Beijing has been suffering from frequent severe air pollution events, with concentrations affected significantly by the mixed-layer height. Major efforts have been made to study the physico-chemical properties, compositions, and sources of aerosol particles at ground level. However, little is known about the morphology, elemental composition, and mixing state of aerosol particles above the mixed layer. In this work, we collected individual aerosol particles simultaneously at ground level (2 m above ground) and above the mixed layer in urban Beijing (within the Atmospheric Pollution and Human Health in a Chinese Megacity, APHH-Beijing, 2016 winter campaign). The particles were analyzed offline by transmission electron microscopy coupled with energy dispersive X-ray spectroscopy. Our results showed that the relative number contribution of mineral particles to all measured particles was much higher during non-haze periods (42.5 %) than haze periods (18.1 %); in contrast, internally mixed particles contributed more during haze periods (21.9 %) than non-haze periods (7.2 %) at ground level. In addition, more mineral particles were found at ground level than above the mixed-layer height. Around 20 % of individual particles showed core–shell structures during haze periods, whereas only a few core–shell particles were observed during non-haze periods (2 %). The results showed that the particles above the mixed layer were more aged, with a larger proportion of organic particles originating from coal combustion. Our results indicate that a large fraction of the airborne particles above the mixed layer come from surrounding areas influenced by coal combustion activities. This source contributes to the surface particle concentrations in Beijing when polluted air is mixed down to the ground level.
Atmospheric aerosols emitted from anthropogenic or natural sources are composed of a variety of chemical components (e.g., organic matter, black carbon, nitrate, sulfate, ammonium, metals, mineral dust) (Merikallio et al., 2011; Guo et al., 2014; Wang et al., 2016; Peng et al., 2016; Shao et al., 2017; Tao et al., 2017). Anthropogenic aerosols have received increasing attention in recent decades due to their effects on climate and the environment. In fact, anthropogenic aerosols affect climate through cloud condensation nuclei activity (Kerminen et al., 2012), hygroscopic growth (Brock et al., 2016), and light scattering and absorption (Jacobson, 2001; Bond and Bergstrom, 2006; Merikallio et al., 2011; China et al., 2013; Peng et al., 2016; Bhandari et al., 2019b). They can also have adverse effects on human health, for example, by carrying toxic and carcinogenic compounds (Chen et al., 2013; Shao et al., 2016, 2017). High concentrations of aerosol particles in urban air can cause cardiovascular, respiratory, and even nervous system diseases (Xia et al., 2018; De Marco et al., 2019; Shou et al., 2019). It is suggested that outdoor air pollution causes 3.3 million premature deaths worldwide each year (Lelieveld et al., 2015). Atmospheric aerosol particles also affect regional and global geochemical cycles as they are transported over long distances (Heald et al., 2006; Li et al., 2017c; Rodriguez-Navarro et al., 2018).
Recently, China has suffered from severe air pollution conditions, like
other countries undergoing rapid social and economic development (Huang et
al., 2014). In China, urban air pollution is characterized by frequent
occurrence of haze events, high PM
As the megacity capital, Beijing has received much attention, being one of
the most polluted cities in China. Atmospheric researchers have been
studying aerosol particles to understand haze formation in China (Sun et
al., 2013; Huang et al., 2014; Zhou et al., 2018b). Measurements and model
analyses highlight the key roles of secondary aerosol formation by trace
gases (e.g., volatile organic compounds, SO
Because the characterization of aerosol particles is mainly focused on
surface level observations, the understanding of aerosol properties at
higher altitudes in urban areas is still insufficient (Zhou et al., 2018a).
Vertical differences between precursors, oxidants, and temperature gradients
might influence gas-particle partitioning and heterogeneous reactions of
N
Vertical comparisons of individual aerosol particles and their morphologies, mixing states, and elemental compositions are very limited. Transmission electron microscopy (TEM) can provide detailed individual-particle characterization and help to explain heterogeneous reactions and the aging process (Li et al., 2016a). In this study, we compare particles simultaneously collected at ground level and above the MLH based on the meteorological tower at the IAP in Beijing as part of the UK–China Atmospheric Pollution and Human Health (APHH) 2016 winter campaign.
Individual aerosol samples were collected at the tower division of the IAP,
Chinese Academy of Science (39
Two DKL-2 single-stage cascade impactors, with a 0.5 mm diameter jet nozzle
and a flow rate of 1 L min
Automatic lidar and ceilometer (ACL) observations of attenuated backscatter were conducted at the site using a Vaisala CL31 sensor. Measurements were corrected to account for instrument-related background and near-range artifacts (Kotthaus et al., 2016). The MLH was derived from profile measurements using the automatic CABAM (Characterising the Atmospheric Boundary layer based on Automatic lidars and ceilometers Measurements) algorithm (Kotthaus and Grimmond, 2018). Since the TEM samples were collected for less than 5 min, the MLH at 15 min resolution was used to determine whether the Z2 observations were located within the MLH or above the MLH (Shi et al., 2019).
Samples were obtained during the periods shown (solid dots and dashed lines)
in Fig. 1. Detailed sample information is provided in Table 1. Other
measurements including PM
Sample information and meteorological conditions.
The dashed lines represent the individual-particle sampling times,
with red lines representing non-haze samples and black lines haze samples.
Individual particles were analyzed using a JEOL JEM-2800 TEM at an
accelerating voltage of 200 kV. The morphology and mixing state of
individual particles were determined from the TEM images. Semi-quantitative
elemental composition was determined using energy-dispersive X-ray
spectroscopy (EDS), by which elements heavier than boron (
The temporal variations in different air pollutants and meteorological
conditions at ground level are shown in Fig. 1. The hourly averaged
PM
Aerosol particles are classified using their morphologies and elemental compositions into seven main types, namely: (1) primary organic aerosols (POAs), (2) sulfur-rich (S-rich) particles, (3) soot particles, (4) mineral particles, (5) metal particles, (6) internally mixed organic and sulfur-rich particles (OP–S), and (7) other mixed particles. The detailed characteristics of each particle type are shown in Table 2.
POA particles are mainly composed of C and O, usually with a small amount of Si, S, Cl, and K. POA particles are relatively stable under the electron beam irradiation. Based on the morphologies, POA particles can be further divided into spherical (Fig. 2a) and irregular shapes (Fig. 2b). They are mainly from the combustion process of biomass and fossil fuel (Li et al., 2016a; Liu et al., 2021).
Examples of morphologies and mixing characteristics of individual
aerosol particles in winter in Beijing at ground level and above the mixed
layer.
Classification and characteristics of individual particle types.
S-rich particles (Fig. 2c and d) are mainly composed of O, S, and N and
sometimes also contain some amount of K. S-rich particles are beam-sensitive
and volatilize under strong beam irradiation. S-rich particles generally
represent secondary inorganic components (e.g., SO
Soot particles are mainly composed of C, a minor amount of O, and sometimes Si. Soot particles consist of a number of C-dominated spherical monomers less than 100 nm in diameter (Fig. 2e and f) and can be easily identified under high-resolution TEM (Buseck et al., 2014; Bhandari et al., 2017). Soot particles, stable under the electron beam, show chain-like or compact morphologies in the atmosphere (Sorensen, 2001; Adachi et al., 2007; China et al., 2013, 2015; Bhandari et al., 2019a). Soot particles are mainly from incomplete combustion of biomass and fossil fuel.
Metal particles (Fig. 2g and h) and mineral particles (Fig. 2i) are stable under the beam irradiation. Mineral particles are mostly irregularly shaped and contain crustal elements (e.g., Si, Al, Ca, Fe, Na, K, Mg, Ti, and S) in addition to O. They can be generated from windblown soil dust or road dust. Metal particles are spherical or near-spherical and are mainly composed of Fe, Zn, Mn, Ti, and Pb. Metal particles mainly originate from industries, coal-fired power plants, and oil refineries (Xu et al., 2019) or vehicle brakes (Hou et al., 2018).
Internally mixed particles (Fig. 2j–p) are particles with at least two of the above components. They usually show relatively larger diameter. We further classify them as internally mixed organic and sulfur-rich particles (OP–S) (Fig. 2j–l) and other mixed particles (Fig. 2m–p).
Haze periods are defined as hourly average PM
Relative number percentage of different particle types at ground level (Z1) and above the mixed-layer height (Z2). The number above each bar represents the total particle number analyzed in each sample.
Relative number percentage of individual particles.
The mineral particles are mainly from re-suspended road dust, soil dust, and construction dust during non-desert transport dust episodes (Sun et al., 2006; Gao et al., 2016; Wang et al., 2017). The relative number percentage of mineral particles was much higher during non-haze periods (42.5 %) than during haze periods (18.1 %), as shown in Fig. 3.
However, the content of mixed particles including OP–S and other mixed particles during haze periods (21.9 %) was much higher than during non-haze periods (7.2 %), suggesting that there was more secondary aerosol formation during haze periods. High secondary aerosol formation in winter in Beijing during the pollution periods was also found in previous studies (Huang et al., 2014; Sun et al., 2016; Li et al., 2017a). Secondary aerosol formation was expected since the RH during the haze periods was relatively higher than during non-haze periods, as shown in Table 1 and Fig. 1, which facilitated chemical reactions of gaseous pollutants (Liu et al., 2016; Wang et al., 2016). Also, the average content of POA particles and S-rich particles was higher during haze periods than during non-haze periods.
Images of core–shell-structured particles.
A total of 1519 individual particles among eight samples above the MLH were analyzed. The results are provided in Table 3 and shown in Fig. 3. We found that the relative number percentage of mineral particles at ground level was larger than that above the MLH. For example, mineral particles at ground level and above the MLH during non-haze periods accounted for 42.5 % and 23.2 %, respectively, and during haze periods the values are 18.1 % and 9.5 %, respectively. S-rich particles during non-haze periods accounted for 20.0 % at ground level, less than the value of 30.7 % above the MLH. However, not all the samples above the MLH during haze periods showed higher relative number percentage of S-rich particles than at ground level. This might be because some of the S-rich particles above the MLH were mixed with other particles, forming mixed particles. Another reason might be that the higher relative number percentage of mixed particles diluted the relative number percentage of S-rich particles. The mixed particles during haze periods accounted for 32.0 % above the MLH, higher than that of 21.9 % at ground level. We also found that POA particles above the MLH accounted for higher relative number percentage than at ground level, although there was some variance. For example, samples 4 and 6 showed a higher relative number percentage of POA particles at ground level. That might be because some of the POA particles were mixed with S-rich particles, and OP–S showed a higher relative number percentage above the MLH than at ground level in samples 4 and 6. Metals and soot only accounted for a few relative number percentages in all samples, and they did not show much difference at ground level and above the MLH. Particles above the MLH were transported either from the surrounding areas or from ground sources. In both cases, they were subject to atmospheric processes, leading to their aging.
Low-magnification images of individual particles. Panels
In the atmosphere, particles are subjected to the aging process. During the
aging process of aerosol particles, secondary species can coat pre-existing
particles (Li and Shao, 2009; Laskin et al., 2016; Li et al., 2016b; Niu et
al., 2016; Tang et al., 2016; Chen et al., 2017; Hou et al., 2018; Unga et
al., 2018; Xu et al., 2019). Using high-resolution TEM images, it is
possible to identify the core–shell structure of particles (Li et al.,
2016a). For example, Fig. 4a and b showed S-rich particles coated by
secondary species. Figure 4c and d were POA particles coated with secondary
species. Figure 4e–h showed core–shell-structured particles with some mixed-particle cores. In this study, we found that the core–shell-structured
particles accounted for 20 % during haze periods, with 17 % at ground
level and 23 % above the MLH, but only 2 % during non-haze periods.
These results demonstrated a general trend that the core–shell-structured
particles during haze periods were much higher than during non-haze periods.
Also, the average
The coating of atmospheric particles is often caused by aging mechanisms
such as coagulation, condensation, and heterogeneous chemical reactions
(Kahnert, 2015; Müller et al., 2017, Zhang et at., 2012). Figure 5 shows
low-magnification images of particles at ground level and above the MLH.
More core–shell particles were found above the MLH. The core
Figure S3 shows the total particle number size distribution; the relative number percentage of the larger-sized particles clearly increased when considering the coatings compared to only considering the core size during haze periods. The change in optical properties due to coating was calculated in various studies by using different methods (Cappa et al., 2012; Scarnato et al., 2013; Liu et al., 2015; Saliba et al., 2016; Unga et al., 2018). When host particles were coated, their optical properties might be amplified (Khalizov et al., 2009; Peng et al., 2016). Also, organic coating can influence the hygroscopic properties and the viscosity of mixed particles (Sharma et al., 2018; Unga et al., 2018) and thus can influence cloud formation activity (Kerminen et al., 2012).
Our results showed a higher relative number percentage of POA particles both
during non-haze (21.1 %) and haze periods (28.3 %) in winter in Beijing,
compared with a tunnel environment (
Most of the POA particles in our study were spherical or nearly spherical in shape according to the projected images, and they were stable under strong electron beam irradiation and appear as dark features in TEM images, which reflected their high thickness and refractory properties (Ebert et al., 2016), suggesting that they were formed through the cooling process after the biomass- or fossil-fuel-combustion pyrolysis products of volatile organic compounds were emitted into the atmosphere (Wang et al., 2015; Chen et al., 2017; Zhang et al., 2017).
These spherical or near-spherical POA particles are considered to be brown carbon (Zhang et al., 2020). Brown carbon plays a significant role in atmospheric shortwave absorption and can cause warming of the atmosphere (Adachi and Buseck, 2011; Hoffer et al., 2016). Some researchers have found that the primary POA particles from coal combustion have more Si than those from biomass burning (Li et al., 2012; Chen et al., 2017). The weight ratio of C–O–Si at ground level and above the MLH is shown in Fig. 7. More coal-burning-related POA particles were found above the MLH. Since the relative number percentage of POA particles affected by coal burning is higher above the MLH than at ground level, POA particles above the MLH are not all from ground level and might originate from surrounding areas influenced by coal combustion. The results were supported by the 24 h backward trajectories, which showed that air masses above the MLH during haze periods were from the northern and western direction of Beijing, as shown in Fig. S4. It is reasonable that Beijing has implemented strict air pollution control measures, including using natural gas to replace domestic coal burning. The particles above the MLH can contribute to Beijing air pollution by mixing down to the ground.
Triangular diagram showing the weight ratio of C–O–Si of primary organic aerosols (POAs) at ground level and above the mixed-layer height (MLH).
Detailed morphologies and elemental compositions of individual aerosol particles at ground level and above the mixed-layer height were analyzed in this study. The following conclusions were reached:
Particles were classified into primary organic particles, S-rich
particles, mineral particles, metal particles, soot, internally mixed
organic and sulfur-rich particles, and other mixed particles. Compared with
non-haze periods, haze periods were associated with a relatively lower number
percentage of mineral particles and a relatively higher number percentage of
mixed particles. Compared with the aerosol samples at ground level, the samples above the MLH
had a lower relative number percentage of mineral particles, a higher number
percentage of coated particles, and a smaller core The relative number percentage of primary organic particles accounted for
21.1 % during non-haze periods and 28.3 % during haze periods in winter
in Beijing. More primary organic particles above the mixed layer were
associated with coal combustion according to the C–O–Si ratio, and the
long-range transportation of air masses from surrounding areas has an
important influence for Beijing air.
Data used in this study are available from the corresponding author upon request (ShaoL@cumtb.edu.cn).
The supplement related to this article is available online at:
WW, LS, CM, JX, and ZS conceived the manuscript. WW, YL, XF, and MZ conducted the sample collection and analysis. SK and SG conducted the MLH measurement. CM and JB conducted manuscript modification.
The authors declare that they have no conflict of interest.
We thank Zifa Wang and Pingqing Fu at the IAP for supporting sample collection.
This research has been supported by the National Natural Science Foundation of China (grant nos. 42075107 and 42065007); the International Cooperation Projects of the National Natural Science Foundation of China (grant no. 41571130031); the China Scholarship Council (grant no. 201806430015); the Yue Qi Scholar Fund of the China University of Mining and Technology (Beijing); the US Department of Energy (DOE), Office of Biological and Environmental Research (OBER), Atmospheric System Research (grant nos. DE-SC0011935 and DESC0018931); and the Natural Environmental Research Council (grant no. NE/N007190/1).
This paper was edited by John Liggio and reviewed by Weijun Li and one anonymous referee.