Hygroscopic aerosols take up water and grow with
increasing relative humidity (RH), giving rise to large changes in light
extinction (
Atmospheric aerosols directly influence global climate forcing by absorbing and scattering solar radiation (IPCC, 2013; Moise et al., 2015; Tang et al., 2016; Shrivastava et al., 2017). The extinction (sum of absorption and scattering) capacity of aerosol particles strongly influences visibility, especially at high relative humidity (RH) (Massoli et al., 2009a; Liu et al., 2012). Hygroscopic particles can take up water from the surrounding atmosphere, modifying their composition, size, complex refractive index (CRI), and mixing state and thereby altering their optical and radiant properties (Covert et al., 1972; Tang and Munkelwitz, 1994; Zhang et al., 2008; Bian et al., 2009; Kuang et al., 2015). Research on the hygroscopicity of aerosols is therefore crucial for assessing their climate and environmental impacts (Pitchford et al., 2007; Cheng et al., 2008; Bian et al., 2009).
Multiple techniques have been developed to characterise aerosol hygroscopic
behaviour over the last few decades and have been described in several
reviews (Kreidenweis and Asa-Awuku, 2014; Titos et al., 2016; Zhao et al.,
2019; Tang et al., 2019). The instrument commonly used to characterise
particle size growth in the laboratory and in field applications is the
hygroscopic tandem differential mobility analyser (H-TDMA). The growth
factor (GF) is obtained by measuring the ratio of particles under humid and
dry conditions (Swietlicki et al., 2008; Tang et al., 2019).
Advances in optical methods have allowed significant progress to be made in
studying
For measuring
Suitable instruments for measuring
In this work, we report the first demonstration of a humidified
cavity-enhanced albedometer (H-CEA) that combines a BBCES albedometer with a
humidigraph system for simultaneous and accurate measurement of
Figure 1 shows a schematic diagram of the H-CEA. The instrument consists of
a BBCES albedometer and a RH-controlled system. The albedometer combined the
BBCES with an integrating sphere (IS) for simultaneous in situ measurements
of
Schematic diagram of the humidified cavity-enhanced albedometer (H-CEA) and the aerosol-generation system. The H-CEA consists of a controllable gas humidifier system, a Nafion humidifier (MD-700, Perma Pure), and a cavity-enhanced albedometer. The relative humidity (RH) of the aerosol sample was controlled by the Nafion humidifier by adjusting the RH of the sheath gas. The sheath gas RH was controlled by adjusting the flow ratio of a dry gas stream and a wet gas stream by two mass controllers. The wet gas stream was generated using a Nafion humidifier (FC-125, Perma Pure), and a water bath was used to control its temperature. Three temperature and RH sensors were used to monitor the temperature and RH of sheath gas and aerosol samples at inlet and outlet of the cavity cell. The cyan, black, and grey lines represent the aerosol sample, humidified gas, and purging gas flows, respectively.
Data retrieval and calibration of the extinction and scattering measurements
have been described elsewhere (Zhao et al., 2014; Xu et al., 2018a). The
albedometer was periodically flushed with particle-free zero air to obtain
the BBCES reference spectrum and reference scattering intensity. High
optical stability was achieved by using a high-performance temperature and
current controller for the LED light source. No obvious drifts in the
transmitted light intensity (fluctuation
The humidigraph system (as shown in Fig. 1) consists of a gas
humidity-adjusting system that generates humid gas and a second Nafion
humidifier (MD-700-24S-3, Perma Pure) to humidify the aerosol sample. Dry
zero air was divided into two paths separately controlled by two mass flow
controllers (MFCs): one was humidified with a Nafion humidifier (FC-125,
Perma Pure, the water was supported by an automatic temperature-controlled
water bath) and the other was used as dry bypass air to adjust the RH of
the air. A program controlled the mixing ratios of the humid air and bypass
air, allowing the RH of the zero air to be varied from 2 % to 98 % RH
(monitored with
Experimental data showing the relatively fast RH control of the humidifier system. A full RH cycle, when the RH varied from 10 % to 90 % and back again, lasted about 20 min. The RH was measured at the inlet of the optical cavity.
The performance evaluation of H-CEA was carried out using laboratory-generated monodispersed particles of ammonium sulfate, sodium chloride, and nigrosin. The aerosol-generation system is shown in Fig. 1. Polydisperse aerosol particles were generated with a constant output atomiser (TSI 3076), dried in a diffusion dryer (TSI 3062), and then charged with an aerosol neutraliser (TSI 3077) (Zhao et al., 2013, 2014). A quasi-monodisperse size distribution of particles was selected using a differential mobility analyser (DMA, TSI 3080L), diluted in dry zero air, and transferred to the H-CEA and a condensation particle counter (CPC, TSI 3776) for measuring optical properties and particle number concentration, respectively. A scanning mobility particle sizer (SMPS) in front of the H-CEA measured the size distribution of size-selected particles.
The extinction, scattering, and absorption coefficients of size-selected
particles can be calculated using the following equation (Pettersson et al.,
2004):
The model calculation of
Particle losses in the H-CEA were evaluated using laboratory-generated
ammonium sulfate particles by measuring the number concentrations of
size-selected particles (as shown in Fig. 3). The particle loss was
characterised based on the difference in concurrent CPC measurements at the
inlet and outlet of the sample tube or cavity, after accounting for dilution
inside the cavity. Losses considered included particle loss in the tube, in
the Nafion humidifier, and in the cavity cell. For particle diameters above
40 nm, the literature-reported losses in the Nafion humidifier were
estimated to be less than 1 % (Bohensky et al., 2014). For particles
larger than 100 nm, the measured total particle loss in the H-CEA was less
than 7 %, with a
Size-resolved particle loss in the newly developed H-CEA instrument, which was the sum of the particle losses in the cavity cell and in the sampling tube and Nafion humidifier. Each point was the result of 25 measurements.
To confirm that the influence of water vapour on the measurements of
Influence of water vapour on measurement of
A power-law dependence of
Simulated dependence of
Figure 5 shows the results of the simulated
A series of laboratory experiments were conducted to evaluate the
performance of the H-CEA. For measurement of
RH dependence of extinction and scattering enhancement for
size-selected
Comparison of the measured RH-dependent extinction and scattering coefficients for the size-selected ammonium sulfate and sodium chloride particles.
Comparison between the measured and E-AIM-calculated extinction and scattering cross sections for the size-selected ammonium sulfate and sodium chloride particles under three selected RH conditions.
Figure 6 shows the RH-dependent extinction and scattering for the
size-selected ammonium sulfate particles of 200, 250, 300, and 350 nm (dry
diameters, with RH of
RH dependence of extinction and scattering enhancement for
size-selected
Figure 7 shows the
The performance evaluation of the H-CEA for
RH dependence of extinction, scattering, absorption, and
In this paper, we report the development and characterisation of a
humidified cavity-enhanced albedometer (H-CEA) for the simultaneous
measurements of multiple optical hygroscopic parameters
(
The data used in this study can be obtained from
XX and WZ designed the research. JZ, XX, WZ, and BF built the device. JZ, QL, and YC conducted the experiment. JZ and XX analysed data. JZ performed the simulation. JZ, XX, WZ, and DSV wrote the paper. All authors discussed the results and commented on the paper.
The authors declare that they have no conflict of interest.
This research has been supported by the National Natural Science Foundation of China (grant no. 4190050321), the Instrument Developing Project of the Chinese Academy of Sciences (grant no. YJKYYQ20180049), the CASHIPS Director's Fund (grant no. BJPY2019B02, YZJJ2019QN3), the Natural Science Foundation of Anhui Province (grant no. 1908085QD157), and the Youth Innovation Promotion Association CAS (grant no. 2016383).
This paper was edited by Mingjin Tang and reviewed by two anonymous referees.