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Charging states on atmospheric aerosol particles affected by meteorological conditions
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Charging states on atmospheric aerosol particles affected by meteorological conditions Yuanping He a,b , Zhaolin Gu a,∗ , Weizhen Lu b,∗ , Liyuan Zhang c , Daizhou Zhang d , Tomoaki Okuda e , Chuck Wah Yu a,f a
Department of Earth and Environmental Sciences, Xi’an Jiaotong University, Xi’an 710049, China Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong, China c Department of Environmental Engineering, Chang’an University, Xi’an 710064, China d Faculty of Environmental & Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto 862-8502, Japan e Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan f International Society of the Built Environment (ISBE), Milton Keynes MK7 8HQ, UK b
a r t i c l e
i n f o
Article history: Received 11 August 2019 Received in revised form 12 December 2019 Accepted 16 December 2019 Available online xxx Keywords: Aerosol particles Atmospheric humidity Charging state Haze day Physicochemical effect Sand–dust day
a b s t r a c t Previous studies on haze formation focused mainly on the various chemical components in aerosol particles and their physicochemical effects on particle behaviour (e.g., generation, growth, and agglomeration). This paper describes the measurement of the charging state on atmospheric aerosol particles, which could be affected by meteorological conditions. A series of experiments on particle charging state and meteorological factors was undertaken on the roof of the west 4th building on the Qujiang Campus at Xi’an Jiaotong University (China). Measurements were conducted approximately 20 m above ground level. Our results showed that most atmospheric particles carried net negative or positive charge and that the electric charge on the particles varied diurnally and seasonally. The average amount of charge on particles was higher in winter than in summer. The number concentration of charged particles was higher during the day than overnight. Obvious difference in the average charge of aerosol particles was found between sand–dust days and haze days. A strong relationship was found between the PM2.5 concentration, charge amount on particles, and humidity. Our findings show that particle formation and growth could partly be attributed to variation in particle charging state, which is related to meteorological conditions including atmospheric humidity. © 2020 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
Introduction The physicochemical processes between aerosol particles and other gaseous pollutants, in combination with the external transport of particles, could increase PM2.5 mass concentration (called accumulative rise), which would always be followed by low–moderate aerosol pollution (Zhang, Gu, Yu, Zhang, & Cheng, 2016). Severe haze episodes are often characteristic of a common atmospheric evolution process, i.e., an abrupt rise in PM2.5 mass concentration during the first 5–20 h of a haze event, subsequent levelling off at a high PM2.5 concentration that persists for three hours to days, followed by rapid decline attributable to the arrival of a cold front (Zhang et al., 2016). Initiation of haze incidents is
characterized by new particle formation and nucleation plays a fundamental role in the phase transformation (e.g., condensation, sedimentation, crystallization, sublimation, and evaporation). Condensable gases of low volatility initially become nucleated clusters that then grow to become nucleation mode particles through condensation and coagulation (Kulmala, 2003). During clear periods with less pollution, this type of nucleation is very common and its environmental effect is remarkable, even though such new particles have a short lifetime. Through further condensation, coagulation, and growth, particles in nucleation mode (<20 nm) and Aitken mode (20–100 nm) are transformed into accumulation mode particles (100–1000 nm). In conjunction with complex physicochemical effects, this further enhances the substantial growth of the PM2.5 mass concentration (Aalto et al., 2001; Curtius, Lovejoy, & Froyd, 2006; Kulmala et al., 2001; Laakso et al., 2004; Zhang, Khalizov, Wang, Hu, & Xu, 2011).
∗ Corresponding authors. E-mail addresses: [email protected] (Z. Gu), [email protected] (W. Lu). https://doi.org/10.1016/j.partic.2019.12.007 1674-2001/© 2020 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
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The formation of severe haze reflects synergetic effects caused by interactions between regional transport, local emissions, and atmospheric physicochemical processes (An et al., 2019). Physical conditions such as temperature, relative humidity (RH), and wind speed and direction could impact the formation and dispersion of pollutants in the ambient air (Cazorla & Tamayo, 2014; Zhang et al., 2018). Elser et al. (2016) measured the size distribution of particles in Beijing and Xi’an using an aerosol mass spectrometer. Their results showed particles were considerably larger during extreme haze events, with a volume-weighted mode at approximately 800–1000 nm that contrasts with that of approximately 400 nm found during periods with lower levels of pollution. The increase of aerosol volume size might be related to the fluctuation of RH under stable atmospheric conditions (Chen, Hong, & Kan, 2004; Li et al., 2014). Aerosol particles absorb moisture that leads to an increase in particle volume size when the particle numbers remain unchanged or decline over a short period (Tie et al., 2017). In fact, atmospheric particles could be charged by contact/collision, ion diffusion, and ion absorption (He et al., 2019). The ratio of charged particles among atmospheric aerosol particles has been examined by He et al. (2019). Their experimental results suggested that most atmospheric particles are charged and that electrical interactions affect their physical behaviour. Jayaratne, Ling, and Morawska (2014) adopted a neutral cluster and air ion spectrometer to measure the number concentration of charged particles (2.8–40.0 nm). They found that the proportion of charged particles near a road with heavy traffic was twice that of a road far from traffic. It was observed that the concentration of free ions in the atmosphere decreased and that of charged particles increased during particle formation. This suggests that the charging state of particles is related to their motion state and the surrounding environment. Atmospheric visibility is mainly related to the decline of the particle number concentration within the visible wavelength range of 0.1–1.0 m and to the scattering and extinction of accumulation mode particles (Huang & Yang, 2013). Thus, there is need to examine the charge characteristics of atmospheric particles in the size range of 0.1–10.0 m. The charge distribution of atmospheric particles is important information for assessing their adverse effect on human health. Cohen, Xiong, Fang, and Li (1998) measured the deposition of singly charged particles and non-charged particles in hollow-cast models of human airways. They found that the deposition ratio of charged particles is about 6 times that of non-charged particles. Azhdarzadeh, Olfert, Vehring, and Finlay (2014) measured the effects of charge on the deposition of monodispersed uniformly charged particles in a child’s oral–extrathoracic airway and
found substantial increase in the particle deposition ratio when the amount of charge on the particles rose. Yang et al. (2012) showed that surface charge could affect the effective stiffness of cellular membranes via increased interaction between the positively charged particles and the negatively charged cellular membrane. Fröhlich (2012) found that although positive charge could improve the efficacy of gene transfer and drug delivery, such constructs might also result in higher cytotoxicity. Their study further confirmed the significance and necessity of measuring the charging state of individual particles in the ambient atmosphere. Our previous work suggested that most aerosol particles could be negatively or positively charged (He et al., 2019). The microscopic mechanism of particle movement was analysed through numerical models and experimental measurements. A strong relationship between PM2.5 concentration and the charge number on the particles was observed in our previous study. To further explore the charge characteristics of atmospheric particles and to analyse the variation characteristics and correlation with haze formation, we conducted a series of measurements on the charge of atmospheric particles during an entire day in different seasons from June 2018 to March 2019. The results provide valuable insight into aerosol characteristics and their evolution mechanism. Methods Sampling site and location in Xi’an Measurement campaigns were conducted from 15 June 2018 to 15 March 2019 on the roof of the west 4th building at the Qujiang Campus of Xi’an Jiaotong University (34◦ 13 44 N, 108◦ 59 39 E, 20 m above ground level) in the city of Xi’an, China. Xi’an, with over nine million inhabitants in 2018, is the largest city in northwestern China and it is typically polluted with soottype particles. The air pollutants mainly comprise total suspended particulates (TSPs) especially PM2.5 and PM10 , SO2 , NOx , and CO. The sources of TSP pollution in the city are mainly “human” and “natural”. Among the former, the main contributors are biomass burning, coal combustion, and vehicle emissions (Huang et al., 2014). Haze days occur most frequently during winter because of emissions associated with winter heating. Natural sources also account for a considerable proportion of air pollutants. As Xi’an is located on the southern edge of the Loess Plateau and dominated by prevailing northerly winds, fine particles on the surface of the northern desert and loess areas (mainly PM10 ) are easily transported into the atmosphere causing dusty weather with obvious seasonality in Xi’an.
Fig. 1. Schematic representation of the experimental apparatus.
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Fig. 2. Meteorological factors on four typical days of sampling.
Xi’an is located within the Guanzhong Basin with the Qinling Mountains to the south, Loess Plateau to the north, open space to the northwest and a narrow space to the southeast. This unique topography promotes accumulation of pollutants in Xi’an when a stagnant air mass is trapped within the Guanzhong Basin. Research has suggested that haze formation is enhanced under stagnant meteorological conditions, characterized by high RH and weak surface winds (An et al., 2019; Zheng, Zhang et al., 2015). Therefore, Xi’an is an appropriate choice of location to study the charge characteristics of atmospheric particles and their relationship with meteorological conditions. Experimental apparatus Fig. 1 shows the particle separator setup with a parallel plate electrode, which is of the same size as used in our previous work (He et al., 2019). The system has three inlets and three outlets. Clean air
from the atmosphere was drawn into two of the inlets and filtered by an in-line filter (SLFA05010, Millipore, Germany). The middle inlet is the aerosol inlet through which air with particles was drawn directly from the ambient environment. At the outlets, three optical particle sizers (OPSs, model 3330, TSI Inc., USA) with an airflow rate of 0.06 m3 /h were used to sample the aerosols. Meanwhile, another OPS (OPS4) was applied directly to measure the number concentration of the particles. Particles in the size range of 0.3–10.0 m were measured by the OPS using up to 16 channels. Laminar flow could be formed inside the parallel plate electrode, and the aerosol particles would flow out via the middle outlet when the parallel plate is out of power. When the electrode plate was powered, charged aerosol particles would change their tracks and flow out via the other outlets. The average particle charge was calculated based on the particle number concentration distribution measured by the three OPSs and the total number of particles measured by OPS4.
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Fig. 3. Examples of PM2.5 concentration and average charge of aerosol particles during four typical days of sampling (e is the elementary charge, 1.602 × 10−19 C).
Results and discussion Fig. 2 shows the meteorological factors (temperature, wind velocity, and RH) on four days of different months: 22–23 June, 20–21 August, 28–29 November 2018 and 5–6 January 2019. The general trends of temperature and RH during the different days were consistent, i.e., RH declined as the temperature rose during the day and then increased as the temperature fell overnight. The RH during 22–23 June and 20–21 August was relatively higher than during 28–29 November and 5–6 January; the highest wind velocity occurred on 5–6 January. Fig. 3 shows the PM2.5 concentration and the average charges on aerosol particles on the four typical days. The PM2.5 concentration and the average charge of particles were higher in winter than summer. When the concentration of particles is relatively high, various physical and chemical reactions could be accelerated by electrical interactions. Therefore, an abrupt rise of PM concentration could be easily initiated. In fact, high wind velocity occurred at 12:00 local time (LT) on 5 January 2019 (Fig. 2(d)), which led to pollution migration. An increase in PM2.5 concentration to around 280 g/m3 was measured at 19:00 LT and this high level was maintained thereafter (Fig. 3(a)). In addition, an obvious rise in the average charge was observed during 22–23 June and 20–21 August (Fig. 3(b)). The meteorological data reveal that wind velocity decreased from 1.2 to 0 m/s and RH rose from 68 % to 97 % from 20:00 LT on 22 June to 07:00 LT on 23 June (Fig. 2(a)). Similarly, wind velocity decreased from 0.9 to 0.1 m/s and RH rose from 78 % to 93 % from 20:00 LT on 20 August to 07:00 LT on 21 August (Fig. 2(b)). The joint effect of high RH and low wind velocity would result in an increase in physicochemical processes, e.g., particle agglomeration due to higher liquid bridge force (He et al., 2019), hygroscopic growth (Liu et al., 2011; Shingler et al., 2016; Titos et al., 2016), uptake kinetics and heterogeneous reactions (Goodman, Bernard, & Grassian, 2001; Sun et al., 2018; Zheng, Duan et al., 2015), which would enhance the electrical interactions of the aerosol particles. Fig. 4 shows the number concentrations of charged particles and non-charged particles. The number concentration of charged parti-
cles was obviously higher than that of neutral particles during the daytime on 22–23 June 2018 and 20–21 August 2018 (Fig. 4(a) and (b)). However, there was no significant diurnal difference during 28–29 November 2018 and 5–6 January 2019 (Fig. 4(c) and (d)). The number concentrations of charged particles and non-charged particles both declined during 28–29 November 2018, which could be attributed to increased wet deposition owing to the rise in RH and the fall of wind velocity (Fig. 2(c)). However, the particle number concentrations of charged particles and non-charged particles both increased during 5–6 January 2019. This was due to the high wind velocity, which hastened pollution migration (Fig. 2(d)). Furthermore, wintertime heating in Xi’an began on 15 November 2018 and ended on 15 March 2019, which contributed to the release of particles into the atmosphere during this period. Huang et al. (2018) conducted source appointment studies of PM. Their results showed that traffic-related emissions dominated PM pollution during less polluted days, while coal combustion and biomass burning become dominant during moderately and severely polluted days (Elser et al., 2016). The diurnal variation of the particle number concentration in June and August might be associated with urban traffic. The decline in the particle number concentration differential during the day relative to overnight in November and January might be attributable to particle pollution caused by winter heating, which would be predominant during the heating season. Higher PM concentration in Xi’an during wintertime would induce a higher charge amount on atmospheric particles, which is consistent with the data presented in Fig. 3. Fig. 5 shows the air quality and average charges on particles during a sand–dust day (1–2 December 2018) and a severe haze day (5–6 January 2019). The chief pollutants during these two events were PM10 and PM2.5 . In winter, the average charge of particles differed between the dusty day and the hazy day. Although the PM2.5 concentration during the sand–dust day was higher than during the severe haze day, the surface charge of the aerosol particles was relatively weak during the sand–dust day, which might be related to the simple composition of dust particles. Moreover, the nature of the charge of these particles was reasonably stable during the severe haze day, while there was a large fluctuation during the sand–dust day. In fact, the collision probability of particles during the sand–dust day was reasonably high, and the enhanced triboelectric charging of wind-blown sand would contribute to the increase of particle charge (Gu & Wei, 2017). Fig. 6 shows the meteorological data and PM concentration during the sand–dust day on 1–2 December 2018. The wind speed and direction data were combined into wind rose plots for this sand–dust event, as shown in Fig. 7. During 08:00–16:00 LT on 1 December 2018, prevailing northwesterly winds were dominant, which facilitated long-range transport of PM from the northern desert and loess areas to Xi’an. The RH declined to around 60 % because of the intrusion of dry air. The strongest wind occurred during 10:00–13:00 LT, accompanied by a large PM concentration. At 16:00 LT, the wind speed started to decline but the PM concentration remained large. Thereafter, the wind became southwesterly and southeasterly bringing humid air that raised the RH in Xi’an. At 18:00 LT, the RH rose rapidly and the average charge on the particles increased, as shown in Fig. 5(c). According to the triboelectric charging mechanism proposed by Gu, Wei, Su, and Yu (2013), the water content and thickness of the absorbed water on the surface of sand particles would increase with atmospheric RH. The ions/electrons are derived from the absorbed water film, and through contact/collision, the ions/electrons would migrate from one sand particle to another. After separation, sand particles would obtain net negative or net positive charge due to the non-uniform distribution of the ions/electrons. Thus, particle electrification would be strengthened in association with the increase of RH and wind speed.
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Fig. 4. Number concentrations of charged particles and non-charged particles during four typical days of sampling.
Fig. 8 shows the meteorological conditions, PM2.5 concentrations and charge amount during a severe haze event on 4–6 January 2019. The wind velocity was reasonably low from 08:00 LT on 4 January 2019 to 08:00 LT on 5 January 2019, which was not conducive to pollution diffusion and caused the PM2.5 concentration to remain at a high value of 150–300 g/m3 . The average charge on the particles did not change significantly (i.e., remaining at 0.55–0.65 e); however, it rose slightly with a decline in the atmospheric RH and rose rapidly thereafter. At 08:00 LT on 5 January 2019, the wind velocity and the temperature both began to rise rapidly, which brought rapid decline in RH. The average charge on the particles and the PM2.5 concentration both increased. The rapid variation of RH would result in an increase in the charge amount, consistent with the measurements of 1–2 December 2018. Regional transport of TSP and enhanced particle contact/collision due to high wind velocity could simultaneously affect the charge characteristics. The above experimental results show that the amount of charge on aerosol particles always has an obvious rise during both rapid increase and rapid decrease of atmospheric RH. In fact, changing the RH of the environment would change the transfer rate of free ions between the water layer absorbed on the surface of the particle and the environment, thereby changing the charge characteristics of the particle surfaces. When the atmospheric pressure/vapour pressure declines, the amount of charge on the surface of a particle can mutate because of the parsing of surface ions. Fluctuation in RH causes continuous change in the vapour pressure on the particle surface, which results in non-thermal equilibrium of particles and charge migration within the particles. When particles collide or contact, charge transfer occurs because of the uneven charge distribution. Ultimately, particles would carry net positive or net negative charge. Therefore, a rise in charge amount occurs when there is rapid variation (rise/decline) in the atmospheric RH.
Conclusions The results of our investigation into the charge of atmospheric particles during 2018–2019 in Xi’an (China) suggest that the electrical charge of atmospheric particles has diurnal and seasonal variations. The average charge amount on particles was higher during winter than summer. The number concentration of charged particles was higher during the day than overnight. Owing to wintertime heating in Xi’an, the PM concentration is reasonably high. Various physical and chemical reactions could be further accelerated by electrical interactions that trigger abrupt rise of PM concentration, which results in extreme haze events. In winter, the charge characteristic of aerosol particles is relatively weaker during sand–dust days than during severe haze days, which might be related to the simple composition of dust particles. The charge nature of the particles is relatively stable during severe haze days, while it fluctuates considerably during sand–dust days. During the sand–dust day of 1–2 December 2018, the prevailing northwesterly winds were dominant, facilitating long-range transport of PM from the northern desert and loess areas to Xi’an. The following southwesterly and southeasterly winds brought moist air and raised the atmospheric RH in Xi’an, and the average aerosol charge increased. During the severe haze event on 4–6 January 2019, a strong relationship between the PM2.5 concentration, charge amount on particles, and RH was observed. Continuous rise in the charge amount occurred during a period of rapid variation in atmospheric RH. Regional transport of TSPs and enhanced particle contact/collision due to high wind velocity affected the charge characteristic of the atmospheric particles. Our findings suggest that particle formation and growth and the occurrence of extreme haze pollution are correlated with the variation of the charging state of particles, which is affected by ambient meteorological conditions. An environmental chamber experiment will be conducted in future work to verify the effects of ambient conditions.
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Fig. 5. Air quality and average charge during a sand–dust day and a severe haze day in winter.
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Fig. 6. Meteorological conditions and PM concentration during a sand–dust day.
Fig. 7. Wind rose plot obtained with 5-min data obtained on 1–2 December 2018.
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Fig. 8. Meteorological conditions, PM2.5 concentration and charge amount during a haze event.
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Charging states on atmospheric aerosol particles affected by meteorological conditions Yuanping He a,b , Zhaolin Gu a,∗ , Weizhen Lu b,∗ , Liyuan Zhang c , Daizhou Zhang d , Tomoaki Okuda e , Chuck Wah Yu a,f a
Department of Earth and Environmental Sciences, Xi’an Jiaotong University, Xi’an 710049, China Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong, China c Department of Environmental Engineering, Chang’an University, Xi’an 710064, China d Faculty of Environmental & Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto 862-8502, Japan e Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan f International Society of the Built Environment (ISBE), Milton Keynes MK7 8HQ, UK b
a r t i c l e
i n f o
Article history: Received 11 August 2019 Received in revised form 12 December 2019 Accepted 16 December 2019 Available online xxx Keywords: Aerosol particles Atmospheric humidity Charging state Haze day Physicochemical effect Sand–dust day
a b s t r a c t Previous studies on haze formation focused mainly on the various chemical components in aerosol particles and their physicochemical effects on particle behaviour (e.g., generation, growth, and agglomeration). This paper describes the measurement of the charging state on atmospheric aerosol particles, which could be affected by meteorological conditions. A series of experiments on particle charging state and meteorological factors was undertaken on the roof of the west 4th building on the Qujiang Campus at Xi’an Jiaotong University (China). Measurements were conducted approximately 20 m above ground level. Our results showed that most atmospheric particles carried net negative or positive charge and that the electric charge on the particles varied diurnally and seasonally. The average amount of charge on particles was higher in winter than in summer. The number concentration of charged particles was higher during the day than overnight. Obvious difference in the average charge of aerosol particles was found between sand–dust days and haze days. A strong relationship was found between the PM2.5 concentration, charge amount on particles, and humidity. Our findings show that particle formation and growth could partly be attributed to variation in particle charging state, which is related to meteorological conditions including atmospheric humidity. © 2020 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
Introduction The physicochemical processes between aerosol particles and other gaseous pollutants, in combination with the external transport of particles, could increase PM2.5 mass concentration (called accumulative rise), which would always be followed by low–moderate aerosol pollution (Zhang, Gu, Yu, Zhang, & Cheng, 2016). Severe haze episodes are often characteristic of a common atmospheric evolution process, i.e., an abrupt rise in PM2.5 mass concentration during the first 5–20 h of a haze event, subsequent levelling off at a high PM2.5 concentration that persists for three hours to days, followed by rapid decline attributable to the arrival of a cold front (Zhang et al., 2016). Initiation of haze incidents is
characterized by new particle formation and nucleation plays a fundamental role in the phase transformation (e.g., condensation, sedimentation, crystallization, sublimation, and evaporation). Condensable gases of low volatility initially become nucleated clusters that then grow to become nucleation mode particles through condensation and coagulation (Kulmala, 2003). During clear periods with less pollution, this type of nucleation is very common and its environmental effect is remarkable, even though such new particles have a short lifetime. Through further condensation, coagulation, and growth, particles in nucleation mode (<20 nm) and Aitken mode (20–100 nm) are transformed into accumulation mode particles (100–1000 nm). In conjunction with complex physicochemical effects, this further enhances the substantial growth of the PM2.5 mass concentration (Aalto et al., 2001; Curtius, Lovejoy, & Froyd, 2006; Kulmala et al., 2001; Laakso et al., 2004; Zhang, Khalizov, Wang, Hu, & Xu, 2011).
∗ Corresponding authors. E-mail addresses: [email protected] (Z. Gu), [email protected] (W. Lu). https://doi.org/10.1016/j.partic.2019.12.007 1674-2001/© 2020 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
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The formation of severe haze reflects synergetic effects caused by interactions between regional transport, local emissions, and atmospheric physicochemical processes (An et al., 2019). Physical conditions such as temperature, relative humidity (RH), and wind speed and direction could impact the formation and dispersion of pollutants in the ambient air (Cazorla & Tamayo, 2014; Zhang et al., 2018). Elser et al. (2016) measured the size distribution of particles in Beijing and Xi’an using an aerosol mass spectrometer. Their results showed particles were considerably larger during extreme haze events, with a volume-weighted mode at approximately 800–1000 nm that contrasts with that of approximately 400 nm found during periods with lower levels of pollution. The increase of aerosol volume size might be related to the fluctuation of RH under stable atmospheric conditions (Chen, Hong, & Kan, 2004; Li et al., 2014). Aerosol particles absorb moisture that leads to an increase in particle volume size when the particle numbers remain unchanged or decline over a short period (Tie et al., 2017). In fact, atmospheric particles could be charged by contact/collision, ion diffusion, and ion absorption (He et al., 2019). The ratio of charged particles among atmospheric aerosol particles has been examined by He et al. (2019). Their experimental results suggested that most atmospheric particles are charged and that electrical interactions affect their physical behaviour. Jayaratne, Ling, and Morawska (2014) adopted a neutral cluster and air ion spectrometer to measure the number concentration of charged particles (2.8–40.0 nm). They found that the proportion of charged particles near a road with heavy traffic was twice that of a road far from traffic. It was observed that the concentration of free ions in the atmosphere decreased and that of charged particles increased during particle formation. This suggests that the charging state of particles is related to their motion state and the surrounding environment. Atmospheric visibility is mainly related to the decline of the particle number concentration within the visible wavelength range of 0.1–1.0 m and to the scattering and extinction of accumulation mode particles (Huang & Yang, 2013). Thus, there is need to examine the charge characteristics of atmospheric particles in the size range of 0.1–10.0 m. The charge distribution of atmospheric particles is important information for assessing their adverse effect on human health. Cohen, Xiong, Fang, and Li (1998) measured the deposition of singly charged particles and non-charged particles in hollow-cast models of human airways. They found that the deposition ratio of charged particles is about 6 times that of non-charged particles. Azhdarzadeh, Olfert, Vehring, and Finlay (2014) measured the effects of charge on the deposition of monodispersed uniformly charged particles in a child’s oral–extrathoracic airway and
found substantial increase in the particle deposition ratio when the amount of charge on the particles rose. Yang et al. (2012) showed that surface charge could affect the effective stiffness of cellular membranes via increased interaction between the positively charged particles and the negatively charged cellular membrane. Fröhlich (2012) found that although positive charge could improve the efficacy of gene transfer and drug delivery, such constructs might also result in higher cytotoxicity. Their study further confirmed the significance and necessity of measuring the charging state of individual particles in the ambient atmosphere. Our previous work suggested that most aerosol particles could be negatively or positively charged (He et al., 2019). The microscopic mechanism of particle movement was analysed through numerical models and experimental measurements. A strong relationship between PM2.5 concentration and the charge number on the particles was observed in our previous study. To further explore the charge characteristics of atmospheric particles and to analyse the variation characteristics and correlation with haze formation, we conducted a series of measurements on the charge of atmospheric particles during an entire day in different seasons from June 2018 to March 2019. The results provide valuable insight into aerosol characteristics and their evolution mechanism. Methods Sampling site and location in Xi’an Measurement campaigns were conducted from 15 June 2018 to 15 March 2019 on the roof of the west 4th building at the Qujiang Campus of Xi’an Jiaotong University (34◦ 13 44 N, 108◦ 59 39 E, 20 m above ground level) in the city of Xi’an, China. Xi’an, with over nine million inhabitants in 2018, is the largest city in northwestern China and it is typically polluted with soottype particles. The air pollutants mainly comprise total suspended particulates (TSPs) especially PM2.5 and PM10 , SO2 , NOx , and CO. The sources of TSP pollution in the city are mainly “human” and “natural”. Among the former, the main contributors are biomass burning, coal combustion, and vehicle emissions (Huang et al., 2014). Haze days occur most frequently during winter because of emissions associated with winter heating. Natural sources also account for a considerable proportion of air pollutants. As Xi’an is located on the southern edge of the Loess Plateau and dominated by prevailing northerly winds, fine particles on the surface of the northern desert and loess areas (mainly PM10 ) are easily transported into the atmosphere causing dusty weather with obvious seasonality in Xi’an.
Fig. 1. Schematic representation of the experimental apparatus.
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Fig. 2. Meteorological factors on four typical days of sampling.
Xi’an is located within the Guanzhong Basin with the Qinling Mountains to the south, Loess Plateau to the north, open space to the northwest and a narrow space to the southeast. This unique topography promotes accumulation of pollutants in Xi’an when a stagnant air mass is trapped within the Guanzhong Basin. Research has suggested that haze formation is enhanced under stagnant meteorological conditions, characterized by high RH and weak surface winds (An et al., 2019; Zheng, Zhang et al., 2015). Therefore, Xi’an is an appropriate choice of location to study the charge characteristics of atmospheric particles and their relationship with meteorological conditions. Experimental apparatus Fig. 1 shows the particle separator setup with a parallel plate electrode, which is of the same size as used in our previous work (He et al., 2019). The system has three inlets and three outlets. Clean air
from the atmosphere was drawn into two of the inlets and filtered by an in-line filter (SLFA05010, Millipore, Germany). The middle inlet is the aerosol inlet through which air with particles was drawn directly from the ambient environment. At the outlets, three optical particle sizers (OPSs, model 3330, TSI Inc., USA) with an airflow rate of 0.06 m3 /h were used to sample the aerosols. Meanwhile, another OPS (OPS4) was applied directly to measure the number concentration of the particles. Particles in the size range of 0.3–10.0 m were measured by the OPS using up to 16 channels. Laminar flow could be formed inside the parallel plate electrode, and the aerosol particles would flow out via the middle outlet when the parallel plate is out of power. When the electrode plate was powered, charged aerosol particles would change their tracks and flow out via the other outlets. The average particle charge was calculated based on the particle number concentration distribution measured by the three OPSs and the total number of particles measured by OPS4.
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Fig. 3. Examples of PM2.5 concentration and average charge of aerosol particles during four typical days of sampling (e is the elementary charge, 1.602 × 10−19 C).
Results and discussion Fig. 2 shows the meteorological factors (temperature, wind velocity, and RH) on four days of different months: 22–23 June, 20–21 August, 28–29 November 2018 and 5–6 January 2019. The general trends of temperature and RH during the different days were consistent, i.e., RH declined as the temperature rose during the day and then increased as the temperature fell overnight. The RH during 22–23 June and 20–21 August was relatively higher than during 28–29 November and 5–6 January; the highest wind velocity occurred on 5–6 January. Fig. 3 shows the PM2.5 concentration and the average charges on aerosol particles on the four typical days. The PM2.5 concentration and the average charge of particles were higher in winter than summer. When the concentration of particles is relatively high, various physical and chemical reactions could be accelerated by electrical interactions. Therefore, an abrupt rise of PM concentration could be easily initiated. In fact, high wind velocity occurred at 12:00 local time (LT) on 5 January 2019 (Fig. 2(d)), which led to pollution migration. An increase in PM2.5 concentration to around 280 g/m3 was measured at 19:00 LT and this high level was maintained thereafter (Fig. 3(a)). In addition, an obvious rise in the average charge was observed during 22–23 June and 20–21 August (Fig. 3(b)). The meteorological data reveal that wind velocity decreased from 1.2 to 0 m/s and RH rose from 68 % to 97 % from 20:00 LT on 22 June to 07:00 LT on 23 June (Fig. 2(a)). Similarly, wind velocity decreased from 0.9 to 0.1 m/s and RH rose from 78 % to 93 % from 20:00 LT on 20 August to 07:00 LT on 21 August (Fig. 2(b)). The joint effect of high RH and low wind velocity would result in an increase in physicochemical processes, e.g., particle agglomeration due to higher liquid bridge force (He et al., 2019), hygroscopic growth (Liu et al., 2011; Shingler et al., 2016; Titos et al., 2016), uptake kinetics and heterogeneous reactions (Goodman, Bernard, & Grassian, 2001; Sun et al., 2018; Zheng, Duan et al., 2015), which would enhance the electrical interactions of the aerosol particles. Fig. 4 shows the number concentrations of charged particles and non-charged particles. The number concentration of charged parti-
cles was obviously higher than that of neutral particles during the daytime on 22–23 June 2018 and 20–21 August 2018 (Fig. 4(a) and (b)). However, there was no significant diurnal difference during 28–29 November 2018 and 5–6 January 2019 (Fig. 4(c) and (d)). The number concentrations of charged particles and non-charged particles both declined during 28–29 November 2018, which could be attributed to increased wet deposition owing to the rise in RH and the fall of wind velocity (Fig. 2(c)). However, the particle number concentrations of charged particles and non-charged particles both increased during 5–6 January 2019. This was due to the high wind velocity, which hastened pollution migration (Fig. 2(d)). Furthermore, wintertime heating in Xi’an began on 15 November 2018 and ended on 15 March 2019, which contributed to the release of particles into the atmosphere during this period. Huang et al. (2018) conducted source appointment studies of PM. Their results showed that traffic-related emissions dominated PM pollution during less polluted days, while coal combustion and biomass burning become dominant during moderately and severely polluted days (Elser et al., 2016). The diurnal variation of the particle number concentration in June and August might be associated with urban traffic. The decline in the particle number concentration differential during the day relative to overnight in November and January might be attributable to particle pollution caused by winter heating, which would be predominant during the heating season. Higher PM concentration in Xi’an during wintertime would induce a higher charge amount on atmospheric particles, which is consistent with the data presented in Fig. 3. Fig. 5 shows the air quality and average charges on particles during a sand–dust day (1–2 December 2018) and a severe haze day (5–6 January 2019). The chief pollutants during these two events were PM10 and PM2.5 . In winter, the average charge of particles differed between the dusty day and the hazy day. Although the PM2.5 concentration during the sand–dust day was higher than during the severe haze day, the surface charge of the aerosol particles was relatively weak during the sand–dust day, which might be related to the simple composition of dust particles. Moreover, the nature of the charge of these particles was reasonably stable during the severe haze day, while there was a large fluctuation during the sand–dust day. In fact, the collision probability of particles during the sand–dust day was reasonably high, and the enhanced triboelectric charging of wind-blown sand would contribute to the increase of particle charge (Gu & Wei, 2017). Fig. 6 shows the meteorological data and PM concentration during the sand–dust day on 1–2 December 2018. The wind speed and direction data were combined into wind rose plots for this sand–dust event, as shown in Fig. 7. During 08:00–16:00 LT on 1 December 2018, prevailing northwesterly winds were dominant, which facilitated long-range transport of PM from the northern desert and loess areas to Xi’an. The RH declined to around 60 % because of the intrusion of dry air. The strongest wind occurred during 10:00–13:00 LT, accompanied by a large PM concentration. At 16:00 LT, the wind speed started to decline but the PM concentration remained large. Thereafter, the wind became southwesterly and southeasterly bringing humid air that raised the RH in Xi’an. At 18:00 LT, the RH rose rapidly and the average charge on the particles increased, as shown in Fig. 5(c). According to the triboelectric charging mechanism proposed by Gu, Wei, Su, and Yu (2013), the water content and thickness of the absorbed water on the surface of sand particles would increase with atmospheric RH. The ions/electrons are derived from the absorbed water film, and through contact/collision, the ions/electrons would migrate from one sand particle to another. After separation, sand particles would obtain net negative or net positive charge due to the non-uniform distribution of the ions/electrons. Thus, particle electrification would be strengthened in association with the increase of RH and wind speed.
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Fig. 4. Number concentrations of charged particles and non-charged particles during four typical days of sampling.
Fig. 8 shows the meteorological conditions, PM2.5 concentrations and charge amount during a severe haze event on 4–6 January 2019. The wind velocity was reasonably low from 08:00 LT on 4 January 2019 to 08:00 LT on 5 January 2019, which was not conducive to pollution diffusion and caused the PM2.5 concentration to remain at a high value of 150–300 g/m3 . The average charge on the particles did not change significantly (i.e., remaining at 0.55–0.65 e); however, it rose slightly with a decline in the atmospheric RH and rose rapidly thereafter. At 08:00 LT on 5 January 2019, the wind velocity and the temperature both began to rise rapidly, which brought rapid decline in RH. The average charge on the particles and the PM2.5 concentration both increased. The rapid variation of RH would result in an increase in the charge amount, consistent with the measurements of 1–2 December 2018. Regional transport of TSP and enhanced particle contact/collision due to high wind velocity could simultaneously affect the charge characteristics. The above experimental results show that the amount of charge on aerosol particles always has an obvious rise during both rapid increase and rapid decrease of atmospheric RH. In fact, changing the RH of the environment would change the transfer rate of free ions between the water layer absorbed on the surface of the particle and the environment, thereby changing the charge characteristics of the particle surfaces. When the atmospheric pressure/vapour pressure declines, the amount of charge on the surface of a particle can mutate because of the parsing of surface ions. Fluctuation in RH causes continuous change in the vapour pressure on the particle surface, which results in non-thermal equilibrium of particles and charge migration within the particles. When particles collide or contact, charge transfer occurs because of the uneven charge distribution. Ultimately, particles would carry net positive or net negative charge. Therefore, a rise in charge amount occurs when there is rapid variation (rise/decline) in the atmospheric RH.
Conclusions The results of our investigation into the charge of atmospheric particles during 2018–2019 in Xi’an (China) suggest that the electrical charge of atmospheric particles has diurnal and seasonal variations. The average charge amount on particles was higher during winter than summer. The number concentration of charged particles was higher during the day than overnight. Owing to wintertime heating in Xi’an, the PM concentration is reasonably high. Various physical and chemical reactions could be further accelerated by electrical interactions that trigger abrupt rise of PM concentration, which results in extreme haze events. In winter, the charge characteristic of aerosol particles is relatively weaker during sand–dust days than during severe haze days, which might be related to the simple composition of dust particles. The charge nature of the particles is relatively stable during severe haze days, while it fluctuates considerably during sand–dust days. During the sand–dust day of 1–2 December 2018, the prevailing northwesterly winds were dominant, facilitating long-range transport of PM from the northern desert and loess areas to Xi’an. The following southwesterly and southeasterly winds brought moist air and raised the atmospheric RH in Xi’an, and the average aerosol charge increased. During the severe haze event on 4–6 January 2019, a strong relationship between the PM2.5 concentration, charge amount on particles, and RH was observed. Continuous rise in the charge amount occurred during a period of rapid variation in atmospheric RH. Regional transport of TSPs and enhanced particle contact/collision due to high wind velocity affected the charge characteristic of the atmospheric particles. Our findings suggest that particle formation and growth and the occurrence of extreme haze pollution are correlated with the variation of the charging state of particles, which is affected by ambient meteorological conditions. An environmental chamber experiment will be conducted in future work to verify the effects of ambient conditions.
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Fig. 5. Air quality and average charge during a sand–dust day and a severe haze day in winter.
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Fig. 6. Meteorological conditions and PM concentration during a sand–dust day.
Fig. 7. Wind rose plot obtained with 5-min data obtained on 1–2 December 2018.
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Fig. 8. Meteorological conditions, PM2.5 concentration and charge amount during a haze event.
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