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 1995~2003 and a drastically drop thereafter. All three events occurred most frequently during 24 the heating season. Benefiting from emission control measures, haze and fog both show 25 decreasing trends in winter during the past 3 decades, while summertime haze displays 26 continuous increasing trends. The distribution of wind speed and wind direction as well as the 27 topography within the NCP has determinative impacts on the distribution of haze and fog. 28 Weakened south-easterly winds in the southern part of the NCP has resulted in high pollutant 29 concentrations and frequent haze events along the foot of the Taihang Mountains. The 1 1 orographic wind convergence zone in the central band area of the southern NCP is responsible 2 for the frequent fog events in this region. Wind speed has been decreasing throughout the entire 3 southern NCP, resulting in more stable atmospheric conditions and weaker dispersion abilities, 4 calling for harder efforts to control emissions to prevent haze events. Haze events are strongly 5 influenced by the ambient RH. RH values associated with haze days are evidently increasing, 6 suggesting that an increasing fraction of haze events are caused by the hygroscopic growth of 7 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 sulphur 13 emission control measures have already resulted in significant declines in haziness and major 14 improvements in visibility during the past 50 years (Schichtel et al., 2001;Doyle and Dorling, 15 2002;Molnár et al., 2008;Vautard et al., 2009). As a rapidly developing country, however, air 16 pollution problems has been haunting China for the past few decades. Visibility has been 17 deteriorating in southeast (Deng et al., 2012) and southwest China (Fu and Wu, 2011) during 18 the past 50 years, with only a few places revealing slight increases in recent years. Six Chinese 19 mega-cities have suffered from continuously decreasing visibilities in the past 30 years (Chang 20 et al., 2009). Overall, sunny day visibilities decreased significantly during 1960~1990 all over 21 China, trends thereafter, however, were not coherent (Wu et al., 2012).
22 The visibility trends were often attributed to the variation of sulphur dioxide emissions or
23 aerosol concentrations, however, no causality could be established so far. Both SO2 and aerosol 24 pollution have been proved most severe in the North China Plain (NCP), due to the rapid 25 economic growth and the high population density (Xu et al., 2011). As a result, low visibility 26 events frequently occur. Many in-situ measurements and studies have already been performed 27 to study the light scattering properties of aerosols and their impact on horizontal visibility under 28 different relative humidity (RH) in the NCP as part of the Haze in China (HaChi) Campaign 29 (Chen et al., 2012;Ma et al., 2011). Results show that under low RH, low visibility events are 30 mostly induced by the heavy aerosol loading, while under high RH, the influence of aerosol 31 hygroscopic growth becomes stronger and can lead to low visibility events even under moderate 32 aerosol pollution levels. However, only few studies have been performed to investigate the
2 1 spatial distribution of low visibility events and their overall variation trend during the past few 2 decades in the NCP. (Zhao et al., 2011) analysed the visibility trends in the NCP, suggesting 3 declining visibilities before 1998 and increasing ones during 2006~2008, with stronger 4 visibility deterioration trends in the summer season due to higher RH and lower wind speeds. 5 Both haze and fog events could be the responsible for low visibility events, however, those 6 were rarely differentiated in the study of visibility trends, which made it difficult to find the 7 reason behind those visibility variations.
8 In this study, a detailed analysis on the decadal variation and spatial distribution of low visibility, 9 fog and haze events in the most polluted southern part of the NCP will be performed and the 10 impact of wind and RH on those events will be revealed.
11 2 Data and Methodology
12 The North China Plain (NCP) is surrounded to the north by the Yan Mountains, to the west by 13 the Taihang Mountains and to the east by the Bohai Sea (Figure 1). The western part of the 14 NCP is affected by the warm and dry wind coming off the eastern Taihang Mountain slopes, 15 which lead to increased surface stability. Under the influence of the Bohai Sea, the east coast 16 of the NCP often experiences large winds. The southern part of the NCP is rather flat and is an 17 important water vapour transport passageway.
18 The location of the 64 meteorological observation sites selected for this study are displayed in 19 Figure 1, with the names of the sites given in Figure 2a. All sites are located in the southern 20 part of the NCP, where the air pollution is most severe. The visibility, RH, wind speed and 21 weather phenomenon observed at 14h local time (LT) during 1981~2010 was used to analyse 22 the long-term temporal and spatial variation of low visibility, haze and fog events.
23 Both haze and fog could lead to low visibility events. The formations of haze and fog are two 24 distinctly different processes. Therefore, it is necessary to differentiate between those two 25 events when analysing low visibility trends, distributions and variations. In this work, low 26 visibility events were defined as days with visibility at 14 LT below 10 km. Those low visibility 27 events that were not associated with fog, precipitation, dust storm, smoke, snow storm etc. were 28 defined as haze events. Fog events mostly occur during early morning, rarely lasting until 14 29 LT. Therefore, a day was defined as a foggy day if the occurrence of fog was recorded at any 30 time during the day.
3 1 For each station the total occurrences of low visibility days, hazy days and foggy days during
2 1981~2010 were counted to analyse the spatial distribution of those 3 events. OMI Level 3 SO2 3 planetary boundary layer column concentrations during 2005-2012 were used to show the
4 correlation between the distribution of SO2 and the occurrence of haze days.
5 The spatial average annual and 10-year moving average occurrence frequencies during 6 1981~2010 were used to analyse the trend of the 3 events over those 30 years. The variation
7 trends are then compared to the SO2 emission trends inferred from the China statistical 8 yearbooks (National Bureau of Statistics of China, 1995-2010). The spatial average monthly 9 occurrence frequencies during 1981~2010 were applied to examine the long-term trend of those 10 3 events in different seasons.
11 The average distribution and the linear trend of wind speeds during 1981~2010 were 12 respectively calculated to reveal their impact on the distribution and variation trend of the 13 occurrence frequency of the 3 events. NCEP final analysis meteorology data during 58 fog days 14 in between Jan 2009 and Feb 2010 was used to reveal the characteristic wind field that has led 15 to fog events in the southern NCP. Surface automatic weather station wind data during May- 16 Dec 2009 were used as supplements to analyse the distribution of the wind field in the entire 17 NCP and how it impacts on haze formation.
18 The average distribution of the count of days with 14h RH > 70% and the count of haze days 19 with 14h RH > 70% were calculated to show its relation to the distribution of low visibility, 20 haze and fog events. To further study the influence of RH on haze events, the decadal variation 21 of the ratio of the annual average haze related RH to the total average RH was compared against 22 the decadal variation in annual haze days. The season-decadal variation trend of the occurrence 23 frequency of haze related RH values falling in the range of RH<50%, 50%
25
26 3 Results and Discussions
27 3.1 The spatial distribution of low visibility, haze and fog
28 Low visibility, haze and fog events are formed through different mechanisms, hence their 29 occurrences are determined by different parameters. This may lead to distinct spatial 30 distributions of occurrence frequencies. The spatial distributions of the counts of low visibility,
4 1 haze and fog days during 1981~2010 in the southern NCP are respectively depicted in Figure 2 2b-d.
3 Haze events are caused by either high aerosol loadings or the strong hygroscopic growth of
4 aerosols. SO2 is the main precursor of sulphate aerosols, which are highly hygroscopic and exist
5 in abundance in the NCP (Liu et al., 2014). The average SO2 planetary boundary layer (PBL)
6 column amount during 2005~2012 is depicted in Figure 2a. High SO2 column concentrations
7 are found on the western border, which is caused on the one hand by the large amount of SO2 8 emissions in this region and on the other hand by the special topography which has led to poor
9 dispersion conditions. The high SO2 concentrations have led to large loadings of highly 10 hygroscopic sulphate aerosol, which can easily lead to haze events in this region. Figure 2c 11 displays the distribution of the count of haze days, which shows a very similar distribution to
12 that of SO2 column concentration. Haze also most frequently occurs on the western border of 13 the NCP, along the eastern slope of the Taihang Mountains.
14 The count of fog days show a very distinct distribution to that of haze days (Figure 2d). Fog 15 events most commonly occur along the centre line of the plain region, parallel to the southwest- 16 northeast ridge of the Taihang Mountain. This suggests that the formation of haze and fog are 17 controlled by different physical processes. While the formation of haze mainly depends on the 18 local aerosol pollution level, the formation of fog is not limited the existence of cloud 19 condensation nucleus under the regional pollution state of the NCP. The special topography and 20 meteorology in the NCP could be the responsible for the spatial distribution of fog event 21 occurrences, which will be further discussed in section 3.3.2.
22 Low visibility events can be caused by both haze and fog. The area with the most low visibility 23 days covers both the area with the most haze and fog days. Low visibility mainly happens in 24 the western part of the region, which is similar to the distribution of haze days. High centres 25 were found around the three major cities Shijiazhuang, Xingtai and Handan. This indicates that 26 low visibility events in the NCP are mostly caused by haze, rather than by fog.
27 Due to the distinct spatial distribution of low visibility, haze and fog days, the spatial coverage 28 of their influence also differ from each other. Table 1 lists the number (percentage) of stations 29 under various count ranges of low visibility, haze and fog days during 1981~2010. In the 30 southern NCP, 34% and 36% of the stations have respectively observed 500-999 and 1000- 31 1999 low visibility days, while another 12% of the stations have gone through more than 2000 32 low visibility days and 17% rather clean stations have only had 100-499 low visibility days.
5 1 Although haze events can be very frequent, its impact is constrained within the limited area in 2 the southwest. 22% of the sites have experienced more than 1000 haze days during the past 30 3 years, while 55% of the sites have had less than 500 haze days. For fog events, however, 73% 4 of the sites have experienced 500~999 fog days during the past 30 years, suggesting that a large 5 area is under a similar influence of fog events.
6 In all, low visibility, haze and fog days are distinctly distributed in the NCP, because their 7 formations are controlled by different mechanisms. The spatial distribution of haze days is 8 determined by the distribution of pollutant emissions and by the topography of the NCP, thus 9 only influencing a small area near the edge of the mountains. The distribution of fog days is 10 mainly determined by the topography and meteorology, affecting a large area parallel to the 11 mountains. Low visibility is mostly induced by haze events, thus showing a similar distribution 12 to that of haze days.
13 3.2 The trends of low visibility, haze and fog during 1981~2010
14 The temporal trends of low visibility, haze and fog are influenced by the variation of many 15 factors such as pollutant emissions, aerosol compositions and meteorological conditions, etc. 16 During the past 30 years, the NCP has undergone rapid economic developments accompanied 17 by growing energy consumptions. In the past decade, policies were made to reduce pollutant 18 emissions in the NCP in hope to improve the air quality. In this section, the decadal trends of 19 low visibility, haze and fog days will be examined to show the combined effect of various 20 influencing factors.
21 3.2.1 The decadal trends of low visibility, haze and fog
22 The decadal variations of the annual and the 10-year moving average occurrence count of low 23 visibility, haze and fog days during 1981~2010 are displayed in Figure 3b. Before 1995, due to 24 the rapid economic development, the increasing energy consumption has led to increasing 25 numbers of haze and fog days, which resulted in more frequent low visibility events. During 26 1995~2003, with the development and employment of waste gas processing techniques, 27 occurrence counts of low visibility, haze and fog days have entered a steady stage. After 2003, 28 in preparation for the 2008 Beijing Olympic Games, under the influence of emission control 29 policies, the number of low visibility, haze and fog days have drastically dropped. The variation
30 trend of low visibility and haze days agrees well with that of the SO2 emissions (Figure 3a),
6 1 suggesting that the control in emissions during the last decade effectively led to the decline of 2 visibility and haze days.
3 The 10-year moving linear slopes of low visibility, haze and fog occurrence days during 4 1981~2010 are shown Figure 3c. The maximum increasing slope for haze events can be found 5 between 1989~1998, reaching 17 days 10a-1. After the period of 1994~2003, haze events have 6 been mostly declining, reaching a maximum decreasing slope of 11 days 10a-1 during 7 1996~2005. After 1987~1996, fog occurrences displays discontinuous decreasing trends. Major 8 decreases happened during the periods of 1990~1999 and 2001~2010, with slopes of 9 10 days 10a-1. The count of low visibility days were increasing before the period of 1996~2005, 10 with a maximum slope of 17 days 10a-1 during 1983~1992. Afterwards, low visibility events 11 were found to be less frequent, the maximum decreasing slope was found during 2001-2010, 12 reaching 16 days 10a-1.
13 Generally, the occurrence frequency of low visibility, haze and fog in the NCP have all
14 increased during 1980~1990. Under the effort of SO2 emission reduction during the past few 15 years, occurrence frequencies of low visibility, haze and fog have all decreased back to a similar 16 level as in 1980.
17 3.2.2 The season-decadal trends of low visibility, haze and fog
18 Low visibility, haze and fog are strongly influenced by meteorological conditions, which in the 19 NCP vary distinctly with season. This may cause different decadal variation in different seasons. 20 Figure 4 shows the season-decadal variation trends of low visibility, haze and fog occurrence 21 frequencies during 1981~2010.
22 During the past 30 years, haze events were most common during the heating season (Nov-Mar) 23 and most rare during the end of spring and early autumn (Figure 4d). After 1990, haze events 24 started to appear not only during the heating season but also during summertime (Figure 4b2). 25 From the 10-year moving average occurrence frequencies of haze in spring, summer, autumn 26 and winter (Figure 4b1) it can be seen that summertime haze occurrence frequencies has been 27 continuously increasing in the past 3 decades, while that in wintertime has been declining since 28 1995. In the past, heating processes during winter has led to high emissions of soot, which were 29 responsible for the degradation of visibility. With the development of central heating and waste 30 gas processing techniques, emissions by heating processes have been gradually declining, 31 leading to less haze events during winter. However, atmospheric pollution has become more
7 1 complex during recent years. During summer, the atmosphere becomes highly oxidative,
2 aerosol pollution coexists with high concentrations of O3 and VOCs (Ran et al., 2011), which 3 contribute to the fast aging and secondary formation of aerosols. Aged aerosols are more 4 hygroscopic and can easily lead to haze events under suitable RH conditions (Chen et al., 2012).
5 Fog events appear most frequently during winter and most rarely during spring until early 6 summer. The fog occurrence frequency during the past 30 years shows no significant variation 7 trend in spring and summer, but shows a declining trend during winter after 1990 (Figure 4c1).
8 Low visibility events are caused by both haze and fog. Their combined effect has resulted in 9 high occurrence frequencies of low visibility during winter and lower ones during spring and 10 summer (Figure 4a1). Low visibility events during summertime has been continuously 11 increasing in the past 3 decades, while those during spring, autumn and winter has been 12 decreasing after 2000.
13 In all, wintertime low visibility, haze and fog days have been declining, while summertime low 14 visibility and haze days have been increasing due to the variation in aerosol composition and 15 the summertime high RH that favors the hygroscopic growth of particles.
16 3.3 The impact of wind on low visibility, haze and fog
17 3.3.1 The impact of wind on haze
18 Wind direction can influence the transport of pollutants, thus determining the spatial 19 distribution of visibility degrading pollutants. Wind speed greatly influences the atmospheric 20 stability and the atmospheric dispersion abilities. Low wind speeds suggest that the atmosphere 21 is rather stable and the dispersion of local emissions is prohibited.
22 Figure 5a shows the spatial distribution of the average 14h (LT) surface wind direction during 23 May-Dec 2009 in the entire NCP. The Yan and Taihang Mountains are governed by northwest 24 winds, while the plain area is dominated by winds from the south. Along the south-western 25 edge of the Taihang Mountains, the winds all come from the south-eastern direction, which can 26 transport the emitted pollutants from the eastern part of the plain area to the western part. The 27 weak north-western winds in the mountains block the way of the pollutant transport, leading to 28 the accumulation of heavy loadings of aerosol and its precursors along the foot of the Taihang 29 Mountains. This explains why haze events are most common in this region.
8 1 Figure 6a displays the distribution of the average 14h (LT) wind speed during 1981~2010. 2 Average wind speeds at 14h (LT) decreases from the southeast to the northwest. Lowest values 3 are found in the northwest corner, reaching below 3 m s-1, while similar values were also 4 detected in Xingtai and Handan, two heavily polluted cities in the southern corner. Average 5 wind speeds near the coast (in the vicinity of Cangzhou) and in the southeast corner are large, 6 reaching over 5 m s-1. The low wind speeds are caused primarily by the Taihang Mountains, 7 which slow down westerly winds before they reach the plain region. As can be seen from Figure 8 5, the geographic height of the Taihang Mountains is higher in the north and lower in the south, 9 which explains why wind speeds are lowest in the northwest corner. Another factor influencing 10 the surface wind speed is the surface roughness. Large cities with densely distributed high 11 buildings will add to the surface roughness and slow down near surface winds.
12 Although dispersion abilities are weakest in the northwest corner, due to relatively lower 13 pollutant emissions in the mountain areas, low visibility and haze events are rare on the 14 northwestern edge. However, the area near Baoding, where the pollution level is slightly higher 15 than the mountain areas and distinctively lower than the polluted region in the southwest, is 16 heavily affected by the low wind speeds, showing frequently occurring low visibility and haze 17 events. The southwestern edge of the southern NCP region suffers from high pollutant 18 emissions and is under relatively weak dispersion conditions. The low average wind speed 19 centers in Shijiazhuang, Xingtai and Handan (Figure 7a) conform to the high haze day count 20 centers (Figure 2c).
21 Figure 6b shows how the linear slope of the annual wind speed during 1981-2010 is distributed 22 in the southern NCP. Over the entire region, wind speeds have been decreasing in the past 30 23 years. Large decreasing slopes were found in those regions with large average wind speeds, 24 but also in parts of the northeast corner and in Shijiazhuang, which is the capital of Hebei 25 Province. The regional average 10-year decreasing slope reaches 0.2 m s-1 10a-1, while that in 26 Shijiazhuang reaches over 0.2 m s-1 10a-1 (Figure 6c). The decrease in wind speed suggests that 27 the atmosphere has become more stable and dispersion abilities has weakened throughout the 28 entire region, which calls for even harder efforts to control emissions in order to prevent haze 29 events.
30
9 1 3.3.2 The impact of wind on fog
2 The formation of fog needs a supersaturated water vapour environment, which requires 3 favourable meteorological conditions. Figure 5b shows the average 8h (LT) 1000 hPa wind 4 field during 58 fog days that occurred between the Jan 2009 and Feb 2010, on which more than 5 10 stations reported the occurrence of fog. Due to the topography of the NCP, an orographic 6 wind convergence line going from southwest to the northeast is formed as indicated by the red 7 line in Figure 5b. The wind convergence line is also parallel to the ridge of the Taihang 8 Mountains, coinciding with the zone with the most frequent fog events (Figure 2d). Fog events 9 usually occur during the night, when the mountain areas cool off faster than the plain area, 10 forming a temperature gradient. To the west of the convergence line, near surface winds are 11 dominated by cold north-westerly mountain winds, while to the east of the convergence line, 12 surface winds are north-easterly or south-easterly. With the north-easterly winds comes the 13 warm humid air from the Bohai Sea, while the south-easterly path is the most typical water 14 vapour transport passage way for the entire NCP region. The wind convergence zone will hence 15 favour the accumulation and convergence of water vapour, and lead to the formation of fog in 16 this area.
17 3.4 The impact of RH on low visibility, haze and fog
18 The ambient RH can influence the visibility by affecting the hygroscopic growth and scattering 19 abilities of atmospheric aerosols. Chen et al. (2012) suggest that, under RH<80%, visibility is 20 highly dependent on dry aerosol volume concentrations, only under high aerosol loadings does 21 the hygroscopic growth become important for visibility impairment. While for RH greater than 22 80%, the hygroscopic growth of aerosols can greatly affect visibility, even under average 23 aerosol pollution levels.
24 The distribution of the count of days with 14h (LT) RH > 70% and the count of haze days with 25 14h (LT) RH > 70% are depicted in Figure 8. High RH days most frequently occur in the 26 southeast, where there is a water vapour transport passageway, and along the orographic wind 27 convergence line, which was observed to be favourable for the formation of fog events in Sect. 28 3.3.2. The distribution of the count of haze days with 14h (LT) RH > 70% (Figure 8b) is similar 29 to that of the count of all days with 14h (LT) RH > 70% (Figure 8a), only with significantly 30 less counts to the east of Shijiazhuang, because haze is not as severe in that region. Compared 31 with the distribution of the total counts of low visibility and haze days (Figure 2b-c), it can be
10 1 noted that the frequent low visibility events along the southern edge of the Taihang mountain 2 were caused primarily by the heavy aerosol pollution and not by haze events associated with 3 high RH. The low visibility events in the vicinity of Shijiazhuang, however, was not only caused 4 by severe pollution but were also associated with high RH events, indicating that the 5 hygroscopic growth of aerosols plays an important role in the visibility impairment in this 6 region. Although aerosol pollution is not as severe in the south-eastern part of our area of study, 7 a large fraction of haze days are associated with high RH events, suggesting that the high RH 8 in this region is able to impair visibility even if the aerosol concentration is not very high. The 9 hygroscopic growth induced light scattering of aerosols plays a dominant role in the degradation 10 of visibility in this region.
11 The 14h (LT) RH values that were accompanied by haze events (haze event RH) were sorted 12 out and its occurrence frequency in the range of RH <50%, 50%
20 The occurrence frequency of haze events under low RH (RH< 50%) in autumn, winter and 21 spring has decreased over the past 30 years, with a slight rebound in the winter time data at the 22 end of the last decade. This indicates that aerosol loadings during winter has declined, which 23 could be the result of effective emission control measures during heating seasons. The 24 occurrence frequency of haze events under higher RH (RH>60%) in autumn, winter and spring 25 show increasing trends during the past 30 years. A possible cause could be the fact that 26 atmospheric pollution has become more complicated over the years, leading to a higher fraction 27 of secondary aerosols, which are more hygroscopic and are more likely to impair visibility 28 through hygroscopic growth processes.
29 To further study if this phenomenon exists throughout the entire region, the decadal variation 30 of haze event RH, annual average RH and annual haze days at four representative stations were 31 analysed as is depicted in Figure 9. The RH associated with haze events are typically higher 32 than the average RH values. Zanhuang station shows continuously increasing number of haze
11 1 days throughout the past 30 years, while Hengshui displays a continuous decline throughout 2 the past 20 years (Figure 9b). Shijiazhuang and Longyao have both undergone an increase 3 before 2000 and a decrease thereafter. Significant increasing trends can be found in the ratio 4 of haze event RH and average RH at all four stations (Figure 9a). This means that a higher 5 fraction of the haze events are now caused by the hygroscopic growth of aerosols. The reduction 6 in primary aerosol emissions further amplifies this effect.
7
8 4 Summary
9 In this study, the spatial distribution and decadal variation of low visibility, fog and haze events 10 in the most polluted southern part of the NCP during the past 30 years were analysed and the 11 impact of wind and RH on those events was investigated.
12 Haze and fog are distinctly distributed, which was determined by the topography of the NCP 13 and the distribution of wind speed and wind direction. Haze occurs mostly along the south- 14 western edge of the plain region, while fog mostly occurs within the central band area parallel 15 to the ridge of the Taihang Mountains.
16 Annual low visibility, haze and fog days have shown increasing trends before 1995, have 17 entered a steady stage during 1995~2003 and have drastically dropped thereafter during the 18 preparation stage for the Beijing Olympic Games in 2008. Low visibility, haze and fog events 19 all occurred most frequently during the heating season in the past 3 decades. Wintertime haze 20 and fog both show decreasing trends, benefiting from the improvements in central heating and 21 desulfurization techniques. Summertime haze, however, displayed continuous increasing trends 22 during 1981~2010.
23 Both the distribution of the wind field and the decadal variation of wind speeds have great 24 impacts on the occurrence of low visibility, haze and fog events. South-easterly winds in the 25 southern part of the NCP which are blocked by the weak north-western winds in the Taihang 26 Mountains has resulted in high pollutant concentrations along the foot of the mountain, which 27 has led to frequent haze events. The orographic wind convergence zone parallel to the ridge of 28 the Taihang Mountains in the central band area of the southern NCP was responsible for the 29 frequent fog events in this region. Wind speed has been decreasing throughout the entire 30 southern NCP, resulting in more stable atmospheric conditions and weaker dispersion abilities, 31 which calls for even harder efforts to control emissions in order to prevent haze events.
12 1 Haze and low visibility events were strongly influenced by the ambient RH. The frequent high 2 RH events near Shijiazhuang and in the south-eastern part of the region favours the hygroscopic 3 growth of aerosols and plays a dominant role in the visibility impairment in those areas. Under 4 RH below 60%, haze events mostly occurred during the heating seasons, while for RH above 5 60%, haze events were more likely to happen during summertime. Although annual RH values 6 displayed no significant trends within the past 30 years, those RH values associated with haze 7 days were evidently increasing, suggesting that an increasing fraction of haze events are caused 8 by the hygroscopic growth of aerosols, rather than simply by high aerosol loadings.
9
10 Acknowledgements
11 This work is supported by the National 973 project of China (2011CB403402), the National 12 Natural Science Foundation of China under Grant No. 41375134, the Beijing Natural Science 13 Foundation (8131003), the Beijing municipal science and technology plan project No. 14 Z131100006113013.
15 16
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 Zhao, P., Zhang, X., Xu, X., and Zhao, X.: Long-term visibility trends and characteristics in the 6 region of Beijing, Tianjin, and Hebei, China, Atmospheric Research, 101, 711-718, 7 http://dx.doi.org/10.1016/j.atmosres.2011.04.019, 2011. 8 9 10
15 1 2 Table 1 The number (percentage) of stations under various count ranges of low visibility, haze 3 and fog days during 1981~2010
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 2005~2012 with the location of the 3 64 stations and the count of b) low visibility, c) haze and d) fog days during 1981~2010 in the 4 southern NCP.
19 )
−1 1.8
ton a 1.6 a)
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 count of low visibility, haze and fog days 5 among the 64 stations during 1981~2010; b) the 10-year moving linear slope of the annual 6 average occurrence count of low visibility, haze and fog days among the 64 stations during 7 1981~2010. 8
20 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 1 Occurence Frequency (%) Occurence Frequency (%) Occurence Frequency (%) 2 Figure 4 1) The trend of the 10-year moving average occurrence frequency of a) low visibility 3 events at 14 LT, b) haze events at 14 LT and c) fog events during the day among the 64 stations 4 during spring (MAM), summer (JJA), autumn (SON) and winter (DJF); 2) The season-annual 5 variation of the average occurrence frequency of a) low visibility events at 14 LT, b) haze 6 events at 14 LT and c) fog events during the day among the 64 stations and d) the seasonal 7 variation of the average occurrence frequency of low visibility events at 14 LT, haze events at 8 14 LT and fog events among the 64 stations during 1981-2010. 9
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 orographic convergence line (red
6 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 occurrence frequency of 14h (LT) 3 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 count of days with 14h (LT) RH above 70% and b) the 3 count 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 count of haze days during 1981~2010 at Shijiazhuang, Hengshui, Longyao 4 and Zanhuang.
5
6
26