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A study of the meteorological causes of a prolonged and severe haze episode in January 2013 over central-eastern China
Atmospheric Environment 98 (2014) 146e157
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
A study of the meteorological causes of a prolonged and severe haze episode in January 2013 over central-eastern China Hong Wang a, b, *, Jiayu Xu c, d, Meng Zhang e, Yuanqin Yang a, Xiaojing Shen a, Yaqiang Wang a, Dong Chen f, Jianping Guo a a
Institute of Atmospheric Composition, Key Laboratory of Atmospheric Chemistry of China Meteorological Administration (CMA), Chinese Academy of Meteorological Sciences (CAMS), Beijing 100081, China Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China c School of Environment, Tsinghua University, Beijing 100084, China d State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China e Beijing Meteorological Bureau, Beijing 100089, China f Chinese Academy of Meteorological Sciences, CMA, Beijing 100081, China b
h i g h l i g h t s Large-scale latitudinal atmospheric circulation is beneficial for the haze forming. Local stable stratification and weak turbulent is favorable for the haze formation. Pollutants transportation from south Hebei on 850e925 hPa favors Beijing's pollution.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2014 Received in revised form 20 August 2014 Accepted 22 August 2014 Available online 23 August 2014
This paper employs meteorological observation data from surface and high-balloon stations, China Meteorological Administration (CMA) model T639 output data, NCEP reanalysis data, PM2.5 observations and modeled HYSPLIT4 trajectory results to study the meteorological causes, including large-scale circulation and planetary boundary layer features, which led to the extended haze episode on January 6e16, 2013 in central-eastern China. It discusses the possible impact of pollutants transported from southern Hebei Province on Beijing. The study's results show that: (1) the re-adjustment of atmospheric circulation from a longitudinal to a latitudinal model provides a valuable interpretation of the large-scale circulation background to the haze episode experienced in the metropolitan regions of Beijing, Tianjin, Hebei and their surrounding regions; (2) the regional atmospheric stratification of the planetary boundary layer is stable and the mixing height is low, suppressing air turbulence in the planetary boundary layer and providing favorable meteorological conditions for the formation of haze; and (3) the southwesterly jet stream with wind speeds of 6e11 m/s at a height of 850e950 hPa and the below-700 m air mass trajectory tracking established using the HYSPLIT4 model interdependently suggest a transport of pollutants from southern Hebei Province to Beijing at 850e950 hPa. © 2014 Published by Elsevier Ltd.
Keywords: Haze episode PM2.5 Atmospheric circulation Meteorological causes Planetary boundary layer Pollutant transport
1. Introduction Atmospheric pollution is an increasingly serious environmental problem faced by developing countries (Chan and Yao, 2008; Zhang
* Corresponding author. Institute of Atmospheric Composition, Key Laboratory of Atmospheric Chemistry of China Meteorological Administration (CMA), Chinese Academy of Meteorological Sciences (CAMS), Beijing 100081, China. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.atmosenv.2014.08.053 1352-2310/© 2014 Published by Elsevier Ltd.
et al., 2009). The deterioration of air quality is caused by the presence of aerosol components and the accumulation of aerosol concentrations (Zhu et al., 2003; Zhang et al., 2012a,b). According to the statistics published by China's Department of Environmental Protection, ca. 85%e90% of the primary pollutants in most Chinese cities throughout the year are of particulate matter. These pollutants are followed in importance by concentrations of SO2 (Che, 1999). In general, particulate matter is suspended in the air for a long time, affecting the optical properties of the atmosphere and
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resulting in reduced visibility (Che et al., 2009; Chen et al., 2009; Guo et al., 2011). During stable weather conditions, particulate matter can accumulate over a sustained period to form haze (Xu et al., 2005), causing severe air pollution (Yang et al., 2009). This may have a profound impact on climate change (Luo et al., 1998; Wang et al., 2007a,b; Shi et al., 2008; Ding et al., 2009). With the rapid urbanization of Beijing municipality and its environs in recent decades, PM10 and PM2.5 have become the main pollutants responsible for the high levels of air pollution in this region (Wang et al., 2007a,b; Liu et al., 2010). After analyzing sulfate, nitrate, ammonium, organic matter, black carbon and other aerosol chemical components in PM1 found in urban Beijing on a seasonal basis, it was found that, due to the combination of dust produced in arid areas and fugitive urban dust, mineral aerosol concentrations in China are equal to, or higher than, the sum of all aerosol concentrations found in the urban areas of Europe and America (Zhang et al., 2012a,b). Annual average PM1 concentrations are ca. 81 mg/m3, with organic aerosols accounting for ca. 41%, sulfate for ca. 16%, nitrate for ca. 13%, ammonium for ca. 8%, black carbon for ca. 11%, chloride for ca. 3% and fine mineral aerosols for ca. 7% of the total (Zhang et al., 2012a,b). At the same time, haze episodes have been observed frequently in Beijing in recent years, especially during cold winter and spring seasons (Sun et al., 2006; Wang et al., 2010; Zhang et al., 2013). In January 2013, central-eastern China experienced severe haze events which affected unusually sizeable areas and were also of long duration. The Jing-Jin-Ji region (contractions of the city names of Beijing and Tianjin, and the regional name for Hebei Province) and its immediate environs were the most polluted regions during these episodes. The haze episode which lasted from January 6 to 16, 2013 was the severest in terms of the size of the region it influenced, its duration and the intensity of the pollution. Multisource observation data covering PM10, SO2 and NO2 concentrations, Moderate Resolution Imaging Spectroradiometer (MODIS), China Aerosol Remote Sensing NETwork (CARSNET) aerosol optical depths (AOD), and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) extinction coefficients were employed to study haze strength, the region affected, and the distribution of pollutants, etc. (Wang et al., 2014). This paper focuses on the meteorological formations and weather conditions leading to this haze pollution, ranging from atmospheric circulation to conditions within the planetary boundary layer (PBL), regional transport, and other meteorological factors closely related to this haze episode. 2. Data and methodology 2.1. Data The data used in this study include meteorological field observations covering haze phenomena, air pressure, geopotential height, wind fields, moisture at surface or high-balloon stations, T639 reanalysis data, and PM2.5 concentration monitoring data as recorded by CMA automatic observation stations (Wang et al., 2008; Zhang et al., 2013). NCEP reanalysis data were also used for running the HYSPLIT4 trajectory model. 2.2. Methodology This paper studies the meteorological causes and formation of the January 6e16, 2013 haze episode by employing a synthetic analysis of atmospheric circulation, physical diagnosis of the PBL's meteorological features, and a three-dimensional structural analysis of wind speeds. The HYSPLIT4 trajectory model developed by the United States National Oceanic and Atmospheric
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Administration (NOAA) (Junce, 1955; Draxler, 1997) was also employed to study the comprehensive transport of haze pollutants from southern Hebei to Beijing from January 10 to 12, 2013. 3. Study results 3.1. Number of hazy days in China during January, 2013 This study indicated that haze phenomena are closely related to an increase in aerosol particles from recent human activity (Zhang et al., 2013). The severest haze episodes and the greatest number of hazy days occurred in January 2013 in central-eastern China (Fig. 1). New post-1961 records for the greatest number of hazy days per month were broken in January 2013 in this region, with 30 hazy days in Hefei (Anhui Province), 29 hazy days in Nanjing (Jiangsu Province) and Hangzhou (Zhejiang Province) and 27 hazy days in Shijiazhuang (Hebei Province), Zhengzhou (Henan Province) and Nanchang (Jiangxi Province). During this prolonged pollutant haze episode in January 2013, PM2.5 concentrations at many monitoring sites also reached a very high level (Fig. 2). The monthly averages of PM2.5 concentrations at stations in Harbin, Benxi, Gucheng, Shijiazhuang, Lanzhou and Changsha were above 150 mg/m3, far exceeding the national standard of 75 mg/m3. Considering the number of hazy days (Fig. 1) and PM2.5 concentrations (Fig. 2) synthetically, it can be seen that, of the polluted cities of eastern China, viz. Beijing (Jing), Tianjin (Jin), Shijiazhuang and Gucheng (southern Hebei Province e Ji), Zhengzhou (northern Henan Province), Jinan (western Shangdong Province) and Taiyuan (eastern Shanxi Province), the highest pollutant concentrations were found in Beijing, Tianjin and Hebei (Jing-Jin-Ji) and their en-vis our virons. In this study, we have focused on this region vis-a discussion of the meteorological causes of this severe haze episode. 3.2. PM2.5 changes in the Jing-Jin-Ji region in January, 2013 Air pollution is closely related to atmospheric aerosol composition and concentrations (He et al., 2001). PM2.5 concentrations exhibit clear seasonal variations, with a peak in winter and a trough in summer (Duan et al., 2008). Particles with an aerodynamic diameter less than 10 mm account for 10%e70% of total suspended particulate matter (PM). Such PM is not easily deposited and may be suspended in the atmosphere for a long period, reducing atmospheric visibility through the scattering and absorption of solar radiation, thus forming a ‘haze episode’ (Zhang et al., 2005a,b; Che et al., 2007). In “static and steady” weather conditions, it is much harder for PM to diffuse, so haze or fog forms and persists much more easily. Weather conditions favorable to severe pollution and low visibility occurred frequently in January, 2013 in central-eastern China, with the Jing-Jin-Ji region being the worst affected by haze pollution. The daily average PM2.5 concentrations in Gucheng, Shijiazhuang, Zhengzhou, Jinan and other stations in the Jing-Jin-Ji region were all higher than 100e150 mg/m3 for most days in January (Figs. 2 and 3). Daily variations in PM2.5 (Fig. 3) show that the air quality was good for a few days only in early January and that PM2.5 concentrations were high thereafter. High concentrations of PM2.5 exceeding 500, 600 and 400 mg/m3 were measured in Shijiazhuang, located southwest of Beijing, on January 7, 11 and 27, 2013, respectively, when the air quality index reached hazardous levels. It is worth noting that the PM2.5 concentration at Shijiazhuang was 540 mg/m3 on January 7, while a PM2.5 concentration of 131 mg/m3 was observed at the Baolian station in Beijing on the same day, indicating that this station had not been affected by the transport of pollutants from southern Hebei at that time. From January 8 to 9, aerosol concentrations in both of the cities
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Fig. 1. Days of haze and fog in China, January 2013.
decreased, though PM2.5 values at Shijiazhuang remained in excess of 300 mg/m3. PM2.5 pollution levels reached another peak of 620 mg/m3 on January 11 at Shijiazhuang. Daily average PM2.5 concentrations at the Baolian station continued to rise from January 10 to 12, 2013, peaking at 397 mg/m3 on January 12, one day later than the Shijiazhuang PM2.5 maximum. This probably implies that the aerosol pollutants in southern Hebei (Shijiazhuang) were transported north-east, affecting Beijing and contributing to the high PM2.5 concentrations experienced at Baolian on January 12, 2013.
The height of the convective mixing layer zic is calculated as follows:
Zzic zic qðzic Þ
Zt qðzÞdz ¼ 1:4
0
0
HðtÞ dt rcp
(2)
where q is potential temperature, and t is the time from sunrise. The height of the mechanical mixing layer zim is calculated thus:
Dt Dt zim ðt þ DtÞ ¼ zim ðtÞe t þ zie ðt þ DtÞ 1 e t
(3)
3.3. PBLH and atmospheric stability Turbulence is a major factor affecting atmospheric diffusion and is thus a good indicator of atmospheric stability (Chen, 1985). Vis-vis methods for calculating the planetary boundary layer height a (PBLH) and atmospheric stability, the MonineObukhov length L method is preferred (Zhao et al., 1991). The thickness of the mixing layer indicates the height in the PBL at which turbulent mixing occurs. The thinner the mixing layer, the weaker is the vertical atmospheric diffusion process. The formulae used to calculate the mixing layer's thickness and the PBLH are introduced in the following section. The MonineObukhov length L is calculated using:
rcp Tref u3* L¼ kgH
(1)
where g is the acceleration due to gravity, cp is the heat capacity at consistent air pressure, and r is air density.
t¼
zim 2u*
zie ¼ 0:4
(4)
u* L f
(5)
where zie is the balanced height of the mechanical mixing layer, f is the Coriolis parameter, and is the timescale controlling change in the height of the mixing layer. The equation used for calculating PBLH is:
zi ¼ max½zic ; zim
(6)
The smaller the positive value of the MonineObukhov length, the more stable is the atmospheric PBL. Table 1 lists the dates for clear, hazy, light fog and foggy weather conditions in January 2013, together with the mean PM2.5, atmospheric stability (MonineObukhov length) and PBLH for the above
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Fig. 2. Monthly averaged PM2.5 (mg/m3) in China, January 2013.
Fig. 3. Daily variations in PM2.5 at stations in the Jing-Jin-Ji region and its hinterland, January 2013.
Table 1 Weather phenomena, Number of days, PM2.5, Atmospheric stability and PBLH. Weather phenomenon
No. of days PM2.5 (mg/ m3)
Atmospheric stability (m)
PBLH (m)
Clear Haze Light fog Fog Haze/fog Clear/haze/fog
5 15 23 4 26 31 (January)
356.8 31.7 52.6 52.0 49.4 99.0
800.4 128.5 184.8 178.8 177.5 278
33.3 153.1 141.0 137.5 131.8 112.0
mentioned four kinds of weather phenomena at the Baolian (Beijing) station, each being calculated using the above formulae. As listed in Table 1, a total of 15 days of haze, 4 days of fog and 23 days of light fog occurred in January 2013 in Beijing. Twelve of the 15 hazy days were accompanied by light fog, showing that haze and light fog commonly occur at the same time. There were, in total, 26 days with haze, light fog or fog in January, constituting 84% of that month. In January 2013, the average MonineObukhov length of the 26 hazy/foggy days was 49.4 m, the average PBLH being 177.5 m. The mean MonineObukhov length and PBLH for the five clear days was 356.8 m and 800.4 m, respectively. The mean PM2.5 was 131.8 mg/m3 and 33.3 mg/m3 for hazy/foggy and clear weather, respectively, showing contrary changes with both MonineObukhov length and PBLH. The MonineObukhov length and PBLH conditions are similar for the three weather conditions, i.e. haze, light fog and fog. Hazy weather corresponds with higher PM2.5 values and a lower MonineObukhov length and PBLH than light fog and foggy weather. The peak monthly PM2.5 concentration of 396.5 mg/m3 occurred on January 12, when the MonineObukhov length was 27.4 m and the PBLH was 131 m, values far below the mean values for hazy/foggy days in January 2013. The January mean values of PM2.5, MonineObukhov length and PBLH were 112.0 mg/m3, 99.0 m and 278 m, respectively, suggesting that generally higher pollution levels and a more stable and steady PBL persisted throughout January. 3.4. Characteristics of regional atmospheric circulation Both pollutant emissions and meteorological conditions play an important role in haze formation and retention (Chen et al., 2009;
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Fig. 4. (a) 500 hPa height field in the Eurasian region at 8:00 on January 1, 2013. (b) 500 hPa height field in the Eurasian region at 8:00 on January 6, 2013. (c) 500 hPa height field in the Eurasian region at 8:00 on January 13, 2013.
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Fig. 5. (a) Surface air pressure pattern in the Eurasian region at 8:00 on January 6, 2013. (b) Surface air pressure pattern in the Eurasian region at 8:00 on January 13, 2013.
Wang et al., 2011). When emission sources remain stable over a fixed period of time, aerosol concentrations can be significantly affected by weather conditions (Yang et al., 2008). Severe pollution in Beijing is closely associated with significant weather events. Weak surface pressure patterns, combined with southerly airflow, provide favorable weather conditions for atmospheric pollution events (Liu et al., 2010). In January 2013, central-eastern China was affected on a number of occasions by a blanket of haze as well as being exposed to severe levels of air pollution, with high concentrations of PM10 and PM2.5 in a number of cities. This situation was closely linked to an adjustment of atmospheric circulation and an abnormal evolution of meteorological factors. Abnormally static and steady weather conditions led to a geographically wide-ranging and long-lasting haze across central-eastern China from January 6 to 16, 2013 (Zhang et al., 2013). The changing characteristics of the threedimensional structure of atmospheric circulation, which led to these regionally static and stable weather conditions, are analyzed in the following section.
3.4.1. Atmospheric circulation features in the middle troposphere Fog, haze and dust episodes are closely associated with certain large-scale circulation patterns (Zhang et al., 2005a,b; Zhou et al., 2008). An analysis of the evolution of atmospheric circulation vis-vis the meteorological background can advance the undera standing of the formation of haze, fog and dust. Based on an analysis of high atmospheric circulation in the period from January 1 to 14, 2013, it was found that the circulation pattern in the mid-troposphere over Eurasia had undergone a longwave adjustment. At a height of 500 hPa, it can be seen that the process evolved from a typical two-trough and one-ridge type on January 1 (Fig. 4a) to a blocking anticyclone near Siberia on January 6 (Fig. 4b), then forming smooth westerly zone circulations in the mid-latitude regions of Asia. This circulation pattern was maintained until January 13, after which the circulation began to change from a smooth westerly to a northwest airflow (Fig. 4c), a process similar to the conditions prevalent during the haze episode experienced in the winter of 2006 (Zhou et al., 2008). On both occasions, the large-scale circulation blocked the invasion of cold polar air
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Fig. 6. Wind speed and direction at 1000 hPa (a), 925 hPa (b) and 850 hPa (c) heights and surface PM2.5 from January 1 to 16, 2013 at the Xingtai and Shijiazhuang stations.
from reaching the middle and lower Asian latitudes, thus weakening the cold air affecting central-eastern China. Hence, a meteorological background developed which allowed the occurrence, development and retention of a wide-ranging haze event in central-eastern China from January 6 to 16, 2013. 3.4.2. Evolution of surface cold air high pressure From January 1 to 3, an intensely cold air mass dominated most of China. With the country being under the influence of two-trough and one-ridge high atmospheric Eurasian circulations, several clear days occurred. On January 6, the surface cold air was situated to the west of Lake Baikal, accompanied by the formation of a blocking anticyclone in high-latitude Siberia at a height of 500 hPa. The surface pressure field became weak, with fewer pressure gradients evident over a large part of central-eastern China (Fig. 5a). This was conducive to the formation of haze. Indeed, haze began to be observed in Hebei and Henan provinces as well as at other locations at this time. As a result of the continuously evolving nature of the
upper air circulation from longitudinal to a smooth westerly latitudinal pattern, in addition to the maintenance of a zonal circulation in mid-latitude, the Lake Baikal surface high pressure system, with its accompanying cold air over China, weakened further (Fig. 5b). This led to an expansion of the haze-affected area. From January 10 to 13, 2013, persistently severe haze phenomena were observed in Beijing, Tianjin, Hebei and most eastern areas of China, resulting in severe air pollution with excessive PM2.5 values across a large area of central-eastern China. 3.5. Meteorological causes of pollutant transport to Beijing from its environs Beijing faces serious PM air pollution issues due to high levels of energy consumption and a rapid increase in the ownership of motor vehicles (15% mean increase per annum) (He et al., 2001). The invasive dust from northern Chinese deserts further contributes to the air pollution in Beijing (Wang et al., 2005). Hebei Province
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Fig. 7. 500 hPa geopotential height (contour) and the surface pressure field (shaded) at 00:00 on January 12, 2013.
surrounds Beijing, its southern plateau region becoming the region subjected to the worst haze pollution of recent years (Wang et al., 2014). It is has been found that atmospheric fine and ultrafine particles in the urban area derive mainly from traffic pollution, involving nucleation of photochemical pollutants and transport from the surrounding suburban districts (Xu et al., 2004). The possible contribution of pollutants from southern Hebei impacting on Beijing haze is a source of increasing concern. From January 10 to 12, 2013, daily average concentrations of PM2.5 at the Baolian station in Beijing continued to rise (Fig. 3), reaching a peak of 396.5 mg/m3 on January 12, one day later than the peak PM2.5 values measured in Shijiazhuang. The possible long-distance regional transport of PM is further analyzed below.
3.5.1. Wind field changes in the lower troposphere Fig. 6 shows wind speed and direction at 1000, 925 and 850 hPa heights as measured by sounding balloons for January 1e16, 2013, at the Xingtai station in Hebei Province (37.04 N, 114.30 E). Fig. 6 also shows PM2.5 concentrations at the Shijiazhuang station (38.04 N, 114.26 E). As sounding balloon data were not available at the Shijiazhuang station, the wind data at the Xingtai station (about 100 km south of Shijiazhuang) were used instead. At a height of 1000 hPa, conditions were calm from January 10 to 11, 2013. At 00:00UTC on January 12, the wind speed reached 2 m/s (Fig. 6a). From January 10 to 12 onwards, PM2.5 concentrations in Shijiazhuang sustained high values between 450 and 600 mg/m3. Wind speed increased from 6 m/s to 8 m/s on January 10 at a height of 925 hPa, and further increased to 11 m/s on January 11, falling to 5 m/s on January 12. However, the wind direction changed from
WSW to SSW at the same time, providing a very favorable airflow for the transport of pollutants from Shijiazhuang to Beijing (Fig. 6b). At a height of 850 hPa, wind speed variation was broadly consistent with that found at 925 hPa. However, the variation in wind direction amplitude prior to January 12 was within the range of 250e300 , i.e. a SW, S to SE direction, but the wind veered SW on January 12, a direction which is more conducive to carrying local pollutants from Shijiazhuang downwind to Beijing (Fig. 6c). The above discussion shows that the weak SW jet flow within the 850e925 hPa range is a very important pollutant transport -vis this episode, the pathway from southern Hebei to Beijing. Vis-a high pollutant concentrations in southern Hebei Province were probably transported northward by the SW jet flow. Added to local pollution, they most likely contributed to the severest pollution episode containing the highest concentration of PM2.5 particles on January 12 in Beijing.
3.5.2. 500 hPa circulation and surface pressure field structure The evolving process of atmospheric circulation from 1 to 13 January was discussed in Sections 3.4.1 and 3.4.2. The January 12 meteorological field features are further studied and discussed in detail in the following section with a view to determining the possible meteorological reasons why the severest pollution and highest PM2.5 occurred on January 12. Fig. 7 displays the 500 hPa geopotential height (contour) and the surface pressure field (shaded) derived from the T639 model at 00:00UTC on January 12, 2013. It also shows that a zonal circulation controlled central-eastern China, indicating that the cold air was weak at this height. A balanced pressure field changing within the 1026e1028 hPa range controlled the area south of the Yellow River.
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Air pressure in the Jing-Jin-Ji area was between 1024 and 1026 hPa. In winter, this circulation structure is conducive to haze formation and air pollution. Aerosol (PM2.5) concentrations in Beijing reached a peak daily average of 396.5 mg/m3 on January 12, 2013 under such a circulation structure.
3.5.3. 850 hPa wind field, humidity and 500 hPa potential height Fig. 8 displays the 850 hPa wind speed (shaded), wind vector and 500 hPa potential height field (contour) from the T639 model at 00:00UTC on January 12, 2013. It can be seen that a WNW airflow existed at 850 hPa height in northeast China from Inner Mongolia to Liaoning Province, while the region from the Hetao area of the Yellow River to the Jing-Jin-Ji region and its hinterland was dominated by a WSW airflow within a velocity range of 8e12 m/s. Such a wind field configuration favors the transport of pollutants from areas south of Beijing such as Taiyuan (Shanxi Province) and Shijiazhuang (Hebei Province). The relative humidity (RH) and wind field at a height of 850 hPa (Fig. 9) show that the RH at 850 hPa is <70% across the whole region polluted by haze (Fig. 2) when high PM2.5 values are under the control of a WSW airflow. This may indicate that, although humidity is very important in the hygroscopic increasing of aerosol particles, not all haze events with high PM2.5 concentrations are accompanied by high environment humidity. Reasons for this include the chemical composition of the particles and the portion of secondary aerosols accounting for the total aerosols etc., all of which demands further and detailed research into other haze episodes.
3.5.4. Air mass backward trajectory analysis Using the HYSPLIT4 backward trajectory model developed by NOAA (Junce, 1955; Draxler, 1997), the 72-h air mass movement is shown at heights of 100 m, 500 m, 700 m, 1000 m and 1500 m (Fig. 10). It can be seen that: (1) The air mass at 1500 m deviated south of Novosibirsk in Russian Siberia at 12UTC on January 9. On January 11, it moved to the contiguous area between Inner Mongolia and Ningxia Province, then moved eastward and arrived in Beijing at 12UTC on January 12 by way of the Hetao area, northern Shanxi and central Hebei. The air mass gradually diminished in height from 3800 m to 1000 m during its flow but remained above the mixing layer (ca. 1000 m), evidently making no contribution to pollutant transport. (2) The 1000 m air mass trajectory shows that the air mass was located near Taiyuan, Shanxi Province, at 12UTC on January 9, and gradually subsided below 1000 m along the airflow's NE pathway, and even entered the mixed layer, which was conducive to transporting pollutants from Gucheng in Hebei Province to Beijing. (3) The air mass trajectories at 700 m, 500 m and 100 m all show that air masses were located in southern Hebei at 12UTC on January 9. The airflow at these heights moved northward and passed through southern Hebei (including Shijiazhuang and Gucheng), arriving in Beijing beneath the mixing layer height, thus proving pollutant transport from centralsouthern Hebei to Beijing at heights between 100 and 700 m.
Fig. 8. 850 hPa wind speed (shaded), vector (wind barb) and 500 hPa height field (contour) at 00:00 on January 12, 2013.
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Fig. 9. 850 hPa wind field (wind barb) and RH (%, shaded).
Fig. 10. 72-h backward trajectory of air mass in Beijing, January 12, 2013.
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The composite results based on mass trajectories, PM2.5 daily concentrations and 925e850 hpa wind fields indicate the possible contribution of pollutant transport from southern Hebei locations such as Shijiazhang, Gucheng and similar localities. The transport along this direction led to an increase in PM2.5 from January 10 to 12 in the Beijing region, agreeing with the results of concentrated particle height determinations reached using the CALIPSO extinction coefficient (Wang et al., 2014). The above discussion indicates that pollutant transport from southern Hebei to Beijing made a definite contribution to the continuously increasing PM2.5 values experienced from January 10 to 12, as well as to the PM2.5 peak reached on July 12. 4. Conclusions Using meteorological data from CMA surface and high-balloon stations, the T639 model, NCEP reanalysis, PM2.5 observation data and results from the NOAA HYSPLIT4 trajectory model, the meteorological causes and features of the haze episode that occurred from January 6 to 16, 2013 are studied by analyzing the atmospheric circulation structure, by diagnosing the features of the local PBL, and by tracking airflow trajectories. A possible meteorological reason for the high PM2.5 levels in Beijing from January 10 to 12, 2013 has also been discussed. In summary, the results are as follows: (1) The re-adjustment in atmospheric circulation from a longitudinal to a latitudinal model provides a valuable insight into the large-scale circulation background that played a part in the haze episode that occurred in January 2013 in the JingJin-Ji cities and their hinterlands. (2) The regional atmospheric stratification of the PBL was stable and the mixing height was low, both of which suppressed turbulence in the PBL and provided favorable meteorological conditions for haze formation. (3) A SW airflow with wind speeds of 6e11 m/s existed in the lower troposphere between heights 925e850 hpa over the central and southern parts of the Jing-Jin-Ji region. The long distance transport of pollutants from southern Hebei Province intensified pollution levels in Beijing. (4) Air mass trajectory tracking using the HYSPLIT4 model also shows that, below 700 m, the air mass in Beijing comes from the southern part of the Jing-Jin-Ji region. This is consistent with the SW airflow in the lower troposphere. These facts, together with the vertical height results from the CALIPSO extinction coefficient (Wang et al., 2014), prove that transported pollutants travel from southern Hebei to Beijing. The RH was <70% across the polluted region during this haze episode. Nevertheless, the relation between RH and PM2.5 during haze episodes is complicated and worthy of further study. More haze cases are needed as a basis for discussing the relation between high PM2.5 density and humidity. More detailed and quantitative evaluations of regional pollutant transport between Beijing and other cities in the Jing-Jin-Ji region are also needed. Acknowledgments This work is supported by the National Basic Research Program (973) (Grant No. 2014CB441201), the National Natural Scientific Foundation of China (Grant No. 41275007), the National Natural Science Foundation of China (Grant Nos. 41130104 and 41075079), and the CAMS key projects program (Grant No. 2013Z007).
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A study of the meteorological causes of a prolonged and severe haze episode in January 2013 over central-eastern China Hong Wang a, b, *, Jiayu Xu c, d, Meng Zhang e, Yuanqin Yang a, Xiaojing Shen a, Yaqiang Wang a, Dong Chen f, Jianping Guo a a
Institute of Atmospheric Composition, Key Laboratory of Atmospheric Chemistry of China Meteorological Administration (CMA), Chinese Academy of Meteorological Sciences (CAMS), Beijing 100081, China Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China c School of Environment, Tsinghua University, Beijing 100084, China d State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China e Beijing Meteorological Bureau, Beijing 100089, China f Chinese Academy of Meteorological Sciences, CMA, Beijing 100081, China b
h i g h l i g h t s Large-scale latitudinal atmospheric circulation is beneficial for the haze forming. Local stable stratification and weak turbulent is favorable for the haze formation. Pollutants transportation from south Hebei on 850e925 hPa favors Beijing's pollution.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2014 Received in revised form 20 August 2014 Accepted 22 August 2014 Available online 23 August 2014
This paper employs meteorological observation data from surface and high-balloon stations, China Meteorological Administration (CMA) model T639 output data, NCEP reanalysis data, PM2.5 observations and modeled HYSPLIT4 trajectory results to study the meteorological causes, including large-scale circulation and planetary boundary layer features, which led to the extended haze episode on January 6e16, 2013 in central-eastern China. It discusses the possible impact of pollutants transported from southern Hebei Province on Beijing. The study's results show that: (1) the re-adjustment of atmospheric circulation from a longitudinal to a latitudinal model provides a valuable interpretation of the large-scale circulation background to the haze episode experienced in the metropolitan regions of Beijing, Tianjin, Hebei and their surrounding regions; (2) the regional atmospheric stratification of the planetary boundary layer is stable and the mixing height is low, suppressing air turbulence in the planetary boundary layer and providing favorable meteorological conditions for the formation of haze; and (3) the southwesterly jet stream with wind speeds of 6e11 m/s at a height of 850e950 hPa and the below-700 m air mass trajectory tracking established using the HYSPLIT4 model interdependently suggest a transport of pollutants from southern Hebei Province to Beijing at 850e950 hPa. © 2014 Published by Elsevier Ltd.
Keywords: Haze episode PM2.5 Atmospheric circulation Meteorological causes Planetary boundary layer Pollutant transport
1. Introduction Atmospheric pollution is an increasingly serious environmental problem faced by developing countries (Chan and Yao, 2008; Zhang
* Corresponding author. Institute of Atmospheric Composition, Key Laboratory of Atmospheric Chemistry of China Meteorological Administration (CMA), Chinese Academy of Meteorological Sciences (CAMS), Beijing 100081, China. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.atmosenv.2014.08.053 1352-2310/© 2014 Published by Elsevier Ltd.
et al., 2009). The deterioration of air quality is caused by the presence of aerosol components and the accumulation of aerosol concentrations (Zhu et al., 2003; Zhang et al., 2012a,b). According to the statistics published by China's Department of Environmental Protection, ca. 85%e90% of the primary pollutants in most Chinese cities throughout the year are of particulate matter. These pollutants are followed in importance by concentrations of SO2 (Che, 1999). In general, particulate matter is suspended in the air for a long time, affecting the optical properties of the atmosphere and
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resulting in reduced visibility (Che et al., 2009; Chen et al., 2009; Guo et al., 2011). During stable weather conditions, particulate matter can accumulate over a sustained period to form haze (Xu et al., 2005), causing severe air pollution (Yang et al., 2009). This may have a profound impact on climate change (Luo et al., 1998; Wang et al., 2007a,b; Shi et al., 2008; Ding et al., 2009). With the rapid urbanization of Beijing municipality and its environs in recent decades, PM10 and PM2.5 have become the main pollutants responsible for the high levels of air pollution in this region (Wang et al., 2007a,b; Liu et al., 2010). After analyzing sulfate, nitrate, ammonium, organic matter, black carbon and other aerosol chemical components in PM1 found in urban Beijing on a seasonal basis, it was found that, due to the combination of dust produced in arid areas and fugitive urban dust, mineral aerosol concentrations in China are equal to, or higher than, the sum of all aerosol concentrations found in the urban areas of Europe and America (Zhang et al., 2012a,b). Annual average PM1 concentrations are ca. 81 mg/m3, with organic aerosols accounting for ca. 41%, sulfate for ca. 16%, nitrate for ca. 13%, ammonium for ca. 8%, black carbon for ca. 11%, chloride for ca. 3% and fine mineral aerosols for ca. 7% of the total (Zhang et al., 2012a,b). At the same time, haze episodes have been observed frequently in Beijing in recent years, especially during cold winter and spring seasons (Sun et al., 2006; Wang et al., 2010; Zhang et al., 2013). In January 2013, central-eastern China experienced severe haze events which affected unusually sizeable areas and were also of long duration. The Jing-Jin-Ji region (contractions of the city names of Beijing and Tianjin, and the regional name for Hebei Province) and its immediate environs were the most polluted regions during these episodes. The haze episode which lasted from January 6 to 16, 2013 was the severest in terms of the size of the region it influenced, its duration and the intensity of the pollution. Multisource observation data covering PM10, SO2 and NO2 concentrations, Moderate Resolution Imaging Spectroradiometer (MODIS), China Aerosol Remote Sensing NETwork (CARSNET) aerosol optical depths (AOD), and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) extinction coefficients were employed to study haze strength, the region affected, and the distribution of pollutants, etc. (Wang et al., 2014). This paper focuses on the meteorological formations and weather conditions leading to this haze pollution, ranging from atmospheric circulation to conditions within the planetary boundary layer (PBL), regional transport, and other meteorological factors closely related to this haze episode. 2. Data and methodology 2.1. Data The data used in this study include meteorological field observations covering haze phenomena, air pressure, geopotential height, wind fields, moisture at surface or high-balloon stations, T639 reanalysis data, and PM2.5 concentration monitoring data as recorded by CMA automatic observation stations (Wang et al., 2008; Zhang et al., 2013). NCEP reanalysis data were also used for running the HYSPLIT4 trajectory model. 2.2. Methodology This paper studies the meteorological causes and formation of the January 6e16, 2013 haze episode by employing a synthetic analysis of atmospheric circulation, physical diagnosis of the PBL's meteorological features, and a three-dimensional structural analysis of wind speeds. The HYSPLIT4 trajectory model developed by the United States National Oceanic and Atmospheric
147
Administration (NOAA) (Junce, 1955; Draxler, 1997) was also employed to study the comprehensive transport of haze pollutants from southern Hebei to Beijing from January 10 to 12, 2013. 3. Study results 3.1. Number of hazy days in China during January, 2013 This study indicated that haze phenomena are closely related to an increase in aerosol particles from recent human activity (Zhang et al., 2013). The severest haze episodes and the greatest number of hazy days occurred in January 2013 in central-eastern China (Fig. 1). New post-1961 records for the greatest number of hazy days per month were broken in January 2013 in this region, with 30 hazy days in Hefei (Anhui Province), 29 hazy days in Nanjing (Jiangsu Province) and Hangzhou (Zhejiang Province) and 27 hazy days in Shijiazhuang (Hebei Province), Zhengzhou (Henan Province) and Nanchang (Jiangxi Province). During this prolonged pollutant haze episode in January 2013, PM2.5 concentrations at many monitoring sites also reached a very high level (Fig. 2). The monthly averages of PM2.5 concentrations at stations in Harbin, Benxi, Gucheng, Shijiazhuang, Lanzhou and Changsha were above 150 mg/m3, far exceeding the national standard of 75 mg/m3. Considering the number of hazy days (Fig. 1) and PM2.5 concentrations (Fig. 2) synthetically, it can be seen that, of the polluted cities of eastern China, viz. Beijing (Jing), Tianjin (Jin), Shijiazhuang and Gucheng (southern Hebei Province e Ji), Zhengzhou (northern Henan Province), Jinan (western Shangdong Province) and Taiyuan (eastern Shanxi Province), the highest pollutant concentrations were found in Beijing, Tianjin and Hebei (Jing-Jin-Ji) and their en-vis our virons. In this study, we have focused on this region vis-a discussion of the meteorological causes of this severe haze episode. 3.2. PM2.5 changes in the Jing-Jin-Ji region in January, 2013 Air pollution is closely related to atmospheric aerosol composition and concentrations (He et al., 2001). PM2.5 concentrations exhibit clear seasonal variations, with a peak in winter and a trough in summer (Duan et al., 2008). Particles with an aerodynamic diameter less than 10 mm account for 10%e70% of total suspended particulate matter (PM). Such PM is not easily deposited and may be suspended in the atmosphere for a long period, reducing atmospheric visibility through the scattering and absorption of solar radiation, thus forming a ‘haze episode’ (Zhang et al., 2005a,b; Che et al., 2007). In “static and steady” weather conditions, it is much harder for PM to diffuse, so haze or fog forms and persists much more easily. Weather conditions favorable to severe pollution and low visibility occurred frequently in January, 2013 in central-eastern China, with the Jing-Jin-Ji region being the worst affected by haze pollution. The daily average PM2.5 concentrations in Gucheng, Shijiazhuang, Zhengzhou, Jinan and other stations in the Jing-Jin-Ji region were all higher than 100e150 mg/m3 for most days in January (Figs. 2 and 3). Daily variations in PM2.5 (Fig. 3) show that the air quality was good for a few days only in early January and that PM2.5 concentrations were high thereafter. High concentrations of PM2.5 exceeding 500, 600 and 400 mg/m3 were measured in Shijiazhuang, located southwest of Beijing, on January 7, 11 and 27, 2013, respectively, when the air quality index reached hazardous levels. It is worth noting that the PM2.5 concentration at Shijiazhuang was 540 mg/m3 on January 7, while a PM2.5 concentration of 131 mg/m3 was observed at the Baolian station in Beijing on the same day, indicating that this station had not been affected by the transport of pollutants from southern Hebei at that time. From January 8 to 9, aerosol concentrations in both of the cities
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Fig. 1. Days of haze and fog in China, January 2013.
decreased, though PM2.5 values at Shijiazhuang remained in excess of 300 mg/m3. PM2.5 pollution levels reached another peak of 620 mg/m3 on January 11 at Shijiazhuang. Daily average PM2.5 concentrations at the Baolian station continued to rise from January 10 to 12, 2013, peaking at 397 mg/m3 on January 12, one day later than the Shijiazhuang PM2.5 maximum. This probably implies that the aerosol pollutants in southern Hebei (Shijiazhuang) were transported north-east, affecting Beijing and contributing to the high PM2.5 concentrations experienced at Baolian on January 12, 2013.
The height of the convective mixing layer zic is calculated as follows:
Zzic zic qðzic Þ
Zt qðzÞdz ¼ 1:4
0
0
HðtÞ dt rcp
(2)
where q is potential temperature, and t is the time from sunrise. The height of the mechanical mixing layer zim is calculated thus:
Dt Dt zim ðt þ DtÞ ¼ zim ðtÞe t þ zie ðt þ DtÞ 1 e t
(3)
3.3. PBLH and atmospheric stability Turbulence is a major factor affecting atmospheric diffusion and is thus a good indicator of atmospheric stability (Chen, 1985). Vis-vis methods for calculating the planetary boundary layer height a (PBLH) and atmospheric stability, the MonineObukhov length L method is preferred (Zhao et al., 1991). The thickness of the mixing layer indicates the height in the PBL at which turbulent mixing occurs. The thinner the mixing layer, the weaker is the vertical atmospheric diffusion process. The formulae used to calculate the mixing layer's thickness and the PBLH are introduced in the following section. The MonineObukhov length L is calculated using:
rcp Tref u3* L¼ kgH
(1)
where g is the acceleration due to gravity, cp is the heat capacity at consistent air pressure, and r is air density.
t¼
zim 2u*
zie ¼ 0:4
(4)
u* L f
(5)
where zie is the balanced height of the mechanical mixing layer, f is the Coriolis parameter, and is the timescale controlling change in the height of the mixing layer. The equation used for calculating PBLH is:
zi ¼ max½zic ; zim
(6)
The smaller the positive value of the MonineObukhov length, the more stable is the atmospheric PBL. Table 1 lists the dates for clear, hazy, light fog and foggy weather conditions in January 2013, together with the mean PM2.5, atmospheric stability (MonineObukhov length) and PBLH for the above
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Fig. 2. Monthly averaged PM2.5 (mg/m3) in China, January 2013.
Fig. 3. Daily variations in PM2.5 at stations in the Jing-Jin-Ji region and its hinterland, January 2013.
Table 1 Weather phenomena, Number of days, PM2.5, Atmospheric stability and PBLH. Weather phenomenon
No. of days PM2.5 (mg/ m3)
Atmospheric stability (m)
PBLH (m)
Clear Haze Light fog Fog Haze/fog Clear/haze/fog
5 15 23 4 26 31 (January)
356.8 31.7 52.6 52.0 49.4 99.0
800.4 128.5 184.8 178.8 177.5 278
33.3 153.1 141.0 137.5 131.8 112.0
mentioned four kinds of weather phenomena at the Baolian (Beijing) station, each being calculated using the above formulae. As listed in Table 1, a total of 15 days of haze, 4 days of fog and 23 days of light fog occurred in January 2013 in Beijing. Twelve of the 15 hazy days were accompanied by light fog, showing that haze and light fog commonly occur at the same time. There were, in total, 26 days with haze, light fog or fog in January, constituting 84% of that month. In January 2013, the average MonineObukhov length of the 26 hazy/foggy days was 49.4 m, the average PBLH being 177.5 m. The mean MonineObukhov length and PBLH for the five clear days was 356.8 m and 800.4 m, respectively. The mean PM2.5 was 131.8 mg/m3 and 33.3 mg/m3 for hazy/foggy and clear weather, respectively, showing contrary changes with both MonineObukhov length and PBLH. The MonineObukhov length and PBLH conditions are similar for the three weather conditions, i.e. haze, light fog and fog. Hazy weather corresponds with higher PM2.5 values and a lower MonineObukhov length and PBLH than light fog and foggy weather. The peak monthly PM2.5 concentration of 396.5 mg/m3 occurred on January 12, when the MonineObukhov length was 27.4 m and the PBLH was 131 m, values far below the mean values for hazy/foggy days in January 2013. The January mean values of PM2.5, MonineObukhov length and PBLH were 112.0 mg/m3, 99.0 m and 278 m, respectively, suggesting that generally higher pollution levels and a more stable and steady PBL persisted throughout January. 3.4. Characteristics of regional atmospheric circulation Both pollutant emissions and meteorological conditions play an important role in haze formation and retention (Chen et al., 2009;
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Fig. 4. (a) 500 hPa height field in the Eurasian region at 8:00 on January 1, 2013. (b) 500 hPa height field in the Eurasian region at 8:00 on January 6, 2013. (c) 500 hPa height field in the Eurasian region at 8:00 on January 13, 2013.
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Fig. 5. (a) Surface air pressure pattern in the Eurasian region at 8:00 on January 6, 2013. (b) Surface air pressure pattern in the Eurasian region at 8:00 on January 13, 2013.
Wang et al., 2011). When emission sources remain stable over a fixed period of time, aerosol concentrations can be significantly affected by weather conditions (Yang et al., 2008). Severe pollution in Beijing is closely associated with significant weather events. Weak surface pressure patterns, combined with southerly airflow, provide favorable weather conditions for atmospheric pollution events (Liu et al., 2010). In January 2013, central-eastern China was affected on a number of occasions by a blanket of haze as well as being exposed to severe levels of air pollution, with high concentrations of PM10 and PM2.5 in a number of cities. This situation was closely linked to an adjustment of atmospheric circulation and an abnormal evolution of meteorological factors. Abnormally static and steady weather conditions led to a geographically wide-ranging and long-lasting haze across central-eastern China from January 6 to 16, 2013 (Zhang et al., 2013). The changing characteristics of the threedimensional structure of atmospheric circulation, which led to these regionally static and stable weather conditions, are analyzed in the following section.
3.4.1. Atmospheric circulation features in the middle troposphere Fog, haze and dust episodes are closely associated with certain large-scale circulation patterns (Zhang et al., 2005a,b; Zhou et al., 2008). An analysis of the evolution of atmospheric circulation vis-vis the meteorological background can advance the undera standing of the formation of haze, fog and dust. Based on an analysis of high atmospheric circulation in the period from January 1 to 14, 2013, it was found that the circulation pattern in the mid-troposphere over Eurasia had undergone a longwave adjustment. At a height of 500 hPa, it can be seen that the process evolved from a typical two-trough and one-ridge type on January 1 (Fig. 4a) to a blocking anticyclone near Siberia on January 6 (Fig. 4b), then forming smooth westerly zone circulations in the mid-latitude regions of Asia. This circulation pattern was maintained until January 13, after which the circulation began to change from a smooth westerly to a northwest airflow (Fig. 4c), a process similar to the conditions prevalent during the haze episode experienced in the winter of 2006 (Zhou et al., 2008). On both occasions, the large-scale circulation blocked the invasion of cold polar air
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Fig. 6. Wind speed and direction at 1000 hPa (a), 925 hPa (b) and 850 hPa (c) heights and surface PM2.5 from January 1 to 16, 2013 at the Xingtai and Shijiazhuang stations.
from reaching the middle and lower Asian latitudes, thus weakening the cold air affecting central-eastern China. Hence, a meteorological background developed which allowed the occurrence, development and retention of a wide-ranging haze event in central-eastern China from January 6 to 16, 2013. 3.4.2. Evolution of surface cold air high pressure From January 1 to 3, an intensely cold air mass dominated most of China. With the country being under the influence of two-trough and one-ridge high atmospheric Eurasian circulations, several clear days occurred. On January 6, the surface cold air was situated to the west of Lake Baikal, accompanied by the formation of a blocking anticyclone in high-latitude Siberia at a height of 500 hPa. The surface pressure field became weak, with fewer pressure gradients evident over a large part of central-eastern China (Fig. 5a). This was conducive to the formation of haze. Indeed, haze began to be observed in Hebei and Henan provinces as well as at other locations at this time. As a result of the continuously evolving nature of the
upper air circulation from longitudinal to a smooth westerly latitudinal pattern, in addition to the maintenance of a zonal circulation in mid-latitude, the Lake Baikal surface high pressure system, with its accompanying cold air over China, weakened further (Fig. 5b). This led to an expansion of the haze-affected area. From January 10 to 13, 2013, persistently severe haze phenomena were observed in Beijing, Tianjin, Hebei and most eastern areas of China, resulting in severe air pollution with excessive PM2.5 values across a large area of central-eastern China. 3.5. Meteorological causes of pollutant transport to Beijing from its environs Beijing faces serious PM air pollution issues due to high levels of energy consumption and a rapid increase in the ownership of motor vehicles (15% mean increase per annum) (He et al., 2001). The invasive dust from northern Chinese deserts further contributes to the air pollution in Beijing (Wang et al., 2005). Hebei Province
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Fig. 7. 500 hPa geopotential height (contour) and the surface pressure field (shaded) at 00:00 on January 12, 2013.
surrounds Beijing, its southern plateau region becoming the region subjected to the worst haze pollution of recent years (Wang et al., 2014). It is has been found that atmospheric fine and ultrafine particles in the urban area derive mainly from traffic pollution, involving nucleation of photochemical pollutants and transport from the surrounding suburban districts (Xu et al., 2004). The possible contribution of pollutants from southern Hebei impacting on Beijing haze is a source of increasing concern. From January 10 to 12, 2013, daily average concentrations of PM2.5 at the Baolian station in Beijing continued to rise (Fig. 3), reaching a peak of 396.5 mg/m3 on January 12, one day later than the peak PM2.5 values measured in Shijiazhuang. The possible long-distance regional transport of PM is further analyzed below.
3.5.1. Wind field changes in the lower troposphere Fig. 6 shows wind speed and direction at 1000, 925 and 850 hPa heights as measured by sounding balloons for January 1e16, 2013, at the Xingtai station in Hebei Province (37.04 N, 114.30 E). Fig. 6 also shows PM2.5 concentrations at the Shijiazhuang station (38.04 N, 114.26 E). As sounding balloon data were not available at the Shijiazhuang station, the wind data at the Xingtai station (about 100 km south of Shijiazhuang) were used instead. At a height of 1000 hPa, conditions were calm from January 10 to 11, 2013. At 00:00UTC on January 12, the wind speed reached 2 m/s (Fig. 6a). From January 10 to 12 onwards, PM2.5 concentrations in Shijiazhuang sustained high values between 450 and 600 mg/m3. Wind speed increased from 6 m/s to 8 m/s on January 10 at a height of 925 hPa, and further increased to 11 m/s on January 11, falling to 5 m/s on January 12. However, the wind direction changed from
WSW to SSW at the same time, providing a very favorable airflow for the transport of pollutants from Shijiazhuang to Beijing (Fig. 6b). At a height of 850 hPa, wind speed variation was broadly consistent with that found at 925 hPa. However, the variation in wind direction amplitude prior to January 12 was within the range of 250e300 , i.e. a SW, S to SE direction, but the wind veered SW on January 12, a direction which is more conducive to carrying local pollutants from Shijiazhuang downwind to Beijing (Fig. 6c). The above discussion shows that the weak SW jet flow within the 850e925 hPa range is a very important pollutant transport -vis this episode, the pathway from southern Hebei to Beijing. Vis-a high pollutant concentrations in southern Hebei Province were probably transported northward by the SW jet flow. Added to local pollution, they most likely contributed to the severest pollution episode containing the highest concentration of PM2.5 particles on January 12 in Beijing.
3.5.2. 500 hPa circulation and surface pressure field structure The evolving process of atmospheric circulation from 1 to 13 January was discussed in Sections 3.4.1 and 3.4.2. The January 12 meteorological field features are further studied and discussed in detail in the following section with a view to determining the possible meteorological reasons why the severest pollution and highest PM2.5 occurred on January 12. Fig. 7 displays the 500 hPa geopotential height (contour) and the surface pressure field (shaded) derived from the T639 model at 00:00UTC on January 12, 2013. It also shows that a zonal circulation controlled central-eastern China, indicating that the cold air was weak at this height. A balanced pressure field changing within the 1026e1028 hPa range controlled the area south of the Yellow River.
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Air pressure in the Jing-Jin-Ji area was between 1024 and 1026 hPa. In winter, this circulation structure is conducive to haze formation and air pollution. Aerosol (PM2.5) concentrations in Beijing reached a peak daily average of 396.5 mg/m3 on January 12, 2013 under such a circulation structure.
3.5.3. 850 hPa wind field, humidity and 500 hPa potential height Fig. 8 displays the 850 hPa wind speed (shaded), wind vector and 500 hPa potential height field (contour) from the T639 model at 00:00UTC on January 12, 2013. It can be seen that a WNW airflow existed at 850 hPa height in northeast China from Inner Mongolia to Liaoning Province, while the region from the Hetao area of the Yellow River to the Jing-Jin-Ji region and its hinterland was dominated by a WSW airflow within a velocity range of 8e12 m/s. Such a wind field configuration favors the transport of pollutants from areas south of Beijing such as Taiyuan (Shanxi Province) and Shijiazhuang (Hebei Province). The relative humidity (RH) and wind field at a height of 850 hPa (Fig. 9) show that the RH at 850 hPa is <70% across the whole region polluted by haze (Fig. 2) when high PM2.5 values are under the control of a WSW airflow. This may indicate that, although humidity is very important in the hygroscopic increasing of aerosol particles, not all haze events with high PM2.5 concentrations are accompanied by high environment humidity. Reasons for this include the chemical composition of the particles and the portion of secondary aerosols accounting for the total aerosols etc., all of which demands further and detailed research into other haze episodes.
3.5.4. Air mass backward trajectory analysis Using the HYSPLIT4 backward trajectory model developed by NOAA (Junce, 1955; Draxler, 1997), the 72-h air mass movement is shown at heights of 100 m, 500 m, 700 m, 1000 m and 1500 m (Fig. 10). It can be seen that: (1) The air mass at 1500 m deviated south of Novosibirsk in Russian Siberia at 12UTC on January 9. On January 11, it moved to the contiguous area between Inner Mongolia and Ningxia Province, then moved eastward and arrived in Beijing at 12UTC on January 12 by way of the Hetao area, northern Shanxi and central Hebei. The air mass gradually diminished in height from 3800 m to 1000 m during its flow but remained above the mixing layer (ca. 1000 m), evidently making no contribution to pollutant transport. (2) The 1000 m air mass trajectory shows that the air mass was located near Taiyuan, Shanxi Province, at 12UTC on January 9, and gradually subsided below 1000 m along the airflow's NE pathway, and even entered the mixed layer, which was conducive to transporting pollutants from Gucheng in Hebei Province to Beijing. (3) The air mass trajectories at 700 m, 500 m and 100 m all show that air masses were located in southern Hebei at 12UTC on January 9. The airflow at these heights moved northward and passed through southern Hebei (including Shijiazhuang and Gucheng), arriving in Beijing beneath the mixing layer height, thus proving pollutant transport from centralsouthern Hebei to Beijing at heights between 100 and 700 m.
Fig. 8. 850 hPa wind speed (shaded), vector (wind barb) and 500 hPa height field (contour) at 00:00 on January 12, 2013.
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Fig. 9. 850 hPa wind field (wind barb) and RH (%, shaded).
Fig. 10. 72-h backward trajectory of air mass in Beijing, January 12, 2013.
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The composite results based on mass trajectories, PM2.5 daily concentrations and 925e850 hpa wind fields indicate the possible contribution of pollutant transport from southern Hebei locations such as Shijiazhang, Gucheng and similar localities. The transport along this direction led to an increase in PM2.5 from January 10 to 12 in the Beijing region, agreeing with the results of concentrated particle height determinations reached using the CALIPSO extinction coefficient (Wang et al., 2014). The above discussion indicates that pollutant transport from southern Hebei to Beijing made a definite contribution to the continuously increasing PM2.5 values experienced from January 10 to 12, as well as to the PM2.5 peak reached on July 12. 4. Conclusions Using meteorological data from CMA surface and high-balloon stations, the T639 model, NCEP reanalysis, PM2.5 observation data and results from the NOAA HYSPLIT4 trajectory model, the meteorological causes and features of the haze episode that occurred from January 6 to 16, 2013 are studied by analyzing the atmospheric circulation structure, by diagnosing the features of the local PBL, and by tracking airflow trajectories. A possible meteorological reason for the high PM2.5 levels in Beijing from January 10 to 12, 2013 has also been discussed. In summary, the results are as follows: (1) The re-adjustment in atmospheric circulation from a longitudinal to a latitudinal model provides a valuable insight into the large-scale circulation background that played a part in the haze episode that occurred in January 2013 in the JingJin-Ji cities and their hinterlands. (2) The regional atmospheric stratification of the PBL was stable and the mixing height was low, both of which suppressed turbulence in the PBL and provided favorable meteorological conditions for haze formation. (3) A SW airflow with wind speeds of 6e11 m/s existed in the lower troposphere between heights 925e850 hpa over the central and southern parts of the Jing-Jin-Ji region. The long distance transport of pollutants from southern Hebei Province intensified pollution levels in Beijing. (4) Air mass trajectory tracking using the HYSPLIT4 model also shows that, below 700 m, the air mass in Beijing comes from the southern part of the Jing-Jin-Ji region. This is consistent with the SW airflow in the lower troposphere. These facts, together with the vertical height results from the CALIPSO extinction coefficient (Wang et al., 2014), prove that transported pollutants travel from southern Hebei to Beijing. The RH was <70% across the polluted region during this haze episode. Nevertheless, the relation between RH and PM2.5 during haze episodes is complicated and worthy of further study. More haze cases are needed as a basis for discussing the relation between high PM2.5 density and humidity. More detailed and quantitative evaluations of regional pollutant transport between Beijing and other cities in the Jing-Jin-Ji region are also needed. Acknowledgments This work is supported by the National Basic Research Program (973) (Grant No. 2014CB441201), the National Natural Scientific Foundation of China (Grant No. 41275007), the National Natural Science Foundation of China (Grant Nos. 41130104 and 41075079), and the CAMS key projects program (Grant No. 2013Z007).
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