Introduction
Incomplete combustion of fossil fuels, bio-fuels and biomass
emit teragram quantities of anthropogenic black carbon to the troposphere
every year . Aerosol containing black carbon is an
important absorber of short wave radiation in the atmosphere, with the
ability to affect the energy balance at the surface and within the
troposphere . Black carbon-containing particles are
estimated to be the third largest contributor to anthropogenic global
warming, after carbon dioxide and methane . However, the
magnitude of their direct effects on radiative balance may become even more
significant when black carbon is internally mixed with other aerosol
components such as sulphate, nitrate and organic species , though the importance of this enhancement for ambient aerosol
remains an open question . The physical and chemical
complexity of these particles compounds the uncertainty associated with the
impacts of black carbon on global and regional climate change and human
health .
A variety of measurement techniques have been used to quantify atmospheric
black carbon. These include direct techniques, such as the single particle
soot photometer (SP2) that uses laser induced incandescence to quantify
refractory black carbon (rBC) and indirect methods using opticaltechniques, sample oxidation, or oxidation combined with optical
methods . When aerosols contain a small black carbon
mass fraction, reported mass concentrations from a range of measurement
techniques can differ significantly (up to 80 %) . This
large range in black carbon mass loadings can be partially attributed to
measurement bias, most often for optical methods, associated with the
presence of other absorbing species in the aerosol. These measurement
uncertainties can make it difficult to understand the spatial and temporal
trends and the climate impacts of black carbon.
The soot-particle aerosol mass spectrometer (SP-AMS) is a recently developed
instrument that combines a near-infrared laser system for volatilization of
rBC with the Aerodyne high-resolution time-of-flight AMS to
quantify aerosol components . Near-IR laser vaporization and
electron impact ionization facilitate mass spectral detection of
light-absorbing rBC and the simultaneous measurement of non-refractory
species associated with rBC. Detection of rBC and NR-PM depends upon the
ability of rBC to absorb the IR energy and produce a heating effect
significant enough to volatilize the rBC coating material and, upon further
heating, the rBC itself. The SP-AMS has been shown to respond linearly and
reproducibly to laboratory generated rBC particles (Regal Black (Cabot R400)
and flame generated soot) . However, in contrast to the Regal
Black particles generally used for SP-AMS calibration, freshly emitted
particulate black carbon tends to be smaller and less compact i.e.
lower mass-mobility exponent,. Over time these small particles
grow and become internally mixed with organic and inorganic species through
condensation and coagulation. With ageing, changes in chemical composition
and physical properties of rBC particles affect their atmospheric lifetime,
radiative properties, and their ability to act as cloud condensation nuclei
and as ice nuclei . Therefore, it is crucial that the
SP-AMS is able to quantify the extent of particle coating and the rBC mass
loading. The ability to simultaneously detect rBC and species that condense
on it with comparable accuracy is crucial for three reasons; first, to assess
the rates at which black carbon processing occurs in the atmosphere; second,
to understand the role that such internally mixed particles may play as cloud
nuclei; and third, to assess direct effects on solar radiation.
Previous research has applied the linear response of the SP-AMS to
measure the size-resolved chemical composition of rBC-containing
particles during roadside measurements of traffic related pollutants
, and to characterize the rBC mixing state for its
relation to black carbon absorption enhancement
. However, the effect of rBC mixing with NR-PM on the
collection efficiency of the SP-AMS laser vaporizer has not been
explicitly addressed. In this work, we first examine the effect of
rBC-particle coating on the SP-AMS sensitivity to the recommended
calibration material, Regal Black. Secondly, we address differences in
collection efficiency for bare and internally mixed rBC particles with
beam width probe (BWP) measurements to demonstrate particle beam
narrowing that occurs due to coating. Finally, using measured
sensitivities for laboratory-generated particles, we demonstrate that
the SP-AMS can quantify the fraction of rBC in coated particles over
a wide range of internally mixed compositions.
Experimental
Particle generation and coating
Experiments were conducted at the University of Toronto (UofT) and at
Aerodyne Research, Inc. (ARI) with similar experimental set-ups. A general
experimental schematic is shown in Fig. . In both cases, rBC
containing particles were generated from a water suspension of Regal Black
(RB) (Regal 400R pigment, Cabot Corp.), with concentrations ranging from 10
to 1000 mgL-1 in de-ionized water (18 MΩ-cm,
TOC <4 ppb), using a constant output atomizer (TSI Inc., model
3076). The polydisperse aerosol flow was dried with a diffusion drier
containing silica gel (<20 % RH) and directed to a differential
mobility analyser (TSI Inc. model 3081) for size selection. The monodisperse
aerosol was characterized “bare” (nascent) or after coating with
bis(2-ethylhexyl)sebacate (BES) via condensation in a thermally
controlled condenser.
Laboratory schematic showing the basic particle generation,
coating and characterization components. Experiments were conducted
independently at the University of Toronto (UofT) and Aerodyne
Research, Inc. (ARI) with similar set-ups, see text for details.
The University of Toronto coating apparatus consisted of
a 1.3 cm outer diameter, 30 cm long glass tube
containing BES. The sample line was connected to the coating region
via 0.63 cm diameter glass inlets. The temperature of the
coating region was controlled using heating tape, monitored using
a thermocouple and varied from room temperature up to
125 ∘C. Aerosol carrier flow through the coating
region was 450 cm3min-1. The ARI coating apparatus
consisted of a 60 cm long, 1.3 cm outer diameter glass
tube wrapped in heating tape and temperature controlled using an Omega
PID controller and a thermocouple. The tube has two temperature zones
(∼30 cm each) and was operated with the downstream zone
set to ∼20 ∘C less than the upstream zone. The
temperature of the upstream zone was varied from room temperature to
150 ∘C. The flow through the coating tube was
750 cm3min-1. Experimental conditions are summarized in Table .
Summary of particle generation systems.
UofT apparatus
ARI apparatus
DMA
TSI model 3081
TSI model 3081
DMA sample flow (sccm)
450
300
DMA sheath flow (sccm)
4500
3000
Coating tube diameter (cm)
1.3
1.3
Coating tube length (cm)
30
60
Coating tube flow rate (sccm)
450
750
Coating tube temperature range (∘C)
25–125
25–150, with two temperature zones (ΔT=20)
Particle characterization
The size selected bare and coated rBC particles were characterized for
particle mass using a mass analyser (aerosol particle mass analyser
(APM model 3600, Kanomax Inc.) or centrifugal particle mass analyser
(CPMA, Cambustion), ), number concentration using
a condensation particle counter (CPC, TSI Inc., model 3776 and 3022a),
and contributions from doubly charged particles using a scanning
mobility particle sizer (SMPS. TSI Inc., model 3936). SMPS measurements
were used to determine appropriate solution concentrations to use in the
atomizer, such that doubly charged particles accounted for less than 10 % of the total particle mass. Particle number
concentration was monitored before and after coating, and used to
monitor particle loss or homogeneous nucleation. The mass analyser was
scanned using both the rotational speed and voltage to measure the
size resolved mass distribution for coated and bare particles. From
the mass analyser measurements of bare and coated Regal Black
particles we define the mass ratio of organic coating to Regal Black
(ROrg/RB=massBES/massRB)
and the mass fraction of Regal Black
(fRB=massRB/(massBES+massRB)),
where massBES=massRB,coated-massRB.
The soot-particle aerosol mass spectrometer (SP-AMS)
The soot-particle aerosol mass spectrometer (SP-AMS) is an Aerodyne
high-resolution AMS equipped with an infrared laser vaporization
module similar to that of the single particle soot photometer (Droplet
Measurement Technologies) as described in detail by
. SP-AMS instruments contain both the traditional tungsten
vaporizer and the IR-laser vaporizer, and can be operated with both tungsten
and IR-laser vaporizers on or with only the tungsten vaporizer on. In order
to operate with the IR-laser vaporizer only, the tungsten vaporizer must
be removed because its lowest possible temperature is ∼200 ∘C due to heating by the filament.
In brief, the SP-AMS laser vaporization module uses
a continuous wave intra-cavity 1064 nm Nd:YAG laser, situated
perpendicular to the particle beam, that allows vaporization and
subsequent detection of rBC species
. The width of the laser beam (fundamental transverse
mode, TEM00, with Gaussian profile) is dictated by the length of the
lasing cavity and the curvature of the optics, which are fixed in
SP-AMS instruments. Particles are focused with an aerodynamic lens
into a narrow beam which crosses the IR laser beam. Particles
containing rBC absorb IR energy and are heated, first vaporizing
non-refractory (NR) coatings (estimated temperature <600 ∘C) and then vaporizing the rBC (estimated
temperature ∼4000 ∘C ). Neutral
gas phase analytes are ionized using 70 eV electron impact
ionization. Three SP-AMS instruments were used in this study (UofT SN 215-121,
ARI SN 215-039 and 215-130), all of which are time-of-flight mass spectrometers
operated in V-mode with both the tungsten vaporizer (∼600 ∘C)
and the laser vaporizer simultaneously, unless otherwise noted.
Particle beam alignment with the tungsten vaporizer and the IR laser beam was
confirmed preceding these experiments and IR laser power was optimized for
saturation of the rBC signal. The physical distance between the centre of the
tungsten vaporizer and the vertical position of maximum sensitivity for Regal
Black in the IR laser was ≤0.5 mm. Data acquisition alternated
between mass spectrum (MS) and particle time-of-flight (pToF) modes. Data
were analysed using the AMS analysis software Squirrel version 1.51H and Pika
version 1.10H. Signals of rBC from Regal Black were quantified by the sum of
C1+ to C9+ peaks, using high resolution (R∼2000 at
m/z 28) MS data. The average ratio of C1+ to C3+ (0.6),
obtained from bare Regal Black, was used to correct the C1+ peak
intensity for interference from non-refractory species in coated particles.
Signals from refractory oxygen containing species can arise during the
vaporization of rBC and can contribute to signals at CO+ and
CO2+ . The signals arising from
CO+ and CO2+ were not included in the organic loading if it
accounted for greater than 10 % of the total organic signal, since the
appropriate chemical fragmentation of these species arising from Regal Black
was not quantified in this work.
For comparison to mass analyser derived mass fractions of Regal Black
(fRB) in coated particles, chemical identification and
quantification was carried out with each SP-AMS instrument. The SP-AMS
derived fRB (or ROrg/RB) was
determined through quantification of rBC and coating, taking into
account NR-PM already present on Regal Black as follows:
fRB=massrBC+massOrg, RBmassrBC+massOrg, RB+massBES,
where massOrg, RB is the mass of organic species present
on bare Regal Black.
Particle beam width measurements
Each SP-AMS was equipped with a beam width probe (BWP) consisting of
a vertical wire of 0.46 or 0.41 mm in width (UofT and ARI,
respectively). The BWP wire was oriented perpendicular to the laser beam axis
and stepped across the particle beam parallel to the laser axis
(horizontally). As detailed in , the BWP wire blocks
a fraction of sampled particles from reaching the vaporizer at each step. By
analysing the results from multiple wire steps across the particle beam, the
particle beam divergence can be determined. In the horizontal orientation,
the laser beam produces uniform vaporization of rBC, thus allowing direct
measurement of rBC particle beam widths. Particle beam widths (σ) in
this work are determined by fitting a one-dimensional Gaussian profile to the
measured transmission profile as a function of the distance the BWP wire
travels along the horizontal axis of the laser beam. These widths are
slightly different widths than those derived by , where
a two-dimensional Gaussian profile was assumed for the particle beam and the
tungsten vaporizer geometry was explicitly included in their modelling. BWP
measurements were also carried out by rotating the BWP such that the wire was
oriented parallel to the laser beam axis and could be stepped across the
particle beam perpendicular to the laser beam (vertically). Measurements in
this orientation can provide some insight into the effective laser beam width
for rBC. Note that the UofT SP-AMS was operated without the tungsten
vaporizer during these vertical BWP measurements. See Supplement Fig. S1 for
an illustration of BWP orientation for particle and laser beam width
measurements.
Results and discussion
The effect of rBC-particle coating on SP-AMS collection efficiency
SP-AMS calibration
The goal of this section is to examine the effect of rBC mixing state
on SP-AMS sensitivity to rBC and associated NR-PM. To do so, we
measure the apparent ionization efficiency of rBC and organic coating
material relative to nitrate (RIErBC, app and
RIEOrg, app) for a variety of particle coating
thicknesses and compare these observations to uncoated values. The
relative ionization efficiency, defined in previous work
, is applied to quantify the mass
concentration of a particulate species. Using the mass based
ionization efficiency for nitrate (mIENO3, in
units of ions per picogram) the mass concentration of species “s” can
be determined as follows:
Cs=1CEs⋅RIEs⋅mIENO3⋅Q∑iIs, i,
where Cs is the mass concentration of species “s,”
RIEs=mIEs/mIENO3,
Q is the flow rate, CEs is the species specific
collection efficiency, and each Is, i is an ion signal associated
with species “s.” The apparent mass based ionization efficiency is
dependent on the collection efficiency (i.e. RIEs,
app=RIEs⋅CEs) for the
particular particle type, which is given by the product of terms associated
with loss due to the aerodynamic lens (EL), beam divergence
(ES), and particle bounce (EB) off of the tungsten
vaporizer (CE=EL⋅ES⋅EB) . For particles with unit lens transmission,
the most important loss mechanism when considering the laser vaporization
module of the SP-AMS is beam divergence, such that CE≅ES .
RIErBC, app and RIEOrg, app
values were determined from independent measurements of
mIErBC, app, mIEOrg, app and
mIENO3. Good agreement was observed between
RIErBC, app values obtained from bare Regal Black
over the particle size range of 250–400 nm, which suggests
minimal difference in collection efficiency for these uncoated particles.
The effect of rBC-particle coating
RIErBC, app and RIEOrg, app
values were measured over a range of BES coating thicknesses, with the
mass ratio of BES to Regal Black (ROrg/RB)
ranging from 0 to 6 (corresponding to a mass fraction of Regal Black,
fRB, ranging from 1 to
0.15). Particles with 200–500 nm mobility diameters, characterized
in terms of their per particle masses and number concentrations, were used
to ensure unit lens transmission (EL= 1). Figure a demonstrates that as
ROrg/RB increases, the apparent sensitivity to
rBC increases by approximately a factor of 2, for two SP-AMS
instruments, and within the uncertainties of our measurements appears to saturate at ROrg/RB>3. A similar trend is shown in Fig. b for the BES
coating from each SP-AMS instrument. A comparable increase in
sensitivity with particle coating was also observed with a third
SP-AMS instrument (see Fig. S2); however, particle mass analyser
measurements were not available during this experiment so the data
were not included in Fig. . The difference in
sensitivity enhancement evident in Fig. a between
the two instruments could be due to differences in the final coated
particle size or to differences in the uniformity of particle coating
provided by each coating apparatus, both of which could result in
differences in particle focusing in the aerodynamic lens and the
resulting particle beam–laser beam overlap. The possibility of
homogeneously nucleated organic particles, which cannot be neglected
in these experiments, could also lead to differences in the overall
RIE enhancement.
(a) RIErBC, app measured as
a function of BES coating thickness, demonstrating an increase in
sensitivity that appears to reach saturation at approximately
ROrg/RB=3. RIErBC, app values at ROrg/RB=0
were obtained using 200 and 300 nm (dm) particles. Data from two instruments is
included: UofT (circles) and ARI data with the laser vaporizer only
(squares, SN 215-130). (b) RIEOrg, app
measured as a function of BES coating thickness, demonstrating
a similar increase with coating thickness. The blue solid lines
represent a best fit to the UofT SP-AMS data used in subsequent analysis (Sect. 3.5).
The relatively large values of RIEOrg, app observed in
this work for BES were higher than the recommended value of 1.4 for all
sampled organics from ambient sources . As noted in
individual organic molecules may exhibit different RIE
values, including values greater than 1.4. Utilizing the results published in
(Fig. 6), organic hydrocarbon compounds have an apparently
higher RIE than highly oxygenated organic compounds (RIE ∼2 for
hydrocarbons vs. ∼1.3 for oxygenated compounds). BES has a relatively
low O : C ratio (4:26∼ 0.15), and thus may exhibit an RIE similar to
hydrocarbons up to ∼2. For these data, the apparent RIE (defined as
SP-AMS measured ions divided by the difference of the coated to uncoated per
particle mass from the mass analyser) measured for BES under thin coating
conditions are highly uncertain due to the difference term in the
denominator, which goes to zero with thin coatings. Under thicker coating
conditions, the observed RIEOrg, app values are
significantly higher than 2 and increase with coating thickness. Reasons for
this may be due to differences between the laser and tungsten vaporizers and
could include; (1) an enhanced ionization efficiency for gas phase molecules
generated in the laser vaporizer due to their position relative to the
filament; (2) an improved ion extraction efficiency of ions originating from
neutral gas-phase molecules vaporized at the laser vaporizer, due to their
position in the ion chamber; or (3) a collection efficiency on the tungsten
vaporizer of less than one (Eb< 1) for organics on rBC
particles. The fact that an RIEOrg, app > 2 for coated
particles is observed not only with dual vaporizers, but also with the laser
vaporizer alone may suggest an enhanced sensitivity to species volatilized
from the laser vaporizer. In addition, the fractional increase in
RIEOrg, app is larger than that of RIErBC,
app, which may indicate the relevance of possibility (3), above, since
organic species are volatilized from both vaporizers in this instrumental
configuration. Further work is required to fully understand the differing
responses to rBC and NR-PM in the SP-AMS.
The observed dependence of SP-AMS sensitivity on particle coating thickness
(Fig. ) is likely caused by decreased particle beam
divergence with coating, resulting in greater particle beam–laser beam
overlap. The saturation behaviour shown in Fig. can be
used to estimate collection efficiency (ES) for bare Regal Black
particles and for the organic species on thinly coated Regal Black particles.
With the assumption that the sensitivity measured at saturation (i.e. highly
coated particles) corresponds to complete particle beam–laser beam overlap
(ES=1 and RIErBC, app=RIErBC), then ES can be inferred by dividing
RIErBC, app (RIEOrg,
app) values for the bare (nascent) Regal Black by the average
maximum value observed in each instrument for coated particles
(RIErBC, app and RIEOrg,
app). From the data sets presented in Fig. ,
for bare (nascent) Regal Black, an ES value of 0.6±0.1 was determined. This result indicates that Regal Black
calibrations used in the past, with mIErBC of
∼150±25 ionspg-1 and corresponding
RIErBC of 0.2±0.1 , are
underestimated by approximately a factor of 2.
rBC particle beam divergence
In this section, the impact of particle coating on SP-AMS collection
efficiency is explored using particle beam width
measurements. Non-spherical particles produce more divergent particle
beams compared to spherical particles, owing to lift and drag on
irregularly shaped particles. As non-spherical rBC-containing
particles are coated with a liquid organic species, they become more
spherical in shape e.g.. Therefore, a change in
rBC-containing particle shape could affect particle beam divergence
enough to alter the extent of particle beam–laser beam overlap, thus
altering the collection efficiency (Es). To investigate
this difference in sensitivity with particle coating, the change in
particle beam width was measured as a function of coating thickness
with a beam width probe (BWP).
In this context, signal transmission is defined as the signal
intensity at a given BWP position relative to that of an unblocked
beam, normalized to particle number concentration. With a BWP centred in the
particle beam, a lower signal transmission will indicate a narrower beam
since a larger fraction of particles are blocked by the BWP wire. Figure compares signal
transmission versus BWP position curves for 200 nm uncoated Regal
Black, BES coated Regal Black particles (ROrg/RB∼ 3) and 300 nm ammonium nitrate (as an example of a well
focused beam), each fit to a 1-D Gaussian distribution. The standard
deviation (σ) of this distribution is used to describe the
particle beam width (i.e. 4σ, or ±2σ, defines
95.4 % of the particle beam width). As shown in
Table BWP measurements indicate that the particle beam
narrows from σ∼ 0.4 mm for bare, 200 and
300 nm Regal Black particles to approximately
0.1–0.2 mm (ARI and UofT measurements, respectively) when the
particles are coated with BES (ROrg/RB∼3). The differences in beam width measured in the two experiments
likely arise from differences in the final coated particle size
(∼ 375 nm and dva∼ 280 nm, see
Table ). The beam width for these coated Regal Black
particles approaches that of pure 230 nm BES particles
(σ = 0.12 mm) . However, it should be
noted that 1-D Gaussian fits to BWP curves at low σ (i.e.
a well focused beam) may overestimate the particle beam width
.
Summary of particle beam width measurements.
Particle type
Beam width σ (mm)
Reference
Flame soot
0.77
Ambient rBC (roadside, Toronto)
0.46±0.03
this work
300 nm Regal Black
0.40±0.08
this work
200 nm Regal Black
0.39±0.04
this work
Ambient rBC (non-roadside, Toronto)
0.26±0.04
this work
300 nm Nitrate (tungsten vaporizer)
0.22±0.05
this work
Coated Regal Black (UofT, 150 nm corea)
0.23±0.08
this work
Coated Regal Black (ARI, 300 nm coreb)
0.11±0.03
this work
230 nm DOS
0.12
Laser vaporizer (upper limit)
≤0.18c
this work
a ROrg/RB∼3, coated dva=280 nm.b ROrg/RB∼3, coated dva≥375 nm.c Derived from BWP attenuation with the probe parallel to laser
beam axis, all other measurements were made with a perpendicular BWP
orientation.
Comparison of BWP signal transmission (signal intensity
relative to that of an unblocked beam, normalized to particle number
concentration) traces for 200 nm bare (fRB=1),
BES coated (ROrg/RB=3) Regal Black and
300 nm ammonium nitrate particles vaporized using the
tungsten vaporizer as an example of a well focused beam. Data from
the UofT SP-AMS.
(a) mIErBC, app as a function
of signal transmission with the beam width probe wire at the centre
of the particle beam. The two sets of data were collected sequentially for each coating thickness by
positioning the BWP wire outside the particle beam to measure mIErBC, app and
then moving the wire into the centre of the beam to measure the associated signal transmission.
Colours indicate organic coating thickness estimated from SP-AMS measurements of rBC and NR-PM (300 nm RB core). (b) Organic
versus rBC signal transmission with a 0.41 mm BWP wire
at the centre of the particle beam, as a function of BES coating
thickness (ROrg/RB). The solid line represents a 1:1
relationship. Data from the ARI SP-AMS (SN 215-039).
rBC signal transmission as a function of BWP position
parallel to the laser beam axis of the SP-AMS. When the
0.46 mm BWP wire was placed in the centre of the particle
beam, an average of 3±2 % signal transmission was
observed indicating that the effective beam width (4σ) is
less than the projected width of the wire on the laser vaporizer
(shaded region). This corresponds to a beam width (σ) ≤0.18 mm. Data from the UofT SP-AMS using uncoated
300 nm (dm) Regal Black particles.
Figure a demonstrates the relationship between
sensitivity to rBC, particle beam width and coating thickness. With
a BWP centred in the Regal Black particle beam, signal transmission
decreases as ROrg/RB increases (i.e. the
particle beam width decreases as coating thickness
increases). Furthermore, with the BWP positioned outside of the
particle beam, mIErBC, app increases as
ROrg/RB increases. These two observations
suggest that as rBC particles become more coated the particle
beam–laser beam overlap becomes more complete, giving rise to enhanced
signal intensity for rBC and the associated
NR-PM. RIEapp vs. SP-AMS derived
ROrg/RB plots for this data set, similar to
those in Fig. , are presented in
Fig. S2. Figure b demonstrates that rBC and organic
signal transmission both decrease due to particle coating, with an
approximately one-to-one relationship. This concept is discussed further in Sect. 3.5.
Measure of effective laser width for rBC
To aid in assessing the importance of incomplete laser beam–particle beam overlap for SP-AMS quantification of rBC, an upper limit
for the effective laser beam width was estimated and compared to
ambient particle beam width measurements.
When the SP-AMS is operated without the tungsten vaporizer and the
BWP is positioned such that the wire could be moved vertically
(perpendicular to the laser beam axis), some information can be gained
about the effective width of the laser beam in much the same manner as
is done for particle beam widths. Since the resulting BWP position
versus signal transmission curve is affected not only by the true laser
beam effective width but also by the particle beam and the BWP wire
width, these measurements only allow for estimation of an upper limit
for the effective SP-laser width for rBC. Figure
demonstrates that with a 0.46 mm BWP centred in front of the
IR laser an average of 97±2 % of the rBC signal was
attenuated. From this information and the projected BWP wire width
(see Fig. ) the effective laser beam width
(4σ) can be estimated as less than 0.73 mm. Therefore,
the effective laser beam width (σ) is at most
0.18 mm. The narrowness of the effective laser beam width
highlights the importance of determining ES for accurate
quantification of rBC mass loadings. Further measurements are required
to unambiguously determine the effective IR laser beam width.
Ambient particle beam width measurements
Beam width probe measurements of ambient rBC-containing particles
provide insight into the morphology, and collection efficiency, of
these particles. Ambient rBC particles sampled at 5 m height
and 16 m from a roadside in downtown Toronto gave wider
particle beams compared to those sampled away from large roadways,
suggesting that fresh rBC emissions produce more divergent beams, and
that particle processing reduces particle beam divergence (see
Table ). Both ambient BWP measurements indicated beam
widths narrower than that of flame soot (σ=0.77 mm)
, and fall within the range of ambient particle beam
widths observed by .
The similarity in beam width for the “fresh” rBC particles measured
here and the beam width for bare Regal Black could suggest similar
collection efficiencies for these types of particles. However,
previous comparisons of SP-AMS rBC mass loadings with those derived
from other instruments, such as the multiangle absorption photometer
(MAAP), have suggested lower overall collection efficiencies for
ambient rBC-containing particles (i.e. 0.1 to 0.3)
. Future ambient BWP measurements, in
conjunction with comparison to other instruments, will further
constrain the SP-AMS collection efficiency for ambient rBC and NR-PM.
Comparison of the estimated upper limit of laser beam width (σ≤0.18 mm) with ambient and laboratory particle beam width
measurements highlights the relevance of incomplete beam overlap for
SP-AMS quantification. It is important to note that this overlap
mismatch is only relevant in the dimension perpendicular to the IR
laser, such that a simple comparison of particle and laser beam widths
is insufficient to provide a quantitative estimate of collection
efficiency.
Quantification of rBC mixing state in organic coated particles
In order to quantify mixing state in terms of the mass fraction of rBC
present in ambient aerosol the SP-AMS must be operated without the
tungsten vaporizer to avoid interference from externally mixed NR-PM;
however, in laboratory experiments when all particles are known to
contain rBC such a quantification is possible. In this section, the
ability of the SP-AMS to quantify mixing state for
laboratory-generated internally mixed rBC particles is examined. It is
important to note that these experiments focused on liquid organic
coatings, which are similar to hydrocarbon-like organic coatings but
may not resemble more oxygenated organic coatings present in ambient aerosol.
To accurately quantify rBC and NR-PM simultaneously from the same
particles, it is important that the effective IR laser beam width for
vaporizing all particle phase species is similar. If, for example,
NR-PM could be vaporized in lower energy regions of the IR laser
(i.e. at the edges of the beam) that do not effectively vaporize
rBC, NR-PM would have a wider effective laser beam width. In such
a case, the contribution of organic species to the overall particulate
mass loading could be overestimated relative to rBC for ES<1. Figure b demonstrates that the rBC and the
organic signal transmission decrease with coating thickness in
a similar manner when a BWP is placed in the centre of the particle
beam. Furthermore, Fig. demonstrates similar
increases in RIErBC and RIEOrg
with coating. Overall, these observations suggest that the effective
IR laser beam width, in terms of its ability to vaporize different
particle phase species, is similar for rBC and BES.
To further assess the ability of the SP-AMS to quantify the mixing state of
rBC-containing particles, the mass fraction of Regal Black
(fRB=massRB/(massBES+massRB))
in BES coated particles of various rBC core sizes (40 to 300 nm
mobility diameter) was measured with the SP-AMS and compared to
fRB obtained from particle mass measurements.
Figure demonstrates that fRB values derived
from the SP-AMS agree well with fRB values from particle mass
measurements when SP-AMS values are corrected for CE. The measured
RIEapp in this case is the product of the
RIErBC and CE, where RIErBC is the
maximum ratio of the ions detected for rBC material sampled
(mIErBC), assuming all rBC particles vaporize in the laser
beam, to the ions detected for ammonium nitrate
(mIENO3). The CE factor, which is unity or less, is a
measure of the missed rBC ion signal due to poor overlap between the particle
beam and the laser vaporizer; that is, it accounts for rBC particles that
miss the laser beam and are not vaporized. To obtain the corrected values,
the upward trend in RIEapp measured for rBC and NR-PM with
increasing coating thickness (Fig. ) was fit to a sigmoid
relationship to obtain the appropriate values for a given coating thickness
(determined by mass analyser measurements for different rBC-core sizes).
The fraction of Regal Black in BES coated particles
(fRB), obtained from SP-AMS mass
loadings using uncorrected RIErBC (0.2) and
RIEOrg (1.4) values (triangles) and RIE values
corrected using the relationship observed in
Fig. , plotted against fRB obtained from particle mass measurements.
Square points refer to RIE data shown in
Fig. that were used to determine the appropriate
RIEapp values to apply to data from separate
experiments at various rBC core sizes (circles). A best-fit line to
the data using corrected RIE values yields a slope of 1.01±0.05 with R2=0.98. The solid line represents a one-to-one
relationship. Data from the UofT SP-AMS. Analogous plots for two ARI
SP-AMS instruments are shown in Fig. S3.
SP-AMS values derived using the RIErBC for uncoated Regal
Black and RIEOrg typically used for organics (i.e. 0.2 and 1.4,
respectively) can underestimate fRB by up to ∼ 50 % and
data in Fig. exhibit a pronounced non-linear relationship.
For the specific case of BES, if the RIEOrg value is
higher than the recommended value of 1.4, the underestimation would be less
than that depicted in Fig. . When RIEapp
values are corrected as described above the relationship in
Fig. shows excellent linearity (R2=0.98, slope =1.01±0.05).
These results indicate that the SP-AMS can accurately measure the fraction of
Regal Black in laboratory generated organic coated particles, down to
fRB of ∼0.05 for small (i.e. 40 nm) inclusions,
when instrumental sensitivity and CE are well known. The ability of the
SP-AMS to quantitatively characterize rBC mixing state depends strongly upon
the RIErBC, app and RIEOrg, app values
used to calculate particulate mass loadings. In addition, knowledge of the
value of ES, or particle loss owing to beam divergence, is
essential for quantitative SP-AMS measurements of mixing state. Future
ambient and laboratory beam width probe measurements may aid in determining
CE for particles with unknown morphology and coating state.
Summary
Our results demonstrate that particle morphology affects the SP-AMS particle
beam width, that in turn affects the collection efficiency through the
overlap of the particle beam and the laser beam. Observations from
laboratory-generated internally mixed rBC-containing particles in three
SP-AMS instruments indicate that particle coating influences CE through
beam divergence (ES). We estimate CE for bare Regal Black
particles, the recommended calibration material, to be 0.6±0.1
suggesting that previous rBC calibrations of the SP-AMS with Regal Black were
underestimated by up to a factor of 2. CE for ambient rBC-containing
particles will depend on their morphology and the resulting particle-beam
width, which is largely unconstrained. Future work will aim to empirically
relate measurements of particle beam width to CE using a number of SP-AMS
instruments, to provide a method to determine the CE of particles with
unknown coating state.
Ambient BWP measurements indicate that fresh rBC containing particles
sampled at the roadside in downtown Toronto have particle beam widths
similar to, but greater than, bare Regal Black, and thus likely
a collection efficiency less than bare Regal Black. More processed rBC
particles have a smaller beam width and therefore a larger value of
CE. The importance of measuring ES is further highlighted
by the upper limit estimate of the effective laser beam width for rBC,
from BWP measurements, of σ≤0.18 mm. Further work
is required to provide an unambiguous measurement of the IR laser beam
width, which could be applied in conjunction with ambient particle
beam width measurements to provide a more specific estimate of CE in
different sampling environments.
This work demonstrates for the first time the ability of the SP-AMS to
quantitatively characterize the levels of internally mixed particulate
rBC and NR-PM in laboratory-generated particles over a wide range of
compositions, when instrumental sensitivity is well known. In
particular, the SP-AMS can quantify the rBC mixing state even for
highly organic-rich particles (fRB<0.05) with
atmospherically relevant rBC core sizes (i.e. ≥40 nm
mobility diameter). The quantitative capability of the SP-AMS,
combined with its ability to chemically characterize NR-PM coated on
rBC-particles, can provide unique insight into the characteristics of
ambient black carbon.