Introduction
Atmospheric aerosols, also known as particulate matter (PM), are defined as
a suspension of fine solid or liquid particles in a gaseous medium. These
particles range in size from a few nanometers in diameter to as much as 100 µm. Aerosols consist of inorganic ions, organic compounds, oxides of
most metals, elemental carbon, and water with organic compounds often being
the dominant fraction of submicrometer PM, with a contribution of 20–90 % depending on the location (Kanakidou et al., 2005).
Fine particles (PM2.5 defined as the dry mass of particles collected
after a particle separator with a 50 % cut point of 2.5 µm
aerodynamic diameter) can affect human health by penetrating into the
respiratory tract and reaching deep into the lungs (Miller et al., 1979;
Dockery et al., 1993; Künzli et al., 2005; Wyler et al., 2000; Cohen et
al., 2005). Toxicological data suggest that ultrafine particles, the
diameter of which is < 100 nm, can have adverse pulmonary and cardiovascular effects
(Donaldson and MacNee, 2001). Ultrafine particles soluble in water and
lipids have a minor contribution to the total aerosol mass and are believed
to have minor effects on human health (Kreyling and Scheuch, 2000) but
insoluble components, like black carbon (BC), could have more significant
contribution (Fuzzi et al., 2015; Terzano et al., 2010). The impacts of
these ultrafine insoluble particles cannot be easily determined with mass
measurements but require studies of the number concentration.
Atmospheric particulate matter can also affect the Earth's radiative budget
and global climate. Particles act as cloud condensation nuclei, form cloud
droplets, and scatter light, leading to cooling of the planet. Absorbing
aerosol components, such as BC, contribute to warming by
strongly absorbing light. BC containing particles, mostly emitted by
combustion sources, evolve due to coagulation or condensation of vapors
(Scheer et al., 2005). This coating of BC particles can increase not only their light
absorption up to a factor of 3 (Andreae and Gelencsér, 2006; Saleh
et al., 2014) but also their hygroscopicity (Kuwata et al., 2008). The
number concentration, size distribution, and mixing state of BC particles are
essential in understanding their effect on climate (Bond et al., 2013).
Since the number concentration of ultrafine atmospheric particles,
especially of the nonvolatile BC particles, plays a key role in both their
health and climate effects, ways to measure these particles have been
proposed. Nonvolatile particles have been indirectly measured by heating up
the aerosol through laminar flow reactors designed to remove semi-volatile
components from particles, called thermodenuders (TDs) (Wehner et al., 2002;
Burtscher et al., 2001). These have been used to quantify the volatility of
particles at low and intermediate temperatures (An et al., 2007; Kim et al.,
2010) as well as their nonvolatile fraction at higher
temperatures (> 250 ∘C). Wehner et al. (2004) suggested that by heating up the
particles at 280 ∘C for residence times up to 9 s, sulfates,
nitrates, and most organics evaporate, and only nonvolatile particle cores
remain. To measure the number particle size distribution in this size range,
scanning mobility particle sizers (SMPS) have been used after the TD (Wehner
et al., 2004; Kim et al., 2010; Saleh et al., 2012). For the determination
of the chemical composition of these particles, aerosol mass spectrometers
(AMSs) have been increasingly applied (Lee et al., 2010; Poulain et al.,
2010; Cappa and Jimenez, 2010).
Despite the increasing focus on nonvolatile particles their contribution to
the total number size distribution remains uncertain and strongly depends on
the different pollution sources. Chemical characterization of these
nonvolatile particles can provide insights about their chemical
composition, information about the contribution of BC to the total number
concentration and improve the characterization of different emission
sources.
In this work an experimental methodology is developed and evaluated to
measure the nonvolatile particle number concentration. The method is tested
with a set of chamber experiments to investigate its ability to evaporate
particles formed from traditional biogenic and anthropogenic secondary organic aerosol (SOA) precursors
and, most importantly, particles coming from nucleation. This experimental
approach was applied during the Athens 2013 winter campaign to examine the
number concentration of nonvolatile particles in an urban environment, their
chemical composition and size distribution, as well as the contribution of
different pollution sources.
Instrumentation
Thermodenuder description
The TD setup used in this study was based on the design of An et al. (2007).
A detailed schematic of the TD apparatus is shown in Fig. S1 in the Supplement. The TD
consists of the bypass (BP) line, which operates at ambient temperature and
the TD line, which was set at 400 ∘C in this study. Automatically
operated three-way valves placed on the aerosol inlet and outlet (Fig. S1)
allow rapid switching between the BP and TD lines and a comparison of the
two, essential for particle evaporation and volatility measurements. The TD
line includes two main parts: (i) the TD heating section and (ii) the TD
cooling section. The TD heating section has a length of 0.5 m and an inner
diameter of 36.4 mm. Fine sand is used to cover the inner stainless steel
tube to avoid temperature fluctuations. A PID controller (model CNi, I/32,
Omega) controls the TD temperature through a heating tape wrapped around the
outer cylinder (diameter of 100 mm) of the TD, based on readings of a
thermocouple (TJ36 Series, Omega) placed in the center of the heating tube.
The heating section is insulated to minimize losses to the surroundings. The
cooling section has the same dimensions as the heating section and consists
of cylindrical stainless steel gauze with an inner diameter of 36.4 mm. The
stainless steel gauze is covered with an activated carbon jacket which is
used to remove organic and inorganic vapors that evaporated from the
particle phase avoiding thus re-condensation during the cooling stage.
The residence time inside the TD has been shown to be a critical parameter
for particle evaporation (Riipinen et al., 2010). In this study, a sample
flow rate of 1 L min-1 was used. The flow in the system is laminar and
results in different residence times, based on radial position. The average
travel time of the particles in the heating section at 400 ∘C
(operating temperature of the TD in this study) was 13 s, significantly
longer than most commercial and research TDs. In the cooling section the
average residence time was approximately 30 s. The cooling section was
cooled by convection and the temperature of the sample exiting the TD was
within 5 ∘C of the room temperature for all conditions.
The temperature profile of the TD when set at 400 ∘C was measured
by a stainless steel thermocouple (50 cm length, Omega) in the center of the
TD tube (Fig. S2). The temperature difference between the center of the TD
tube and the walls was less than 10 ∘C. The walls of the TD were
warm even before the heating section, due to conduction, resulting in
preheating of the particles before entering the heating tube. The aerosol
sample already has a temperature of 150 ∘C when introduced in the
heating section, which increases to the set temperature of 400 ∘C
20 cm from the TD inlet. Similar results were reported by Wehner et al. (2002) using a similar TD setup.
During the Athens 2013 winter campaign
the TD was operating at different temperatures, ranging from 40 to
400 ∘C (Fig. S4). More specifically, seven temperature steps were
chosen for the campaign (40, 70, 100, 120, 200, and 400 ∘C). One
cycle, from 40 to 400 ∘C and back, required 10 h, which resulted in
roughly two temperature cycles per day. The starting time of each cycle
changed from day to day, to early morning (02:30–05:00 LT), morning
(07:00–10:00 LT), afternoon (16:00–18:00 LT), and midnight
(21:00–00:00 LT), so that measurements at each temperature were performed
during different periods of the day. The temperature of interest for the
present work is 400 ∘C and during the campaign there were 32 periods
of measurements at this temperature, resulting in 20 h of data. It should be
noted that throughout this work, nonvolatile particles are defined as the
particles that did not evaporate completely under the conditions of these
measurement (TD set at 400 ∘C) and therefore had diameters above the
detection limit of the used SMPS system (approximately 10 nm). According to
Riipinen et al. (2010) the saturation concentration of the corresponding
compounds at room temperature should be less than
10-5 µg m-3 which, based on the terminology of Murphy et
al. (2014) for atmospheric organics, should be categorized as extremely low
volatility organic compounds. If these particles consist of BC or other
material they should be categorized as practically nonvolatile.
SMPS
A scanning mobility particle sizer (model 3936, TSI Classifier
model 3080, TSI DMA 3081, TSI Water CPC 3787) was coupled to the TD to
provide the number size distribution of the aerosol in the diameter range
from 10 to 500 nm. The SMPS was operated with a sheath air flow of 5 L min-1. The CPC was modified with the addition of a clean air line with
a HEPA filter and operated at an effective sample flow rate of 1 L min-1. The aerosol number size distribution (10 to 500 nm) was measured
every 3 min (SMPS scanning time). Measurements of both the TD and the BP
line were achieved by automatic switching (every 3 min, synchronized with
the SMPS) of the three-way valves (Fig. S1).
Aerosol mass spectrometer
The AMS (Jayne et al., 2000) is commonly used
for measurement of submicron non-refractory aerosol particles, which
includes organics and most inorganic salts but not black carbon. The
high-resolution time-of-flight AMS (HR-ToF-AMS) instrument used here and its
data analysis procedures have been already described in depth in previous
works (Drewnick et al. 2005; DeCarlo et al., 2006; Canagaratna et al.,
2007). In brief, aerosol particles are introduced into the AMS through a
critical orifice and an aerodynamic lens focuses the particles into a narrow
beam. Then, the particles are flash-vaporized (impaction on a resistively
heated tungsten surface, at 600 ∘C) and ionized (electron impact
ionization by 70 eV electrons). The HR-ToF-AMS can be operated in two
ion optical modes (V or W). In this work it was operated in the V mode. The
AMS data analysis software packages SQUIRREL (v1.53) and PIKA (v1.10C) were
used for the analysis of the high-resolution mass spectra.
In all measurements in this work, the AMS was coupled to the TD measuring the
chemical composition and mass size distribution of the non-refractory
components of particles with vacuum aerodynamic diameters less than
approximately 1 µm. This corresponds roughly to a physical diameter
of 0.7 µm assuming a density of 1.4 g cm-3. A time-dependent
AMS collection efficiency was calculated using the Kostenidou et al. (2007)
algorithm combining the AMS and SMPS size distributions. The average
collection efficiency was 0.85 ± 0.23. The AMS data were
corrected for losses in the
thermodenuder as described in Sect. 3.1 below.
During the Athens 2013 winter campaign, positive matrix factorization (PMF)
was applied to the ambient measurements of the AMS organic spectra,
following the approach of Ulbrich et al. (2009), to estimate the
contributions of the various sources to the organic aerosol (OA) levels.
Multi-angle absorption photometer (MAAP)
To estimate the PM2.5 black carbon mass concentration a MAAP (model
5012, Thermo-Scientific) was used. The MAAP applies a constant mass
absorption coefficient of 6.6 m2 g-1 to internally convert the
absorption coefficient measured at a wavelength of 637 nm to the soot mass
concentration (Petzold and Schönlinner, 2014). The absolute value of the
mass absorption coefficient depends, however, on the morphology of the carbon
particles and their interactions with other compounds (Fuller et al., 1999),
implying a systematic uncertainty of the BC derived from the MAAP.
During the Athens 2013 winter campaign the MAAP was characterizing the
PM2.5 fraction. To estimate the particulate mass (PM1) BC concentration a correction
factor of 0.9 was used following Poulain et al. (2014). The measurement
accuracy of an individual MAAP yields a variability of less than 5 %
around the mean value (Müller et al., 2011). Taking into account
uncertainties of the mass absorption coefficient, the different particle cut
size, and instrumentation accuracy, a total uncertainty of approximately 30 % is expected for the estimated BC mass concentrations. It should be
noted though that these BC concentrations are used in a relative sense
throughout this work and therefore the corresponding uncertainties do not
affect our conclusions.
Thermodenuder characterization
Particle wall losses
Aerosol losses inside the TD at 400 ∘C were determined in a
separate set of experiments, using NaCl particles, since their evaporation
is negligible even at high temperatures (Burtscher et al., 2001). NaCl
aerosol was produced by atomizing (constant output atomizer, aerosol
generator 3076, TSI) aqueous NaCl solutions of known concentration in the
range between 0.5 and 3 g L-1 (NaCl purity ≥ 99.5 %, Sigma).
The produced NaCl droplets flowed through a silica dryer (RH < 20 %) and then the dry particles passed through the TD. Experiments were
conducted in a wide range of particle mass and number concentrations ranging
from 50 to 300 µg m-3 and 104 to 105 cm-3, respectively. Particle wall loss inside the TD line was determined
relative to the BP line using
Loss(Dp)=1-dNdlogDpTDdNdlogDpBP,
where dN/dlogDp is the number concentration for the TD and BP measurements, and Dp is the
mobility diameter of the particles.
Particle wall loss was determined at 400 ∘C and the average
result from all nine experiments indicates that up to 70 % of the particles
smaller than 30 nm are lost on the walls while for particles larger than 50 nm the losses are around 50 % (Fig. S3). Small particles are lost to the
walls mostly due to Brownian diffusion, while for larger particles
thermophoretic losses prevail (Burtscher et al., 2001). The precision of the
measurements (± 2 σ) for particles smaller than 100 nm is
< 10 % while for larger particles increases up to around 20 %. The focus
of this work is on particles smaller than 100 nm so the corresponding losses
have been determined quite precisely.
The average determined particle wall loss function was fitted with a triple
exponential function:
(%)Loss(Dp)=1.28exp(-0.1Dp)+0.47exp(2×10-4Dp)-0.64exp(-0.11Dp),
where Dp is in nm. The resulting fit
parameters (Fig. S3) were used to correct all the measurements (SMPS and AMS)
throughout this work.
Evaporation of SOA
The TD methodology was tested in a set of laboratory experiments performed
using the indoor smog chamber facility at ICE-HT/FORTH, Patras. The chamber
is a 10 m3 Teflon reactor suspended in a 30 m3 temperature-controlled room with aluminum-coated walls. The room walls are covered with
UV/vis lamps (320–450 nm wavelength range). The photolytic rate of
NO2 was measured using the well-established chemical actinometry method
(Bohn et al., 2005), resulting in JNO2=0.59 min-1. In all
experiments the temperature and the RH inside the chamber were kept constant
at 25 ∘C and < 30 %, respectively.
Experimental conditions used in α-pinene dark ozonolysis and
toluene photooxidation experiments.
Initial
Initial
α-Pinene
Temperature
RH
α-pinene
O3
SOA Mass a
experiments
(∘C)
(%)
(ppb)
(ppb)
(µ m-3)
1
25
30
5.5
280
3.1
2
25
30
10
300
8.7
Initial
Initial
Toluene
Temperature
RH
Toluene
NOx
H2O2
[OH]b
SOA Mass a
experiments
(∘C)
(%)
(ppb)
(ppb)
(ppb)
(moleculescm-3)
(µ m-3)
1
25
30
50
2
2
2.4×106
1.7
2
25
30
50
7
2.1
3.1×106
2.2
a Determined from the SMPS measurements using a density of 1.4 g cm-3 (Kostenidou et al., 2007).
b Initial [OH] after UV lights were turned on, as measured by the decay
of toluene.
Biogenic SOA
For the generation of biogenic SOA, a set of experiments utilizing the dark
ozonolysis of α-pinene was conducted following the experimental
conditions given by Pathak et al. (2007), shown in Table 1. First, O3
was introduced inside the chamber and then an injection of α-pinene
initiated the production of SOA. The experiment took place at NOx mixing
ratio < 2 ppb in the absence of particle seeds. The SOA mass concentration was
determined from the measured SMPS size distribution using a density of 1.4 g cm-3 (Kostenidou et al., 2007).
The particle number distributions of SOA particles formed from α-pinene
ozonolysis at ambient temperature (BP) and 400 ∘C (TD) for
experiment 1 (Table 1) are shown in Fig. 1. After the introduction of
O3 at 00:45 and injection of α-pinene at 01:20, new particles
were formed and grew rapidly up to 100 nm in less than 1 h, reaching a
concentration of around 2000 cm-3. These fresh SOA particles evaporated
almost completely (2 % by number remained) after passing through the TD
line at 400 ∘C (Fig. 1). It should be noted that the TD removal
efficiency reported in this work corresponds to particles that either
evaporated completely or had a residue no larger than the minimum detectable
size of 10 nm.
The particle number size distribution in the chamber at the end of the
experiment is shown in Fig. 1c. A significant decrease of total particle
number concentration, from 1700 to 35 cm-3 (number fraction remaining
∼ 2.2 %), after sampling through the TD line was observed.
After new particle formation occurred the number fraction remaining (NFR)
immediately dropped to 2 %, indicating that 98 % of the particles
evaporated in the TD line. This shows that fresh SOA formed from α-pinene ozonolysis evaporates almost completely after heated at 400 ∘C. The few remaining particles after the TD were mainly
preexisting particles in the chamber. Background particle number
concentration after the TD was around 20 cm-3 before the reactions
started and this value was relatively stable during the experiment.
Time evolution of particle number size distribution during the dark
ozonolysis of α-pinene, measured after (a) the bypass line
at 25 ∘C and (b) the TD set at 400 ∘C. The color
scale represents particle number concentration. (c) Average particle
number size distribution of α-pinene SOA (15 scans, 3 min time
span), for chamber measurements, at 25 ∘C (blue), and loss-corrected
TD measurements, at 400 ∘C (red), for Exp. 1 (see Table 1 for
details). Vertical lines correspond to confidence intervals.
Anthropogenic SOA
As a typical example of anthropogenic SOA, the photooxidation of toluene was
examined to test the ability of the proposed TD methodology to completely
evaporate anthropogenic SOA. H2O2 and toluene were injected in
the chamber and after reactants were mixed, the UV lights were turned on,
initiating the photochemistry. OH radicals were produced from H2O2
photolysis and OH concentration inside the chamber was determined following
the decay of toluene. The experiments were conducted at NOx mixing ratio
of < 10 ppb in the absence of particle seeds. A summary of the experimental
conditions is given in Table 1.
The time evolution of the number size distribution of experiment 1 is shown
in Fig. 2. After injecting toluene and H2O2 at 00:30, UV lights
were turned on (00:45), forming fresh particles at 25 ∘C. The
growth of these particles up to 100 nm took place in less than 1 h,
reaching a concentration of 1300 cm-3. When heated at 400 ∘C
these fresh particles evaporated practically completely (NFR of 2 to 4 %).
The average number size distributions from 07:00 to 09:00 are shown in Fig. 2c. Again, a significant decrease of the particle number concentration was
observed from 1350 to 60 cm-3 (NFR = 4 %). After new particle
formation NFR was less than 5 % and as particle number concentration
increased NFR decreased to 2 %, suggesting that 98 % of the particles
evaporated at 400 ∘C. Background particle number concentration
after the TD was around 30 cm-3 before the reactions started. This
concentration increased during the experiment to 70 cm-3, suggesting
that a small fraction (1–2 %) of the newly formed particles did not
evaporate through the TD. This shows that fresh SOA particles formed from
toluene photooxidation evaporate almost completely after heating at 400 ∘C.
Time evolution of particle number size distribution during
the photooxidation of toluene, measured after (a) the bypass line set
at 25 ∘C and (b) the TD set at 400 ∘C. The color
scale represents particle number concentration. (c) Average particle
number size distribution (from 07:00 to 09:00; 40 scans, 3 min time span) of
toluene SOA for chamber measurements, at 25 ∘C (blue), and TD
loss-corrected measurements at 400 ∘C (red) (Exp. 1, Table 1).Vertical lines correspond to confidence intervals.
Evaporation of freshly nucleated particles
A set of measurements was conducted to explore the ability of the TD to
evaporate freshly nucleated particles that originate from photo-oxidation of
ambient air mixtures enclosed in a chamber. Experiments were performed in a
10 m3 Teflon chamber, while OH radicals were generated from
H2O2 photolysis (JNO2∼0.59 min-1).
Ambient air present in typical summer days in Patras (Kostenidou et al.,
2015) was introduced inside the chamber and characterized under dark
conditions with the full suite of instruments as given in Table 2. After the
initial characterization, the ambient air was exposed to UV light,
initiating photochemistry. OH levels ranged from 3.1 to 4.2 × 106
molecules cm-3, as determined by the decay of a suitable tracer
(n-butanol, d9) detected by proton transfer reaction/mass spectrometry (Barmet et al., 2012).
Initial species concentration for the induced ambient
nucleation experiment.
Gas phase
C8 and C9
O3
NOx
H2O2
Isoprene
Toluene
aromatics
OHa
(ppb)
(ppb)
(ppm)
(ppt)
(ppt)
(ppt)
(moleculescm-3)
34
6.4
1.34
100
400
900
3.7×106
Aerosol phase
AMS
SMPS
Organics
Sulfate
Nitrate
Ammonium
Number
Massb
(µgm-3)
(µgm-3)
(µgm-3)
(µgm-3)
(cm-3)
(µgm-3)
0.45
0.27
<0.1
0.1
400
0.6
a Average [OH] during the experiment as measured by the decay of
n-butanol(d9).
b Assumed density of 1.5 g cm-3.
The time series for particle number size distribution for a typical
experiment is shown in Fig. 3. After the chamber background measurement,
ambient air was introduced inside the chamber and characterized. In most
experiments the initial total particle number ranged from 300 to 500 cm-3, about half of what was measured in the atmosphere outside the chamber
facility during the chamber filling stage, mostly due to dilution and losses
through the transfer setup. PM1 ranged from 2 to 5 µg m-3 and as determined from AMS its chemical composition was
organics (55 %), sulfate (30 %), ammonium (10 %), and nitrate (5 %), while its O : C ratio was about 0.6. A large fraction (40–45 %)
of these aged particles was persistent at 400 ∘C (Fig. 3b),
indicating that they contained some extremely low volatility components.
(a) Time evolution of particle number size distribution
during the induced ambient nucleation experiment, measured after the BP line
at 25 ∘C. The stages of the experiment were measurement of
chamber background (00:00 to 02:30), introduction of ambient air in the
chamber (02:00 to 06:00), nucleation event (06:00 to 09:00), and fresh SOA
formation (09:00 to 23:00). (b) Total number concentration of ambient
particles introduced in the chamber (04:45 to 06:15) and of particles after
nucleation occurs (06:15 to 07:45) as a function of time. (c) the
number size distribution during the nucleation event (at 06:15) at 25 ∘C (blue)
and 400 ∘C (red). The Aitken mode (30–300 nm) consists mainly of ambient particles.
After the initiation of photochemistry, significant new particle formation
occurred (∼ 4000 cm-3 new particles formed), followed by
significant particle growth. AMS measurements showed that these new
particles mainly consisted of organics, however, a small contribution of
particulate sulfate and ammonium cannot be excluded. Figure 3c shows the
number size distributions a little after the nucleation event occurred.
Although a significant increase in the number of nucleation mode particles
(< 20 nm) was observed during the nucleation phase, these particles were
completely evaporated after passing through the TD line at 400 ∘C.
In the last phase of the experiment, nucleated particles grew up to 100 nm,
growing for more than 10 h. After the nucleation event particle number
concentration was stabilized and then started decreasing due to wall losses
inside the smog chamber. Particles reached a maximum concentration of 6000 cm-3 and during aging decreased to 1000 cm-3 inside the chamber.
The particle number concentration decreased again significantly (NFR = 2 %) after the TD, suggesting that more than 98 % of the particles
evaporated at 400 ∘C. This is an indication that fresh SOA
particles produced from the complex ambient organic mixture in a suburban
area of Patras evaporate completely after passing through the TD.
Overall, from this set of chamber experiments, the experimental method
proposed was able to evaporate almost completely (> 97 %) particles formed
from traditional biogenic and anthropogenic SOA precursors and most
important, particles coming from a nucleation event.
Ambient measurements
During the winter of 2013 (7 January to 5 February) an intensive field
campaign was conducted at the premises of the National Observatory of
Athens, located in the center of the city (37∘58′21.37′′ N,
23∘42′59.94′′ E). The major objective of this campaign was to determine
local and regional air pollution sources in the Greek capital (3.8 million
people) during wintertime with a main focus on air pollutants originating
from the increasing burning of biomass for residential heating. The
measurement site was in the center of Athens, 200 m from the nearest road,
and is considered an urban background site. The measurements took place on
top of a rocky cliff, opposite from the temple of Thissio. Details of
instrumentation and operation are given in Sect. 2.
(a) Box-and-whisker plot showing median and 25th and
75th percentile levels for NFR during the Athens 2013 campaign when the
TD was set at 400 ∘C for the different periods of the day.
Vertical lines represent range from 0 to 100 %. (b) BC mass
concentration measured with MAAP. The color scale represents number fraction
remaining. The different periods of the day were early morning hours (02:30–05:00),
morning hours (07:00–10:00), afternoon hours (16:00–18:00), and
midnight hours (21:00–00:00).
Chemical characterization of ambient and thermodenuded aerosol
During the campaign there were 32 periods of measurements at 400 ∘C, resulting in 20 h of data. These 32 measurement periods were
analyzed with a focus on number concentration and size distribution of
particles with a nonvolatile core. A summary of the particle NFR after
passing through the TD is shown in Fig. 4. NFR for all time periods
measured, was highly variable, especially during midnight and early morning
hours with values ranging from 0.3 to 0.9. The morning NFR ranged from 0.2
to 0.7 while the afternoon values were more stable, giving a NFR ranging from
0.3 to 0.5. A correlation of the NFR with BC mass concentration was evident
as shown in Fig. 4b. The higher the BC mass concentration the higher the NFR
measured. During certain morning periods BC mass concentration reached
values more than 3 µg m-3 while the NFR was < 0.65. During these
periods particles were mostly originating from traffic emissions and are
discussed in detail in Sect. 4.3.
Average mass concentration of major species when the TD was
set at 400 ∘C (given for ambient conditions and after 400 ∘C) during the Athens 2013 campaign.
Temperature
Organics
Sulfate
Nitrate
Ammonium
BC
(µgm-3)
(µgm-3)
(µgm-3)
(µgm-3)
(µgm-3)
Ambient
10
0.74
0.55
0.36
2.5
aBBOA
HOA
OOA
COA
31 %
23 %
29 %
17 %
400 ∘C
2.1
0.18
0.1
0.04
2.5b
BBOA
HOA
OOA
COA
13 %
23 %
58 %
6 %
a Source contribution in (%) for the organic aerosol fraction
estimated by PMF analysis.
b Assuming zero evaporation of BC at 400 ∘C.
The average mass concentration of the major PM1 species during the time
that TD was set at 400 ∘C, combined with the PMF analysis of the
organics for ambient (BP line) and thermodenuded conditions (TD line), is
given in Table 3. During ambient measurements, organics dominated the
PM1 mass concentration with a 70 % contribution, while BC was responsible for another 17 %. The remaining 13 % consisted of
sulfate, nitrate, and ammonium.
Since organics dominated the total mass, further characterization was
performed using PMF analysis. Organics were separated in four categories:
hydrocarbon-like OA (HOA) coming from traffic, biomass burning OA (BBOA) due
to fireplace and wood stove use for residential heating, oxygenated organic
aerosol (OOA) corresponding to long range transport OA, and organic aerosol
related to cooking activity (COA). During ambient measurements, biomass
burning aerosol contributed 30 % of the organic mass due to increased
burning of wood for domestic heating. BBOA had high levels mostly during the
night. A high contribution of aged organic aerosol (28 %) was observed
during low concentration periods. During these periods, mostly rainy days,
most of the OA is transported to Athens from other areas. Aerosol emitted by
traffic was 25 % of the total organic mass, occurring mostly during the
morning and afternoon weekday rush-hour traffic. Finally, cooking organic
aerosol contributed 17 %. These high levels could be due to the
restaurants close to the sampling site.
The aerosol composition changed when particles passed through the TD. BC
dominated the remaining PM1 mass with a contribution of 51 % of the
total particulate mass while organics were responsible for another 45 %
(fractions were estimated assuming negligible BC evaporation through the TD
at 400 ∘C). The remaining < 5 % consisted mainly of sulfate. The
organic mass fraction remaining (MFR) was 20 %, suggesting that part of
the organics was of extremely low volatility. Sulfate had a MFR of 10 %, which is a
low volatility response seen also in other field studies (Huffman et al.,
2009). The MFR of ammonium and nitrate is subject to a larger relative error
since the corresponding concentration values after the TD were close to the
detection limit of the instrument (MFR 0.03 to 0.05).
From PMF analysis, organics after 400 ∘C were mostly OOA, around
60 %, an expected result, since usually the more oxygenated organic
compounds in the aerosol tend to have lower volatility (Florou, 2014). HOA
coming from traffic sources (23 %), biomass burning (13 %), and COA
(6 %) were responsible for the rest of the remaining OA. The increased
OOA contribution to the TD organic mass could also be due to oxidation of the
organic particles when passing through the TD at 400 ∘C. This could
lead to artifacts in the measurement of the contribution of nonvolatile
particles with TD systems. These artifacts could include (1) pyrolysis of
lower volatility organic species at 400 ∘C potentially leading to an
overestimation of the nonvolatile number and mass concentration and
(2) oxidation of BC and nonvolatile particles leading to a possible
underestimation. These processes have been examined in detail by Novakov and
Corrigan (1995). Their work suggested that constituents such as potassium can
act as catalysts for the combustion processes and therefore lower the
combustion temperature of BC and organics by as much as 100 ∘C.
Applying these to our measurements, for high concentrations of potassium
(5 % of total mass) we estimated a < 5 % overestimation of
the nonvolatile particle mass concentration due to pyrolysis and
< 10 % underestimation due to oxidation. To further investigate
whether these processes highly contribute during our campaign measurements,
PMF analysis was performed twice: once using only the ambient AMS
measurements and the second time combining the ambient and thermodenuded
measurements at all temperatures. The resulting factor spectra were all
practically identical to each other (the angles between the corresponding
vectors were less than 2 ∘) except for OOA, for which the theta
angle was 6 ∘. However, this discrepancy is considered within the
uncertainty of the PMF analysis, suggesting that if there is chemical change
during the processing of the organic aerosol by the thermodenuder then it is
minor. A similar conclusion has been reached in other studies (Huffman et
al., 2009). These results suggest that although oxidation processes might
introduce errors in our measurements, their magnitude is probably small, thus
increasing our confidence on the resulting nonvolatile particle number
concentration. Further work is needed in order to explore these processes
which might constitute a source of error for high temperature TD
measurements.
Given the variability of the measured NFR, we focus next on periods when the
site was dominated by aerosol from a specific source to gain insights into
the behavior of the corresponding particles. When the fractional source
contribution to organic aerosol was higher than 50 %, the measurement
periods were considered “representative” of the emission source. Two major
anthropogenic sources were investigated: (1) biomass burning and (2) traffic.
Biomass burning periods
The correlation between NFR and biomass burning contribution to OA is shown
in Fig. 5. As the BBOA fraction increased the NFR increased also (R2 = 0.76). For periods when the BBOA was dominant, NFR exceeded 80 %, an
indication that most biomass burning particles did not evaporate completely
in the TD system at 400 ∘C. Periods when the BBOA contribution
was < 10 % were dominated by aerosol coming from other sources, mostly
traffic (Fig. S6).
The number fraction remaining as a function of the
fractional contribution of biomass burning to the organic aerosol mass. Each
point corresponds to 0.5–1.2 h.
Average particle number size distribution (17 scans; 3 min time
span) for a major biomass burning period (BBOA was around 75 % of the OA)
at ambient conditions (black) and 400 ∘C corrected for losses (red).
Vertical lines correspond to confidence intervals.
The average particle number size distributions during a major biomass
burning period (BBOA was 50 µg m-3 and NFR > 90 %) at ambient conditions and at 400 ∘C are shown in
Fig. 6. During this biomass burning event there was a significant shift of
the size distribution mode during heating from 100 to 40 nm, but only a few
particles evaporated completely (10 %). This is a strong indication that
biomass burning particles have a nonvolatile core that survived after 400 ∘C and were coated with compounds that evaporated through the
system and led to a decrease of particle size. Although > 90 % of the
particle number concentration was in the size range from 10 to 100 nm, the
chemical characterization of these particles was based on the mass
concentration that reflects mainly particles larger than 100 nm. This
asymmetry may add uncertainties to this characterization.
During ambient measurements (BP mode) organics dominated the PM1 mass
concentration with a 70–75 % contribution, while BC was
responsible for another 20 %. The remaining 5–10 % consisted of
sulfate, nitrate, and ammonium. The aerosol composition changed when
particles passed through the TD. BC dominated the PM1 mass
concentration, with a contribution of 60 %, while organics were
responsible for another 35–40 %. The remaining < 5 % consisted mainly
of sulfate. The MFR of sulfate was 25 % after 400 ∘C, a low
volatility response seen also in other field studies (Huffman et al., 2009).
The organic mass fraction remaining was 17 %, suggesting that roughly 80 % of the organics evaporated through the TD. Detailed results are given
in Table S2 in the Supplement.
The PMF analysis results for the four biomass burning events were averaged
for ambient measurements (BP mode) and after 400 ∘C (TD mode).
BBOA was dominant in the BP and TD measurements with a 75 and 50 %
contribution to the organic mass, respectively. HOA and OOA mass fractions
remaining were the highest with 33 and 75 %, respectively; this suggests
that, although BBOA particles dominated the TD mass, HOA and OOA were the
hardest to evaporate completely, while 97 % of the COA mass evaporated in
the TD. Detailed results are given in Table S1.
The volatility of the organics that survived the intense heating was
estimated using the TD model of Riipinen et al. (2010). Using the average
size of particles, Dp=200 nm, the organic saturation mass
concentration was estimated to be less than 10-5 µg m-3 at
298 K, categorizing these organics as extremely low volatility OA (ELV-OA)
(Murphy et al., 2014).
Average particle number size distribution (14 scans; 3 min time
span) for a major traffic period (HOA was around 70 % of the OA), at
ambient conditions (black) and 400 ∘C, corrected for losses (red).
Vertical lines correspond to confidence intervals.
Traffic periods
The average particle number size distributions during a traffic period (HOA
was 60 % of the OA) at ambient conditions and at 400 ∘C are
shown in Fig. 7. During this period the ambient particle number size
distribution had a peak at 45 nm. After heating at 400 ∘C the
distribution was separated in two: one with a peak at 20 nm and one with a
70 nm peak. The NFR of 60 % indicated that 40 % of the particles
evaporated completely in the TD, while the remaining 60 % had a
nonvolatile core.
During the three representative traffic events when the HOA contribution was
higher than 50 %, BC dominated the ambient PM1 mass concentration
with a 50 % contribution, while organics were responsible for another 35 %. The remaining 10–15 % consisted of sulfate, nitrate, and
ammonium. The aerosol composition changed when particles passed through the
TD. The BC contribution increased to 80 %, while organics were
responsible for another 17 %. The remaining < 5 % consisted
mainly of sulfate with a MFR of 18 % at 400 ∘C. The organic
MFR was around 25 %, suggesting that a quarter of the organic mass had
extremely low volatility. Detailed results are given in Table S4.
By averaging the PMF analysis from the three traffic events for ambient
measurements (BP mode) and after 400 ∘C (TD mode), it was
estimated that HOA contributed 70 % of the ambient OA. After passing
through the TD, HOA and OOA were the dominant components remaining with a
contribution of 55 and 40 %, respectively. BBOA and COA evaporated almost
completely with < 5 % contribution to the organic mass. These organics were
again categorized as ELV-OA with a saturation mass concentration estimated
to be less than 10-5 µg m-3. Detailed results are given in
Table S3.
Conclusions
Nonvolatile particle number concentration and size distribution were
measured using a TD operating at 400 ∘C. The TD
is based on the design of An et al. (2007), providing high residence times
for the aerosol. The TD maximum temperature was set at 400 ∘C.
The TD setup was coupled with a HR-ToF-AMS, measuring the chemical
composition and mass size distribution of the PM1 aerosol and a SMPS
that provided the number size distribution of the aerosol in the range from
10 to 500 nm.
The ability of this system to measure nonvolatile particle number
distributions was evaluated with a set of smog chamber experiments. It
achieved almost complete evaporation (> 98 %) of biogenic and anthropogenic
secondary organic aerosol derived from ozonolysis of α-pinene and OH
photooxidation of toluene, respectively. In a different test, the introduction of
ambient air in the chamber and exposure to OH radicals induced an ambient
nucleation event. The TD was able to fully (99 %) evaporate all the
particles coming from the nucleation event as well as the fresh ambient SOA
that condensed on them after nucleation.
This experimental approach was applied in a winter field campaign in Athens
and provided a direct measurement of nonvolatile particle levels. During
periods in which the contribution of biomass burning sources was dominant
(> 60 %) more than 80 % of the particles survived the intensive heating,
suggesting that nearly all biomass burning particles had a nonvolatile
core. The particles that did not evaporate consisted of 60 % BC and the
rest was mostly organics. Organics surviving through the TD were mostly BBOA
and OOA, contributing 90 % of the organic mass concentration, while 10 –
15 % was from HOA and COA.
For periods during which traffic contributed the majority of the OA 50–60 % of the particles had a nonvolatile core, while the rest 40–50 %
evaporated at 400 ∘C. The remaining particles consisted mostly
from BC (80 % of the mass) while organics were responsible for another 15–20 %. Organics were mostly HOA and OOA, with a contribution of
> 95 % to the organic mass concentration.
These results suggest that it is very difficult or almost impossible to
evaporate all organics from ambient aerosol particles using thermodenuders.
Therefore, the assumption that has been used in previous studies that the
particles coming from these devices are organic free is not valid. This
could bias studies attempting to quantify the physical and/or chemical
properties of ambient particles without including their organic content
(Lack et al., 2012).
Overall, this methodology can be applied to measure the nonvolatile
particle number size distribution and provide a chemical characterization of
their mass. Assuming that all particles remaining after the TD have a black
carbon core, the methodology also provides an indirect way of measuring the
upper limit of the BC contribution to the particle number concentration.