ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-17735-2018Angular scattering of the Sahara dust aerosolAngular scattering of the Sahara dust aerosolHorvathHelmuthhelmuth.horvath@univie.ac.atAlados ArboledasLucashttps://orcid.org/0000-0003-3576-7167Olmo ReyesFrancisco JoséUniversity of Vienna, Faculty of Physics, Aerosol Physics and
Environmental Physics, 1090 Vienna, AustriaUniversity of Granada, Department of Applied Physics, 18071 Granada,
SpainAndalusian Institute for Earth System Research (IISTA-CEAMA), Granada,
SpainHelmuth Horvath (helmuth.horvath@univie.ac.at)13December2018182317735177449May201812June201826November201828November2018This 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/18/17735/2018/acp-18-17735-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/17735/2018/acp-18-17735-2018.pdf
Soil erosion aerosols can be transported considerable distances, the Sahara
being one of the major sources in the world. In June 2016 the volume
scattering function of the atmospheric aerosol was determined in the Sierra
Nevada, Spain, at an altitude of 2500 m. Measurements were performed with a
polar nephelometer permitting measurements between scattering angles of 5 to
175∘. The values at the missing angles could be estimated to a high
accuracy, using the shape of the scattering function adjacent to the missing
angles, and thus a complete volume scattering function was available. During
the measuring period intrusions of long-range transported Sahara aerosol
happened several times. The classification of the aerosol was done by back
trajectories and by the Angström exponent of the wavelength-dependent
scattering coefficient, which was determined by a three-wavelength
Integrating Nephelometer. The phase function of the Sahara aerosol had a
stronger forward scattering, and less backscattering compared to the
non-Sahara aerosol, which is in agreement with other findings for irregular
particles. The asymmetry parameter of the phase function is the best
characteristic to distinguish Sahara aerosol from non-Sahara aerosol. In this
study the asymmetry parameter for the Sahara aerosol was larger than 0.65,
whereas the non-Sahara aerosol had an asymmetry parameter below 0.6. A
comparison with measurements performed with long-range transported Gobi
desert aerosols observed in Kyoto, Japan, showed very similar results.
Introduction
Deserts are a major source of aerosol particles. On a global scale desert
aerosol contributes 60–1800 Tg yr-1 of the total yearly aerosol
production of 2900–4000 Tg (Jaenicke, 1988). Junge (1979) estimates the
global source strength of deserts as 260 to 400 Tg yr-1, with the
Sahara contributing 60 to 200 Tg yr-1. Zender et al. (2004), using all
estimates published between 2001 and 2004, give a range of global dust
emissions of between 1000 and 2150 Tg yr-1. Recently the total PM10
(particles smaller than 10 µm) emission of desert dust particle
emissions was estimated as 1700 Tg yr-1; the atmospheric loading of
dust particles PM10 is estimated as 20 Tg (Kok et al., 2017). For
PM20 (particles <20µm) the values are 3000 Tg yr-1
and 23 Tg. The clay-sized particles (diameter smaller than 2 µm)
are an important subgroup due to their efficient light scattering, but
account only for 3.5 % to 5.7 % of the PM10 emissions.
In contrast to most of the other particles present in the atmosphere, desert
aerosol particles are produced from minerals by mechanical processes, which
obviously led to irregularly shaped particles. This is documented by
innumerous electron micrograph studies (see e.g. Falkovich et al., 2001;
Iwasaka et al., 2003; Kandler et al., 2007, 2011). The mechanical production of particles results in
particle sizes usually larger than 1 µm. Mean sizes range up to
10 µm (Kontratyev et al., 2006; Alfaro et al., 2003; Cheng et al.,
2005; Xin et al., 2005; Alfaro and Gomez, 2001; Falkovich et al., 2001), and during atmospheric transport
the size distribution is modified, mainly due to the shorter lifetime of
larger particles: the average residence time in the troposphere is estimated
as 10 days for 1 µm particles and 3 days for 10 µm
particles (Jaenicke, 1988). Other estimates of the lifetime of the
size-dependent desert particles are 11 days for 1 µm and 0.4 days
for 15 µm (Kok et al., 2017). Obviously the particle size
distribution is modified when transported in the atmosphere, since larger
particles have a considerable sedimentation velocity; therefore, the
clay-sized particles amount to 15 % of the global atmospheric load, while
accounting only for 4.3 % of the emitted mass. With the global wind
system the smaller Sahara aerosol particles can be transported considerable
distances, have been observed in the Amazonas basin (Formenti et al., 2001)
and are a major source of mineral supply to the Amazonas or Congo basins
(Okin et al., 2004).
Among many other effects, the desert aerosol is expected to have an influence
on the radiative balance; see e.g. Obregón et al. (2015), Valenzuela et al. (2012a, b), and Antón et
al. (2014). Due to the irregular shape of the particles, they have a larger
surface-to-volume ratio, leading to a higher extinction efficiency compared
to volume-equivalent spherical particles (Kocifaj and Horvath, 2005; Kok et
al., 2017). The maximum efficiency is around an equivalent diameter of
1 µm; thus, the clay-sized particles are especially important.
Recently it has been found that the cooling effect of desert aerosol might be
smaller than expected (Kok et al., 2017): in the source region coarse dust
particles (>5µm) dominate and they absorb both solar and
thermal radiation; furthermore, the effect is augmented by the bright surface
of the desert; distant from source regions the cooling effect of the fine
dust seems to be less than previously anticipated.
The dust particles have an irregular shape; thus, their scattering properties
are difficult to model. If the shape, size, and possible inhomogeneities of
the particles are known, the scattering matrix of the particle could be
calculated (Lindqvist et al., 2014). Since it is not possible to have an
electron micrograph of every particle, simplifications are used: Lindqvist et
al. (2014) considered the following models: homogeneous particles of
irregular shape, Gaussian random spheres, spheroids and spheres. Whereas the
scattering properties of homogeneous irregular particles do not differ much
from the reality of inhomogeneous particles, the other models give
considerable deviations; e.g. when assuming spheres instead of irregular
chrysotile particles, the asymmetry parameter is 0.66 instead of 0.82, and
the lidar ratio is 4 instead of 48 sr-1. Obviously this discrepancy
cannot be tolerated. Thus several indirect methods are in use, such as
inverting sun and sky radiation measurements (e.g. Estellés et al.,
2007), using measured size distributions and Mie theory to calculate the
scattering, or using an integrating nephelometer with two scattering ranges
and a Henyey–Greenstein model phase function (Andrews et al., 2006). Model
assumptions always have to be made in order to obtain the complete scattering
properties of the aerosol containing irregularly shaped particles, which may
create considerable uncertainties as shown above. But the exact scattering
properties, especially the asymmetry parameter, are urgently needed to
estimate the effect on climate. Below we report the first direct measurements
of the complete volume scattering function of the Sahara aerosol transported
to the Iberian Peninsula, which were performed during several dust outbreaks.
No assumption or model is needed in order to obtain the volume scattering
function and the asymmetry parameter.
Definitions, units, and nomenclature
The volume scattering function γ(θ) of the aerosol is defined
as follows. See Fig. 1: let a volume dV of aerosol be illuminated
by radiation with a flux density S. The light flux dΦ
scattered into a solid angle dω at the scattering angle
θ is obtained as
dΦ=S⋅γ(θ)⋅dω⋅dV.
The unit is [γ]= m-1 sr-1. The total scattering
coefficient σs is obtained by integrating the volume
scattering function γ(θ) over a whole solid angle:
σs=∫04πγ(θ)dω,
or for scattering with rotational symmetry with respect to the incident beam:
σs=2π∫0πγ(θ)sin(θ)dθ.
The scattering coefficient can be understood as the fraction of the light
flux scattered per unit length out of a parallel beam of light; its unit is
[σs] = m-1. Both the volume scattering function and
the scattering coefficient are extensive properties.
Definition of the volume scattering function.
The phase function is an intensive property and describes the relative
angular dependence of the scattered light of a volume element of particles.
For the phase function P(θ) we have used the following definition:
P(θ)=4⋅π⋅γ(θ)/σs.
The angular distribution of the scattered light frequently is characterized
by two parameters.
The asymmetry parameter g is obtained by folding the phase function with
cos (θ); therefore,
g=1/2∫0πP(θ)sin(θ)cos(θ)dθ.
The asymmetry parameter is zero for symmetric scattering such as Rayleigh
scatter of the air molecules and g=1 for only forward scatter. Another
characteristic is the fraction b of the backscattered radiation. It is
obtained by integrating
1/2P(θ)sin(θ)between1/2πandπ.
Both parameters g and b are intensive. The lidar ratio, S, is defined
as the ratio of the extinction coefficient, σe, and the
volume scattering function at 180∘, γ(180∘), i.e. S=σe/γ(180∘), or S=4π/([P(180∘)⋅ω], with ω the single scattering albedo, i.e. the ratio
of the scattering coefficient to the extinction coefficient. The single
scattering albedo cannot be measured directly with the polar nephelometer. We
have used values for ω, which were obtained by inverting data from
sun and sky photometers during this study. The average for the Sahara aerosol
was ω=0.928 and ω=0.943 for the non-Sahara aerosol. These
values are in agreement with previous findings (Valenzuela et al., 2012a, b).
The measurement of the scattering coefficient at three wavelengths (e.g. with
an integrating nephelometer, Charlson et al., 1967) permitted the
determination of the wavelength dependence of the scattering coefficient. In
many cases it can be represented by a power law relation σ(λ)=σ(λ0)⋅(λ/λ0)α with α
the Ångström exponent (Ångström, 1929, 1930). The value of
α is independent of the absolute magnitude of the scattering
coefficient, i.e. the concentration of the particles. But it is different for
different types of aerosols and mainly influenced by the size distribution of
the particles. For a power law number size distribution given by
dn/dr=n0⋅(r/r0)-ν, the exponent
α can be obtained by α=3-ν (Junge, 1963). For an aerosol with a size distribution
with ν>3, the smaller particles dominate and α<0; i.e. the scattering
coefficient decreases with increasing wavelength, which is the normal case.
If the particles are larger than a few micrometres, then ν<3 and the
Ångström exponent is zero or even positive. A strong wavelength
dependence of the scattering coefficient (α<-1) is an indication of
the non-Sahara aerosol, whereas little or almost no dependence on wavelength
is a sign of an aerosol containing larger particles, in this study mainly
desert particles.
Instrument and methods
The volume scattering function of the aerosol has been determined by a
custom-made polar nephelometer; its design is similar to the one of
Waldram (1945). See Fig. 2: light from a 532 nm solid-state laser of 10 mW
power shines into a light trap and illuminates the particles within the beam.
A photomultiplier with a collimation optic is mounted on a goniometer and can
measure scattered light between 5 and 175∘. The scattering volume is
approximately 70 mm3 at a 90∘ scattering angle, increasing to
800 mm3 at 5 and 175∘ respectively. Measurements are taken at
intervals of 5∘, in the near-forward direction at 1 to 2∘ intervals.
For a one-volume scattering function a scan from 175 to 5∘ and back
is made and the average is used. One full scan takes 35 min, which can pose
a problem with rapidly changing aerosols. Therefore a check for differences
between the forward and back scans was made and if the difference was larger
than a factor of 1.5, the data were not used. The instrument is enclosed in
an airtight housing; by sucking air out of the enclosure, air from outside
can be brought into the instrument. Calibration is done by filling the
instrument with carbon dioxide, whose volume scattering function is well
known. For all data reported below, the scattering of the air molecules has
been subtracted; i.e. all scattering functions, phase functions, asymmetry
parameters, or backscattered fractions are for aerosol particles only.
Principle of the polar nephelometer.
Experimentally it is impossible to measure the volume scattering function
from 0 to 5∘, and 175 to 180∘, but the contribution of this
range cannot be neglected, especially the forward region. The measured volume
scattering function and its shape between 10 and 5∘ and 170 and
175∘ can be used to extrapolate the missing regions. This can be done
with an accuracy of better than 5 % (Horvath, 2015), and thus the
complete scattering function is available.
Classification of back trajectories.
NameAir passes overRemark“Atlantic”60∘ W < longitude < 0∘, 35∘ N < latitude < 40∘ N“Atlantic North”40∘ W < longitude < 0∘, 60∘ N < latitude < 40∘ N“Sahara”10∘ W < longitude < 5∘ E, 37∘ N < latitude < 26∘ N“Sahara high”5∘ W < longitude < 5∘ E, 37∘ N < latitude < 26∘ N“Sahara/Atlantic”, “Atlantic/Sahara” resp.Up to 500 km west of the coast of AfricaDistinction between the two by Angström exponent“North Africa/Mediterranean”6∘ W < longitude < 8∘ E, 35∘ N < latitude < 37∘
The flow rate through the instrument was 3.3×10-4 m3 s-1. The connection to the sampling inlet of the
field laboratory was by a slightly downwards inclined hose of a diameter of
10 mm and a length of 2.5 m. Using data given by Hinds (1999, chap. 10),
the loss of particles due to sedimentation in the tube amounted to 10 %,
2 %, and 0.3 % for particles with diameters of 10, 5, and
2 µm. Losses due to diffusion were below 0.02 %. Thus it can be
concluded that for the most important particle sizes below 5 µm the
sampling losses are negligible.
A TSI 3563 Integrating Nephelometer was used to measure the scattering
coefficient for red (700 nm), green (550 nm), and blue (450 nm) light. A
detailed description of the instrument and its operation can be found at the
NOAA website:
https://www.esrl.noaa.gov/gmd/aero/instrumentation/neph_desc.html (last
access: 5 December 2018).
Sampling took place in the Albergue Universitario of the University of
Granada, located in the Sierra Nevada at an elevation of 2505 m a.s.l. Its
coordinates are 37∘5′43.72′′ N, 3∘23′12.57′′ W. The
surrounding mountains had elevations of approx. 3400 m, extending to
distances of 20 km from the sampling location. All instruments were located
in a room below the flat roof of the building. A pipe with a diameter of
10 cm extended 2.1 m above the roof and into the laboratory. A blower
maintained a flow of 0.00167 m3 s-1; all instruments used in the
SLOPE campaign (Integrating Nephelometer, Aerodynamic Particle Sizer (APS),
Multi Angle Absorption Photometer (MAAP), Scanning Mobility Particle
Spectrometer (SMPS), Aethalometer (A33)), and the polar nephelometer) sampled
from this pipe. The residence time of the air in the pipe was 0.6 s. Losses
in the pipe can be considered negligible.
Back trajectories used for classification of aerosol
types.
Measurements were made in the framework of the SLOPE (Sierra Nevada Lidar
AerOsol Profiling Experiment) campaign between 6 and 30 June 2016. This
campaign mainly was intended to determine the vertical structure of the
aerosol by remote sensing instruments and test the various retrieval schemes
for obtaining microphysical and optical properties. So the main instruments
were sun and sky photometers, multiwavelength lidar, and an airplane.
Obviously ground-based instruments were also used, as described above. The
polar nephelometer could not be operated continuously, due to instrument
failures and the absence of the operator; still, in total 120 phase functions
were determined. During this time several intrusions of Sahara dust occurred:
usually the dust was layered and could be recognized with the unaided eye.
From May to September 2016 there were a total of 15 Sahara dust events, on
96 days out of 153. For details, see
https://www.mapama.gob.es/es/calidad-y-evaluacion-ambiental/temas/atmosfera-y-calidad-del-aire/episodiosnaturales2016_tcm30-379284.pdf
(last access: 8 December 2018). In June 2016 the events were on the following
days: 2–3, 6–11, and 21–30. To distinguish dust aerosols from others, two
methods have been applied. (1) The size of the dust particles is larger than
1 µm; thus, the Angström exponent normally is larger than -1,
whereas the non-Sahara aerosol has Angström exponents below -1.
(2) Using 72 h back trajectories (from the NOAA HYSPLIT website, Draxler and
Rolph, 2003), the likely origin of the particles can be estimated. Since the
Sierra Nevada is a small mountain range, most of the time the air masses
reached the measuring location (at an elevation of 2500 m a.s.l.) without
admixing of particles of possible nearby sources. The origin of the air mass
was classified in a total of six groups. Figure 3 shows the typical
situations: Sahara, Sahara high, Atlantic, Atlantic North, Atlantic/Sahara,
North Africa/Mediterranean. From the source region in the Sahara to the
receptor region in the Sierra Nevada, the particles travelled around
1500 km. Table 1 characterizes the six types which occurred during this
campaign.
Angström exponent of the spectral scattering coefficient of the
aerosol measured and classification by back trajectories. For the desert
aerosol the Angström exponent is larger than -1. In addition, the
asymmetry parameter is plotted too; for desert aerosol it is larger than
0.65.
Measured scattering coefficient of the aerosol during the three
periods of observation. The solid red line is the signal of the Integrating
Nephelometer; the points are scattering coefficients obtained by integrating
the measured volume scattering function.
Comparison of the phase function attributed to Sahara and non-Sahara
aerosols.
A plot of the Angström exponent as a function of time measured with the
three-wavelength Integrating Nephelometer is shown in Fig. 4. The
classification using the back trajectories is given above. It is evident that
for aerosols influenced by the Sahara, the Angström exponent is larger
than -1; thus, the two methods of determining Sahara aerosols agree in most
of the cases.
Results
An overview of the scattering coefficient obtained by integration of the
measured volume scattering function and the value measured with the
Integrating Nephelometer is shown in Fig. 5, and only the light scattering of
the particles is plotted; i.e. the Rayleigh scattering of the air was
subtracted. The data obtained with the Integrating Nephelometer are values,
taken at intervals of minutes, and depict also the variability of the layered
aerosol. The scattering coefficient obtained by integration of the polar
nephelometer data are averages of about 35 min. Agreement between the two
datasets is evident. Three periods of measurements can be seen. During
Period 1 mostly a distinct intrusion of Sahara aerosol was observed, except
for the beginning; in Period 2 mainly aerosol from the Atlantic reached the
site, whereas during Period 3 again Sahara particles dominated the aerosol.
The air masses passing over the Atlantic had a much lower scattering
coefficient, being about twice the one of pure air.
A plot of all Sahara and non-Sahara phase functions is depicted in Fig. 6.
There is some scatter in the data due to the layered aerosol, but it is
evident that the phase functions of the Sahara and non-Sahara aerosol are
different. The averages of all phase functions of definite Sahara origin and
of definite non-Sahara origin and the standard deviations are shown in
Fig. 7.
Average of the Sahara and non-Sahara aerosol-phase
functions.
Discussion
The comparison between the two types of average phase functions of Fig. 7
shows definite differences. The non-Sahara (mainly Atlantic) phase function
has less forward scatter (on average 31 sr-1 at 0∘) than the
Sahara phase function (62 sr-1). This is readily explained by the
larger size of the desert particles. Whereas spherical particles are a good
approximation for the near-forward scattering, the smaller backscattering of
the Sahara phase function compared to the non-Sahara particles only can be
explained by the irregular shape of the particles, for
spherical large-particle
interferences and resonances are most pronounced, which leads to a
considerable increase in backscattering and a low side scattering, both of
which have not been observed. For irregularly shaped particles which
furthermore are randomly oriented by Brownian rotation, the backscattering is
by far less, up to a factor of 10 compared to spherical particles; see e.g.
Von Hoyningen-Huene and Posse (1997) or Mishchenko et al. (1997),
Mishchenko (2000), and Nousiainen and Kandler (2015).
Relationship between the measured Angström exponent and the
asymmetry parameter using the measured phase function.
The complete volume scattering function or phase function can be used to
determine derived properties. The scattering coefficient is obtained by
integrating the volume scattering function over the full solid angle. A
comparison with the calculated and measured scattering coefficients was shown
in Fig. 5; the agreement is evident.
For modelling of e.g. radiative transfer, the asymmetry parameter of an
aerosol is an important parameter. It is obtained by folding the phase
function with the cosine of the scattering angle. For the average phase
functions shown in Fig. 7 the asymmetry parameter for the Sahara aerosol
particles is 0.71 with a standard deviation of 0.03; for the non-Sahara
particles it is 0.56±0.04. The difference is significant; therefore, the
asymmetry parameter is a good indicator for the desert aerosol. In Fig. 4 the
measured asymmetry parameter is added to the graph of the Angström
exponent and the origin of the aerosol. Whenever the origin of the aerosol
indicates desert particles, the asymmetry parameter is high. At the same time
the Angström exponent also is high. Figure 8 is a plot of all the data
points and shows the relationship between the Angström exponent and the
asymmetry parameter for all measured phase functions for which Integrating
Nephelometer data and polar nephelometer data were available simultaneously.
Evidently a larger asymmetry parameter is associated with a larger
Angström exponent, but the relationship is not very pronounced.
Backscattered fraction measured with the Integrating Nephelometer
(red curve) and values obtained by the measured volume scattering function.
Simulation of the truncation is shown by the hollow points.
The scattering function of the desert aerosol has a low back scattering,
which is typical for non-spherical particles. Therefore it is to be expected
that a characterization of desert aerosol particles could be achieved by
considering the fraction of backscattered light. It is defined as the ratio
of the integral of the volume scattering function between 90 and 180∘
divided by the integral over the full angle and is readily available once the
volume scattering function is known. A time series of the backscattered
fraction obtained in this way is shown in Fig. 9, black squares; the
backscattered fraction obtained from polar nephelometer measurements is lower
for the aerosol dominated by desert particles, as expected, but less distinct
than the asymmetry parameter. Furthermore the backscattered fraction obtained
with the Integrating Nephelometer (red line) is systematically larger, which
has the following reason: the backscattered fraction for the Integrating
Nephelometer is obtained by dividing the signal BbsG (signal of the
Nephelometer in the backscattering range for green light) by BsG (signal of
the total scattered light). Both signals are proportional to the light flux
scattered by the aerosol, but they are truncated, since it is experimentally
impossible to integrate the scattered light flux from 90 to 180∘ and
0 to 180∘ respectively. But the measured volume scattering functions
permits the simulation of the truncation effect; and for a range of the
scattering angles between 8 and 170∘ the simulated BbsG is obtained
by integrating the measured volume scattering function from 90 to
170∘ and BsG is obtained by integrating the measured volume
scattering function from 8 to 170∘. The simulated truncated
backscatter fraction is calculated by dividing the simulated BbsG by the
simulated BsG. The expected signal is also shown (blue circles), which is in
much better agreement with the Integrating Nephelometer data.
Asymmetry parameter and backscattered fraction calculated for the
aerosols measured during this study. A clear separation between Sahara and
non-Sahara aerosols is evident. The data points are in excellent agreement
with data for the Gobi desert aerosol.
For the Sahara-particle-dominated aerosol the asymmetry parameter is larger
and the backscattered fraction is smaller than for the non-Sahara aerosol. So
it is obvious to use both parameters to characterize the Sahara aerosol. A
plot of all data points in the (b,g) plane is shown in Fig. 10. The points
representing the Sahara aerosol and the non-Sahara aerosol are well
separated. The curve gives the relationship between the backscattered
fraction and the asymmetry parameter calculated for monomodal spherical
particles. For other refractive indices or ellipsoidal particles an almost
identical curve is obtained; the point of inflection is at a slightly
different location. For bimodal size distributions the data points are
located to the right of the curve (Horvath et al., 2016), as is the case for
the data of this study. A clear distinction between the points representing
Sahara and non-Sahara particles is possible. Additionally, points are
plotted, which were obtained in Kyoto, Japan, during an event of long-range
transport dust intrusion from the Gobi. These data perfectly fit together
with the Sahara data.
Conclusion
The volume scattering function of the atmospheric aerosol was measured in the
Sierra Nevada, where intrusions of Sahara aerosol are frequent. The origin of
the aerosol particles was determined by back trajectories and/or by the
Angström exponent of the wavelength dependence of the scattering
coefficient of the aerosol. Usually it took 48 h or more from the origin to
the measuring site. Thus particles larger than 8 µm have been lost
by sedimentation and the remaining coarse dust particles were reduced
considerably.
The phase function of the Sahara aerosol has more forward scatter, and less
back scatter; it is more asymmetric than the non-Sahara aerosol, which in
this study was mainly marine, with little continental influence. A few
parameters of the aerosol are listed in Table 2.
Characteristics of the two types of aerosols.
Sahara Non-Sahara aerosol aerosol ValueSDValueSDAsymmetry parameter (–)0.710.030.560.05Backscattered fraction (–)0.0940.0140.1530.027Average scattering coefficient (Mm-1)712222.37.3Phase function at 0∘ (sr-1)67273110Phase function at 90∘ (sr-1)0.260.030.350.04Phase function at 180∘ (sr-1)0.310.110.590.21Lidar ratio (sr-1)4416238
The best distinction between Sahara and non-Sahara aerosol is possible when
using the asymmetry parameter, as is evident also in Figs. 4 and 10. This
study suggests that particles causing an asymmetry parameter of the phase
function above a value of 0.65 could be considered of Sahara desert origin.
The asymmetric scattering of the Gobi desert aerosol is very similar to the
one of the Sahara aerosol.
The asymmetry parameter can be used as a distinction between aerosol
dominated by desert particles and the other aerosol particles. In this work
the desert-dominated particles have an average asymmetry parameter of
g=0.71, whereas the mainly marine aerosol has g=0.56. The asymmetry
parameter of the aerosol measured in Vienna is below 0.65. This aerosol is
little influenced by maritime particles and not at all by particles of desert
origin; it could be considered as continental background plus traffic-related
particles (Horvath et al., 2016). The PM10 is regulated in Europe's air
quality standards to a 24 h average of 50 µg m-3 with 35
permitted exceedances per year, and a yearly average of
40 µg m-3. Frequent dust intrusions might make it impossible
to comply with the regulations; e.g. in 2016 Sahara dust intrusions in the
vicinity of the Sierra Nevada were on 96 days. But natural sources are
allowed to be excluded from the rules governing the air quality standards if
their origin can definitely be proven. In that case the measurement of the
asymmetry parameter, possibly in combination with the backscattered fraction,
definitely will create clarity.
All measured phase functions are available in the
Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-18-17735-2018-supplement.
LAA and FJOR planned the SCOPE project and its financing, supplied the
infrastructure in the Sierra Nevada site, took care of the operation and data
analysis of the Integrating Nephelometer as well as many other instruments,
and supplied data on dust events. HH designed and operated the polar
nephelometer, reduced the data and performed the data analysis, wrote the
draft and after through discussion with all authors finalized the paper.
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by the Andalusia Regional Government through project
P12-RNM-2409, by the Spanish Agencia Estatal de Investigación, AEI,
through projects CGL2016-81092-R and CGL2017-90884-REDT. We acknowledge the
financial support by the European Union's Horizon 2020 research and
innovation programme through project ACTRIS-2 (grant agreement no. 654109).
The authors thankfully acknowledge the FEDER programme for the
instrumentation used in this work and the University of Granada that
supported this study through the Excellence Unit Program. Edited by: Paola Formenti Reviewed by: two
anonymous referees
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