1 The distribution and trends of fog and haze in the North China
2 Plain over the past 30 years
3 G. Q. Fu1,3, W. Y. Xu2,4, R. F. Rong1, J. B. Li5, C. S. Zhao2,*
4
5 [1]{Hebei Province Meteorological Service Centre, Shijiazhuang, Hebei, China}
6 [2]{Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, 7 Beijing, China}
8 [3]{Key Laboratory for Meteorology and Ecological Environment of Hebei Province, 9 Shijiazhuang, Hebei, China}
10 [4]{Key Laboratory for Atmospheric Chemistry, Institute of Atmospheric Composition, 11 Chinese Academy of Meteorological Sciences, Beijing, China}
12 [5]{Hebei Province Meteorological Observatory, Shijiazhuang, Hebei, China}
13
14 * Correspondence to: C. S. Zhao ([email protected])
15
16 Abstract
17 Frequent low visibility, haze and fog events were found in the North China Plain (NCP). Data 18 throughout the NCP during the past 30 years were examined to determine the horizontal 19 distribution and decadal trends of low visibility, haze and fog events. The impact of 20 meteorological factors such as wind and RH on those events was investigated. Results reveal 21 distinct distributions of haze and fog days, due to their different formation mechanisms. Low 22 visibility, haze and fog days all display increasing trends of before 1995, a steady stage during 23 the period 1995 to 2003 and a drastically drop thereafter. All three events occurred most 24 frequently during the heating season. Benefiting from emission control measures, haze and fog 25 both show decreasing trends in winter during the past three decades, while summertime haze 26 displays continuous increasing trends. The distribution of wind speed and wind direction as 27 well as the topography within the NCP has determinative impacts on the distribution of haze 28 and fog. Weakened south-easterly winds in the southern part of the NCP has resulted in high 29 pollutant concentrations and frequent haze events along the foot of the Taihang Mountains. The 1 1 orographically generated boundary layer wind convergence line in the central area of the 2 southern NCP is responsible for the frequent fog events in this region. Wind speed has been 3 decreasing throughout the entire southern NCP, resulting in more stable atmospheric conditions 4 and weaker dispersion abilities, calling for harder efforts to control emissions to prevent haze 5 events. Haze events are strongly influenced by the ambient RH. RH values associated with haze 6 days are evidently increasing, suggesting that an increasing fraction of haze events are caused 7 by the hygroscopic growth of aerosols, rather than simply by high aerosol loadings.
8 1 Introduction
9 Low visibility events caused by fog and severe haze events can be a heavy burden for air 10 transport and on-road traffic. The severe aerosol pollution that has led to the visibility 11 impairment is also highly damaging for human health.
12 In developed countries such as the US and the European countries, the employment of emission 13 control measures for sulphur, nitrogen, hydrocarbon compounds and particulate matter have 14 already resulted in significant declines in haziness and major improvements in visibility during 15 the past 50 years (Schichtel et al., 2001;Doyle and Dorling, 2002;Molnár et al., 2008;Vautard 16 et al., 2009). As a rapidly developing country, however, air pollution problems has been 17 haunting China for the past few decades. Visibility has been deteriorating in southeast (Deng et 18 al., 2012) and southwest China (Fu and Wu, 2011) during the past 50 years, with only a few 19 places revealing slight increases in recent years. Six Chinese mega-cities have suffered from 20 continuously decreasing visibilities in the past 30 years (Chang et al., 2009). Overall, visibilities 21 on clear days decreased significantly during the period 1960 to 1990 over all of China, trends 22 thereafter, however, have not been consistent (Wu et al., 2012).
23 The visibility trends were often attributed to the decrease in sulphur dioxide emissions or
24 aerosol concentrations, however, no causality could be established so far. Both SO2 and aerosol 25 pollution have been proved most severe in the North China Plain (NCP), due to the rapid 26 economic growth and the high population density (Xu et al., 2011). As a result, low visibility 27 events frequently occur. Many in-situ measurements and studies have already been performed 28 to study the light scattering properties of aerosols and their impact on horizontal visibility under 29 different relative humidity (RH) in the NCP as part of the Haze in China (HaChi) Campaign 30 (Chen et al., 2012;Ma et al., 2011). Results show that under low RH, low visibility events are 31 mostly induced by the heavy aerosol loading, while under high RH, the influence of aerosol 32 hygroscopic growth becomes stronger and can lead to low visibility events even under moderate
2 1 aerosol pollution levels. However, only few studies have been performed to investigate the 2 spatial distribution of low visibility events and their overall trends during the past few decades 3 in the NCP. (Zhao et al., 2011) analysed the visibility trends in the NCP, suggesting declining 4 visibilities before 1998 and increasing ones during the period 2006 to 2008, with stronger 5 visibility deterioration trends in the summer season due to higher RH and lower wind speeds. 6 Both haze and fog events could thus be responsible for low visibility events, however, those 7 were rarely differentiated in the study of visibility trends, which made it difficult to find the 8 reason behind those visibility variations.
9 In this study, a detailed analysis on the decadal variation and spatial distribution of low visibility, 10 fog and haze events in the most polluted southern part of the NCP will be performed and the 11 impact of wind and RH on those events will be revealed.
12 2 Data and Methodology
13 The North China Plain (NCP) is surrounded to the north by the Yan Mountains, to the west by 14 the Taihang Mountains and to the east by the Bohai Sea (Figure 1). The western part of the 15 NCP is affected by the warm and dry wind coming off the eastern Taihang Mountain slopes, 16 which lead to increased surface stability. Under the influence of the Bohai Sea, the east coast 17 of the NCP often experiences moderate to strong winds. The southern part of the NCP is rather 18 flat and is an important water vapour transport passageway.
19 The location of the 64 meteorological observation sites selected for this study are displayed in 20 Figure 1, with the names of the sites given in Figure 2a. All sites are located in the southern 21 part of the NCP, where the air pollution is most severe. The visibility, RH, wind speed and 22 weather phenomenon observed at 14h local time (LT) during the period 1981 to 2010 were used 23 to analyse the long-term temporal and spatial variation of low visibility, haze and fog events.
24 Both haze and fog can lead to low visibility events. The formation of haze and fog are two 25 distinctly different processes. Therefore, it is necessary to differentiate between those two 26 events when analysing low visibility trends, distributions and variations. In this work, low 27 visibility events were defined as days with visibility at 14 LT below 10 km. Those low visibility 28 events that were not associated with fog, precipitation, dust storms, smoke, snow storms etc. 29 were defined as haze events. Fog events mostly occur during early morning, rarely lasting until 30 14 LT. Therefore, a day was defined as a foggy day if the occurrence of fog was recorded at 31 any time during the day.
3 1 For each station the occurrence of low visibility days, hazy days and foggy days during the 2 period 1981 to 2010 were counted to analyse the spatial distribution of those three types of
3 events. OMI Level 3 SO2 (OMSO2 Readme file v1.1.1, 4 http://so2.gsfc.nasa.gov/Documentation/OMSO2ReleaseDetails_v111_0303.htm) planetary 5 boundary layer column concentrations during 2005-2012 were used to show the correlation
6 between the distribution of SO2 and the occurrence of haze days.
7 The spatial average annual and 10-year moving average frequency of occurrence during the 8 period 1981 to 2010 were used to analyse the trend of the three types of events over those 30
9 years. The trends are then compared to the SO2 emission trends inferred from the China 10 statistical yearbooks (National Bureau of Statistics of China, 1995-2010). The spatial average 11 monthly frequency of occurrence during the period 1981 to 2010 were applied to examine the 12 long-term trend of those threfre types of events in different seasons.
13 The average distribution and the linear trend of wind speeds during the period 1981 to 2010 14 were respectively calculated to reveal their impact on the distribution and trend of the frequency 15 of occurrence of the three types of events. NCEP final analysis meteorology data during 58 fog 16 days in between Jan 2009 and Feb 2010 was used to reveal the characteristic wind field that has 17 led to fog events in the southern NCP. Surface automatic weather station wind data during May- 18 Dec 2009 were used as supplements to analyse the distribution of the wind field in the entire 19 NCP and how it impacts haze formation.
20 The average distribution of the number of days with 14h RH > 70% and the number of haze 21 days with 14h RH > 70% were calculated to show its relation to the distribution of low visibility, 22 haze and fog events. To further study the influence of RH on haze events, the decadal variation 23 of the ratio of the annual average haze related RH to the total average RH was compared against 24 the decadal variation in annual haze days. The season-decadal trend of the frequency of 25 occurrence of haze related RH values falling in the range of RH<50%, 50%
27
4 1 3 Results and Discussions
2 3.1 The spatial distribution of low visibility, haze and fog
3 Low visibility, haze and fog events are formed through different mechanisms, hence their 4 occurrences are determined by different parameters. This may lead to distinct spatial 5 distributions of the frequency of occurrence. The spatial distributions of the occurrences of low 6 visibility, haze and fog days during the period 1981 to 2010 in the southern NCP are depicted 7 in Figure 2b-d.
8 Haze events are caused by either high aerosol loadings or the strong hygroscopic growth of
9 aerosols. SO2 is the main precursor of sulphate aerosols, which are highly hygroscopic and exist
10 in abundance in the NCP (Liu et al., 2014). The average SO2 planetary boundary layer (PBL)
11 column amount during the period 2005 to 2012 is depicted in Figure 2a. High SO2 column 12 concentrations are found on the western border, which is caused on the one hand by the large
13 amount of SO2 emissions in this region and on the other hand by the special topography which
14 leads poor dispersion conditions. The high SO2 concentrations have led to large loadings of 15 highly hygroscopic sulphate aerosol, which can easily lead to haze events in this region. Figure 16 2c displays the distribution frequency of haze days, and which shows a very similar distribution
17 to that of SO2 column concentration. Haze also most frequently occurs on the western border 18 of the NCP, along the eastern slope of the Taihang Mountains.
19 The distribution of fog days is distinctly different than that of haze days (Figure 2d). Fog events 20 most commonly occur along the centre line of the plain region, parallel to the southwest- 21 northeast ridge of the Taihang Mountain. This suggests that the formation of haze and fog are 22 controlled by different physical processes. While the formation of haze mainly depends on the 23 local aerosol pollution level, the formation of fog is not limited to the presence of cloud 24 condensation nucleus under the regional pollution state of the NCP. The special topography and 25 meteorology in the NCP could be the responsible for the spatial distribution of fog event 26 occurrences, which will be further discussed in section 3.3.2.
27 Low visibility events can be caused by both haze and fog. The area with the most low visibility 28 days covers both the area with the most haze and fog days. Low visibility mainly happens in 29 the western part of the region, which is similar to the distribution of haze days. High centres 30 were found around the three major cities Shijiazhuang, Xingtai and Handan. This indicates that 31 low visibility events in the NCP are mostly caused by haze, rather than by fog.
5 1 Due to the distinct spatial distribution of low visibility, haze and fog days, the spatial coverage 2 of their influence is also different. Table 1 lists the number (percentage) of stations within 3 various count ranges of low visibility, haze and fog days during the period 1981 to 2010. In the 4 southern NCP, 34% and 36% of the stations have respectively observed 500-999 and 1000- 5 1999 low visibility days, while another 12% of the stations have gone through more than 2000 6 low visibility days and 17% rather clean stations have only had 100-499 low visibility days. 7 Although haze events can be very frequent, their impact is largely constrained within the limited 8 area in the southwest. 22% of the sites have experienced more than 1000 haze days during the 9 past 30 years, while 55% of the sites have had less than 500 haze days. For fog events, however, 10 73% of the sites have experienced 500-999 fog days during the past 30 years, suggesting that a 11 large area is under a similar influence of fog events.
12 In all, low visibility, haze and fog days are distinctly distributed in the NCP, because their 13 formations are controlled by different mechanisms. The spatial distribution of haze days is 14 determined by the distribution of pollutant emissions and by the topography of the NCP, thus 15 only influencing a small area near the edge of the mountains. The distribution of fog days 16 mainly determined by the topography and meteorology, affecting a large area parallel to the 17 mountains. Low visibility is mostly induced by haze events, thus showing a similar distribution 18 to that of haze days.
19 3.2 The trends of low visibility, haze and fog during the period 1981 to 2010
20 The temporal trends of low visibility, haze and fog are influenced by the variation of many 21 factors such as pollutant emissions, aerosol compositions and meteorological conditions, etc. 22 During the past 30 years, the NCP has undergone rapid economic developments accompanied 23 by growing energy consumptions. In the past decade, policies were made to reduce pollutant 24 emissions in the NCP in hope of improving the air quality there. In this section, the decadal 25 trends of low visibility, haze and fog days will be examined to show the combined effect of 26 various influencing factors.
27 3.2.1 The decadal trends of low visibility, haze and fog
28 The decadal variations of the annual and the 10-year moving average of the occurrence of low 29 visibility, haze and fog days during the period 1981 to 2010 are displayed in Figure 3b. Before 30 1995, due to the rapid economic development, increasing energy consumption has led to 31 increasing numbers of haze and fog days, which resulted in more frequent low visibility events. 6 1 During the period 1995 to 2003, with the development and employment of waste gas processing 2 techniques, occurrences of low visibility, haze and fog days have entered a steady stage. After 3 2003, in preparation for the 2008 Beijing Olympic Games, under the influence of emission 4 control policies, the number of low visibility, haze and fog days have drastically dropped. The
5 trend of low visibility and haze days agrees well with that of the SO2 emissions (Figure 3a), 6 suggesting that the control in emissions during the last decade effectively led to increased 7 visibility and the decline in the number of hazy days.
8 The slope of the linear fits to the 10-year moving averages of low visibility, haze and fog 9 occurrence days during the period 1981 to 2010 are shown Figure 3c. The maximum increasing 10 slope for haze events can be found between 1989 and 1998, reaching 17 days 10a-1. After the 11 period of 1994 to 2003, haze events have been generally declining, reaching a maximum 12 decreasing slope of 11 days 10a-1 during the period 1996 to 2005. After the period of 1987 to 13 1996, fog occurrences show a variable but generally decreasing trend. Major decreases occurred 14 during the periods of 1990 to 1999 and 2001 to 2010, with slopes of 10 days 10a-1. The number 15 of low visibility days increased before the period of 1996 to 2005, with a maximum slope of 17 16 days 10a-1 during the period 1983 to 1992. Afterwards, low visibility events were found to be 17 less frequent, the maximum decreasing slope was found during the period 2001 to 2010, 18 reaching 16 days 10a-1.
19 Generally, the frequency of occurrence of low visibility, haze and fog in the NCP have all
20 increased during the period 1980 to 1990. Due to the effect of SO2 emission reduction during 21 the past few years, the occurrences of low visibility, haze and fog have all decreased back to a 22 similar level as in 1980.
23 3.2.2 The season-decadal trends of low visibility, haze and fog
24 Low visibility, haze and fog are strongly influenced by meteorological conditions, which in the 25 NCP vary distinctly with season. This may cause different decadal variation in different seasons. 26 Figure 4 shows the season-decadal trends of the frequency of occurrence of low visibility, haze 27 and fog during the period 1981 to 2010.
28 During the past 30 years, haze events were most common during the heating season (Nov-Mar) 29 and least common during the end of spring and early autumn (Figure 4d). After 1990, haze 30 events started to appear not only during the heating season but also during summertime (Figure 31 4b2). From the 10-year moving average frequency of occurrence of haze in spring, summer,
7 1 autumn and winter (Figure 4b1) it can be seen that summertime haze occurrence frequency has 2 been continuously increasing in the past three decades, while that in wintertime has been 3 declining since 1995. In the past, residential and commercial heating during winter has led to 4 high emissions of soot, which were responsible for the degradation of visibility. With the 5 development of central heating and waste gas processing techniques, emissions from heating 6 processes have been gradually declining, leading to less haze events during winter. However, 7 atmospheric pollution has become more complex during recent years. During summer, the 8 atmosphere becomes highly oxidative, aerosol pollution coexists with high concentrations of
9 O3 and VOCs (Ran et al., 2011), which contribute to the fast aging and secondary formation of 10 aerosols. Aged aerosols are more hygroscopic and can easily lead to haze events under suitable 11 RH conditions (Chen et al., 2012). The increased use of air conditioning during summertime 12 has resulted in increasing energy consumption trends (Zhang, 2004), which could also lead to 13 increased aerosol loading and reduced visibility during summer.
14 Fog events appear most frequently during winter and least frequently during spring until early 15 summer. The frequency of occurrence of fog during the past 30 years shows no significant trend 16 in spring and summer, but shows a declining trend during winter after 1990 (Figure 4c1).
17 Low visibility events are caused by both haze and fog. Their combined effect has resulted in 18 high frequency of occurrence of low visibility during winter and lower ones during spring and 19 summer (Figure 4a1). Low visibility events during summertime have been continuously 20 increasing in the past three decades, while those during spring, autumn and winter have been 21 decreasing since 2000.
22 In all, wintertime low visibility, haze and fog days have been declining, while summertime low 23 visibility and haze days have been increasing due to the variation in aerosol composition and 24 the summertime high RH that favors the hygroscopic growth of particles. Additionally, the 25 increased energy consumption due to air conditioning might also lead to increased summertime 26 aerosol loadings and decreased visibilities.
27 3.3 The impact of wind on low visibility, haze and fog
28 3.3.1 The impact of wind on haze
29 Wind direction can influence the transport of pollutants, thus determining the spatial 30 distribution of visibility degrading pollutants. Wind speed greatly influences the atmospheric
8 1 stability and the atmospheric dispersion capability. Low wind speeds suggest that the 2 atmosphere is rather stable and the dispersion of local emissions is limited.
3 Figure 5a shows the spatial distribution of the average 14h (LT) surface wind direction during 4 May-Dec 2009 in the entire NCP. The Yan and Taihang Mountains are governed by northwest 5 winds, while the plain area is dominated by winds from the south. Along the south-western 6 edge of the Taihang Mountains, the winds all come from the south-eastern direction, which can 7 transport the emitted pollutants from the eastern part of the plain area to the western part. The 8 weak north-western winds in the mountains block pollutant transport, leading to the 9 accumulation of heavy loadings of aerosol and its precursors along the foot of the Taihang 10 Mountains. This explains why haze events are most common in this region.
11 Figure 6a displays the distribution of the average 14h (LT) wind speed during the period 1981 12 to 2010. Average wind speeds at 14h (LT) decrease from the southeast to the northwest. Lowest 13 values are found in the northwest corner, decreasing below 3 m s-1, while similar values were 14 also detected in Xingtai and Handan, two heavily polluted cities in the southern corner. Average 15 wind speeds near the coast (in the vicinity of Cangzhou) and in the southeast corner are large, 16 reaching over 5 m s-1. The low wind speeds are caused primarily by the Taihang Mountains, 17 which slow down westerly winds before they reach the plain region. As can be seen from Figure 18 5, the Taihang Mountains are higher in the north and lower in the south, which explains why 19 wind speeds are lowest in the northwest corner. Another factor influencing the surface wind 20 speed is the surface roughness. Large cities with densely distributed high buildings will add to 21 the surface roughness and slow down near surface winds.
22 Although dispersion abilities are weakest in the northwest corner, due to relatively lower 23 pollutant emissions in the mountain areas, low visibility and haze events are rare on the 24 northwestern edge. However, the area near Baoding, where the pollution level is slightly higher 25 than in the mountain areas and distinctively lower than over the polluted region in the southwest, 26 is heavily affected by the low wind speeds, showing frequently occurring low visibility and 27 haze events. The southwestern edge of the southern NCP region suffers from high pollutant 28 emissions and is subject to conditions that lead to relatively weak dispersion. The low average 29 wind speed centers in Shijiazhuang, Xingtai and Handan (Figure 7a) conform to the high haze 30 day count centers (Figure 2c).
31 Figure 6b shows how the linear slope of the annual wind speed during 1981-2010 is distributed 32 in the southern NCP. Over the entire region, wind speeds have been decreasing in the past 30
9 1 years. Large decreasing slopes were found in those regions with high average wind speeds, but 2 also in parts of the northeast corner and in Shijiazhuang, which is the capital of Hebei Province. 3 The regional average 10-year decreasing slope reaches 0.2 m s-1 10a-1, while that in 4 Shijiazhuang reaches over 0.2 m s-1 10a-1 (Figure 6c). The decrease in wind speed suggests that 5 the atmosphere has become more stable and dispersion capabilities has weakened throughout 6 the entire region, which calls for even more extreme efforts to control emissions in order to 7 prevent haze events.
8
9 3.3.2 The impact of wind on fog
10 The formation of fog needs a supersaturated water vapour environment, which requires 11 favourable meteorological conditions. Figure 5b shows the average 8h (LT) 1000 hPa wind 12 field during 58 fog days that occurred between the Jan 2009 and Feb 2010, on which more than 13 10 stations reported the occurrence of fog. Due to the topography of the NCP, an orographically 14 generated, boundary layer wind convergence line going from southwest to the northeast is 15 formed as indicated by the red line in Figure 5b. The convergence line is also parallel to the 16 ridge of the Taihang Mountains, coinciding with the zone with the most frequent fog events 17 (Figure 2d). Fog events usually occur during the night, when the mountain areas cool off faster 18 than the plain area, forming a temperature gradient. To the west of the convergence line, near 19 surface winds are dominated by cold north-westerly mountain winds, while to the east of the 20 convergence line, surface winds are north-easterly or south-easterly. With the north-easterly 21 winds comes the warm humid air from the Bohai Sea, while the south-easterly path is the most 22 typical water vapour transport passage way for the entire NCP region. The convergence zone 23 will hence favour the accumulation and convergence of water vapour, and lead to the formation 24 of fog in this area.
25 3.4 The impact of RH on low visibility, haze and fog
26 The ambient RH can influence the visibility by affecting the hygroscopic growth and scattering 27 abilities of atmospheric aerosols. Chen et al. (2012) suggest that, under RH<80%, visibility is 28 highly dependent on dry aerosol volume concentrations. The hygroscopic growth of aerosols in 29 this RH range only becomes important for visibility impairment if the aerosol loading is high. 30 While for RH greater than 80%, the hygroscopic growth of aerosols can greatly affect visibility, 31 even under average aerosol pollution levels. 10 1 The distribution of the number of days with 14h (LT) RH > 70% and the number of haze days 2 with 14h (LT) RH > 70% are depicted in Figure 8. High RH days most frequently occur in the 3 southeast, where there is a water vapour transport passageway, and along the convergence line, 4 which was observed to be favourable for the formation of fog events in Sect. 3.3.2. The 5 distribution of the number of haze days with 14h (LT) RH > 70% (Figure 8b) is similar to that 6 of the number of all days with 14h (LT) RH > 70% (Figure 8a), only with significantly smaller 7 numbers to the east of Shijiazhuang, because haze is not as severe in that region. Compared 8 with the distribution of the total number of low visibility and haze days (Figure 2b-c), it can be 9 noted that the frequent low visibility events along the southern edge of the Taihang mountain 10 were caused primarily by the heavy aerosol pollution and not by haze events associated with 11 high RH. The low visibility events in the vicinity of Shijiazhuang, however, were not only 12 caused by severe pollution but were also associated with high RH events, indicating that the 13 hygroscopic growth of aerosols plays an important role in the visibility impairment in this 14 region. Although aerosol pollution is not as severe in the south-eastern part of our area of study, 15 a large fraction of haze days are associated with high RH events, suggesting that the high RH 16 in this region is able to impair visibility even if the aerosol concentration is not very high. The 17 hygroscopic growth induced light scattering of aerosols plays a dominant role in the degradation 18 of visibility in this region.
19 The 14h (LT) RH values that were accompanied by haze events (haze event RH) were sorted 20 out and its frequency of occurrence in the range of RH <50%, 50%
28 The frequency of occurrence of haze events under low RH (RH< 50%) in autumn, winter and 29 spring has decreased over the past 30 years, with a slight rebound in the winter time data at the 30 end of the last decade. This indicates that aerosol loadings during winter havet declined, which 31 could be the result of effective emission control measures during heating seasons. The 32 frequency of occurrence of haze events under higher RH (RH>60%) in autumn, winter and
11 1 spring show increasing trends during the past 30 years. A possible cause could be the fact that 2 atmospheric pollution has become more complicated over the years, leading to a higher fraction 3 of secondary aerosols, which are more hygroscopic and are more likely to impair visibility 4 through hygroscopic growth processes.
5 To further study if this phenomenon exists throughout the entire region, the decadal variation 6 of haze event RH, annual average RH and annual haze days at four representative stations were 7 analysed as is depicted in Figure 9. The RH associated with haze events is typically higher than 8 the average RH values. Zanhuang station shows continuously increasing number of haze days 9 throughout the past 30 years, while Hengshui displays a continuous decline throughout the past 10 20 years (Figure 9b). Shijiazhuang and Longyao have both undergone an increase before 2000 11 and a decrease thereafter. Significant increasing trends can be found in the ratio of haze event 12 RH and average RH at all four stations (Figure 9a). This means that a higher fraction of the 13 haze events are now caused by the hygroscopic growth of aerosols. The reduction in primary 14 aerosol emissions further amplifies this effect.
15
16 4 Summary
17 In this study, the spatial distribution and decadal variation of low visibility, fog and haze events 18 in the most polluted southern part of the NCP during the past 30 years were analysed and the 19 impact of wind and RH on those events was investigated.
20 Haze and fog are distinctly distributed, which was determined by the topography of the NCP 21 and the distribution of wind speed and wind direction. Haze occurs mostly along the south- 22 western edge of the plain region, while fog mostly occurs within the central band area parallel 23 to the ridge of the Taihang Mountains.
24 Annual low visibility, haze and fog days have shown increasing trends before 1995, have 25 entered a steady stage during the period 1995 to 2003 and have drastically dropped thereafter 26 during the preparation stage for the Beijing Olympic Games in 2008. Low visibility, haze and 27 fog events all occurred most frequently during the heating season in the past three decades. 28 Wintertime haze and fog both show decreasing trends, as a benefit of the improvements in 29 central heating and desulfurization techniques. Summertime haze, however, displayed 30 continuously increasing trends during the period 1981 to 2010.
12 1 Both the distribution of the wind field and the decadal variation of wind speeds have great 2 impacts on the occurrence of low visibility, haze and fog events. South-easterly winds in the 3 southern part of the NCP which are blocked by the weak north-western winds in the Taihang 4 Mountains has resulted in high pollutant concentrations along the foot of the mountain, which 5 has led to frequent haze events. The convergence zone parallel to the ridge of the Taihang 6 Mountains in the central area of the southern NCP was responsible for the frequent fog events 7 in this region. Wind speed has been decreasing throughout the entire southern NCP, resulting 8 in more stable atmospheric conditions and thus stronger inversions and reduced dispersion 9 capability, which calls for even increased efforts to control emissions in order to prevent haze 10 events.
11 Haze and low visibility events were strongly influenced by the ambient RH. The frequent high 12 RH events near Shijiazhuang and in the south-eastern part of the region favours the hygroscopic 13 growth of aerosols and plays a dominant role in the visibility impairment in those areas. Under 14 RH below 60%, haze events mostly occurred during the heating seasons, while for RH above 15 60%, haze events were more likely to happen during summertime. Although annual RH values 16 displayed no significant trends within the past 30 years, those RH values associated with haze 17 days were evidently increasing, suggesting that an increasing fraction of haze events are caused 18 by the hygroscopic growth of aerosols, rather than simply by high aerosol loadings.
19
20 Acknowledgements
21 This work is supported by the National 973 project of China (2011CB403402), the National 22 Natural Science Foundation of China under Grant No. 41375134, the Beijing Natural Science 23 Foundation (8131003), the Beijing municipal science and technology plan project No. 24 Z131100006113013.
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13 1 References
2 Chang, D., Song, Y., and Liu, B.: Visibility trends in six megacities in China 1973–2007, 3 Atmospheric Research, 94, 161-167, http://dx.doi.org/10.1016/j.atmosres.2009.05.006, 2009. 4 Chen, J., Zhao, C. S., Ma, N., Liu, P. F., Göbel, T., Hallbauer, E., Deng, Z. Z., Ran, L., Xu, W. 5 Y., Liang, Z., Liu, H. J., Yan, P., Zhou, X. J., and Wiedensohler, A.: A parameterization of low 6 visibilities for hazy days in the North China Plain, Atmos. Chem. Phys., 12, 4935-4950, 7 10.5194/acp-12-4935-2012, 2012. 8 Danielson, J. J., and Gesch, D. B.: Global multi-resolution terrain elevation data 2010 9 (GMTED2010): U.S. Geological Survey Open-File Report 2011-1073, 26 2011. 10 Deng, J., Du, K., Wang, K., Yuan, C.-S., and Zhao, J.: Long-term atmospheric visibility trend 11 in Southeast China, 1973–2010, Atmospheric Environment, 59, 11-21, 12 http://dx.doi.org/10.1016/j.atmosenv.2012.05.023, 2012. 13 Doyle, M., and Dorling, S.: Visibility trends in the UK 1950–1997, Atmospheric Environment, 14 36, 3161-3172, http://dx.doi.org/10.1016/S1352-2310(02)00248-0, 2002. 15 Fu, C., and Wu, J.: The Different Characteristics of Sunny Visibility over Southwest China in 16 Recent 50 Years, Procedia Environmental Sciences, 10, Part A, 247-254, 17 http://dx.doi.org/10.1016/j.proenv.2011.09.041, 2011. 18 Liu, H. J., Zhao, C. S., Nekat, B., Ma, N., Wiedensohler, A., van Pinxteren, D., Spindler, G., 19 Müller, K., and Herrmann, H.: Aerosol hygroscopicity derived from size-segregated chemical 20 composition and its parameterization in the North China Plain, Atmos. Chem. Phys., 14, 2525- 21 2539, 10.5194/acp-14-2525-2014, 2014. 22 Ma, N., Zhao, C. S., Nowak, A., Müller, T., Pfeifer, S., Cheng, Y. F., Deng, Z. Z., Liu, P. F., 23 Xu, W. Y., Ran, L., Yan, P., Göbel, T., Hallbauer, E., Mildenberger, K., Henning, S., Yu, J., 24 Chen, L. L., Zhou, X. J., Stratmann, F., and Wiedensohler, A.: Aerosol optical properties in the 25 North China Plain during HaChi campaign: an in-situ optical closure study, Atmos. Chem. 26 Phys., 11, 5959-5973, 10.5194/acp-11-5959-2011, 2011. 27 Molnár, A., Mészáros, E., Imre, K., and Rüll, A.: Trends in visibility over Hungary between 28 1996 and 2002, Atmospheric Environment, 42, 2621-2629, 29 http://dx.doi.org/10.1016/j.atmosenv.2007.05.012, 2008. 30 China Statistical Yearbook: http://www.stats.gov.cn/tjsj/ndsj/, access: 26 Mar 2014, 1995-2010. 31 Ran, L., Zhao, C. S., Xu, W. Y., Lu, X. Q., Han, M., Lin, W. L., Yan, P., Xu, X. B., Deng, Z. 32 Z., Ma, N., Liu, P. F., Yu, J., Liang, W. D., and Chen, L. L.: VOC reactivity and its effect on 33 ozone production during the HaChi summer campaign, Atmos. Chem. Phys., 11, 4657-4667, 34 10.5194/acp-11-4657-2011, 2011. 35 Schichtel, B. A., Husar, R. B., Falke, S. R., and Wilson, W. E.: Haze trends over the United 36 States, 1980–1995, Atmospheric Environment, 35, 5205-5210, 37 http://dx.doi.org/10.1016/S1352-2310(01)00317-X, 2001. 38 Vautard, R., Yiou, P., and van Oldenborgh, G. J.: Decline of fog, mist and haze in Europe over 39 the past 30 years, Nature Geosci, 2, 115-119, 40 http://www.nature.com/ngeo/journal/v2/n2/suppinfo/ngeo414_S1.html, 2009. 41 Wu, J., Fu, C., Zhang, L., and Tang, J.: Trends of visibility on sunny days in China in the recent 42 50 years, Atmospheric Environment, 55, 339-346, 43 http://dx.doi.org/10.1016/j.atmosenv.2012.03.037, 2012. 14 1 Xu, W. Y., Zhao, C. S., Ran, L., Deng, Z. Z., Liu, P. F., Ma, N., Lin, W. L., Xu, X. B., Yan, P., 2 He, X., Yu, J., Liang, W. D., and Chen, L. L.: Characteristics of pollutants and their correlation 3 to meteorological conditions at a suburban site in the North China Plain, Atmos. Chem. Phys., 4 11, 4353-4369, 10.5194/acp-11-4353-2011, 2011. 5 Zhang, Q.: Residential energy consumption in China and its comparison with Japan, Canada, 6 and USA, Energy and Buildings, 36, 1217-1225, 7 http://dx.doi.org/10.1016/j.enbuild.2003.08.002, 2004. 8 Zhao, P., Zhang, X., Xu, X., and Zhao, X.: Long-term visibility trends and characteristics in the 9 region of Beijing, Tianjin, and Hebei, China, Atmospheric Research, 101, 711-718, 10 http://dx.doi.org/10.1016/j.atmosres.2011.04.019, 2011. 11 12 13
15 1 2 Table 1 The number (percentage) of stations under various count ranges of low visibility, haze 3 and fog days during the period 1981 to 2010
Days >=3000 2000-2999 1000-1999 500-999 100-499 <100 Stations
Low Visibility 2 (3%) 6 (9%) 22(34%) 23 (36%) 11 (17%) 0 (0%)
Haze 0 (0%) 2 (3%) 12(19%) 15 (23%) 30 (47%) 5 (8%)
Fog 0 (0%) 0 (0%) 8 (13%) 47 (73%) 9 (14%) 0 (0%) 4
5
16 1
17 1 2 Figure 1 The location of the measurement sites (circle), the area of study (red line) and the 3 regional topography by (Danielson and Gesch, 2011) (color). 4
18 40° N 40° N 2 Langfang a) Zhuozhou b) Gu'an langfang Laiyuan Yixian Xincheng 3200 Baxian Rongcheng 1.8 2900 Fuping Baoding Anxin baoding Shunping Wenan 2600 Renqiu Quyang Gaoyang Qingxian Dingzhou 2400 Xingtang Hejian Huanghua 1.6 Xinle Anguo Raoyang 2200 Pingshan Wuji Cangzhou cangzhou Xianxian shijiazhuang Jingxing ZhengdingGaocheng Yanshan 1800 ° Shenxian ° 38 N Shijiazhuang 38 N Xinji Dongguang 1.4 1400 Luancheng Zhaoxian Fucheng hengshui Zanhuang Ningjin Hengshui Wuqiao 1100 Jixian Lincheng Longyao 900 Neiqiu Nangong Gucheng 1.2 PBL Column Amount (D.U.) xingtai Julu 700 Xingtai Qinghe 2 Shahe Weixian Linxi 500 Quzhou SO Wu'a n Handan handan Shexian Guantao 1 300 Fengfeng Feixiang 100 Cixian Daming ° ° 36 N 0.8 36 N 1981-2010 during days visibility low of Count 114° E 116° E 118° E 114° E 116° E 118° E ° 40° N 40 N c) d) langfang langfang 3200 3200 baoding 2900 2900 2600 baoding 2600 2400 2400 cangzhou 2200 shijiazhuang 2200 cangzhou 1800 shijiazhuang 1800 ° ° 38 N hengshui 38 N 1400 1400 hengshui 1100 1100 900 xingtai 900 xingtai 700 700 500 500 handan 300 300 handan
100 1981-2010 during days fog of Count 100
Count of haze days during 1981-2010 during days haze of Count
° ° 36 N 36 N ° ° ° 114° E 116° E 118° E 114 E 116 E 118 E 1
2 Figure 2 a) Average OMI SO2 PBL column amount during the period 2005 to 2012 with the 3 location of the 64 stations and the number of b) low visibility, c) haze and d) fog days during 4 the period 1981 to 2010 in the southern NCP. 5
19 )
−1 1.8 a)
ton a 1.6
6
1.4
1.2
emission (10 2 1
SO 1985 1990 1995 2000 2005 2010
b) 40
20
Occurence count (days) 0 1985 1990 1995 2000 2005 2010
) 20 c)
−1 10 0 Low visibility at 14h LT −10 Haze at 14h LT
Slope (days 10a −20 Fog during the day 81−90 86−95 91−00 96−05 01−10 1 Year
2 Figure 3 a) The annual SO2 emissions in Hebei Province inferred from the China statistical 3 yearbooks (National Bureau of Statistics of China, 1995-2010); b) the annual (solid lines) and 4 the 10-year moving average (dashed lines) occurrence of low visibility, haze and fog days 5 among the 64 stations during the period 1981 to 2010; b) The slope of the linear fits to the 10- 6 year moving averages of low visibility, haze and fog days among the 64 stations during the 7 period 1981 to 2010. 8
20 1
30 15 15 a1) b1) c1) Spring Summer 20 10 10 Autumn Winter Low visibility at 14h LT 10 5 5 Haze at 14h LT Fog during the day 0 0 0 Occurence Frequency (%) 2 2 2 2 1 a2) 1 b2) 1 c2) 1 d) 12 12 12 12 11 11 11 11 10 10 10 10 9 9 9 9 8 8 8 8 Month 7 7 7 7 6 6 6 6 5 5 5 5 4 4 4 4 3 3 3 3 1985 1990 1995 2000 2005 1985 1990 1995 2000 2005 1985 1990 1995 2000 2005 0 10 20 30 Year Year Year Occurence Frequency (%)
0 10 20 30 40 0 5 10 15 20 25 0 10 20 30 40 2 Occurence Frequency (%) Occurence Frequency (%) Occurence Frequency (%) 3 Figure 4 1) The trend of the 10-year moving average frequency of occurrence of a) low 4 visibility events at 14 LT, b) haze events at 14 LT and c) fog events during the day among the 5 64 stations during spring (MAM), summer (JJA), autumn (SON) and winter (DJF); 2) The 6 season-annual variation of the average frequency of occurrence of a) low visibility events at 7 14 LT, b) haze events at 14 LT and c) fog events during the day among the 64 stations and d) 8 the seasonal variation of the average frequency of occurrence of low visibility events at 14 LT, 9 haze events at 14 LT and fog events among the 64 stations during 1981-2010. 10
21 1
a) S b) 50° ° 42.5 1800 1000hPa wind field SE -1 Vmax = 8 m s 1600 E Orographic convergence line 45° 1400
NE 1200 40.0° N 1000 40° 800 NW 600
W ° 37.5° Average surface wind direction 35 400 SW 200
Surface Geopotential Height (gpm) 0 S 30° 2 115.0° 117.5° 120.0° 105° 110° 115° 120° 125°
3 Figure 5 a) Averaged 14h surface wind direction during May-Dec 2009 given by 307 AWS
4 stations. b) The orographic geopotential height above sea level (shading), the average 8h
5 1000 hPa NCEP final analysis wind field (black arrows) and the orographically generated wind
6 convergence line (red line) of 58 fog days during Jan 2009 - Feb 2010.
7
8
22 1
40 40 4.5
10a
11
--
-1
a) b) ) c)
-1
(m s ) 4
m s 39.5 5.1 langfang 4.9 langfang 3.5 39 baoding 39 -0.1 4.7 baoding 4.5 3 38.5 -0.2 cangzhou 4.3 cangzhou shijiazhuang shijiazhuang 2.5 4.1 38 38 -0.3 3.9 hengshui hengshui 2 3.7 37.5 -0.4 xingtai 3.5 xingtai 1.5 37 3.3 37 -0.5 1 3.1 Regional Average:10a slope = −0.2 m s-1 10a-1 handan handan 2.9 36.5 -0.6 0.5 Cangzhou:10a slope = −0. 2 m s-1 10a-1
Annual average 14h (LT) wind speed (m s 2.7 Shijiazhuang:10a slope = −0. 3 m s-1 10a-1 36 ( 1981-2010 during speed wind Average ) 36 0 114 115 116 117 114 115 116 117 1985 1990 1995 2000 2005 2010 2 1981-2010 during trend speed wind of slope 10a Year
3 Figure 6 a) Distribution of the average 14h (LT) wind speed during 1981-2010 in the southern
4 NCP; b) distribution of the linear slope of the annual wind speed during 1981-2010 in the
5 southern NCP; c) Variation (dashed lines) and linear trend (solid lines) of the regional average
6 14h (LT) wind speed and that of Cangzhou and Shijiazhuang during 1981-2010.
7
23 ) 100 80 80 80 Spring Spring a1) b1) c1) d1) Summer Summer 60 60 60 Autumn Autumn Winter Winter 50 40 40 40 RH <50% 50%
0 50 100 0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 1 Occurence Frequency (%) Occurence Frequency (%) Occurence Frequency (%) Occurence Frequency (%) 2 Figure 7 1) The decadal trend of the 10-year moving average frequency of occurrence of 14h 3 (LT) haze event RH in the range of a) RH <50%, b) 50%
24 40 40 a) b) langfang 700 langfang 39 baoding 39 baoding 300 660 260 620 220 cangzhou 580 cangzhou 38 shijiazhuang 540 38 shijiazhuang 180 hengshui 500 hengshui 140 460 xingtai xingtai 100 37 420 37 380 60 handan handan 340 20
Count of days with RH above 70% above RH with days of Count
36 36 70% above RH with days haze of Count 1 114 115 116 117 114 115 116 117 2 Figure 8 The distribution of a) the number of days with 14h (LT) RH above 70% and b) the 3 number of haze days with 14h (LT) RH above 70% during 1981-2010 in the southern NCP.
25 Shijiazhuang Hengshui Longyao Zanhuang
1.6 a) Annual average 10a moving average 1.4 /RH
Haze 1.2 RH
1
150 b)
100
50
Annual count of haze days 0 1985 1990 1995 2000 2005 2010 1 Year 2 Figure 9 a) The decadal variation of the haze event associated RH divided by the average RH 3 and b) the annual number of haze days during the period 1981 to 2010 at Shijiazhuang, 4 Hengshui, Longyao and Zanhuang.
5
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