Water adsorption and hygroscopicity are among the most important physicochemical properties of aerosol particles, largely determining
their impacts on atmospheric chemistry, radiative forcing, and climate. Measurements of water adsorption and hygroscopicity of
nonspherical particles under subsaturated conditions are nontrivial because many widely used techniques require the assumption of
particle sphericity. In this work we describe a method to directly quantify water adsorption and mass hygroscopic growth of atmospheric
particles for temperature in the range of 5–30
Atmospheric aerosol particles, directly emitted by natural and anthropogenic processes or secondarily formed in the atmosphere, have
significant impacts on air quality, visibility, human health, and radiative and energy balance of the Earth system (Pöschl, 2005;
Seinfeld and Pandis, 2006). The ability to uptake water is among the most important physicochemical properties of aerosol particles,
and it largely determines their impacts on atmospheric chemistry and climate (Martin, 2000; Rubasinghege and Grassian, 2013; Farmer
et al., 2015; Tang et al., 2016). The ability of aerosol particles to uptake water depends on particle composition, relative humidity
(RH), and temperature (Martin, 2000; Tang et al., 2016). Under subsaturated conditions (RH
Hygroscopicity of atmospheric particles has been extensively investigated by a large number of studies, and many experimental
techniques have been developed. These techniques have been summarized and discussed by a very recent review paper (Tang et al., 2016),
and here we only mention widely used ones. For airborne monodisperse particles typically produced by a differential mobility analyzer
(DMA), the hygroscopicity can be determined by measuring their diameters at dry (typically at RH
There are several techniques which can be applied to quantify the amount of water associated with nonspherical particles at given
temperature and RH. For example, adsorbed water can be measured by Fourier-transform infrared spectroscopy by its infrared absorption at
around 3400 and 1645
In this work we have developed an experimental method to investigate water adsorption and hygroscopicity of atmospheric particles,
using a vapor sorption analyzer which is commercially available. We note that two groups have used similar techniques to measure water
adsorption by
The instrument used in this work is a vapor sorption analyzer (Q5000SA) manufactured by TA Instruments (New Castle, DE, USA). The first part of this section provides a general description of this instrument, and the second part describes experimental methods used in this work.
Figure 1 shows the schematic diagram of the vapor sorption analyzer used in this work to measure hygroscopicity and water adsorption of particles of atmospheric relevance. This instrument consists of two main parts: (1) a high-precision balance used to measure the mass of samples and (2) a humidity chamber in which temperature and RH can be precisely regulated and also monitored online.
Schematic diagram of Q5000SA used in this work. MFC: mass flow controller. High-purity
The balance simultaneously measures the mass of an empty pan (serving as a reference) and a sample pan which contains particles under
investigation. Each pan is connected to the balance by a hang-down wire which has a hook at the lower end to hold the pan. The balance
is housed in a chamber which is temperature regulated. To avoid moisture condensation, the balance chamber is purged with
a 10
The balance has a dynamic range of 0–100
The humidity chamber is used to regulate the temperature and RH under which hygroscopicity and/or water adsorption of particles are
investigated. Inside the humidity chamber are housed a reference chamber (in which an empty pan is connected to the balance) and
a sample chamber (in which a sample pan is connected to the balance). A dry
The main advantage of using a reference chamber and a sample chamber is that the amount of water adsorbed by the empty pan and the
attached wire can be simultaneously determined (and automatically subtracted using the provided software) under the same condition when
water uptake by particles under investigation is being measured. In addition, the effect of buoyancy, which varies with RH (Beyer
et al., 2014; Schroeder and Beyer, 2016), is also automatically taken into account by using an empty pan as the reference.
Semispherical quartz crucibles with a volume of 180
Q5000SA is equipped with a programmable autosampler designed to deliver sample pans into the humidity chamber. The autosampler can host
up to 10 sample pans; however, in order to minimize contamination by lab air, only one sample pan is uploaded into the autosampler
immediately prior to the measurement. The instrument status is displayed on a touch screen for local operation. Q5000SA can also
communicate with a computer via Ethernet. Two software packages are provided by the manufacturer: (1) TA Instrument Explorer Q Series
is used to control the instrument, program measurement procedures, and log experimental data; (2) TA Universal Analysis can be used for
graphing experimental data in real time, data analysis, and exporting data. Experimental data can be sampled with frequencies up to
1
In our work two major experimental protocols are developed to (1) determine the DRH and (2) quantify water adsorption and/or mass hygroscopic growth. Corresponding experimental procedures are detailed below.
Based on the standard recommended by American Society for Testing and Materials International (ASTM, 2007) and TA Instruments
(Waguespack and Hesse, 2007), an experimental method has been developed in this work to determine the DRH of a given sample, and it
consists of the following steps. After the sample pan is properly located in the humidity chamber, temperature is set to the given
value. After temperature is stabilized, RH is set to a value which is
Figure 2a shows changes of RH and sample mass (normalized to that at 0
Typical experimental data in determination of DRH at a given temperature (NaBr at 25
A second method has been developed to measure DRH at a given temperature. The particle sample is first dried at 0 % RH until its
relative mass change is
Typical experimental data in determination of DRH at a given temperature by stepwise increasing RH. The experiment displayed
in this figure was conducted to measure DRH of
The following experimental procedures are used to determine the amount of water adsorbed by a material (i.e., MGFs):
(1) a sample pan is delivered into the humidity chamber and temperature in the humidity chamber is set to a given value. (2)
After temperature becomes stable, RH in the humidity chamber is set to 0 % and the sample is equilibrated with the environment
until its mass change is
All the processes are programmed, with the flexibility to choose the number of RH steps and the corresponding RH values. Experimental
data such as RH and sample mass are recorded with a time resolution of 30
Sodium bromide, provided by TA Instruments as a reference material for RH calibration, was supplied by Alfa Aesar with a stated purity
of
RH in vapor sorption analyzers and/or thermogravimetric analyzers can be calibrated/verified by determining the DRH of a reference
material with a well-defined DRH (ASTM, 2007). In this work, NaBr provided by TA Instruments is used as the reference material
(Waguespack and Hesse, 2007). We compare our measured DRHs of NaBr at six different temperatures with those reported by a previous
study (Greenspan, 1977). The results are summarized in Table 1, suggesting that the differences between our measured and previous
reported DRHs is
Comparison of DRHs (in %) of NaBr at different temperatures
measured in our study with those reported in literature
(Greenspan, 1977). The uncertainties for our measured DRH
values are estimated to be
Comparison of our measured and previous reported DRHs (Greenspan, 1977).
Comparison of mass hygroscopic growth factors measured in this work with these predicted by the E-AIM model.
DRH values reported by Greenspan (1977), widely accepted as standard values, are recommended by the instrument manufacturer (Waguespack
and Hesse, 2007) and also used in this study to calibrate our measure RH by taking into account the difference between our measured
DRHs and those reported by Greenspan (1977) for NaBr at different temperatures. All the RHs reported in this work (except measured DRHs
of NaBr listed in Table 1) have been calibrated. In our work we have not verified RH for temperature higher than 30
Comparison of DRHs measured by our study with those reported in literature (Greenspan, 1977) for
Using the experimental method detailed in Sect. 2.2.1, we have measured DRHs of
In addition, we repeated the measurements of the DRH of
We have also measured the MGFs of
Figure 6b displays change of RH and normalized sample mass with time during the measurement, suggesting that within 6
The ability to uptake water vapor under subsaturated conditions is one of the most important physicochemical properties of atmospheric
particles, largely determining their impacts on atmospheric chemistry and climate. In this work, we have developed a new experimental
method to investigate interactions of particles with water vapor under subsaturated conditions at different temperatures from 5 to
30
To test the ability of this instrument to measure hygroscopic growth of compounds with low hygroscopicity, we have determined MGFs
of
Atmospheric aging processes are known to alter water
adsorption, hygroscopicity, and cloud condensation nucleation activity of mineral dust and soot particles (Kelly and Wexler, 2005;
Laskin et al., 2005; Zhang et al., 2008; Sullivan et al., 2009b; Han et al., 2013; Denjean et al., 2015; Tang et al., 2016). In the
future, this instrument will be used to investigate water adsorption and hygroscopicity of mineral dust and soot particles before and
after chemical processing. We note that this technique also has a few drawbacks: (1) this technique cannot be used to examine
supersaturated droplets or determine efflorescence relative humidities due to the contact of particles with the sample pan; (2)
substantial amount of particles, typically around or larger than 1
Experimental data presented in this work are available upon request (Mingjin Tang: mingjintang@gig.ac.cn).
The authors declare that they have no conflict of interest.
Financial support provided by Chinese National Science Foundation (grant nos. 91644106 and 41675120), Chinese Academy of Sciences international collaborative project (grant no. 132744KYSB20160036) and State Key Laboratory of Organic Geochemistry (grant no. SKLOGA201603A) is acknowledged. Mingjin Tang would like to thank the CAS Pioneer Hundred Talents program for providing a starting grant and Yongjie Li would like to acknowledge funding support of the start-up research grant from University of Macau (SRG2015-00052-FST). This is contribution no. IS-2438 from GIGCAS. Edited by: Hartwig Harder Reviewed by: four anonymous referees
Normalized mass of