The multiwavelength Mie–Raman–fluorescence lidar of the University of Lille has the
capability to measure three aerosol backscattering coefficients, two extinction
coefficients and three linear depolarization ratios, together with
fluorescence backscattering at 466 nm. It was used to characterize aerosols
during the pollen season in the north of France for the period March–June
2020. The results of observations demonstrate that the presence of pollen
grains in aerosol mixture leads to an increase in the depolarization ratio.
Moreover, the depolarization ratio exhibits a strong spectral dependence
increasing with wavelength, which is expected for the mixture containing
fine background aerosols with low depolarization and strongly depolarizing
pollen grains. A high depolarization ratio correlates with the enhancement of
the fluorescence backscattering, corroborating the presence of pollen
grains. Obtained results demonstrate that simultaneous measurements of
particle depolarization and fluorescence allows for the separation of dust, smoke
particles and aerosol mixtures containing the pollen grains.
Introduction
Pollen grains represent a significant fraction of primary biological
particles emitted from the biosphere into the atmosphere in certain seasons
and locations (Fröhlich-Nowoisky et al., 2016). There has been a growing
interest in pollen study in recent years because they can affect human
health by causing allergy-related diseases and contribute to cloud
formation by acting as giant cloud condensation nuclei (CCN) (Diehl et al.,
2001; Pope, 2010; Griffiths et al., 2012; D'Amato et al., 2014; Steiner et al., 2015; Lake et al.,
2017; Mack et al., 2020). To investigate the processes of pollen transport
and dispersion, information about the vertical distribution of pollen grains
is needed, and this information can be obtained from lidar measurements.
Pollen grains are large irregularly shaped particles of complicated
morphology (Frenguelli, 2003; Katifori et al., 2010) causing strong depolarization of
backscattered laser radiation, which provides a basis for their
identification. The first profiling of pollen with depolarization lidar was
reported by Sassen (2008, 2011). His measurements over Alaska revealed that the
linear depolarization ratio of birch pollen plumes at 694 nm can exceed
30 %. Further studies of pollen with elastic backscatter lidar at 532 nm
were reported by Noh et al. (2013a, b) and by Sicard et al. (2016). Their
measurements confirmed the high depolarization ratio of pollen grains (particle
depolarization ratios as high as 43 % were observed for aerosol mixtures
containing Platanus and Pinus pollen). Moreover, pollen grain backscattering
demonstrates a strong diurnal cycle, being the highest near noon. The use of
multiwavelength observations increases the capability of the lidar technique for
aerosol characterization. In recent studies by Bohlmann et al. (2019) and
Shang et al. (2020), measurements performed with PollyXT lidar allowed
for the estimation of mean values of lidar ratios (about 45 and 55 sr at 355
and 532 nm, respectively, for birch pollen grains). Lidar measurement of
the pollen depolarization ratio at 355, 532 and 1565 nm was also reported
in the recent work of Bohlmann et al. (2021), revealing a strong decrease in the
depolarization ratio at 355 nm.
Atmospheric biological particles efficiently produce wideband fluorescence
emissions when exposed to UV radiation (Pöhlker et al., 2012; Pan,
2015; Miyakawa et al., 2015), which offers an opportunity for monitoring
them with fluorescence lidars. Nowadays, lidar spectrometers based on
multi-anode photomultipliers allow simultaneous detection of fluorescence
backscattering in 32 spectral bins (Sugimoto et al., 2012; Reichardt,
2014; Reichardt et al., 2017; Saito et al., 2018; Richardson et al., 2019). In particular,
such a lidar spectrometer was used in the recent work of Saito et al. (2018) for
remote measurement of the fluorescence spectrum of atmospheric pollen
grains. The results demonstrate that, for the 355 nm stimulating wavelength, the
fluorescence spectra of different pollen grains have maxima in the 400–600 nm range and a intensity peak at around 460 nm.
To achieve the highest sensitivity of fluorescence detection, in many tasks
it is preferable to use single-channel monitoring, whereby a part of the
fluorescence spectrum is selected with a wideband interference filter
(Immler et al., 2005; Rao et al., 2018; Li et al., 2019). In our recent
publication (Veselovskii et al., 2020a) we reported the results obtained
from a modified Mie–Raman lidar system at the Laboratoire d'optique
Atmosphérique with one additional fluorescence channel at 466 nm. Such
an approach has proved high sensitivity, allowing for the detection of fluorescence
signals from weak aerosol layers and the calculation of the fluorescence
backscattering coefficient from the ratio of fluorescence and nitrogen Raman
backscatters, thus making it potentially attractive for pollen monitoring.
In the present research we combine the capability of multiwavelength Mie–Raman
lidar for providing three backscattering coefficients, two aerosol extinction
coefficients and the linear depolarization ratio at three wavelengths with
single-channel fluorescence measurements for characterization of aerosol
mixtures containing pollen grains. The measurements reported were performed
during the March–June 2020 period at the Lille Atmospheric Observation Platform
(https://www-loa.univ-lille1.fr/observations/plateformes.html?p=apropos, last access: 30 June 2021) hosted by the Laboratoire d'Optique Atmospherique, University of
Lille, Hauts-de-France region.
InstrumentationMie–Raman–fluorescence lidar
The measurements were performed using the LILAS (LIlle Lidar AtmosphereS) system – a multiwavelength Mie–Raman lidar based on a
tripled Nd:YAG laser with a 20 Hz repetition rate and pulse energy of 70 mJ
at 355 nm. The backscattered light is collected by a 40 cm aperture
Newtonian telescope. The full geometrical overlap of the laser beam and the
telescope field of view (FOV) is achieved at approximately 1000 m. To obtain the
information about particles at lower altitudes, some of the measurements
were performed at an angle of 30∘ to the horizon. No overlap
correction of lidar signals was performed. The system is designed for
simultaneous detection of elastic and Raman backscattering, allowing the so-called 3β+2α+3δ data configuration, including
three particle backscattering coefficients (β355, β532, β1064), two extinction coefficients (α355, α532)
and three particle depolarization ratios (δ355, δ532, δ1064). The particle
depolarization ratio δ, determined as the ratio of cross- and
co-polarized components of the particle backscattering coefficient, was
calculated and calibrated the same way as described in Freudenthaler et al. (2009). The description of the system can be found in the recent publication
of Hu et al. (2019).
To perform fluorescence lidar measurements, the water vapor Raman channel at
408 nm was replaced by a fluorescence channel, whose spectrum is captured by
a wideband filter centered at 466 nm and of 44 nm width (Veselovskii et al.,
2020a). The fluorescence measurements were performed during nighttime only.
The aerosol extinction and backscattering coefficients at 355 and 532 nm
were calculated from Mie–Raman observations (Ansmann et al., 1992), while
β1064 was derived by the Klett method (Klett, 1985). The
fluorescence backscattering coefficient βF is calculated from
the ratio of fluorescence and nitrogen Raman backscattering, as described in
Veselovskii et al. (2020a). This approach allows for the evaluation of the absolute
values of βF if the relative sensitivity of the channels is
calibrated and the nitrogen Raman scattering cross section is known.
We estimate the corresponding uncertainty to be below 50 %. Parameters of
detectors were not changed during the campaign, so the uncertainty of relative
variations of βF was significantly lower and was determined by
the statistical errors of fluorescence measurements.
To characterize the efficiency of the fluorescence with respect to elastic
scattering, the fluorescence capacity,
GF=βFβ532,
is also used. This parameter depends on the relative humidity (RH), so
information about RH is important for data analysis. Radiosonde measurements
are used to monitor water vapor, as the water vapor channel is replaced by
the fluorescence channel in the current lidar configuration. The closest
available radiosonde data are from Herstmonceux (UK) and Beauvecchain
(Belgium) stations, located 160 and 80 km away from the observation site,
respectively. These radiosonde data are not collocated with the lidar
measurements, so only qualitative analysis of humidification effects was
possible.
Pollen in situ sampling
Airborne pollen grains and spores were collected by a Hirst-type volumetric
sampler (Lanzoni VPPS 2000). The pollen sampler was located on the
campus of the University of Lille (France) on the rooftop of a 20 m high
building where the lidar instrument was operated. Ambient air was sampled at
a 10 L min-1 flow rate, allowing the impaction of pollen and spores on an
adhesive strip mounted on a rotating clockwork-driven drum. The impaction
surface moves at 2 mm h-1 behind the entrance slit, allowing a temporal
resolution of 2 h. The adhesive strip was substituted every 7 d after
a full rotation of the drum, which is split into seven parts, each
corresponding to a day of monitoring. And then they are fixed on a
microscope glass slide with gelatin and fuchsine dye. Pollen taxa were
identified by light microscopy on the basis of their characteristic shape
and size. Airborne pollen concentrations were expressed as a daily and dual
hourly number of pollen grains per cubic meter of air.
Daily concentration of the most abundant pollen taxa for the period
March–June 2020 in Lille from in situ measurements on the rooftop. Open
symbols show fluorescence backscattering βF measured by lidar. Lidar measurements are averaged overnight, and the maximal value in the 500–1000 m range is shown. Vertical arrows mark the sessions, which are further analyzed in Figs. 8, 10 and 11.
Figure 1 shows the most abundant pollen taxa for the period from March to June
2020 in Lille. These include Betula (54.8 % of total pollen taxa over the
period), Fraxinus (8.2 %), Quercus (5.8 %), Urticaceae (4.6 %), Salix (4.5 %) and Cupressaceae (4.1 %). The
same figure also shows the fluorescence backscattering βF
measured by lidar. The results presented are obtained by averaging all
available data during the night, and the maximal values in the 500–1000 m
height range are shown. The highest fluorescence was observed at the end of
March when ash (Fraxinus) is the main pollinator. The period of intense birch
(Betula) pollination (3–15 April 2020) also correlates with high βF.
Strong fluorescence observed for the 5–10 May and 28 May–2 June periods can be
due to grass (Poaceae) pollen contribution. By the end of June, βF
decreases and becomes comparable with the fluorescence backscattering of
background aerosol. From Fig. 1, we can conclude that there is no direct
correlation between in situ and fluorescence lidar measurements; thus, pollen
grains observed in the boundary layer by the lidar are probably transported from
other regions. Comparing lidar and in situ observations, we should also keep
in mind that maximum of pollen emission occurs near noon, while lidar
measurements were performed in the night.
Discrimination of pollen from other types of aerosolSpecific features of pollen-containing aerosol mixture
In contrast to the observations performed over Alaska (Sassen, 2008, 2011)
and in Kuopio, Finland (Bohlmann et al., 2019), where the pollen concentration
was high due to boreal forests surrounding, the pollen loading in the north
of France is significantly lower. Long-term lidar and sun photometer
observations performed at the University of Lille demonstrate that local aerosol is
mainly of continental type, with a predominance of fine-mode particles and
a low depolarization ratio. The emission of large pollen grains should lead
to strong spectral dependence of the depolarization ratio because the
backscattering at 1064 nm is less sensitive to fine background particles
than at shorter wavelengths; thus, the particle depolarization ratio at 1064 nm
(δ1064) should be more sensitive to the presence of pollen
grains compared to δ355 and δ532. The particle
depolarization ratio δ of the mixture containing background aerosol
(b) and pollen (p), with corresponding depolarization ratios δb
and δp, can be calculated as
δ=δp1+δpβp+δb1+δbβbβp1+δp+βb1+δb.
Here total backscattering β=βb+βp.
To estimate the dependence of depolarization of the aerosol mixture on the
contribution of pollen to the backscattering coefficient β532pβ532 at 532 nm, a simplified simulation was
performed. Assuming that the depolarization ratios of pollen and background
aerosol are spectrally independent and that δp= 30 % when
δb= 3 %, the mixture depolarization ratios δ355, δ532 and δ1064 were calculated as a
function of β532pβ532 using Eq. (2).
For pollen the backscattering Ångström exponents (BAE) A355/532β=A532/1064β=0 were used. The backscattering Ångström exponents
of background aerosol are assumed to be the same for both wavelength pairs
(A355/532β=A532/1064β), and computations were
performed for the values of BAE Aβ= 1.0, 1.5 and 2.0. Results of the
simulation are shown in Fig. 2. For a low pollen contribution the
depolarization ratio δ1064 increases faster with β532pβ532 compared to δ532 and δ355, and the slope of this increase depends on the BAE value. Spectral
properties of the real mixture can be more complicated due to possible
spectral dependence of both δp and δb. Information
on laboratory-measured spectral dependence of depolarization ratios of
pollen is rare. Cao et al. (2010) measured the linear depolarization ratio
of several types of pollen in a chamber at 355, 532 and 1064 nm wavelengths.
The results demonstrate strong variation of spectral dependence for
different taxa, and for most of the samples δ532 exceeded both
δ355 and δ1064. In particular, for birch pollen,
depolarization ratios at 355, 532 and 1064 nm are 8 %, 33 % and
28 %. Using the depolarization ratios from Cao et al. (2010) in the analysis,
we should also keep in mind that measurements in the chamber were performed
at low RH, and depolarization ratios at higher RH may be different.
Depolarization ratios at 355, 532 and 1064 nm as a function of pollen contribution to the total backscattering coefficient β532pβ532. Depolarization ratios of pollen (δp) and background aerosol (δb) are assumed to be spectrally independent (δp= 30 % and δb= 3 %). The backscattering coefficient of pollen is spectrally independent. The backscattering Ångström exponents of background aerosol were assumed to be the same for both pairs of wavelengths (A355/532β=A532/1064β), and results are shown for the values Aβ= 1.0, 1.5 and 2.0.
The Ångström exponents A355/532β and A532/1064β depend on the particle size and the refractive index in different ways
(Veselovskii et al., 2015), so their values can be different. Thus, spectral
dependence of the depolarization ratio of an aerosol–pollen mixture will differ
from the simplified modeling shown in Fig. 2. However, for a moderate contribution
of pollen to the total backscattering β532pβ532below≈0.5, the depolarization ratio at 1064 nm should
be higher than that at shorter wavelengths. Hence, an increase in the particle
depolarization ratio with wavelength can be an indication of the presence of
large, irregularly shaped pollen grains in an aerosol mixture.
The presence of pollen should also lead to a decrease in the extinction
and backscattering Ångström exponents. The extinction Ångström exponent (EAE)
depends mainly on particle size, while BAE is also sensitive to the particle
complex refractive index and shape; thus, the measured profiles of EAE and
BAE can present significant differences (Veselovskii et al., 2015, 2020b). In
our study we analyze the EAE and BAE for the wavelength pair 355 nm and 532 nm
(A355/532α and A355/532β) only because the
extinction coefficient and backscattering coefficient involved are calculated from
Mie–Raman observations.
When analyzing Mie–Raman–fluorescence lidar measurements of pollen-containing aerosol mixtures, numerous factors should be taken into
account. These factors include the fluorescence of background aerosol and
other non-pollen aerosols that have strong fluorescence capacity: for
example, smoke particles. Dust particles can contribute to the increase in the
depolarization ratio, and, finally, hygroscopic growth can modify the
particle parameters. All these factors will be considered in the following
sections.
Characteristics of background aerosol over the observation
site
Long-term observations at the University of Lille demonstrate that aerosol over the
observation site is mainly of continental type with a predominance of fine-mode particles. Typical vertical profiles of the background aerosol
parameters, observed on 3 June 2020, are given in Fig. 3, showing aerosol
elastic and fluorescence backscattering coefficients, lidar ratios, Ångström
exponents and depolarization ratios at three wavelengths. The RH from
Beauvecchain (Belgium) radiosonde observations was below 50 % in the
height range considered. Particle depolarization ratios at all three
wavelengths are below 7 %, indicating that the contribution of pollen to the
total backscattering was low. This agrees with the low values of pollen
concentration provided by in situ measurements (Fig. 1). The lidar ratios at
both wavelengths (S355, S532) are close, varying in the 50–60 sr
range, and the fluorescence capacity GF is below 0.35 × 10-4. The EAE and BAE (A355/532α, A355/532β) are in the 1.5–2.0 range. The presence of pollen should lead to a
deviation of the particle intensive parameters, such as the fluorescence
capacity, depolarization ratio, EAE and BAE, from the typical values of
background aerosol.
Measurements in the condition of background aerosol predominance.
Vertical profiles of (a) backscattering coefficients β355,
β532 and β1064 as well as lidar ratios S355 and S532. (b) Fluorescence backscattering βF and fluorescence capacity GF=βF/β532. (c) Particle linear depolarization ratios δ355, δ532 and δ1064 together with extinction and backscattering Ångström exponents
A355/532α and A355/532β on 3 June 2020 for 20:30–23:00 UTC. Measurements were performed at 30∘ to the horizon.
Identification of the smoke particles
During the observation period the smoke elevated layers transported over
the Atlantic were frequently detected. Smoke particles are characterized by
a high fluorescence cross section (Reichardt et al., 2017; Veselovskii et al.,
2020a) and can interfere with pollen fluorescence measurements. The temporal
evolution of the range-corrected lidar signal, volume depolarization ratio
at 1064 nm and fluorescence backscattering for the smoke episode on the
night of 23–24 June 2020 are shown in Fig. 4. During the night the smoke
layer, with a low depolarization and high fluorescence, is observed at
approximately 5000 m of height. Back trajectories (not shown) indicate that the
layer is transported from Canada. Vertical profiles of the particle
parameters for this episode are shown in Fig. 5. The lidar ratio is about 50 sr at 355 nm, while the lidar ratio at 532 nm increases within the smoke
layer from 60 to 80 sr. This increase in S532 occurs simultaneously
with decrease in A355/532α from 1.5 to 0.75, indicating that
the particle size inside the layer growths with height. Higher values of
S532 with respect to S355 are typical for
aged smoke (e.g., Müller et al., 2005; Nicolae et al., 2013; Hu et
al., 2019). The depolarization ratio decreases with wavelength from δ355= 10 % to δ1064= 1.5 %. A strong spectral
dependence of the depolarization ratio and, in particular, low values of δ1064 are the features allowing for the identification of smoke layers. We
should also recall that an increase in the particle depolarization ratio at
355 nm is more typical for aged smoke layers in the high troposphere
(Haarig et al., 2018), though we observed this increase at lower altitudes
over Lille during smoke episodes in summer–autumn 2020.
Range-corrected lidar signal at 1064 nm, volume depolarization ratio at 1064 nm and fluorescence backscattering coefficient (in 10-4 Mm-1 sr-1) on 23–24 June 2020.
Vertical profiles of (a) backscattering coefficients β355, β532 and β1064 as well as lidar ratios S355 and S532.
(b) Fluorescence backscattering coefficient βF, fluorescence capacity GF, and (c) particle depolarization ratios δ355, δ532 and δ1064 together with the extinction and backscattering Ångström exponents A355/532α and A355/532β on the night of 23–24 June 2020 at 21:30–02:30 UTC.
The extinction Ångström exponent A355/532α in the center of the
layer is about 0.75, while A355/532β is about 1.9 and shows no
significant variation through the layer. High values of A355/532β compared to A355/532α represent another feature that will be
used for aged smoke discrimination. Smoke fluorescence capacity is high,
reaching up to GF= 5 × 10-4 for the period of observations, and
this is one more feature allowing for the separation of smoke from other types of
aerosol.
Identification of the dust particles
The presence of dust particles and pollen in the fine background aerosol leads
to some common characteristics in the lidar data, such as decreased
Ångström exponents and increased depolarization ratios. However, pollen
and dust can be separated by the fluorescence capacity. The vertical
profiles of particle parameters during a dust episode on 27 May are shown in
Fig. 6. The dust-containing layer extends from 2000 to 7000 m, and the
particle depolarization ratios δ1064 and δ532 in
this layer are close to 20 %. These values are lower than the depolarization
of pure dust. For example, Freudenthaler et al. (2009) for pure dust provide
values of 27 % and 31 % at 1064 and 532 nm wavelengths,
respectively; thus, in our case transported dust particles may be mixed with
local aerosols. The particle depolarization at 355 nm is not shown in the
figure because the scattering ratio in the dust layer was too low to
compute δ355 reliably. The fluorescence capacity of particles
in the dust layer is about 0.1 × 10-4 at 4000 m, which is
a factor of 50 lower than GF of the smoke in Fig. 5. There is also a weak
aerosol layer at 1600 m with β532 of about 0.035 Mm-1 sr-1. The fluorescence capacity in this layer is high
(GF≈ 2.0 × 10-4), suggesting that this layer may contain
smoke or pollen particles.
Lidar measurements during a dust episode. Vertical profiles of particle
β1064 and fluorescence βF backscattering
coefficients, fluorescence capacity GF, and particle depolarization ratios δ1064 and δ532 on 27 May 2020 at 21:00–23:00 UTC.
Impact of particle hygroscopic growth
The vertical variation of observed aerosol properties may be a result of
particle water uptake, which should be separated from the features related
to pollen presence. Figure 7 shows the profiles of the particle parameters for
the episode on 15 June 2020 when aerosol hygroscopic growth took
place. In the height range 900–1500 m the fluorescence backscattering
βF is stable, while the elastic backscattering β532
increases by a factor of 3. Radiosonde measurements in Herstmonceux (UK) in
this height range demonstrate an increase in RH from about 75 % to 85 %,
while lidar-measured extinction and backscattering Ångström exponents
decrease from 1.5 to 1.3, corroborating the occurrence of particle
hygroscopic growth. The depolarization ratio δ1064 at low
altitudes exceeds δ355 and δ532, which can be an
indication of pollen presence. This is supported by significant fluorescence
capacity (GF= 0.9 × 10-4 at 750 m).
Lidar measurements in the condition of aerosol hygroscopic growth in the 900–1500 m height range. Vertical profiles of (a) particle β532 and fluorescence βF backscattering coefficients, fluorescence capacity GF, lidar ratios S355 and S532, and (b) particle depolarization ratios δ355, δ532 and δ1064 together with extinction A355/532α and backscattering A355/532β Ångström exponents measured on 15 June 2020 at 22:00–24:00 UTC.
The number of fluorescent particles in the 900–1500 m range does not
present significant changes (βF is stable), so observed vertical
variations, i.e., the decrease in depolarization ratios at all three
wavelengths and the increase in lidar ratios S355 and S532 from 50 to 65 sr, are probably the result of water uptake by the particles. Water
uptake does not change the number of fluorescent molecules; however, the
fluorescence capacity decreases in the process of hygroscopic growth, so
GF can be a representative parameter of aerosol types only in the
condition of low RH.
Results of lidar measurements in the presence of pollen
During March–June 2020, we had numerous measurement cases demonstrating
features in the profiles of the particle parameters that can be attributed
to pollen. For representative cases we have chosen observations with a high
depolarization ratio and high fluorescence backscattering. At the same time, we
omitted the days with high relative humidity to minimize the impact of
hygroscopic growth effects. Below we consider several measurement cases
representing different scenarios, in particular the episodes when pollen
concentration decreases with height (30–31 May, 1–2 June) and the episodes
when pollen grains are well mixed inside the boundary layer (27–28 March and
21 April).
30–31 May and 1–2 June 2020 observations
The results of lidar measurements during the campaign in many episodes can
be interpreted as a decrease in the pollen concentration with height. Vertical
profiles of the main particle parameters for two representative cases on the
nights of 30–31 May and 1–2 June 2020 are shown in Fig. 8. The atmospheric
conditions for these nights were stable, so the profiles presented are
averaged over an approximately 5 h interval. The HYSPLIT back-trajectory
analysis (Stein et al., 2015) demonstrates that in the 1000–2000 m height range
the air masses were transported from northern Europe. At the ground
level, grass could be the main pollen contributor for this period, as
shown in Fig. 1. On 31 May (at 00:00 UTC) the RH measured by the radiosonde
in Herstmonceux (UK) was about 40 % at 500 m, and it increased up to 70 %
at 2000 m. On 2 June the RH increased from approximately 40 % to 60 % in
the same height range. For both nights the fluorescence backscattering
decreases with height, indicating a decrease in the concentration of
fluorescent particles (presumably pollen). This decrease in βF
correlates with a decrease in the depolarization ratio at all three
wavelengths. Particle depolarization δ1064 is the highest
(about 15 % at 750 m), while δ355 and δ532 are
significantly lower. Such spectral dependence of the depolarization ratio can be
partly due to the contribution of the background aerosol, as follows from
the model calculation in Fig. 2. The lidar ratios are available above 1250 m, and
for both cases, S355 and S532 increase with height. It indicates
that the lidar ratios of pollen in the two considered cases can be quite
low: below 40 sr at 355 nm and below 30 sr at 532 nm, considering that
the pollen concentration decreases with height, which is inferred from the
features of the depolarization ratio and fluorescence backscattering
Vertical profiles of (a, d) particle backscattering coefficients β355, β532 and β1064 as well as lidar ratios
S355 and S532. (b, e) Fluorescence backscattering coefficient βF, fluorescence capacity GF, pollen backscattering coefficient β532p and its contribution to the total backscattering β532pβ532. (c, f) Particle depolarization ratios δ355, δ532 and δ1064 together with extinction A355/532α and backscattering A355/532β Ångström exponents on (a–c) 30–31 May 2020 at 21:00–02:00 UTC and on (d–f) 1–2 June 2020 at 21:00–02:30 UTC.
Profiles of β532p and β532pβ532 were computed with the assumption that δ532p=30 %. The depolarization ratios of the background aerosol δ532b are measured or assumed to be 3 % on 30 May and 5 % on 1 May.
The EAE for both nights varied in the 1.75–2.0 range and did not show
significant changes with height. The BAE is lower (about 1.5 at 1000 m), and
for both nights it shows some increase in the 1250–2250 m range. The BAE, in
contrast to EAE, depends strongly on the particle refractive index and
shape; thus, it may demonstrate higher sensitivity to the changes in aerosol
mixture composition. Recall that backscattering and extinction Ångström
exponents are related as
A355/532β=A355/532α+ln(S532/S355)ln(355/532).
Thus, for S355>S532, which has been observed during
pollen episodes, the A355/532β is lower than
A355/532α. This is in contrast with smoke episodes in which
S355<S532 and A355/532β>A355/532α (Fig. 5).
If the depolarization ratios of pollen δp and background
aerosol δb are known, the pollen backscattering coefficient
βp can be calculated. Such an approach is widely used for the
separation of contributions of dust and smoke particles (Sugimoto and Lee,
2006; Tesche et al., 2009), and the same technique was applied to separate
pollen and background aerosol (Noh et al., 2013a; Sicard et al., 2016; Shang
et al., 2020). For height-independent depolarization ratios of pollen and
background aerosol the pollen backscattering coefficient can be calculated
as suggested by Tesche et al. (2009):
βp=βδ-δbδp-δb1+δp1+δ,
Here β and δ are the backscattering coefficient and particle
depolarization ratio of the mixture. The profiles of β532p and
the relative contribution β532pβ532 are
shown in Fig. 8b and e. Computations were performed assuming that height-independent δ532p=30 %. For background aerosol, the values δ532b=3 % for 30–31 May and δ532b=5 % for 1–2 June were used. On 30–31 May the contribution of pollen β532pβ532 at 750 m is estimated as 30 %.
The profiles of βF and β532p in Fig. 8b and e behave
similarly, decreasing with height. Above 2000 m the decrease in βF slows down due to the fluorescence of background aerosol. The
profiles of the fluorescence capacity GF and relative contribution
β532pβ532 also demonstrate a good
correlation. Thus, both the depolarization and fluorescence techniques lead to
the same conclusion: pollen concentration in the boundary layer for the
considered episodes decreases with height.
27–28 March and 21 April 2020 observations
According to the in situ pollen sampling at rooftop level, the maximal
pollen content was detected during the birch pollination period on 4–20 April.
However, the maximal fluorescence backscattering of lidar data was observed
at the end of March when sampling shows an increase in ash (fraxinus) pollen
emission. The temporal evolution of the range-corrected lidar signal, volume
depolarization ratio at 1064 nm and fluorescence backscattering on the night of 27–28 March is shown in Fig. 9. The main part of the aerosol is localized
below 2000 m. The back-trajectory analysis demonstrates that the air masses in
this episode were transported from eastern Europe. In contrast with Fig. 8,
where fluorescence decreases with height, on 27–28 March the fluorescent
particles are rather well mixed inside the PBL (planetary boundary layer).
The fluorescence backscattering is high, exceeding 2.5 × 10-4 Mm-1 sr-1, and the volume depolarization at 1064 nm is about 15 %.
The vertical profiles of the particle parameters, averaged for the period 19:20–04:30 UTC, are shown in Fig. 10. In spite of temporal variations of the
lidar signal and the fluorescence backscattering inside the PBL (Fig. 9), the
averaged overnight profiles of particle parameters are representative.
Radiosonde measurements (at both Beauvecchain and Herstmonceux sites) show
that RH gradually increased with height from approximately 40 % to 70 %
in the 500–1750 m range. The depolarization ratios δ532 and
δ1064 inside the PBL are close, which is in contrast with
results in Fig. 8, where δ1064 exceeds δ532. This
difference can be due to the different types of pollen that were present, which is
probably grass in Fig. 8 and birch in Fig. 10. Also, the BAE in Fig. 10 is
lower than that in Fig. 8, which decreases the influence of background
aerosol on the spectral dependence of the depolarization ratio, as follows
from Fig. 2.
Range-corrected lidar signal at 1064 nm (a), volume
depolarization ratio at 1064 nm (b) and fluorescence
backscattering coefficient (in 10-4 Mm-1 sr-1, c)
measured on 27–28 March 2020.
Vertical profiles of (a) particle backscattering coefficients β355, β532 and β1064 as well as lidar ratios S355 and S532. (b) Fluorescence backscattering coefficient βF, fluorescence capacity GF, pollen backscattering coefficient β532p and its contribution to the total backscattering β532pβ532. (c) Particle depolarization ratios δ355, δ532 and δ1064 together with extinction A355/532α and backscattering A355/532β Ångström exponents measured on 27–28 March 2020 at 19:20–04:30 UTC. Profiles of β532p and β532pβ532 were computed with the assumption that δ532p=30 % and δ532b= 3 %.
Both the fluorescence backscattering and depolarization ratios do not
demonstrate strong variations inside the 600–1500 m range. The maximum
fluorescence capacity exceeds 1.2 × 10-4, which is
significantly higher than GF for background aerosol in Fig. 3. The profiles of GF and β532pβ532 behave reasonably similar, and the slight decrease in β532pβ532 with height with respect to GF can be due to dependence of the
depolarization ratio of pollen on RH.
Agreement between results obtained from the depolarization and fluorescence
techniques in Figs. 8 and 10 corroborates the suggestion that the observed
fluorescence is mainly due to the presence of pollen. However, in some
episodes the particles with a high fluorescence cross section, other than
pollen, could interfere. In particular, such interference occurred in the 20–23 April 2020 period. Figure 11 shows the vertical profiles of particle parameters
measured on 21 April. The depolarization ratio δ1064= 22 %
at 750 m was one of the highest during campaign. The RH was low, increasing
from 30 % to 45 % in the 800–1500 m range according to Herstmonceux radio
sounding. The back-trajectory analysis demonstrates that below 1500 m the
air masses are transported from Spain, while at 2000 m the transportation is
from northern Europe.
The same particle parameters as in Fig. 10 for 21 April 2020,
20:00–23:00 UTC. Measurements were performed at 30∘ to the horizon.
Fluorescence backscattering is stable in the 500–1500 m range, and the
fluorescence capacity at 1000 m is about 1.5 × 10-4, which is a typical value for pollen. However, above 1250 m GF starts to rise, reaching the value of 2.5 × 10-4 at 1750 m. Such high GF is
more typical for smoke particles. The depolarization ratio δ355
is about 15 %, and this is higher than corresponding values shown in
Figs. 8 and 10, which again may corroborate the presence of smoke particles. We
should recall that smoke particles are small, so, in contrast to pollen,
their presence influences δ355 more strongly than δ1064. Enhanced values of δ355 and GF were
observed during the 20–23 April period, indicating the possible presence of
biomass burning particles in the aerosol mixture.
Separation of pollen and smoke layers
During the campaign we observed narrow layers with strong fluorescence. Two
examples of such observations, on the nights 13–14 April and 16–27 May 2020,
are shown in Fig. 12. The white arrows in this figure point to the
fluorescent layers. On 13 April, a weak aerosol layer (β532≈ 0.6 Mm-1 sr-1 for 23:00–00:00 UTC) is
observed at the top of the PBL. This layer demonstrates volume
depolarization ratios of about 10 % and high fluorescence backscattering. On
26–27 May a weak layer with high fluorescence backscattering occurs between
3 and 4 km. However, in contrast with the first case, it has a low
depolarization ratio, so the layers may have a different nature. Figure 13 shows
the vertical profiles of the particle parameters for these two cases. On
13–14 April the fluorescence backscattering below 1000 m is stable, while
β532 rises, which can be the result of the particle water
uptake. Above 1000 m, the depolarization ratio δ1064 increases
up to 8 %. Results in Fig. 13a are averaged over the 21:15–00:40 UTC temporal
interval, but peak values of δ1064 between 23:00 and 00:00 UTC
exceeded 12 %. Fluorescence backscattering increases simultaneously with
the depolarization. The aerosol backscattering coefficient of the fluorescent
layer is too low for a reliable calculation of δ355 and δ532, so only the profile of δ1064 is provided.
Range-corrected lidar signal at 1064 nm, volume depolarization ratio δ1064v and fluorescence backscattering coefficient (in 10-4 Mm-1 sr-1) measured on 13–14 April (a, c, e) and
26–27 May 2020 (b, d, f). Arrows point to the fluorescent layers.
Vertical profiles of elastic β532 and fluorescence
βF backscattering coefficients, fluorescence capacity GF, and particle depolarization ratio δ1064 measured on (a) 13–14 April at 21:00–01:00 UTC and (b) 26–27 May 2020 at 23:30–00:40 UTC. Values of β532 are multiplied by a factor of 5.
On 26–27 May the backscattering coefficient of the fluorescent layer at 3400 m is lower than in Fig. 13a (β532≈ 0.14 Mm-1sr-1), so the depolarization ratio δ1064 can be
calculated only in the center of the layer and is about 2 %, which is
significantly lower than that on 13–14 April. However, the fluorescence
capacity on 26–27 May is up to 3.5 × 10-4, which is typical for smoke. Thus, we can conclude that the fluorescent layer on 26–27 May
contains smoke particles due to high GF and low δ1064.
On 13–14 April, the fluorescence capacity is significantly lower (about
0.9 × 10-4) and the depolarization ratio δ1064 exceeds
10 %, which is more typical for pollen. Due to low backscattering
coefficients of the fluorescent layers in Fig. 12, we are not able to provide
a complete set of intensive parameters, such as Ångström exponents and
particle depolarization ratios at three wavelengths. However, based on the
obtained fluorescence capacities and δ1064 values, we conclude
that the fluorescent layers probably contain pollen grains in Fig. 12a and
smoke particles in Fig. 12b.
Aerosol classification based on polarization and fluorescence measurements
Table 1 summarizes the results in the campaign, showing the aerosol
parameters (particle depolarization and lidar ratios, extinction
Ångström exponent, fluorescence backscattering, and capacity) for several days
in the March–June 2020 observation period when the contribution of pollen to
the total particle backscattering was significant. All available night
observations were averaged, and results are given for heights with the
highest particle depolarization. Lidar ratios varied approximately in the 40–70 sr range, wherein S355 is normally greater than S532. It must be
emphasized that pollen lidar ratios may differ for different taxa and that
the observed lidar ratios are not attributed to pure pollen but to the
aerosol–pollen mixture, so the values provided are influenced by the
properties of background aerosol. Moreover, the shape of pollen grains
depends on RH (Heidemarie et al., 2018), which may also lead to the
variation of pollen lidar ratios. In most of the cases, the depolarization
ratio presents strong spectral dependence and increases with wavelength.
This spectral dependence is probably the result of mixing of strongly
depolarizing pollen grains with fine background aerosol. The maximal value
of observed fluorescence capacity of the pollen–aerosol mixture is 1.6 × 10-4, which is significantly higher than that of background aerosol
but lower than the fluorescence capacity of smoke.
Lidar-measured aerosol parameters, such as particle depolarization
ratios (δ355, δ532, δ1064), lidar
ratios (S355,S532), extinction Ångström exponent
(A355/532α), fluorescence backscattering coefficient (βF) and fluorescence capacity (GF), for several days during the March–June 2020 period when the contribution of pollen to the total particle backscattering could be significant.
The simultaneous observations of depolarization ratio and fluorescence
capacity for different types of aerosol are summarized in Fig. 14. Particle depolarization δ532 is plotted versus GF.
The diagram allows for the separation of four types of the particles: (i) dust
particles – high δ532 and low GF; (ii) pollen – high
δ532 and high GF; (iii) smoke – low δ532 and
high GF; and (iv) background aerosol (continental type) – low δ532 and low GF. Points corresponding to the pollen mixture provide an
extended pattern because parameters depend on the concentration of pollen
in the aerosol mixture. The dust measurements are also scattered because
dust over the instrumentation site is transported long-range and mixed with local
aerosol. Minimum GF for dust is about 0.1 × 10-4, while for
smoke maximal GF is about a factor of 50 higher. The fluorescence capacity
depends on the relative humidity, so strong scattering of measurement points
can also be partly due to RH variations. Maximal values of GF for pollen
mixture were about 1.5 × 10-4, and the corresponding
depolarization ratios δ532 are about 18 %. Thus, assuming
that the depolarization ratio of pure pollen is 30 %, we can expect GF for
pure pollen to be about 2.5 × 10-4, which is comparable with
values for smoke.
Particle depolarization δ532 versus fluorescence
capacity GF. This diagram allows for the identification of dust (blue), smoke
particles (black) and aerosol mixtures containing pollen (red).
Conclusion
We analyzed the measurements from a multiwavelength Mie–Raman–fluorescence
lidar during March–June 2020 in the north of France to reveal the features
that can be attributed to pollen grains. The lidar system allowed us to measure
depolarization ratios at three wavelengths simultaneously with the
fluorescence backscattering at 466 nm. In numerous episodes during the
campaign, high values of the particle depolarization ratio at 1064 nm,
exceeding 15 %, were observed. Moreover, the depolarization ratio had strong
spectral dependence, being the highest at 1064 nm and lowest at 355 nm,
which is expected for big particles of irregular shape mixed with fine,
low-depolarizing background aerosol. The increase in particle depolarization
correlated with enhancement of the fluorescence backscattering, corroborating the fact
that in these episodes we observed aerosol mixtures containing pollen.
The lidar ratios of aerosol–pollen mixtures observed during the campaign varied
in a wide range. At low altitudes, where particles presented strong
depolarization and fluorescence, in many cases we observed lidar ratios
below 40 sr at both wavelengths. However, we also had cases when the lidar
ratios at both wavelengths were in the 50–60 sr range. Thus, at the moment we
are not able to specify lidar ratios for pure pollen, and additional
measurement campaigns in locations with high pollen content are strongly
desirable.
Obtained results demonstrate that simultaneous measurements of particle
depolarization and fluorescence allows for the separation of dust, smoke particles and
pollen grains. Moreover, the fluorescence measurements provide additional
information that can be used in aerosol classification schemes. However,
further studies are needed to make this technique applicable for
quantitative pollen characterization. In the data analysis it is important
to account for the process of water uptake by the particles because
hygroscopic growth increases the backscattering of background aerosol and
influences the pollen grain shape. In the presented lidar configuration, the
water vapor channel was absent and radiosonde RH data were not collocated
with lidar, which prevented us from performing a quantitative analysis of
the hygroscopic effects. Since December 2020, we have recovered the water vapor
channel in an upgraded configuration of the lidar. Moreover, we added one more
fluorescence channel centered at 549 nm, which will be used in the next
pollen campaign in 2021. This additional channel should improve the
discrimination of pollen from other aerosols. In the coming campaign we will
try to correlate our results with pollen concentrations at different locations
in Europe by using a transport model, e.g., SILAM (System for Integrated
modeLling of Atmospheric coMposition) (Sofiev et al., 2015). The use of
this model should help in the identification of pollen type in our observations.
Data availability
Lidar measurements are available upon request (philippe.goloub@univ-lille.fr).
Author contributions
IV processed the data and wrote the
paper. QH and TP performed the measurements. PG supervised the project and
helped with paper preparation. MC and NV performed in situ pollen analysis,
and MK developed software for data processing.
Competing interests
The authors declare that they have no conflict of interest.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The Réseau National de Surveillance Aérobiologique (RNSA) and the Association pour la Prévention de la Pollution Atmosphérique (APPA) are gratefully acknowledged for providing Hirst-collected pollen grain identification and for assistance with the pollen data handling. Development of lidar retrieval algorithms was supported by the Russian Science Foundation (projects 16-17-10241 and 21-17-00114).
Financial support
This research has been supported by the Agence Nationale de la Recherche through the PIA under contract R-LABEX0-20-007-CAPPA, the “Hauts de France” Regional Council and the European Regional Development Fund (FEDER).
Review statement
This paper was edited by Daniel Perez-Ramirez and reviewed by three anonymous referees.
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