by the Poisson counting statistics, up to about ±60 % for
concentrations smaller than 10-2 cm-3. Also, the uncertainties in
size calibration are ±0.025 µm for particles smaller than 0.6 µm, 5 % for particles
in the 0.7–2 µm range, and 10 % for
particles greater than 2 µm.
Inlet sampling efficiency
LOAC will be used in different conditions, mainly on the ground and under
balloons. Depending on the chosen inlet and the relative speed between the
inlet and the wind, the isokinetic sampling is respected or not, and the
efficiency of collecting the largest particles can change.
On the ground, a total suspended particulate (TSP) inlet can be used,
ensuring an efficiency close to 100 % for collecting all the particles up
to a few tens of µm. For some specific studies where very large
particles dominate, as measurements inside fog or clouds, or because of
mechanical constraint if a TSP inlet cannot be mounted, the particles can be
collected by a tube having a bevelled metal inlet and oriented downwards. In
this case, the largest particles are generally under-sampled, and a
corrective coefficient must be applied, taking into account the direction
and the speed of the wind.
Laboratory tests have shown that the LOAC counting can be underestimated
when using a collecting pipe longer than about 50 cm, even if the pipe is
vertical. Due to the low air flow, some carbonaceous particles can stick to
the walls of the pipe (as shown by the analysis of typology measurements),
and for this reason it is then recommended to use a short collecting system.
For measurements under balloons floating at constant altitude, the relative
speed between ambient air and the inlet is close to zero. The sampling
efficiency assessed using the Agarwal and Liu (1980) criterion for an
upward-facing inlet shows that the sampling is unbiased for particles with a
diameter below 20 µm.
The sampling line used during the meteorological balloon flights is composed
of a thin wall metallic probe and antistatic tubing. The thin wall aerosol
probe has an inlet diameter equal to 5.4 mm and is connected to a tube of
about 20 cm length and 6.7 mm internal diameter. The sampling line is
connected vertically to the LOAC. Nevertheless, due to the tube stiffness,
the line can be inclined with a maximum sampling angle of 30∘ from
vertical. The sampling efficiency of the line was assessed using modelling
calculations in order to account for changes in atmospheric pressure,
temperature and possible changes of the probe orientation during these
flights. For that purpose, the values of pressure and temperature as a
function of altitude are taken from the international standard atmosphere
(ISO 2533–1975). Sampling efficiency calculations have been made by
considering a mean balloon ascending velocity of 5 m s-1, which is a
typical value for meteorological balloons, a LOAC sampling flow rate equal
to 1.7 L min-1 and two angles of the sampling line from the vertical
(0 and 30∘). According to these parameters, the inlet
aspiration velocity of the probe is equal to 1.24 m s-1
(sub-isokinetic) and the flow is laminar in the tubing for all altitudes.
Efficiency of the sampling line at different altitudes from the
surface up to 30 km; dashed lines: isoaxial conditions; full lines:
30∘ deviation from isoaxial conditions.
The mechanisms considered to calculate the sampling efficiency are the inlet
efficiency of the probe in isoaxial and isokinetic sampling conditions
(Belyaev and Levin, 1974; Hangal and Willeke, 1990) and particle losses in
the tubing due to gravitational settling when the line is not perfectly
vertical (Heyder and Gehbart, 1977). Calculations have been conducted for
particles with diameters ranging from 0 to 20 µm, and from the ground
to an altitude of 30 km. Figure 8 presents the sampling efficiency for a
0∘ deviation (isoaxial) and for a 30∘ deviation of the
sampling line with respect to the vertical. Data are plotted according to
the particle aerodynamic diameter which describes particle settling and
inertia phenomenon.
In isoaxial conditions for all altitudes, results show an increase of
sampling efficiency with the particle diameter, up to a factor of > 3 for the largest particles. In this case, there is no particle deposition
in the sampling line and the sampling is dominated by sub-isokinetic
conditions (apparent wind velocity higher than inlet probe velocity). A
sampling efficiency higher than unity is explained by the particle inertial
effect resulting from the divergence of the flow field at the inlet of the
probe. The increase in sampling efficiency with altitude is due to changes
in air viscosity and gas mean free path with temperature and pressure.
When the tube is inclined by 30∘ from the vertical, the sampling
efficiency is between 1 and 2. The sampling efficiency is lower than for the
0∘ isoaxial conditions. Firstly, the sub-isokinetic effect is
reduced by the orientation of the tube, and secondly, deposition can occur
in the tubing due to particle settling.
Since the tube has always a deviation of about 30∘ during the
balloon flights, we consider only the results at 30∘ from the
vertical. The over-sampling effect is negligible for particles smaller than
5 µm up to the lower stratosphere and for particles smaller than 2 µm in the middle stratosphere. Thus, this effect will just affect the
retrieved concentrations of the largest particles by about 50 % (which is
similar to the Poisson statistic uncertainty in case of low concentrations),
increasing their errors bar.
The results of these theoretical calculations are not yet fully validated by
an experimental approach with LOAC itself. Thus, all meteorological balloon
measurements are not corrected at present for this aerodynamic effect. This
effect should be taken into account in future work involving large
particles, for example when converting concentrations to extinction by
comparison with remote sensing instruments, or to estimate the real
concentration of the interplanetary dust in the middle atmosphere.
Conditions of measurements for evaluation exercises.
Campaign
Location
Date
Installation
Instruments for validation
ParisFog
SIRTA Observatory,
November 2012–April 2013
Continuous ground
– WELAS counter
Palaiseau (France)
September 2013–January 2014
measurements
– Fog monitor counter
– Scanning Mobility Particle Sizer (SMPS)
Cloud
Puy de Dôme
May 2013
Continuous ground
Well-known atmospheric conditions
measurements
(France)
measurements
for typology identification
ChArMEx
Minorca
17 June 2013
Tropospheric pressurized
Well-known atmospheric conditions
(Spain)
balloon flight
for the typology identification
ChArMEx
Ile du Levant
22 July 2013
Tropospheric pressurized
Well-known atmospheric conditions
(France)
balloon flight
for the typology identification
ChArMEx
Minorca
15 June 2013–
Continuous ground
HHPC-6 counter
(Spain)
2 July 2013
measurements
ChArMEx
Minorca
16 and 17
Meteorological sounding
Well-known atmospheric conditions
(Spain)
June 2013
balloon flights
for the typology identification
ChArMEx
Minorca
16 and 19
Meteorological and pressurized
WALI lidar
(Spain)
June 2013
tropospheric balloon flights
QAIDOMUS
Orléans
September–
Indoor air
TEOM microbalance
(France)
November 2013
VOLTAIRE-LOAC
Reykjavik
7 November 2013
Meteorological
Well-known atmospheric conditions
(Iceland)
balloon flight
for the typology identification
Observatoire
Paris
January–April 2014
Permanent measurements
– TEOM microbalances
Atmosphérique Generali
(France)
on tethered balloon flight
(Airparif air quality network)
(at ground and up to an
– Well-known atmospheric conditions
altitude of 270 m)
for the typology identification
SIRTA5
Gif-sur-Yvette
3–13 February 2014
Continuous ground
– Grimm counter
campaign
(France)
measurements at SIRTA
– HHPC-6 counter
– SMPS
Sea spray chamber tests
Stockholm
12–14 August 2015
Laboratory
– FIDAS counter
chamber tests
(Sweden)
measurements
– DMPS
Cross-comparison with other instruments
Various cross-comparisons have been conducted in the laboratory, in ambient
air at ground and during balloon flights for concentrations and typology
identification, to evaluate the real LOAC performances. For all the cases,
the inlet is vertical or close to vertical to ensure the best sampling. LOAC
concentrations have been compared to other commercial particle counter
instruments and photometer measurements. Nevertheless, none of them are an
absolute reference, since they use different technical approaches and
calibration procedures. The LOAC typologies are validated during
well-identified atmospheric events of liquid and solid particles. Finally,
the LOAC particle concentrations are converted to mass concentrations to be
compared to commercial microbalance mass instruments used as reference
instruments in air quality monitoring. Table 2 summarizes the conditions of
measurements.
The LOAC was used under different conditions. An autonomous version for
automatic ground-based applications uses an on-board computer to record the
data. When deployed underneath meteorological balloons, tropospheric
balloons, and transportable tethered balloons, the data are transmitted in
real time by telemetry. For deployments under large stratospheric balloons,
the data are stored on board using a specific module. For a tethered
touristic balloon, the data are sent to the ground using a Wi-Fi link and
are stored on a computer.
Laboratory concentrations and size distribution (sea spray
aerosols)
A laboratory cross-comparison of LOAC with the FIDAS 200 (Palas GmbH)
aerosol counter and a custom built DMPS (differential mobility particle
sizer; Salter et al., 2014) has been conducted using a
temperature-controlled sea spray chamber at Stockholm University, Sweden,
from 12 to 14 August 2015. All the three instruments were sampling in
parallel. The aerosol generation and the air flow were well controlled, thus
the instruments have sampled the same air masses.
The sea spray chamber is fabricated from stainless steel components and
incorporates temperature control so that the water temperature can be held
constant between -1 and 30 ∘C. Air is entrained using a plunging
jet that exits a stainless steel nozzle held in a vertical position above
the water surface. Water is circulated from the centre of the bottom of the
tank back through this nozzle using a peristaltic pump (more technical
details on the simulation chamber can be found in Salter et al., 2014). The
parameterization of the sea spray aerosol production as a function of water
temperature in the chamber can be found in Salter et al. (2015).
Comparison of LOAC measurements with DMPS and FIDAS measurements
performed at the sea spray aerosol simulation chamber at Stockholm
University. Top: concentration size distributions for sea spray aerosol
particles still containing water (droplets; upper left panel) and at dry or
crystalline state (salt; upper right panel). Bottom: integrated number
concentration for the 0.2 to 0.9 µm (lower left panel) and 0.3 to 0.9 µm (lower right panel) vs. time of the experiment while the water
temperature decreased; the transition from seawater droplets to crystalline
salt particles (at T= 23 ∘C) is indicated as well.
Dry zero-sweep air entered the tank at 8 L min-1 after passing through
an ultrafilter and an activated carbon filter. Aerosol particle-laden air
was sampled through a port in the lid of the sea spray chamber and
subsequently passed through a dilution chamber where the aerosols were dried
through the addition of dry particle-free air. Following this the aerosol
flow was split and transferred under laminar flow to all aerosol
instrumentation. To prevent contamination by room air, the sea spray
simulator was operated under slight positive pressure by maintaining the
sweep air flow several L min-1 greater than the sampling rate.
Particles produced by the sea spray generation chamber are mainly cubes with
rounded edges with dynamic shape factors below those expected for pure
cubes.
The measurements were conducted while the water temperature was decreasing.
The instruments determined liquid droplets for temperatures above
23 ∘C at the beginning of the measurement session, and then pure
salt crystals (dry state) for lower temperatures, as shown by the LOAC
typology measurements. This observation is in accordance with the hypothesis
of Salter et al. (2015) that the salt particles above 23 ∘C water
temperature (which leads to an increase of relative humidity in the
headspace of the simulation chamber) are not yet fully effloresced and thus
still contain water. Figure 9 presents two examples of the size distribution
for the three instruments in the case of liquid droplets and in the case of salts
(top), and the time evolution of the total particle number concentrations in
the 0.2–0.9 and 0.3–0.9 µm range (bottom). The lower limit of LOAC
begins at 0.2 and 0.9 µm represents the upper limit of the
DMPS. Taking into account the LOAC errors bars, the agreement with the DMPS
is very good for the number size distribution and the time evolution of the
total particle number concentration, although LOAC might slightly
overestimate the concentration in its first size class. The FIDAS seems to
slightly underestimate the concentrations of the sub-micronic particles
above 0.3 µm. The particle size distribution measured by the FIDAS
below 0.3 µm is strongly influenced by a decrease in the instrument's
sensitivity and thus should be generally disregarded. It should be noted
that LOAC has well captured the size distribution and total concentration of
droplets, which indicates that the assumption concerning the LOAC ability to
detect liquid particles is valid.
Ambient air concentration and size distribution
Continuous measurements have been conducted in ambient air at the SIRTA
observatory (Site Instrumental de Recherche par Télédétection
Atmosphérique, http://sirta.ipsl.fr/) at Palaiseau, south of Paris, France
(48.713∘ N, 2.208∘ E), during the ParisFog campaign,
http://parisfog.sirta.fr/), from November 2012 to April 2013. During this period, the total
concentrations of aerosols have been monitored by a WELAS aerosols counter
and a fog monitor (counter for large droplets).
Cross-comparison of LOAC with two other instruments (WELAS and fog monitor) for the total concentrations of aerosols in the size range domain
in common, during the ParisFog campaign. The LOAC uncertainties are ±20 %. The peaks of high concentrations correspond to fog events.
Strong fog events were observed in November 2012. Total particle concentrations measured by LOAC, WELAS and the fog monitor are in very good
agreement during these events (Fig. 10). This result validates the
correction procedure applied to the LOAC measurements in the case of dense
medium of liquid particles. Figure 11 presents the size distribution at the
beginning of a fog event, with the typical enhancement around a diameter of
10 µm (e. g. Singh et al., 2011), and at the end of the event. Both LOAC
and WELAS found a bimodal size distribution but disagree for the size and
the position of the second mode. Conversely, LOAC and the fog monitor were in
good agreement for the position of the second mode, although the population
of the first size class of the fog monitor was obviously underestimated.
Finally, for the largest sizes, LOAC concentrations are between those of
the WELAS and the fog monitor.
The shape of the size distribution of the WELAS instrument is unusual, as
for the FIDAS measurements presented in Sect. 3.1, with a decrease of the
sub-µm aerosol concentrations with decreasing size (the opposite
trend is expected for background aerosol conditions). The LOAC size
distributions are often below those of the WELAS, which could be due to a
calibration problem of the WELAS as proposed by Heim et al. (2008)
and Rosati et al. (2015).
Between the fog events, LOAC and WELAS were most of the time in
disagreement, which was due to the difference in the concentration values
obtained by the two instruments for the particles smaller than
∼ 0.5 µm, partly attributed to the WELAS undercounting.
Cross-comparison of the three instruments at the beginning of the fog
event (top) and at the end (bottom), during the ParisFog campaign on 20
November 2012 during a fog event. The LOAC uncertainties are ±20 % for the higher concentrations and ±40 % for the lower
concentrations.
A ground-based measurement session was conducted from Minorca (Spain) during
the ChArMEx campaign (Chemistry Aerosol Mediterranean Experiment,
https://charmex.lsce.ipsl.fr/) in parallel with measurements of an HHPC-6 aerosol counter in the period
12 June–2 July 2013. The orders of magnitude for the different size
classes were in good agreement. In particular, both instruments captured an
aerosol enhancement of large solid particles between 18 and 21 June 2013, as
shown in Fig. 12 for the size distribution.
Example of size distribution for LOAC and HHPC-6 during an event
of solid particles during the ChArMEx campaign at Minorca on 20 June 2013.
The LOAC uncertainties are ±20 % for the higher concentrations
and ±60 % for the lower concentrations.
The last cross-comparison exercise was conducted during an ambient air
campaign at SIRTA observatory, site #5 near Gif-sur-Yvette, south of
Paris, France (48.709∘ N, 2.149∘ E), in the beginning of
2014. LOAC performed measurements from 3 to 13 February 2014 in parallel
with a SMPS, a Grimm aerosol counter and a HHPC-6 aerosol counter. Due to
the sampling conditions that vary from one instrument to another (direct
sampling, TSP inlet, dryer, direct or curved tubes), the analysis is limited
to the smallest particles (diameter < 1 µm) which are expected
to be not too sensitive to the sampling techniques. Nevertheless, LOAC used a
2 m longer pipe to carry the particles inside the optical chamber, with a
risk of losing some (small) carbonaceous particles, as said in Sect. 2.6. Figure 13 presents
the temporal cross-comparison for four size-classes: 0.2–0.3, 0.3–0.5, 0.5–0.7
and 0.7–1. µm. In fact, the size classes of
the four instruments are not always the same, thus the closest ones have been
considered for the comparison.
Globally, all the instruments give similar concentrations for all size
classes, the better agreement being for the 0.5–0.7 µm diameter range.
Some discrepancies appear for some time periods between the various
instruments. For particles greater than 0.3 µm, LOAC has missed just one concentration peak detected both by the SMPS and the Grimm, at the end of February 3.
The peak detected on 10 February by the SMPS was not really detected both by
the Grimm and the LOAC instruments (these two instruments are in good
agreement here). On the opposite, LOAC and Grimm have detected a peak on
February 13 for particles greater than 0.5 µm, which was not observed
by the SMPS. Several reasons can explain these discrepancies. First, the
SMPS instrument determines the electric mobility diameter that can depend on
the nature of the aerosols, whereas the other instruments determine optical
diameters. SMPS measurements could lead to some uncertainties in size
determination, and thus in concentrations, when compared to other kinds of
instruments for irregular particles (e. g. Gulijk et al., 2003). This could
explain why LOAC has missed some concentration peaks detected by SMPS.
Secondly, the particles size distribution of sub-µm particles strongly decreases while the diameter increases. Thus the
uncertainty in the size calibration of a few hundredths of µm could
induce concentration differences of at least a factor of 2. This is presented
in Fig. 13 for the 0.7–1 µm comparison with the Grimm instrument for
which both 0.65–1 and 0.8–1 µm concentrations are plotted.
Finally, the Grimm and HHPC-6 instruments are sensitive to the nature of the
particles, and changes in the type of aerosol (for example mineral or carbon
particles) could partially affect their size determination.
Comparison (in linear scale) between the ambient air measurements
obtained during the campaign at the SIRTA-5 station south of Paris.
Nevertheless, it appears that the agreements are not as strong during ambient
air measurements compared to those during the sea spray laboratory measurements, where the
inlets were the same for all the instruments. This is the limit of such
cross-comparison in ambient air where the instruments are sensitive to their
sampling efficiency and to the complexity of the environment.
An indirect evaluation of the LOAC size calibration has been conducted
during the ChArMEx campaign on the Balearic island of Minorca, Spain. A
total of nine flights of LOAC have been performed under a meteorological
sounding balloon launched from Sant Lluís airfield (39.865∘ N, 4.254∘ E) in the 15–19 June 2013 period during a desert dust
transport event. The aerosol concentration has been integrated for all size
classes from the ground to the highest altitude reached by the balloon, i.e.
an altitude of about 30 km, to be compared to ground-based remote sensing
measurements provided by the AERONET photometer network
(http://aeronet.gsfc.nasa.gov/) station of Cap d'En Font (39.826∘ N, 4.208∘ E), which
performed measurements close the trajectory of the LOAC balloon
measurements. AERONET provides the vertically integrated volume
concentration of aerosols (in dV/dln(r), where r is the radius of the
particles) in the 0.13–30 µm radius range (Nakajima et al., 1983;
Dubovik and King, 2000; Dubovik et al., 2000).
The LOAC-integrated concentrations are converted to volume concentrations by
using the mean volumetric diameter Dv calculated for each size class by
the formula:
Dv=0.5×[(Dmin3+Dmax3)](1/3),
where Dmin and Dmax are the lower and upper diameter of a given
size class, respectively. With such a formula, the mean volumetric diameter
is at about 60 % of the size class width instead of 50 % for the mean
geometric diameter. For each size class, the volume of the particles is
calculated assuming sphericity. To be consistent with the AERONET data, the
LOAC results are presented in radius instead of diameter.
Comparison between integrated LOAC volume size distribution from
vertical profiles obtained under meteorological balloons and AERONET
measurements during an African dust transport event during the ChArMEx 2013
campaign (note that the LOAC data are given in radius to match the AERONET
format).
Figure 14 presents two examples of comparison between LOAC and AERONET
volume size distributions for two different amounts of sand particles in the
troposphere (the contribution of the stratospheric particles is negligible).
The bi-modal distribution is typical for a desert dust or sand plume event.
The two instruments are in good agreement, both in size distribution and
volume concentration. This comparison is just to evaluate the LOAC
calibration. Since the volume concentrations are proportional to the cube of
the size of the particles, an error in the LOAC calibration would lead to
strong discrepancies both in size distribution and volume concentrations,
which is not the case.
The cross-comparison measurements presented above have been conducted for
different air temperature, including day–night cycles and seasonal
temperature variations. No effect of the temperature on the accuracy of the
retrieved concentrations has been pointed out. These results confirm that
the LOAC real-time noise-checking process works well.
All of these cross-comparison exercises have shown that the LOAC measurements
are consistent with those of the other instruments considered here,
accounting for the errors and the limitation of the various techniques. This
confirms the LOAC calibration and the concentration retrievals are
acceptable. Nevertheless, the concentrations could be sometimes
underestimated when the length of inlet pipe is longer than a few tens of cm
or when the high concentration of large particles affects the detection of
the smallest particles.
Extinction profiles of the WALI lidar and extinction profiles
calculated from LOAC measurements under meteorological and pressurized
tropospheric balloons, from Minorca Island during the ChArMEx campaign.
Tropospheric vertical distribution
Cross-comparison exercises have been also conducted for balloon-borne LOAC
measurements.
LOAC has performed tropospheric flights during the ChArMEx campaign from
Minorca Island in time coincidence with the WALI aerosols lidar measurements
(Chazette et al., 2014) at a few tens of km apart. One LOAC flight was
conducted under a meteorological balloon on 16 June 2014; two LOAC flights
were conducted on 19 June 2013 at the same time, the first one being under a
meteorological balloon and the second being under a drifting pressurized tropospheric
balloon (see the companion paper for more information of the balloons and
the gondolas). The LOAC data were converted to extinction using Mie
scattering theory, assuming spherical sand particles with a refractive
index of n=1.53+0.02i (e.g. Wagner et al., 2012), to be compared to
lidar extinction data at 350 nm. Uncertainties of the refractive index
values are included in the errors bars calculations of the retrieved LOAC
extinctions. Figure 15 presents the tropospheric vertical profile of LOAC
and WALI lidar extinctions. Taking into account the instrumental errors
bars, LOAC and WALI have captured the same main vertical structures and the
extinction values are, on average, in good agreement in the lower troposphere.
Outside the plume, the LOAC extinctions are smaller than the WALI ones,
because the LOAC extinctions are calculated from 0.2 µm, thus missing
the contribution of the smallest particles. The extinction presented here
must be considered as lower limits. Also, the location of the balloon
measurements move away from the lidar location as the altitude increases,
due to the balloon motion and the wind direction. Thus the discrepancy
between the measurements can increase with altitude.
Example of the detection of carbon particles in urban air, in
the south-west of Paris on 29 December 2013 around 07:30 UT, at the Observatoire
Atmosphérique Generali (OAG); upper panel: size distribution; lower panel:
typology, the LOAC data are in black dots. The LOAC uncertainties are ±20 % for the higher concentrations and ±60 % for the
lower concentrations.
Typology of the particles
The speciation zones, obtained from laboratory measurements, must be
validated in real atmospheric conditions.
Urban ambient air measurements are proper for the detection of carbon
particles (black and organic carbon), especially during well-identified
pollution events. Permanent LOAC measurements have been conducted at
“Observatoire Atmosphérique Generali” (OAG) in the south-west of Paris
since May 2013 (48.841∘ N, 2.274∘ E). This observatory
is a recreational tethered balloon operated in a public park; the LOAC
measurements nominal maximum altitude is 120 m but some flights could be
conducted up to an altitude of 270 m. The measurements can be sorted out
between measurements with the balloon at ground level and measurements
during flight. Figure 16 presents an example of light-absorbing particles
(probably carbonaceous ones) detected at the OAG on 29 December 2013 around
07:30 UT. In this example, the speciation index curve is well inside the
carbon speciation zone in the whole size range up to ∼ 10 µm.
Balloon flights from Minorca Island were also
conducted during several well-identified desert dust events above the
Mediterranean sea during the summer ChArMEx campaign. Figure 17 presents an
example on 17 June 2013, around 14:30 UT (approximate balloon position:
41.9∘ N, 4.1∘ E) at an altitude of 2050 m under a low
altitude pressurized drifting balloon. The speciation curve is well inside
the mineral dust zone, showing that LOAC has indeed detected the desert dust
particles.
Example of the detection of sand particles above Mediterranean
Sea (longitude of 39∘55′, latitude of 4∘14′, close to
Minorca) from a drifting pressurized tropospheric balloon on 17 June 2013
around 14:30 UT at an altitude of 2050 m, during the ChArMEx campaign; upper
panel: size distribution; lower panel: typology, the LOAC data are in black
dots. The LOAC uncertainties are ±20 % for the higher
concentrations and ±60 % for the lower concentrations.
Measurements in the marine atmospheric boundary layer were also conducted
with a low altitude balloon on 22 July 2013 drifting in an altitude range of
250–400 m, launched from the French Levant Island on the
Mediterranean French coast (43.021∘ N, 6.461∘ E). Figure 18 presents the measurements at 21:25 UT (approx. balloon position:
43.0∘ N, 6.55∘ E, alt. ∼ 275 m), and the
typology is mainly in the “salt zone”, as expected for a measurement close
to the sea surface.
Droplet typology was validated in fog events during the ParisFog campaign,
but also during cloud measurements conducted in May 2013 at the Puy de
Dôme observatory (45.772∘ N, 2.964∘ E, alt. 1465 m).
Figure 19 presents an example of measurements inside a cloud on 15 May 2013
at 10:30 UT. Globally, the typology identification is inside the droplets
zone, which indicates that all of the particles were indeed liquid. In
addition, measurements were conducted inside haze or thin cloud at an
altitude of 1.2 km during a flight under a meteorological balloon launched
from Reykjavik, Iceland (64.127∘ N, 21.904∘ W), on 7
November 2013 at 12:30 UT in the frame of the VOLTAIRE-LOAC campaign for the
study of the stratospheric aerosol trend. The presence of the droplets was
confirmed by the on-board humidity sensor, with a hygrometry of 90 %. The
typology in Fig. 20 is well inside the droplets zone.
Example of the detection of salt particles above Mediterranean
Sea (longitude of 40∘00′, latitude of 6∘40′, close to
Minorca, Spain) from balloon on 22 July 2013 at 21:25 UT at an altitude of
275 m during the ChArMEx campaign; upper panel: size distribution; lower
panel: typology, the LOAC data are in black dots. The LOAC uncertainties are ±20 % for the higher concentrations and ±60 % for
the lower concentrations.
Finally, most of the measurements under meteorological balloons in the
middle atmosphere show that (pure) liquid water and sulphuric acid droplets
largely present in the stratosphere are close to the lower part of the
droplets zone, and sometimes slightly below. Vertical profiles of LOAC
concentration and typology measurements are presented in the companion
paper.
These examples show that the typology determination works well in the case of
homogeneous aerosol media. Nevertheless, there are two limitations of this
process. First, the analysis of measurements conducted in heterogeneous
media could be difficult or even inaccurate, in particular when different
size modes are present. In this case, the speciation curve exhibits unusual
oscillations that match none of the speciation zones. Secondly, some high
porosity aerosols can exhibit high values for the speciation index, even if
they are not black (as fluffy silica). Thus, the typology determination usually provides an estimate of the nature of the particles, but we must be
cautious in the analysis when the speciation curves are non-typical.
Mass concentrations
Our final test to evaluate both the calibration of LOAC and the retrieval of
concentrations in all size classes (but especially large particles) is to
convert the number size distribution measurements to mass concentrations and
to compare the results to reference mass measurements. This is the most
complete test to evaluate LOAC because it combines the use of a parameter
proportional to the cubic diameter of the particles, and the use of the
particle typology determination, so that simultaneous measurements by both
channels have to intervene. The typology helps to determine the type of
aerosols from which a density can be deduced. The density determination is
necessary for the conversion of number concentrations (in cm-3) to
mass concentrations (in µg m-3).
Example of measurements inside a cloud at Puy de Dôme
observatory (France) on 15 May 2013 at 10:30 UT; upper panel: size
distribution; lower panel: typology, the LOAC data are in black dots. The
LOAC uncertainties are ±20 % for the higher concentrations and ±60 % for the lower concentrations.
Measurements were conducted first in indoor air (in the “pollution room”
at the LPC2E laboratory) in autumn 2013, by injecting in the air of the room
different kinds of carbonaceous and mineral particles (smaller than 20 µm) in various concentrations to produce a large range of mass
concentrations. The reference mass measurements were achieved with a
calibrated TEOM microbalance. An air flow system was used (when needed) to
prevent sedimentation of the particles in the room. Also, some measurements
have been conducted without injecting particles, to detect only the smallest
particles present in the ambient air, in particular during the night without
convection in the room.
The volume concentration is calculated for each size class, using the mean
volumetric diameter, assuming spherical particles, and is multiplied by the
corresponding concentrations. The mass concentration is obtained by
multiplying these results by the particle density. The mass densities were
determined for each size class by identifying the typology of the particles
though their speciation index. The mass densities chosen here are as follows.
2.2 g cm-3 for salt – This value corresponds to NaCl particles.
2.2 g cm-3 for mineral particles – This value is a compromise for
common mineral particles present in ambient air: compact sand (2.1 g cm-3), quartz (2.7 g cm-3), limestone (2.5 g cm-3) and
silicon (2.3 g cm-3).
1.4 g cm-3 for carbonaceous particles – This value was derived after
detailed tests during the comparison between LOAC and microbalance
measurements in the laboratory. It lies well within the range of values
proposed in the literature for such particles (e.g. Chen et al., 2010;
Virtanen et al., 2006; Spencer et al., 2007). Sensitivity tests have shown
that a 10 % variation of this value will not induce strong changes in the
results presented below.
A value of 0.0 g cm-3 was used for water droplets, to compare the LOAC
measurements to those of the TEOM instrument, which evaporates condensed
water and thus cannot provide mass for water droplets.
Example of measurements inside a haze or thin cloud at an
altitude of 1.2 km during a flight under meteorological balloon from
Reykjavik (Iceland) on 7 November 2013 at 12:30 UT; upper panel: size
distribution; lower panel: typology, the LOAC data are in black dots. The
LOAC uncertainties are ±20 % for the higher concentration and ±40 % for the lower concentrations.
Correlation between LOAC and TEOM microbalance mass
concentrations in indoor air (averaged over 24 h); particles have been
injected with various concentrations to document a large range of mass
concentration.
The duration of the sessions ranged from several hours to several days. Figure 21 presents the mass measurements for particles smaller than 20 µm,
averaged on 24 h for the two instruments. The variability of the
concentrations is related to the amount of particles injected into the room.
The lowest values correspond to measurements without injection. In this
case, LOAC indicates that only particles smaller than 2 µm were present
in the air. The LOAC and TEOM measurements are in very good agreement, with
a correlation of 0.97. The correlation curve has the slope of 0.98, with an
offset at the origin of 2.2 µg.m-3, and a mean error of 4.8 µg.m-3.
PM2.5 (upper panel) and PM10 (lower panel) LOAC mass
concentrations measurements in 2013 during the ParisFog campaign at SIRTA
Observatory in Palaiseau, south of Paris, and at the OAG in the south-west of Paris, and comparison
with reference TEOM data from the Airparif air quality monitoring network.
Sessions of ambient air measurements were conducted in Paris and in its
suburbs, to test the retrieval of PM2.5 and PM10 mass concentrations, with
pumps working at 2.7 L min-1. The first location of measurements is at the OAG in Paris (latitude
48.8417∘ N, longitude 2.2736∘ E). The LOAC measurements
were conducted using a vertical TSP inlet. The second location is at SIRTA
observatory at Palaiseau (48.7180∘ N, 2.2075∘ E) during
the ParisFog campaign. The LOAC measurements were conducted with the
vertical inlet directed towards the ground. The OAG and SIRTA measurements
considered here were conducted in the periods September 2013–April 2014
and September 2013–December 2013, respectively. The PM2.5 and PM10 LOAC
mass concentrations were retrieved by combining the results for particles
smaller than 3 µm and smaller than 10 µm, respectively, taking
into account the sampling efficiency of the PM2.5 and PM10 inlets
currently used by the air quality networks (cut-off at 2.5 µm for
PM2.5 inlet and cut-off at 10 µm for PM10 inlet).
PM2.5 (upper panel) and PM10 (lower panel) LOAC mass
concentrations measurements in 2014 at the OAG (south-west of Paris) and comparison with reference TEOM data
from the Airparif air quality monitoring network.
Reference mass concentrations data of urban ambient air in the Paris region
are provided by the Airparif network (http://www.airparif.asso.fr/)
operating TEOM microbalance instruments. Unfortunately, there is no Airparif
station very close to the OAG site nor to the SIRTA site at the time of the
measurements. Therefore, we decided to use data recorded at three stations that
have environmental conditions close to those at OAG and SIRTA: the “Paris
Centre” station (latitude 48.8528∘ N, longitude 2.3600∘ E),
Vitry-sur-Seine (latitude 48.7820∘ N, longitude 2.3992∘ E) in the south-eastern suburb area of Paris, and the “Rural South” station at
Bois-Herpin (latitude 48.3725∘ N, longitude 2.2258∘ E)
in the south of Paris region; the last station provides background
conditions measurements.
Figures 22 and 23 present the comparison of PM2.5 and PM10
concentrations, for the 2013 and 2014 period, respectively. The LOAC
measurements being most of the time between the background and the urban
conditions, the small discrepancies with the reference mass concentrations
are probably due to a difference in the wind direction and to the
regional-scale transport of the particles. It is worth noting that LOAC did
capture well the 10–15 December 2013 and the 11–14 March 2014 pollution
peaks.
These measurement sessions have been conducted with different kinds of pumps
and vertical inlet systems. The agreement with reference mass
concentration measurements is very good. This confirms that no obvious bias
is present in LOAC observations for the sizes of particles considered here
(∼ 0.2–20 µm), and that the typology procedure is
providing useful information to convert the LOAC concentrations for the 19
size classes to mass concentrations.