The influence of surface charge on the coalescence of ice and dust particles in the mesosphere and lower thermosphere
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Atmos. Chem. Phys., 21, 8735–8745, 2021
https://doi.org/10.5194/acp-21-8735-2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.
The influence of surface charge on the coalescence of ice and dust
particles in the mesosphere and lower thermosphere
Joshua Baptiste1 , Connor Williamson1 , John Fox1 , Anthony J. Stace1 , Muhammad Hassan2 , Stefanie Braun2 ,
Benjamin Stamm2 , Ingrid Mann3 , and Elena Besley1
1 School of Chemistry, University of Nottingham, University Park NG7 2RD, UK
2 Centerfor Computational Engineering Science, Mathematics Department, RWTH Aachen University,
Schinkelstr. 2, 52062 Aachen, Germany
3 UiT – The Arctic University of Norway, Space Physics Group, Postboks 6050 Langnes, 9037 Tromsø, Norway
Correspondence: Ingrid Mann (ingrid.b.mann@uit.no) and Elena Besley (elena.besley@nottingham.ac.uk)
Received: 3 November 2020 – Discussion started: 10 December 2020
Revised: 27 April 2021 – Accepted: 10 May 2021 – Published: 9 June 2021
Abstract. Agglomeration of charged ice and dust particles 2011). Part of the meteoroid material vaporises, and the re-
in the mesosphere and lower thermosphere is studied using leased small solid particles and gaseous species are incorpo-
a classical electrostatic approach, which is extended to cap- rated into the atmosphere where they grow further to form
ture the induced polarisation of surface charge. Collision out- MSPs (see, e.g., Megner et al., 2006; Brooke et al., 2017).
comes are predicted whilst varying the particle size, charge, The coalescence or condensation mechanisms leading to dust
dielectric constant, relative kinetic energy, collision geome- agglomerates is considered to be an important aspect of at-
try and the coefficient of restitution. In addition to Coulomb mospheric physics and chemistry. A better understanding of
forces acting on particles of opposite charge, instances of at- these mechanisms could help to establish the significance of
traction between particles of the same sign of charge are dis- particles containing refractory materials that are present in
cussed. These attractive forces are governed by the polarisa- the upper mesosphere and lower thermosphere, in short, the
tion of surface charge and can be strong at very small sepa- mesosphere and lower thermosphere (MLT) region of 60 to
ration distances. In the mesosphere and lower thermosphere, 130 km. These small solid particles could also play a role
these interactions could also contribute to the formation of in the formation of ice clouds by providing a core for het-
stable aggregates and contamination of ice particles through erogeneous condensation that is more effective than homo-
collisions with meteoric smoke particles. geneous nucleation. During summer, at high and mid lati-
tudes, the temperature near the mesopause reaches a mini-
mum and can fall below the freezing point of water (Lübken,
1999), and clouds of ice particles, polar mesospheric clouds
1 Introduction (PMCs), can form at heights of 80 to 85 km (Hervig et al.,
2001). These are also observed from Earth after sunset and
A significant fraction of the cosmic dust and meteoroid ma- are known as noctilucent clouds (NLCs). Because NLCs may
terial that hits the Earth remains in the atmosphere for ex- be an indicator of climate change (Lübken et al., 2018), it is
tended periods of time and is a source of solid dust particles, important to understand the possible role of meteoric smoke
denoted as meteoric smoke particles (MSPs) (Megner et al., in the coalescence of ice particles, although the growth of the
2006; Rapp et al., 2012). MSPs are formed by an ablation meteoric smoke is an interesting topic of research in itself.
process, whereby meteoroids colliding with atmospheric par- Models of coagulation (Megner et al., 2006; Bardeen et al.,
ticles experience strong deceleration and are heated to evapo- 2008; Brooke et al., 2017) take into consideration the con-
ration temperatures. Meteoric and atmospheric species form vection of dust particles in global atmospheric circulation,
an expanding column of partially ionised gas behind the me- the influence of gravitational force and the Brownian motion.
teoroid, which is observed as a meteor (see, e.g., Mann et al.,
Published by Copernicus Publications on behalf of the European Geosciences Union.8736 J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles
The models also assume that particles stick together after a The focus of this work is on aggregation processes rele-
collision, which is not always the case. The outcome can de- vant to mesospheric conditions and, in particular, at high lat-
pend on the relative velocity of the colliding particles and the itudes. The MLT region offers unique conditions in terms of
elasticity of a collision as defined by the coefficient of resti- the electrostatic environment, composition and physical pa-
tution, which can vary according to the composition of a par- rameters such as temperature and pressure. The pressure at
ticle. Dust charging, which can cause particles to experience 60 km is less than 1 hPa and decreases further with increasing
either strong attractive or repulsive forces, could also play a altitude; therefore, particles interact essentially in a vacuum,
role in the growth process. This consideration has not previ- and consequently, in these simulations the dielectric constant
ously been included in modelling the collisional dust growth of the surrounding medium is taken to be 1. To investigate
in the MLT but has been studied for droplets in tropospheric the growth of meteoric smoke particles, we consider charged
clouds (Zhang et al., 2019). and neutral metal oxides particles with radii ranging from
In this paper, we investigate the influence of surface charge 0.2 to 5 nm, as shown in Table 1. To simulate the growth of
on the particle agglomeration processes. We apply models ice onto the meteoric smoke, we examine the interactions be-
that are developed to describe electrostatic interactions be- tween metal oxide particulates and large ice particles ranging
tween charged dielectric spheres and are based on solutions in size from 10 to 100 nm and with charges 0 to −5e. As these
presented by Bichoutskaia et al. (2010) and Filippov et al. particles typically possess a low charge (or single charge aris-
(2019). These theories predict collision outcomes according ing, for example, from either a photoionisation event that re-
to the variables of particle size, charge, dielectric constant, moves a single electron from a molecule on the particle or
relative kinetic energy, collision geometry and the coefficient the attachment of an ambient air ion) the charge distribution
of restitution. The presence of negative, positive and neu- is best represented by a point free charge residing on the sur-
tral particles in the MLT region implies that Coulomb forces face. For this case, we have extended the numerical method
between oppositely charged objects are the main attractive developed in Lindgren et al. (2018a) to allow for description
component of any electrostatically driven dust agglomeration of particle charge in the form of point charge(s) residing on
process. However, in addition to the strong attractive interac- its surface, similar to a solution proposed in Filippov et al.
tion between oppositely charged particles, attractive interac- (2019) but based on a numerical method. Comparisons with
tions between particles of the same sign of charge can also a uniform distribution of free surface charge, as described in
take place at small separation distances, leading to the forma- Bichoutskaia et al. (2010), shows that, for particles with radii
tion of stable aggregates. This attractive force is governed by greater than 10 nm, the choice of a specific form of surface
the polarisation of surface charge, leading to regions of neg- charge distribution does not affect the calculated electrostatic
ative and positive surface charge density close to the point of energy between particles; however, the difference does be-
contact between colliding particles (Stace et al., 2011). The come important for sub-nanometre particles.
strength of the resulting attractive electrostatic force depends The remaining parts of the paper are organised as follows.
on the particle composition, as the value of the dielectric con- In Sect. 2, we describe the ionospheric dusty plasma in the
stant determines the extent of polarisation of the bound sur- region where we study dust growth. In Sect. 3, the range of
face charge. Previously, the model has successfully explained relative velocities for collisions leading to aggregation is cal-
the effects of like-charge attraction in a range of coalescence culated for all collision scenarios that are considered suitable
processes such as the agglomeration of single particles and to describe the interactions between ice and dust particles in
small clusters derived from a metal oxide composite (Lind- the mesosphere. These velocity ranges are subsequently used
gren et al., 2018b), aerosol growth in the atmosphere of Ti- to calculate the percentage aggregation outcome. The orien-
tan (Lindgren et al., 2017) and the self-assembly behaviour tational geometry of the collisions is discussed, and a quan-
of charged micro-colloids (Naderi Mehr et al., 2020). Inter- titative estimation of the electrostatic interaction energy pro-
actions between pairs of neutral and charged particles also file is presented for collisions between like-charged particles.
depend on the polarisation of surface charge, but these take Section 4 focuses on specific cases of aggregation between
place in the absence of a Coulomb barrier (see below). In at- like-charged dust and ice particles, and Sect. 5 deals with ag-
mospheric science, the method of image charges is routinely gregation between small charged dust particulates. A brief
used to study collision outcomes if particles can be approx- discussion of the results is provided separately in Sect. 6.
imated by conducting spheres (or having the dielectric con-
stant greater than 80). The image charge model can also be
applied to study qualitatively the interaction between dielec- 2 Ionospheric dusty plasma conditions
tric particles if the value of the image charge is corrected as
q 0 = 11 +
−2
2
q, where 1 and 2 are the dielectric constants, q 0 The MLT overlaps with the D-region ionosphere, where the
is image charge, and q is real charge (Jackson, 1999). In con- major ionisation process under quiet conditions above 80 km
trast, quantitatively accurate theoretical studies of interacting is due to solar radiation and geo-coronal UV radiation, and
dielectric spheres began only quite recently. galactic cosmic rays become an important source of ionisa-
tion below 80 km. At high latitudes, where ice clouds can
Atmos. Chem. Phys., 21, 8735–8745, 2021 https://doi.org/10.5194/acp-21-8735-2021J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles 8737
Table 1. Common particulates found in the MLT region which are considered in this study.
Particle Dielectric constant Density/g cm−3 Size range/nm Charge/e
Ice, H2 O 100 0.92 3–100 0, −1 to −5
Silicon dioxide, SiO2 3.9 2.65 0.2–5 0, −1, −2
Magnesium oxide, MgO 9.6 3.58 0.2–5 0, −1, −2
Iron oxide, FeO 14.2 5.74 0.2–5 0, −1, −2
form, the precipitation of high-energy electrons and protons observations to constrain the work function of the MSP ma-
that form in the Sun–magnetosphere interactions and enter terial and to infer Fe and Mg hydroxides as possible initial
the ionosphere during geo-magnetically perturbed conditions compounds from that. Robertson et al. (2009) pointed out
promote further ionisation. This increases the ionisation rate that rocket measurements of dust particles using charge de-
by up to several orders of magnitude (Gumbel et al., 2003). tection can be misinterpreted when there are approximately
The dust particles in the MLT are, hence, embedded in a equal numbers of positively and negatively charged MSPs;
low-pressure atmosphere with a small but highly variable de- they also point out the importance of charge interactions for
gree of ionisation. They collect and emit electrons and ions understanding the coagulation process.
and some of the dust particles acquire a net equilibrium sur-
face charge in a balance when the currents of incoming and
outgoing charged particles are equal. The number density of 3 Collision dynamics
plasma particles is variable. The impact of photons causes
photoelectron emission and the detachment of electrons from Temperatures close to the mesopause at high latitudes fall
negatively charged dust. Photoionising solar X-ray, extreme during summer to the range of 130 to 150 K; however, ob-
UV (EUV) and UV fluxes can be variable, and other sources servational studies have shown this to be variable (Lübken,
of ionising radiation include aurora and geo-corona, as well 1999). Such low temperatures have a significant effect on
as elves and sprites formed in the atmosphere (Barrington- the nature of water droplets, as according to the appropri-
Leigh et al., 2001). The dust number density is of a sim- ate phase diagram (Journaux et al., 2020; Hudait and Mo-
ilar order as that of the free electrons and ions; all parti- linero, 2016), ice particles are in a soft ice state and may
cles with charge collectively form a dusty plasma. In model absorb some of the kinetic energy present during a collision.
calculations, the dust charge has been simulated by taking This possibility has implications for the outcome of all col-
into account the capture of plasma particles, photo detach- lisions between small metal oxide particulates and ice parti-
ment and photoionisation (Baumann et al., 2013, 2015). The cles, which, at short separation distances, can exhibit a strong
time to reach equilibrium charge varies from around 100 s attraction, even when both particles have a charge of the same
in quiet conditions to less than 1 s in a meteor (Mann et al., sign (Bichoutskaia et al., 2010). However, for like-charged
2011, 2019). particles with low velocities, this attractive region is largely
Different assumptions have been made regarding the com- inaccessible due to the presence of a large repulsive Coulomb
position of particles. Hervig et al. (2012) describe the polar energy barrier (ECoul ) which prevents their aggregation. In
mesospheric cloud particles that are observed with extinction addition to the Coulomb barrier, other factors affect aggrega-
measurements as a mixture of ice and meteoric smoke and tion during a collision; these include the binding energy as
suggest wüstite and magnesiowüstite as possible smoke ma- defined by the interaction energy at the point of contact (E0 ),
terials. To simulate dust conditions in the laboratory, Plane the coefficient of restitution (kr ), the Maxwell–Boltzmann
et al. (2015) consider olivine and pyroxene and Duft et al. distribution of particle velocities at a defined temperature and
(2019) iron silicate. A number of laboratory experiments and the composition of colliding particles (as defined by the di-
ion chemistry considerations could also constrain expected electric constant and particle density).
MSP dust composition, revealing that FeO and MgO are The total kinetic energy of a system containing two col-
rapidly oxidised by O3 and O2 and recombine with H2 O and liding particles is the sum of the relative kinetic energy with
CO2 (Plane et al., 2015). Hence, the existence of these oxides respect to the centre of mass (Krel ), and the kinetic energy of
as pure particles is unlikely. Bearing this in mind, we chose the centre of mass (Kcm ) is as follows:
our sample materials mentioned above as analogues for ma- 1 2 1 2
terials with different dielectric constants. There has been no Ktot = µvrel + Mvcm , (1)
2 2
successful analysis made so far of the composition of col-
lected MSPs, which is because of difficulties in the collection m1 m2
where µ = m1 +m2 is the reduced mass of the colliding par-
process and because of different sources of contamination P
m v
ticles, M = m1 + m2 , vrel = v1 − v2 , and vcm = Mj j (j =
(Hedin et al., 2014). Rapp et al. (2012) used in situ rocket
1, 2). The kinetic energy of the centre of mass is unaffected
https://doi.org/10.5194/acp-21-8735-2021 Atmos. Chem. Phys., 21, 8735–8745, 20218738 J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles
by changes in the inter-particle interaction energy; however,
due to the law of conservation of energy, the loss or gain
of electrostatic interaction energy between the colliding par-
ticles leads to corresponding changes in the relative kinetic
energy. At the point where the electrostatic interaction en-
ergy is at the maximum (ECoul ), the relative kinetic energy
of the colliding pair is at the minimum. Once over the bar-
rier and immediately before the collision the kinetic energy
before = K initial −E , and in an inelastic
is at its highest, i.e. Krel rel 0
collision, it is reduced to Krelafter = k 2 × K before . If k = 1, the
r rel r
collision is elastic, and the kinetic energy does not change
during the collision. The minimum relative initial velocity
colliding particles require to overcome the Coulomb barrier
is therefore as follows:
s
min 2ECoul
vrel = . (2) Figure 1. Possible outcomes for a collision between like-charged
µ particles. The total energy is schematically split into two compo-
nents, i.e. the electrostatic interaction energy (solid) and the relative
If the loss of kinetic energy during a collision (Krel before −
kinetic energy (dashed). The electrostatic interaction energy profile
after
Krel ) is greater than the excess kinetic energy as compared is calculated for a collision between an ice particle (r1 = 3 nm) and
to the Coulomb barrier (Krel initial − E a SiO2 particle (r2 = 0.5 nm), with both particles carrying a charge
Coul ), then the particles
are trapped behind the barrier. The maximum relative initial of q1 = q2 = −1e.
max ), above which coalescence is not possible, is
velocity (vrel
derived from the situation where, during a collision, insuf-
ficient kinetic energy is removed through the action of the In Fig. 2, representative examples for the Maxwell–
coefficient of restitution and the particles fly apart. This max- Boltzmann distribution of the relative velocities are shown
imum initial velocity is given by the following: for collisions between SiO2 particles carrying a charge of
s q2 = −1e and ice particles with q1 = 0, −1e and −2e at
2 (ECoul − E0 )/kr2 + E0
T = 150 K. If the surface charge is represented by a point
max
vrel = . (3) charge residing on the particle’s surface, then the orienta-
µ
tional geometry of a collision becomes important. Figure 3
The above collision scenarios are illustrated in Fig. 1 based shows the geometries considered in this study, both for col-
on an example case of a small SiO2 particle colliding with a lisions between ice particles and small metal oxide particu-
larger ice particle, with both carrying a negative charge of lates (Fig. 3a) and for collisions between metal oxide parti-
q1 = q2 = −1e. A total of three possible outcomes are de- cles (Fig. 3b and c).
scribed. If the relative kinetic energy of the colliding particles Previous studies by Bichoutskaia et al. (2010) have shown
is smaller than the height of the Coulomb barrier (case 1) conclusively that, between like-charged particles, attraction
then the particles always repel one another without energy is strongly size dependent, such that particles carrying the
loss. If the particles collide inelastically with a relative ki- same amount of charge should have dissimilar sizes. This ef-
netic energy sufficient to overcome the Coulomb barrier, the fect becomes more noticeable with the increase in the ratio
loss of kinetic energy during a collision may prevent their of particle radii, r1 /r2 ; as the ratio increases, surface charge
subsequent separation and lead to the formation of a stable, polarisation becomes more pronounced, leading to strong at-
or metastable, aggregate (case 2). If the energy loss during traction at short separation distances and a reduction in the
such a collision is not sufficient to stabilise the pair, the par- Coulomb barrier. This effect is illustrated in Fig. 4a, which
ticles rebound and separate (case 3). The latter case may be shows electrostatic interaction energy profiles as a function
applicable in warmer regions of the atmosphere where parti- of separation distance for collisions between like-charged ice
cles move with higher velocities. In this work, we consider a and SiO2 particles (q1 = q2 = −1e) as the size of the ice
wide range of particle velocities in order to identify a wide particle varies between r1 = 10, 20 and 30 nm. As the ice
range of possible collision outcomes. The probability distri- particle becomes larger, the height of the Coulomb barrier
bution for the relative velocity of two colliding particles in decreases, which in turn can affect the outcome of a colli-
the form of a Maxwell–Boltzmann distribution at tempera- sion. Note that Fig. 4 refers to a collision geometry shown in
ture T is given by Müller-Kirsten (2013) as follows: Fig. 3a, which favours the attractive interaction between two
particles, each with a point charge located on their surface.
r
2 µ 3/2 2 − µvrel 2 In this example, the SiO2 particle approaches the ice par-
P (vrel ) = vrel e 2kT . (4) ticle from the direction opposite to the location of the point
π kT
Atmos. Chem. Phys., 21, 8735–8745, 2021 https://doi.org/10.5194/acp-21-8735-2021J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles 8739
Figure 2. Aggregation probability, indicated by the shaded area, for a collision between a SiO2 particle (r2 = 0.2 nm, q2 = −1e) and an ice
particle (r1 = 30 nm), as defined by the Maxwell–Boltzmann distribution of the relative velocity at T = 150 K. (a) In the case of a neutral ice
particle (q1 = 0), the probability of aggregation is 1 as P (vrel ) is integrated in the velocity range of [0, 1192] m s−1 . (b) For q1 = −1e, the
probability of aggregation is 0.293 as P (vrel ) is integrated in the velocity range of [295, 1219] m s−1 . (c) For q1 = −2e, the probability of
min and v max are taken from Table 2.
aggregation is 0.034 as P (vrel ) is integrated in the velocity range of [450, 1260] m s−1 . The values of vrel rel
Figure 3. Position of the point charge on the surface of colliding
particles depicted by a small open circle. Panel (a) shows an ice
particle (1) and a small oxide particulate (2), and panels (b, c) show
two oxide particles (1 and 2).
Figure 4. Electrostatic interaction energy as a function of the
surface-to-surface separation distance, s, between an ice particle
charge on the latter, and this collision corresponds to the least
and a SiO2 particle (r2 = 0.2 nm; q2 = −1e) in the geometry shown
repulsive interaction. An equivalent scenario has been con-
in Fig. 3a, as calculated by the point charge model analogous to
sidered assuming a uniform distribution of surface charge on Filippov et al. (2019). Horizontal lines indicate the value of the
both particles, following the approach described in Bichout- Coulomb energy barrier obtained using the uniform surface charge
skaia et al. (2010). The height of the Coulomb barrier ob- model. (a) The charge of the ice particle is q1 = −1e, and the radius
tained using a uniform distribution of surface charge is de- varies as r1 = 10 nm (line 1), 20 nm (line 2) and 30 nm (line 3). (b)
picted in Fig. 4 by horizontal lines. For the size of particles The radius of the ice particle is r1 = 30 nm, and the charge varies as
considered in this work, these two approximations give very q1 = −1e (line 3), −2e (line 4) and −5e (line 5). Note the change
similar results. Although the height of the Coulomb barrier in scale on the y axis.
is strongly influenced by the size of the large ice particle
(Fig. 4a), it shows no change with variations in the sizes of
the SiO2 particles considered here. variation in the electrostatic energy with particle size shown
The height of the Coulomb barrier is affected even more in Fig. 4a is a more subtle effect related to surface charge po-
greatly when the charge of colliding particles is changed. In larisation (note the change of scale along y axis in Fig. 4b).
the case considered in Fig. 4b, the charge on the ice parti-
cle was increased from q1 = −1e to −5e to show the almost
linear dependence of the barrier on charge variation, in accor-
dance with the leading Coulomb energy term E ∝ q1rq2 . The
https://doi.org/10.5194/acp-21-8735-2021 Atmos. Chem. Phys., 21, 8735–8745, 20218740 J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles
4 Aggregation of like charged metal oxide and ice
particles
Consider first the aggregation of negatively charged metal
oxides and ice particles. Table 2 shows values of vrel min and
max
vrel calculated using Eqs. (2) and (3) with kr = 0.9. Integrat-
ing the probability distribution shown in Fig. 2 between these
limits gives the probability of aggregation, and the results are
presented in Table 2, where aggregation is expressed as a per-
centage of all collisions. Table 2 summarises results for the
aggregation of a metal oxide particle, with a fixed size and
charge, with ice particles of varying size and charge. These
data show that large ice particles with low charge have the Figure 5. Aggregation probability, presented as percentage, for a
highest probability of coalescence with like-charged metal collision between a SiO2 particle (r2 = 0.2 nm; q2 = −1e) and an
oxide particles. However, in many cases the Coulomb barrier ice particle (q1 = −1e and q1 = −2e) whose size varies from r1 =
1 to 100 nm.
prevents the aggregation of particles with the kinetic ener-
gies typically found in the MLT region (kT = 12.9 meV at
T = 150 K), assuming that thermal motion is the predomi- ence of meteoric smoke in ice particles. Our results also point
nant contribution to velocity. The barrier can be overcome to coagulation rather than condensation as a possible growth
by a small number of high kinetic energy particles found in mechanism. Further studies are, however, required to help us
the tail of the Maxwell–Boltzmann distribution of molecu- understand how the collision probabilities influence the mag-
lar speeds at 150 K. For these particular interactions, the free nitudes of rate coefficients for coagulation.
charge on the surface of both colliding particles is described
by a point charge with the geometry shown in Fig. 3a, and
the change in electrostatic interaction energy is due to a re- 5 Aggregation of metal oxide and silica particles
distribution of bound charge (polarisation effects). Note that,
for ice particles with higher charges, a uniform distribution The abundant presence of metal oxide and silica particles in
of free charge might be more appropriate. As mentioned pre- meteoric smoke in the MLT region (Plane et al., 2015) leads
viously, if the initial relative velocity of the incoming parti- to a possibility that these may also aggregate, and with radii
cles is smaller than vrelmin , the two like-charged particles repel ranging from 0.2 to 5 nm, these are amongst the smallest par-
(case 1 shown in Fig. 1); however, if it is greater than vrel max , ticles found in this region of atmosphere. Their size means
the particles do not coalesce but instead fly apart due to the that if the point charge approximation is used to describe the
residual excess kinetic energy (case 3). Therefore, only col- surface charge, then the exact location of the point charge on
lisions with a relative initial velocity greater than vrel min and the surface of each colliding particle becomes very important
max
smaller than vrel lead to coalescence. In these examples, a because, as shown previously by Filippov et al. (2019), col-
change in the coefficient of restitution would not affect the lision geometry can alter the strength of the electrostatic in-
probability of aggregation as it only reduces vrel max , and values teraction. This statement does not apply to most like-charged
of the latter that fall within the temperature range appropriate interactions because, as shown in Table 3, the height of the
for these calculations have extremely low probabilities. Coulomb barrier prevents very small like-charged particles
Figure 5 shows coalescence results where the size of the (less than 5 nm radius) from aggregating. Note that colli-
ice particle has been increased to 100 nm. These data rein- sions between like-charged silica particles have lower en-
force the fact that, for like-charged collisions, an increase in ergy barriers than those calculated for collisions between
the size of the ice particle from 10 to 100 nm can lead to an iron oxide particles. For collisions involving larger particles
order of magnitude increase in the probability of aggrega- (r1 = 5 nm), despite the lower energy barriers, the minimum
tion. Also given in Fig. 5 are data calculated for a charge of min ) required to overcome the barriers for
initial velocity (vrel
−2e on the ice particle. In this case, the probability of ag- SiO2 are still higher for than those for FeO particles. These
gregation is increased from zero (for r1 < 20 nm) to more effects arise from differences in density and mass.
than 40 % (for r1 ≈ 100 nm), thus providing a mechanism For collisions between charged and neutral particles, the
whereby ice particles can increase their charge, but still par- Coulomb barrier is always zero, and their aggregation is
ticipate in aggregation processes. The results in Table 2 and driven by polarisation effects. Again, orientation of the par-
Fig. 5 demonstrate that there are several routes whereby ice ticles becomes important, and here, two limiting cases are
particles can become contaminated by both neutral and like- considered. Table 3 corresponds to the case where the point
charged MSPs. These calculations on the coalescence of ice charge on the surface of particle 2 faces the neutral parti-
particles and dust are supported by the experimental obser- cle 1 (geometry shown in Fig. 3b, but we now assume that
vations of Hervig et al. (2012), who have identified the pres- particle 1 is neutral). In this configuration, there is strong at-
Atmos. Chem. Phys., 21, 8735–8745, 2021 https://doi.org/10.5194/acp-21-8735-2021J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles 8741
Table 2. Energetic considerations and the percentage of aggregation for SiO2 –ice collisions at T = 150 K and kr = 0.9 (the surface point
charge model). The SiO2 particle has a fixed radius and charge (r2 = 0.2 nm, q2 = −1e), and the size and charge of ice particle is varied.
The collision geometry is shown in Fig. 3a. The interactions of the MgO and FeO particles with ice show the same trend (see Tables A1 and
A2 in the Appendix).
Ice particle Coulomb barrier, min , m s−1
vrel max , m s−1
vrel Aggregation, %
ECoul , meV
r1 = 30 nm; q1 = 0 0 0 1192 100
r1 = 30 nm; q1 = −1e 23.8 293 1219 29.9
r1 = 30 nm; q1 = −2e 55.3 447 1260 3.57
r1 = 20 nm; q1 = 0 0 0 1235 100
r1 = 20 nm; q1 = −1e 35.7 361 1275 13.7
r1 = 20 nm; q1 = −2e 82.9 547 1333 0.50
r1 = 10 nm; q1 = 0 0 0 1251 100
r1 = 10 nm; q1 = −1e 71.3 511 1330 1.15
r1 = 10 nm; q1 = −2e 165.8 780 1441 0
Table 3. Energetic considerations and the percentage of aggregation for SiO2 –SiO2 and FeO–FeO collisions at T = 150 K and kr = 0.9 (the
surface point charge model). Particle 2 has a fixed radius and charge (r2 = 0.2 nm; q2 = −1e), and the size and charge of particle 1 is varied.
The collision geometry is shown in Fig. 3b.
SiO2 –SiO2 Coulomb barrier, min , m s−1
vrel max , m s−1
vrel Aggregation, %
ECoul , meV
r1 = 0.2 nm; q1 = 0 0 0 8112 100
r1 = 1.0 nm; q1 = 0 0 0 3914 100
r1 = 5.0 nm; q1 = 0 0 0 2187 100
r1 = 0.2 nm; q1 = −1e 2889 4566 9168 0
r1 = 1.0 nm; q1 = −1e 622 1504 4156 0
r1 = 5.0 nm; q1 = −1e 125 671 2273 0.02
FeO–FeO
r1 = 0.2 nm; q1 = 0 0 0 2876 100
r1 = 1.0 nm; q1 = 0 0 0 1811 100
r1 = 5.0 nm; q1 = 0 0 0 1307 100
r1 = 0.2 nm; q1 = −1e 3056 3175 4150 0
r1 = 1.0 nm; q1 = −1e 679 1068 2055 0
r1 = 5.0 nm; q1 = −1e 136 476 1376 0.03
traction as the point charge approaches the neutral particle outcome, even though the maximum relative initial velocity
leading to a redistribution (polarisation) of surface charge on of colliding particles required for coalescence is significantly
the latter. This leads to a significant increase in the binding lower. When the charged particle is very small, the interac-
energy between the particles (E0 ) and results in coalescence tion resembles a point charge – a neutral particle case, which
through the subsequent action of the coefficient of restitu- is always attractive. Note that the aggregation remains almost
tion. Irrespective of particle composition, the absence of a complete (100 %) even when both charged and neutral parti-
Coulomb barrier results in aggregation for all of the exam- cles are extremely small (r1 = r2 = 0.2 nm) and highly po-
ples examined in Table 3. larisable (FeO and MgO). In general, there are distinct dif-
The data displayed in Table 4 correspond to the case least ferences between the aggregation outcomes for SiO2 parti-
favourable to aggregation between neutral and charged parti- cles and the more polarisable FeO particles, with the FeO
cles. Here, the point charge on the surface of particle 2 faces collisions consistently having higher percentage aggregation
away from the neutral particle 1 (the geometry is shown in and MgO particles lying somewhere between the two. For
Fig. 3c, but particle 1 is neutral). In this orientation, collisions the geometry shown in Fig. 3c, the aggregation percentage
with the smallest charged particles (r2 = 0.2 nm) strongly drops very significantly as the size of the charged particle 2
favour aggregation often resulting in a 100 % coalescence grows. This is because any surface polarisation response on
https://doi.org/10.5194/acp-21-8735-2021 Atmos. Chem. Phys., 21, 8735–8745, 20218742 J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles
Table 4. Energetic considerations and the percentage of aggregation for SiO2 –SiO2 and FeO–FeO collisions at T = 150 K and kr = 0.9 (the
surface point charge model). Particle 2 has a fixed charge (q2 = −1e), particle 1 is neutral (q1 = 0) and the size of both particles is varied.
The collision geometry is shown in Fig. 3c.
SiO2 –SiO2 FeO–FeO MgO–MgO
max , m s−1
vrel Aggregation,% max , m s−1
vrel Aggregation,% max , m s−1
vrel Aggregation, %
r2 = 0.2 nm; r1 = 0.2 nm 364 58.3 445 96.0 495 93.1
r2 = 0.2 nm; r1 = 1.0 nm 569 99.7 625 100 714 100
r2 = 0.2 nm; r1 = 5.0 nm 737 100 748 100 869 100
r2 = 1.0 nm; r1 = 0.2 nm 34.2 0.29 29.8 0.49 29.3 0.29
r2 = 1.0 nm; r1 = 1.0 nm 14.6 9.75 18.0 36.3 20 30.4
r2 = 1.0 nm; r1 = 5.0 nm 22.8 57.2 25.2 91.4 28.7 88.4
r2 = 5.0 nm; r1 = 0.2 nm 9.00 0.01 0.0∗ 0.0∗ 0.0∗ 0.0∗
r2 = 5.0 nm; r1 = 1.0 nm 1.42 0.02 1.15 0.04 1.24 0.03
r2 = 5.0 nm; r1 = 5.0 nm 0.59 1.01 0.72 4.78 0.80 3.81
∗ Zero within the accuracy of our calculations.
the neutral particle due to the presence of a point charge on and neutral-charged pairs driven by the induced polarisation
the surface of particle 2 is now hindered by the volume of the of the particle surface charge. The latter interactions can be
charged particle itself. Finally, when the charged particle is significant at short separation distances between interacting
large and the neutral one is very small, surface polarisation particles. Low temperatures in the MLT region imply that
effects on the neutral particle are negligible and aggregation the colliding particles are not very energetic, and for a like-
does not occur. This can be illustrated by comparing the fol- charged pair, the relative kinetic energy is often insufficient
lowing two examples: if r2 /r1 = 10 (the radius of the charged to overcome the Coulomb barrier. However, the high energy
particle is 10 times bigger than that of neutral particle), then tail of the Maxwell–Boltzmann distribution of the relative
the aggregation is 0 %, and if r1 /r2 = 10 (the radius of the velocity at T = 150 K provides an adequate amount of colli-
neutral particle is 10 times bigger than that of charged parti- sions leading to aggregation both between like-charged parti-
cle), then the aggregation is 100 % (Table 4). cles of ice and dust and between dust particulates themselves.
Finally, if the results given in Tables 3 and 4 for the per- The like-charged attraction is more common (and
centage of aggregation are compared, it can be seen that there stronger) between particles with low charge. This collision
are differences that depend on how the point charges are ori- scenario can be described by a localised, point surface charge
entated on these particles, all of which have comparatively model and one where the charge is assumed to be uniformly
low dielectric constants. In all instances where a charge is distributed over the entire surface of a particle. An earlier
pointing towards a large polarisable particle (Table 3; when study by Filippov et al. (2019) of the interaction between
q1 = 0 and q2 = −1e), the aggregation is 100 %. However, positively charged particles, showed that, for particles with
when in Table 4 the charge is located at 180◦ from the adja- low dielectric constants, there is a difference in the predicted
cent particle (case 3c in Fig. 3), the aggregation drops to 58 % behaviour between these two models. As the dielectric con-
when, in the least polarisable particle pair, SiO2 , the neutral stant increased in value, results from the two models became
particle has a radius of 0.2 nm. As the dielectric constant in- equivalent. Similarly, differences in orientational geometry
creases on moving to MgO and FeO, the particles become of a collision (extreme scenarios are shown in Fig. 3b and c)
more polarisable and the percentage aggregation increases. were also found to be evident at low dielectric constants, but
again, these disappeared as the value of the dielectric con-
stant increased. The presented results provide a basis for fu-
6 Brief discussion of main results and conclusions ture work to estimate the coagulation rates between particles
of a given size and charge and their variation with tempera-
This work is focused on the description of the basic prin-
ture.
ciples underpinning the coalescence of ice and dust particles
in thermal motion. Specific examples considered in this study
examine the coalescence between particles, commonly found
in the mesosphere, at the temperature T = 150 K, which is
typical of this region of the atmosphere. Pair interactions of
charged particulates follow the Coulomb law with an addi-
tional contribution from the attraction between like-charged
Atmos. Chem. Phys., 21, 8735–8745, 2021 https://doi.org/10.5194/acp-21-8735-2021J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles 8743
Appendix A
Table A1. Energetic considerations and the percentage of aggregation for FeO–ice collisions at T = 150 K and kr = 0.9 (the surface point
charge model). The FeO particle has the fixed radius and charge (r2 = 0.2 nm, q2 = −1e), and the size and charge of the ice particle is varied.
The collision geometry is shown in Fig. 3a.
Ice particle Coulomb barrier, min , m s−1
vrel max , m s−1
vrel Aggregation, %
ECoul , meV
r1 = 30 nm; q1 = 0 0 0 1007 100
r1 = 30 nm; q1 = −1e 23.7 199 987 34.7
r1 = 30 nm; q1 = −2e 55.3 303 1012 5.2
r1 = 20 nm; q1 = 0 0 0 1094 100
r1 = 20 nm; q1 = −1e 35.7 244 1059 17.4
r1 = 20 nm; q1 = −2e 82.9 372 1092 0.91
r1 = 10 nm; q1 = 0 0 0 1267 100
r1 = 10 nm; q1 = −1e 71.3 345 1165 1.91
r1 = 10 nm; q1 = −2e 165.9 526 1225 0
Table A2. Energetic considerations and the percentage of aggregation for MgO–ice collisions at T = 150 K and kr = 0.9 (the surface point
charge model). The MgO particle has the fixed radius and charge (r2 = 0.2 nm, q2 = −1e), and the size and charge of ice particle is varied.
The collision geometry is shown in Fig. 3a.
Ice particle Coulomb barrier, min , m s−1
vrel max , m s−1
vrel Aggregation, %
ECoul , meV
r1 = 30 nm; q1 = 0 0 0 1341 100
r1 = 30 nm; q1 = −1e 23.7 252 1311 29.9
r1 = 30 nm; q1 = −2e 55.3 384 1340 3.57
r1 = 20 nm; q1 = 0 0 0 1481 100
r1 = 20 nm; q1 = −1e 35.7 309 1425 13.7
r1 = 20 nm; q1 = −2e 82.9 470 1465 0.50
r1 = 10 nm; q1 = 0 0 0 1776 100
r1 = 10 nm; q1 = −1e 71.3 436 1607 1.15
r1 = 10 nm; q1 = −2e 165.9 665 1676 0
https://doi.org/10.5194/acp-21-8735-2021 Atmos. Chem. Phys., 21, 8735–8745, 20218744 J. Baptiste et al.: The influence of surface charge on the coalescence of ice and dust particles
Data availability. All data are provided in the paper and Ap- Bichoutskaia, E., Boatwright, A. L., Khachatourian, A., and
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have revised the paper. EB supervised the research. 2017.
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Competing interests. The authors declare that they have no conflict Chem. Phys., 19, 2871–2879, https://doi.org/10.5194/acp-19-
of interest. 2871-2019, 2019.
Filippov, A. V., Chen, X., Harris, C., Stace, A. J., and
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Acknowledgements. We acknowledge the International Space Sci-
of dication molecular clusters, J. Chem. Phys., 151, 154113,
ence Institute (ISSI) Bern, Switzerland, that supported the team, led
https://doi.org/10.1063/1.5119347, 2019.
by Elena Besley, through a project titled “Electrostatic Manipula-
Gumbel, J., Siskind, D. E., Witt, G., Torkar, K. M., and
tion of Nano-Scale Objects in Planetary Environments”. This work
Friedrich, M.: Influences of ice particles on the ion chemistry
benefitted from discussions during our stays at ISSI in Bern (2018–
of the polar summer mesosphere, J. Geophys. Res.-Atmos., 108,
2019), and we thank the ISSI staff for their hospitality during our
8436, https://doi.org/10.1029/2002JD002413, 2003.
visits. Ingrid Mann has been supported by the Research Council of
Hedin, J., Giovane, F., Waldemarsson, T., Gumbel, J., Blum, J.,
Norway (grant no. 275503). Elena Besley acknowledges a Royal
Stroud, R. M., Marlin, L., Moser, J., Siskind, D. E., Jansson, K.,
Society Wolfson Fellowship for financial support. Anthony J. Stace
Saunders, R. W., Summers, M. E., Reissaus, P., Stegman, J.,
would like to thank the Leverhulme Trust for the award of an Emer-
Plane, J. M. C., and Horányi, M.: The MAGIC meteoric
itus Fellowship.
smoke particle sampler, J. Atmos. Sol.-Terr. Phy., 118, 127–144,
https://doi.org/10.1016/j.jastp.2014.03.003, 2014.
Hervig, M., Thompson, R. E., McHugh, M., Gordley, L. L., Russell
Financial support. This research has been supported by the Re- III, J. M., and Summers, M. E.: First confirmation that water ice
search Council of Norway (grant no. 275503) and a Royal Society is the primary component of polar mesospheric clouds, Geophys.
Wolfson Fellowship. Res. Lett., 28, 971–974, https://doi.org/10.1029/2000GL012104,
2001.
Hervig, M. E., Deaver, L. E., Bardeen, C. G., Russell, J. M.,
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