This paper describes the aerosol measurement setup and
results obtained during the BEXUS18 (Balloon-borne Experiments for University Students) stratospheric balloon within the
A5-Unibo (Advanced Atmospheric Aerosol Acquisition and Analysis)
experiment performed on 10 October 2014 in northern Sweden (Kiruna).
The experimental setup was designed and developed by the University of
Bologna with the aim of collecting and analyzing vertical profiles of
atmospheric ions and particles together with atmospheric parameters
(temperature, relative humidity, and pressure) all along the stratospheric
ascent of the BEXUS18 stratospheric balloon. Particle size distributions
were measured with the MeteoModem Light Optical Aerosol Counter (LOAC) and
air ion density was measured with a set of two commercial and portable ion
counters. Though the experimental setup was based upon relatively low-cost
and light-weight sensors, vertical profiles of all the parameters up to an
altitude of about 27 km were successfully collected. The results obtained
are useful for elucidating the relationships between aerosols and charged
particles between ground level and the stratosphere, with great potential in
collecting and adding useful information in this field, also in the
stratosphere where such measurements are rare. In particular, the equipment
detected coherent vertical profiles for particles and ions, with a
particularly strong correlation between negative ions and fine particles,
possibly resulting from proposed associations between cosmic rays and ions
as previously suggested. In addition, the detection of charged aerosols in
the stratosphere is in agreement with the results obtained by a previous
flight and with simulations conducted with a stratospheric ion–aerosol
model. However, further measurements under stratospheric balloon flights
equipped with a similar setup are needed to reach general conclusions about
such important issues.
Introduction
It is well recognized that aerosols play a fundamental role in the lower
atmosphere as they may affect climate with both a direct effect on
absorption and scattering of solar radiation, but also an indirect effect
through cloud processing (Yu and Turco, 2001; Forster et al., 2007).
Aerosols are tightly involved in the atmospheric chemical mass balance,
including the stratospheric chemistry through the heterogeneous reactions
with nitrogen and halogen species triggering the austral ozone hole through
polar stratospheric clouds (PSCs) (Hanson et al., 1994; Deshler, 2008).
Aerosol still represents the largest uncertainty in the correct estimate and
interpretation of the ongoing change in Earth's energy budget (Boucher et
al., 2013; IPCC, 2013; Myhre et al., 2013). In this framework, it is
therefore of paramount importance to accurately and systematically collect
experimental data such as particle number densities, as well as all the
properties shedding light on their nature, their size distribution, and
their source in order to define both qualitatively and quantitatively their
role in the troposphere as well as stratosphere.
Stratospheric aerosols are contributed by several sources which determine
particle size, composition, and morphology, as well as their mean residence
time. Historically, the first measurements of stratospheric aerosol were
carried out by Junge (Junge, 1961; Junge et al., 1961); stratospheric
aerosols drew the attention of scientists during the Cold War owing to the
artificial radioactivity released into the stratosphere and returned to the
troposphere through the stratosphere-to-troposphere exchange processes
(Feely et al., 1966; Corcho Alvarado et al., 2014). The monitoring of
radioactive fallout from nuclear weapons testing (and in 1964 from the
accident of SNAP9A, a nuclear-fueled satellite which released 238Pu into
the upper atmosphere upon navigation failure, Eisenbud and Gesell, 1997)
not only brought about the understanding of the basic dynamic processes
coupling the troposphere and stratosphere, but also the discovery of
cosmogenic radionuclides which have their maximum production in the
stratosphere, mostly in the form of aerosols, still largely employed in the
study of vertical exchange between the innermost atmospheric layers
(Cristofanelli et al., 2018). Beside radionuclides, the main source of
aerosol particles in the stratosphere is through the flux of sulfur-bearing
molecules into the stratosphere from the troposphere, primarily, OCS
(atmospheric carbonyl sulfide), during non-volcanic times, and SO2
from volcanic eruptions: after release, sulfur is oxidized and converted to
sulfuric acid which then condenses, forming the bulk of the stratospheric
aerosol layer (e.g., Kremser et al., 2016). Secondary sources encompass the
outer space, contributing an array of mineral micrometeoritic particles
mainly in the solid phase and in situ emission from aircrafts (Murphy et al., 1998)
and the troposphere itself, through active upward transportation; the
troposphere may also act as a source through more localized exchange
processes likely mediated by cumulonimbus dynamics.
Tropospheric sources include volcanic eruptions, usually occurring on an
event basis and with a strong dependency on the event energy (see for
example Deshler, 2008, or Murphy et al., 2014), as well as the transport in
the tropical tropopause layer (TTL; ∼12–18 km) of air and
(water-insoluble) gases and particles driven by cumulonimbus cloud and
further upward transport within the TTL (and in the tropical lower
stratosphere) due to the Brewer–Dobson circulation (e.g., Buchart, 2014).
Fromm et al. (2000) also showed that major forest fires can locally inject
large numbers of carbonaceous and potassium-rich aerosols in the lower
stratosphere, often internally mixed with meteoritic smoke (Hervig et al.,
2009; Neely et al., 2011) and solid grains surviving from their atmospheric
entry (Cziczo et al., 2001; Renard et al., 2005). In this framework,
additional evidence of tropospheric contribution has been shown lately by Yu
et al. (2017), who indicated a major role of the Asian summer monsoon in
contributing more than 15 % of the particles in the stratosphere,
potentially emphasizing anthropogenic aerosol contributions.
Overall, while recent progress in both stratospheric observations and
research suggests the need for increasing attention and consequently for
data collection on stratospheric aerosol, most of the investigations on this
topic are still mainly focused on volcanic contributions which constitute a
sort of baseline to assess behavior and properties of stratospheric aerosols
themselves, but also inspire potential as well as very arguable global
warming countermeasures under the broad term of geoengineering (Launder and
Thompson, 2009). Indeed, volcanic eruptions strongly influence particle
populations in the stratosphere, in particular those from El Chichon in 1982
and from Mt. Pinatubo in 1991 (e.g., Russell et al., 1996), whose monitoring
in the course of the past decades brought assessment of how stratospheric
aerosols may be strongly affected by extreme events until their slow
removal, when they reach background concentration levels. Recently, it was
shown that even weaker volcanic events, similar to biomass burning, are
capable of accessing at least the lower stratosphere, suggesting the need for
further investigation and monitoring (Robock, 2000). As a result, using
various measurement techniques such as remote sensing observations from
satellites and in situ measurements is needed in order to get an updated view of
the stratospheric system and its connections with upper and lower layers of
the atmosphere (e.g., Steele et al., 1999; Deshler et al., 2003).
It is commonly assumed that stratospheric aerosols are mostly liquid
consisting of sulfuric acid from volcanic eruptions, while the upper
stratosphere is free of aerosols except for some instances of residual
particles from meteoritic disintegration and for interplanetary grains in
very low concentrations (Murphy et al., 2014). Nevertheless, it seems that
the stratospheric aerosol content is more complex, in terms of both aerosol
concentrations and nature (Renard et al., 2008). Vernier et al. (2011)
showed that moderate volcano eruptions can inject a significant number of
aerosols into the stratosphere, refilling the aerosol layer episodically.
Though less investigated than tropospheric aerosols, extensive details on
the properties of stratospheric aerosols are provided in the recent review
by Murphy et al. (2014).
In this framework, another physical property still highly underscored, if
not in very specialized science fields, is represented by aerosol electrical
characteristics. Air conductivity due to the presence of differently sized
ions has long been recognized and studied as reviewed in Clement and
Harrison (1991), Hirsikko et al. (2011), and Harrison and Carslaw (2003).
Charged tropospheric aerosols were detected not only in disturbed weather,
but also in a fair weather atmosphere, as resulting from ion diffusion.
Charged particles have also been detected in volcanic ashes (Gilbert et al.,
1991; Harrison et al., 2010) and in Saharan dust layers (Nicoll et al.,
2011). In the mesosphere, smoke and ice particles are part of the plasma in
the D-region and carry positive and negative charges (Rapp, 2009). In the
stratosphere, electrified aerosols were detected in situ for the first
time during balloon-borne aerosol counting measurements (Renard et al.,
2013). The observations carried out during that stratospheric flight showed
that most of the aerosols are charged in the upper troposphere from
altitudes below 10 km and in the stratosphere from altitudes above 20 km,
while the aerosols seem to be uncharged between 10 and 20 km. The
electrification of the aerosols could originate from ion clusters produced
mainly in the atmosphere by the interaction of galactic cosmic rays with the
atmospheric gases, especially in the dense regions of the planetary
atmospheres where solar extreme ultra violet radiation is absent (Harrison
and Carslaw, 2003).
Major sources of ions in the atmosphere include radon isotopes, cosmic rays,
and terrestrial gamma radiation, with a variable relative contribution
depending on the altitude and latitude (Tinsley, 2008): while ionization
from turbulent transport of radon and gamma radiation prevail near the
Earth's surface and over the continents, ionization due to cosmic rays
dominates far away from the continental surface (i.e., over the oceans and
in the upper stratosphere/lower troposphere) (Hirsikko et al., 2011) and
where the production of cosmogenic radionuclides is also highest (Tositti et
al., 2014, and references therein). Both primary and secondary ionization may
therefore interact with air components to produce air ions.
While the overall air ion population is largely responsible for the
so-called “atmospheric global electric circuit” (Tinsley, 2008), the
detection of charged particles in the stratosphere is extremely important
since they might have affected both sprite formation and stratospheric
photochemistry (e.g., Belikov and Nikolayshvili, 2016, and references
therein).
In this framework, the detection of ions and charged particles across the
atmospheric column is ever increasingly drawing attention due to the
experimental evidence linking ions to nucleation mechanisms, firstly proposed
by Raes and Janssens (1985). Requiring a smaller supersaturation of the
involved gases, ion-induced nucleation is thermodynamically advantaged over
homogeneous nucleation. Even though experiments still do not agree on the
relative contributions of ions and neutral nucleation (e.g., Eisele et al.,
2006; Suni et al., 2008; Yu, 2010), this effect is at the basis of the
proposed link between the flux of ionizing galactic cosmic rays modulated by
solar activity and the global cloud cover (Svensmark and Friis-Christensen,
1997; Carslaw et al., 2002) which predicts that an increase in
cosmic-ray intensity will cause an enhancement in CCN (cloud condensation
nuclei) abundance and therefore in cloud reflectivity and lifetime (by
suppressing rainfall). This hypothesis stems from the observed enhancement
of cloud cover during peaks of high energy radiation leading to enhanced
particle formation and growth in the presence of ions: depending on the
competition between condensation growth and processes reducing particle
concentrations (i.e., coagulation, surface deposition, and in-cloud
scavenging), a fraction of those particles may eventually grow into the size
of CCN. This mechanism, unlike the aerosol indirect effect, is only driven
by changes in the rates of microphysical processes and acts on a global
scale, being stronger in regions of low aerosol concentrations.
A second link between galactic cosmic rays and global cloud cover, the
ion–aerosol near-cloud mechanism (Tinsley et al., 2000), involves the
effects of cloud microphysical properties due to the accumulation of space
charge on the tops of clouds. This mechanism is less understood than the
former one, but is linked to the hypothesis that variations in cosmic ray
ionization might modulate the fair-weather current generated by the
electrical current flowing into clouds leading to a sequence of both micro-
and macro-physical responses in cloud processing (Dunne et al., 2012).
Observations conducted to study and quantify these effects (e.g., Laakso et
al., 2007; Svensmark et al., 2007; Pierce and Adams, 2009) are often
incomplete and non-conclusive, leaving model simulations as the most
convenient source of information. The search for a link between CRs (cosmic
rays) and cloud formation is also one of the main drivers for the CLOUD
experiment being conducted at CERN since 2009 where a chamber filled with
atmospheric gases is crossed by charged pions that simulate ionizing CRs.
While some preliminary results suggested that IIN (ion-induced
nucleation) is indeed a relevant factor in determining nucleation rates in the upper
troposphere (e.g., Kirkby et al., 2011), recent results show that cosmic ray
intensity cannot meaningfully affect climate via nucleation (Dunne et al.,
2016), while others indicate that IIN of pure organic particles constitutes a
potentially widespread source of aerosol particles in terrestrial
environments with low sulfuric acid pollution (Kirkby et al., 2016).
Stratospheric balloon research devoted to the collection of aerosol profiles
vs. height has traditionally been carried out in the last decades with the
aim of elucidating properties and processes of this fundamental air
component. While as a rule, in situ observations, either onboard
stratospheric balloons or onboard aircraft, are fairly demanding and
costly (e.g., Sugita et al., 1999; Matsumura et al., 2001; Hervig and Deshler, 2002; Deshler et al., 2003; Kasai et al., 2003;
Watanabe et al., 2004; Hunton et al., 2005; Curtius et al., 2005; Shiraishi et al., 2011; Andersson et al.,
2013; Murphy et al., 2014), the present paper aims at
promoting the forming of young researchers in the spirit of the BEXUS
initiative (Balloon-borne Experiments for University Students; see below),
but also in elaborating effective and “relatively” cheap experiments to
fulfill the need for experimental vertical profiles of aerosol data useful
for filling knowledge gaps in atmospheric and climatological research. The
A5-Unibo (Advanced Atmospheric Aerosol Acquisition and Analysis)
experiment designed by the University of Bologna has been developed with the
purpose of collecting and studying vertical profiles of atmospheric ions and
particles in addition to atmospheric parameters (temperature, relative
humidity, and pressure) all along the flight path of the BEXUS18
stratospheric balloon, using a relatively low-cost and lightweight setup
compared to the conventional instrumentation onboard stratospheric balloons.
This paper describes the setup of the experiment and the measurements
obtained during the flight.
Instrumentation
The A5-Unibo experiment was flown from the Swedish Space Corporation (SSC) Esrange Space Center in
northern Sweden (Kiruna; 67∘53′ N, 21∘04′ E) on 10 October 2014
with the BEXUS18 stratospheric balloon under the REXUS/BEXUS
program. The program was realized under a bilateral Agency Agreement between
the German Aerospace Centre (DLR) and the Swedish National Space Board (SNSB). The Swedish share of the payload is available through collaboration
with the European Space Agency (ESA).
The balloon was a Zodiac BL-DD-12SF-404-ZIT filled with helium gas with a
volume of 12 000 m3 and a diameter of 14 m. The flight lasted 3 h from 08:48 to 12:00 and reached a maximum altitude of 27.2 km with an
average ascending speed during climbing of 3.5 m s-1. Due to onboard
electric problems, no data were available between the altitude range of 18.5 and 20 km. The
floating time in the stratosphere was 68 min. The balloon eventually landed in Finland where it was promptly
retrieved and safely brought back to the Esrange Space Center facilities the
day after the flight.
Apart from the A5-Unibo experiment whose results and setup are herein
described, the whole payload of the stratospheric balloon was quite big and
included a wide range of different experiments: the AFIS-P (Antiproton Flux
In Space-Prototype), ARCA (Advanced Receiver Concepts for ADS-B), COUGAR
(Control of Unmanned Ground Vehicle from Higher Altitude in near Real Time),
and POLARIS (POLymer-Actuated Radiator with Independent Surfaces).
Aerosol measurements
Particles' size distribution vertical profiles were measured by the Light
Optical Aerosol Counter (LOAC) (MeteoModem Inc.), an optical particle
counter/sizer (Renard et al., 2016a, b, based on scattering measurements at
angles of 12 and 60∘). The instrument is light (250 g
total weight including the pump) and compact enough to perform measurements
onboard all kinds of balloons. As described in detail in Renard et al. (2016a, b), the combination of the measurements at two scattering angles
provides both the determination of the particle size distribution and an
estimation of the typology of particles in 19 size classes from 0.2 to 100 µm: briefly, while the measurement at 12∘ scattering angle
does not depend on the refractive index of the particles and enables
accurate size determination and counting, the measurement at 60∘ scattering angle is strongly sensitive to the refractive index of the
particles, giving information on the nature of the particles. LOAC has
already performed more than 150 flights in the stratosphere since 2013.
The LOAC vertical resolution is linked to the total concentrations of
aerosols, as due to the Poisson counting statistics and the capability of
the instrument to detect the smallest particles. A detailed analysis of the
raw measurements has shown that the data must be integrated over 5 min
to remove the oscillations due to the measurements' uncertainty. Considering
the balloon ascent speed, this procedure provides a resolution of about 1 km.
Ion measurements
Air ion densities' vertical profile was measured by means of two air ion
counters (ALPHALAB Inc.), respectively, for positive and negative ions. These
instruments are handheld meters designed to measure ion density, i.e., the
number of ions per cubic centimeter (ions / cc) in air. The instrument is a
ion density meter, based on a Gerdien tube condenser design, and containing
a fan which draws air through the meter at a calibrated rate and is able to
count the number of positive/negative ions when the voltage applied to the
outer cylinder is positive/negative, respectively. The air ion counter can be
used for the detection of natural and artificial ions. Natural ions include
those generated from the decay of radioactive minerals and radon gas, fires,
lightning, and evaporating water, and finally ions associated with storm
activity. These devices are light and small enough (305 g each; 160×100×55 mm) to be mounted on balloons; their measurement resolution extends up to
200 000 ions cm-3. These probes are equipped with a fan to create an air
flow of 24 L min-1 throughout the gondola. Air ambient ions are
diverted from the flow and collected on a plate which returns a voltage
output proportional to the number of ions. The air is then expelled
downwards through the bottom plate.
The use of these small commercial probes based on a Gerdien tube meter (see
for example Aplin and Harrison, 1999) for stratospheric balloon experiments
is unprecedented, and therefore quality control procedures for ion
quantification are not yet available; however, the instrumental performance
of the air ion counters under stratospheric conditions was tested in pre-flight
lab experiments simulating stratospheric conditions. Those tests indicate
that the average MAD (median absolute deviation) of ions' measurements was
equal to 15 ions at 200 hPa for negative ions and 7 ions for positive ions
at the same pressure level.
The ion counters were operated for offset values first in a vacuum changing
the pressure between 1000 and 5 hPa and subsequently in a thermal chamber
between 15 and -60∘C.
The result of these tests was that both of the air ion counters worked
properly in low-pressure environments, while the offset was found to be
independent of external pressure. It is also worth pointing out that during
this experiment the fan flow rate was expected to remain constant during the
balloon's ascent phase. Even though the flow rate actually could
monotonously decrease with increasing altitude, thus leading to
underestimations in ion density concentrations with increasing altitude,
preliminary tests performed to assess the flow rate dependence on pressure
were not conclusive and did not provide a satisfactory working curve.
However, even if ion concentration variations might be biased by the
pressure dependence of the air flow rate, this bias does not affect the
relative variations of concentrations (local strong increases or decreases),
where in fact variations seem to be consistent, as shown and discussed later
on. To compare directly with the aerosols' measurements, the ion
measurement data were integrated with a 1 km vertical resolution.
Temperature, relative humidity measurements and other instrumentation
The BEXUS18 gondola was also equipped with a Parallax MS5607 altimeter
module for pressure readings, which was successfully tested at 36 576 m.
A humidity sensor, HIH9120-021, was used to record the vertical profile of
relative humidity. A temperature sensor, LM35DZ, was mounted on the electronic
board to control the internal temperature. External temperature and GPS data
were instead retrieved by the Esrange Balloon Service System (EBASS), a
telemetry/telecommand (TM/TC) service system for stratospheric balloons
developed by SSC and DLR in 1998.
All the sensors and the instruments onboard (especially the air ion counters
and the LOAC) were tested prior to the stratospheric flight in a vacuum
chamber to ensure their proper functioning at ambient pressures from 1000 to
5 mbar. In addition, after complete assembly of all the probes, the whole
experiment was put into a thermal chamber to ensure its proper working under
low-temperature conditions. All the tests confirmed the correct performance
of the experiment under stratospheric conditions. For a detailed test
related to the LOAC performance, the reader is referred to Renard et al. (2016a).
An Arduino MEGA 2560 microcontroller was used for data acquisition from
sensors and instruments. The sensors were connected to Arduino through a
hardware interface and two stacked boards. An Arduino Ethernet Shield was
used to connect the Arduino MEGA 2560 board to the BEXUS telemetry system.
Two additional electronic boards were designed for the respective control of
the heating system which kept the temperature of the key components above
0 ∘C and of a power control unit which fed the probes and sensors
with the required voltage and current.
Data acquired by the onboard unit, including ambient data and internal
sensors, were collected with a 10 s time resolution; the data were
transmitted to the ground station and displayed via HID through a graphical
interface. The data were then integrated over 60 s and only data
acquired during the ascent and floating phases were analyzed.
Numerical simulations
Model calculations have been used to quantify the electrification of
aerosols with a stratospheric ion–aerosol model in the altitude range of
LOAC measurements. Ion clusters in the atmosphere are produced primarily by
interaction of galactic cosmic rays with atmospheric gases, especially in
the dense regions of planetary atmospheres where extreme solar ultraviolet
radiation is absent (Harrison and Carslaw, 2003). A high fraction of the
cosmic-ray (1 GeV) energy flux is typically carried by particles of
high-kinetic energy. The peak ion production rate by this process has been
found to be generally located at altitudes between 14 and 17 km (Rawal et
al., 2013), which is our major study area, and the ion pair production rate
is calculated using the statistical model of O'Brien (2005), considering
SO42- and NH4+ to be the most abundant ion clusters
produced by this process (Renard et al., 2013). Other sources (radon
isotopes and terrestrial gamma radiation) can be included for the further
improvement of the model simulation as one of the future scopes of the
study.
This ion pair production rate is calculated using the statistical model
developed by O'Brien (2005), with the major ions considered being
SO42- and NH4+. Electrons are not included in the model as
they recombine with positive ions and uncharged molecules very rapidly and
are consequently not available to interact with aerosols. The charging of
aerosols is calculated using charge-balance equations as described by
Michael et al. (2007, 2008) and Tripathi et al. (2008). The charged
particles are constantly interacting with each other, resulting in changes in
initial charge and size distribution with time. As this is a bipolar
interaction, it is expected that charge distribution will be wider than
initial distribution with time (Ghosh et al., 2017).
1dn+dt=q-αn+n--[n+∑j=rminrmax∑i=-ppβi,j+Ni,j]2dn-dt=q-αn+n--[n-∑j=rminrmax∑i=-ppβi,j-Ni,j]
In Eqs. (1) and (2), n+ and n- represent positive and negative
ion concentrations, respectively, q is the ion pair production rate, β
is the ion–ion recombination coefficient, N is the aerosol concentration,
and α is the ion–aerosol attachment rate. Here the radii of the
aerosols vary from size rmin to rmax, and the maximum number of elementary charges
an aerosol can have is p. The aerosol concentration for any size and charge is
calculated by Eq. (3), where i represents the number of elementary charges on
a particle for j, the associated radius bin.
dNi,jdt=βi-1,j+Ni-1,jn++βi+1,j-Ni+1,jn--βi,j+Ni,jn+-βi,j-Ni,jn-+12∑l,m=-pl,m=p∫0vKj-v,vl,mNj-vlNvmdv3-Ni,j∑q=-pp∫0vKj,vi,qNvqdv
In Eq. (3), the first two terms on the right-hand side are the
probability of interaction between the ions and aerosols of any particular
charge and size, and the last term is for growth of that particle due to the
charge-particle coagulation process. K is the charge-particle coagulation
coefficient (probability of collision between two charged particles). v is
the aerosol particle volume assuming all particles have a spherical shape;
Ns is the number of aerosol particles for any particular size. Full
details are provided in Ghosh et al. (2017).
The model is run for number of charges of any particular size of particle
(q) running from +20 to -20. The ion–aerosol attachment coefficients
(β) are calculated in different ways depending on the relative size
of the particles with the ionic mean free path. The calculation depends in
particular on the different regimes, i.e., diffusion, free molecular and
transition. Hoppel and Frick (1986) developed a method to calculate in all
three different regimes. The major requirements for this calculation are the
ionic mobility and mean free path, which are calculated using the
expressions given by Borucki et al. (1982). The charge coagulation
coefficient K was calculated from diffusional force (including vertical
diffusion), turbulent shear force, turbulent inertial force along with
electrostatic force due to charge on particles (Ghosh et al., 2017).
A polydispersed distribution of aerosols is used in the model and is
obtained from the LOAC observation. The LOAC measured data were used for
calculation of different input parameters, like ionic mobility, charge-particle mobility, charge-particle coagulation, ion–aerosol attachment
coefficient, and ion–ion recombination, which are the global input parameters
for the overall model (Global Electrical Circuit Model, GEC, as described
in Rawal et al., 2013). The model also uses temperature and pressure
measured during the experiments. Sensitivity tests to temperature and
pressure indicate that the change in the input temperature and pressure
profile affects only the ion–aerosol attachment coefficient (approximately
10 % change for 20 % change in temperature and pressure) and charge-particle coagulation coefficient (approximately 6 % change for 20 %
change in temperature and pressure). This does not affect the final model
results drastically, as results show that steady-state conditions are
reached in more or less a couple of hours. Only 1 % change in aerosol
concentration is observed for an input of 20 % higher/lower temperature
profile into the model (reported in the Supplement). Overall, no
significant differences are observed for 20 % change in the T–p profile. The
T–p profile only changes the rate of the reaction, but not steady-state
concentrations. We added the charge-particle coagulation model to the Renard et al. (2013) model, as it is close to an accurate simulation scenario. The charge-balance equations are solved by implicit numerical method to obtain
concentrations of positive ions, negative ions, uncharged aerosols and
charged aerosols, for the steady state.
Results
Figure 1 shows the comparison between the vertical profiles of relative
humidity and temperature measured during the flight with those measured with
the radiosonde sounding at Kandalashka (67.15∘ N, 32.35∘ E; 25 m a.s.l.; Russia),
12:00 UTC. The comparison of the temperature profiles shows good agreement in
the troposphere, with a small inversion layer close to the ground and the
starting of the inversion typical of the tropopause located at about 11 km up. The comparison of the temperature profile in the stratosphere presents
instead major differences: in fact, while onboard the BEXUS flight the
temperature remains almost constant in the stratosphere up to an altitude of
26 km and presents a sudden and strong increase around 26–27 km; the
temperature profile measured at Kandalashka presents a small decrease until
about 25–26 km, typical of profiles of this time of the year in the Arctic
region. The reason for such a discrepancy in the stratosphere might be due to
solar heating and perhaps to heat/solar light reflection from other
instruments/structures of the gondola. The comparison of the relative
humidity profiles presents instead major differences already in the
troposphere: in particular, the strong dryness in the planetary boundary
layer (PBL) detected onboard the BEXUS flight (less than 20 %) with a
further decrease up to the altitude of about 5 km is probably due to the
slow response of the relative humidity sensors used onboard. Since standard
radiosonde measurements of relative humidity are only reliable in the
troposphere above temperatures near -40∘C, whereas below these
temperatures and in the stratosphere special instrumentation for
stratospheric water vapor measurements is needed (Berthet et al., 2013;
Tomikawa et al., 2015), measurements of relative humidity above the
tropopause are reported only for the Kandalaksha profile, which have to be
treated with care nevertheless.
Vertical profiles of ambient external temperature (T) and
relative humidity (RH) as measured along the BEXUS18 stratospheric flight on
10 October 2014 and by the radiosounding from the Kandalashka station
(67.15∘ N, 32.35∘ E) on 11 October 2014 at 00:00 UTC.
Figure 2 reports the vertical profiles for the cumulative aerosol particle
number density obtained by summing up the data from all the size bins
collected by the LOAC (in black) together with the negative (blue) and
positive (red) ions during the ascent. A sliding smoothing (i.e., each point
is simply replaced with the average of m adjacent points) is applied to
suppress small-scale fluctuations. It is important to note first of all that
since the lower limit of detection of LOAC is for particles presenting an
optical diameter of 200 nm, we can expect to have ion concentrations
greater than the aerosol detected total concentrations. In particular, since
the number of ions is greater than the detected aerosol, we can infer that
aerosols smaller than 200 nm are the main contributors to negative ions.
Vertical profiles of integrated aerosol concentration, for
aerosols greater than 200 nm (black line), and of positive (red line) and
negative ions (blue line).
Figure 3 reports the vertical profiles of aerosol size distribution for each
size bin measured by the LOAC instrument. Most of the particles have size
below 1 µm, as expected in a clean free troposphere and in the
stratosphere. Few particles greater than 1 µm and smaller than 15 µm and just one 50 µm particle were detected in the stratosphere. All
fine particles (< 1 µm) presented the same vertical variation,
with a global trend of decreasing concentrations at heights higher than the
tropopause. Larger particles, besides presenting lower number concentrations
as expected, presented a different vertical profile, with the presence of an
abrupt increase in the PBL and then at 10 km less evident in finer
particles. However, it is important to note that with these measurements it
is difficult to derive information on the PBL, which is beyond the focus of
this paper.
Vertical profiles of particle size distributions for the 19 size
classes of the LOAC particle counter as part of the A5-Unibo experiment on
the BEXUS18 stratospheric flight. The notation dN/dlog(D) used in the x axis
stands for the number concentration of particles in the various size classes
divided by the width of the size classes.
In Fig. 4 we report the average variation of particle size distribution with
altitude. The five size distributions depicted have been determined
empirically by averaging the LOAC data over five temperature intervals along
the height profile (see Fig. 1) according to roughly coherent atmospheric
layers, as obtained during the BEXUS18 experiment. The intervals chosen
were, respectively, 0–404, 650–1519, 1765–10 118, 10 650–25 044, and 25 289–27 191 m.
Average size distribution of aerosol particles at various height
layers during the BEXUS18 stratospheric flight. The five curves are obtained
by averaging the aerosol number densities as a function of the atmospheric
layers pointed out by the temperature profile as follows: 1: 0–404 m (black
line); 2: 650–1519 m (red line); 3: 1765–10 118 m (blue line); 4: 10 650–25 044 m (pink line); 5: 25 289–27 191 m (green line). x and y axes are in
log-normal scales.
In practice, they correspond, respectively, to lower and upper PBL, free
troposphere, tropopause/lower stratosphere, and mid stratosphere, which as
known is characterized by a marked increase in temperature owing to the
ozone absorption of longer wavelength UV radiation. However, the marked
temperature increase recorded at float in our measurements is probably due
to instrumental errors since the relative speed between the balloon and the
air is close to 0 there (no ventilation). While there is a steady decrease
in particles in all the size bins as the altitude increases, in the
intermediate tropospheric groupings an increase in the coarse particle bins
around and above 10 µm is observed.
Finally, Fig. 5 reports the vertical profiles of simulated fractions of
charged particles, which can be used for comparison with measured profiles
and with previous simulations and observations presented by Renard et al. (2013) with completely different instrumentation.
Vertical profile of the simulated fraction of charged particles:
(a) total; (b) in the different size classes. Because of the very low and even
zero concentration particles in the largest size classes, only particles with
diameter smaller than 5 µm are considered.
Discussion
The data collected show that there is a steady decrease in the particle
number density with height for all the size bins determined. At all heights
sub-micron particles are the most numerous, though coarse particles show a
relative, sensitive increase in the upper PBL and free troposphere. At an
altitude of 1 km, LOAC typology measurements indicate the presence of a thin
layer, less than 100 m in thickness, of transparent particles, possibly droplets. At
the other altitudes the typology measurements indicate optically absorbing
and semi-absorbing particles, probably related to the presence of minerals
(dust). In particular, above the tropopause, almost all of the particles
detected by LOAC were smaller than 1 µm, and their typology
measurements indicate both the occurrence of stratospheric liquid droplets
and the presence of optically absorbing material (i.e., internally or
externally mixed particles): even though LOAC typology measurements cannot
provide precise information on particles' chemical composition, it is in
agreement with results from aircraft observations in the lower stratosphere
(e.g., Schwartz et al., 2008; Murphy et al., 2014), which showed that while
sulfate particles dominate the aerosol composition in the stratosphere,
other sources producing absorbing particles also contribute. The vertical
profiles of integrated concentration of aerosols > 200 nm and of
ions (Fig. 2) present interesting features. Firstly, positive charges are
only present relatively close to the ground, which is in agreement with
previous observations (e.g., Li et al., 2015), even though we cannot exclude
the possibility that the complete absence of positive charges at upper levels derives from a
failure of the positive ions' counter during the flight. Even though we are
performing more flights with a similar instrumental setup in order to
compare and provide evidence of our findings, this general behavior is also
consistent with previous observations, showing that ionization from
turbulent transport of radon (positively charged product ions) and gamma
radiation (negative ions) prevails close to the Earth's surface, whereas
ionization from cosmic rays (negative ions) dominates away from the
continental surface (upper troposphere and above) (Hirsikko et al., 2011),
both ion distributions playing a basic role in the terrestrial global circuit
(Tinsley and Zhou, 2006). Indeed, preliminary results of a stratospheric
flight with the ion counters performed on 8 April 2017 in Australia show and
confirm the detection of only positive ions in the lower troposphere, while
both polarities, with a prevalence of negative ions, were present at upper
levels. Secondly, vertical profiles of particles and ions present the same
general structure above the tropopause, including an enhancement in the
20–25 km altitude range. None of these profiles is similar to the
temperature, humidity, and pressure profiles, which can exclude the
possibility of instrumental contamination by these atmospheric parameters.
Moreover, the ion concentration variations between 10 and 20 km cannot be
linked to the decrease in the airflow fan, whose precise dependence on
pressure cannot be correctly estimated for the time being, as previously
pointed out in the material and methods section. Even though the absolute
values could be biased from this effect, the relative variations seem to be
real. In particular, Spearman's correlation coefficient (a nonparametric
measure of rank correlation, where nonparametric means not based on
parameterized families of probability distributions) (Table 1) indicates a
strong negative correlation (i.e., anticorrelation) of negative ions with
fine particles, a behavior which might have resulted from ion-induced
nucleation and in particular from the proposed association between cosmic
rays and ions (“ion–aerosol clear-air” mechanism), although we are aware
that nucleation concerns particles in the 1–2 nm range, and further growth
is governed by condensation. Our results are also in agreement with previous
observations showing that, in general, negative ions more efficiently
promote nucleation than positive ions (Svensmark and Friis-Christensen, 1997; Eisele et al., 2006; Suni et al.,
2008).
Spearman's correlation coefficient between ions and number of
particles in the different size classes. The asterisk * indicates
significant (0.05 significance level, i.e., p<0.05) correlation
coefficients; bold indicates strong (R>0.6 in absolute value)
correlation, while italics indicate weaker (0.4<R<0.6
in absolute value) correlation.
The weaker but positive correlation between positive ions and coarse
particles might instead arise from their simultaneous detection closer to
the Earth's surface.
The number of (negative) charges as well as of particles strongly increases
above 20 km. The maximum value around 20 km corresponds to the region of
maximum ionization (Regener–Pfotzer maximum; Regener and Pfotzer, 1935) and
was previously observed by Harrison et al. (2014) observing count rates
through Geiger counters on standard meteorological balloons. Stratospheric
ion–aerosol model simulations can be used to quantify and explain the
electrification of the aerosols (Rawal et al., 2013). The simulated profile
shows that more than 75 % of aerosols are charged above the altitude of 5 km (Fig. 5). This result is in agreement with the presence of the charged
liquid and/or solid particles detected by ion detectors and could be used as
an estimate of the vertical variability of their percentage. The
measurements presented here are also in general good agreement with the
unique previous direct detection of charged stratospheric aerosols of Renard
et al. (2013). In addition, they reveal a “depletion layer” of poorly
charged aerosols from the tropopause to an altitude of about 20 km, where
the charged fraction drops at about 1 %, similar to the one previously
detected by Renard et al. (2013).
In particular, as from Fig. 5b, it is clear that as from model
simulations, fine particles are the ones contributing to the largest
variations in the fraction of the charged fraction, while coarse particles,
when present, are mostly charged, confirming the calculations made by Renard
et al. (2013) (see their Fig. 4).
Summarizing, our observations first of all demonstrate the effectiveness of
the adopted instrumental setup in measuring vertical profiles of particles'
size distributions and particles' typology together with ions. In addition,
they can also provide interesting results in terms of the association
between cosmic rays and ions, and further reveal novel features in terms
of the charged fraction, from new stratospheric flights with a similar
instrumental setup.
Conclusions
The A5-Unibo experiment flown under a stratospheric balloon seems to have
confirmed the previous detection of charged aerosols in the stratosphere and
a possible vertical variability. In particular, the results show coherent
vertical profiles for particles and ions, with a particularly strong
correlation between negative ions and fine particles, possibly resulting
from proposed associations between cosmic rays and ions as previously
suggested. Due to the important implications of charged aerosols for the
high-energy phenomena (sprites, blues jets, and elves) in the middle
stratosphere (Füllekrug et al., 2016) and of ions in nucleation mechanisms,
further stratospheric balloon-borne measurements of charged particles are
necessary. Poly-instrumented gondolas with aerosol counters for the
estimate of the percentage of charged particles, positive and negative ions
counters, and Geiger counter, will help to better evaluate the direct link
between cosmic rays, ions, and the charged aerosols. Furthermore, the
addition of small condensation particle counters able to characterize
particles in the 1–2 nm range could help to gain precise information on
nucleation, which here was only derived and could not directly observed. In
particular, since both the present flight and the Renard et al. (2013)
flight were performed for altitudes below 27 km, an experimental campaign
involving many flights at higher altitude, up to the maximum altitude
reachable with stratospheric balloons (around 40 km), is necessary to better
document the vertical evolution of the charged aerosols. Also, the analysis
of the results obtained during such future flights will be helpful in
answering open questions raised in this and previous flights.
Are the stratospheric aerosols always charged?
Is there variability in the percentage of charged aerosols, for example with
latitude, season, or solar eruptions?
Is the percentage of charged aerosols dependent on the nature of the
aerosols (liquid droplet, ash from volcanic eruptions, meteoritic mineral
material, or carbonaceous particles from Earth and space)?
Is the “depletion layer” of poorly charged aerosols above the tropopause,
not expected from modeling, a transient phenomenon or a permanent feature?
Such aerosol measurements could have implications for climate and atmospheric
chemistry issues, but also for the atmospheric electricity and high-energy
phenomena such as sprites, blue jets and elves that are not yet well
understood.
Data availability
A description of the observational data and model simulations used in this paper can be found in Sects. 2 and 3. Observational data are available upon request by contacting Laura
Tositti (laura.tositti@unibo.it), while model simulations are available by contacting Sachi N. Tripathi (snt@iitk.ac.in).
The supplement related to this article is available online at: https://doi.org/10.5194/angeo-37-389-2019-supplement.
Author contributions
EB conceived and designed the analysis, contributed data or analysis tools, performed the analysis, and wrote the paper; ESC collected the data and as the team leader of the A5-UNIBO experiment was responsible of all the system, electronics and thermal engineering of this experiment; FG supported the young researchers when developing the experiment; J-BR performed the analysis of aerosols data and supported in the writing of the paper; SNT and KG contributed the simulations and the writing up of the related part in the text; GB and DV supported in the realization of the testing of the instrumental setup and supported in data analysis; LT conceived the scientific idea of the experiment and wrote the paper.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors wish to thank two anonymous reviewers for their useful comments which improved the quality of this paper. This work was designed and developed within the collaboration of the Flight
Mechanics Laboratory (Fabrizio Giulietti) and the Environmental Chemistry and
Radioactivity Laboratory of the Department of Chemistry “G. Ciamician”
(Laura Tositti) of the University of Bologna as main supporters. In
particular, the experiment was flown onboard the BEXUS18 stratospheric
balloon under the REXUS/BEXUS program, supported under a bilateral Agency
Agreement between the German Aerospace Centre (DLR) and the Swedish National
Space Board (SNSB). The Swedish share of the payload is available through
collaboration with the European Space Agency (ESA). Every team member of the
A5-Unibo experiment is acknowledged for his/her essential role in the
success of the experiment: Encarnaciòn Serrano Castillo (team leader and
system engineer), Riccardo Lasagni Manghi (verification and testing
engineer), Erika Brattich (data analysis, scientific expert), Igor Gai
(ground station engineer), Danilo Boccadamo (power engineer), Paolo Lombardi
(mechanics), Alice Zaccone (software engineer), Abramo Ditaranto (electronics
engineer), Luca Mella (software engineer), and Marco Didonè (thermal
engineer). We also acknowledge (a) institutional supporters: DLR,
Rymdstyrelsen, SSC, ESA Education Office, EuroLaunch, ZARM; and (b) private
companies and associations: AlphaLab Inc., Boxer, Iacobucci HF Aerospace,
Icos, CNA Forlì-Cesena, Dogcam, Gruppo SDS, Società Italiana di
Medicina Generale, Plastica Panaro, Bustaplast, Bellini Tiziana, Mascherpa.
We also thank Sergio Brattich for a thorough revision of the English language
of the manuscript. Erika Brattich also thanks the Department of Biological,
Geological and Earth Sciences of the University of Bologna for grant support
during her PhD study, during which the experiment was developed, and the
Department of Chemistry “G. Ciamician” of the University of Bologna, for
support during her post-doc.
Review statement
This paper was edited by Christoph Jacobi and reviewed by
two anonymous referees.
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