STOTEN-12587; No of Pages 14
Science of the Total Environment xxx (2011) xxx–xxx
Contents lists available at ScienceDirect
Science of the Total Environment
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s c i t o t e n v
Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and
Mie calculation over Milan
L. Ferrero a,⁎, G. Mocnik b, B.S. Ferrini a, M.G. Perrone a, G. Sangiorgi a, E. Bolzacchini a
a
b
POLARIS Research Center, Department of Environmental Sciences, University of Milan-Bicocca, Piazza della Scienza 1, 20126, Milan, Italy
Aerosol d.o.o., Kamniska 41, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history:
Received 29 August 2010
Received in revised form 27 January 2011
Accepted 11 April 2011
Available online xxxx
Keywords:
Black carbon
Vertical profile
Absorption coefficient
Aethalometer
Optical particle counter
Particulate matter
Air pollution
a b s t r a c t
Vertical profiles of aerosol number–size distribution and black carbon (BC) concentration were measured
between ground-level and 500 m AGL over Milan. A tethered balloon was fitted with an instrumentation
package consisting of the newly-developed micro-Aethalometer (microAeth® Model AE51, Magee Scientific,
USA), an optical particle counter, and a portable meteorological station. At the same time, PM2.5 samples were
collected both at ground-level and at a high altitude sampling site, enabling particle chemical composition to
be determined. Vertical profiles and PM2.5 data were collected both within and above the mixing layer.
Absorption coefficient (babs) profiles were calculated from the Aethalometer data: in order to do so, an optical
enhancement factor (C), accounting for multiple light-scattering within the filter of the new microAeth®
Model AE51, was determined for the first time. The value of this parameter C (2.05 ± 0.03 at λ = 880 nm) was
calculated by comparing the Aethalometer attenuation coefficient and aerosol optical properties determined
from OPC data along vertical profiles. Mie calculations were applied to the OPC number–size distribution data,
and the aerosol refractive index was calculated using the effective medium approximation applied to aerosol
chemical composition. The results compare well with AERONET data.
The BC and babs profiles showed a sharp decrease at the mixing height (MH), and fairly constant values of babs
and BC were found above the MH, representing 17 ± 2% of those values measured within the mixing layer. The
BC fraction of aerosol volume was found to be lower above the MH: 48 ± 8% of the corresponding ground-level
values.
A statistical mean profile was calculated, both for BC and babs, to better describe their behaviour; the model
enabled us to compute their average behaviour as a function of height, thus laying the foundations for valid
parametrizations of vertical profile data which can be useful in both remote sensing and climatic studies.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Aerosols affect the climate due to their ability to scatter and absorb
sunlight (direct effect), and to act as cloud condensation nuclei (CCN),
thus modifying the lifetime of clouds, droplet size and precipitation
rate (indirect effect) (Ramanathan and Feng, 2009; IPCC, 2007; Koren
et al., 2004, 2008; Kaufman et al., 2002; Ramanathan et al., 2001;
Penner et al., 2001; Ackerman et al., 2000). Different aerosol species
(black carbon, sulfate, organics and dust) contribute to surface dimming (Ramanathan and Carmichael, 2008; IPCC, 2007); the absorbing
species (i.e. black carbon, dust) can absorb atmospheric sunlight thus
“masking” (and cooling) the surface whilst warming the atmosphere
in the process (Ramanathan and Carmichael, 2008). This can affect
⁎ Corresponding author. Tel.: + 39 0264482814; fax: + 39 0264482839.
E-mail address: [email protected] (L. Ferrero).
atmospheric thermal structure and regional circulation systems such
as monsoons (Ramanathan and Feng, 2009).
These processes depend strongly on the vertical distribution of
aerosols throughout the whole atmospheric column. Consequently,
measurements of vertical profiles are required in order to better
understand the effect of aerosols on climate (Corrigan et al., 2008;
Ramana et al., 2007; Podgorny and Ramanathan, 2001). These data
can be obtained by direct and indirect methods such as tethered
balloons (Ferrero et al., 2007, 2010; McKendry et al., 2004; Stratmann
et al., 2003; Maletto et al., 2003), aircraft (Taubman et al., 2006),
unmanned aerial vehicles (UAVs) (Corrigan et al., 2008; Ramana et al.,
2007), lidars (Kim et al., 2007; Amiridis et al., 2007; Eresmaa et al.,
2006) or sunphotometers (Schuster et al., 2005). Direct methods
enable the aerosol's physical–chemical and optical (scattering and
absorption) properties to be measured at the same time, and of such
methods, only balloons and UAVs can be used to collect long-term
measurements at a reasonable cost (Ferrero et al., 2010; Corrigan et al.,
2008). However, the payload limitations of these platforms require a
0048-9697/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.scitotenv.2011.04.022
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
2
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
new generation of light-weight, battery-powered miniaturized instruments (Ferrero et al., 2010).
Until recently, it was necessary to adapt commercial instruments
(e.g. Aethalometers) that were heavy and primarily designed for
installation in ground stations (Corrigan et al., 2008).
However, a new portable, lightweight, battery-powered microAethalometer (microAeth® Model AE51, Magee Scientific, USA) has
been developed. Its primary intended application is to measure
personal exposure to emissions such as diesel exhaust, but its small
size and light weight suggest its use for vertical profile measurements.
In this context, special attention has to be paid to the interpretation of
its data in terms of calculating an absorption coefficient.
Past studies of filter-based methods have clearly shown that
calculation of the absorption coefficient (babs) requires the application
of various different kind of compensatory factors to the light
attenuation signal measured by filter based absorption photometers,
due to the multiple light-scattering effect of the filter fibers, and to
aerosol loading on the sampling filter itself (Schmid et al., 2006;
Arnott et al., 2005; Weingartner et al., 2003; Bond et al., 1999). This
procedure, and the optical enhancement factor values, were basically
calculated for Aethalometers using synthetic, laboratory-generated
aerosols (Arnott et al., 2005; Weingartner et al., 2003) and from a
limited number of ambient aerosol measurement campaigns conducted at ground-level (Schmid et al., 2006; Arnott et al., 2005;
Weingartner et al., 2003); as a result, the estimated uncertainty in the
retrieved absorption coefficients may range from 5% to 40% (Corrigan
et al., 2008). Vertical profile application introduces another uncertainty factor, due to the sampling, on the same filter, of different kinds
of aerosols collected at different heights in different atmospheric
layers.
To investigate these effects, we measured vertical profiles of
aerosol properties above Milan, one of the most polluted areas of
Europe. We also derive an optical enhancement factor suitable for use
along vertical profiles, calculated for the new microAeth® Model
AE51. We also discuss vertical profiles of BC and absorption coefficient
measured in this location.
Fig. 1. Tethered balloon (a) fitted with: the OPC 1.108 “Dustcheck” (Grimm Aerosol
Technick), the meteorological station (LSI-Lastem) and the microAeth® Model AE51
(Magee Scientific) (b).
atmosphere, and rendering the standardized results directly comparable with those obtained at different locations, heights, dates and
times; UTC time is used throughout this article.
2. Experimental
2.1. Aerosol characterization
2.1.1. Vertical aerosol profiles
Measurements of vertical aerosol and BC profiles were carried out
at the Torre Sarca site in Milan (University of Milan-Bicocca; 45°31′
19″N, 9°12′46″E), situated in the Po Valley basin, on the 2nd and 3rd
of December 2008 (11 profiles). The instrument package consisted of:
1) an optical particle counter (OPC, 1.108 “Dustcheck” Grimm, 15
class-sizes ranging from 0.3 μm to 20 μm); 2) the new microAethalometer microAeth® Model AE51 (Magee Scientific); 3) a
meteorological station (BABUC-ABC, LSI-Lastem: pressure, temperature
and relative humidity). The sampling platform was carried aloft by a
helium-filled tethered balloon (diameter 4 m, volume 33.5 m3, payload
15 kg), both shown in Fig. 1. An electric winch controlled the ascent and
descent rate, which was set at a fixed value of 30.0±0.1 m/min; a
measurement time resolution of 6 s was chosen for each instrument,
giving 3.0 m of vertical resolution for each measurement. The maximum
height reached during each launch depended on atmospheric conditions;
for the majority of profiles, maximum height was 510 m AGL. Tethered
balloon soundings have already been successfully used in the Po Valley to
measure aerosol property changes (number size distribution and
chemical composition) as a function of height in the low troposphere
over Milan; further details of the experimental approach can be found in
Ferrero et al., 2010.
All vertical profile data were normalized to standard conditions
(0 °C, 1013 hPa), as suggested by Hänel (1998), making measurements independent of the actual thermodynamic state of the
2.1.2. Aerosol sampling and chemical characterization
During the course of vertical profile measurements, PM2.5 samples
were also collected at ground-level (at the Torre Sarca site) and at a
remote, high-altitude site (Alpe S. Colombano, 2280 m ASL, 46°27′17″N,
10°18′53″E). The Alpe S. Colombano site lies on the Italian side of the
Alps, facing northern Italy. During winter, it is located in the free
troposphere; during summer, the site is influenced by regional
transport, a higher mixing height and valley breezes bringing polluted
air masses from the Po Valley up to the site (Ferrero et al., 2005; Belis
et al., 2006).
The Torre Sarca and Alpe S. Colombano sites have been active since
2005; PM2.5 is constantly monitored (in accordance with EN-14907
standards), and was sampled at both sites using the dual channel FAIHydra gravimetric system (PM2.5 sampling head, flow 2.3 m3/h;
WHATMAN pre-fired quartz fiber filters, Ø = 47 mm).
PM2.5 chemical composition was assessed at both sites. PM2.5
samples were analyzed using an ion chromatography coupled system
(Dionex ICS-90–ICS-2000), in order to determine the ionic inorganic
fraction (Ferrero et al., 2010; Perrone et al., 2010), and using the
Thermal Optical Transmission method (TOT, Sunset Laboratory inc.;
NIOSH 5040 procedure, http://www.cdc.gov/niosh/nmam/pdfs/
5040f3.pdf) to quantify elemental and organic carbon (EC and OC)
content (Gualtieri et al., 2009). The ionic fraction is denoted here as
the water soluble (WS) fraction of the aerosol. The organic matter
(OM) fraction was estimated from OC using different coefficients to
account for the presence of hetero-atoms (H, O, N, etc.). Following
the work of Turpin and Lim (2001), the chosen factors were 1.6 for
the urban Torre Sarca site, and 2.1 for the remote site of Alpe S.
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
Colombano. Table 1 shows the particle chemical composition measured at the two sites.
2.2. Micro-Aethalometer
The new microAeth® Model AE51 was developed to measure
personal exposure to BC. Due to its light weight (250 g), small size
(117 × 66 × 38 mm3) and long-life battery (~ 24 h), it can be used in
vertical profile measurements on tethered balloons or UAVs. The
measured signal is the attenuation of light (ATN) from a LED
source at 880 nm, transmitted through a PTFE-coated borosilicate
glass fiber filter (Fiberfilm™ Filters, T60 material, Pall Corporation)
while being continuously loaded by the aerosols. The ATN is given
by:
ATN = 100⁎lnðI0 = IÞ
ð1Þ
where I0 and I are the light intensities transmitted throughout a
reference blank spot, and throughout the aerosol-laden 3 mm
diameter sample spot of the filter respectively. These data enable us
to estimate the absorption coefficient (babs) (Schmid et al., 2006;
Arnott et al., 2005; Weingartner et al., 2003) from:
babs =
A⋅Δ ATN
1
⋅
100Q ⋅Δt C ⋅Rð ATN Þ
ð2Þ
1
σATN
⋅
wavelength) operating in a test chamber with different BC concentrations at low attenuation values. The comparison was then repeated
using ambient air.
Eqs. (2) and (3) allow one to estimate babs, given BC and σATN; in
Eqs. (2) and (3), the expression (A ⁎ ΔATN)/(100Q ⁎ Δt) is also referred
to as the attenuation coefficient (bATN).
C and R(ATN) constitute the investigated parameters. Briefly, C is a
constant optical enhancement factor (≥1) which compensates for the
enhanced optical path through the filter caused by multiple scattering
induced by the filter fibers themselves; R(ATN), on the other hand,
rectifies the “shadowing” effect due to the enhanced absorption of
scattered light caused by an increase in aerosol loading over time,
which in turn results in a reduction in the optical path (Schmid et al.,
2006; Weingartner et al., 2003). Due to the different filter material
used in the microAeth® Model AE51, compared with that of other
Aethalometers, a new C value has to be calculated.
A full mathematical treatment of the aforesaid optical enhancement procedure is given in Schmid et al. (2006), Arnott et al. (2005)
and Weingartner et al. (2003).
In the aforementioned studies, optical enhancement factors are
calculated for other types of Aethalometer, and as reported in Schmid
et al. (2006), Eq. (2) is suggested for the purpose of calibrating the
Aethalometer's response. Hence, Eq. (2) is used in this present study
to calculate a C value for the new microAeth® Model AE51.
2.3. Mie calculation of absorption coefficient along vertical profiles
and the BC concentration from:
BC =
3
A⋅ΔATN
b
= ATN
100Q ⋅Δt
σATN
ð3Þ
where A is the sample spot area (7.1.10− 6 m2), Q is the volumetric
flow rate (1.7 10− 6 m3/s), ΔATN indicates the ATN variation during
the time period Δt (6 s), C is the multiple scattering optical
enhancement factor, and R(ATN) is the aerosol loading factor, which
depends on light attenuation. σATN (12.5 m2/g) is the apparent mass
attenuation cross-section for the black carbon collected on the PTFEcoated borosilicate glass fiber filter, considering the optical components of the instrument; σATN is provided by the manufacturer and
was obtained by comparing the BC values measured with the
microAeth® Model AE51, with an AE31 Aethalometer (880 nm
Aerosol optical properties were calculated using a Mie code, based
on the work of Bohren and Huffman (1983), along vertical profiles
from OPC data. As Guyon et al. (2003) and Howell et al. (2006) have
reported, OPC data can be used to estimate the aerosol optical
properties, and thus Mie theory can be applied to aerosol vertical
profile data.
The scattering efficiency is calculated on the basis of the
integration of the scattered power over all directions, and the
extinction efficiency results from the extinction theorem (Mätzler,
2002; Seinfeld and Pandis, 1998; Bohren and Huffman, 1983; Van de
Hulst, 1981; Ishimaru, 1978). The absorption efficiency (Qabs) is
calculated as the difference between the extinction and scattering
efficiencies. The absorption coefficient (babs) at 880 nm is calculated
Table 1
Aerosol properties considered in this study: aerosol mass fraction measured at the Milan and Alpe S. Colombano (ASC) sites (WS: water soluble; OM: organic matter; EC: elemental
carbon), density of each component (ρ) and the complex refractive index (m). The figures marked ts are those chosen for the purposes of this study, while the figures in brackets are
the ones most commonly found in studies of densities; those figures found in existing literature are marked (by letters) as follows.
ρ (g cm− 3)
Aerosol
component
Mass fraction (%)
Milan (ground)
ASC (2300 m asl)
WS
OM
EC
Missing mass
36.06
35.86
11.83
16.25
23.60
49.81
1.37
25.22
1.75ts (1.7a, 1.72b, 1.77b,c)
1.45ts (1.4c, 1.5a)
2.0ts,d,e (1.5f,g, 1.8c, 2d,e, 2.05h-2.25h)
1.0ts,i (H2O), 2.6ts,l (Dust)
m (n + ik)
m (n + ik)
λ = 880 nm
λ = 780 nm
1.520 + i0.012ts,m,n,o
1.520 + i0.012ts,m,n,o
2.09 + i0.60ts,a,p,q
1.324 + i4.05e− 7 ts,r (H2O), 1.520 +
i0.008ts,m,n,q (Dust)
1.525 + i0.010ts,m,n,o
1.525 + i0.010ts,m,n,o
2.07 + i0.61ts,a,p,q
1.326 + i1.41e− 7 ts,r (H2O), 1.525 +
i0.008ts,m,n,q (Dust)
a
Chazette and Liousse (2001).
Fierz-Schmidhauser et al. (2010).
c
Schmid et al. (2009).
d
Schuster et al. (2005).
e
Hand and Kreidenweis (2002).
f
Horvat (1993).
g
Janzen (1979).
h
Hess and Herd (1993).
i
Pesava et al. (2001).
l
Dong et al. (2010).
m
Shettle and Fenn (1979, 1976).
n
D'Almeida et al. (1991).
o
World Climate Programme (1986).
p
Ackerman and Toon (1981).
q
Raut and Chazette (2008).
r
Velazco-Roa and Thennadil (2007).
b
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
4
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
from the integration of Qabs over the whole number-size distribution
(Seinfeld and Pandis, 1998):
Dpmax
babs = ∫
0
πD2p
Qabs ðm; αÞn Dp dDp
4
ð4Þ
where m and α are the aerosol complex refractive index and the size
parameter at 880 nm respectively, and n(Dp) represents the numbersize distribution.
The aerosol refractive index was calculated from measured aerosol
chemical composition (Section 2.3.1). The aerosol size distribution
function was obtained from the log-normal interpolation of aerosol
number-size distribution data measured by the OPC. For this, the OPC
size channels were corrected in agreement with the ambient aerosol
refractive index. The size correction procedure and the log-normal
interpolation are discussed in Section 2.3.2.
2.3.1. Aerosol refractive index
Mie calculations require the aerosol complex refractive index
(m = n+ik) as an input parameter. In this study, coarse (dp N 1 μm)
and fine (dp ≤ 1 μm) particles have different refractive indexes. Coarse
particles (dp N 1 μm) are assumed to be composed of dust, while the
mean aerosol complex refractive index m of fine particles is calculated
from the aerosol chemical composition measured and reported in
Table 1.
The calculation of the mean aerosol refractive index m of the fine
particles, using chemical composition data, must be performed using a
mixing rule.
Many authors (Fierz-Schmidhauser et al., 2010; Liu and Daum,
2008; Stier et al., 2006, 2007; Riemer et al., 2003; Guyon et al., 2003;
Ebert et al., 2002; Pesava et al., 2001; Chazette and Liousse, 2001;
Horvath, 1998; Levoni et al., 1997) have used a linear volume-average
mixing rule or linear mass-average mixing rule (Raut and Chazette,
2008) in their studies. However, it is well known that a linear volume
(mass)-average mixing rule can overestimate the imaginary part (k)
of m in the presence of highly absorbing inclusions (i.e. BC) in a nonabsorbing medium (i.e. NH4NO3 and (NH4)2SO4) (Stier et al., 2007;
Lesins et al., 2002; Chýlek et al., 1995). A more correct approach would
be to consider all possible positions of the inclusions relative to the
host medium. Aspnes (1982) formulated such a mixing rule as
follows:
n
εeff −εh
ε −εh
= ∑ fi i
εeff + 2εh
ε
i=1
i + 2εh
ð5Þ
where εeff is the complex effective dielectric constant of the mixture
pffiffiffiffiffiffiffi
(meff ≈ εeff ), εh represents the dielectric function of the host medium,
εi and fi are the complex dielectric constant, and the volume fraction,
of the i-th component respectively.
Depending on the choice of host medium, we may obtain three
different mixing rules: 1) Maxwell–Garnett (MG) if the host medium
is one of the components (εi = εh) (Stier et al., 2007; Schuster et al.,
2005; Bohren and Huffman, 1983; Aspnes, 1982; Heller, 1965);
2) Lorentz–Lorenz (LL) if the host medium is the vacuum (εh = 1) (Liu
and Daum, 2008; Aspnes, 1982; Heller, 1965); 3) Bruggeman (BR) if
no choice of host medium is made, and inclusions are considered
embedded in the effective medium itself (Stier et al., 2007; Aspnes,
1982; Heller, 1965; Bruggeman, 1935): this approach is also known as
“effective medium approximation” (EMA).
Stier et al. (2007) and Aspnes (1982) point out that the BR mixing
rule overcomes the dilemma of the choice of host medium among the
various aerosol components. From this point of view, the BR mixing
rule considers all possible positions of each component (BC, dust,
water soluble materials…) in an aerosol particle, thus simulating the
real complexity of aerosols and making the BR mixing rule suitable for
use in calculating the aerosol meff. For this reason, the BR mixing rule
has been chosen here to calculate the aerosol effective complex
refractive index.
The refractive index within the mixing layer has been estimated
starting from ground-level particle chemical composition; above the
mixing layer, the refractive index is calculated from the particle
chemical composition considered as an average of that measured at
ground-level in Milan, and that measured in the free troposphere at
Alpe S. Colombano. The reliability of this assumption will be discussed
in Section 3.2: briefly, the BC content in aerosol volume, as well as the
ionic fraction, decreased above the mixing layer, approaching the
average value of Milan and Alpe S. Colombano.
The refractive indexes of pure aerosol components used in the
calculation (λ = 880 nm), as well as the pure densities (ρ) for each
aerosol component, are reported in Table 1 (together with the
respective references). Pure densities were used to estimate the volume
fraction of each aerosol component from the aerosol's chemical
composition, as reported in the literature (Fierz-Schmidhauser et al.,
2010; Pesava et al., 2001; Chazette and Liousse, 2001; Heller, 1965). The
value of BC refractive index m (2.09 + i0.60) was chosen because it lies
at the mid-point of the published data, and is consistent with the value
estimated in a smog chamber at 700 nm (Riemer et al., 2003). The value
of BC density ρ is the same as used by Schuster et al. (2005) in AERONET
data retrieval, and once again it is to be found in the mid-range of the
data reported in Table 1. The OM refractive index was assumed to be the
same as WS (the total aerosol ionic content). This assumption derives
from the observation of those values reported in the literature, which
are close to WS ones in the green region of the visible spectrum (Raut
and Chazette, 2008; Randriamiarisoa et al., 2006; Schmid et al., 2009);
furthermore, as Moosmüller et al. (2009) point out, the OM is not
absorbing in the infrared part of the spectrum, as brown carbon
absorption mainly takes place in the blue and ultraviolet parts of the
spectrum.
Many scholars presumed the missing mass was essentially water
adsorbed on particles (Pesava et al., 2001; Marley et al., 2001; Gebhart
and Malm, 1994), while others assigned the refractive index of dust to
the missing mass (Raut and Chazette, 2008). Since it has been shown
that a certain amount of water is chemically bound to particles
(Subramanian et al., 2007; Hueglin et al., 2005; Rees et al., 2004), and
also that this amount is comparable to dust in winter (Hueglin et al.,
2005; Rees et al., 2004), we have assigned the missing mass to both
water and dust.
Finally, the aerosol refractive index was modulated point by point
along vertical profiles, considering the hygroscopic growth of the
aerosol:
rw = r ⋅
1−RH
1−RHref
−ε
ð6Þ
where RH is ambient relative humidity, r is the aerosol radius at
RH = RHref (dry conditions) and rw at ambient RH; ε is the coefficient
controlling the aerosol's hygroscopic growth. The value of coefficient ε
was 0.26, as reported in both Randriamiarisoa et al. (2006) and in Raut
and Chazette (2008); this value was chosen because the ground-level
chemical composition of dry aerosol measured in Milan (Table 1), is
similar to that reported in the aforementioned studies. RHref was 65%,
and it was estimated from the aerosol chemical composition following
the work of Potukuchi and Wexler (1995a, 1995b); this value is also in
accordance with other mutual deliquescence relative humidity figures
reported in literature (Badger et al., 2006; Randriamiarisoa et al.,
2006).
From hygroscopic growth, the BR mixing rule was applied point by
point along height to calculate the final aerosol refractive index every
3.0 m for each profile.
Finally, the calculated aerosol refractive indexes were used to
calculate the absorption coefficient profiles (Eq. (4)): the results
thus obtained were compared to the Aethalometer data in order to
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
estimate the optical enhancement factor C (Section 3.3). The accuracy
of m estimation is discussed in Section 3.1.1.
2.3.2. Aerosol size distribution
Mie calculations also require the aerosol number size distribution
as an input parameter. The size distribution is measured using an OPC
(Grimm 1.108), whose size channels are calibrated using polystyrene
latex spheres (PLS, m = 1.59) which are non absorbing and whose
refractive index has a larger real part compared to ambient aerosol
(see Section 3.1.2). This may result in an “undersizing” of atmospheric
aerosols (Guyon et al., 2003; Liu and Daum, 2000; Schumann, 1990).
Moreover, the OPC has a PLS equivalent size range between 0.3 μm
and 20 μm, which renders the coarse mode clearly defined, while the
accumulation mode is only partially measured: no measurements are
available for Aitken mode particles. As many authors report (Randriamiarisoa et al., 2006; Guyon et al., 2003; Liu and Daum, 2000), if we
neglect the Aitken mode (dp b 0.1 μm) this may give rise to a ~2–4% error
in the calculation of aerosol optical properties, while the “truncation
effect” of the OPC in the accumulation mode (~0.3 μm, ~half of the
accumulation mode) can not be neglected, because particles in the
accumulation mode are highly efficient in absorbing light.
Thus, to avoid the first problem (“undersizing”), the OPC size
channels were corrected to account for the ambient aerosol refractive
index m. The size correction procedure requires the calculation of the
OPC response function (S) which describes the intensity of light
scattered into a given angular interval normalized to the incident
radiation; S represents the partial light scattering cross section of the
particle (cm2) related to the specific optical design of the OPC. For a
plane, linearly polarized, monochromatic wave λ, the response
function S can be computed as follow (Baron and Willeke, 2005;
Heyder and Gebhart, 1979):
5
size corrected channels: these results agree with literature studies (Liu
and Daum, 2000; Schumann, 1990). Table 2 shows that correction
resulted in an increase of the lower cutoff diameter, from 0.30 μm up to
0.33 μm; thus, if the size correction solves the OPC “undersizing”
problem, the “truncation effect” is accentuated by the same correction.
The complete aerosol size distribution function n(Dp) was therefore
obtained from the log-normal interpolation of aerosol number-size
distribution data measured by the OPC. This procedure makes the
calculated babs (Eq. (2)) closely dependent on the reliability of the
particle number-size distribution interpolation procedure. However,
the log-normal interpolation of OPC data has already been successfully
used by Deshler et al. (2003), and extinction profiles calculated from
balloon borne OPC have been successfully compared with extinction
profiles estimated using an automated lidar ceilometer (Vaisala LD40,
λ = 855 nm) installed at the Torre Sarca site at the same time as our
balloon campaign (Angelini et al., 2009). We discuss the accuracy of the
log-normal interpolation of the accumulation mode in Section 3.1.2
Moreover, to achieve the effect of size correction on the calculated
babs, the log-normal interpolation was conducted on both the two size
distributions: uncorrected (original OPC sizes); and corrected (for
SAMB). The results thus obtained were used to calculate uncorrected
and corrected babs that were compared to the Aethalometer data,
allowing to estimate uncorrected and corrected optical enhancement
factors C, namely CUSD (USD = “uncorrected size distribution”) and
CCSD (CSD = “corrected size distribution”) respectively (Section 3.3).
The final optical enhancement factor C for the microAeth® Model
AE51 is given in Section 3.3.
3. Results and discussion
3.1. Data quality
2
λ
∬ iðθ; ϕ; x; mÞ sinθdθdϕ
4π2 ΔΩ
ð7Þ
where θ0 represents the mean scattering angle of the optical arrangement, ΔΩ the receiver aperture, x the dimensionless size parameter,
m the refractive index and i(θ,ϕ,x,m) the Mie scattering function
composed by the perpendicular and parallel components: i1(θ,x,m) and
i2(θ,x,m) respectively.
The OPC 1.108 uses 780 nm polarized laser light to illuminate the
aerosols in the airflow, and the PLS calibration curve is calculated by
the manufacturer under the plane wave approximation (we gratefully
acknowledge the manufacturer for this information). The optical
arrangement of the OPC 1.108 is the same as that of the Grimm 1.109
OPC described in Heim et al. (2008), and consists of: 1) a wide angle
parabolic mirror (121°, from 29.5° to 150.5°, θ0 = 90°) that focuses
scattered light on the photodetector located on the opposite side;
2) 18° of direct collected scattered light on the photodetector (from
81° to 99°, θ0 = 90°).
Knowledge of the specific optical design of the OPC 1.108 allows us
to calculate the response function S (Eq. (7)) following the
methodology reported in Heyder and Gebhart (1979); the response
function was calculated both for PLS (SPLS) and for ambient aerosol
(SAMB) along each vertical profile, within and above the mixing layer.
The refractive indexes of ambient aerosol used in SAMB calculations
were calculated as reported in Section 2.3.1: 1) dust refractive index
for coarse particles (dp N 1 μm), 2) m for fine particles calculated
applying the EMA (Section 2.3.1) to the aerosol chemical composition
along vertical profiles, at the OPC laser wavelength (780 nm; Table 1
also reports m of pure aerosol components at 780 nm).
Calculations of SPLS and SAMB allow us to correct the size channels of
the OPC according to the ambient m. Fig. 2 shows the response curves
SPLS and SAMB for the average refractive index of the whole campaign
(m = 1.480 + 0.034i at 780 nm). Table 2 shows the corresponding new
3.1.1. Aerosol optical properties
The reliability of the calculated aerosol optical properties can be
evaluated by considering some parameters such as the refractive
1.0E-04
S_PLS
1.0E-05
S_AMB
1.0E-06
1.0E-07
S (cm2)
Sðθ0 ; ΔΩ; x; mÞ =
1.0E-08
1.0E-09
1.0E-10
1.0E-11
1.0E-12
1.0E-13
0.10
1.00
10.00
100.00
Fig. 2. Response curves for 1.108 Grimm OPC in terms of partial light scattering cross
sections for PLS (SPLS, m = 1.59) and ambient aerosol (SAMB, m = 1.480 + i0.034 for fine
fraction and m = 1.525 + i0.008 for coarse fraction).
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
6
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
index and the single scattering albedo (SSA) which, once averaged
along vertical profiles, can be compared with the same data measured
by the Aerosol Robotic Network (AERONET; Table 3). The closest
AERONET station is the Ispra site (E 8°37′36″,N 45°48′11″, 57 km from
the Torre Sarca site); AERONET data for the 2nd and 3rd of December
were not available, and we used the averaged values for December as
a reference in order to assess the accuracy of our optical estimation.
The average refractive index calculated in this study along vertical
profiles was 1.476(±0.032) + i0.034(±0.008), which is in keeping
with the columnar AERONET estimation of 1.457(±0.087) + i0.034
(±0.011).
Considering the SSA, the average values calculated in this study
along vertical profiles for the CSD and USD cases were SSACSD = 0.803 ±
0.040 and SSAUSD = 0.728 ± 0.074 respectively; these were in keeping
with the columnar AERONET estimation of 0.721 ± 0.069.
Table 2
Size channels of OPC 1.108 Grimm for PLS (m = 1.59) and ambient aerosol (m = 1.480 +
i0.034 for fine fraction: dp ≤ 1 μm; m = 1.525 + i0.008 for coarse fraction: dp N 1 μm).
OPC Channel
PLS (μm)
Ambient Aerosol (μm)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0.30
0.40
0.50
0.65
0.80
1.00
1.60
2.00
3.00
4.00
5.00
7.50
10.00
15.00
20.00
0.33
0.46
0.62
0.91
1.15
1.38
1.86
2.60
3.85
5.50
7.00
13.18
20.89
35.89
49.55
Table 3
Aerosol optical and size distribution properties calculated in this study at Torre Sarca site (TS) in Milan for both the USD (uncorrected size distribution) and CSD (corrected size
distribution) cases. Ispra site is located ~ 57 km far from Torre Sarca, while MB (Milano-Bresso) site is is located ~ 2 km far from Torre Sarca. n and k are the real and immaginary part
of the complex index of refraction. SSA indicates the Single Scattering Albedo, while Dg and σg the geometric mean diameter and the geometric standard deviation respectively. ML
indicates the mixing layer.
Parameter
This study — TS
USD
n columnar
k columnar
SSA columnar
Dg (μm) within ML
σg within ML
Dg (μm) columnar
σg columnar
1.476(± 0.032)
0.034(± 0.008)
0.728(± 0.074)
0.099(± 0.074)
1.802(± 0.102)
0.110(± 0.040)
1.810(± 0.156)
AERONET
Van Dingenen et al. (2004)
CSD
Ispra
MB morning
MB afternoon
MB night
0.803(± 0.040)
0.166(± 0.030)
1.894(± 0.257)
0.201(± 0.095)
1.851(± 0.351)
1.457(± 0.087)
0.034(± 0.011)
0.721(± 0.069)
–
–
0.152(± 0.036)
1.766(± 0.160)
–
–
–
0.114
1.83
–
–
–
–
–
0.096
2.00
–
–
–
–
–
0.123
1.74
–
–
Fitted particle conc. (cm -3)
1100
1000
n(Dp)_USD
900
n(Dp)_CSD
800
700
600
500
y = 1.091x - 7.760
R² = 0.998
400
300
y = 1.105x - 11.741
R² = 0.997
200
100
0
0
100
200
300
400
500
600
700
Measured particle conc.
800
900
1000
1100
(cm -3)
Fig. 3. Linear regression between measured and fitted particle number concentrations in the OPC size range (N0.3 μm). Blue circles indicate average particle number concentration
measured and fitted within and above the mixing layer for the corrected size distribution (CSD); red diamonds indicate average particle number concentration measured and fitted
within and above the mixing layer for the uncorrected size distribution (USD). Error bars indicate the variability of particle number concentration (as standard deviation) around
corresponding average concentrations, for measured and fitted concentrations. An F test was conducted, which revealed the absence of any statistical difference between variances
associated to measured and fitted concentrations both within the mixing layer and above it. (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
2–3 December 2008
December 2008
mean
σ
mean
σ
p (hPa)
t (°C)
RH (%)
ws (m/s)
1001.3
1.1
1008.8
10.3
4.0
1.1
3.7
2.7
77.4
7.6
74.6
14.1
1.5
0.5
1.6
0.8
3.1.2. Log-normal interpolation
The estimation of the absorption coefficient (babs) from OPC data
depends very much on the reliability of the particle number-size
distribution interpolation procedure. Several factors have to be taken
into account here, and the accuracy of n(Dp)USD and n(Dp)CSD have to
be considered.
The reliability of the OPC data interpolation procedure is firstly
guaranteed by a comparison of the interpolated particle number
concentrations with the measured ones within the OPC size ranges
(uncorrected and size corrected). Considering the whole profile
dataset, interpolated data showed a good fit with measured ones both
for n(Dp)USD and n(Dp)CSD (R2 0.997 and 0.998 for n(Dp)USD and
n(Dp)CSD respectively; Fig. 3); the average difference between
interpolated and measured particle number concentration was
found to be only + 6.56 ± 0.83% for n(Dp)USD and + 6.18 ± 0.71% for
n(Dp)CSD: these are small differences compared to the ones reported
in other works (~ 8–15% in Schmid et al., 2006). Additionally, the
geometric mean diameter (Dg) and the geometric standard deviation
(σg) of the accumulation mode were calculated, for both n(Dp)USD and
n(Dp)CSD, as mixing layer and columnar averaged values, and were
compared each other (Table 3): as expected, n(Dp)CSD shows a larger
Dg compared with n(Dp)USD due to size correction of OPC channels
(Table 2). Finally, these values were compared with AERONET (Ispra
site) and literature data (Van Dingenen et al., 2004; Milan-Bresso site:
~ 2 km from the Torre Sarca site, during year 2004); Table 3 reports all
the data.
Size distribution data (both n(Dp)USD and n(Dp)CSD) agree well
with AERONET data. For n(Dp)CSD the agreement is better if the Dg
value within the mixing layer is also considered: the optical signal
originated mainly within the mixing layer. Conversely, n(Dp)USDDg
agree better with literature data (Van Dingenen et al., 2004). Since
AERONET data were collected during the same period of balloon
launches, and the size correction procedure reported in Section 2.3.2
is more appropriate from a physical point of view, we consider the
n(Dp)CSD size distribution most reliable to calculate aerosol optical
properties along vertical profiles.
The reported values, as well as the agreement between the
calculated aerosol optical properties and AERONET data, underline the
accuracy of optical estimations from OPC data, and their appropriateness for the calculation of the Aethalometer optical enhancement
factor (Section 3.3).
a
600
BC
Particle conc.
500
Height (m AGL)
Date
very close to that measured along vertical profiles: 76.8 ± 6.0% (range:
61–88%); as a result, the columnar average value of the calculated
hygroscopic growth factor, applied in the refractive index determination (Section 2.3.1, Eq. (6)), was 1.128 ± 0.082 (range considering
each single 3.0 m measuring point along vertical profiles: 1.000–
1.318). The average ground-level wind speed was very low (less then
2 m/s) and favoured the accumulation of pollution within the mixing
layer. Ground-level meteorological conditions during the vertical
profile measurements (2–3 December) are summarized in Table 4 and
400
300
200
100
0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
BC (µg/m3)
0.0
100.0 200.0 300.0 400.0 500.0 600.0 700.0
Particle conc. (cm-3)
b
600
Potential temperature
Relative humidity
500
Height (m AGL)
Table 4
Ground meteorological data measured in Milan both on 2nd–3rd December 2008 and
along the whole month of December 2008; σ respresent the standard deviation.
7
400
300
200
100
3.2. Vertical profiles
0
3.2.1. Meteorology and mixing height determination
We present the first BC vertical profile measurements performed
in Italy; the data show a vertical distribution of BC (the main
absorbing aerosol species) over Milan, in the middle of the Po Valley,
which represents Italy's largest urban and industrial area. Vertical
profiles of BC, aerosol parameters and meteorological measurements
were made on the 2nd and 3rd of December 2008, from the morning
to the afternoon. Measurements were conducted under a synoptic low
pressure system (average pressure was 1001.3 ± 1.1 hPa), thus
causing an alternation of cloudy and clear sky during measurements.
Under these conditions, the relative humidity average value at ground
on the 2nd and 3rd of December was 77.4 ± 7.6% (range: 57–89%)
280.6 280.8 281.0 281.2 281.4 281.6 281.8 282.0 282.2 282.4
T (K)
62.7
64.2
65.7
67.2
68.7
70.2
71.7
73.2
RH (%)
Fig. 4. (a). Vertical BC (blue line) and aerosol (light blue line) profiles measured on the
3rd of December 2008 (9:41–9:58 UTC). The main horizontal axis shows BC
concentrations (μg/m3), while the secondary x-axis shows total particle number
concentration (cm− 3). (b) Potential temperature (T, red line) and relative humidity
(RH, green line) vertical profiles for the same balloon launch. The main horizontal axis
shows potential temperature (K), while the secondary x-axis shows relative humidity
(%). (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
8
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
Table 5
a–b. BC concentrations and absorption coefficients averaged every 50 m along height for each profile measured both on the 2nd of December 2008 (Table 5a) and on the 3rd of
December 2008 (Table 5b); particle, BC, T and RH derived mixing heights (p-MH, BC-MH, T-MH and RH-MH) are also reported for each profile.
2/12/2008
h 9.53–10.20
Parameter
BC
(μg/m3)
2/12/2008
h 10.25–10.42
babs
(Mm−1)
BC
(μg/m3)
2/12/2008
h 10.57–11.14
babs
(Mm−1)
BC
(μg/m3)
2/12/2008
h 14.30–14.47
babs
(Mm−1)
BC
(μg/m3)
2/12/2008
h 14.50–15.07
babs
(Mm−1)
BC
(μg/m3)
babs
(Mm−1)
Height (m a.g.l.)
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
0–50
50–100
100–150
150–200
200-250
250–300
300–350
350-400
400–450
450–500
p-MH (m AGL)
BC-MH (m AGL)
T-MH (m AGL)
RH-MH (m AGL)
13.2
11.3
10.8
8.9
2.9
1.4
1.0
0.9
1.0
1.2
196
199
227
224
1.2
0.2
0.3
0.7
0.8
0.2
0.1
0.1
0.1
0.1
80.4
68.7
65.6
54.2
17.7
8.3
6.3
5.6
6.2
7.2
7.5
1.1
2.0
4.2
4.9
1.0
0.5
0.1
0.2
0.8
12.2
11.8
10.0
4.0
1.1
0.9
0.9
1.0
1.0
1.2
157
149
147
140
0.6
1.1
1.1
1.8
0.3
0.1
0.1
0.1
0.1
0.1
74.5
72.2
60.9
24.4
6.8
5.5
5.5
5.9
6.2
7.2
3.9
6.6
6.4
10.8
1.6
0.4
0.2
0.3
0.5
0.2
15.9
13.0
12.4
10.5
6.2
2.2
1.1
219
222
216
199
3.0
0.2
0.1
1.4
1.1
1.1
0.1
–
-
96.8
79.5
75.7
64.1
38.0
13.3
6.5
–
-
18.1
1.4
0.6
8.2
6.7
6.6
0.3
–
-
12.8
9.6
9.5
10.0
9.7
9.3
9.7
9.0
5.4
2.7
396
396
412
406
1.9
0.1
0.2
0.4
0.2
0.2
0.1
0.9
0.4
1.1
78.1
58.5
58.0
61.1
59.1
56.8
59.4
55.0
32.9
16.6
11.8
0.8
1.0
2.4
1.0
1.1
0.9
5.3
2.6
6.6
11.0
8.4
8.3
8.7
9.5
9.8
8.2
5.9
4.4
1.9
443
440
431
452
1.4
0.3
0.1
0.2
0.4
0.6
0.4
0.8
0.9
0.2
67.0
51.2
50.4
53.0
57.8
59.8
50.0
35.9
26.8
11.8
8.3
1.6
0.4
1.3
2.2
3.6
2.7
4.7
5.5
1.5
a
compared with that of the whole December month: from a
meteorological point of view, the vertical profile measurements
were conducted during typical winter days, and can be considered
representative of this period.
Fig. 4a shows, as a case study, BC and aerosol profiles measured on
the 3rd of December 2008 (9:41–9:58 UTC); 50 m averaged BC
concentrations as a function of height, are summarized for the whole
campaign in Table 5a–b.
On the 3rd of December 2008 (9:41–9:58 UTC), aerosol and BC
pollution loading was restricted to the first 264 m in the lower
troposphere (Fig. 4a–b). At 264 m the aerosol concentration and BC
concentration, as well as potential temperature (T) and relative
humidity (RH) (Fig. 4b), showed a clearly defined mixing height
boundary (MH). The MH was inferred using the gradient method
200
180
160
bATN (Mm-1)
140
120
100
y = 2.221x
R² = 0.970
80
60
40
20
0
0
10
20
30
40
50
60
70
80
babs_USD(Mm-1)
b
600
Abs from BC
Abs calculated
180
500
y = 2.050x
R² = 0.985
160
Height (m AGL)
bATN (Mm-1)
140
120
100
80
60
40
400
300
200
20
100
0
0
10
20
30
40
50
60
70
80
babs_CSD(Mm-1)
0
0.0
Fig. 5. Linear regression between bATN calculated from the microAeth® Model AE51
measurements, and the aerosol babs calculated on the basis of Mie theory as described in
Section 2.3 for: (a) uncorrected size distribution (babs_USD); (b) corrected size
distribution, (babs_CSD). Full dots and diamonds indicate averages within the mixing
layer, while open circles and diamonds indicate averages above the mixing layer. Error
bars indicate data variability (as standard deviation) regarding the corresponding
averages both within and above the mixing layer.
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
Mm-1
Fig. 6. Vertical profiles of the absorption coefficient calculated from aerosol (light blue
line) and BC (blue line) profiles measured on the 3rd of December 2008 (9:41–9:58
UTC). (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
2/12/2008
h 15.18–15.35
BC
(μg/m3)
babs
(Mm− 1)
2/12/2008
h 15.37–15.54
3/12/2008
h 8.46–9.03
BC
(μg/m3)
BC
(μg/m3)
babs
(Mm− 1)
3/12/2008
h 9.10–9.27
babs
(Mm− 1)
BC
(μg/m3)
9
3/12/2008
h 9.41-9.58
babs
(Mm− 1)
BC
(μg/m3)
3/12/2008
h 10.05–10.22
babs
(Mm− 1)
BC
(μg/m3)
babs
(Mm− 1)
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
m
σ
9.8
10.7
7.8
7.7
8.1
7.2
6.2
3.9
1.7
1.8
374
377
413
419
0.9
0.7
0.6
0.2
0.2
0.2
0.5
1.0
0.4
0.3
60.0
65.3
47.6
46.8
49.3
43.9
38.0
23.9
10.4
11.2
5.7
4.3
3.7
1.0
1.2
1.4
2.8
6.1
2.2
1.9
7.3
7.3
7.2
5.9
3.7
2.8
3.7
2.5
325
322
340
297
0.2
0.2
0.1
0.6
0.9
0.2
0.3
0.5
44.7
44.3
44.0
36.0
22.4
17.4
22.3
15.5
1.2
1.2
0.7
3.4
5.7
1.5
1.8
3.2
7.4
9.7
8.3
7.9
5.2
2.2
1.7
1.5
1.1
1.0
213
217
162
241
0.5
0.7
0.2
0.3
1.4
0.2
0.1
0.2
0.1
0.1
45.1
59.3
50.7
48.2
31.6
13.2
10.6
8.9
6.9
5.9
3.3
4.0
1.0
1.7
8.7
1.3
0.9
1.0
0.3
0.3
7.8
4.8
3.3
3.8
2.7
1.6
1.4
1.2
1.2
1.0
221
221
235
244
1.1
0.3
0.2
0.2
0.3
0.3
0.1
0.1
0.1
0.1
47.7
29.5
20.3
22.9
16.5
9.8
8.2
7.3
7.1
6.3
6.7
1.8
1.3
1.0
2.0
1.7
0.2
0.3
0.1
0.5
6.1
4.8
3.9
3.5
4.5
3.1
1.0
1.4
0.9
0.6
264
264
287
284
0.5
0.4
0.5
0.4
0.3
1.7
0.1
0.3
0.1
0.1
37.0
29.0
23.9
21.2
27.7
19.1
6.4
8.4
5.4
3.6
3.2
2.3
2.9
2.6
1.6
10.2
0.8
2.1
0.6
0.3
7.6
6.2
6.1
5.8
5.6
4.8
3.1
4.4
1.1
0.7
404
401
407
400
1.0
0.1
0.1
0.1
0.4
0.7
0.3
1.1
0.3
0.1
46.6
37.8
37.1
35.5
34.2
29.0
18.7
26.9
6.5
4.1
6.1
0.8
0.3
0.9
2.5
4.6
1.6
6.6
1.7
0.6
(Ferrero et al., 2010) from particle (p-MH), BC (BC-MH), T (T-MH)
and RH (RH-MH) profiles; Table 5a–b reports values of p-MHs, BCMHs, T-MHs and RH-MHs. These values coincide for all measured
profiles, and the MHs calculated from different parameters, either
meteorological and pollution tracers, showed a marked correlation each
other, together with very low root mean squared errors (RMSE) and
with linear best fits close to the ideal ones (BC-MH= 0.996*p-MH+
0.774, R 2 = 0.999, RMSE = 3.5 m; T-MH = 1.049*p-MH-8.468,
R 2 = 0.948, RMSE = 24.4 m; RH-MH = 1.014*p-MH + 4.470,
R2 = 0.950, RMSE= 23.7 m): these data are in agreement with that
reported in Ferrero et al. (2010) and underline the reliability and
physical inter-consistence of using particle, BC, T and RH soundings
when determining MH. Angelini et al. (2009) also showed that balloon
derived MH was in keeping with MH estimation performed using an
automated lidar ceilometer (Vaisala LD40, λ = 855 nm) installed at the
Torre Sarca site at the same time as the balloon launching. All of these
results underline the accuracy of BC-MH and p-MH in estimating the
mixing height. Hereinafter we are going to refer to the mixing height as
that derived from BC concentration gradient, indicated simply as MH.
3.2.2. Black carbon profiles
Fig. 4a and the results shown in Section 3.2.1 indicate that aerosol
and BC profiles were shaped in the same way by atmospheric
turbulence (either thermal or mechanical forces), and that the
majority of BC lies within the MH, as would be expected over a
large urban area. To a smaller degree, BC profiles within the mixing
layer revealed a layer of high concentration close to the ground,
probably due to proximity to combustion sources (traffic, domestic
heating, industry). This ground-level layer (~50 m) is evident in most
profiles (Fig. 7) with concentrations + 24 ± 4% higher than the
average BC concentration measured within the whole mixing layer.
These BC concentrations near ground-level are of particular importance due to the health effects of BC (Hesterberg et al., 2006; Mar et al.,
2000; Vedal, 1997).
Above the MH, both BC and aerosol concentrations were fairly
constant from the MH to the highest elevation of the balloon (Fig. 4).
Over the course of the entire campaign, the BC concentrations ranged
between 0.82 and 2.06 μg/m3 (average value: 1.39 ± 0.14 μg/m3)
above the MH. These values represent 17 ± 2% of BC concentrations
measured within the mixing layer (8.40 ± 0.95 μg/m3), and indicate
the presence of stable BC concentrations above the MH, at least during
the measuring campaign period. At the same time, the average
particle concentration above the MH was 206 ± 40 cm− 3, representing 36 ± 5% of the total particle number concentration measured
within the mixing layer (588 ± 67 cm− 3); these data confirm the
winter averages (Dec., Jan. and Feb.) reported in Ferrero et al. (2010),
indicating that, also from the pollution point of view, the campaign
was conducted during days which, on average, were representative of
the mean seasonal conditions (compare Section 3.2.1 and Table 4).
Another sign that vertical profile measurements were conducted
during typical winter days is given by the PM2.5 ground-level
concentrations measured at Torre Sarca on the 2nd and 3rd of
December 2008: 52 and 50 μg/m3 , respectively, and are thus in
agreement with the December average of 53 ± 1 μg/m3.
Regardless of the mechanism responsible for the BC profiles, BC
experienced a higher concentration drop at the MH than did the
aerosol number concentration. This means that the BC content of the
aerosol was not constant along the vertical profiles, but sharply
decreased above the MH; the aerosol chemical composition is
different above the MH than below it.
The BC fraction of total aerosol volume was found on average to be
48± 8% lower above MH than within the mixing layer. Ferrero et al.
(2010) showed, after measuring vertical profiles for a period of three
years, that during winter the inorganic ionic aerosol content also
behaves similarly, and is 26± 6% lower than it is when measured near
the ground. These data are in accordance with the average aerosol
chemical composition measured in the free troposphere at the Alpe S.
Colombano site (Table 1), which showed lower BC and inorganic
aerosol mass contents than those measured at the same time at groundlevel in Milan (at the Torre Sarca site, see Table 1), while organic matter
content was found to be higher in the more remote site.
Because BC particles are emitted from primary sources, this
difference in aerosol content could be due both to the presence of
these sources within the mixing layer, and to secondary particles
formed above the MH. This would be in agreement with the results
reported by many authors (Morgan et al.;, 2009; Schneider et al.,
2006; Hueglin et al., 2005), which pointed out a lower vertical
gradient of organic species than of inorganic ones; the organic matter
appeared to be more important above the MH, as has also been
reported by Sun et al. (2009).
Because the aerosol volume content of a chemical species is a
proxy of the aerosol mass fraction of the same species (without
considering particle density), we applied the fraction of BC reduction
above the MH (48 ± 8%) to the BC mass fraction reported in Table 1 for
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
10
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
600
2-12-2008-h 9:53 -10:20
2-12-2008-h 10:25 -10:42
2-12-2008-h 10:57 -11:14
500
2-12-2008-h 14:30 -14:47
2-12-2008-h 14:50 -15:07
Height (m AGL)
400
2-12-2008-h 15:18 -15:35
2-12-2008-h 15:37 -15:54
3-12-2008-h 8:46 -9:03
300
3-12-2008-h 9:10 -9:27
3-12-2008-h 9:41 -9:58
3-12-2008-h 10:05 -10:22
200
100
0
0
5
10
20
15
BC µg/m3
0
25
50
75
100
125
150
babs(Mm-1)
Fig. 7. Vertical BC profiles measured over Milan on the 2nd and 3rd of December 2008 (11 profiles altogether) with BC concentrations (μg/m3 main x-axis), and the corresponding
absorption coefficients (Mm− 1, secondary x-axis) calculated as described in Section 3.2.
Milan (Torre Sarca PM2.5 chemical composition). By adopting this
procedure, we were able to estimate the BC mass fraction above the
MH on the 2nd and 3rd of December 2008: the estimated BC mass
fraction above the MH was 6.2%, which is very close to the value of
6.6% calculated as the average of the Milan and Alpe S. Colombano EC
data reported in Table 1. This result substantiates the assumption
made in Section 2.3.1, whereby the aerosol refractive index above the
MH is calculated from the averaged chemical composition between
that at ground-level in Milan, and that in the free troposphere at Alpe
S. Colombano.
We also used PM2.5 chemical composition at ground-level in order to
estimate a refractive index for the whole mixing layer (Section 2.3.1).
This was verified by comparing the BC profile data measured within the
mixing layer, with EC concentrations measured in ground-level PM2.5
samples. We averaged the BC concentrations measured within the
mixing layer for each profile for the 2nd and 3rd of December 2008, and
estimated the BC content in ground-level PM2.5 samples. The averages
were calculated within the mixing layer to account for the atmospheric
turbulence that continuously mixes the aerosol from ground-level to the
MH; even if vertical BC profiles are discontinuous during the day, these
averages are a first estimation of daily BC concentrations calculated on
the basis of readings of the microAeth® Model AE51.
The estimated BC mass fraction in the PM2.5 samples was 14.9 ±
1.2%, that is, slightly higher than the EC mass fraction (11.8 ± 0.9%)
measured using the TOT method (see Section 2.1.2 and Table 1); the
difference of ~ 3% could be due to the fact that the BC measurements
were conducted during the daytime only, when primary emission
sources are more active than they are at night.
3.3. Aethalometer optical enhancement factor and absorption coefficient
profiles
The Aethalometer optical enhancement factor C was calculated for
the microAeth® Model AE51 in order to derive an estimation of babs
along vertical profiles over Milan. The optical enhancement factor C
was not previously estimated for the microAeth® Model AE51, and its
determination is of crucial importance since the C value depends on
the filter material (PTFE-coated borosilicate glass fiber) and instrument specifications, which are completely new in this particular case.
The experimental design of vertical profiles does not require any
estimation of the aerosol loading factor R(ATN): all vertical BC profiles
were conducted by changing the filter ticket after each profile. Every
Aethalometer measurement cycle (ascent and descent of the balloon)
took less than 40–50 min. As a result, ATN never achieved values
higher than 20 during all profiles, meaning that the bATN measurements were not affected by the “shadowing” effect due to filter
loading. The average ATN measured along vertical profiles was 5 ± 1,
hence there was no need to use R(ATN) in the determination of the
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
optical enhancement factor C (Schmid et al., 2006; Arnott et al., 2005;
Weingartner et al., 2003).
In order to calculate the C factor, first bATN was calculated from BC
profiles by means of (3); the reference absorption coefficients (here
denoted babs_USD and babs_CSD) were calculated by applying Mie theory
to aerosol number size distribution data, n(Dp)USD and n(Dp)CSD
respectively. Coefficients bATN, babs_USD and babs_CSD were averaged
within and above the mixing layer, for every balloon profile, and were
compared to each other. A similar averaging procedure had been used
previously (Corrigan et al., 2008; Schmid et al., 2006; Arnott et al.,
2005; Weingartner et al., 2003) to smooth out short-term signal
variations.
The linear regression between averaged bATN and babs_USD and
babs_CSD is shown in Fig. 5a–b, and shows a high degree of correlation
(R2 = 0.970 and 0.985 for babs_USD and babs_CSD respectively). The
estimated optical enhancement coefficients CUSD and CCSD were 2.22 ±
0.06 (95% confidence limits: 2.10–2.34) and 2.05 ± 0.03 (95%
confidence limits: 1.98–2.12) respectively. CCSD is ~8% higher than
CUSD; this reflects the effect of the OPC size correction (Section 2.3.2),
which results in an increase of the geometric mean diameter (Dg,
Section 3.1.2, Table 3), and consequently in an absorption coefficient
babs_CSD on average ~ 13% higher than babs_USD. The OPC size correction
does not significantly affect the result and 95% confidence limits of CCSD
and CUSD overlap; this is because the size distribution is interpolated to
prevent the “truncation effect” which, as reported in Section 2.3.2, can
not be neglected considering Dg values reported in Table 3 (for both n
(Dp)USD and n(Dp)CSD) compared to the OPC lower cutoff (~0.3 μm).
Thus the interpolation procedure compensates for the highest source
of possible bias (the “truncation effect”), while the size correction
allows to improve the whole algorithm performance (from interpolation to the optical properties calculations); this turns in a higher R2
for CCSD, in a narrower standard deviation and confidence width.
Moreover, CCSD calculation is more appropriate from a physical point of
view, thus we consider CCSD the most reliable estimation, and
hereinafter C = CCSD = 2.05 ± 0.03.
This C factor is specific to the new micro-Aethalometer microAeth® Model AE51.
We wanted to compare the new optical enhancement factor to
those reported in the literature; we took account of the fact that the
new microAeth® Model AE51 has a mass attenuation cross section of
12.5 m2/g, rather than the 16.6 m2/g, reported for other Aethalometers
at the 880 nm wavelength (e.g. AE22, AE31, Magee Scientific).
In order to compare the C value calculated for the microAeth®
Model AE51 with that of other Aethalometers reported in the
literature, C was recalculated according to a mass attenuation crosssection of 16.6 m2/g, in order to compare it with the data currently
available in other studies; the recalculated C factor (Cr) amounted to
2.73, which agrees with those reported by Weingartner et al. (2003),
who found optical enhancement factors in the 2.13–3.90 range (from
pure soot particles to internally mixed coated particles) at 660 nm,
and with those given by Schmid et al. (2006), who calculated the
wavelength dependence of optical enhancement factors to give a C
value of 3.14 at 880 nm.
Our Cr value (2.73 at 880 nm) was found to be close to those values
reported in other studies (Schmid et al., 2006; Arnott et al., 2005;
Weingartner et al., 2003), when soot particles were coated with
secondary compounds.
This fits well with the aerosol chemical composition measured in
Milan during balloon launches: 36 ± 3% of the mass fraction of PM2.5
was due to NH4NO3 and (NH4)2SO4, while a further 36 ± 2% was due
to organic matter (OM).
3.3.1. Absorption coefficient profiles
By using the mass attenuation cross-section σATN (12.5 m2/g) and
the optical enhancement factor C (2.05), the micro-Aethalometer BC
data can be used to calculate aerosol absorption coefficient profiles.
11
The reliability of this form of estimation using short-term
measurements (6 s) conducted along vertical profiles, is clearly
shown in Fig. 6, where the absorption coefficient profiles resulting
from the Mie calculation and from the micro-Aethalometer measurements are compared to each other. Fig. 6 reports the babs for the same
aerosol and BC profiles shown in Fig. 4a.
All BC and absorption profiles measured using the microAeth®
Model AE51 during the field campaign are reported in Fig. 7, while
Table 5a–b also contains 50 m averaged babs values as a function of
height.
The values of babs reported in Table 5a–b and Fig. 7, reflect the BC
behaviour with respect to height previously discussed in Section 3.2.2.
Obviously, babs profiles are shaped in the same way as BC profiles, and
the main atmospheric absorbing layer was found within the MH, with
an average absorption coefficient of 51.2 ± 5.8 Mm− 1 (range: 26.3–
81.3 Mm− 1). From the MH to the top of the profile, babs remained
fairly constant (see Fig. 7), on average 8.5 ± 0.8 Mm− 1 (range: 5.0–
12.5 Mm− 1), representing 17 ± 2% of those values measured within
the mixing layer (as for BC, Section 3.2). The shape of the babs (and BC)
profiles, characterized by a convergence value above the MH, led us to
a
1.00
0.80
Measurements
0.60
Interpolation
0.40
0.20
Hs 0.00
-0.20
-0.40
-0.60
-0.80
-1.00
0
20
40
60
80
100
120
Abs % (or BC%)
b
9.00
8.00
Measurements
7.00
Interpolation
6.00
Alpe S. Colombano (2280 m asl)
5.00
Hs 4.00
3.00
2.00
1.00
0.00
-1.00
0
20
40
60
80
100
120
Abs % (or BC%)
Fig. 8. (a) the statistical mean profile of babs (or BC) along standardized height Hs, as a
percentage in relation to the value of babs (or BC) measured at ground-level (denoted as
Abs% or BC%); (b) the statistical mean profile of Abs% (or BC%) extrapolated up to
2280 m ASL near the Alpe S. Colombano site. Blue dots indicate points of the statistical
mean profile averaged from measured babs (or BC) profiles; the red line indicates the
behaviour of the interpolant function; while the asterisk (*) indicates the percentage of
EC measured at Alpe S. Colombano compared to that calculated at ground-level in
Milan. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
12
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
calculate a statistical mean profile in order to account for this
behaviour. A simple average of the data collected at the same height is
not the proper metric, as such an average does not take into account
the difference in MH at different times, and can only produce
smoothed profiles not clearly shaped by atmospheric turbulence, as is
the case in Morgan et al. (2009). One way of avoiding this problem is
to average vertical profile data for a standardized height (here
denoted as Hs) which assumes a value of 0 at the MH, and values of
−1 and 1 at ground-level and at twice the MH, respectively (Ferrero
et al., 2010). The standardized height (Hs) enables us to average out
data in relation to their position at the MH, and can be defined as:
Hs =
z−MH
MH
ð8Þ
where z is the absolute height above ground. Averaging babs (or BC)
data along Hs enables us to compute the statistical mean profile by
taking the daily evolution of MH into account. Fig. 8a shows the
behaviour of babs (or BC) along Hs: this behaviour is reported as a
percentage, and refers to the value of σabs (or BC) measured at
ground-level, denoted as Abs% (or BC%); this enables us to draw the
shape of a generic profile regardless of the initial ground value of babs
(or BC). Fig. 8a shows that the mean profile of Abs% is characterized
by: 1) a sharp decrease at the point where Hs = 0 (which corresponds
to the MH); 2) higher values near ground-level compared to the
whole mixing layer (Section 3.2.2); 3) a lower variance above the MH,
pointing to the presence of a relatively constant value of babs (or BC),
as already mentioned in this section and in Section 3.2.2.
The presence of a clearly defined mean profile enabled us to
interpolate it in order to find a generic function to describe the
behaviour of Abs% (or BC%) along the new standardized height Hs. This
is a realistic approach considering that, as was mentioned in
Sections 3.2.1 and 3.2.2, meteorological parameters and aerosol
number concentration along vertical profiles on the 2nd and 3rd of
December, were very close to the seasonal mean over Milan, and so
these two days could be considered to be representative of winter
over the city of Milan.
One way of building a generic function for Abs% (or BC%) is to
consider a weighted sum of powers of Hs (up to four), in order to
describe the non-linear relationship between Abs% (or BC%) and the
standardized height (Ferrero et al., 2010).
The statistical mean profile of Abs% (or BC%) can be described using
the following equation:
2
ð2−iÞ
∑ p2−i Hs
Abs% =
i=0
4
ð9Þ
ð4−jÞ
∑ q4−j Hs
j=0
where p2 − i and q4 − j are specific parameters designed to represent
the shape of Abs% (or BC%) (Table 6). The rational of the two
polynomials in Eq. (9) enables us to shape the profile by taking the
MH into account, and also to estimate a ~ 1/H2s dependence of Abs% (or
BC%) when Hs is NN1 (going towards the free troposphere). The
effectiveness of Eq. (9) in parametrizing Abs% (or BC%) is shown in
Fig. 8a, by comparing it with the experimental mean profile
(R2 = 0.993, RMSE = 3.17%): Eq. (8) clearly describes the aforementioned characteristics 1), 2) and 3) of the babs (or BC) mean profile.
Moreover, we tested Eq. (9) by trying to predict BC% at 2280 m ASL
at the Alpe S. Colombano site, in the free troposphere; in this process,
we considered the median value of the MH estimated during the
campaign (264 m). The result, portrayed in Fig. 8b, clearly shows how
the parametrization of BC% (predicted BC% = 1.13%) is close to the
percentage of EC measured at the Alpe S. Colombano site compared
with the ground-level EC measured in Milan (EC% = 1.10 ± 0.10%). A
second test was conducted by predicting the absorption Aerosol
Optical Depth (AODabs) integrating the Abs% along the atmosphere up
to 5000 m; AODabs was 0.020, in keeping with the estimate from
AERONET data: 0.015.
Therefore, Eq. (9) can be used to describe the behaviour of black
carbon and the absorption coefficient as a function of height, at least
over Milan, once the MH and the absolute ground-values of babs (or BC)
are known. This lays the basis for the development of valid
parameterizations of vertical profile data, which are useful both in
remote sensing and in climatic studies; this is of vital importance over
highly built-up areas such as the Po Valley, where BC aerosol is one of
the main aerosol components, as Putaud et al. (2004) and Baltensperger
et al. (2002) have shown. The parameterization that emerges is also
based on results derived from robust, inexpensive techniques, which are
very useful in collecting statistically significant long-term data series
to be used to calculate the main atmospheric aerosol behaviour (Ferrero
et al., 2010).
4. Conclusions
Vertical profiles of black carbon and aerosol number-size distribution were measured on the 2nd and 3rd of December 2008, using a
tethered balloon fitted with the newly-developed micro-Aethalometer (microAeth® Model AE51) and an OPC (Grimm 1.108).
BC profiles clearly identified the mixing layer boundary (MH),
which was characterized by a strong vertical concentration gradient.
BC profiles also showed a shallow layer of increased concentrations
close to the ground, due to the proximity of combustion sources. This
ground-layer was +24 ± 4% higher than the average BC concentrations measured within the whole mixing layer. Fairly constant concentrations of BC were found above the MH, representing 17 ± 2% of
those BC concentrations measured within the mixing layer.
The BC fraction of aerosol volume fell to 48 ± 8% above the MH,
compared to ground-level data. This caused a change in the optical
absorption properties of the aerosol at different heights. This result
was confirmed by separate analyses of the chemical composition of
particles taken from PM2.5 samples, collected at the same time at both
ground-level and at a high altitude site.
Profiles of the absorption coefficient (babs) were calculated from the
Aethalometer measurements. In order to do so, an optical enhancement
factor (C) for the new microAeth® Model AE51 was calculated for the
first time. Mie calculations were applied to the OPC to correct the
number-size distribution data, and to calculate the aerosol optical
properties as a function of height. Aerosol chemical composition data
was used to calculate the aerosol refractive index by means of the
Bruggeman mixing rule. The comparison between the Aethalometer
attenuation coefficient and the aerosol optical properties estimated
from OPC enabled us to calculate the optical enhancement factor
(C = 2.05 ± 0.03) for the new microAeth® Model AE51 at 880 nm, and
thus to calculate the absorption coefficient profiles using this factor.
Table 6
Estimated parameters for the parameterization of Abs% and BC% vertical profiles over Milan.
Model parameters
p0
p1
p2
q0
q1
q2
q3
q4
Estimated values
Model performance
9.95
RSS = 120.70
18.59
105.40
R2 = 0.993
0.25
0.58
R2adj = 0.990
4.35
3.71
RMSE = 3.171
1.00
Please cite this article as: Ferrero L, et al, Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation
over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022
L. Ferrero et al. / Science of the Total Environment xxx (2011) xxx–xxx
A statistical mean profile was established for babs and BC data, in
order to better describe their behaviour. The statistical mean profile
referred to a standardized height (Hs). A simple model, based on the
same standardized height, was successfully created in order to simulate
the vertical behaviour of babs and BC. This model was able to represent
the main characteristics of the mean profile, namely: 1) a sharp decrease
at the MH, 2) higher values of babs (and BC) near the ground than in the
entire mixing layer, 3) a lower degree of variance above the MH, thus
suggesting the presence of fairly constant values of babs (and BC) 4) in
the free troposphere: the model estimated a ~1/H2s dependence of babs
(and BC).
The validity of this parametrization was confirmed by comparing
the predicted BC% at 2280 m asl at the Alpe S. Colombano site in the
free troposphere (predicted BC% = 1.13%) to the percentage of EC
measured at Alpe S. Colombano compared to the ground-level EC
readings taken in Milan (EC% = 1.10 ± 0.10%). This formed the basis
for the development of parametrizations of vertical profile data to be
used in remote sensing and climatic studies.
Acknowledgements
This paper presents some results from the Italian SATMAP
(Mapping particulate matter from satellite) project. Grisa Mocnik
would like to thank the Slovenian Ministry of Higher Education,
Science and Technology for the financial support (grant 3211-09000304). We thank Giuseppe Zibordi for his effort in establishing and
maintaining the AERONET Ispra site. We acknowledge Friedhelm
Schnider for information concerning the optical and laser parameters
of the Grimm 1.108 OPC.
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over Milan, Sci Total Environ (2011), doi:10.1016/j.scitotenv.2011.04.022