Long-term trend and spatiotemporal variations of haze over China by satellite observations from 1979 to 2013

Atmospheric Environment 119 (2015) 362e373

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Long-term trend and spatiotemporal variations of haze over China by satellite observations from 1979 to 2013 Xingying Zhang a, *, Ling Wang a, Weihe Wang a, Dongjie Cao a, Xi Wang a, Dianxiu Ye b a b

National Satellite Meteorological Center, China Meteorological Administration, Beijing, 100081, China National Climate Center, China Meteorological Administration, Beijing, 100081, China

h i g h l i g h t s  Long-term observation of haze in China by satellite.  Satellite AAI products are demonstrated to be used in haze monitoring in China.  Haze shows significant increase over east and northeast of China in recent ten years.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 February 2015 Received in revised form 17 August 2015 Accepted 19 August 2015 Available online 22 August 2015

With the fast development of economy and industry in the past thirty years, many large cities in the eastern and southwestern areas of China are experiencing increased haze events and atmospheric pollution, which causes significant impacts on the regional environment, human health, and even climate. The long-term trend and spatiotemporal variations of haze over China during recent 30 years are investigated using TOMS AAI products. In addition, the heavy haze events that occurred in January 2013 over eastern China are explored using AAI products from TOU on board FY-3A. Validation results show that satellite AAI products can be used for haze monitoring since it is sensitive to the carbonaceous aerosol, which is one of the main components of haze. In China, the high AAI values (>1.0) mainly located in the main four areas with intense anthropogenic activities, except for the desert region in Northwestern China. In the eastern and northeastern region, AAI peaks dominate in spring before 2005 since those areas were always affected by dust in spring. However, after 2005, AAI peaks appear in winter over eastern China because of haze. Moreover, in the northeastern region, AAI peaks dominate in winter with a secondary peak in spring because this area is affected by both dust and haze. In the southern region, the AAI peaks always dominate in spring since the high-level air pollution often occur in spring, but a decreasing trend is acquired during recent ten years. Over eastern China and northeastern China, AAI shows an increasing trend during recent 30 years in winter, which reveals that the haze over these areas is strengthen. A case study result shows that the heavy haze events occurred in January 2013 in eastern China can be clearly identified from the AAI products of TOU/FY-3A. The daily coverage area with AAI > 3.0 peaks at five periods at this time, i.e. Jan. 7e8, Jan. 13, Jan. 18, Jan. 23, and, Jan. 28e29, which agrees well with the haze events recorded by in-situ measurements. © 2015 Published by Elsevier Ltd.

Keywords: Haze Satellite observations Long-term trend China

1. Introduction Heavy haze shrouded most parts of central-eastern China in January 2013, which attracts a lot of attention over the world. Haze is a worldwide phenomenon that has garnered a lot of attention for

* Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.atmosenv.2015.08.053 1352-2310/© 2015 Published by Elsevier Ltd.

its significant effects on visibility, cloud formation, public health, and even global climate (Okada et al., 2001; Menon et al., 2002; Yadav et al., 2003; Andre, 2005). The urbanization of China, coupled with rapid industrial development, has led to serious air pollution problems in many large cities in China (Che et al., 2009; Tan et al., 2009a,b; Ma et al., 2010; Li et al., 2011; Zhao et al., 2013). Sulfate, nitrate and carbonaceous aerosols are thought to be the most important contributors of haze and they were significantly correlated during haze events (Brown et al., 2002; Jacobson, 2001;

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Kang et al., 2004; Alves et al., 2007; Kim et al., 2008; Hou et al., 2011). Carbonaceous aerosols are released by human activities over the globe, but are attracting special concern in areas with rapid economic growth and high population density (Zhang et al., 2008). Most previous studies on haze were based on the in-situ measurement and chemical analysis, which have shown that in many Chinese cities carbonaceous pollutants account for 20e50% of mass of particulate matter less than 2.5 mm in diameter (PM2.5) during the haze (He et al., 2001; Cao et al., 2003, 2005; Dan et al., 2004; Shen et al., 2007, 2009; Che et al., 2007, 2014). However, the limited sampling sites could not distribute equally over a large area such as China. In addition, there is not a long-term record from the in-situ measurement. Haze trend was only inferred from long-term records of visual range from limited in-situ measurements (Fan et al., 2005; Che et al., 2009). Satellite observations can provide wide coverage data and long-term records by serial satellite products. But traditionally, aerosol optical thickness measurements from the visible and infrared could not generate the aerosol result because the large surface albedos of many land types and cloud make retrieval of aerosols difficult over these regions (de Graaf et al., 2005). Usually the monitoring results of aerosol optical thickness are absence during heavy haze events. The Absorbing Aerosol Index (AAI) from satellite measurements in the ultraviolet (UV) show positive values generally represent absorbing aerosols (mineral dust and carbonaceous aerosols) while small or negative values represent non-absorbing aerosols and clouds, which can give a light sight to monitor the aerosol during haze events because carbonaceous aerosol is the main comment of haze (Torres et al., 2007). The Absorbing Aerosol Index (AAI) can been gotten by separating the spectral contrast at two ultraviolet (UV) wavelengths caused by absorbing aerosols from that of other effects, including molecular Rayleigh scattering, surface reflection, gaseous absorption and aerosol, and cloud scattering (Torres et al., 1998). In this study, the AAI products from TOMS (de Graaf et al., 2005) have been used to analysis the trend and spatiotemporal variations of haze in China. Special case study of the characteristics of heavy haze occurred in the central-eastern China in January 2013 was explored by AAI from Fengyun-3A Meteorological Satellite, which can provide daily coverage of China.

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2. Data 2.1. Satellite data The Absorbing Aerosol Index (AAI) is a well-known measure of UV-absorbing aerosols (Chiapello et al., 1999; Alpert and Ganor, 2001; Pandithurai et al., 2001; Spichtinger et al., 2001; Prospero et al., 2002; Colarco et al., 2002; Moulin and Chiapello, 2004; Hsu et al., 1996; Gleason et al., 1998; Hsu et al., 1999; Goloub and Arino, 2000; Darmenova et al., 2005). The Absorbing Aerosol Index (AAI) product from TOMS is the longest available aerosol record, which is used extensively for the analysis of aerosol impact on climate and studies of heavy dust, smoke, and volcanic eruption events (Hsu et al., 1996; Herman et al., 1997; Seftor et al., 1997; Chiapello et al., 1999; Pandithurai et al., 2001; Alpert and Ganor, 2001; Moulin and Chiapello, 2004). However, from May 1993 to June 1996 no TOMS observations were available. Because of the latest TOMS AAI data from Ozone Monitoring Instrument (OMI) onboard Aura satellite launched in 2004 shows poor performance with daily global coverage for the degradation of the scanners after 2012, AAI from Total Ozone Unit (TOU) onboard FY-3A was used to investigate the heavy haze event in China occurred in January 2013. The TOU is an experimental ultraviolet (UV) spectrometer onboard FY-3A, the second generation of Chinese meteorological polar-orbiting satellites launched in May 2008. The TOU was designed to measure the solar ultraviolet radiation backscattered from Earth and its atmosphere in six discrete bands, which show good performance to monitor global total ozone (Weihe Wang et al., 2010, 2011). The AAI products from TOU were obtained based on the same retrieval algorithm of TOMS products, which show a good performance for daily global coverage. Hence, for better analyzing the day-by-day changes of the short-term haze event, the AAI products from TOU instead of OMI were applied for the case study on the heavy haze event occurred in January 2013. Since the index product of AAI is different in different payloads, the long serial TOMS data was used for the long trend analysis. In order to compare with the haze record of the color picture from satellite, the picture result from MERSI/FY-3A is also used in this study. MERSI (Medium Resolution Spectral Imager) is one of the instruments onboard FY-3A satellite just like MODIS.

Fig. 1. Map showing the locations of two typical in-situ sites, Shangdianzi (117.120E, 40.650N) and Shijiazhuang (114.210E, 38.040N), in the study area with the mean tropospheric NO2 column concentration during 2012, which is derived from The Global Ozone Monitoring Experiment-2 (GOME -2) onboard MetOp-A satellite (http://www.temis.nl/ airpollution/no2.html).

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Fig. 2. Two typical haze events results from satellite and in-situ measurement.

2.2. In-situ measurements

3. Results and discussions

In order to assure that AAI data can provide a good signal for haze observation, PM2.5 and visibility data from Shangdianzi (117.12 E, 40.65 N) and Shijiazhuang (114.21 E, 38.04 N) sites as shown in Fig. 1 were used for comparison with AAI data in the North China Plain (112e120 E, 36e43 N) during the heavy haze event. Shijiazhuang is the capital of Hebei province. Shangdianzi is a regional background station of atmospheric compositions used as the representative site of North China Plain, which is 100 km northeast to the urban area of Beijing and located in the boundary of plain area. It is also one of the Global Atmosphere Watch (GAW) stations, and the detailed information for Shangdianzi had been previously reported (Zhao et al., 2009).

3.1. The relationship between AAI and haze aerosol Three important UV-absorbing aerosol types are desert dust aerosols, biomass burning aerosols, and volcanic mineral aerosols (Martin, 2006). Mineral dust aerosols are UV-absorbing, mainly due to the presence of hematite and other iron oxides. Black carbon (BC), or soot, is the major contributor of biomass burning. In addition to natural biomass burning, some anthropogenic incomplete combustion such as residential coal and motor vehicle fuel can also generate BC (Bond et al., 2013). Ground-based sampling and remote sensing measurements both reveal that mineral aerosols and carbon aerosols (including BC and non-absorbing organic carbon aerosol) are the major constituents of haze aerosol particles

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Fig. 4. Spatial distributions of AAI in China.

in china (Hou et al., 2011; Zhang et al., 2008; Li et al., 2013a,b). Therefore, the AAI product can be used as an index to monitor haze in China. Fig. 2 shows two typical haze events results. The Fig. 2 (a) is the results of AAI from TOU/FY-3A (left) and the artificial in-situ haze monitoring results (right) on Feb. 25 2013. The Fig. 2 (b) is the results of AAI from TOU/FY-3A (left) and the picture from MERSI/FY3A (right) on Mar. 10 2014. The spatial distribution pattern of AAI from TOU/FY-3A corresponds well with the in-situ haze monitoring results and the picture from satellite. In order to further make sure that the satellite AAI data can provide a good index for haze observation, comparison AAI data with the in-situ measurements (detailed information in Section 2.2) has been done. The result reveals (Fig. 3) that AAI shows significant positive correlation with PM2.5, and inverse correlation with visibility during the heave haze in Jan. 2013. In addition, the AAI from TOU is more sensitive than that from OMI since OMI shows poor performance due to the degradation of the scanners.

3.2. Spatial distributions of AAI in China The spatial distributions of annual TOMS mean AAI for two periods of January 1979 to December 2004 and January 2005 to January 2013 over China are shown in Fig. 4. Except the desert region in Northwestern China, the high AAI values (>1.0) are found over the four areas with intense anthropogenic activities, i.e. Pearl River Delta region in Southern China, the North China Plain and Yangtze River Delta in east of China, and the Northeast Plain of China. The AAI values show significant changes before and after the year of 2005 over these three areas. Except the Pearl River region, the AAI becomes larger after 2005 and the area with high AAI value (>1.0) expands further. In contrast, the AAI becomes smaller after 2005 in the Pearl River region. The increasing pattern of AAI in eastern China and the decreasing pattern in southern China indicate that the outbreak of haze event significantly increased in eastern China, but decreased in southern China. The increasing anthropogenic emissions caused by the industrialization and urbanization in coastal

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areas of East China could be the primary contributions to the increasing trend of haze event in eastern China. Ground-based measurements carried out by Zhang et al.(2009) confirmed that emissions of carbonaceous aerosols increased by 14% from 2001 to 2006. Another research about the haze event in Hangzhou, a city in the Yangzi River Delta in eastern China, reported that the occurrence of hazy weather has become more frequent over the past eight years in Hangzhou (Xiao et al., 2011). 3.3. Seasonal variations of AAI in China Fig. 5 shows the seasonally averaged distributions of the AAI measurements in the eastern region of China from TOMS for the period of January 1979 to December 2004 (left) and the period of January 2005 to January 2013 (right). There is a similar seasonal variation of AAI before and after 2005. AAI peaks in spring before 2005 since the dust dominate the high AAI value in spring (Zhou et al., 1996; Fang, 1997; Zhang et al., 1997; Zhuang et al., 2001; Lu et al., 2003; Zhai, 2003; Zhang et al., 2003; Sun et al., 2004; Han et al., 2005). The secondary high value appears in winter because of the “heating season” from November to the following March in northern cities of China (Zhang et al., 2012). Moreover, the low mixing layer height and little precipitation during winter also resulted in the high AAI during cold seasons. The minimum of AAI in summer is mainly ascribed to decreasing pollution emissions and diffusion of pollutants due to better meteorological conditions such as more precipitation. However, after 2005 AAI peaks emerge in winter, which demonstrates the haze enhances in winter in the eastern region of China. The result revels that AAI shows the most significant increase in winter than in other seasons in the eastern region of China before and after 2005. It indicates that haze enhance in winter after 2005. For the southern region of China, high AAI values only occur in spring because of the high amounts of the carbonaceous aerosol in this season (Zhang et al., 2008). Nevertheless, there is a significant decrease after 2005 because of the air-pollution control in this area. Fig. 6 shows the time series of AAI over eastern China (112e120 E, 30e40 N), southern China (105e115 E, 21e25 N), and northeastern China (118e128 E, 41e46 N). The result reveals that two peaks of AAI exist over eastern and northeastern China due to the dust in spring and the haze in winter alternately. But for southern China, there is only one peak in spring because of the high amounts of carbonaceous aerosols in this season. 3.4. Long-term trend of haze in China 3.4.1. Long-term trend of haze in China from in-situ measeurment Rapid economic growth and urbanization in China have sharply increased the quantities of fossil fuels burned for energy production, and this has led to serious air pollution and other related problems (Ye et al., 2003; Chen et al., 2012). Fig. 7 shows the longterm record of haze days from the in-situ measurement of CMA (http://mdss.cma.gov.cn) since 1961 in eastern China. It also can be seen clearly that the haze days increase very fast during recent ten years, just like the trend of AAI record (Fig. 9). In addition to the increase in emissions of air pollutants (Huang et al., 2015), the weather and meteorological conditions, such as wind speeds and precipitation, are also primary contributors to haze. High wind speed and more precipitation are beneficial for the air pollutants to diffuse, on the contrary making it difficult for the air pollutants to diffuse. Fig. 8 exhibits the number of precipitation days and the average wind speed since 1961 in eastern China. The data is from the in-situ measurement supported by China Meteorological Administration based on the WMO measure standard (http://mdss.cma.gov.cn). The results show that wind speeds and

precipitation decreased significantly. The number of precipitation days is more than 160 days per year from 1961, but only 130 days by 2012. Meanwhile, the average wind speed decreases from 2.7 m/s down to 2.0 m/s, leading to the accumulation of pollutants more easily and the hazy weather more frequently. 3.4.2. Long-term trend of haze in China from AAI indicator The AAI values in eastern China and northeastern China are generally below 1.0 before 1997. However, AAI values increase remarkably after 2005, with the peak values generally within the range of 1.0e1.5 (Figs. 6 and 9). But for southern China, a decreasing trend is presented during recent three years (the green line in Fig. 6 (in web version). This is mainly due to the government policy of Guangdong province, which is the major developed province in southern China and chosen as the atmospheric pollution control pilot by the Chinese government since the end of 1990. After 20-years control in the emissions of air pollutants, the hazy weather occurs less frequently. Concerning to the regional AAI average, the annual mean AAI values in eastern China and northeastern China are larger than that in the southern China, indicating that the impacts of hazy weather is more stronger in eastern China and northeastern China. Jingjin Ji (including Beijing, Tianjin and some cities in Hebei provinces, 111e120 E, 34e40 N), Yangzi River delta (118e123 E, 28.5e33.5 N), Pearl River delta (111e116 E, 21e24 N), and SiChuan Basin (104e107 E, 29e31 N) are the four major economic hubs, as well as centers for science and technology, and industrial bases in China. Hazy weather is frequently observed in these regions, especially in winter (Tan et al., 2009a,b; Li et al., 2012, 2013a,b). Fig. 10 presents the yearly variations of annual mean AAI in these four regions from 1979 to 2013. Before 1992, the variations in annual mean AAI of these four regions are relatively stable, and of the range is within 0.6 ~ 1.0, with SiChuan basin > Jingjin Ji > Yangzi River delta > Pear River delta. For Sichuan Basin, it is located in a basin surrounded by mountains with low wind speeds (1.3 m/s in January and 2.0 m/s in July), high relative humidity (84% in January and 81% in July), and stable atmospheric conditions. The pollutants are more difficulty to diffuse in Sichuan Basin compared with other regions (Wang et al., 2013a,b). From 1997 to 2005, the AAI values in these four regions show a similar variation trend (a growing trend after 1997 and a decreasing trend from 2002 to 2005). But for the period of 2005e2013, the variation trends turn to be different between Jingjin Ji and other three regions. The AAI values in Jingjin Ji continue to increase after 2005, while the values of AAI in other three regions get to peak in 2008 then decrease since then. In Jingjin Ji region, located in the northeastern part of China, hazy weather mainly occurs in winter (Zhang et al., 2012; Li et al., 2013a,b). The monthly mean AAI of January of each year from 1979 to 2013 in this region is illustrated in Fig. 11. In general, the mean AAI of January shows an increasing trend during the last 30 years, with a slow growth rate before 2005, but a dramatic increase after 2005. The mean AAI of January in Jingjin Ji increases from 1.0 in 1980 to 1.2 in 2001, with a growth rate nearly 1% per year. However, it increases from 0.8 in 2005 to 1.6 in 2013, with a growth rate nearly 12.5% per year. Before 2005, the hazy days in January in Jingjin Ji are normally less than 20 days and less than 10 days during the period of 1981e1991. But after 2005, the days with haze are normally more than 20 days, especially for recent two years almost the whole month suffering the hazy weather (Fig. 12). 3.5. Case study of the heavy haze in January 2013 in China In January 2013, the entire eastern China suffered severe haze events, drawing a great deal of attention in China. Five severe hazyweather processes in Jingjin Ji region, which includes January 6e8, 10e14, 18e19, 21e23 and 26e31, are reported at this time (Li et al.,

Fig. 5. Seasonal variations of AAI in the eastern region of China.

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Fig. 5. (continued).

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2013a,b; Wang et al., 2013a,b). Based on the in-situ measurements data from Jingjin Ji region (the information in Section 2.2), these five haze-pollution events can be identified (Fig. 13). During the period of these five haze-pollution events, nitrogen dioxide (NO2),

Fig. 7. The long-term record of haze days from the in-situ measurement since 1961 in eastern China.

sulfur dioxide (SO2), and fine particulate matter (PM2.5) significantly increased, with distinct decrease of visibility and increase of relative humidity. The spatial distribution of monthly averaged AAI, obtained from TOU/FY-3A, in January 2013 in eastern China is shown in Fig. 14. Almost the entire eastern China has AAI larger than 2.0, indicating that the eastern China has experienced heavy haze pollution. High AAI (larger than 3.6) values can be found in Sichuan basin and north China. Based on the daily spatial distribution of AAI over eastern China, the coverage area with AAI greater than 3.0 of each day in January 2013 (except Jan. 17) is demonstrated in Fig. 15. The daily coverage area with AAI > 3.0 peaks at five periods, that is Jan. 6e8, Jan. 18, and Jan. 23 with coverage area larger than 1000,000 km2, Jan. 28e29 with coverage area larger than 1200,000 km2, and Jan. 13 with coverage area around 1000,000 km2. This agrees well with the five severe hazy-weather processes in Jingjin Ji region reported by the media and other published literatures (Li et al., 2013a,b; Wang et al., 2013a,b). In order to better understand the sources and formation of the haze aerosol occurred in eastern China in January 2013, the Cloud-

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Fig. 8. The number of precipitation days (left) and the average wind speed (right) since 1961 in eastern China. The red line is the fitted trend line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Aerosol Lidar with Orthogonal Polarization (CALIOP) measurements obtained in January over this region are used here. CALIOP is the primary instrument on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite, which has flown in formation with the NASA A-train constellation of satellites since May 2006. It is a two-wavelength (1064 and 532 nm)

polarization lidar that performs global profiling of aerosols and clouds in the troposphere and lower stratosphere (Hunt & McGill, 2007). CALIPSO level 2 version 2 data generate vertically resolved distributions of aerosol types and their respective optical characteristics (Omar et al., 2009). CALIPSO satellite has 22 tracks through eastern China in

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January 2013 (Fig. 16). The Aerosol classification results in different height obtained from one track over Jingjin Ji region are exhibited in Fig. 17. The aerosol types over Beijing (39.93 N, 116.4 E) and the regions near Beijing are mixed aerosol types of polluted dust, indicating that mineral dust transported from northern China or emitted from local bared ground, industrial combustion and home heating is a major component of the haze

aerosol. The dust aerosols mixed with the local pollutants under the stagnant conditions, exacerbate the haze pollution further, reducing visibility greatly. 4. Conclusions The long-term trend and spatiotemporal variations of haze over

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Fig. 16. The Orbits of CALIPSO over east of China during January 2013. Fig. 14. The distribution of heavy haze in East of China by AAI indicator in January 2013.

(3) In eastern and northeastern China, the AAI shows increasing haze pollution during the recent 30 years. However, due to the atmospheric-pollution control pilot by the Chinese government at the end of 1990, a decreasing trend is found in the southern China in the recent three years. (4) Gernally The haze pollution show a rapid growing trend after 2005 in the major economic centers in China. But after 2008 haze continue to increase in Jingjin Ji region, while decrease in the other regions. (5) Not only the increasing air pollutants emissions contribute much to haze in China, but also the change of weather and meteorological conditions are also primary contributors to haze. (6) Case study shows the eastern China has experienced heavy haze pollution in January 2013 with AAI larger than 2.0. There were five heavy haze events (Jan. 7e8, Jan. 13, Jan18, Jan. 23, and, Jan 28e29) with daily AAI > 3.0, which agrees well with the five severe hazy-weather processes reported by the media and other published literature.

China during recent 30 years are investigated using TOMS AAI products. In addition, the heavy haze event occurred in January 2013 over eastern China are explored using AAI products from TOU on board FY-3A. The major findings of this study can be summarized as follows: (1) In China, haze pollution have been found in the four major economic areas with intense anthropogenic activities with high satellite AAI values (>1.0). (2) The AAI follows a clear seasonal pattern in the eastern and northeastern of China. The peak in winter because of haze and a secondary peak in spring for the dust were found. A typical summer minimum and autumn-to-winter increasing pattern is also observed.

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Fig. 15. Daily coverage area of heavy haze in East of China by AAI (>3.0) in January 2013.

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Fig. 17. The aerosol type from CALIPSO on 13 January 2013.

Acknowledgment Thank the TOMS AAI data product. This work has received research funding from the European Community's Seventh Framework Programme under grant agreement no 606719 and also supported from China Earth Observation Project (Grant No. E310/ 1112), the National High Technology Research and Development Program (“863”Program) of China(Grant No. 2011AA12A104-3)and the Public industry-specific fund for meteorology (Grant No. GYHY201106045). References Alpert, P., Ganor, E., 2001. Sahara mineral dust measurements from TOMS: comparison to surface observations over the middle east for the extreme dust storm, March 14e17, 1998. J. Geophys. Res. 106 (D16), 18,275e18,286. Alves, C., Oliveira, T., Pio, C., Silvestre, A.J.D., Fialho, P., Barata, F., Legrand, M., 2007. Characterization of carbonaceous aerosols from the Azorean Island of Terceira. Atmos. Environ. 41 (7), 1359e1373. Andre, N., 2005. Air pollution-related illness: effects of particles. Science 308 (5723), 804e806. Bond, T.C., Doherty, S.J., Fahey, D.W., Forster, P.M., Berntsen, T., DeAngelo, B.J., Flanner, M.G., Ghan, S., Karcher, B., Koch, D., Kinne, S., Kondo, Y., Quinn, P.K., Sarofim, M.C., Schultz, M.G., Schulz, M., Venkataraman, C., Zhang, H., Zhang, S., Bellouin, N., Guttikunda, S.K., Hopke, P.K., Jacobson, M.Z., Kaiser, J.W., Klimont, Z., Lohmann, U., Schwarz, J.P., Shindell, D., Storelvmo, T., Warren, S.G., Zender, C.S., 2013. Bounding the role of black carbon in the climate system: a scientific assessment. J. Geophys. Res. Atmos. 118, 5380e5552. http:// dx.doi.org/10.1002/jgrd.50171. Brown, S.G., Herckes, P., Ashbaugh, L., Hannigan, M.P., Kreidenweis, S.M., Collett, J.L., 2002. Characterization of organic aerosol in Big Bend National Park, Texas. Atmos. Environ. 36 (38), 5807e5818. Cao, J.J., Wu, F., Chow, J.C., Lee, S.C., Li, Y., Chen, S.W., An, Z.S., Fung, K.K., Watson, J.G., Zhu, C.S., Liu, S.X., 2005. Characterization and source apportionment of atmospheric organic and elemental carbon during fall and winter of 2003 in Xi’an, China. Atmos. Chem. Phys. 5, 3127e3137. Cao, J.J., Lee, S.C., Ho, K.F., Zhang, X.Y., Zou, S.C., Fung, K., Chow, J.C., Watson, J.G., 2003. Characteristics of carbonaceous aerosol in Pearl River Delta Region, China during 2001 winter period. Atmos. Environ. 37, 1451e1460. Che, H., Zhang, X., Li, Y., Zhou, Z., Qu, J.J., 2007. Horizontal visibility trends in China 1981-2005. Geophys. Res. Lett. 34, L24706. http://dx.doi.org/10.1029/ 2007GL031450. Che, H.Z., Zhang, X.Y., Li, Y., Zhou, Z.J., Qu, J.J., Hao, X.J., 2009. Haze trends over the capital cities of 31 provinces in China, 1981e2005. Theor. Appl. Climatol. 97 (3e4), 235e242. http://dx.doi.org/10.1007/s00704-008-0059-8. Che, H., Xia, X., Zhu, J., Li, Z., Dubovik, O., Holben, B., Goloub, P., Chen, H., Estelles, V., , E., Blarel, L., Wang, H., Zhao, H., Zhang, X., Wang, Y., Sun, J., Cuevas-Agullo Tao, R., Zhang, X., Shi, G., 2014. Column aerosol optical properties and aerosol radiative forcing during a serious haze-fog month over North China Plain in 2013 based on ground-based sunphotometer measurements. Atmos. Chem. Phys. 14, 2125e2138. http://dx.doi.org/10.5194/acp-14-2125-2014. Chen, B., Du, K., Wang, Y., Chen, J.S., Zhao, J.P., Wang, K., Zhang, F.W., Xu, L.L., 2012. Emission and transport of carbonaceous aerosols in urbanized coastal areas in China. Aerosol Air Qual. Res. 12, 371e378. Chiapello, I., Prospero, J.M., Herman, J.R., Hsu, N.C., 1999. Detection of mineral dust over the North Atlantic Ocean and Africa with the Nimbus 7 TOMS. J. Geophys. Res. 104 (D8), 9277e9291. Colarco, P.R., Toon, O.B., Torres, O., Rasch, P.J., 2002. Determining the UV imaginary

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