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Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations
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Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations Qiong Liu a,b , Xiaojun Ma a , Yanrong Yu a,c , Yan Qin a , Yonghang Chen a,d,∗ , Yanming Kang a , Hua Zhang e , Tiantao Cheng b , Yan Ling a , Yujie Tang a a
College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China c College of Physics and Electrical Information Engineering, Ningxia University, Yinchuan 750021, China d Institute of Desert Meteorology, China Meteorology Administration, Urumqi 830002, China e National Climate Center, China Meteorology Administration, Beijing 100081, China b
a r t i c l e
i n f o
Article history: Received 7 January 2016 Received in revised form 15 February 2017 Accepted 22 February 2017 Available online xxx Keywords: CALIPSO Aerosols Haze Vertical distribution Seasonal variation Urban agglomerations
a b s t r a c t Using CALIPSO (cloud-aerosol lidar and infrared pathfinder satellite observation) vertical observation data during haze periods from January 2007 to December 2008, we analyzed differences in aerosol characteristics near the surface, as well as in the middle troposphere between the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B) in China. One significant difference was that haze pollution in Area A was related to local and non-local aerosols, while in Area B it was related to local anthropogenic sources. In all seasons apart from autumn, aerosol pollution in Area A was more severe than in Area B, both near the surface and at higher altitudes. In Area A, non-spherical aerosols were dominant from 0 to 4 km in spring, summer, and winter; while in autumn, there were considerably high numbers of non-spherical aerosols below 0.5 km, and near-spherical aerosols from 0.5 to 4 km. In Area B, both near-spherical and non-spherical aerosols were common in all seasons. Moreover, aerosols with attenuated color ratios of 0–0.2 were more common in all seasons in Area A than in Area B, indicating that fine particle pollution in Area A was more serious than in Area B. Finally, relatively large aerosols linked to gravity settling appeared more frequently near the surface in Area A than in Area B. © 2017 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Haze pollution in urban agglomerations has become the most important environmental problem in China. Given the reduction of air quality (Chang, Song, & Liu, 2009; Tie & Cao, 2009; Wu et al., 2007) and its influence on human health (Tie, Wu, & Brasseur, 2009; Zhao et al., 2013), the Chinese government has adopted a series of mitigation measures, but needs more quantitative information on haze pollution to make further plans to control air pollution. Haze is a complex pollution problem, resulting from a combination of various factors, including fine particles, gaseous contaminants, and specific meteorological conditions (Tie et al., 2015; Zhang et al., 2015). In 2013, there were several long-lasting and severe haze
∗ Corresponding author at: College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China. E-mail address: [email protected] (Y. Chen).
pollution episodes within China. For example, when haze occurred in January 2013 in eastern China (Bi, Huang, Hu, Holben, & Guo, 2014), the maximum hourly mass concentration of PM2.5 observed in Beijing was higher than 600 g/m3 , although instantaneous concentrations at some sites reached 1000 g/m3 . In early December 2013, another severe haze episode occurred, covering almost half of China, and affecting more than 100 cities in 25 provinces. Shanghai was one of the most severely affected cities, having maximum hourly mass concentrations of PM2.5 close to 500 g/m3 . It is therefore evident that strategies aimed at educating the public about the effects of regional atmospheric pollution (especially haze in urban agglomerations), as well as further mitigation methods, are urgently needed. In this context, it is worthwhile to analyze and compare aerosol characteristics in various urban agglomerations. Such information will increase the accuracy of air quality forecasting and facilitate the control of regional atmospheric pollution. The Beijing–Tianjin–Hebei metropolitan region and the Yangtze River Delta region (hereafter Area A and Area B, respectively) are
http://dx.doi.org/10.1016/j.partic.2017.02.001 1674-2001/© 2017 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 1. Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B) selected to compare optical and microphysical properties of aerosols and the CALIPSO satellite ground track shown as lines (solid lines for daytime and dotted lines for nighttime).
two of the largest urban agglomerations in China. Urbanization, and in particular changes to urban surface characteristics, affect meteorological conditions, such as precipitation, air temperature, and wind speed, which are essential for atmospheric pollutant diffusion (Kaufmann et al., 2007; Rosenfeld, 2000; Tao et al., 2014; Wang, Zhang, Zhang, Cao, & Wang, 2013). Thus, air contaminants produced by industrial activities often accumulate in the urban atmosphere under adverse weather conditions. Given their different climatic and social-economic characteristics, the optical and physical properties of aerosols in these two urban agglomerations are found dissimilar. In addition, weather conditions differ markedly between these two regions. Although previous studies have analyzed the characteristics of aerosols in a single city or area within these agglomerations using surface-based lidar or space-based passive remote sensing instruments, few studies have compared the vertical distribution of aerosols in these two regions (Cheng, Chen, Li, Wang, & Guo, 2007; Chen et al., 2012; He & Mao, 2005; Liu, Geng et al., 2012). In contrast to observations from surface-based lidar and spacebased passive remote sensing instruments, the superior CALIPSO (cloud-aerosol lidar and infrared pathfinder satellite observation) data provides high-resolution vertical profiles of aerosol characteristics, including their extinctive and microphysical properties. The 532-nm and 1064-nm vertical resolutions are 30 m and 60 m within the range from −0.5 to 8.3 km. Moreover, CALIPSO is able to observe aerosols above bright surfaces, such as deserts and snow, and above bright clouds (Liu et al., 2004, 2008; Winker, Hunt, & McGill, 2007; Winker, Pelon, & McCormick, 2006) over the entire globe, day and night. It is worthwhile to note that under bright daytime conditions, these data have serious solar contamination. Thus, aerosol measurements are most reliable at nighttime under thin cloud. To profit from the advantages of using CALIPSO data, both temporal and spatial distributions of aerosols are used; this information is useful for improving model prediction accuracy and estimating aerosol radiative forcing (Huang, Huang, Bi, Zhang, & Zhang, 2008). In addition, although CALIPSO products have been
widely used to analyze the characteristics of clouds and aerosols (Chen et al., 2009; Chiang, Das, Shih, Liao, & Nee, 2011; Huang, Ge, & Weng, 2007; Huang, Minnis et al., 2007, 2008; Huang et al., 2009; Liu, Key, Ackerman, Mace, & Zhang, 2012; Liu et al., 2013; Ma, Gong, Wang, & Hu, 2011), studies have rarely focused on differentiating the characteristics of atmospheric aerosols during haze periods within urban agglomerations. Given that CALIPSO has become an effective tool to extract vertical information on aerosols in different cities at nearly the same time, without a large amount of capital, we used data from CALIPSO to assess temporal and spatial distributions of haze in two major urban agglomerations of China. Data and methods CALIPSO data Level 1B data products, processed by the National Aeronautics and Space Administration (NASA) CALIPSO Science Team, were derived from the CALIOP (cloud-aerosol lidar with orthogonal polarization) instrument, onboard the CALIPSO environmental satellite. CALIOP is a two-wavelength polarization sensitive lidar that provides vertical profiles of backscatter at 532 and 1064 nm during daytime and nighttime. The nearest CALIOP orbits pass over Beijing and Shanghai at approximately 02:00 LST (local standard time) and 14:00 LST each day. In this study, three parameters describing aerosol optical and microphysical properties are used: the total attenuated backscatter coefficient (TABC); the volume depolarization ratio (VDR); and the attenuated color ratio (ACR). TABC is a proxy for aerosol content. When the value of TABC is large, more aerosols are present within the atmosphere and pollution is more severe. TABCs are calculated using Eqs. (1)–(3) below (Chen et al., 2012; Hostetler et al., 2006): 2 ˇ532,Total (z) = [ˇ (z) + ˇ⊥ (z)]T532 (z),
(1)
2 ˇ532,⊥ (z) = ˇ⊥ (z)T532 (z),
(2)
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 2. Seasonal frequency distributions of the total attenuated backscatter coefficient (TABC) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B). 2 ˇ1064 (z) = ˇ1064 (z)T1064 (z),
(3)
where ˇ (z) and ˇ⊥ (z) are the horizontal and vertical components of the backscatter coefficients at altitude z; ˇ532 and ˇ1064 are the backscatter coefficients for wavelengths 532 and 1064 nm, respectively; and T2 523 (z) and T2 1064 (z) are the two-way atmospheric transmittances, given by (Chen et al., 2012; Hostetler et al., 2006):
T 2 (z) = exp
⎧ ⎨
zsat
m (z ) + a (z ) + O3 (z ) dz
−2
⎩
z
⎫ ⎬ ⎭
,
(4)
where m , a , and O3 are the extinction coefficients related to molecular scattering, aerosol scattering, and ozone absorption; and zsat is the altitude of the satellite. VDR and ACR reflect the regularity of aerosol shape and aerosol size, respectively (Liu et al., 2013). VDR is defined as the ratio of the perpendicular and parallel components of the received lidar signals at 532 nm (see Eq. (5)), while ACR is defined as the ratio of the intensity of backscatter at 1064 nm and of total backscatters at 532 nm (see Eq. (6)) (Liu et al., 2008; Xia & Zhang, 2006). VDR(z) =
ACR(z) =
(z) ˇ532,⊥ ˇ532, (z) (z) ˇ1064 ˇ532,Total (z)
(5)
Methods To obtain explicit information from these observational data, the concept of a “haze hour” is introduced. It is defined as an hour in which the averaged visibility is less than 10 km and the averaged relative humidity is less than 95%, without precipitation, floating dust, and smoke, dust or snow storms (Chen et al., 2012; China Meteorological Administration, 2010; Wu, 2008). Sixty cases were selected during haze periods from January 2007 to December 2008, in which the satellite passed over Area A (38.0◦ –41.5◦ N, 113.4◦ –119.9◦ E) or Area B (27.1◦ –35.0◦ N, 114.9◦ –122.4◦ E) (see Fig. 1). Data were excluded, when only a few detection points were obtained, or when the laser failed to detect the lower troposphere because of thick cloud. Generally, clouds have larger backscatter coefficients and higher attenuated color ratios, while aerosols have smaller backscatter coefficients and lower attenuated color ratios. Thus, when TABC and ACR values were greater than 4.5 × 10−3 km−1 sr−1 and 2, respectively, the corresponding altitude was noted, and all the data below this altitude were considered unreliable, and were excluded. To explore the differences in seasonal variation in aerosol characteristics, data were grouped into four seasons (spring: March– April–May; summer: June–July–August; autumn: September– October–November; and winter: December–January–February).
(6)
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 3. Seasonal vertical distributions of the total attenuated backscatter coefficient (TABC) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 4. Seasonal frequency distributions of the volume depolarization ratio (VDR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
Results and discussion Differences in the total attenuated backscatter coefficient between the two regions According to a previous study (Liu et al., 2008), the aerosol-related TABC of 532 nm varies between 8.0 × 10−4 and 4.5 × 10−3 km−1 sr−1 , except when there is optically thick aerosol layers or optically thin clouds (particularly cirrus). Fig. 2 shows the seasonal variations in TABC between the two urban regions. The xaxis gives four range gates for TABC values, while the y-axis shows the frequency of each range gate. For Area A, in Fig. 2(a), (b), and (d), the highest frequency TABCs were between 3.5 × 10−3 and 4.5 × 10−3 km−1 sr−1 , while the smallest were between 8 × 10−4 and 1.5 × 10−3 km−1 sr−1 in spring, summer, and winter. In autumn, values of TABC were mainly concentrated within the range of 1.5 × 10−3 –2.5 × 10−3 km−1 sr−1 , while values between 3.5 × 10−3 –4.5 × 10−3 km−1 sr−1 were least represented, as shown in Fig. 2(c). In Area B, TABCs showed a similar trend in spring, summer, autumn, and winter, where the smallest and largest frequencies ranged from 1.5 × 10−3 to 2.5 × 10−3 km−1 sr−1 and from 3.5 × 10−3 to 4.5 × 10−3 km−1 sr−1 , respectively. In comparison with Area A, Area B had more aerosols in spring, summer, and winter, than in autumn. Understanding the vertical characteristics of aerosols assists with their source apportionment. Vertical distributions of TABCs for all four seasons for the two urban regions are shown in Fig. 3. TABC ranges in Area A were much larger than in Area B along a verti-
cal direction, especially during summer and winter. Compared with Area A, the vertical distributions of TABCs in Area B decreased with increasing altitude. Thus, the average value of TABC below 1.5 km in each season was larger than above 1.5 km. Taking winter as an example, the average TABCs below 1.5 km and above 1.5 km were (24.6 ± 1.5) and (21.9 ± 0.5) km−1 sr−1 , respectively. In contrast, in Area A (except for Autumn), values of aerosol TABC were almost the same from the surface to 4 km, indicating that both local and nonlocal aerosol sources contributed to the haze pollution. As shown in Fig. 3(c), TABCs, which could represent aerosol loading, were quite low near the surface in autumn because of good air quality. In autumn, clouds were higher and the weather was mainly fine. Under these conditions, the heat island effect was conducive to the reduction of pollutant loads (Sun, Zhao, & Wu, 2012). As a result, aerosol loadings in autumn were considerably less than in other seasons. In contrast, it is often hot and rainy in summer, which is beneficial to the diffusion of contaminants. Under these conditions, the urban heat island causes low pressure over the city and produces atmospheric flow between urban and suburban areas, which transports pollutants back into the city. Results from Song et al. (2002) indicate that the mass concentrations of fine particles during continuous high temperature periods are 2–3 times higher than during lower temperature periods. Winter is typically very cold. Therefore, the consumption of energy for heating, such as burning of coal, produces plenty of aerosols. Sun, Wang, and Zhang (2009) showed that PM10 concentrations were relatively high during haze periods in Beijing.
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 5. Seasonal vertical distributions of the volume depolarization ratio (VDR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 6. Seasonal frequency distributions of the attenuated color ratio (ACR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
Volume depolarization ratio differences between the two regions The values of VDR indicate the shape irregularity of particles. VDR values for four common aerosol types (i.e., dust, maritime, smoke, and continental aerosols, Liu et al., 2008) are typically 0%–5% for maritime and continental aerosols, 0%–8% for smoke, and the largest range of 5%–40% for dust. As shown in Fig. 4, in Area A, aerosols consistent with each of these VDR ranges contributed more or less the same fractions in spring, summer, and winter. In contrast, in autumn, there were considerably more aerosols with smaller VDRs (ranging from 0% to 10%), accounting for a fraction of 67.01%, consistent with observations of Liu et al. (2014). Higher values are likely related to floating or blowing dust fractions, which often occur in Beijing during spring (Zhou & Zhang, 2003), yielding aerosols with more irregular shape. In contrast, values of VDR over Area B were almost all within the range of 0%–5% during all four seasons. Aerosols in Area B were clearly much more regular in shape than in Area A. As shown in Fig. 5, VDRs had different seasonal vertical variations in the two regions. It is clear that in Area B, the seasonalaveraged VDR values from 0 to 4 km were smaller than in Area A. The vertical distributions of VDRs in Area A and B were similar, except in autumn. In autumn, values of VDR in Area B were close to zero, indicating that maritime and continental aerosols were the main aerosol type. In Area A, values of VDR in autumn above 0.5 km were much smaller than those below 0.5 km, indicating that surface pollution contained irregularly-shaped aerosols from a local source. Compared with Area A, the VDRs in all seasons in Area B
were much smaller, while the volatilities of VDR from 0 to 4 km were relatively stable. Generally, when haze occurred, aerosols in Area A were considerably more irregular in shape below 4 km than in Area B.
Attenuated color ratio differences between the two regions Frequency distributions for ACRs for all four seasons are shown in Fig. 6 for Areas A and B. Typically, the larger the values of ACR, the larger the size of the particles (Liu et al., 2013). In Area A, the highest frequency ACR values were 0–0.2 in summer, autumn, and winter, while they were 0.6–0.8 in spring. Given that larger aerosols appeared more frequently in spring, suggests they are related to dust from North or Northwest China. However, in Area B, ACR values ranging from 0 to 0.2 were most frequent in all four seasons. Thus, fine particles were more common in haze in Area B. Seasonal variations of ACRs at different altitudes are shown in Fig. 7. Over Area A, aerosols with larger ACRs in spring, summer, autumn, and winter were concentrated in layers at 1.5–2, 0–1.2, 1–1.5, and 1.5–2.2 km, respectively. The seasonal vertical distributions of ACRs over Area B were markedly different. Aerosols with larger ACRs appeared below 0.5 km and above the surface in all four seasons. Moreover, the size of particles became smaller with increasing altitude, related to the gravity settling. There were also larger aerosols over Area B than Area A at lower altitudes (below altitudes of 1.5, 0.2, 0.8, and 1.2 km in spring, summer, autumn, and winter, respectively). Aside from aerosols at those altitudes,
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 7. Seasonal vertical distributions of the attenuated color ratio (ACR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
aerosols with smaller ACRs over Area A were generally more than those over Area B. According to previous research (Liu et al., 2008), continental and smoke aerosols containing black-carbon particles are almost spherical in shape and are usually small. In contrast, maritime aerosols are coarse-mode particles. With a large VDR value of around 17% and a peak ACR value of 0.8, dust aerosols are highly irregular in shape and larger than other aerosols (Dong
et al., 2007; Liu et al., 2008). Based on our analysis, we surmise that continental, smoke, and dust aerosols are the main aerosol types over Area A, while continental, smoke, and maritime aerosols are the main aerosol types over Area B. However, more detailed research involving ground-based laser radar and satellite would be required to quantify specific aerosol types in these regions.
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Conclusions Statistical features of aerosol optical and microphysical parameters (TABC, VDR, and ACR) in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B) during haze periods from January 2007 to December 2008 were analyzed. In particular, their seasonal and vertical variations in these two urban agglomerations were compared. The main results are as follows. (1) Although enormous numbers of aerosols occurred at altitudes of 0–4 km in both urban regions during haze periods, there were several distinctions between them. A major finding in this paper was that aerosols from local anthropogenic pollution sources were more numerous than from non-local sources in Area B, while aerosols from local and non-local sources played equivalent roles in haze pollution over Area A. Compared with Area B, aerosol pollution in Area A during spring, summer and winter was more serious than in autumn. (2) VDR values in Area A were generally larger than in Area B. This implied that aerosols with irregular shapes were more frequent in Area A. In Area A, both regular- and irregular-shaped aerosols contributed to spring, summer and winter aerosol pollutions, while in autumn there were considerably more regular-shaped aerosols, mainly concentrated above 0.5 km. In Area B, regularshaped aerosols were the main type in all four seasons. (3) In Area A, aerosols with relatively large ACR values occurred more frequently in spring, summer, and winter, likely related to the high-levels of dust in Beijing or dust events from Northwest China. In contrast, fine-particle pollution episodes occurred more frequently in Area B, where relatively small aerosols dominated in all four seasons. Moreover, the vertical distribution of ACRs in Area B suggested that aerosols at higher altitudes (1.5–4 km) were smaller than at lower altitudes (0–1.5 km). Acknowledgments The CALIPSO data were obtained from the NASA Langley Atmospheric Science Data Center. This work was funded by the Major Research plan of the National Natural Science Foundation of China (No. 91644211), the General Program of the National Natural Science Foundation of China (No. 41590870), the Chinese Universities Scientific Fund (CUSF-DH-D-2016055), the Natural Science Foundation of Shanghai (No. 16ZR1431700) and the opening project of the Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ). References Bi, J., Huang, J., Hu, Z., Holben, B. N., & Guo, Z. (2014). Investigating the aerosol optical and radiative characteristics of heavy haze episodes in Beijing during January of 2013. Journal of Geophysical Research: Atmospheres, 119 http://dx.doi.org/10. 1002/2014jd021757 Chang, D., Song, Y., & Liu, B. (2009). Visibility trends in six megacities in China 1973–2007. Atmospheric Research, 94(2), 161–167. Chen, Y. H., Mao, X. Q., Huang, J. P., Zhang, H., Tang, Q., Pan, H., et al. (2009). Vertical distribution characteristics of aerosol during a long-distance transport of heavy dust pollution. China Environmental Science, 29(5), 449–454 [in Chinese]. Chen, Y., Liu, Q., Geng, F., Zhang, H., Cai, C., Xu, T., et al. (2012). Vertical distribution of optical and micro-physical properties of ambient aerosols during dry haze periods in Shanghai. Atmospheric Environment, 50, 50–59. Cheng, S., Chen, D., Li, J., Wang, H., & Guo, X. (2007). The assessment of emissionsource contributions to air quality by using a coupled MM5-ARPS-CMAQ modeling system: A case study in the Beijing metropolitan region, China. Environmental Modelling & Software, 22(11), 1601–1616. Chiang, C. W., Das, S. K., Shih, Y. F., Liao, H. S., & Nee, J. B. (2011). Comparison of CALIPSO and ground-based lidar profiles over Chung-Li, Taiwan. Journal of Quantitative Spectroscopy and Radiative Transfer, 112(2), 197–203. China Meteorological Administration. (2010). Observation and forecasting levels of haze. QX T113-2010. China Meteorological Administration [in Chinese].
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Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations Qiong Liu a,b , Xiaojun Ma a , Yanrong Yu a,c , Yan Qin a , Yonghang Chen a,d,∗ , Yanming Kang a , Hua Zhang e , Tiantao Cheng b , Yan Ling a , Yujie Tang a a
College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China c College of Physics and Electrical Information Engineering, Ningxia University, Yinchuan 750021, China d Institute of Desert Meteorology, China Meteorology Administration, Urumqi 830002, China e National Climate Center, China Meteorology Administration, Beijing 100081, China b
a r t i c l e
i n f o
Article history: Received 7 January 2016 Received in revised form 15 February 2017 Accepted 22 February 2017 Available online xxx Keywords: CALIPSO Aerosols Haze Vertical distribution Seasonal variation Urban agglomerations
a b s t r a c t Using CALIPSO (cloud-aerosol lidar and infrared pathfinder satellite observation) vertical observation data during haze periods from January 2007 to December 2008, we analyzed differences in aerosol characteristics near the surface, as well as in the middle troposphere between the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B) in China. One significant difference was that haze pollution in Area A was related to local and non-local aerosols, while in Area B it was related to local anthropogenic sources. In all seasons apart from autumn, aerosol pollution in Area A was more severe than in Area B, both near the surface and at higher altitudes. In Area A, non-spherical aerosols were dominant from 0 to 4 km in spring, summer, and winter; while in autumn, there were considerably high numbers of non-spherical aerosols below 0.5 km, and near-spherical aerosols from 0.5 to 4 km. In Area B, both near-spherical and non-spherical aerosols were common in all seasons. Moreover, aerosols with attenuated color ratios of 0–0.2 were more common in all seasons in Area A than in Area B, indicating that fine particle pollution in Area A was more serious than in Area B. Finally, relatively large aerosols linked to gravity settling appeared more frequently near the surface in Area A than in Area B. © 2017 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Haze pollution in urban agglomerations has become the most important environmental problem in China. Given the reduction of air quality (Chang, Song, & Liu, 2009; Tie & Cao, 2009; Wu et al., 2007) and its influence on human health (Tie, Wu, & Brasseur, 2009; Zhao et al., 2013), the Chinese government has adopted a series of mitigation measures, but needs more quantitative information on haze pollution to make further plans to control air pollution. Haze is a complex pollution problem, resulting from a combination of various factors, including fine particles, gaseous contaminants, and specific meteorological conditions (Tie et al., 2015; Zhang et al., 2015). In 2013, there were several long-lasting and severe haze
∗ Corresponding author at: College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China. E-mail address: [email protected] (Y. Chen).
pollution episodes within China. For example, when haze occurred in January 2013 in eastern China (Bi, Huang, Hu, Holben, & Guo, 2014), the maximum hourly mass concentration of PM2.5 observed in Beijing was higher than 600 g/m3 , although instantaneous concentrations at some sites reached 1000 g/m3 . In early December 2013, another severe haze episode occurred, covering almost half of China, and affecting more than 100 cities in 25 provinces. Shanghai was one of the most severely affected cities, having maximum hourly mass concentrations of PM2.5 close to 500 g/m3 . It is therefore evident that strategies aimed at educating the public about the effects of regional atmospheric pollution (especially haze in urban agglomerations), as well as further mitigation methods, are urgently needed. In this context, it is worthwhile to analyze and compare aerosol characteristics in various urban agglomerations. Such information will increase the accuracy of air quality forecasting and facilitate the control of regional atmospheric pollution. The Beijing–Tianjin–Hebei metropolitan region and the Yangtze River Delta region (hereafter Area A and Area B, respectively) are
http://dx.doi.org/10.1016/j.partic.2017.02.001 1674-2001/© 2017 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Fig. 1. Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B) selected to compare optical and microphysical properties of aerosols and the CALIPSO satellite ground track shown as lines (solid lines for daytime and dotted lines for nighttime).
two of the largest urban agglomerations in China. Urbanization, and in particular changes to urban surface characteristics, affect meteorological conditions, such as precipitation, air temperature, and wind speed, which are essential for atmospheric pollutant diffusion (Kaufmann et al., 2007; Rosenfeld, 2000; Tao et al., 2014; Wang, Zhang, Zhang, Cao, & Wang, 2013). Thus, air contaminants produced by industrial activities often accumulate in the urban atmosphere under adverse weather conditions. Given their different climatic and social-economic characteristics, the optical and physical properties of aerosols in these two urban agglomerations are found dissimilar. In addition, weather conditions differ markedly between these two regions. Although previous studies have analyzed the characteristics of aerosols in a single city or area within these agglomerations using surface-based lidar or space-based passive remote sensing instruments, few studies have compared the vertical distribution of aerosols in these two regions (Cheng, Chen, Li, Wang, & Guo, 2007; Chen et al., 2012; He & Mao, 2005; Liu, Geng et al., 2012). In contrast to observations from surface-based lidar and spacebased passive remote sensing instruments, the superior CALIPSO (cloud-aerosol lidar and infrared pathfinder satellite observation) data provides high-resolution vertical profiles of aerosol characteristics, including their extinctive and microphysical properties. The 532-nm and 1064-nm vertical resolutions are 30 m and 60 m within the range from −0.5 to 8.3 km. Moreover, CALIPSO is able to observe aerosols above bright surfaces, such as deserts and snow, and above bright clouds (Liu et al., 2004, 2008; Winker, Hunt, & McGill, 2007; Winker, Pelon, & McCormick, 2006) over the entire globe, day and night. It is worthwhile to note that under bright daytime conditions, these data have serious solar contamination. Thus, aerosol measurements are most reliable at nighttime under thin cloud. To profit from the advantages of using CALIPSO data, both temporal and spatial distributions of aerosols are used; this information is useful for improving model prediction accuracy and estimating aerosol radiative forcing (Huang, Huang, Bi, Zhang, & Zhang, 2008). In addition, although CALIPSO products have been
widely used to analyze the characteristics of clouds and aerosols (Chen et al., 2009; Chiang, Das, Shih, Liao, & Nee, 2011; Huang, Ge, & Weng, 2007; Huang, Minnis et al., 2007, 2008; Huang et al., 2009; Liu, Key, Ackerman, Mace, & Zhang, 2012; Liu et al., 2013; Ma, Gong, Wang, & Hu, 2011), studies have rarely focused on differentiating the characteristics of atmospheric aerosols during haze periods within urban agglomerations. Given that CALIPSO has become an effective tool to extract vertical information on aerosols in different cities at nearly the same time, without a large amount of capital, we used data from CALIPSO to assess temporal and spatial distributions of haze in two major urban agglomerations of China. Data and methods CALIPSO data Level 1B data products, processed by the National Aeronautics and Space Administration (NASA) CALIPSO Science Team, were derived from the CALIOP (cloud-aerosol lidar with orthogonal polarization) instrument, onboard the CALIPSO environmental satellite. CALIOP is a two-wavelength polarization sensitive lidar that provides vertical profiles of backscatter at 532 and 1064 nm during daytime and nighttime. The nearest CALIOP orbits pass over Beijing and Shanghai at approximately 02:00 LST (local standard time) and 14:00 LST each day. In this study, three parameters describing aerosol optical and microphysical properties are used: the total attenuated backscatter coefficient (TABC); the volume depolarization ratio (VDR); and the attenuated color ratio (ACR). TABC is a proxy for aerosol content. When the value of TABC is large, more aerosols are present within the atmosphere and pollution is more severe. TABCs are calculated using Eqs. (1)–(3) below (Chen et al., 2012; Hostetler et al., 2006): 2 ˇ532,Total (z) = [ˇ (z) + ˇ⊥ (z)]T532 (z),
(1)
2 ˇ532,⊥ (z) = ˇ⊥ (z)T532 (z),
(2)
Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001
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Fig. 2. Seasonal frequency distributions of the total attenuated backscatter coefficient (TABC) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B). 2 ˇ1064 (z) = ˇ1064 (z)T1064 (z),
(3)
where ˇ (z) and ˇ⊥ (z) are the horizontal and vertical components of the backscatter coefficients at altitude z; ˇ532 and ˇ1064 are the backscatter coefficients for wavelengths 532 and 1064 nm, respectively; and T2 523 (z) and T2 1064 (z) are the two-way atmospheric transmittances, given by (Chen et al., 2012; Hostetler et al., 2006):
T 2 (z) = exp
⎧ ⎨
zsat
m (z ) + a (z ) + O3 (z ) dz
−2
⎩
z
⎫ ⎬ ⎭
,
(4)
where m , a , and O3 are the extinction coefficients related to molecular scattering, aerosol scattering, and ozone absorption; and zsat is the altitude of the satellite. VDR and ACR reflect the regularity of aerosol shape and aerosol size, respectively (Liu et al., 2013). VDR is defined as the ratio of the perpendicular and parallel components of the received lidar signals at 532 nm (see Eq. (5)), while ACR is defined as the ratio of the intensity of backscatter at 1064 nm and of total backscatters at 532 nm (see Eq. (6)) (Liu et al., 2008; Xia & Zhang, 2006). VDR(z) =
ACR(z) =
(z) ˇ532,⊥ ˇ532, (z) (z) ˇ1064 ˇ532,Total (z)
(5)
Methods To obtain explicit information from these observational data, the concept of a “haze hour” is introduced. It is defined as an hour in which the averaged visibility is less than 10 km and the averaged relative humidity is less than 95%, without precipitation, floating dust, and smoke, dust or snow storms (Chen et al., 2012; China Meteorological Administration, 2010; Wu, 2008). Sixty cases were selected during haze periods from January 2007 to December 2008, in which the satellite passed over Area A (38.0◦ –41.5◦ N, 113.4◦ –119.9◦ E) or Area B (27.1◦ –35.0◦ N, 114.9◦ –122.4◦ E) (see Fig. 1). Data were excluded, when only a few detection points were obtained, or when the laser failed to detect the lower troposphere because of thick cloud. Generally, clouds have larger backscatter coefficients and higher attenuated color ratios, while aerosols have smaller backscatter coefficients and lower attenuated color ratios. Thus, when TABC and ACR values were greater than 4.5 × 10−3 km−1 sr−1 and 2, respectively, the corresponding altitude was noted, and all the data below this altitude were considered unreliable, and were excluded. To explore the differences in seasonal variation in aerosol characteristics, data were grouped into four seasons (spring: March– April–May; summer: June–July–August; autumn: September– October–November; and winter: December–January–February).
(6)
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Fig. 3. Seasonal vertical distributions of the total attenuated backscatter coefficient (TABC) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
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Fig. 4. Seasonal frequency distributions of the volume depolarization ratio (VDR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
Results and discussion Differences in the total attenuated backscatter coefficient between the two regions According to a previous study (Liu et al., 2008), the aerosol-related TABC of 532 nm varies between 8.0 × 10−4 and 4.5 × 10−3 km−1 sr−1 , except when there is optically thick aerosol layers or optically thin clouds (particularly cirrus). Fig. 2 shows the seasonal variations in TABC between the two urban regions. The xaxis gives four range gates for TABC values, while the y-axis shows the frequency of each range gate. For Area A, in Fig. 2(a), (b), and (d), the highest frequency TABCs were between 3.5 × 10−3 and 4.5 × 10−3 km−1 sr−1 , while the smallest were between 8 × 10−4 and 1.5 × 10−3 km−1 sr−1 in spring, summer, and winter. In autumn, values of TABC were mainly concentrated within the range of 1.5 × 10−3 –2.5 × 10−3 km−1 sr−1 , while values between 3.5 × 10−3 –4.5 × 10−3 km−1 sr−1 were least represented, as shown in Fig. 2(c). In Area B, TABCs showed a similar trend in spring, summer, autumn, and winter, where the smallest and largest frequencies ranged from 1.5 × 10−3 to 2.5 × 10−3 km−1 sr−1 and from 3.5 × 10−3 to 4.5 × 10−3 km−1 sr−1 , respectively. In comparison with Area A, Area B had more aerosols in spring, summer, and winter, than in autumn. Understanding the vertical characteristics of aerosols assists with their source apportionment. Vertical distributions of TABCs for all four seasons for the two urban regions are shown in Fig. 3. TABC ranges in Area A were much larger than in Area B along a verti-
cal direction, especially during summer and winter. Compared with Area A, the vertical distributions of TABCs in Area B decreased with increasing altitude. Thus, the average value of TABC below 1.5 km in each season was larger than above 1.5 km. Taking winter as an example, the average TABCs below 1.5 km and above 1.5 km were (24.6 ± 1.5) and (21.9 ± 0.5) km−1 sr−1 , respectively. In contrast, in Area A (except for Autumn), values of aerosol TABC were almost the same from the surface to 4 km, indicating that both local and nonlocal aerosol sources contributed to the haze pollution. As shown in Fig. 3(c), TABCs, which could represent aerosol loading, were quite low near the surface in autumn because of good air quality. In autumn, clouds were higher and the weather was mainly fine. Under these conditions, the heat island effect was conducive to the reduction of pollutant loads (Sun, Zhao, & Wu, 2012). As a result, aerosol loadings in autumn were considerably less than in other seasons. In contrast, it is often hot and rainy in summer, which is beneficial to the diffusion of contaminants. Under these conditions, the urban heat island causes low pressure over the city and produces atmospheric flow between urban and suburban areas, which transports pollutants back into the city. Results from Song et al. (2002) indicate that the mass concentrations of fine particles during continuous high temperature periods are 2–3 times higher than during lower temperature periods. Winter is typically very cold. Therefore, the consumption of energy for heating, such as burning of coal, produces plenty of aerosols. Sun, Wang, and Zhang (2009) showed that PM10 concentrations were relatively high during haze periods in Beijing.
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Fig. 5. Seasonal vertical distributions of the volume depolarization ratio (VDR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
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Fig. 6. Seasonal frequency distributions of the attenuated color ratio (ACR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
Volume depolarization ratio differences between the two regions The values of VDR indicate the shape irregularity of particles. VDR values for four common aerosol types (i.e., dust, maritime, smoke, and continental aerosols, Liu et al., 2008) are typically 0%–5% for maritime and continental aerosols, 0%–8% for smoke, and the largest range of 5%–40% for dust. As shown in Fig. 4, in Area A, aerosols consistent with each of these VDR ranges contributed more or less the same fractions in spring, summer, and winter. In contrast, in autumn, there were considerably more aerosols with smaller VDRs (ranging from 0% to 10%), accounting for a fraction of 67.01%, consistent with observations of Liu et al. (2014). Higher values are likely related to floating or blowing dust fractions, which often occur in Beijing during spring (Zhou & Zhang, 2003), yielding aerosols with more irregular shape. In contrast, values of VDR over Area B were almost all within the range of 0%–5% during all four seasons. Aerosols in Area B were clearly much more regular in shape than in Area A. As shown in Fig. 5, VDRs had different seasonal vertical variations in the two regions. It is clear that in Area B, the seasonalaveraged VDR values from 0 to 4 km were smaller than in Area A. The vertical distributions of VDRs in Area A and B were similar, except in autumn. In autumn, values of VDR in Area B were close to zero, indicating that maritime and continental aerosols were the main aerosol type. In Area A, values of VDR in autumn above 0.5 km were much smaller than those below 0.5 km, indicating that surface pollution contained irregularly-shaped aerosols from a local source. Compared with Area A, the VDRs in all seasons in Area B
were much smaller, while the volatilities of VDR from 0 to 4 km were relatively stable. Generally, when haze occurred, aerosols in Area A were considerably more irregular in shape below 4 km than in Area B.
Attenuated color ratio differences between the two regions Frequency distributions for ACRs for all four seasons are shown in Fig. 6 for Areas A and B. Typically, the larger the values of ACR, the larger the size of the particles (Liu et al., 2013). In Area A, the highest frequency ACR values were 0–0.2 in summer, autumn, and winter, while they were 0.6–0.8 in spring. Given that larger aerosols appeared more frequently in spring, suggests they are related to dust from North or Northwest China. However, in Area B, ACR values ranging from 0 to 0.2 were most frequent in all four seasons. Thus, fine particles were more common in haze in Area B. Seasonal variations of ACRs at different altitudes are shown in Fig. 7. Over Area A, aerosols with larger ACRs in spring, summer, autumn, and winter were concentrated in layers at 1.5–2, 0–1.2, 1–1.5, and 1.5–2.2 km, respectively. The seasonal vertical distributions of ACRs over Area B were markedly different. Aerosols with larger ACRs appeared below 0.5 km and above the surface in all four seasons. Moreover, the size of particles became smaller with increasing altitude, related to the gravity settling. There were also larger aerosols over Area B than Area A at lower altitudes (below altitudes of 1.5, 0.2, 0.8, and 1.2 km in spring, summer, autumn, and winter, respectively). Aside from aerosols at those altitudes,
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Fig. 7. Seasonal vertical distributions of the attenuated color ratio (ACR) during haze periods in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B).
aerosols with smaller ACRs over Area A were generally more than those over Area B. According to previous research (Liu et al., 2008), continental and smoke aerosols containing black-carbon particles are almost spherical in shape and are usually small. In contrast, maritime aerosols are coarse-mode particles. With a large VDR value of around 17% and a peak ACR value of 0.8, dust aerosols are highly irregular in shape and larger than other aerosols (Dong
et al., 2007; Liu et al., 2008). Based on our analysis, we surmise that continental, smoke, and dust aerosols are the main aerosol types over Area A, while continental, smoke, and maritime aerosols are the main aerosol types over Area B. However, more detailed research involving ground-based laser radar and satellite would be required to quantify specific aerosol types in these regions.
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Conclusions Statistical features of aerosol optical and microphysical parameters (TABC, VDR, and ACR) in the Beijing–Tianjin–Hebei metropolitan region (Area A) and the Yangtze River Delta region (Area B) during haze periods from January 2007 to December 2008 were analyzed. In particular, their seasonal and vertical variations in these two urban agglomerations were compared. The main results are as follows. (1) Although enormous numbers of aerosols occurred at altitudes of 0–4 km in both urban regions during haze periods, there were several distinctions between them. A major finding in this paper was that aerosols from local anthropogenic pollution sources were more numerous than from non-local sources in Area B, while aerosols from local and non-local sources played equivalent roles in haze pollution over Area A. Compared with Area B, aerosol pollution in Area A during spring, summer and winter was more serious than in autumn. (2) VDR values in Area A were generally larger than in Area B. This implied that aerosols with irregular shapes were more frequent in Area A. In Area A, both regular- and irregular-shaped aerosols contributed to spring, summer and winter aerosol pollutions, while in autumn there were considerably more regular-shaped aerosols, mainly concentrated above 0.5 km. In Area B, regularshaped aerosols were the main type in all four seasons. (3) In Area A, aerosols with relatively large ACR values occurred more frequently in spring, summer, and winter, likely related to the high-levels of dust in Beijing or dust events from Northwest China. In contrast, fine-particle pollution episodes occurred more frequently in Area B, where relatively small aerosols dominated in all four seasons. Moreover, the vertical distribution of ACRs in Area B suggested that aerosols at higher altitudes (1.5–4 km) were smaller than at lower altitudes (0–1.5 km). Acknowledgments The CALIPSO data were obtained from the NASA Langley Atmospheric Science Data Center. This work was funded by the Major Research plan of the National Natural Science Foundation of China (No. 91644211), the General Program of the National Natural Science Foundation of China (No. 41590870), the Chinese Universities Scientific Fund (CUSF-DH-D-2016055), the Natural Science Foundation of Shanghai (No. 16ZR1431700) and the opening project of the Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ). References Bi, J., Huang, J., Hu, Z., Holben, B. N., & Guo, Z. (2014). Investigating the aerosol optical and radiative characteristics of heavy haze episodes in Beijing during January of 2013. Journal of Geophysical Research: Atmospheres, 119 http://dx.doi.org/10. 1002/2014jd021757 Chang, D., Song, Y., & Liu, B. (2009). Visibility trends in six megacities in China 1973–2007. Atmospheric Research, 94(2), 161–167. Chen, Y. H., Mao, X. Q., Huang, J. P., Zhang, H., Tang, Q., Pan, H., et al. (2009). Vertical distribution characteristics of aerosol during a long-distance transport of heavy dust pollution. China Environmental Science, 29(5), 449–454 [in Chinese]. Chen, Y., Liu, Q., Geng, F., Zhang, H., Cai, C., Xu, T., et al. (2012). Vertical distribution of optical and micro-physical properties of ambient aerosols during dry haze periods in Shanghai. Atmospheric Environment, 50, 50–59. Cheng, S., Chen, D., Li, J., Wang, H., & Guo, X. (2007). The assessment of emissionsource contributions to air quality by using a coupled MM5-ARPS-CMAQ modeling system: A case study in the Beijing metropolitan region, China. Environmental Modelling & Software, 22(11), 1601–1616. Chiang, C. W., Das, S. K., Shih, Y. F., Liao, H. S., & Nee, J. B. (2011). Comparison of CALIPSO and ground-based lidar profiles over Chung-Li, Taiwan. Journal of Quantitative Spectroscopy and Radiative Transfer, 112(2), 197–203. China Meteorological Administration. (2010). Observation and forecasting levels of haze. QX T113-2010. China Meteorological Administration [in Chinese].
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Please cite this article in press as: Liu, Q., et al. Comparison of aerosol characteristics during haze periods over two urban agglomerations in China using CALIPSO observations. Particuology (2017), http://dx.doi.org/10.1016/j.partic.2017.02.001