ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-4885-2018Clear-air lidar dark bandClear-air lidar dark bandDi GirolamoPaolodigirolamo@unibas.itScoccioneAndreahttps://orcid.org/0000-0003-4137-700XCaccianiMarcoSummaDonatohttps://orcid.org/0000-0002-0867-4144De RosaBenedettoSchweenJan H.https://orcid.org/0000-0001-6686-1207Scuola di Ingegneria, Università degli Studi della Basilicata, Potenza, ItalyDipartimento di Fisica, Università di Roma “La Sapienza”, Rome, ItalyInstitut fuer Geophysik und Meteorologie, Universität zu Köln, Cologne, GermanyPaolo Di Girolamo (digirolamo@unibas.it)10April20181874885489617October201721November20176March20189March2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/4885/2018/acp-18-4885-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/4885/2018/acp-18-4885-2018.pdf
This paper illustrates measurements carried out by the
Raman lidar BASIL in the frame of the HD(CP)2 Observational Prototype
Experiment (HOPE), revealing the presence of a clear-air dark band phenomenon
(i.e. a minimum in lidar backscatter echoes) in the upper portion of the
convective boundary layer. The phenomenon is clearly distinguishable in the
lidar backscatter echoes at 532 and 1064 nm, as well as in the particle
depolarisation data. This phenomenon is attributed to the presence of lignite
aerosol particles advected from the surrounding open pit mines in the
vicinity of the measuring site. The paper provides evidence of the phenomenon
and illustrates possible interpretations for its occurrence.
Locations of the Raman lidar system BASIL within the Jülich
Research Centre (pink dot). The figure also indicates the location of the
Hambach open-pit lignite mine (approximately 3 km East of the
Research Centre), the large artificial hill Sophienhöhe (both within the
angle cone 29–114∘, red shaded area) and a second open-pit lignite
mine (approximately 3 km south-west of the research centre, within the angle
cone 180–240∘, blue shaded area).
Introduction
In the frame of the HD(CP)2 Observational Prototype Experiment (HOPE), the
Raman lidar system BASIL was deployed and operated over a two-month period
(April–May 2013) in the Atmospheric Supersite JOYCE, located within the
Jülich Research Centre. This site is approximately 3 km West of the
Hambach open-pit lignite mine, which represents the largest operational lignite
mine on Earth, with a maximum depth of ∼ 200 m (Fig. 1). The
dump of this mine forms a large artificial hill, called Sophienhöhe, which reaches 302 m and is partially re-cultivated with forest. A second open-pit lignite
mine, named the Inden mine, is located approximately 3 km south-west of the Supersite JOYCE.
The Hambach and Inden mines lie in the sectors 29–114∘ (red shaded area in
Fig. 1) and 180–240∘ (blue shaded area in Fig. 1),
respectively, relative to the location of the Raman lidar. When wind blows
from these directions, lignite particles from the two open-pit mines or the
surrounding hill are lifted up from the ground and transported over the
lidar site, with appreciable effects on the measurements.
Evidence of this particle transportation was found in lidar elastic
backscatter echoes in a variety of case studies during HOPE, with the
appearance of a specific odd feature in the upper portion of the convective
boundary layer (CBL). Specifically, a minimum in lidar backscatter echoes at
532 and 1064 nm, with a backscatter reduction of approximately 10 % is
observed. This feature is found to have a vertical extent of approximately
100 m and persist over a period of several hours, with an alternation of
intensifications and attenuations of the phenomenon. Similar features with a
comparable temporal duration and backscatter reduction had been reported by
Sassen and Chen (1995) in the presence of light precipitation events; this
phenomenon, referred to as lidar dark band, was demonstrated to be ascribable to changes in
scattering properties of precipitating particles taking place during the
snowflake-to-raindrop transition in the proximity of the melting level
(Sassen et al., 2005; Demoz et al., 2000; Di Girolamo et al., 2003, 2012b).
Instead, the phenomenon reported in the present research effort appears in
clear-air conditions and in the presence of strong convective activity
within the boundary layer: we will refer to it in the following as the
clear-air dark band phenomenon or the convective dark band phenomenon. In
the following, we provide experimental evidence of this
phenomenon and a possible physical interpretation for its occurrence.
The outline of the paper is as follows: Sect. 2 provides a description
of the experimental set-up and a brief overview of the HOPE field campaign.
Sect. 3 illustrates the measurements collected for a selected case study,
providing remarks on the meteorological conditions occurring during these
periods. Section 4 illustrates the hygroscopic and scattering properties of
the sounded particles, while Sect. 5 formulates possible hypotheses for
the interpretation of the observed phenomena. Finally, Sect. 6 summarises
all results and provides some indications for possible future measurements
and analysis.
BASIL and the HOPE field campaign
The University of Basilicata Raman lidar system (BASIL) is a ground-based
Raman lidar hosted in a transportable “seatainer”. BASIL performs
high-resolution and accurate measurements of the vertical profiles of
atmospheric temperature and water vapour, both in the daytime and at
night-time, exploiting both the rotational and vibrational Raman lidar
techniques in the UV (Di Girolamo et al., 2004, 2006, 2009a, 2016; Bhawar et al.,
2011). Aside from temperature and water vapour, BASIL also measures the
vertical profiles of particle backscatter at 354.7, 532 and 1064 nm,
as well as particle extinction and depolarisation at 354.7 and 532 nm (Griaznov et al.,
2007; Di Girolamo et al., 2009b, 2012a, b). BASIL makes use of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser
source, equipped with second and third harmonic generation crystals, which
emits pulses at 354.7, 532 and 1064 nm. The receiver is built around a large
aperture Newtonian telescope (primary mirror diameter: 0.45 m, focal length:
2.1 m) and two small-aperture telescopes (50 mm diameter lenses). The large
aperture receiver incorporates eight channels for the detection of eight
different signals (primarily Raman lidar signals), while the two
small-aperture receivers include another three measurement channels for the
detection of additional lidar signals. These eleven detected signals allow
for the
determination of the atmospheric variables listed above, plus additional
ancillary parameters as the atmospheric boundary layer depth and the
geometric (cloud base and top height, the latter in case of optically thin
clouds) and optical (cloud optical depth for optically thin clouds)
properties of clouds. More details on the experimental set-up of the system
are provided in Di Girolamo et al. (2009a, 2017).
In this paper we illustrate measurements carried out in the frame of the
High-Definition Clouds and Precipitation for advancing Climate Prediction
(HD(CP)2) Observational Prototype Experiment (HOPE, Macke et al., 2017). For the
purposes of HOPE, BASIL was deployed in the Supersite JOYCE, located within
the Jülich Research Centre (Central Germany, Lat.: 50∘54′ N;
Long.: 6∘24′ E, Elev. 105 m). The system operated between 25 March and 31 May 2013, collecting more than 430 h of measurements
distributed over 44 days and 18 Intensive Operation Periods (IOPs).
Time–height cross-section of β1064(a) and the
vertical wind speed (b) in the time interval 12:00–13:00 UTC on
18 April 2013. The black dashed line in panel (a) around 1200 m
highlights the presence of a persistent lidar backscatter reduction
(clear-air dark band), with alternating intensity fluctuations. The red and
blue areas in between the two plots indicate the up-draught and down-draught time
intervals, respectively, identified in Fig. 6.
Results
The weather at the lidar site in Jülich on 18 April 2013 was
characterised by the presence of clear sky conditions in the morning until
06:00 UTC and by the passage of a cold front shortly afterwards. The passage of
the cold front was followed by a circulation change from a south-westerly to
west/north-westerly marine flow, with the sky clearing up in the late morning
and the onset of a strong convective activity. Boundary layer clouds were
found to form in the late morning and early afternoon, while broken cirrus
clouds were observed throughout the day.
Figure 2 illustrates the time–height cross-section of the particle
backscattering coefficient at 1064 nm, β1064, as measured by BASIL
(Fig. 2a), as well as the vertical wind speed, as measured by the University of
Cologne wind lidar (Fig. 2b), in the time interval 12:00–13:00 UTC on
18 April 2013. For the purpose of these measurements, the two lidars were
located within a distance of ∼ 80 m. Figure 2a clearly reveals the
presence of a significant aerosol loading within the boundary layer, which is
tracing the presence of a well-mixed and quasi-stationary CBL at this time of
the day, extending up to approximately 2000 m. The figure also reveals the
presence of alternating up-draughts and down-draughts. The largest variability of
β1064 is observed in the interfacial layer, as a result of the
penetration of aerosol-rich air rising from the ground and the entrainment of
aerosol-poor air sinking from the free troposphere.
A persistent minimum in lidar backscatter is observed around 1200 m (black
dashed line) with alternating intensity fluctuations. This minimum persists
albeit the clear presence of up-draughts (orange eddies, with positive
vertical wind speed values) and down-draughts (blue eddies, with negative
vertical wind speed values), and thus cannot be related to an aerosol
layered structure in the mixing layer. Note that the backscatter minimum
occurs preferably during up-draughts but not during down-draughts. This
behaviour is clearly highlighted in Fig. 2, where the black dashed lines
indicate that the lidar backscatter minima only appear in temporal coincidence
with the vigorous up-draughts, which are testified by the positive vertical
speed values (up to 2–3 m s-1) measured by the wind lidar, but do not
appear in coincidence with the down-draughts (negative vertical speed values
down to -2–3 m s-1). The presence of a persistent minimum in lidar
backscatter at 1200 m, preferably during up-draughts, is also clearly visible in
the particle backscattering coefficient data at 532 nm (not shown here).
While we are concentrating on the time interval 12:00–13:00 UTC on 18 April
2013, additional evidence of this phenomenon was observed earlier and later
in the day (i.e. 11:25–11:55, 13:25–13:40, 13:50–14:05, 14:15–14:25, 14:35–15:00 UTC).
The clear-air dark band phenomenon was also visible on other days (i.e. 20 April 2013) during HOPE, when the wind was blowing from directions
overpassing the Hambach and Inden mines.
Figure 3 illustrates the vertical profile of β1064 for the time
interval 12:56:41–13:00:45 UTC on 18 April 2013 (4 min average, green line),
together with the vertical profiles of temperature, relative humidity (RH)
and wind direction and speed, as measured by the radiosonde launched at
13:00 UTC from the nearby station of Hambach (4 km E-SE). The clear-air
dark band is found to extend from 1150 to 1275 m with a vertical extension
of 125 m and a minimum in particle backscattering at 1225 m (backscatter
reduction is 8 %, corresponding to 0.4 dB). This band takes place few
hundred metres below both the lifting condensation level (LCL, at 1725 m or
814 mbar) and the freezing level (at 1630 m or 823.2 mbar). The figure
reveals that wind is blowing from directions in the interval from
265∘ (at surface) to 232∘. More specifically, the
particle backscattering reduction is located in the same height region
(1125–1450 m) where wind is found to blow from directions in the interval
232–240∘, i.e. from the directions where the Tagebau Inden
open-pit lignite mine is located. In general, CBL wind direction
measurements by radiosondes may be difficult to interpret as they may
reflect rotations taking place within the convective plumes. However, wind
direction values (236–242∘) similar to those measured by the
radiosonde are also present in the same height interval in the 1 h
(12:00–13:00 UTC) average wind direction profile measured by the wind lidar
(Fig. 4) with values throughout the whole profile from 270∘ (at
surface) to 230∘ (at 1600 m).
Vertical profile of β1064 at 13:00 UTC on 18 April 2013
(12:56:41–13:00:45 UTC, green line), together with the vertical profiles of
temperature (blue line), RH (black line), wind direction (red line) and
speed (purple line) as measured by the radiosonde launched at 13:00 UTC from
the nearby station of Hambach (4 km E-SE).
Figure 3 also reveals that the air at this height is characterised by RH
values in the range 60–62 %. Lignite particles advected by the wind over
the lidar site are captured and ingested within the up-draughts and down-draughts
associated with the intensive convective activity present at the lidar site.
As a result of the adiabatic cooling associated with the uplift, air parcels
undergo a sudden RH increase from values in the range of 60–62 %
(environmental RH values at the base of the dark band) to values in the
range of 75–80 % (these being the values reached within the lifting
air parcel assuming an ideal adiabatic cooling with no air entrainment into
the convective plumes or external air ingestion within the lifting
air parcel). This sudden increase of RH has important effects on the size
growth of the uplifted lignite aerosols.
Vertical profile of wind speed and direction averaged over the time
interval 12:00–13:00 UTC on 18 April 2013, as measured by the wind lidar
located in the proximity of BASIL at the Supersite JOYCE. Profiles are
reported with error bars, corresponding to ±1 standard deviation.
Time–height cross-section of particle depolarisation at 532 nm in
the time interval 12:00–13:00 UTC on 18 April 2013.
Figure 5 illustrates the time–height cross-section of the particle
depolarisation ratio at 532 nm, δ532, as measured by BASIL in
the time interval 12:00–13:00 UTC on 18 April 2013, i.e. the same time
interval considered in Fig. 2. Particle depolarisation ratio, defined as
the power ratio of the cross-polarised to the co-polarised components of the
particle backscattering coefficient, provides an indication of the degree of
asphericity of sounded particles. Particle depolarisation depends not only
on particles' shape, but also on their size and refractive index (among
other, Burton et al., 2015). Water-coated aerosols, wet haze, fog, cloud
droplets and small raindrops can be assumed to be almost spherical and are
characterised by very small values of δ532, typically not
exceeding 0.03. Low depolarising particles, usually smoke or urban aerosol,
have depolarisation ratios between 0.03 and 0.1 (e.g. Burton et al., 2012),
while high depolarising particles, such as desert or volcanic dust, have
depolarisation ratios that vary between 0.25 and 0.35 (e.g. Mona et
al., 2012).
A proper calibration of particle depolarisation measurements requires
accurate measurements of the cross-polarised and co-polarised components of the
particle backscattering coefficient. However, accurate measurements of these
quantities may be difficult to obtain, often as a result of the depolarising
properties of different optical devices included in the receiver
(Freudenthaler, 2016). This translates into a non-negligible uncertainty
affecting particle depolarisation measurements, which includes both a
systematic component (bias) and a random component (statistical error). For
the present lidar system, these two components were estimated to be 10 % and
20 %, respectively (Di Girolamo et al., 2012a).
Vertical profiles of β1064, δ532 and RH (as
measured by BASIL) and the wind direction and vertical wind speed (as measured
by the wind lidar) for eight consecutive up-draughts and down-draughts time intervals
during the time period 12:00–13:00 UTC on 18 April 2013:
(a) 12:01:34–12:04:37 UTC, (b) 12:04:37–12:06:39 UTC,
(c) 12:21:01–12:23:03 UTC, (d) 12:23:03–12:24:04 UTC,
(e) 12:35:17–12:36:18 UTC, (f) 12:36:18–12:37:19 UTC,
(g) 12:56:41–13:00:45 UTC and (h) 12:55:39–12:56:41 UTC).
Green-dashed ellipses highlight dark band features during the up-draught
intervals. The up-draught and down-draught time intervals considered in the
present figure are identified in Fig. 2 with red and blue areas.
Figure 5 reveals a decrease in particle depolarisation at the same height
and time intervals of the dark band. More specifically, δ532 decreases from values of 0.05–0.07 below the dark band to values of
0.02–0.03 within and above the dark band. A decrease of δ532 within and above the dark band is compatible with the conjectured size
growth of the uplifted dry lignite particles, initially having a more
irregular shape, and then getting a more regular spherical shape as a result
of the water uptake. Additionally, as previously observed for β1064, the decrease of δ532 occurs during up-draughts, but
not during down-draughts, as these latter values of δ532 are in the range of 0.02–0.04 both below and within the dark band.
However, both below and within the dark band, values of δ532 are rather low, which is typical of aerosols including a large portion of
carbonaceous species as those resulting from fossil fuel combustion that
have
a rather spherical shape (Dieudonné et al., 2017; Müller et al., 2007).
Particle depolarisation ratio measurements, while providing some information
on particle shape, may also be used for aerosol typing and mass
concentration studies (among others, Petzold, 2011; Burton et al., 2012).
The presence of the clear-air dark band phenomenon during
up-draughts is also well documented in Fig. 6, as it illustrates the simultaneous
vertical profiles of β1064, δ532 and RH as measured by
BASIL, and the wind direction and vertical wind speed as measured by the wind
lidar for a number of consecutive up-draught and down-draught time intervals.
Sharp lidar backscatter minima are only observed around 1200 m in temporal
coincidence with positive vertical speed values (Fig. 6a, c, e, g), while
slowly variable backscatter values are observed at these heights in temporal
coincidence with negative vertical speed values (Fig. 6b, d, f, h). Wind
direction values in the time intervals and vertical regions characterised by
the presence of backscatter minima are very similar to those observed in
these vertical regions during the down-draught periods. This observation
supports the hypothesis that the observed backscatter minima are not caused
by the presence and sounding of different types of particles that might
originate from different aerosol sources, as sounded air masses are
coming from the same direction both during up-draughts and down-draughts. However,
wind direction measurements by Doppler wind lidar
require a minimum integration time of 5 min, as in fact a number of
off-zenith measurements are needed to determine the horizontal wind
component. This implies that a perfect time matching between BASIL
measurements of β1064 and RH and wind lidar measurements of wind
direction was not possible in Fig. 6, as in fact the integration time for
BASIL measurements was taken as coincident (within 10 s, which is the
maximum time resolution for BASIL measurements) with the duration of the
up-draughts and down-draughts, typically lasting 1–2 min, while the
5 min integration time for wind direction measurements may superimpose to
consecutive up-draughts and down-draughts. Additionally, the approach of
determining
wind directions by Doppler wind lidar measurements is affected by a large
uncertainty (typically around 25∘ in the vertical regions
characterised by the presence of backscatter minima).
Similar considerations apply for RH measurements. Accounting for the error affecting these measurements, RH values, observed by BASIL in the altitude
region where backscatter minima take place, are very similar when sounded
during the up-draught and down-draught, which would support the hypothesis of the
presence of a reversal (evaporation) process in the down-draughts, which instead
is not observed. Vertical profiles of RH are
obtained from water vapour mixing ratio and temperature profile measurements
by BASIL, which are based on the application of the vibrational and
rotational Raman lidar technique, respectively. Both techniques rely on Raman
backscatter phenomena characterised by cross-sections that are several
orders of magnitude smaller than the elastic backscatter cross-section. This
makes the water vapour mixing ratio and temperature measurements, and
consequently RH measurements, very difficult to perform, especially in
daytime around noon, as is the case for the measurements illustrated in this
paper, as a result of the large solar irradiance affecting the measurements
during this portion of the day. This translates into a large statistical
uncertainty affecting RH measurements with a random error of 4–8 %
(error bars in Fig. 6) in the altitude region (∼ 1200 m) where the
particle backscatter minima are observed.
Clear-air dark bands were mostly
observed in the absence of a cloud topped CBL. However, few clouds were
observed for this specific case study in the upper portion of the CBL at
12:53–13:00 UTC (orange-brown features in Fig. 2a and strong backscattering
enhancement observed above 1600 m in Fig. 3). The occurrence of these clouds
is discussed in more detail in the final portion of Sect. 4.
Hygroscopic and scattering behaviour of lignite particles
Aerosol particles can be classified according to their affinity for water as
hygroscopic, neutral or hydrophobic. The characterisation of particle
hygroscopicity is of primary importance in climate monitoring and
prediction. Model studies have demonstrated that RH has a critical influence
on aerosol climate forcing (Pilinis et al., 1995), with hygroscopic growth at
large RH values having important implications in terms of aerosol direct
effect (Wulfmeyer and Feingold, 2000).
Lignite, often referred to as brown coal, is a combustible sedimentary
rock formed from naturally compressed peat with a carbon content around
60–70 %. The high moisture content of lignite (approximately 50–60 %)
is an undesirable inert component, which significantly reduces its calorific
value. Consequently, when employed in conventional power plants, a
considerable portion of lignite's energy content is typically required prior
to combustion to evaporate this large portion of water. For this reason,
following the mining process, raw lignite usually undergoes effective drying
processes. This is indeed the case for the two open-pit lignite mines of
Hambach and Inden in the proximity of the lidar station, where a drying process based on
the pulverisation of the lignite particles is applied.
Dried lignite particles produced in open pit lignite mines have a very
marked hygroscopic behaviour (Schobert, 1995; Krawczykowska and
Marciniak-Kowalska, 2012) and, as a result of this behaviour, effectively
absorb moisture from the atmosphere. Measurements of the particle size
distribution of lignite particles escaped from heavy industrialised areas
(mining and power stations operations) in the form of fly ash or fugitive
dust have been reported by several authors (among others, Triantafyllou et al.,
2006; Civiš and Hovorka, 2010). Specifically, Triantafyllou
et al. (2006) were able to measure the particle size distribution of fly ash
injected into the atmosphere from elevated stacks in power stations,
thus identifying a prominent particle mode at ∼ 8 µm, with
approximately 80 % of the particles smaller than 10 µm. Civiš and
Hovorka (2010) reported size distribution measurements for brown coal with
an average particle size of 1.84 µm. All of these authors revealed a
limited degree of poly-dispersion of atmospheric lignite particles. When
considering a log-normal size distribution, the degree of poly-dispersion or
width of the particle size distribution is expressed in terms of the
percentage standard deviation of the logarithm of the distribution,
σ. Narrow size distributions for brown coal particles, with values
of σin the interval 5–10 %, have been reported by a variety of
authors (Mujuru et al., 2009; Civiš and Hovorka, 2010; Wang and Tichenor,
1981).
The solution effect typically dominates hygroscopic particles' growth when
the radius is small (smaller than the critical radius rc), which
results in small solution droplets being in equilibrium with water vapour at
RH values less than 100 % (Yau and Rogers, 1989). At this stage, small
increases in RH determine particles' size growth until equilibrium is newly
reached. This mechanism is possibly responsible for the lignite particle
growth below the LCL, ultimately leading to the appearance of a minimum in
lidar backscatter echoes (i.e. the above mentioned clear-air dark band
phenomenon). The increase in particles' radius associated with the relative
humidity change experienced by the adiabatically uplifted air parcel can be
estimated based on the application of the Köhler equation. When RH
values are smaller than 100 %, the Köhler equation is dominated by
the solution term, which depends on the mass and molecular weight of the
solute species and the so called van't Hoff factor. Based on literature
values of these quantities, the above specified increase of RH from 60–62 to
75–80 % would result in a particle size growth in radius by 10–20 %.
In this study, we are considering an initial size for the dry lignite
particles of 1.84 and 8 µm, as reported by Civiš and Hovorka (2010)
and Triantafyllou et al. (2006), respectively.
Simulations of the single-particle backscattering coefficient as a function of
particle radius for lignite particles. Simulations consider a log-normal size
distribution with a percentage standard deviation of 5 %.
(a) Selection of a minimum radius of 1.84 µm, as given by
Civiš and Hovorka (2010) and a maximum radius of 10 µm;
(b) selection of a minimum radius of 8 µm, as given by
Triantafyllou et al. (2006) and a maximum radius of 50 µm. Both
simulations consider a sounding wavelength of 1.064 µm.
Scattering properties of lignite particles have been simulated based on the
application of a light scattering code for spheres based on Mie theory
(http://philiplaven.com/mieplot.htm, last access: 28 December 2017). In this respect, the small values
of δ532 characterising the observed aerosol particles call for a
very limited degree of asphericity, which makes Mie theory still successfully
applicable for the simulation of particles' scattering properties (Martin,
1993; Mishchenko and Lacis, 2003). In order to properly simulate the
scattering processes, accurate information on particle refractive index are
required, beyond those on particle size distribution already provided above.
Accurate measurements of lignite refractive index were reported by Lohi et
al. (1992), who observed values of the real and imaginary part of the
complex refractive index of 1.70 and 1 × 10-6, respectively.
Similar values were reported by McCartney and Ergun (1962) and
Read (2008). Simulations of the scattering properties of lignite particles
are illustrated in Fig. 7. The figure shows the variability of the quantity
Qback×r2 as a function of r, with Qback being the
backscattering efficiency and r the particle radius. These
simulations are obtained by considering a log-normal size distribution with a
value of σ of 5 %. The simulation in Fig. 7a considers a minimum
radius of 1.84 µm, as measured by Civiš and Hovorka (2010),
while the simulation in Fig. 7b considers a minimum radius of 8 µm,
as measured by Triantafyllou et al. (2006). Both simulations consider a
sounding wavelength of 1.064 µm, which is the laser wavelength used
for the dark band lidar measurements illustrated in Fig. 2. The quantity
Qback×r2 represents the single-particle backscattering
coefficient, assuming a constant particle number density n. Figure 7
reveals the presence of marked oscillations in particle backscattering
efficiency. As a result of these oscillations, for specific radius values of
the dry lignite particles (for example, 6.5, 7.5, 18, 28.5 or 41 µm),
a reduction in Qback×r2 of 8–27 % (0.35–1.4 dB)
is observed for a particle size growth by 10–16 %, which is compatible
with the size growth experienced by these particles during their adiabatic
ascent. Thus, we believe that the observed dark band phenomenon is associated
with the oscillations in the particle backscattering coefficient, ultimately
leading to Mie back-scattered signal intensity fluctuations. These
backscattering coefficient oscillations are to be attributed to the limited
degree of poly-dispersion of atmospheric lignite particles. It is to be
specified that these oscillations smooth down and finally disappear in the
case of
larger values of σ, thus wider particle size distributions
(distributions with a higher degree of poly-dispersion) are considered. This
is clearly highlighted in Fig. 8, which illustrates the simulated values of
Qback for lignite particles as a function of particle radius,
considering a log-normal size distribution with different values of σ
(0.1, 5, 10 and 20 %) again considering a sounding wavelength of
1.064 µm. As for Fig. 7, the simulation in panel (a) considers a
minimum radius of 1.84 µm, as measured by Civiš and
Hovorka (2010), while the simulation in panel (b) considers a minimum radius
of 8 µm, as measured by Triantafyllou et al. (2006). The figure
clearly reveals that both in the smaller and larger particle domains, the
consideration of progressively larger values of σ leads to a
progressive smearing down of the Qback oscillations, which are
still present for values of σ≤10 % but are almost absent for
σ=20 %.
Simulations of the backscattering efficiency at 1064 nm as a
function of particle radius for lignite particles, considering a log-normal
size distribution with values of σ equal to 0.1, 5, 10 and 20 %.
(a) Selection of a minimum radius of 1.84 µm, as given by
Civiš and Hovorka (2010) and a maximum radius of 10 µm;
(b) selection of a minimum radius of 8 µm, as given by
Triantafyllou et al. (2006) and a maximum radius of 100 µm. Both
simulations consider a sounding wavelength of 1.064 µm.
An additional quantity, namely the backscatter colour ratio, BCR, i.e. the
ratio of total backscattering coefficients at 1064 and 532 nm, was
determined from BASIL measurements. Colour ratio profiles measured during the
time interval considered in the present study (12:00–13:00 UTC on 18 April
2013, not shown here) indicate values in the range 0.40–0.45 below the dark
band and in the range 0.33–0.36 within the dark band region. The colour ratio
decrease is an indication of the increase of particle size. This represents
an additional experimental evidence of the conjectured particles' growth,
which represents the basis of the given interpretation of the observed
phenomenon. Furthermore, small backscatter colour ratio values, as those found
both below and within the dark band, are indicating relatively large
particles (Burton et al., 2013), compatible with those conjectured in the
present study and presently considered in our simulations. The variability of
backscatter colour ratio as a function of particle radius has been simulated
with the same Mie scattering code already used above, with simulations
revealing that values of BCR in the range of 0.33–0.45 are compatible with
particle size in the range of 7–11 µm. Finally, backscatter colour
ratio values in the range of 0.33–0.45 combined with values of δ532
in the range of 0.02–0.07 are in agreement with previously observed values of
these quantities as reported by a variety of authors (de Villiers et al.,
2010:
BCR = 0.3–0.5 and δ532= 0.02–0.08; Burton et al.,
2014: BCR = 0.55 and δ532= 0.07; Burton et al., 2015:
BCR = 0.47 and δ532= 0.06–0.09). Similar values
(BCR = 0.35–0.54 and δ532 < 0.05) were also
reported by Franke et al. (2003) and Müller et al. (2007) for
Southeast Asian aerosols, which were argued to possess a pronounced coarse
mode with large particles originating mainly from coal and dried plants
used for domestic heating and cooking (Müller et al., 2007).
The comparison of simulated values of single-particle backscattering
coefficient Qback×r2
(∼ 3 × 10-11 m2 sr-1 for a particle radius of
4 µm and ∼ 1 × 10-9 m2 sr-1 for a
particle radius of 20 µm) with measured values of the volume
backscattering coefficient β1064 (in Fig. 6, in the range
2.5–3.5 × 10-6 m-1 sr-1 within the dark band) leads
to an estimate of particle number density n of 0.8–1.2 × 105
and 2.5–3.5 × 103 m-3 in the small and large particles'
domain, respectively. These values of n are in agreement with literature
values for continental and urban polluted aerosols (e.g.
Mészáros, 1991; 0.8–3.5 × 105 m-3 for a particle
radius of 4 µm and 1–2 × 103 m-3 for a
particle radius of 20 µm).
The solution effect growth of particles to equilibrium size associated with
increasing RH can be continued up to a RH value of 100 % and slightly
beyond. Cloud formation at the top of the CBL will finally take place above
the LCL if the critical saturation ratio, Sc, corresponding to
the peak of the Koehler curve, is reached. Sc is typically
reached for super-saturation values of 0.5–1 %, depending on the
composition (and consequently the level of hygroscopicity) and size of the
aerosol particle acting as condensation nuclei. In the case of lignite
particles, typical values of Sc and of the critical radius,
rc, are in the range 0.5–1 % and 1–10 µm,
respectively. Up to this point RH had to be increased in order for the
droplet to grow. However, if RH slightly exceeds Sc, the particle
is enabled to grow beyond rc and its saturation ratio falls below
Sc. As a consequence, the water vapour condensates on the
droplet, which will continue to grow without the need for a further increase
in saturation ratio (Yau and Rogers, 1989). When this occurs, clouds can form
on the top of the CBL. These processes are responsible for the clouds
observed in the upper portion of the CBL at 12:53–13:00 UTC (orange-brown
features in Fig. 2a). In the clouds, the droplet growth process does not
continue indefinitely as many droplets are present and all of them compete
for the same available water vapour.
Discussion of the observed phenomena
Raman lidar measurements illustrated in this paper reveal the presence of a
persistent minimum (dark band) in lidar elastic backscatter echoes in the
upper portion of the CBL. This phenomenon appears in clear sky conditions, in
the presence of strong convective activity, and is mostly confined to
up-draughts. Adiabatic cooling within the up-draughts leads to an RH increase and
a consequent particle growth, especially in the presence of hygroscopic particles. If we
assume that most of the particles we observe are dry hygroscopic lignite
particles from the surrounding lignite open-pit mines and that their size
distribution is mono-disperse or very narrow, we must conclude that the
observed dark band is related to the oscillations of the backscatter
efficiency as described by Mie-theory, ultimately leading to intensity
fluctuations of the Mie back-scattered radiation. In the presence of a wider
particle size distribution the backscatter oscillations should smear out, if
not disappear. This interpretation is also supported by the outcome of the
lidar depolarisation measurements. In fact, water uptake by uplifted dry
lignite aerosols, initially having a more irregular shape, confers a more
regular spherical shape to these particles, this shape change being
responsible for the decrease in particle depolarisation observed at the same
height and time intervals of the dark band (Fig. 6), again mostly confined
to up-draughts.
The fact that the dark band and the depolarisation decrease are confined to
the up-draughts can be explained in two ways: either the adiabatic warming, and
the consequent decrease in RH, in down-draughts does not lead to an inversion
of the particle growth (i.e. there is a hysteresis, and humidified particles
do not evaporate the water amount they incorporated during their way up) or
down-draughts transport different and/or modified particles to the up-draughts. These
particles might be less hygroscopic and thus change their size less with RH.
The possibility that particles within the down-draughts are different from
those within the up-draughts increases in the interfacial layer due to the
entrainment effects and is possibly testified by the presence of smaller
particle backscatter values within the down-draughts with respect to those
observed within the up-draughts (see Fig. 2). This is possibly associated with
the entrainment of air from the free-troposphere at the top of the CBL, which
may ultimately lead to changes in particle size distribution and scattering
properties. Evidence of the sharp entrainment of air pockets from the free
troposphere into the boundary layer, which gradually mix with the
environmental air, has been reported by a variety of authors (Couvreux et
al., 2005, 2007; Wulfmeyer et al., 2010, 2016; Turner et al.,
2014). Additionally, particle size distribution within the down-draught could be not as
narrow as in the up-draughts, resulting in a smear out of backscatter efficiency
oscillations.
The hygroscopic growth of
particles is dependent upon aerosol composition and may be subject to monotonic
(smoothly varying) or deliquescent (step change) growth. A dry hygroscopic
aerosol transforms into a solution droplet when RH increases beyond the so
called deliquescence point. Particle deliquescent growth, as the one
characterising lignite particles (Brooks et al., 2004), shows a hysteresis
behaviour during the uptake and loss of water, i.e. exhibits difference values
for the deliquescence and efflorescence relative humidity (Sjogren et al., 2007);
this hysteresis behaviour ultimately determines a less efficient evaporation
process (Seinfeld and Pandis, 2006). More specifically, when RH decreases,
the solution droplet starts reducing in size through the evaporation of the
previously taken up water at the efflorescence point, which is found at a
much lower RH value than the deliquescence point (Oatis et al., 1998).
Summary and final remarks
This paper illustrates measurements carried out by the Raman lidar system
BASIL in the frame of HOPE, revealing the presence of a persistent minimum
in clear-air backscatter echoes in the upper portion of the convective
boundary layer. Backscatter reduction is approximately 10 %, has a
vertical extent of approximately 100 m and persists over a period of several
hours. We refer to this phenomenon as the clear-air dark band or the convective dark band. This has to be
distinguished from a similar phenomenon, with comparable temporal duration
and backscatter reduction, observed in the presence of light precipitation
events (Sassen and Chen, 1995; Demoz et al., 2000), the so called lidar dark band, ascribable to
changes in precipitating particles' scattering properties taking place
during the snowflake-to-raindrop transition.
Dark bands illustrated in this paper are observed in the presence of strong
convective activity within the boundary layer, when dry lignite aerosol
particles are advected from the surrounding open pit mines, the bands are
mostly confined to the convective up-draughts. The phenomenon is interpreted as
being related to the oscillations characterising lignite particle
backscatter efficiency, ultimately leading to Mie back-scattered signal
intensity fluctuations. These backscatter efficiency oscillations are
attributed to the limited degree of poly-dispersion and the high
hygroscopicity of atmospheric lignite particles. Adiabatic cooling within
the up-draughts leads to an RH increase and a consequent particle growth.
Adiabatically warming and thus a decrease in RH in down-draughts does not lead
to an inversion of the particle growth and humidified particles do not or
only partially evaporate the water they took up during the up-draught.
Additionally, down-draughts may transport different particles with respect to the up-draughts.
These are possible motivations for having clear-air dark bands mostly
confined to up-draughts. Observations and results illustrated in this paper
support the interpretation of the phenomenon as a purely microphysical
growth mechanism; however, the possibility that other mechanisms (e.g. dynamics) may also participate and contribute to the appearance of
the phenomenon cannot be completely excluded.
Data used in this study, together with the related metadata, are available from the public
data repository HD(CP)2 Data Archive (Stamnas et al., 2016), which is freely accessible by all users from the HD(CP)2 Web Portal
(https://hdcp2data.rrz.uni-koeln.de:8443/thredds/catalog/hdcp2/rlid/00/any/l1/hope/unibas/2013/catalog.html, last access: 27 July 2015). The details for the
data structure and organization can also be found in Stamnas et al. (2016).
The authors declare that they have no conflict of
interest.
Acknowledgements
Measurements illustrated in this paper were supported on the basis of a specific cooperation agreement between Scuola di Ingegneria – Università degli
Studi della Basilicata, Leibniz Institute for Tropospheric Research and the Max Planck Institute. We also wish to thank Dario Stelitano, from Scuola di Ingegneria –
Università degli Studi della Basilicata, for his support during the HOPE field deployment.
Edited by: Matthias Tesche
Reviewed by: three anonymous referees
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