Individual aerosol samples were collected at the tower division of the IAP, Chinese Academy of Science (39∘58′ N, 116∘22′ E), in Beijing from 1 to 9 December 2016. The site, located between the north 3rd and 4th Ring Roads in Beijing, is influenced by surrounding and regional traffic, and commercial as well as residential activities (Sun et al., 2016). There is a highway 250 m east of the IAP.

Two DKL-2 single-stage cascade impactors, with a 0.5 mm diameter jet nozzle and a flow rate of 1 L min-1, were used. The sampler collection efficiency is ∼100 % at an aerodynamic diameter of 0.5 µm if the particle density is 2 g cm-3 (Li et al., 2016b). Copper (Cu) TEM grids, coated with carbon film (300 mesh, Tianld Co., Beijing, China), were used to collect the aerosol samples. The sampling duration varied from 30 s to less than 5 min, depending on the air pollution load. Simultaneous observations at ground level (Z1; 2 m above ground) and an elevated altitude (Z2; 280 m above ground) enabled us to obtain the vertical profile of the particles. The collected samples were stored in a dry plastic tube and placed in an air dryer to minimize particle changes before analysis.

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 PM2.5, SO2, NO2, and O3 mass concentrations at ground level were obtained from the Olympic Park monitor site, which is the closest national air quality monitor station to the IAP (∼1.5 km) (Shi et al., 2019). City average temperature (T) and relative humidity (RH) at ground level were obtained from the Ministry of Ecology and Environment of China (https://www.aqistudy.cn/, last access: 31 March 2021). In this study, the particles were all collected in the morning and midnight, when the MLH was the lowest, and the height of the tower could reach the MLH at that time.

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 (Z>5) can be detected. Cu was not included because the TEM grids were made of copper. The EDS collection duration of each individual particle was about 15 s to reduce damage of particles from the electron beam. For most particles, only one spectrum of each particle was collected, and the spot size of the beam would be adjusted according to the size of the particles. Therefore, we obtained the average elemental compositions of each particle. However, more than one spot per particle was collected if the particles were inhomogeneous particles according to the TEM images. The aerosol particles were not evenly distributed on the TEM grids; the coarser particles occurred near the center, and the finer particles occurred on the periphery. To ensure a representative data analysis, three or four meshes from the center to the periphery were selected and analyzed. The projected areas of individual particles were determined using the Image-J software (Schneider et al., 2012), which was commonly used for counting and measuring the projected area of atmospheric particles acquired by electron microscopes (Unga et al., 2018). First, the grayscale images of the particles were converted into binary images, in which black pixels represented the particles, and white pixels represented the background. The area-equivalent diameters (DAeq) of the particles are calculated by the following formula: DAeq=2⋅(A/π)1/2, where A is the projected area of the particles shown in the TEM images (Bhandari et al., 2019a). Most of the particles with a diameter larger than 100 nm were analyzed in this study.

The temporal variations in different air pollutants and meteorological conditions at ground level are shown in Fig. 1. The hourly averaged PM2.5 mass concentration at the Olympic Park monitoring site ranged from 3 to 530 µg m-3, with a sample period average of 113.3 µg m-3, significantly exceeding the safe level of 75 µg m-3 according to the Chinese National Ambient Air Quality Standard (GB3095-2012). The MLH ranged from 54 to 1496 m, with an average of 397 m. The MLH showed obvious diurnal variation. The hourly mean RH ranged from 17 % to 97 %, with a 9 d mean of 50.3 %. The RH and PM2.5 were positively correlated (correlation coefficient = 0.75; Fig. S1) according to the 216 groups of hourly data, suggesting that higher RH favors the formation of haze (Sun et al., 2014; Wang et al., 2016). As expected, RH and temperature were negatively correlated (correlation coefficient =-0.51; Fig. S2). The variation trend of SO2 was similar to that of NO2. However, the average concentration of NO2 (83.2 µg m-3) was nearly 5.5 times higher than that of SO2 (15.2 µg m-3). The concentration of O3 showed a different trend compared with NO2 and SO2 (Fig. 1), with a 9 d hourly mean concentration of 20 µg m-3.

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).

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., SO42+, NO3-, and NH4+) (Xu et al., 2019).

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 PM2.5 mass concentration greater than 75 µg m-3 during collection time; the rest are defined as non-haze periods. A total of 1538 individual particles among eight samples at ground level were analyzed based on the TEM results. The relative number percentage (N(type i) / N(total)⋅100) of particles in each sample was calculated. The results are provided in Table 3 and shown in Fig. 3. During non-haze periods, the particles were composed of mineral particles (42.5 %), POA particles (21.1 %), S-rich particles (20.0 %), soot particles (6.4 %), other mixed particles (5.6 %), metal particles (2.83 %), and OP–S (1.6 %) in descending order. During haze periods, the particles were composed of POA particles (28.3 %), S-rich particles (23.5 %), mineral particles (18.1 %), OP–S (13.1 %), other mixed particles (8.8 %), soot particles (6.6 %), and metal particles (1.7 %) in descending order.

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.

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.

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 DAeq of particles was larger during haze periods than during non-haze periods, as shown in Fig. S3. These results confirmed that particles during haze periods underwent more extensive aging than during non-haze periods.

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 / shell ratio (R), which is the ratio of the DAeq of the core to the DAeq of the whole particle including the coating, has been used to evaluate the aging process of aerosol particles in different studies (Niu et al., 2012, 2016; Hou et al., 2018). The value of R ranged from 0 to less than 1. A smaller R value means the particles are more coated and thus are subjected to a more extensive degree of aging (Hou et al., 2018). Because a high number percentage of core–shell-structured particles were only found during haze periods, we only measured R of core–shell-structured particles during the haze periods (including the samples 2, 4, 5, 6, and 7). Figure 6a shows the R value of each sample during the haze periods. We can see from Fig. 6a that all the samples showed a smaller average R value above the MLH compared with those from the ground level. The average R value above the MLH (0.54) was smaller than at ground level (0.59). Additionally, the relative number percentage of core–shell-structured particles was higher above the MLH than at ground level, except for sample 4. These findings indicated that the particles above the MLH were more aged than those at ground level.

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 (∼5 %), where the vehicle emissions were the main pollution sources (Hou et al., 2018). Also, recent studies did not find abundant POA particles in North China during spring and summer (Yuan et al., 2015; Li et al., 2016b; Xu et al., 2019). Instead, a larger number percentage of POA particles has been found in winter using electron microscopy in previous studies, including an outflow of a haze plume in East Asia (Zhu et al., 2013), a coal-burning region in China's Loess Plateau (Li et al., 2012), three sampling sites in the North China Plain (Chen et al., 2017), and urban and rural sites in Northeast China (Xu et al., 2017; Zhang et al., 2017). These results suggested that POA particles accounted for a large number percentage of the particles in North China in winter.

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.