Many studies have focused on the physicochemical properties of aerosol particles in unusually severe haze episodes in North China instead of the more frequent and less severe hazes. Consistent with this lack of attention, the morphology and mixing state of organic matter (OM) particles in the frequent light and moderate (L & M) hazes in winter in the North China Plain (NCP) have not been examined, even though OM dominates these fine particles. In the present work, morphology, mixing state, and size of organic aerosols in the L & M hazes were systematically characterized using transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and nanoscale secondary ion mass spectrometer, with the comparisons among an urban site (Jinan, S1), a mountain site (Mt. Tai, S2), and a background island site (Changdao, S3) in the same hazes. Based on their morphologies, the OM particles were divided into six different types: spherical (type 1), near-spherical (type 2), irregular (type 3), domelike (type 4), dispersed-OM (type 5), and OM-coating (type 6). In the three sampling sites, types 1–3 of OM particles were most abundant in the L & M hazes and most of them were internally mixed with non-OM particles. The abundant near-spherical OM particles with higher sphericity and lower aspect ratio indicate that these primary OM particles formed in the cooling process after polluted plumes were emitted from coal combustion and biomass burning. Based on the Si-O-C ratio in OM particles, we estimated that 71 % of type 1–3 OM particles were associated with coal combustion. Our result suggests that coal combustion in residential stoves was a widespread source from urban to rural areas in NCP. Average OM thickness which correlates with the age of the air masses in type 6 particles only slightly increased from S1 to S2 to S3, suggesting that the L & M hazes were usually dry (relative humidity < 60 %) with weak photochemistry and heterogeneous reactions between particles and gases. We conclude that the direct emissions from these coal stoves without any pollution controls in rural areas and in urban outskirts contribute large amounts of primary OM particles to the regional L & M hazes in North China.
Atmospheric particulate matter is composed of diverse chemical compounds, both organic and inorganic matters. Organic aerosol particles are of two types: primary organic aerosol (POA), directly emitted from fossil fuel combustion, biomass burning, vehicular exhaust, and cooking; and secondary organic aerosol (SOA), formed from the oxidation of gaseous volatile organic compounds (Kanakidou et al., 2005). Organic aerosols account for 18–70 % of the non-refractory submicron aerosol particles in the atmosphere (Zhang et al., 2007). It is well known that organic aerosols affect the atmosphere through the interaction with reactive trace gases, water vapor, clouds, precipitation, and radiation (Fuzzi et al., 2006). Organic aerosols also influence the physical and chemical properties (e.g., size, light absorptivity, and hygroscopicity) of other particles; they directly affect visibility and climate by scattering and absorbing solar radiation (Pöschl, 2005; Kanakidou et al., 2005; Kulmala et al., 2004). Although most organic aerosol components are known to have a cooling effect on global climate, brown carbon in organic aerosols can absorb solar radiation at shorter wavelengths and lead to warming (Alexander et al., 2008). Moreover, many organic compounds (e.g., benzene, polycyclic aromatic hydrocarbons (PAHs), toluene) which are toxic to humans and other biological species have been found in atmospheric particles (Mauderly and Chow, 2008).
In recent years, haze episodes have become one of the most serious
environmental problems in China, following the rapid urbanization and
population growth in eastern China. The Ministry of Environmental Protection
of the People's Republic of China, on 1 January 2013, started to monitor daily
PM
Because of their unusually high PM
Various “bulk” analytical instruments have been used to study organic
aerosol particles during haze episodes. High-resolution time-of-flight
aerosol mass spectrometry (HR-AMS) was applied to determine the mass
concentrations and bulk composition of organic aerosols (Sun
et al., 2010). Gas chromatography–mass spectrometry (GC-MS) provided
chemical composition and structures of organics in aerosols (Fu et al.,
2012; Wang et al., 2009). It should be noted that bulk analytical techniques
only provide average properties of PM
To characterize organic aerosols in greater detail in L & M hazes, individual particles in the NCP in winter were analyzed using different individual particle instruments. Morphology, mixing state, and size of organic aerosols were systematically characterized and compared at the three sampling sites (background island site, mountain site, and urban site) in the same haze. This information enables the discussion of source and ageing mechanisms of OM particles, which leads to insights about the formation of regional wintertime L & M hazes in the NCP.
Regional haze layer covering the North China Plain: S1 (urban Jinan), S2 (top of Mt. Tai ), and S3 (Changdao Island) sites. The MODIS images on 14 and 19 December show a grey haze layer during the light and moderate regional hazes over the NCP.
PM
S1, an urban site in Jinan (53.9 m a.s.l.; 36.67
S2, at the top of Mt. Tai (1534 m a.s.l.; 36.251
S3, Changdao Island, the National Station for Background Atmospheric
Monitoring site (153 m a.s.l.; 38.19
PM
The JEOL JEM-2100 transmission electron microscopy operated at 200 kV with energy-dispersive X-ray spectrometry (TEM/EDX) was used to analyze individual particles. An energy-dispersive X-ray spectrometer (EDX) can detect elements heavier than carbon. EDX spectra were acquired for 15 s to minimize the potential beam damage. TEM grids are made of copper (Cu), so Cu is excluded in the analyses. The distribution of particles on the TEM grids was not uniform: coarser particles were deposited near the center and finer particles dispersed on the fringe. To make sure that the analyzed particles were representative of the entire size range, three to four areas were chosen from the center and periphery of the sampling spot on each sample.
After the TEM analysis, three typical samples were chosen for nanoscale
secondary ion mass spectrometer analysis (NanoSIMS 50L, CAMECA Instruments,
Geneviers, France), an ultrahigh vacuum technique for surface and thin-film
analysis at the Institute of Geology and Geophysics, Chinese Academy of
Sciences. In this study,
Atomic force microscopy (AFM) with a tapping mode analyzed aerosol particles
under ambient conditions. AFM, using a digital Nanoscope IIIa instrument, can
determine the three-dimensional morphology of particles. The AFM settings
contain imaging forces between 1 and 1.5 nN, scanning rates between 0.5 and
0.8 Hz, and scanning range sizes at 10
The definition and relationship of equivalent circle diameter (ECD,
Flowchart of individual aerosol particles classification in L & M haze episodes in NCP based on TEM/EDX. A total of 5090 individual particles were analyzed using TEM/EDX.
NanoSIMS-based ion intensity mappings of
Typical TEM images of different types of OM particles.
Aerosol particles were collected in three regional L & M hazes during 13–23
December 2014 (Fig. S4). Moderate Resolution Imaging Spectroradiometer
(MODIS) images on 14 and 19 December 2014 clearly display a regional haze
layer covering the three sampling sites in the NCP (Fig. 1). The average
PM
The average concentrations of OC, EC, OC
Based on morphology and chemical composition of individual particles using
TEM/EDX, we identified five types of particles: sulfates (including K-rich
sulfate and ammonium sulfate), fly ash/metal, mineral, soot, and OM-like
particles (Figs. 2 and S6). These results are consistent with previous
studies during the haze episodes in the NCP (Li et al., 2012; W. J. Li et al.,
2011b). In order to remove the interference of the carbon substrate on TEM
grids, a nanoSIMS was employed to verify OM-like particles through
Based on the morphology of OM particles, they were divided into six different types: spherical (type 1, Fig. 4a), near-spherical (type 2, Fig. 4b), irregular (type 3, Fig. 4c), domelike (type 4, Fig. 4d), dispersed-OM (type 5, Fig. 4e), and OM-coating (type 6, Fig. 4f). Here, the domelike particles look like transparent droplet-like particles in TEM images.
Because the high-resolution TEM images of individual particles can clearly display particle interior mixing structures, it allows us to identify OM particles based on their different shapes in OM-containing particles (Fig. S6). Figure 2 shows that the proportions of types 1–3 in OM particles was 73 %, following type 4 at 5 %, type 5 at 13 %, and type 6 at 14 % for the three sites as a whole. Further, we measured the projected area, the perimeter, the maximum projected length, and the maximum projected width of 967 selected OM particles. From these data, the sphericity (Sph) and aspect ratio (AR) of different types of OM particles were calculated, which characterize their shape and thereby imply their ageing during transport and their emission sources (W. Li et al., 2016). The Sph and AR were defined by the following formulas referred to by Li et al. (2013).
Sphericity describes the sphericity or
“roundness” of the measured object by using central moments. A sphericity
of 1 (the highest value) indicates a particle is perfectly spherical.
The maximum ratio of length and width of a
bounding rectangle for the measured object is the aspect ratio. An aspect
ratio of 1 (the lowest value) indicates a particle is not elongated in any
direction.
Average size, number, sphericity, and aspect ratio for different OM types at the three sampling sites.
Table 1 displays the Sph and AR of individual OM particles measured by the
iTEM software. At the three different sampling sites, OM particles were in
the fine range with diameters < 1
Typical TEM images of OM internally mixed particles
Number fractions of OM internally mixed particles at
Although we identified different types of OM particles in individual particles, 86 % of OM particles were internally mixed with non-OM particles, such as soot, mineral, fly ash, metal, and sulfate particles (Figs. 2 and S6). Based on their morphological mixing state, we discriminated four OM internally mixed particles: OM-soot (Figs. 5a and S8), OM-mineral (Fig. 5b), OM-fly ash/metal (Fig. 5c), and OM-sulfate particles (Fig. 5d–e). Our results show that 83 % of type 1–4 OM particles were attached to soot, mineral, sulfate, and metal particles; only 17 % of type 1–4 OM particles were externally mixed particles; and all the type 5–6 OM particles were internally mixed with sulfate particles (Fig. 2). In addition, the major OM internally mixed particles include 54 % OM-sulfate particles and 38 % OM-soot particles, followed by 5 % OM-mineral particles and 3 % OM-fly ash/metal particles (Fig. 2). Based on these analyses, a flowchart of classification of individual aerosol particles is summarized in Fig. 2.
Size distributions of OM-containing particles and OM particles
during L & M haze episodes.
Figure 6 shows number fractions of OM internally mixed particles in
different size bins from 0.04 to 4.5
Figure 7a shows size distributions of OM-containing particles at the three
sampling sites. Aerosol particles collected at S2 and S3 display a similar
peak at
TEM adequately characterized the morphology and mixing state of OM-containing particles in wintertime L & M hazes. We found that the type 1–3 OM (Fig. 4a–c) particles were most abundant in the hazes and that most of them were internally mixed with non-OM particles (Fig. 2). This result is consistent with one previous study which found abundant amorphous spherical OM particles in the outflow of a haze plume in East Asia (Zhu et al., 2013). Moreover, Li et al. (2012) found large amounts of type 1 OM particles in a coal-burning region in China's Loess Plateau in winter. However, some studies have only found abundant type 5–6 OM particles in the atmosphere and only a few type 1–3 OM particles in urban and remote mountain air in China (Li and Shao, 2010; Li et al., 2015). Based on these comparisons, we conclude that those type 1–3 OM particles were not directly emitted by vehicular emissions in the NCP.
It should be noted that recent studies did not find abundant type 1–3 OM particles at three sampling sites in haze episodes caused by industries, coal-fired power plants and vehicular emissions in spring and summer (W. J. Li et al., 2011a; Yuan et al., 2015). However, these abundant type 1–3 OM particles occurred in haze episodes over a coal-burning haze caused by house heating, heavy industries, and residential stoves in the China Loess Plateau in winter (Li et al., 2012). Based on the comparisons, we may exclude that large amount of type 1–3 OM particles could be directly emitted from coal-fired power plants and heavy industries. Zhang et al. (2008) suggested that industrial boilers had cleaner combustion with much less by-product of particulate carbon and with much lower levels of OM, while residential stoves had significantly higher emissions of carbonaceous particulate matter with emission rates 100 times higher than that of industrial boilers. In addition, recent studies have found that uncontrolled solid fuels combustion in households had a major effect on haze episode in Beijing through aerosol modeling and satellite monitoring (Ru et al., 2015; J. Liu et al., 2016). As a result, we believe that the type 1–3 OM particles can mainly be emitted by coal combustion and biomass burning in households while not emitted from vehicular, heavy industries, or coal-fired power plants in wintertime. In particular, the abundant near-spherical OM particles with higher Sph and lower AR indicate that these OM particles formed in cooling process after polluted plumes emitted from coal combustion and biomass burning.
Triangular diagram of weight ratios of Si-O-C based on TEM/EDX
data. A total of 39 OM particles are from coal combustion and 28 OM particles from corn
stalks combustion, as well as 281 OM particles produced from the haze samples in
this study. The three lines represent the connection of Si
Biomass burning and coal combustion both can produce types 1–3 OM particles and all contain a certain amount of Si in addition to C and O (Li et al., 2012; Posfai et al., 2004; Hand et al., 2005; Adachi and Buseck, 2011). Li et al. (2012) found that primary OM particles contain much higher Si from coal combustion than biomass burning. Although EDX can only obtain semiquantitative C, O, and Si from OM particles, the ratios of C-O-Si were comparable in different OM particles (Fig. 8). To evaluate OM sources in this study, we compared ratios of Si, O, and C in individual OM particles collected in haze (281 OM particles) and fresh OM particles from corn stalks (28 OM particles) and coal combustion (39 OM particles) conducted in the laboratory (see the Supplement). Figure 8 shows that the haze OM particles were more associated with coal combustion compared with corn stalks from the point of coverage. The haze OM line is located between corn stalks and coal combustion (Fig. 8). This result revealed that the Si ratio in individual OM particles is ordered as coal combustion > haze particles > corn stalks, and that 71 % of haze OM particles are associated with coal combustion. Based on the result, we can estimate that coal combustion contributes more type 1–3 OM particles than biomass burning in the wintertime L & M haze. This result is consistent with the source apportionment of OM particles based on their mass concentrations (Elser et al., 2016; Sun et al., 2013).
The relationship between the size of individual particles and their sulfate cores based on 366 OM-coating particles at sites S1, S3, and S3. The smaller slope represents the thicker OM coating. The number fractions of OM coating particles to OM-containing particles at three sampling sites are shown in the pie charts.
The complicated mixing structures of individual particles can be used to evaluate particle ageing mechanisms (W. Li et al., 2016). It is well known that S2 and S3, as the polluted background sites, received aged particles after long-range transport and that the urban site, S1, received more fresh particles. Indeed, OM-soot particles at S2 and S3 sites were internally mixed with sulfates but not at S1 (Figs. 6 and S8). To evaluate particle ageing processes, we measured OM coating thickness in type 6 OM-coating particles (e.g., Fig. 5e) because coating thickness on secondary particles can be used to infer particle ageing during transport (Moffet et al., 2010, 2013). In this study, OM coating thickness increased with particle size and their average values at the three sampling sites were ordered as S3 > S2 > S1 (i.e., 249.4 nm at S1, 274.5 nm at S2, and 305.6 nm at S3) (Fig. 9). The results suggest that particles with larger sizes underwent more ageing than the fine particles and that the particles at S3 underwent the most ageing. However, number fractions of type 6 OM particles were small at three sampling sites (2.9 % at S1, 14.8 % at S2, and 12 % at S3) (Fig. 9). This phenomenon may be caused by the weak atmospheric reactions for SOA formation in the whole haze layer.
Abundant type 1–3 OM particles at S1, S2, and S3 suggested that coal
combustion and biomass burning used for cooking and heating in the residential
sector in winter significantly contributed to the haze layer over the NCP.
Although heavy industrial and coal-fired plants emitted large amount of
gases such as SO
Our microscopic observations of individual particles provide direct evidence on the regional L & M haze formation in NCP, which is mainly caused by the residential coal stoves used for heating and cooking in winter. The result is consistent with the large-scale modeling studies and the field campaign in the NCP from recent studies (P. Liu et al., 2016; J. Liu et al., 2016; Ru et al., 2015). Therefore, we can conclude that these studies prove that these residential coal stoves in rural areas and in the urban outskirts have no pollution controls and directly emit particulate carbon and other pollutants. The emission control of residential coarse coal combustion is simply not regulated by the national environmental protection bureau, even though this bureau has made much recent progress in controlling emissions from heavy industries and coal-fired power plants.
Our study indicated that measures should be taken to control the wide range
of residential coal stoves in the NCP in wintertime. The type 1–3 OM
particles from residential stoves mainly consist of PAHs (Zhang
et al., 2008). In this study, we found that type 1–3 OM particles not only
occurred in 68 % of OM-containing aerosol particles but were also
concentrated on fine particles (< 1
All the data presented in this paper are available upon request. Please contact the corresponding author (liweijun_atmos@gmail.com or liweijun@sdu.edu.cn).
The authors declare that they have no conflict of interest.
We appreciate Peter Hyde's comments and proofreading. This work was funded by the National Natural Science Foundation of China (41575116 and 41622504), the National Key Project of MOST (JFYS2016ZY01002213), International Cooperation and Exchange project, the National Natural Science Foundation of China (41571130033), and the Shandong Provincial Science Fund for Distinguished Young Scholars, China (JQ201413). Edited by: T. Zhu Reviewed by: three anonymous referees