Atmospheric aerosol is a key component of the chemistry and climate of the
Earth's atmosphere. Accurate measurement of the concentration of atmospheric
particles as a function of their size is fundamental to investigations of
particle microphysics, optical characteristics, and chemical processes. We
describe the modification, calibration, and performance of two commercially
available, Ultra-High Sensitivity Aerosol Spectrometers (UHSASs) as used on
the NASA DC-8 aircraft during the Atmospheric Tomography Mission (ATom). To
avoid sample flow issues related to pressure variations during aircraft
altitude changes, we installed a laminar flow meter on each instrument to
measure sample flow directly at the inlet as well as flow controllers to
maintain constant volumetric sheath flows. In addition, we added a compact
thermodenuder operating at 300 ∘C to the inlet line of one of the
instruments. With these modifications, the instruments are capable of making
accurate (ranging from 7 % for Dp < 0.07 µm to 1 %
for Dp > 0.13 µm), precise
(< ±1.2 %), and continuous (1 Hz) measurements of size-resolved particle number
concentration over the diameter range of 0.063–1.0 µm at ambient
pressures of > 1000 to 225 hPa, while simultaneously providing
information on particle volatility.
We assessed the effect of uncertainty in the refractive index (n) of ambient
particles that are sized by the UHSAS assuming the refractive index of
ammonium sulfate (n= 1.52). For calibration particles with n between 1.44 and
1.58, the UHSAS diameter varies by +4/-10 % relative to ammonium
sulfate. This diameter uncertainty associated with the range of refractive
indices (i.e., particle composition) translates to aerosol surface area and
volume uncertainties of +8.4/-17.8 and +12.4/-27.5 %,
respectively. In addition to sizing uncertainty, low counting statistics can
lead to uncertainties of < 20 % for aerosol surface area and
< 30 % for volume with 10 s time resolution. The UHSAS reduction
in counting efficiency was corrected for concentrations > 1000 cm-3.
Examples of thermodenuded and non-thermodenuded aerosol number and volume
size distributions as well as propagated uncertainties are shown for several
cases encountered during the ATom project. Uncertainties in particle number
concentration were limited by counting statistics, especially in the
tropical upper troposphere where accumulation-mode concentrations were
sometimes < 20 cm-3 (counting rates ∼ 5 Hz) at
standard temperature and pressure.
Introduction
The concentration of particles as a function of size is fundamentally
related to both direct (aerosol–radiation) and indirect (aerosol–cloud)
effects of aerosol on climate. Particles with diameters (Dp)
> 0.1 µm efficiently scatter and absorb solar radiation
(e.g., Charlson et al., 1992). Particles with Dp
> 0.05 µm serve as cloud condensation nuclei (CCN;
Clarke and Kapustin, 2002; Dusek et al., 2006; Köhler
1936). CCN play a role in cloud formation and in altering
radiative properties and lifetime of existing clouds (Albrecht, 1989;
Twomey, 1974, 1977). Measurement of aerosol size-resolved number
concentration is crucial for understanding aerosol sources and sinks, optical
properties, cloud nucleation potential and chemical transformations, and
consequently to constrain models of aerosol–cloud–climate interactions.
There is currently a variety of techniques available for measuring aerosol
size distributions (McMurry, 2000), but only some of these are fast enough
to sample aboard aircraft. The Ultra-High Sensitivity Aerosol Spectrometer
(UHSAS; Droplet Measurement Techniques (DMT) Inc., Longmont, CO, USA) is one
such instrument. The UHSAS is an optical particle counter for measuring
particles from 0.06 to 1 µm, which is often used for laboratory,
ground-based, and airborne measurements. It counts and sizes particles by
measuring the amount of light scattered by individual particles as they
traverse a focused laser beam. A fraction of the side-scattered light is
then collected by the optical system and focused onto two photodetectors
where it is converted to a size-proportional voltage pulse. The size of
particle is determined from the height of the voltage pulse by using a
calibration curve obtained from measurements of spherical particles with
known size and composition. Size distributions are obtained by accumulating
the individual pulse magnitudes of a population of particles into a
histogram.
Two versions of UHSAS are currently commercially available. One, designed
for airborne measurements, is enclosed in an underwing canister for in situ
sampling, while the other one is intended for ground-based aerosol sampling.
Here we focus on the modification, accuracy, and operation of two UHSAS
instruments (hereafter referred to as UHSAS-1 and UHSAS-2) during the first
and second Atmospheric Tomography Mission (ATom) field campaigns in summer
2016 and winter 2017, respectively. The ground-type UHSAS instruments were
chosen for this study over the wing-mounted version because we wished to dry
the air sample and to install a thermodenuder used to distinguish
non-volatile particles. These sample treatments are not possible with the
compact, wing-mounted instrument. The ground-type UHSAS has been deployed in
various airborne-based campaigns (Brock et al., 2011, 2016; Kassianov
et al., 2015; Yokelson et al., 2011). However, as reported by Brock
et al. (2011), modifications to the flow system are required to make them
suitable for airborne sampling.
Cai et al. (2008) reported a laboratory evaluation of the
UHSAS, and Brock et al. (2011) reported modifications to the flow system;
however, a complete evaluation of the accuracy and precision of the UHSAS
instrument for airborne operation is lacking. Here we describe modifications
to the ground-based UHSAS for airborne operation, detail the installation of
a compact thermodenuder in a second UHSAS for aerosol volatility studies,
and evaluate the accuracy, precision, and in-flight performance of both UHSAS
instruments during the first two of four ATom airborne campaigns.
The ATom mission
The ATom mission uses a DC-8 aircraft to survey the remote atmosphere over
the Pacific and Atlantic oceans from ∼ 80∘ N to
∼ 65∘ S while making repeated vertical profiles from
0.15 to 12 km to provide information on greenhouse gases, reactive and tracer
species, and aerosol composition and size distribution. At the conclusion of
the ATom project in spring 2018, the DC-8 will have made four global
circuits, one circuit for each season. The UHSAS instruments are a part of a
suite of fast-response aerosol size distribution instruments focusing in
particular on the spatial variation in the abundance of particles sized 0.003–4.8 µm
(Brock et al., 2018; Williamson
et al., 2018). Scientific goals for these instruments include
identifying the spatial extent of new particle formation in the remote
troposphere and the associated mechanisms and controlling parameters,
quantifying the growth of newly formed particles to cloud-active sizes, and
determining the importance of aerosols from continental sources to the
remote troposphere.
By operating two well-calibrated UHSAS instruments, one with a thermodenuder
(UHSAS-1) and one without (UHSAS-2), the size-dependent particle volatility
can be determined continuously, which is particularly useful for airborne
sampling where fast time response is needed. Volatility is an important
physical property defined by the chemical composition of the condensed
species and may reflect the origin of the particle (Huffman et al.,
2008; Jonsson et al., 2007). Most secondary compounds
(such as sulfates, nitrates, or organics) are expected to volatilize below
300 ∘C while primary particles such as soot, sea salt, and soil
dust survive heating (e.g., Clarke, 1991; Clarke and
Kapustin, 2002; DeCarlo et al., 2008). Measurements of particle
volatility help identify the contribution of secondary particles formed in
the free troposphere (FT) to the budget of CCN-sized particles in the marine
boundary layer (MBL), and how this contribution varies with altitude and
location in the remote atmosphere.
The Ultra-High Sensitivity Aerosol Spectrometer (UHSAS)Operating principles
The UHSAS (Cai et al., 2008) measures aerosol size-resolved
number concentration between 0.06 and 1 µm in diameter in 99
logarithmically spaced bins with user-selected time resolution. The UHSAS
uses a high-intensity infrared laser (semiconductor-diode-pumped
solid-state neodymium-doped yttrium lithium fluoride, Nd3+:Y
LiF4, operating at 1054 nm with intra-cavity circulating power of
∼ 1 kW cm-2), an inlet jet assembly, and two detection
systems: a highly sensitive avalanche photodiode (APD) to detect and size
the smallest particles, and a less sensitive secondary PIN photodiode to
size larger particles (Fig. 1). These detectors are
located at 90∘ on either side of the laser beam, aligned with the
intersection of the aerosol stream and the laser beam.
When particles exit the inlet jet assembly in the optical block, they
traverse the center of the focused laser beam and scatter light into the
detection system. Scattered light collected by two pairs of Mangin optics
(over solid angle of 33–147∘) is imaged onto the APD and PIN
photodetectors. The center region of the solid angle (72.5–104.8∘) is not sampled due to the hole cut out in the outer of the mirrors, with
the detector size being a negligible fraction of this hole area (Brock et
al., 2016). This geometry contrasts with that reported by Cai
et al. (2008), who used scattering angles from 22 to 158∘ to simulate
UHSAS response. The geometry we report was determined in consultation with
the manufacturer and agrees with values given by Petzold et al. (2013). The amount of scattered light reaching the detectors is a function
of not only particle size but also refractive index (n) and shape.
Each photodiode produces a photocurrent pulse, which is converted to a
voltage pulse through analog amplifiers. The signal from each detector is
amplified by two different gain circuits (high and low), providing a total
of four independent gain stages with some overlap (one particle may be
separately sized by each of the two adjacent gain stages). The outputs from
these gain stages are combined by linear regression in the overlap regions
to provide a single scale for accurate sizing across the full range of the
UHSAS response.
UHSAS with its modified flow system including schematic of the
UHSAS-1 and UHSAS-2 inlets. The sample air at a flow rate of 60 cm3min-1 enters the inlet and in UHSAS-2 goes directly through the
laminar flow element, while in UHSAS-1 additionally a thermodenuder (TD,
T= 300 ∘C) was installed so the flow first enters the switching
(Hanbay) valve and either bypasses or passes through the TD before it enters
the laminar flow element. Optical block schematic adopted from UHSAS User
Manual.
Modified flow system
Because the ground-based version of the UHSAS was not designed for operation
on aircraft where pressure changes, flow system modifications are essential
for airborne use. In the standard UHSAS configuration the aerosol sample
flow is controlled and measured by a mass flow controller mounted on the
exhaust side of the pump (Fig. 1). If mass flow were
maintained in flight, the volumetric flow rate would change inversely with
air density, leading to changes in particle velocity through the laser beam
and thus pulse width. A further issue is associated with transient sample
flow response to pressure changes during aircraft altitude changes. Because
the inlet nozzle restricts the sample flow entering the optical block, there
is a time lag between any external pressure change and the pressure within
the UHSAS optics block. This pressure disequilibrium changes the inlet flow
to the optics block in a way that is dependent on the rate of pressure
change and the fluid dynamics of the nozzle flow, which may vary with
altitude because it depends upon Reynolds number (Re). Because the
particle number concentration is calculated from the measured count rate and
sample flow rate, it is essential to account for these transient effects and
directly measure the flow rate at the inlet. Finally, using a needle valve
to control the split between the aerosol and sheath flows results in the
sheath–aerosol flow ratio varying with changing pressure because pressure
drop through the valve is also Reynolds-number-dependent and will vary with
pressure, even at a constant volumetric flow rate.
Because of the above issues, the flow system of both UHSAS instruments was
modified (Fig. 1; Table S1 in the Supplement). The modifications
include installation of a laminar flow element with a differential pressure
transducer to directly and precisely measure the time-varying sample
volumetric flow rate at the optics block inlet, and replacement of the
sheath flow valve with a volumetric flow controller (VFC) to directly
monitor and control sheath flow. The Alicat mass flow controller on the
exhaust side of the instrument, which is connected to an exhaust line near
inlet pressure to control the exhaust flow, was switched to operate in
volume flow control mode. The inlet laminar flow meter and differential
pressure transducer were calibrated together over a flow range of 0–0.1 L min-1 using a volumetric flow calibration standard (DryCal DC-Lite,
Bios, Inc., Butler, NJ, USA). The modified UHSAS is operated at
∼ 0.06 L min-1 total inlet flow and 0.7 L min-1 sheath
flow. The original UHSAS LabView software was modified to accommodate these
changes.
Detection efficiency of UHSAS-1 and UHSAS-2. n/a: not applicable.
ParticleReal refractiveWavelength,ReferenceDp50 (nm) index, nλ (nm)UHSAS-1UHSAS-2PSL1.58780Yoo et al. (1996)n/an/a(NH4)2SO41.5271054Hand and Kreidenweis (2002)72.8 +1.2/-5.962.8 +1.0/-5.9DOS1.44532Pettersson et al. (2004)75.9 +1.2/-6.068.2 +1.1/-5.9Limonene oxidationunknownn/an/a78.9 +1.3 /-6.069.7 +1.1/-5.9productsLaboratory performanceAerosol generation method
The sizing performance of the UHSAS and the effects of particle composition
and concentration were investigated in the laboratory
(Fig. 2). Particles with diameters between 0.05 and
1 µm were generated in two ways: (1) by using an atomizer to produce
ammonium sulfate ((NH4)2SO4), polystyrene latex (PSL)
spheres, or di-2-ethylhexyl (dioctyl) sebacate (DOS) particles
(Table 1) or (2) from new particle formation and
condensational growth from limonene ozonolysis products in a flow tube
reactor.
Atomized aerosol and DMA
Particles were generated using an HPLC-grade water (or HPLC-grade
isopropanol in the case of DOS) solution and a custom-built Collison-type
atomizer (May, 1973). Atomized droplets were dried in a silica
gel diffusion drier, charged by a Po210 radioactive source, and (except
for the PSL) size-selected in a custom-built differential mobility analyzer
(DMA) with a recirculating sheath flow. The sizing uncertainty (σS) of the DMA was ±1.6 %, estimated from the sum in
quadrature (square root of the sum of squares) of the sheath flow (σQ), pressure (σP), temperature (σT), and
voltage (σV) uncertainties as described in Eq. (1).
σS=σQ2+σP2+σT2+σV2
A sizing bias to smaller diameters was identified when using NIST-traceable
polystyrene latex (PSL) microspheres with diameters between 0.07 and 0.4 µm (Thermo Scientific, Inc. Waltham, MA, US). This DMA sizing bias is
estimated to be about 7 % at sizes below 0.07 µm and decreases to
1 % for sizes above 0.13 µm. However, we believe that the actual
bias is < 7 % as these PSLs were checked against an independent
DMA by P. Campuzano-Jost of the University of Colorado, and the results were
similar, suggesting a surfactant coating on the smaller PSL sizes rather
than a DMA sizing error. No adjustments were made to the DMA diameters, but
the potential biases when compared to the PSL sizes are propagated through
to the aerosol surface and volume concentration uncertainties discussed
below.
The calibration DMA operated at a 1:10 aerosol-to-sheath-flow ratio and
sheath flow rates of 3 to 5 L min-1. The monodisperse aerosol flow
exiting the DMA was diluted using particle-free air to match the flow rate
of the instruments located downstream. The incoming particle-free air was
homogeneously mixed with calibration particles in a short section of
turbulent (Re > 4000) flow and sampled by the two UHSAS instruments
and a condensation particle counter (CPC; Model 3022A; TSI Inc., St. Paul, Minnesota, USA). The relative
humidity (RH) of the aerosol flow was monitored by two RH sensors (Vaisala HMP60)
installed in the DMA, one on the sample flow exiting the DMA and the other
on the sheath flow exiting the DMA column, and was typically < 10 %. It was important to dry the atomized (NH4)2SO4 aerosol
prior to size classification to avoid sizing biases due to the uncontrolled
evaporation of water in the DMA and UHSAS and refractive index effects in
the UHSAS.
Flow tube reactor and DMA
A secondary organic aerosol (SOA) from limonene ozonolysis was generated in
a borosilicate glass (Pyrex) flow tube reactor as described in Williamson et al. (2018). Particles formed from limonene oxidation were size
selected in a DMA as described above.
A schematic diagram of the aerosol generation and measurement setup
at atmospheric pressure conditions. The calibration aerosol was generated
either in a flow tube reactor or the atomizer. Apart from PSL, all atomized
particles were sent through a diffusion drier to DMA for size selection,
while PSL particles were delivered from the atomizer directly to both UHSAS
instruments following dilution with dry air.
The effect of composition on particle sizing
Particle sizing in the UHSAS is a function of the amount of light scattered
onto the instrument's photodetectors. The quantity of scattered light,
however, is a function not only of size but also of the
composition-dependent aerosol refractive index (Bohren and Huffman,
1983). Particles of (NH4)2SO4 were used to relate scattered
light intensity to particle size, since their refractive index at 1054 nm
(n= 1.527; Hand and Kreidenweis, 2002) lies in the middle of the
typical range of refractive indices for atmospheric particles composed of
mixed sulfate salts and organic compounds. However, the composition of the
atmospheric particles is not known a priori. The refractive index of organic aerosol
in particular is not well constrained (Dick, 2007; Kanakidou et al.,
2005). Kim and Paulson (2013) suggest values for refractive index (at
λ= 532 nm) for biogenic and anthropogenic secondary organic
aerosol (SOA) of 1.44 and 1.55, respectively. To constrain the effects of
particle refractive index on UHSAS sizing, we investigated a range of nearly
monodisperse calibration particles having different known refractive indices
(Table 1, Fig. 3),
including (NH4)2SO4, PSL microspheres, and DOS, as well as
limonene oxidation products (with an unknown refractive index). For
particles of Dp < 0.6 µm and a real refractive index (n) of
1.44–1.58, the diameter measured by the UHSAS may vary by +4/-10 % relative to the one based on (NH4)2SO4. The propagation
of this potential bias to reported aerosol surface and volume concentrations
uncertainties is discussed in Sect. 5.1.
The refractive index of soil dust may exceed the range of real refractive
indices considered here. In addition, dust can be both absorbing and
aspherical. When dust is an important component of the atmospheric aerosol,
uncertainties in both the denuded and thermodenuded UHSAS instruments should
be evaluated on a case-by-case basis using best estimates of refractive
index and shape based on other measurements, coupled with optical
simulations of instrument response. Also, because the thermodenuded UHSAS
instrument volatilizes non-refractory particles, the refractive indices in
the aerosol measured by the two instruments will differ. This problem is
probably minor in the MBL because sea salt aerosol has a refractive index
within the range of the calibrants. For the free troposphere, however, there
may be substantial sizing biases between the two instruments that should be
considered case by case using additional information on aerosol composition.
Finally, we note that light-absorbing black carbon (BC) particles are
mis-sized in the UHSAS. The optical cavity laser power is ∼ 1 kW cm-2 at 1054 nm, similar to that in the single-particle soot
photometer (SP2; Schwarz et al., 2010), and some limited laboratory
studies we performed suggest that BC incandesces and vaporizes in the UHSAS.
Even without incandescing, the complex refractive index of BC particles
(n= 2.26–1.26i at λ= 1064 nm; Moteki et al., 2010) substantially
alters UHSAS sizing compared with the calibration aerosol. Because the
number concentration of BC cores with volume-equivalent diameter (assuming
void-free density of 1.8 g cm-3) in the range 90–550 nm accounted for
less than 5 % of the particle concentration in the same size range during
the ATom-1 mission (except for the case of biomass burning plumes off the
coast of Africa), mis-sizing due to BC is a minor effect in general in ATom.
For cases of specific plumes from combustion sources in which BC is an
abundant aerosol component this assumption should be re-evaluated.
Calibration particle diameter as a function of UHSAS-2 (not
thermodenuded) bin number for particles composed of PSL,
(NH4)2SO4, DOS, and limonene ozonolysis products. Solid lines
represent fits to the data. Uncertainties are shown for
(NH4)2SO4 and PSL but are often obscured by the symbols.
Detection efficiency of the non-thermodenuded UHSAS-2 instrument as a
function of mobility equivalent diameter for (NH4)2SO4 and
DOS aerosol. Data are corrected for coincidence. Solid lines are fits
presented to guide the eye.
Particle detection efficiency
The detection efficiency, the ratio of concentration of particles of a given
size measured by the sum of all bins of the non-thermodenuded UHSAS-2 to
that measured by a TSI 3022A CPC, depends on the refractive index of the
calibration particles used. Figure 4 presents the
detection efficiency for the non-thermodenuded UHSAS as a function of
mobility equivalent diameter for (NH4)2SO4 and DOS particles,
which varies due to the differing refractive indices of these compounds. The
diameter uncertainties were calculated as described in Eq. (1), and were
corrected for the possible sizing bias observed using PSL standards. In a
similar manner the uncertainties in the efficiency were calculated using the
UHSAS and CPC uncertainties from the flow and pressure measurements and
counting statistics. Detection efficiencies for both UHSASs are provided in
Table 1. The thermodenuded UHSAS begins detecting
particles at larger diameters than the other instrument.
The effect of concentration on particle counting
The UHSAS sensitivity to particle concentration was quantified using
atomized (NH4)2SO4 particles with diameters > 0.1 µm and concentrations between 1 and 104 cm-3. All
concentrations and flow rates presented in this paper are at STP conditions.
The UHSAS exhibited a nonlinear reduction in counting efficiency relative
to the reference CPC at concentrations > 1000 cm-3 due to
particle coincidence in the optical sensing volume
(Fig. 5). Since the UHSAS software does not monitor
and correct for coincidence effect, or run live-time correction, while the CPC software
does, we determined a phenomenological correction based on the observed
counting efficiency as a function of count rate (Eq. 2):
Ntrue=Nmeas1-τ×NmeasQsamp,
where Ntrue is the corrected number concentration (equal to the CPC
concentration), Nmeas is the measured number concentration, and
Qsamp is the measured volumetric sample flow rate. Based on fitting the
data in Fig. 5 to Eq. (2), for the UHSAS-1, τ= 7.81 × 10-5 s while for the UHSAS-2, τ= 5.36 × 10-5 s.
These values represent the average particle pulse width for each instrument.
Relationship between the UHSAS-2 and the CPC particle number
concentration (cm-3) for nearly monodisperse (NH4)2SO4
aerosol of various sizes (> 0.1 µm) at ambient pressure.
Dashed line represents 1:1 correspondence line.
The effect of pressure on sample flow and particle sizing
Laboratory evaluation of the UHSAS operation at reduced pressure conditions
is important for the interpretation and validation of the airborne data
during the ATom flights. To investigate possible pressure dependencies, a
needle valve and an external pump were used to reduce the instrument
pressure. The flow passing through a needle valve downstream of the atomizer
was split into sample and bypass flows, the latter of which was connected to
the pump. The exhausts of the UHSAS instruments were also connected to the
bypass flow line to keep them at near-inlet pressure. A mixture of four PSL
sizes was atomized and measured as instrument pressure was adjusted to as
low as 250 hPa. The sizing of the UHSAS instruments showed no statistically
significant pressure dependence (Fig. S1). The mean bin number and replicate
standard deviation associated with each of the four PSL sizes at various
pressure settings is 10.5 ± 0.19, 24.5 ± 0.24, 47.6 ± 0.2,
and 64.5 ± 0.2 for the 81, 125, 240, and 400 nm PSL particles,
respectively. Using the standard (NH4)2SO4 calibration curve
(Fig. 3), which relates bin number to particle
diameter, the equivalent relative standard deviations in diameter were
±0.6, 0.7, 0.5, and 0.7 % for the four diameters, respectively.
Using the same setup, we investigated the effect of changing pressure on
the sample flow. The aerosol volumetric flow rate showed a pressure
dependence, decreasing from 60 at around 850 hPa to
about 35 cm3 min-1 at 250 hPa (near the minimum pressure
encountered during ATom). This flow reduction is caused by a small leak in
the optics block downstream of the detection region. It was impractical to
disassemble the complex optics assembly to find the source of this leak.
Therefore, we directly measure the sample flow to account for this effect on
concentration, and the leak does not affect UHSAS sizing characteristics
(Fig. S1).
Thermodenuder
A compact thermodenuder was designed and installed in UHSAS-1 to determine
the number and volume fraction of volatile particles
(Fig. 6; Table S2). This measurement is used to
identify particles that are formed from secondary products (e.g., sulfates,
nitrates, and organics) from primary particles (e.g., soil dust and
sea salt; Clarke, 1991; Huffman et al., 2008). Quantifying
the volatile-to-non-volatile aerosol fraction during ATom may help improve
understanding of the importance of secondary particles relative to sea salt
such as CCN in the MBL, an area of active scientific inquiry (e.g., Bates et
al., 2016; Quinn et al., 2017).
Design
We constructed a custom thermodenuder based on the design principles
outlined by Fierz et al. (2007), who improved denuder
performance by providing a heated adsorption section. This thermodenuder
operates at a lower flow rate and is of a smaller size compared to previous
designs. An electric actuator (MDM-060DT, Hanbay Inc., Pointe-Claire,
Quebec, Canada), driving a Swagelok valve (SS-43YF2) is used to
automatically switch between sampling through the thermodenuder or bypassing
it. The thermodenuder consists of a heated section (length, L= 10.16 cm,
inner diameter (ID) = 0.48 cm) held at a fixed temperature
(T= 300 ∘C) followed by an adsorption section of same dimensions
(Fig. 6; Table S2). Both sections are housed in
stainless steel tubing (L= 30.48 cm, OD = 1.27 cm) which contains an inner
porous, perforated tube of the same length constructed from two pieces and
manufactured using a metallic 3-D printing technique, direct metal laser
sintering (Xometry, Gaithersburg, MD, USA). This perforated tubing is
wrapped with activated carbon fabric (Zorflex; 4.066 g). The outer tube
passes through an aluminium housing which holds the tube and temperature
sensor in place and is wrapped with a heating tape and fiberglass insulation
material. Two fans installed in the outer casing of the heating section the
entrance and exit sections of the thermodenuder cool these sections of the
outer tube. A thermal process controller monitors a resistance thermal
detector (RTD) and controls the temperature of the aluminium block housing
using a cylindrical cartridge heater. The temperature of the housing is
maintained in flight at 300 ± 0.5 ∘C. The residence time of
the aerosol in the thermodenuder as well as the temperature profile in part
determine thermodenuder performance. Fierz et al. (2007)
developed simple guidelines for selecting an appropriate thermodenuder
heated section length for a particular sample flow rate. Our thermodenuder
meets these recommendations and provides a residence time in the heated
section between 1.59 and 3.7 s. We do not directly measure the thermal
profile within the compact thermodenuder.
Schematic cross section of the thermodenuder and conceptual
temperature profile. Temperature is measured at a single point with a
platinum RTD sensor inside the aluminium housing around the heated section.
The thermal diffusion length estimate assumes standard pressure and
temperature and typical flow in the thermodenuder, small perturbations in
temperature, and is used only for qualitative understanding of heat flow in
the thermodenuder.
Thermodenuder performance
Particle losses through the thermodenuder were determined relative to either
a TSI 3022A CPC or the second UHSAS instrument. With the thermodenuder
operating at room temperature, losses through the sample selection valve and
heater plumbing were < 13 % for particles with
Dp > 0.15 µm. The mechanism and size dependence of this
particle loss is currently unclear and requires further investigation. With
the heater on and the thermodenuder operating at 300 ∘C, losses
of non-volatile NaCl particles did not change significantly.
The efficiency of volatilizing particles in the thermodenuder was tested
using DMA-size-selected particles from the generation of NaCl,
(NH4)2SO4, and limonene oxidation products at concentrations
< 1000 cm-3. The UHSAS-1 alternated sampling between the
thermodenuded and unheated sample lines every 2–3 min. The temperature
of the thermodenuder was increased in steps from room temperature up to
310 ∘C and the fraction of particles exiting the thermodenuder
(relative to the unheated sample) at three different particle sizes was
determined (Fig. 7). Particles composed of
(NH4)2SO4 were most volatile, limonene oxidation products
were less volatile, while NaCl was not volatile at the temperatures
investigated. Smaller particles of (NH4)2SO4 and limonene
oxidation products volatilized at lower temperatures than larger particles
of the same material, suggesting the particles were highly viscous, glassy,
or solid. The effect of particle concentration on performance was
checked with particles generated from limonene oxidation products at 0.15 µm in diameter and concentrations of up to 11 000 cm-3. All
particles at these concentrations were effectively volatilized, with no
“break-through” effects observed. In no cases was there any evidence of
recondensation of volatilized material to form new particles or to add
material onto partially volatilized or non-volatile particles.
Particle response to heating as a function of temperature of the
thermodenuder, particle size, and composition. Data normalized to the number
measured at ambient temperature. Solid lines are used to guide the eye. The
stability of the set temperature (dashed line) was within ±0.5 ∘C.
UncertaintiesUncertainties due to refractive index
Uncertainties in the aerosol volume and surface calculated from atmospheric
dry size distributions depend on possible biases associated with the actual
refractive index and shape of the particles vs. the calibration aerosol, as
well as on random uncertainties associated with counting statistics, flow
rate, pressure, sizing precision, and calibration accuracy. Since the
ATom project focuses on the remote atmosphere where well-aged particles are
expected to dominate the submicron aerosol (outside of sea salt and dust
cases), we did not investigate the effect of particle shape on sizing
accuracy. Since the refractive index of organic compounds in the atmosphere
is unknown but is likely bounded by our different calibration materials
(e.g., Kim and Paulson, 2013), we use the range of instrument
responses to the different calibration aerosols to estimate the likely effect
of potential refractive index biases on aerosol volume and surface area
derived from the UHSAS measurements.
As an example of the effect of these potential sizing biases on measured
size distributions, we have selected a period of time from one of the ATom-2
flights (10 February 2017, Christchurch–Punta Arenas) while in the free troposphere
(P∼ 200–400 hPa). Using the range of instrument response
curves for (NH4)2SO4 (n= 1.52), DOS (n= 1.44), or PSL
(n= 1.58), the reasonable range of possible particle diameters associated
with each UHSAS channel (bin) could vary by as much as +4/-10 % (as
described in Sect. 3.2). These diameter uncertainties propagate into aerosol
volume and surface uncertainties of +12.4/-27.5 and +8.4/-17.8 %,
respectively, as calculated from each 1 s size distribution
(Fig. 8). Examples from this and other cases
representative of conditions encountered during ATom flights are summarized
in Table S3.
Uncertainties due to flow and pressure
Random uncertainties may arise from uncertainty in sample flow rates and
uncertainty in the pressure measurement used to convert instrument
concentrations to standard temperature and pressure (STP; 0 ∘C and
1013 hPa). Uncertainty in the sample flow rate is ± 0.86 % based on
repeated calibrations of the sample flow meter over a range of 0–0.1 L min-1 using a reference calibration device (DryCal DC-Lite, Bios,
Inc., Butler, NJ, USA). The uncertainty in the STP flow rate is the sum in
quadrature of the flow calibration variation, the uncertainty of the DryCal
flow calibration device (±0.25 %), the uncertainty in the
differential pressure transducer reading (±0.25 %), and the
uncertainty in the sample pressure (Eq. 1). The uncertainty of the
measurement of the UHSAS-2 sample pressure at sea-level pressure is better
than 0.38 % when comparing to a reference pressure gauge. At < 300 hPa, this pressure uncertainty was 3.8 % due to the lower accuracy of the
pressure reference standard used for lower pressures. The total propagated
random uncertainty for the STP sample flow is < 3.9 %.
Uncertainties due to counting statistics
Very low concentrations of accumulation-mode particles were often
encountered in the free troposphere during the ATom mission. Uncertainties
associated with resulting poor counting statistics at 1 s resolution are
reduced by averaging over longer time intervals. The uncertainty caused by
the counting statistics was estimated for 1, 10, and 60 s data-averaging times
using various STP concentrations (20–440 cm-3) representative of
typical MBL and the upper-FT conditions encountered (Table S3). As an
example, uncertainties for STP concentrations of ∼ 150 and ∼ 30 cm-3 as measured in the MBL for 1 s
acquisition intervals were ±8.7 and ±18 %, respectively. In
the FT the uncertainties were much greater: ±14 and ±41 %
for STP concentrations of ∼ 440 and ∼ 25 cm-3, respectively. Actual instrument counting rates in the FT were
much lower than for equivalent STP concentrations measured in the MBL
because of lower air density.
Comparison of the calculated aerosol volume from the UHSAS-2 measured
dry size distributions based on calibration particles with refractive
indices between 1.44 and 1.58: (NH4)2SO4 (n= 1.52), DOS
(n= 1.44), and PSL (n= 1.58). Flight data shown (10 February 2017) are from 1 s
measurements. Solid lines represent double-sided orthogonal distance
regression linear fits.
Uncertainties due to instrument stability and calibration
repeatability
Although careful calibrations undertaken using a DMA in the laboratory
provide a precise assessment of UHSAS sizing characteristics, a method to
validate the calibration stability of the UHSAS instruments in the field,
where the DMA could not be carried out, is critical. A solution of four PSL
sizes (81, 125, 240, and 400 nm) in HPLC-grade water was atomized producing
an aerosol with four distinct concentration peaks that could be measured by
the UHSAS (Fig. 9). The sizing channel associated
with each PSL diameter was determined by fitting a Gaussian curve to each
peak in the size distribution histogram. The standard deviation of the
identified peak bin was determined for a total of 84 calibrations taken
before and after each flight, and at high altitude during test
flights. The mean bin number and replicate standard deviation associated
with each of the four PSL sizes is 10.8 ± 0.4, 25.2 ± 0.3, 47.4 ± 0.2, and 64.6 ± 0.2 for the 81, 125, 240, and 400 nm PSL
particles, respectively. Using the standard (NH4)2SO4
calibration curve (Fig. 3), which relates bin number
to particle diameter, for the UHSAS-2 instrument the equivalent precisions
in diameter were ±1.2, 0.8, 0.7, and 0.7 % for the four PSL sizes,
respectively (Fig. 9). Because the power in the
optical cavity is sensitive to contamination of the optics, the UHSAS sizing
calibration may shift over time. This was observed during the middle of the
Atom-1 mission in the UHSAS-1 when optical power dropped by 27 %. Because
of the repeated calibration checks with the PSL particles, we were able to
correct the observed size distribution with minimal errors despite the shift
in calibration. Upon return to the laboratory, the instrument was
recalibrated, then cleaned until laser power was restored and then
calibrated again.
Fitted peak bin number for four PSL size standards and as a function
of time from July 2016 to February 2017, showing calibration precision and the
stability of the UHSAS-2 sizing during ATom-1 and -2.
Total uncertainties
The total relative uncertainties for aerosol number concentration, surface,
and volume for cases of low and high particle number concentration measured
in MBL and FT during ATom-2 mission are summarized in Table S3. The total
uncertainty consists of random uncertainties due to the counting statistics,
sample flow, and pressure measurements, and possible systematic uncertainties
due to sizing biases from the unknown refractive index of the atmospheric
aerosol. The total uncertainty for aerosol number, surface, and volume
represents the sum in quadrature (Eq. 1) of the random uncertainties plus the
linear addition of possible systematic sizing biases propagated through the
surface and volume calculation.
We have not considered particle shape and homogeneity as a potential source
of uncertainty. Given the laser wavelength of 1054 nm, and because most
particles encountered in ATom were aged and likely only modestly aspherical,
we do not expect shape sizing biases to be significant except for some
larger sea salt and fresh dust particles.
In-flight performance
In this section, we describe the performance of the modified UHSAS
instruments measuring dry aerosol size distributions, both directly sampled
and thermodenuded, on the DC-8 aircraft during the ATom-1 (July–August 2016)
and ATom-2 (January–February 2017) missions. Brock et al. (2018)
more thoroughly describe the inlet and sampling configuration and provide
comparisons between several aerosol instruments on the ATom payload. The
measured internal UHSAS instrument pressures varied between ∼ 1100 (due to ram pressure) and 225 hPa, which corresponded to 0.15–13 km in
altitude. The two UHSAS instruments sampled in parallel at 1 Hz downstream
of a Nafion dryer that reduced sample RH to < 20 %. Periods of
in-cloud measurement were excluded from the reported data due to aerosol
sampling artifacts caused by droplets or ice crystals impacting the inlet,
which produced spurious counts in the UHSAS instruments.
Consistency of aerosol number concentration, surface, and volume measured
by UHSAS-1 and UHSAS-2
During the ATom-1 deployment the thermodenuder on the UHSAS-1 instrument was
not operated, allowing for direct comparison between the two UHSAS
instruments. We compare number, surface, and volume concentrations over the
diameter range from 0.1 to 0.9 µm to see if the measurements agree
within the estimated uncertainties. We focus on the first five flights of
ATom-1, between 29 July and 8 August 2016, before the laser power on the
UHSAS-1 instrument shifted. The number, surface, and volume concentrations
were highly correlated between the two instruments (r2 > 0.98), with slopes within 5 % of 1 (Fig. 10).
This agreement is well within the propagated uncertainties over the full
dynamic range of 0–3000 cm-3, 0–380 µm2 cm-3, and 0–16 µm3 cm-3 for number, surface area, and volume
concentrations, respectively.
During ATom-2, when the UHSAS-1 was operated with the thermodenuder, the two
UHSAS instruments could be compared by periodically switching the UHSAS-1
flow to bypass the thermodenuder when in MBL. During the non-thermodenuded
sampling intervals, the agreement in concentration measured during first
three flights (29 January–3 February 2017) over the Pacific was found to be between
0.97 ± 0.011 and 1.04 ± 0.01 (for 1 s data). The corresponding
slopes for aerosol surface and volume concentration varied between 0.97 and 1.02
and between 0.95 and 1.08, respectively.
Comparison between the UHSAS-1 and UHSAS-2 instruments on ATom-1
from 29 July to 8 August 2016 for (a) dry particle number, (b) surface
area, and (c) and volume concentrations for diameters from 0.1 to 0.9 µm. Each point is a 10 s average. The r2 values indicated here refer to
one-sided linear fit, while the solid lines represent double-sided
orthogonal distance regression linear fits to non-transformed data.
Estimated uncertainty is shown on a subset of points.
Measurements of non-volatile aerosol fraction
The thermodenuded UHSAS was developed to help identify the fraction of
particle number and volume (roughly proportional to mass) associated with
primary particles such as sea salt and dust as opposed to those that are
produced by secondary processes (most particles composed of organic,
nitrate, and sulfate species). Several measurements and modeling studies
(Clarke and Kapustin, 2002; Korhonen et al., 2008; Mericanto et al.,
2010; Quinn et al., 2017; Raes, 1995) suggest that secondary
particles formed in the FT play an important role in governing CCN abundance
in the MBL, despite the presence of sea salt. It is possible that sea salt
may dominate aerosol mass in the MBL, but that CCN concentrations may be
controlled by secondary processes, even those occurring in the FT above
(Clarke et al., 2013; Raes, 1995; Twomey, 1977; Quinn and Bates, 2011).
To demonstrate the utility of the ATom UHSAS measurements for such
investigations, we present examples of thermodenuded and non-thermodenuded
aerosol number and volume size distributions for a single MBL case (measured
for 50 s at 22∘ N latitude over the central Pacific) and a single
FT case (measured for 360 s at 3∘ N over the central Pacific)
during ATom-2 on 26 January 2017 (Fig. 11). In the
MBL (Fig. 11a, b) volatile aerosol species
dominate number concentrations, while non-volatile particles (presumably
sea salt) comprise ∼ 52 % of aerosol volume (or mass) for
Dp between 0.1 and 0.9 µm. The non-volatile (sea salt) mode was
largely > 0.3 µm in diameter, clearly distinct from the
smaller mode of volatile particles centered at ∼ 0.15 µm volume mean diameter. Small amounts of non-volatile (sea salt) mass
extended down to diameters < 0.1 µm, consistent with prior
studies (Bates et al., 1998; Clarke et al., 1997;
Mericanto et al., 2010; Middlebrook et al., 1998;
Murphy et al., 1998; Quinn et al., 2017).
Example of an averaged dry aerosol size distribution from UHSAS-1-TD
and UHSAS-2 as sampled in the MBL (21.74∘ N, 1080 hPa; 27 January 2017,
01:15:26–01:16:16 UTC) showing (a) number and (b) volume; and for a separate
size distribution sampled in the FT (3.4∘ N, 293 hPa; 26 January 2017,
21:32:47–21:38:47 UTC) showing (c) number and (d) volume. For particles with
Dp between 0.1 and 0.9 µm, in the MBL case (a, b) 94 % of
the total number and 52 % of the total volume volatilized in the
thermodenuder, while in the FT case (c, d) 96 % of the number and 74 % of the volume volatilized.
The number concentration of accumulation-mode particles with Dp between
0.1 and 0.9 µm in the clean air of the FT
(Fig. 11c, d) was ∼ 7 cm-3 and
96 % of these were volatile. The peak modal diameter was smaller than
could be detected by the UHSAS, implying the dominance of the Aitken-mode
aerosol (0.012–0.06 µm). These particles were recently formed from
gas-phase precursors (Williamson et al., 2018). In
the FT, ∼ 26 % of the particle volume was non-volatile,
dominated by a few particles with Dp > 0.5 µm and
uncertain due to poor counting statistics.
Summary and context
Two UHSAS instruments were modified, calibrated, tested in the laboratory,
and operated during the first and second deployments of the ATom mission.
The instruments are capable of continuous 1 s measurements of size-resolved
particle number concentration with high accuracy and precision over a
diameter range of 0.063–1.0 µm from > 1100 to 225 hPa,
while simultaneously measuring particle volatility. Precision is limited by
counting statistics, especially in the remote FT. The modified flow system
of the UHSAS allowed direct monitoring of the sample flow rate and
eliminated flow measurement issues associated with the pressure variations
during aircraft altitude changes. The sizing of the UHSAS instruments showed
no statistically significant pressure dependence, crucial for consistent
airborne sampling. Detailed calibrations with laboratory aerosols spanning a
range of refractive indices (1.44–1.58) representative of the atmosphere
allowed us to constrain the uncertainty associated with the unknown
composition of the atmospheric aerosol. An equation to correct for particle
coincidence was derived to improve the quantification of the counting
accuracy at concentrations from ∼ 1000 to > 20 000 cm-3. Two UHSAS instruments agreed in flight to within 5 % for
integrated number, surface, and volume concentrations from sea level to
∼ 13 km altitude. We developed a compact thermodenuder for one
of the UHSAS instruments, characterized its performance, and demonstrated
its utility for quantifying the size distribution of the nonvolatile fraction of
the aerosol. Both modified UHSAS instruments worked well with no significant
failures while flying on a DC-8 aircraft during the ATom missions.
The ATom observations taken with these instruments provided representative
(non-targeted) measurements, across an unprecedented latitude range over
both ocean basins, of vertically resolved, size-dependent aerosol properties
that are related to radiative effects, to the ability of aerosols to act as
CCN, and to the sources and abundance of primary vs. secondary particles in
the MBL and FT. Hence, the size distribution data gathered by the UHSAS
instruments over altitudes between ∼ 0.2 and ∼ 13 km will improve our understanding of global aerosol characteristics in
the under-sampled regions of the atmosphere that closely resemble natural
conditions minimally perturbed by pollution. These new measurements may be
placed in the context of similar data gathered over more than 2 decades by
Clarke (1991), Clarke et al. (1997, 1998, 2013), Clarke
and Kapustin (2002, 2010),
and others (e.g., Anderson et al., 1996) to help fill gaps in knowledge
of aerosol properties, processes, sources, sinks, and aerosol–cloud–climate
interactions.
The ATom measurements in October 2017 and May 2018 will provide data from fall and spring seasons in the Northern Hemisphere. The
past and future ATom measurements, placed in the context of chemical and
meteorological conditions and combined with size distribution measurements
from 0.003 to 4.8 µm (Brock et al., 2018;
Williamson et al., 2018), will help constrain model
simulations of the processes that govern particle formation and their
evolution in remote regions (Lee et al., 2013;
Hamilton et al., 2014). Only if aerosol production mechanisms, sinks, and
transformations are understood can models accurately simulate global CCN
distributions in the pre-industrial, modern, and future atmosphere, and the
resulting effects on climate through aerosol–cloud interactions.
Data availability
Calibration and laboratory testing data are available upon request to the
corresponding author. In-flight data are available at the ATom data archive:
https://doi.org/10.5067/Aircraft/ATom/TraceGas_Aerosol_Global.
The supplement related to this article is available online at: https://doi.org/10.5194/amt-11-369-2018-supplement.
Author contributions
All authors contributed substantially to the work presented in this paper.
AK and CAB modified the instruments. NLW and CAB
developed the thermodenuder. MR developed the software. AK
designed, carried out experiments, and analyzed data. AK and CW calibrated instruments and collected data during ATom-1 and -2
missions. AK prepared the manuscript with contributions from all
co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Disclaimer
This publication's contents do not necessarily represent the official views
of the respective granting agencies. The use or mention of commercial
products or services does not represent an endorsement by the authors or by
any agency.
Acknowledgements
The authors acknowledge support by NASA's Earth System Science Pathfinder
Program under award NNH15AB12I and by NOAA's Health of the Atmosphere and
Atmospheric Chemistry, Carbon Cycle, and Climate Programs. Agnieszka Kupc is
supported by the Austrian Science Fund FWF's Erwin Schrodinger Fellowship
J-3613. Droplet Measurement Technologies kindly provided permission and
source code to allow modification of the UHSAS control software. We would
like to thank Joshua (Shuka) Schwarz and Joseph Katich for access to data
and their helpful comments. We would also like to thank Bernadett Weinzierl,
Maximilian Dollner, T. Paul Bui, and Glenn S. Diskin for access to their
preliminary data. Finally, we would like to thank David Fahey, Karl Froyd,
and Daniel M. Murphy for insightful discussions, and Pedro Campuzano-Jost
and Jason C. Schroder for checking PSL standards.
Edited by: Eric C. Apel
Reviewed by: three anonymous referees
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