We present a humidity-controlled fast integrated mobility
spectrometer (HFIMS) for rapid particle hygroscopicity
measurements. The HFIMS consists of a differential mobility analyzer
(DMA), a relative humidity (RH) control unit and a water-based FIMS
(WFIMS) coupled in series. The WFIMS (Pinterich et al., 2017)
combines the fast integrated mobility spectrometer (Kulkarni and
Wang, 2006a, b) with laminar flow water condensation methodologies
(Hering and Stolzenburg, 2005; Spielman et al., 2017). Inside the
WFIMS, particles of different electrical mobilities are spatially
separated in an electric field, condensationally enlarged and imaged
to provide 1
The performance of the HFIMS was evaluated using NaCl particles with
well-known hygroscopic growth behavior and further through
measurements of ambient aerosols. Results show that the HFIMS can
reproduce, within 2 %, the literature values for hygroscopic
growth of NaCl particles. NaCl deliquescence was observed between 76
and 77 % RH in agreement with the theoretical value of
76.5 % (Ming and Russell, 2001), and efflorescence relative
humidity (43 %) was found to lie within the RH range of 41 to
56 % reported in the literature. Ambient data indicate that
the HFIMS can measure the hygroscopic growth of five standard dry
particle sizes ranging from 35 to 165
The hygroscopicity of atmospheric aerosols is a key parameter in determining their impact on global climate. The uptake of water by individual particles increases the light scattering, enhances heterogeneous chemical transformations important to secondary aerosol formation (e.g., Surratt et al., 2010) and is important in the formation of cloud droplets. The abundance of hygroscopic particles that act as cloud condensation nuclei affects cloud formation and cloud droplet number concentrations, which in turn influences cloud albedo, coverage and lifetime (Twomey, 1977; Albrecht, 1989). These “indirect effects” of atmospheric aerosols on the Earth's radiation balance remain one of the largest uncertainties in understanding climate change (IPCC, 2013). Hygroscopicity is among the key determinants of the ability of aerosol particles to form cloud droplets and therefore the aerosol indirect effects (e.g., Mei et al., 2013; Liu and Wang, 2010).
Most commonly particle hygroscopic growth is measured using
hygroscopicity tandem differential mobility analyzer (HTDMA) systems,
which consist of two differential mobility analyzers (DMAs) in series,
separated by a means to control the sample flow relative humidity
(RH). HTDMA systems first select a single particle size using the
first DMA, change its relative humidity environment, and then scan the
classifying voltage of the second DMA to measure the distribution of
particle sizes resulted from the change in RH. The HTDMA method is
accurate, but slow. Typically the time required to complete
a measurement cycle for determining the growth factor (GF) at a single
relative humidity (such as 90 %) for 5 different particle sizes is
about 30
Several investigators have worked to increase the speed of HTDMA measurements
by replacing the second DMA with an instrument that is capable of fast size
distribution measurements. Sorooshian et al. (2008) developed a differential
aerosol sizing and hygroscopicity spectrometer probe (DASH-SP), in which wet
particle size is measured by an optical particle counter (OPC). By replacing
the second DMA with an optical counter, DASH-SP accelerates the measurement
significantly. However, the optical counting limits DASH-SP measurements to
particles larger than
To address the need for fast and precise measurements of particle hygroscopic
growth, we have developed a humidity-controlled water-based fast integrated
mobility spectrometer (HFIMS), which replaces the second DMA of the HTDMA
systems with a water-based FIMS (WFIMS; Pinterich et al., 2017). By detecting
particles of different sizes simultaneously, the WFIMS provides rapid
measurements of the size distribution of humidified particles. Unlike the
final optical sizing of Sorooshian et al. (2008) or the final aerodynamic sizing
of Leinert and Wiedensohler (2008), the WFIMS measures particle sizes based on
electrical mobility. This removes the uncertainty introduced by the particle
refractive index or density and provides the same, precise growth factor
measurements of the HTDMA systems, but with a much faster measurement speed.
Compared to systems based on optical or aerodynamic sizing, the HFIMS extends
fast measurements to particles with diameters below 150
The HFIMS consists of three individual units (see Fig. 1): a TSI Inc. DMA (either long-column or nano-column DMA, depending on the particle size) classifying particles at a desired dry size under a low RH, an RH control unit providing independent controls of the size-selected particle sample and WFIMS sheath flow RH, and a WFIMS measuring size distributions of particles after being exposed to a different RH. The WFIMS used here is identical to the original WFIMS (Pinterich et al., 2017), except the high-voltage (HV) electrode is replaced with one that provides a uniform electrical field with a small offset from the aerosol inlet slit, as described below. This modification is made to optimize the measurements of humidified particle size distributions. In essence, the WFIMS deployed in this study is similar to the alcohol-based FIMS reported in Kulkarni and Wang (2006a), except that particle growth is achieved by condensation of water instead of butanol, which is key to hygroscopicity measurements.
Schematic diagram of the HFIMS.
An automated RH control system was constructed to independently
control the RH of the size-selected aerosol sample flow
(
The WFIMS is identical to the original version presented in Pinterich
et al. (2017) except that the HV electrode is replaced with one that
provides a uniform electric field with a slight offset from the aerosol
inlet slit. The WFIMS consists of a parallel plate mobility separator
followed by a three-stage condensational growth channel and an imaging
system. A particle-free sheath flow
The WFIMS is configured with a single-voltage electrode that has an
offset in the direction of the flow (
Schematic diagram of the offset electrode used in the HFIMS with the aerosol inlet on the left.
The single high-voltage electrode was configured with an offset such
that the high-voltage region (red area in Fig. 2) begins slightly
downstream (32
A heater and thermistor were attached near the bottom of the separator
to compensate for the heat loss to the adjacent cooled conditioner stage
(see details in the next paragraph) and to avoid a corresponding
change in RH due to this gradient. The heater was driven to equalize
the temperatures within the separator. Without heating, the
temperature at the bottom of the separator is about 1.0
Upon exiting the separator the particles continue along their flow
trajectories through the three-stage growth channel, consisting of
the conditioner, initiator and moderator, all with wetted walls (Spielman
et al., 2017). Particles are enlarged through water condensation
without being diverted from their trajectories. The WFIMS' three-stage
growth channel design provides supersaturation levels of
In this study, the WFIMS separating voltages ranged from 70 to
4500
Dry particle diameter range (
Experimental setup for laboratory characterization of the HFIMS.
Experimental setup for measuring hygroscopic growth of ambient particles.
The capability of the HFIMS to accurately characterize particle
hygroscopicity is examined by measuring sodium chloride particles, for
which hygroscopic growth has been well characterized in prior
studies. The experimental setup is shown in Fig. 3. NaCl particles
were generated by atomizing a dilute NaCl solution (1.7
For the measurements of the deliquescence and hygroscopic growth of NaCl
particles, we matched the relative humidity of aerosol and sheath
flows (
The measurement speed of the HFIMS was evaluated by sampling ambient
aerosols outside of our laboratory at Brookhaven National Laboratory
(Upton, New York). Figure 4 shows the schematic of the experimental
setup. We obtained ambient particle growth factors at
Mean growth factor of NaCl particles
(
Growth factors of 50
WFIMS mobility resolution (solid line) and typical DMA
mobility resolution (dashed line) as a function of growth factor
(
The NaCl deliquescence transition observed by the HFIMS is just over 76 %, which is in agreement with the theoretical value of 76.5 % (Ming and Russell, 2001) and measurements by Hämeri et al. (2001) and Cruz and Pandis (2000) of 76 and 75.6 %, respectively. It should be mentioned that around the deliquescence transition two distinct size modes are observed (see Fig. 6). This suggests some heterogeneity in the RH of the aerosol sample (i.e., some particles experienced slightly higher RH than others), which is likely due to temperature variations among different parts of the system. Deliquescence transition data shown in Fig. 5 represent the number-weighted mean growth factor for the two modes. Improved RH and temperature control could minimize the RH heterogeneity and will be a topic of future study. Above the deliquescence transition, growth factors measured by the HFIMS are within 2 % of theoretical values, suggesting the RH heterogeneity has negligible impact on measured particle growth factors above the deliquescence RH (e.g., at 85 %).
Figure 5 also shows the efflorescence curve (blue circles), that is
the size change when the relative humidity environment decreases. Data
were obtained by maintaining
Using the HFIMS operating conditions listed in the supplement (Sect. S2) we calculated the resolution of its sizing unit, i.e., the WFIMS, as a function of the hygroscopic growth factor for nondiffusing particles and compared it to the typical resolution of a DMA, i.e., 10. As shown in Fig. 7 the mobility resolution of the HFIMS is equal to, or exceeds that of, the HTDMA over the measured growth factor range, i.e., 1–2.27, shown in Fig. 5.
Size-dependent average growth factor at RH
Comparison of GF distributions of size-selected ambient
particles humidified to 85 % for short (20
Characterization of ambient aerosol hygroscopicity often requires measurements at multiple particle sizes within a reasonable time. This is often challenging for measurements using traditional TDMA systems, especially for the larger particles which are low in number concentration and the smallest particles which have low charging efficiency.
Figure 8 shows results from ambient measurements with the HFIMS, where we
evaluated the relative standard error of the mean growth factor (SEM
of
Figure 9 shows the average growth factors of ambient aerosol (red circles)
and corresponding minimum sample duration (black crosses) measured at
a constant RH of 85 % on 3 December 2015. Larger particles
(
In addition to the average growth factor, the GF distribution of the
humidified aerosol, its width, and whether it is unimodal or bimodal
are examined. Figure 10 compares size distributions of the humidified
aerosol obtained by the HFIMS at five particle sizes recommended by the
EUSAAR project for 20
These analyses indicate that the hygroscopic growth factors at the
five particle sizes could be captured by the HFIMS within 3 min,
including a 15 s waiting time to ensure the system reaches steady state
following the switching between different
We present a humidity-controlled water-based fast integrated mobility spectrometer for rapid measurement of particle hygroscopicity.
The HFIMS consists of a DMA, an RH control unit and a water-based fast integrated mobility spectrometer (Pinterich et al., 2017). The WFIMS combines a single high-voltage FIMS (Kulkarni and Wang, 2006a) with the laminar flow water condensation methodologies developed by Hering and coworkers (e.g., Hering and Stolzenburg, 2005). By detecting particles of different sizes simultaneously, the WFIMS provides rapid-mobility-based measurements of particle size distributions over a factor of 3 or more in particle diameter, which is sufficient to cover the entire range of growth factor for ambient aerosol particles. Thus, with the combination of a DMA, relative humidity control and WFIMS, the HFIMS can capture the complete growth factor distribution of size selected particles with much improved speed.
Laboratory experiments with NaCl particles showed that the HFIMS can reproduce theoretical growth factors within 2 %. The deliquescence transition was observed just over 76 %, which is in excellent agreement with the theoretical value of 76.5 % (Ming and Russell, 2001). The measured efflorescence relative humidity (43 %) was found to lie within the range of 41 to 56 % reported in the literature.
The hygroscopicity of ambient aerosols was characterized by keeping
the sample and sheath RH at 85 % and varying the dry particle size. We
found that growth factors of ambient particles ranging from 35 to
165
Data sets used in this article will be provided by the corresponding author (Jian Wang, jian@bnl.gov) upon request.
The authors declare that they have no conflict of interest.
We thank Andrew McMahon for his help with the development of the offset high-voltage electrode. This work was supported by the US Department of Energy, Office of Science, Small Business Technology Grants DE-SC0006312 and DE-SC0013103. Edited by: Mingjin Tang Reviewed by: two anonymous referees