Elucidating the relationship between characteristics of aerosol particles and optical absorption is important to deepen the understanding of atmospheric chemistry. Aerosol particles play significant roles in climate forcing via their optical
absorption properties. However, the relationship between characteristics of
aerosol particles and optical absorption remains poorly understood. Aerosol
optical properties and morphologies were measured by a transmission electron
microscope (TEM), cavity ring-down spectrometer (CRDS), a nephelometer and an
Aethalometer in a urban site of Beijing from 24 May to 22 June. Five episodes
were categorized according to the meteorological conditions and composition.
The results showed that the clear episode (EP-2 and EP-4) featured as the low
aerosol optical depth (AOD
Aerosol particles are ubiquitous in the troposphere and exert an important
influence on global climate and the environment (Ramana et al., 2010). They
affect climate through direct scattering, transmission, and absorption of
radiation, or indirectly by acting as nuclei for cloud formation (Buseck and
Posfai, 1999). In addition, light extinction by aerosol particles can impair
visibility, both during extreme events such as dust storms and more widely in
the vicinity of urban regions, frequently leading to regional haze and fog
events (Wang et al., 2009a; Chameides et al., 1999; Sun et al., 2006; Saleh
et al., 2016). Inorganic salts and light-coloured organic carbon have a
“cooling effect” on the climate due to a decrease in the solar radiation
that reaches the Earth's surface (Buseck and Posfai, 1999). Soot aerosols,
mineral dust, and brown carbon are important absorbing aerosols that can lead
to global and regional warming effects (Buseck and Posfai, 1999; Bahadur et
al., 2012; Wang et al., 2014). The impact of aerosols on the Earth's climate
is a major uncertainty in climate change models as was emphasized in the
latest Intergovernmental Panel on Climate Change (IPCC) report (Solomon,
2007). It follows that understanding aerosol optical behaviour and associated
spatial and temporal variability is a necessary prerequisite to understanding
its role in climate and the environment (Langridge et al., 2012; Che et al.,
2014). Soot is a major contributor to Earth's radiative balance (Ramana et
al., 2010). Recent investigations involving direct atmospheric measurements
of soot aerosols suggest that these aerosols may have a global warming
potential second only to CO
Biomass burning is by far the largest source of primary, fine carbonaceous aerosols in the atmosphere (Habib et al., 2008). It is estimated to contribute 20 % of soot aerosols from biomass burning. Besides strongly absorbing soot particles, high amounts of brown organic carbon, such as “tar balls” or humic-like substances, can be emitted from biomass burning (Roden et al., 2006; Hand et al., 2005; Hoffer et al., 2006). Brown carbon has a significant absorbing component at short wavelengths that may be comparable to the soot absorption (Alexander et al., 2008; Bahadur et al., 2012). Consequently, organic carbon from biomass burning may also contribute to the warming potential of aerosols (Alexander et al., 2008). These large quantities of climate-related aerosols can persist in the atmosphere for several weeks and be transported over long distances. As a result, biomass burning aerosols have a significant impact on climate, which was considered to provide a major uncertainty in accurately predicting the effects of light-absorbing aerosols on the climate (Bahadur et al., 2012). Many field measurements in East Asia, South Asia and Africa have shown extensive biomass burning in these regions causes important perturbations in the Earth's atmosphere (Gustafsson et al., 2009; Alexander et al., 2008; Hand et al., 2005). Once biomass burning particles are mixed with other atmospheric components during aging and transport, such as sulfate and dust, solar absorption is further amplified due to the formation of internally mixed particles (Ramanathan et al., 2005). Such mixtures of absorbing and scattering aerosols at the regional scale are referred to as “ABCs” (abbreviated from “atmospheric brown clouds”) (Ramanathan et al., 2007). ABCs' radiative forcing can cool the surface, stabilize the atmosphere, and reduce evaporation and monsoonal rainfall. The large influence of ABCs on the climate and hydrological cycle changes has recently been demonstrated through model simulations (Ramanathan et al., 2005, 2007).
In the farmlands of eastern China such as those near Beijing, most wheat straw is burned in the field within 1 week after harvesting in preparation for rice cultivation during May and June. Emissions from the biomass burning are often transported and mixed with urban pollution, leading to degradation of air quality, impairment of visibility, and regional haze events (Li et al., 2010). Stagnation occurs during episodes of urban haze, when there is insufficient wind velocity to carry pollutants away from the city (Katrinak et al., 1993; Sun et al., 2006). During these periods of pollutant retention, haze particles aggregate, continue to collide and combine, resulting in larger average sizes and altered morphology (Li et al., 2010). Enhanced absorption is mainly brought about in the presence of high levels of non-absorbing hygroscopic aerosols such as sulfates, nitrates, and water-soluble organic carbon, as their hygroscopic nature favours internal mixing/core–shell formation (Bahadur et al., 2012). On the other hand, under the conditions of high atmospheric relative humidity (RH), the initially hydrophobic soot particles can become associated with hygroscopic materials, leading to increased scattering due to particle growth. In an extreme case, the coating material can cause the absorbing fractal soot to collapse, potentially changing optical behaviour, which further complicates this picture (Zhang et al., 2008; Langridge et al., 2012; Lei et al., 2014; Tan et al., 2016). Such changes cause both positive and negative effects on the interplay between the direct and indirect aerosol effects, making overall prediction of the radiative forcing difficult. At present, large uncertainties exist in estimates of the radiative forcing of haze particles because of the lack of detailed in situ measurements of the mixing state and the associated optical properties as a function of particle size and composition (Moffet and Prather, 2009). These uncertainties limit our ability to quantify the relative impacts of soot on climate, thus limiting our ability to make effective policy decisions.
In an attempt to address this knowledge gap, and in the absence of the opportunity for widespread field studies in eastern China, the experiments in this study were designed to simultaneously measure mixing states and optical properties of haze particles. The present analysis focused on the Beijing plume, which in addition to strong urban emissions is influenced by local agricultural emissions (Li et al., 2010). The light extinction and scattering coefficients were measured with a cavity ring-down spectrometer (CRDS) and a nephelometer, respectively. Absorption was calculated from the difference between extinction and scattering. Individual aerosol particles were identified with transmission electron microscopy (TEM). Back trajectory analyses suggest flow patterns consistent with long-range transport of agricultural smoke to the study site during periods when the sampling site was engulfed in severe haze and fog.
All of the ambient investigations of aerosol optical properties and TEM samplings
were conducted at the Institution of Atmospheric Physics (39
A in-house-designed cavity ring-down spectrometer (CRDS) was used to measure
the extinction coefficient of aerosols at 1 min intervals with an accuracy
of 0.1 Mm
An integrating nephelometer (TSI, model 3563) was operated to obtain aerosol
scattering coefficient at three different wavelengths (450, 550, and 700 nm)
and the flow rate was set at 5 L min
It is known that RH also has a profound impact on visibility (Chow et al., 2002); however, in this study the aerosols passed through a diffusion drying tube before the measurement of optical properties, and thus aerosol optical property measurements and TEM observations were both performed under dry conditions.
An Aethalometer (model AE-31, Magee Scientific) was employed to
simultaneously quantify BC concentration by calculating the optical
attenuation (absorbance) of light from light-emitting diode lamps emitting at
seven different wavelengths (370, 470, 520, 590, 660, 880 and 950 nm) every
5 min, with a typical half-width of 0.02
The uncertainty in measurement might originate from the multiple scattering in the filter fibres in the unloaded filter and in those particles embedded in the filters (Clarke et al., 2007; Jeong et al., 2004). The attenuation values were within the limit of an acceptable uncertainty, that is, no greater than 150 in the range of 75–125 at various wavelengths, verifying the reliability of the measurement. Moreover, the BC concentration was compared with the results of a multi-angle absorption photometer (MAAP, model 5012) and a particle soot absorption photometer (PSAP, Radiance Research), which shows great consistence.
Aerosol optical depth (AOD) data at the sampling site were based on the MODIS
(Moderate Resolution Imaging Spectroradiometer) retrieved data from a CIMEL
CE-318 sun photometer (AERONET/PHOTONS) at the Institute of Atmospheric Physics,
reflecting the amount of direct sunlight prevented from reaching the ground
by aerosol particles, by measuring the extinction of the solar beam. The AOD
value of the sampling site was downloaded from the AERONET website
(
Samples were made by collecting airborne particles onto copper TEM grids
coated with carbon film (carbon type-B, 300-mesh copper, Tianld Co., China)
using a single-stage cascade impactor with a 0.5 mm diameter jet nozzle at a
flow rate of 1.0 L
Sampling time and instantaneous meteorological state.
Individual aerosol samples were analysed using a high-resolution TEM (JEOL 2010, Japan) operated at 200 kV. TEM can obtain morphology, size, and mixing state of individual aerosol particles. Energy-dispersive X-ray spectrometer (EDS) can get the chemical compositions of the targeted particles. Cu and C were excluded from the copper TEM grid with carbon film. Details have been described in previous papers (Fu et al., 2012; Guo et al., 2014a). Particle sizes on the grid decrease from the centre to the periphery due to the limitation of sampler, and three to four round meshes were chosen from the centre to the periphery in a line to ensure the representativeness of the entire size range. Each mesh analyses three to four views. The average values of each mesh were used for statistics. The analysis was done by labour-intensive manual sorting of the particles. Nine grids, with a total of 1173 particles, have been analysed by TEM.
The NOAA/ARL Hybrid Single-Particle Lagrangian Integrated Trajectory model
(available at
Meteorological data were downloaded from Weather Underground
(
Categorization of five episodes. EP-1 features haze caused mainly by transportation of southern industrial pollution; EP-2 is clear, EP-3 has a frequent transition among haze, fog and clear conditions; EP-4 is mainly clear with rain interrupting; and EP-5 haze resulted mainly from the biomass burning (brown, green, and orange colours mean the haze, clear, and fog conditions, respectively).
Haze is usually defined as a weather phenomenon that lasts at least 4 h when the visibility is less than 10 km and RH is lower than 80 % (Sun et al., 2006), while fog was characterized with a higher RH, larger than 90 %, according to the Chinese Meteorological Administration. The sampling period was categorized into 5 episodes to observe the optical properties between different weather phenomena (Fig. 1). Although every episode contains a mixture of different types of pollution, the main origin can be discerned by studying the weather conditions, back trajectories and fire maps. The first episode (EP-1) was from 24 to 29 May, when a haze event occurred with the south wind bringing in the industrial pollution from the heavily polluted cities in the south, which conformed to the 3-day back trajectories shown in Fig. 2a, showing the air masses passing through Henan, Shandong, Hebei and Tianjin before arriving at the sampling site. Only scattered fire spots were observed during these days along the air mass pathway, suggesting little biomass burning emission interference. The second episode (EP-2) was in clear weather on 30 May. A heavy rain interrupted the previous haze; hence the air was cleaned up by rain washout. It was impacted by the air mass from the north region (Fig. 2b) as the air parcel from the north was relatively clean and the time was insufficient for a heavy accumulation. This episode could be viewed as the background. The third episode (EP-3) from 31 May to 9 June was changeable, with a variety of transitions between fog, haze and clear days. This was partly caused by the variable wind directions and air mass transference (Fig. 2c). When the wind is from the east, the back trajectories are across the Bohai Sea, and the air mass carries a high content of water vapour, facilitating the formation of fog, whereas when southerly wind is dominant, haze is likely to occur (Wang and Chen, 2014; Zhang et al., 2010). The following fourth episode (EP-4) from 10 to 16 June consists mainly of clear days with a small amount of dust. The dust's back trajectories originate from the north part (Fig. 2d), mostly travelling from the Siberian region, across eastern Mongolia and Inner Mongolia and finally arriving at the sampling site with little pollution. The last episode (EP-5) was from 17 to 21 June. Severe haze was observed during this time. Figure 2e shows that the air parcel pathway crossed dense fire spots, indicating a severe impact of the biomass burning. Every year after harvest, crop residue burning is extremely frequent in Anhui, Shandong and Henan provinces, which serve as important centres for the rice supply (Li et al., 2010). Therefore, the biomass burning emissions can be the main contributor to the haze formation in this episode.
The 3-day back-trajectory clusters of each episode, arriving at Beijing at the height of 100 m, together with the fire spot distribution of these periods (green, purple, red, and blue lines denote the air parcel with the height of 500, 1000, 2000, and 3000 m).
Variation in optical parameters during the study period.
Aerosol optical depth (AOD) is representative of the airborne aerosol loading
in the atmospheric column, which was also verified by a significant related
coefficient with PM
Nine categories of particles under the TEM view. The inserted
spectra are obtained by the EDS, and the grid-like images are acquired from
the selected-area diffraction.
Ångström exponent (Å) is a good indicator of aerosol size distribution, which decreases with the increase in particle size (Eck et al., 1999). The value is computed from pairs of AOD measurements at 700 nm with 450 nm, 700 nm with 550 nm and 550 nm with 450 nm, respectively. A high accordance is observed between each pair (Fig. 3b). The Å increases sharply to its highest value above 2.0 at EP-2, 45 times of the minimum value 0.044 observed in EP-5. This could be explained by the wet removal impact of the heavy rain. It is well known that rains wash out the coarse particles, resulting in a fine size distribution (Dey et al., 2004). The Å value during EP-4 fluctuated between 0.08 and 0.2. Since the rains are light and short, the clear days in EP-4 are more impacted by the northern air mass, which brings in a larger fraction of coarse dust particles. Comparatively, the Å value was lower in both the haze and fog periods, including EP-1, EP-3 and EP-5. Especially in the case of EP-5, the low Å value indicated that the biomass burning emission could contain more coarse particles. Such a scene is in contrast to the conclusion that the haze days were dominated by fine particles (Yan et al., 2008). It is likely caused by the high collision occurrences of fine particles along the long-range transport from the fire spots (Wang et al., 2009b). In comparison, the Å value during 2001 to 2005 in Beijing varied from between 0.04 and 1.06 (Yu et al., 2006). The lower limit is similar to the present field measurement, while the upper limit is much higher than this study. This could be attributed to the increase in fine particle emission contributed by more vehicles, waste incineration and industrial plants in the past years.
Single-scattering albedo (SSA),
Based on morphology and chemical composition, 1173 particles were classified into nine categories: S-rich (Fig. 4a), N-rich (Fig. 4b), mineral (Fig. 4c), K-rich (Fig. 4d), soot (Fig. 4e), tar ball (Fig. 4f), organic (Fig. 4g), metal (Fig. 4h) and fly ash (Fig. 4i). The classification is similar to the work reported by Li and Shao (2009).
The most common particles are sulfates and nitrates (Fig. 4a and b), which
are around 1.0
On the clear days, as the result of effects of northern air mass, dust
particles were relatively abundant. The size of dust particles (Fig. 4c) was
large, usually bigger than 1.0
As for the haze episode, K-rich particles (Li et al., 2010; Duan et al.,
2004; Engling et al., 2009), soot (Li et al., 2010), tar balls (Chakrabarty et
al., 2010; Bond, 2001) and organics (Lack et al., 2012) were more observed
under the TEM. K-rich particles (Fig. 4d) often existed as sulfate or
nitrate. A larger fraction of K-rich particles was observed in EP-5 than in the other periods. Together with the back trajectories and fire spot
maps, it was supposed that the regional haze that occurred in EP-5 was contributed
significantly by biomass burning. K-rich particles were characterized by
the irregular shape, which was unstable when exposed to the electron beam. KCl
was barely detected in the samples, even though it has been recommended that
KCl was internally mixed with K
It is well documented that soot (Fig. 4e) is vital to light absorption, which could alter regional atmospheric stability and vertical motions, large-scale circulation and precipitation with significant regional climate effects (Ramanathan et al., 2001; Jacobson, 2002). It was well characterized by a structure like an onion ring, resembling a fractal long chain as agglomerates of small spherical monomers (Li and Shao, 2009). The fresh soot was loose and externally mixed. However, after undergoing long-range transportation and aging in the atmosphere, soot became more compacted, with a slight increase in O concentration because of the photochemistry (Stanmore et al., 2001; Krasowsky et al., 2016). Meanwhile, soot generally attached to other particles on the surface or serves as the core for the formation of other particles.
Tar balls (Fig. 4f) were present as spherical carbon balls with a small fraction of O. These were thought to originate from smouldering combustion and have relatively strong absorption effects (Chakrabarty et al., 2010; Bond, 2001). Tar balls constituted a large fraction of the freshly emitted wildfire carbonaceous particles (China et al., 2013; Lack et al., 2012). However, they were seldom observed in the present work, even in EP-5, when there was severe biomass burning emission, which may be due to the difference in burning species and conditions.
Organic matter (Fig. 4g) identified by high-resolution TEM was an amorphous species, and was stable under the strong electron beam exposure. It could be traced to direct emission such as biomass burning (Lack et al., 2012) or to the second reaction between volatile organic compounds with ozone (Wang et al., 2012a). It can absorb radiation in the low visible and UV wavelengths (Chakrabarty et al., 2010; Clarke et al., 2007; Lewis et al., 2008; Hoffer et al., 2006). In addition, when attaching to soot as the core, organic matter can enhance absorption by internal mixing (Adachi and Buseck, 2008).
For the common haze and fog episodes, stagnated weather favours the
accumulation of pollutants, especially metal particles and fly ash (Hu et
al., 2015). Metal particles (Fig. 4h) were generally round and stable under
the TEM. Fly ash (Fig. 4i) was a dark sphere with a large size of
> 1
Percentages of nine particle components under clear, haze and fog conditions with different mixing states.
TEM typical views of the particles in clear (upper row), haze
(middle row) and fog episodes (bottom row). Ten components are marked with the coloured arrows. Images
Figure 5 shows the percentage of nine components in clear, haze and fog episodes under external mixing, internal mixing and adjacent states (partially internal mixing). About 28 % of particles were internally mixed during the foggy days, while about 52 % of particles exhibited external mixing state on clear days based on the TEM analysis. Mineral particles were likely to be externally mixed with K-rich particles and organic matter on clear days, while the external ratio of other particles was relatively lower, particularly during the haze and fog days. Li et al. (2010) showed that mineral particles generally displayed external association with organic matter or other particles. However, many fine particles including metal-bearing particles, fly ash and soot were often internally mixed with S-rich and K-rich particles, particularly during the fog-haze episodes. Shi et al. (2008) reported that rapid aging of fresh soot tended to appear during the fog-haze days, which were generally associated with ammonium sulfate. Heavily polluted air generally promoted the coagulation between S/K-rich particles and fine particles such as those of metal, soot, and fly ash (Li and Shao, 2009), which could explain the results. Additionally, haze and fog episodes held a higher possibility of collision and attachment due to the heavy particle loading and prolonged their remaining in the atmosphere (Li and Shao, 2009; Li et al., 2010), leading to a higher internal mixed-state percentage around 65 %.
BC concentrations converted from the data measured by the AE-31 and MAAP. Good correlation is observed.
The different morphologies of the particles collected from the different weather can be easily identified under the TEM, as shown in Fig. 6. Due to the washout effect of the heavy rain, the particles collected in the typical clear period of EP-2 were much smaller in size (Figs. 6a, b), which was in good agreement with the larger Ångström exponent. Coarse particles, such as dusts, were hardly observed, whereas a few K-rich particles were detected, whose shapes were small and cubic. Such particles could be explained by the coal combustion around the sampling site due to the slight presence of fire spots. In addition, the cubic shape of K-rich particles suggested that they had not undergone long-range transportation or severe photochemical reaction because cubic K-rich particles were generally generated as a result of the molten nature of the material at high temperatures (Ault et al., 2012). Likewise, soot was generally less oxidized in the EP-2 periods, maintaining fractional morphologies and external mixing. Small metal particles and amorphous Zn particles dominated the fine particles, which was ascribed to the industrial activity and/or waste incineration (Choël et al., 2006; Moffet et al., 2008).
In the EP-5 episode, the increased aerosol loading played a remarkable role in the enhancement of scattering coefficient and decrease in visibility (Kang et al., 2013; Charlson et al., 1987; Deng et al., 2008). Because of the high rate of aerosol collision, particles were larger than those on the clear days (Fig. 6c, d), leading to a smaller Ångström exponent. Almost all of the soot particles observed under the TEM were compact and adhesive. These were internally mixed with the K-rich particles, which were larger, rounder or with a coating of high-S components. As discussed above, they were probably transported from the southern crop residual burning and had undergone aging in the atmosphere, which was confirmed by the trajectories passing through intense fire spots. Due to the high concentration of soot, EP-5 were characterized by a high absorption coefficient, shown in Fig. 3.
The BC variations in the different weather types during the sampling period
are illustrated in Fig. 7. The preliminary component of BC could be viewed as
soot. High BC concentration was easily recognized in EP-5, with a mean value
of 12.8
The relationship between characteristics of aerosol particles and optical
properties is of importance to atmospheric chemistry research. However,
the relationship between characteristics of aerosol particles and optical
absorption remains poorly understood. Characteristics of aerosol optical
properties, morphologies and their relationship were studied in urban Beijing
during the clear, haze and fog episodes, sampled from 24 to 22 June 2012.
A transmission electron microscope (TEM), a cavity ring-down spectrometer
(CRDS), a nephelometer and an Aethalometer were employed to investigate the
corresponding changes in aerosol properties. Five episodes were
categorized according to the meteorological conditions and composition. The
results indicated that the clear episode (EP-2 and EP-4) was characterized by low aerosol optical depth (AOD
The authors declare that they have no conflict of interest.
This work was supported by the National Natural Science Foundation of China (no. 21577022, 21190053, 40975074), the Ministry of Science and Technology of China (2016YFC0202700), and the international cooperation project of Shanghai municipal government (15520711200). Edited by: T. Zhu Reviewed by: three anonymous referees