The study of aerosols in the troposphere and in the stratosphere is
of major importance both for climate and air quality studies. Among
the numerous instruments available, aerosol particles counters
provide the size distribution in diameter range from few hundreds of
nm to few tens of
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 affectingregional and global climate (Ramanathan et al., 2001; Diner et al., 2004; Kanakidou et al., 2005; Quaas et al., 2008). When very high concentrations of ashes after volcanic eruptions are present at cruise altitude, they can affect air traffic (e.g. Chazette et al., 2012). In the middle atmosphere, aerosols play a significant role in stratospheric chemistry through heterogeneous reactions with nitrogen and halogen species (e.g. Hanson et al., 1994, 1996), and they can affect climate through their role in the global radiative balance of the Earth (e.g. Hansen et al., 1992; Ammann et al., 2003). 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 relatively short residence time in the atmosphere. 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 (UAV), 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 (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, 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 size distributions of the particles.
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
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
Another approach was proposed by Renard et al. (2010a); in this case,
measurements are conducted at small scattering angles, below
20
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, 2010b). We propose here a new optical particle counter concept,
called LOAC (Light Optical Aerosols 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
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 a companion paper, we illustrate first scientific results from airborne observations on-board balloons and unmanned aircraft.
LOAC is a modular instrument, for which some parts can be changed
depending on the measurements conditions. For measurements under
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 by a small
pump (having a life-time of 3 weeks in continuous operation) working
at
The sampled air crosses a laser beam of 25
The electronic sampling is at 40
The maximum of the intensity pulse is obtained after subtracting the
stray-light contamination. Figure 2 presents an example of real
measurements of the time evolution of the flux scattered by
a 5
To minimize its weight, the optical chamber is in plastic
Delrin
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).
Latex beads, which are perfect transparent spheres, have been used for
diameter calibration below 2
For the calibration in the 5–45
Figure 3 presents the calibration curve; where the scattered flux is
given in mV, which corresponds to the photodiode output voltage
(updated from Lurton et al., 2014). 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. The electronic noise is taken into account, and acts as
an offset in the output voltage. The calibration captures well the
large-amplitude Mie oscillations calculated by integrating the
scattered fluxes over the whole LOAC field of view. In particular, the
amplitude of the
Particles found in ambient air are not perfectly spherical and have
some irregularity on their surface, even for the sub-micrometre
(sub-
LOAC, with its present calibration procedure, is operated to the detection of irregular grains and droplets, but not to perfect spherical solid grains, such as latex or metal beads for which uncertainties arise from the smoothing of Mie oscillations by the calibration curve.
Overall, a total of 19 size classes are defined for diameters between
0.2 and 100
Counting is conducted while the particles cross the laser beam one by
one, and are classified in size classes corresponding to the scattered
flux. The measurements are integrated during 10
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, 10
LOAC can count up to
For the LOAC integration time of 10
LOAC is designed to be used in various atmospheric conditions. The
temperature can dramatically change, in particular during balloon
flights up to the middle stratosphere. The electronic offset 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 10
The scattered flux recorded at 60
This procedure works for a large enough number of detected particles
per size class, because of the irregular shape of the particles. 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 speciation with an integration
time of 15
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 4 families: carbonaceous particles, minerals, salts and liquid droplets. 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, carbononaceous 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 having 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
The speciation indices obtained from LOAC observations in the atmosphere are compared to the reference charts obtained in the laboratory. The position of the data points in the various speciation zones provides the main nature 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 suze 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 nature of the particles works well in case of an 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 it is an evolving data base. Additional laboratory measurements can be conducted to retrieve the speciation zone for specific particles in case of measurements in new specific environments.
The instrument is industrially produced by Environnement-SA
(Environnement-SA,
The variation of the laser flux from one copy to another is less than
5 %, which has no significant effect on the flux scattered by the
particles. The variability of the pump flow was less than
Taking into account all these uncertainties, we can expect a total
uncertainty of about
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 efficiency close to 100 % for collecting all the
particles up to a few tens of
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 particle with diameter below 20
The sampling line used during the 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
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 diameter ranging
from 0 to 20
In isoaxial conditions, results show for all altitudes an increase of
sampling efficiency with the particle diameter, up to a factor
When the tube is inclined by 30
Since the tube always has a deviation of about 30
The results of these theoretical calculations are not yet fully validated by an experimental approach with LOAC itself. Thus, all balloon measurements in the stratosphere will not be corrected from this aerodynamic effect. It could 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.
Various cross-comparisons have been conducted in ambient air at ground and during balloon flights for concentrations and speciation. LOAC concentrations are compared to other commercial particle counter instruments and photometer measurements, although there is no absolute reference, many of them are using different technical approaches and calibration procedures. The LOAC speciations 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.
Continuous measurements have been conducted in ambient air at the
SIRTA observatory (Site Instrumental de Recherche par
Télédétection Atmosphérique,
Figure 8 presents the cross-comparison of the instruments in
January 2013. Most of the measurements were conducted in background
aerosol conditions, although some small fog events were detected and
can be identified by concentration peaks in the Fog Monitor
measurements. Roughly speaking, the order of magnitude of the
concentrations is similar, although some significant discrepancies are
present. To investigate their possible origin, the size distribution
obtained in different conditions of aerosol content can be
compared. Figure 9 (upper panel) presents an example where the
agreement in total concentration during background aerosol conditions
is very good between LOAC and SMPS. On the other hand, the shape of
the size distribution of the WELAS instrument is unusual with
a decrease of the sub-
Strong fog events were observed in November 2012. LOAC, WELAS and Fog
Monitor are in very good agreement during these events (Fig. 10). This
result is validating the correction procedure applied to the LOAC
measurements in case of dense medium of liquid particles. Between fog
events, LOAC and WELASwere sometimes in disagreement. This was due to
the difference in the concentration values obtained by the two
instruments for the particles smaller than
A ground-based measurement session was conducted from Minorca (Spain)
during the ChArMEx campaign (Chemistry Aerosol Mediterranean
Experiment,
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
Globally, all the instruments give similar concentrations for all size
classes, the better agreement being for the 0.5–0.7
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 9 flights of LOAC have been performed under
a meteorological sounding balloon launched from Sant Lluís
airfield (39.865
The LOAC integrated concentrations are converted to volume
concentrations by using the mean volumetric diameter
Figure 14 presents two examples of comparison between LOAC and AERONET 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 excellent agreement, both in size distribution and volume concentrations It is worth noting that 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, which is not the case.
All 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 that no systematic bias are present in the LOAC calibration and in the data analysis.
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
In addition to sounding balloons mentioned above, measurements under
drifting balloons launched from Sant Lluís on Minorca Island were
also conducted during several well-identified desert dust events above
the Mediterranean Sea during the summer ChArMEx campaign. Figure 16
presents an example on 17 June 2013, around 14:30 UT (approximative
balloon position: 41.9
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
Droplet speciation 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
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 speciation measurements are presented in paper 2.
These examples show that the speciation determination works well in 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 exhibits high values for the “speciation index”, even if they are not black (as fluffy silica). Then, the speciation determination is providing most of the time the main nature of the particles, but one has to be cautious in the analysis when the speciation curves are non-typical.
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 sensitive test to evaluate LOAC, since
mass concentrations are proportional to the cubic diameter of the
particles (and to their density). The speciation 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
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
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
nature of the particles though their speciation index. The mass
densities chosen here are:
2.2 2.2 1.4 A value of 0.0
The duration of the sessions was from several hours to several
days. Figure 20 presents the mass measurements for particles smaller
than 20
Sessions of ambient air measurements were conducted in Paris and in
its suburbs, to test the retrieval of
Reference mass concentrations data of urban ambient air in the Paris
region are provided by the AirParif network
(
Figures 21 and 22 present the comparison of
These measurement sessions have been conducted with different kinds of
pumps and of 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 (
LOAC is a modular optical particle counter/sizer, of which the pump
and the air inlet can be changed, depending on the conditions of
measurements. Extensive tests performed in different atmospheric
conditions have shown that LOAC can be used to retrieve the size
distribution of irregular-shaped or liquid aerosols with
a satisfactory accuracy at ground level and from all kinds of
balloons. The uncertainty (at 1
The LOAC project was funded by the French National Research Agency's ANR ECOTECH. The instrument and the gondola are built by Environnement-SA and MeteoModem companies. The balloons flights of the ChArMEx campaigns were funded and performed by the French Space Agency CNES. The Icelandic flights were conducted by the Iceland Meteorological Office. The various copies of LOAC used in the campaigns were funded with the support of CNES, ADEME, and INSU-CNRS in the framework of the MISTRALS Programme, and of the French VOLTAIRE Labex (Laboratoire d'Excellence ANR-10-LABX-100-01). The QAIDOMUS laboratory tests were funded by the French Ministry of Industry. Some calibration tests were conducted at the Aerolab Company.
This work is in memory of Jean-Luc Mineau.
The 19 size classes of LOAC for concentration measurements.
Conditions of measurements for evaluation exercises.
The LOAC instrument; upper panel: principle of measurement; lower panel: picture of the instrument (the inlet tube is not presented here).
Example of the flux scattered by particles while 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. In this
example, the small particle causing the small secondary peak at
2.1
Calibration curve of the scattered flux at 12
Typical size distribution in a suburban ambient air with carbon particles (Palaiseau, South of Paris) on 14 October 2013 during ParisFog campaign; the data are integrated during 15 min; the last points are not related because of zero concentration measured between them.
Monte-Carlo modelling for the response of the counting system
for particles larger than 1
Principle of the determination of the “speciation index” D2/D1 (the example presented here uses real measurements).
Efficiency of the sampling line at different altitudes from
the surface up to 30
Cross-comparison of LOAC with 3 other instruments (WELAS, Fog
Monitor and SMPS) for the total concentrations of aerosols in the
size range domain in common, during the ParisFog campaign south of
Paris. The LOAC uncertainties are of
Cross-comparison of the 4 instruments during background
conditions, in case of good agreement for the total concentrations
measurements, during the ParisFog campaign. Upper panel: 10
January 2013, good agreement between the instruments; lower panel:
12 January 2013, poor agreement. The LOAC uncertainties are of
Cross-comparison of LOAC with 2 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 of
Cross-comparison of the 3 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 of
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.
Comparison (in linear scale) between the ambient air measurements obtained during the campaign at the SIRTA-5 station South of Paris.
Comparison between integrated LOAC measurements 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).
Example of the detection of carbon particles in urban air, in South-West of Paris on 29 December 2013 around 07:30 UT, at the “Observatoire Atmosphérique Generali”; upper panel: size distribution; lower panel: speciation.
Example of the detection of sand particles above
Mediterranean Sea (close to Minorca) from a drifting pressurized
tropospheric balloon on 17 June 2013 around 14:30 UT at an altitude
of 2050
Example of the detection of salt particles above
Mediterranean Sea (close to Minorca, Spain) from balloon on 22
July 2013 at 21:25 UT at an altitude of 275
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: speciation.
Example of measurements inside a haze or thin cloud at an
altitude of 1.2
Comparison of coincident LOAC and TEOM microbalance
measurement in indoor air (averaged over 24