The Distribution and Trends of Fog and Haze

icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Atmos. Chem. Phys. Discuss., 14, 16123–16149, 2014 www.atmos-chem-phys-discuss.net/14/16123/2014/ doi:10.5194/acpd-14-16123-2014 ACPD © Author(s) 2014. CC Attribution 3.0 License. 14, 16123–16149, 2014

This discussion paper is/has been under review for the journal Atmospheric Chemistry The distribution and and Physics (ACP). Please refer to the corresponding ﬁnal paper in ACP if available. trends of fog and haze

The distribution and trends of fog and G. Q. Fu et al. haze in the North China Plain over the past 30 years Title Page Abstract Introduction 1,3 2 1 4 2 G. Q. Fu , W. Y. Xu , R. F. Rong , J. B. Li , and C. S. Zhao Conclusions References

1Hebei Province Meteorological Service Centre, Shijiazhuang, Hebei, China Tables Figures 2Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China J I 3Key Laboratory for Meteorology and Ecological Environment of Hebei Province, Shijiazhuang, Hebei, China J I 4 Hebei Province Meteorological Observatory, Shijiazhuang, Hebei, China Back Close

Received: 29 April 2014 – Accepted: 12 June 2014 – Published: 19 June 2014 Full Screen / Esc Correspondence to: C. S. Zhao ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion

16123 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Abstract ACPD Frequent low visibility, haze and fog events were found in the North China Plain (NCP). Data throughout the NCP during the past 30 years were examined to determine the 14, 16123–16149, 2014 horizontal distribution and decadal trends of low visibility, haze and fog events. The 5 impact of meteorological factors such as wind and RH on those events was investi- The distribution and gated. Results reveal distinct distributions of haze and fog days, due to their diﬀerent trends of fog and formation mechanisms. Low visibility, haze and fog days all display increasing trends haze of before 1995, a steady stage during 1995–2003 and a drastically drop thereafter. All three events occurred most frequently during the heating season. Beneﬁting from G. Q. Fu et al. 10 emission control measures, haze and fog both show decreasing trends in winter during the past 3 decades, while summertime haze displays continuous increasing trends. The distribution of wind speed and wind direction as well as the topography within Title Page the NCP has determinative impacts on the distribution of haze and fog. Weakened Abstract Introduction south-easterly winds in the southern part of the NCP has resulted in high pollutant Conclusions References 15 concentrations and frequent haze events along the foot of the Taihang Mountains. The orographic wind convergence zone in the central band area of the southern NCP is Tables Figures responsible for the frequent fog events in this region. Wind speed has been decreasing

throughout the entire southern NCP, resulting in more stable atmospheric conditions J I and weaker dispersion abilities, calling for harder eﬀorts to control emissions to prevent 20 haze events. Haze events are strongly inﬂuenced by the ambient RH. RH values asso- J I ciated with haze days are evidently increasing, suggesting that an increasing fraction Back Close of haze events are caused by the hygroscopic growth of aerosols, rather than simply by high aerosol loadings. Full Screen / Esc

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16124 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 1 Introduction ACPD Low visibility events caused by fog and severe haze events can be a heavy burden for air transport and on-road traffic. The severe aerosol pollution that has led to the 14, 16123–16149, 2014 visibility impairment is also highly damaging for human health. 5 In developed countries such as the US and the European countries, the employment The distribution and of sulphur emission control measures have already resulted in significant declines in trends of fog and haziness and major improvements in visibility during the past 50 years (Schichtel et al., haze 2001; Doyle and Dorling, 2002; Molnár et al., 2008; Vautard et al., 2009). As a rapidly developing country, however, air pollution problems has been haunting China for the G. Q. Fu et al. 10 past few decades. Visibility has been deteriorating in southeast (Deng et al., 2012) and southwest China (Fu and Wu, 2011) during the past 50 years, with only a few places revealing slight increases in recent years. Six Chinese mega-cities have suffered from Title Page continuously decreasing visibilities in the past 30 years (Chang et al., 2009). Overall, Abstract Introduction sunny day visibilities decreased significantly during 1960–1990 all over China, trends Conclusions References 15 thereafter, however, were not coherent (Wu et al., 2012). The visibility trends were often attributed to the variation of sulphur dioxide emissions Tables Figures or aerosol concentrations, however, no causality could be established so far. Both SO2 and aerosol pollution have been proved most severe in the North China Plain (NCP), J I due to the rapid economic growth and the high population density (Xu et al., 2011). As 20 a result, low visibility events frequently occur. Many in-situ measurements and studies J I have already been performed to study the light scattering properties of aerosols and Back Close their impact on horizontal visibility under different relative humidity (RH) in the NCP as part of the Haze in China (HaChi) Campaign (Chen et al., 2012; Ma et al., 2011). Full Screen / Esc Results show that under low RH, low visibility events are mostly induced by the heavy Printer-friendly Version 25 aerosol loading, while under high RH, the influence of aerosol hygroscopic growth becomes stronger and can lead to low visibility events even under moderate aerosol Interactive Discussion pollution levels. However, only few studies have been performed to investigate the spatial distribution of low visibility events and their overall variation trend during the past

16125 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion few decades in the NCP. (Zhao et al., 2011) analysed the visibility trends in the NCP, suggesting declining visibilities before 1998 and increasing ones during 2006–2008, ACPD with stronger visibility deterioration trends in the summer season due to higher RH and 14, 16123–16149, 2014 lower wind speeds. Both haze and fog events could be the responsible for low visibility 5 events, however, those were rarely differentiated in the study of visibility trends, which made it difficult to find the reason behind those visibility variations. The distribution and In this study, a detailed analysis on the decadal variation and spatial distribution of trends of fog and low visibility, fog and haze events in the most polluted southern part of the NCP will be haze performed and the impact of wind and RH on those events will be revealed. G. Q. Fu et al.

10 2 Data and methodology Title Page The North China Plain (NCP) is surrounded to the north by the Yan Mountains, to the west by the Taihang Mountains and to the east by the Bohai Sea (Fig. 1). The western Abstract Introduction part of the NCP is affected by the warm and dry wind coming off the eastern Taihang Conclusions References Mountain slopes, which lead to increased surface stability. Under the influence of the Tables Figures 15 Bohai Sea, the east coast of the NCP often experiences large winds. The southern part of the NCP is rather flat and is an important water vapour transport passageway. The location of the 64 meteorological observation sites selected for this study are J I displayed in Fig. 1, with the names of the sites given in Fig. 2a. All sites are located in J I the southern part of the NCP, where the air pollution is most severe. The visibility, RH, Back Close 20 wind speed and weather phenomenon observed at 14:00 LT during 1981–2010 was used to analyse the long-term temporal and spatial variation of low visibility, haze and Full Screen / Esc fog events. Both haze and fog could lead to low visibility events. The formations of haze and fog Printer-friendly Version are two distinctly different processes. Therefore, it is necessary to differentiate between Interactive Discussion 25 those two events when analysing low visibility trends, distributions and variations. In this work, low visibility events were defined as days with visibility at 14:00 LT below 10 km. Those low visibility events that were not associated with fog, precipitation, dust storm, 16126 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion smoke, snow storm etc. were defined as haze events. Fog events mostly occur during early morning, rarely lasting until 14:00 LT. Therefore, a day was defined as a foggy day ACPD if the occurrence of fog was recorded at any time during the day. 14, 16123–16149, 2014 For each station the total occurrences of low visibility days, hazy days and foggy days 5 during 1981–2010 were counted to analyse the spatial distribution of those 3 events. OMI Level 3 SO2 planetary boundary layer column concentrations during 2005–2012 The distribution and were used to show the correlation between the distribution of SO2 and the occurrence trends of fog and of haze days. haze The spatial average annual and 10 year moving average occurrence frequencies dur- G. Q. Fu et al. 10 ing 1981–2010 were used to analyse the trend of the 3 events over those 30 years. The variation trends are then compared to the SO2 emission trends inferred from the China statistical yearbooks (National Bureau of Statistics of China, 1995–2010). The spatial Title Page average monthly occurrence frequencies during 1981–2010 were applied to examine the long-term trend of those 3 events in different seasons. Abstract Introduction 15 The average distribution and the linear trend of wind speeds during 1981–2010 were Conclusions References respectively calculated to reveal their impact on the distribution and variation trend of the occurrence frequency of the 3 events. NCEP final analysis meteorology data dur- Tables Figures ing a continuous fog event in between 9 and 19 December 2002 was used to reveal the characteristic wind field that has led to fog events in the southern NCP. Surface J I 20 automatic weather station wind data during May–December 2009 were used as sup- plements to analyse the distribution of the wind field in the entire NCP and how it J I impacts on haze formation. Back Close The average distribution of all RH and the haze related RH were calculated to show Full Screen / Esc its relation to the distribution of low visibility, haze and fog events. To further study the 25 influence of RH on haze events, the decadal variation of the ratio of the annual aver- Printer-friendly Version age haze related RH to the total average RH was compared against the decadal variation in annual haze days. The season-decadal variation trend of the occurrence fre- Interactive Discussion quency of haze related RH values falling in the range of RH < 50 %, 50 % < RH < 60 %,

16127 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 60 % < RH < 70 %, RH > 70 % was analysed to show the impact RH has on haze in diﬀerent seasons. ACPD 14, 16123–16149, 2014 3 Results and discussions The distribution and 3.1 The spatial distribution of low visibility, haze and fog trends of fog and

5 Low visibility, haze and fog events are formed through diﬀerent mechanisms, hence haze their occurrences are determined by diﬀerent parameters. This may lead to distinct G. Q. Fu et al. spatial distributions of occurrence frequencies. The spatial distributions of the counts of low visibility, haze and fog days during 1981–2010 in the southern NCP are respectively

depicted in Fig. 2b–d. Title Page 10 Haze events are caused by either high aerosol loadings or the strong hygroscopic Abstract Introduction growth of aerosols. SO2 is the main precursor of sulphate aerosols, which are highly hygroscopic and exist in abundance in the NCP (Liu et al., 2014). The average SO2 plan- Conclusions References etary boundary layer (PBL) column amount during 2005–2012 is depicted in Fig. 2a. Tables Figures High SO2 column concentrations are found on the western border, which is caused on 15 the one hand by the large amount of SO2 emissions in this region and on the other hand by the special topography which has led to poor dispersion conditions. The high J I SO concentrations have led to large loadings of highly hygroscopic sulphate aerosol, 2 J I which can easily lead to haze events in this region. Figure 2c displays the distribution Back Close of the count of haze days, which shows a very similar distribution to that of SO2 column 20 concentration. Haze also most frequently occurs on the western border of the NCP, Full Screen / Esc along the eastern slope of the Taihang Mountains.

The count of fog days show a very distinct distribution to that of haze days (Fig. 2d). Printer-friendly Version Fog events most commonly occur along the centre line of the plain region, parallel to the southwest-northeast ridge of the Taihang Mountain. This suggests that the formation of Interactive Discussion 25 haze and fog are controlled by different physical processes. While the formation of haze mainly depends on the local aerosol pollution level, the formation of fog is not limited 16128 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion the existence of cloud condensation nucleus under the regional pollution state of the NCP. The special topography and meteorology in the NCP could be the responsible ACPD for the spatial distribution of fog event occurrences, which will be further discussed in 14, 16123–16149, 2014 Sect. 3.3.2. 5 Low visibility events can be caused by both haze and fog. The area with the most low visibility days covers both the area with the most haze and fog days. Low visibility The distribution and mainly happens in the western part of the region, which is similar to the distribution of trends of fog and haze days. High centres were found around the three major cities Shijiazhuang, Xingtai haze and Handan. This indicates that low visibility events in the NCP are mostly caused by G. Q. Fu et al. 10 haze, rather than by fog. Due to the distinct spatial distribution of low visibility, haze and fog days, the spatial coverage of their influence also differ from each other. Table 1 lists the number (per- Title Page centage) of stations under various count ranges of low visibility, haze and fog days during 1981–2010. In the southern NCP, 34 % and 36 % of the stations have respec- Abstract Introduction 15 tively observed 500–999 and 1000–1999 low visibility days, while another 12 % of the Conclusions References stations have gone through more than 2000 low visibility days and 17 % rather clean stations have only had 100–499 low visibility days. Although haze events can be very Tables Figures frequent, its impact is constrained within the limited area in the southwest. 22 % of the sites have experienced more than 1000 haze days during the past 30 years, while 55 % J I 20 of the sites have had less than 500 haze days. For fog events, however, 73 % of the sites have experienced 500–999 fog days during the past 30 years, suggesting that J I a large area is under a similar influence of fog events. Back Close In all, low visibility, haze and fog days are distinctly distributed in the NCP, because Full Screen / Esc their formations are controlled by different mechanisms. The spatial distribution of haze 25 days is determined by the distribution of pollutant emissions and by the topography of Printer-friendly Version the NCP, thus only influencing a small area near the edge of the mountains. The distribution of fog days is mainly determined by the topography and meteorology, affecting Interactive Discussion a large area parallel to the mountains. Low visibility is mostly induced by haze events, thus showing a similar distribution to that of haze days.

16129 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3.2 The trends of low visibility, haze and fog during 1981–2010 ACPD The temporal trends of low visibility, haze and fog are influenced by the variation of many factors such as pollutant emissions, aerosol compositions and meteorological 14, 16123–16149, 2014 conditions, etc. During the past 30 years, the NCP has undergone rapid economic de- 5 velopments accompanied by growing energy consumptions. In the past decade, poli- The distribution and cies were made to reduce pollutant emissions in the NCP in hope to improve the air trends of fog and quality. In this section, the decadal trends of low visibility, haze and fog days will be haze examined to show the combined effect of various influencing factors. G. Q. Fu et al. 3.2.1 The decadal trends of low visibility, haze and fog

10 The decadal variations of the annual and the 10 year moving average occurrence count Title Page of low visibility, haze and fog days during 1981–2010 are displayed in Fig. 3b. Before 1995, due to the rapid economic development, the increasing energy consumption has Abstract Introduction led to increasing numbers of haze and fog days, which resulted in more frequent low Conclusions References visibility events. During 1995–2003, with the development and employment of waste Tables Figures 15 gas processing techniques, occurrence counts of low visibility, haze and fog days have entered a steady stage. After 2003, in preparation for the 2008 Beijing Olympic Games, under the influence of emission control policies, the number of low visibility, haze and J I fog days have drastically dropped. The variation trend of low visibility and haze days J I agrees well with that of the SO2 emissions (Fig. 3a), suggesting that the control in Back Close 20 emissions during the last decade effectively led to the decline of visibility and haze days. Full Screen / Esc The 10 year moving linear slopes of low visibility, haze and fog occurrence days during 1981–2010 are shown Fig. 3c. The maximum increasing slope for haze events Printer-friendly Version can be found between 1989–1998, reaching 17 days 10 a−1. After the period of 1994– Interactive Discussion 25 2003, haze events have been mostly declining, reaching a maximum decreasing slope of 11 days 10 a−1 during 1996–2005. After 1987–1996, fog occurrences displays discontinuous decreasing trends. Major decreases happened during the periods of 16130 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 1990–1999 and 2001–2010, with slopes of 10 days 10 a−1. The count of low visibility days were increasing before the period of 1996–2005, with a maximum slope of ACPD −1 17 days 10 a during 1983–1992. Afterwards, low visibility events were found to be 14, 16123–16149, 2014 less frequent, the maximum decreasing slope was found during 2001–2010, reaching −1 5 16 days 10 a . Generally, the occurrence frequency of low visibility, haze and fog in the NCP have The distribution and trends of fog and all increased during 1980–1990. Under the effort of SO2 emission reduction during the past few years, occurrence frequencies of low visibility, haze and fog have all decreased haze back to a similar level as in 1980. G. Q. Fu et al.

10 3.2.2 The season-decadal trends of low visibility, haze and fog

Low visibility, haze and fog are strongly influenced by meteorological conditions, which Title Page in the NCP vary distinctly with season. This may cause different decadal variation in Abstract Introduction different seasons. Figure 4 shows the season-decadal variation trends of low visibility, haze and fog occurrence frequencies during 1981–2010. Conclusions References 15 During the past 30 years, haze events were most common during the heating sea- Tables Figures son (Nov-Mar) and most rare during the end of spring and early autumn (Fig. 4d). After

1990, haze events started to appear not only during the heating season but also during J I summertime (Fig. 4b2). From the 10 year moving average occurrence frequencies of haze in spring, summer, autumn and winter (Fig. 4b1) it can be seen that summertime J I 20 haze occurrence frequencies has been continuously increasing in the past 3 decades, Back Close while that in wintertime has been declining since 1995. In the past, heating processes during winter has led to high emissions of soot, which were responsible for the degra- Full Screen / Esc dation of visibility. With the development of central heating and waste gas processing techniques, emissions by heating processes have been gradually declining, leading to Printer-friendly Version 25 less haze events during winter. However, atmospheric pollution has become more com- Interactive Discussion plex during recent years. During summer, the atmosphere becomes highly oxidative, aerosol pollution coexists with high concentrations of O3 and VOCs (Ran et al., 2011),

16131 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion which contribute to the fast aging and secondary formation of aerosols. Aged aerosols are more hygroscopic and can easily lead to haze events under suitable RH conditions ACPD (Chen et al., 2012). 14, 16123–16149, 2014 Fog events appear most frequently during winter and most rarely during spring until 5 early summer. The fog occurrence frequency during the past 30 years shows no significant variation trend in spring and summer, but shows a declining trend during winter The distribution and after 1990 (Fig. 4c1). trends of fog and Low visibility events are caused by both haze and fog. Their combined eﬀect has haze resulted in high occurrence frequencies of low visibility during winter and lower ones G. Q. Fu et al. 10 during spring and summer (Fig. 4a1). Low visibility events during summertime has been continuously increasing in the past 3 decades, while those during spring, autumn and winter has been decreasing after 2000. Title Page In all, wintertime low visibility, haze and fog days have been declining, while summertime low visibility and haze days have been increasing due to the variation in aerosol Abstract Introduction 15 composition and the summertime high RH that favors the hygroscopic growth of parti- Conclusions References cles. Tables Figures 3.3 The impact of wind on low visibility, haze and fog

J I 3.3.1 The impact of wind on haze J I Wind direction can influence the transport of pollutants, thus determining the spatial Back Close 20 distribution of visibility degrading pollutants. Wind speed greatly influences the atmospheric stability and the atmospheric dispersion abilities. Low wind speeds suggest Full Screen / Esc that the atmosphere is rather stable and the dispersion of local emissions is prohibited. Figure 5a shows the spatial distribution of the average 14:00 LT surface wind direc- Printer-friendly Version tion during May–December 2009 in the entire NCP. The Yan and Taihang Mountains Interactive Discussion 25 are governed by northwest winds, while the plain area is dominated by winds from the south. Along the south-western edge of the Taihang Mountains, the winds all come from the south-eastern direction, which can transport the emitted pollutants from the 16132 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion eastern part of the plain area to the western part. The weak north-western winds in the mountains block the way of the pollutant transport, leading to the accumulation of ACPD heavy loadings of aerosol and its precursors along the foot of the Taihang Mountains. 14, 16123–16149, 2014 This explains why haze events are most common in this region. 5 Figure 6a displays the distribution of the average 14:00 LT wind speed during 1981– 2010. Average wind speeds at 14:00 LT decreases from the southeast to the northwest. The distribution and Lowest values are found in the northwest corner, reaching below 3 m s−1, while similar trends of fog and values were also detected in Xingtai and Handan, two heavily polluted cities in the haze southern corner. Average wind speeds near the coast (in the vicinity of Cangzhou) −1 G. Q. Fu et al. 10 and in the southeast corner are large, reaching over 5 m s . The low wind speeds are caused primarily by the Taihang Mountains, which slow down westerly winds before they reach the plain region. As can be seen from Fig. 5, the geographic height of Title Page the Taihang Mountains is higher in the north and lower in the south, which explains why wind speeds are lowest in the northwest corner. Another factor influencing the Abstract Introduction 15 surface wind speed is the surface roughness. Large cities with densely distributed high Conclusions References buildings will add to the surface roughness and slow down near surface winds. Although dispersion abilities are weakest in the northwest corner, due to relatively Tables Figures lower pollutant emissions in the mountain areas, low visibility and haze events are rare on the northwestern edge. However, the area near Baoding, where the pollution J I 20 level is slightly higher than the mountain areas and distinctively lower than the polluted region in the southwest, is heavily affected by the low wind speeds, showing frequently J I occurring low visibility and haze events. The southwestern edge of the southern NCP Back Close region suffers from high pollutant emissions and is under relatively weak dispersion Full Screen / Esc conditions. The low average wind speed centers in Shijiazhuang, Xingtai and Handan 25 (Fig. 7a) conform to the high haze day count centers (Fig. 2c). Printer-friendly Version Figure 6b shows how the linear slope of the annual wind speed during 1981–2010 is distributed in the southern NCP. Over the entire region, wind speeds have been Interactive Discussion decreasing in the past 30 years. Large decreasing slopes were found in those re- gions with large average wind speeds, but also in parts of the northeast corner and

16133 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion in Shijiazhuang, which is the capital of Hebei Province. The regional average 10 year decreasing slope reaches 0.2 m s−1 10 a−1, while that in Shijiazhuang reaches over ACPD 0.2 m s−1 10 a−1 (Fig. 6c). The decrease in wind speed suggests that the atmosphere 14, 16123–16149, 2014 has become more stable and dispersion abilities has weakened throughout the entire 5 region, which calls for even harder eﬀorts to control emissions in order to prevent haze events. The distribution and trends of fog and 3.3.2 The impact of wind on fog haze

The formation of fog needs a supersaturated water vapour environment, which requires G. Q. Fu et al. favourable meteorological conditions. Figure 5b shows the average 08:00 LT 1000 hPa 10 wind ﬁeld during a continuous fog event that occurred between the 9 and 19 December 2002. Due to the topography of the NCP, an orographic wind convergence line going Title Page from southwest to the northeast is formed as indicated by the red line in Fig. 5b. The Abstract Introduction wind convergence line is also parallel to the ridge of the Taihang Mountains, coinciding with the zone with the most frequent fog events (Fig. 2d). Fog events usually occur Conclusions References

15 during the night, when the mountain areas cool oﬀ faster than the plain area, form- Tables Figures ing a temperature gradient. To the west of the convergence line, near surface winds are dominated by cold north-westerly mountain winds, while to the east of the conver- J I gence line, surface winds are north-easterly or south-easterly. With the north-easterly winds comes the warm humid air from the Bohai Sea, while the south-easterly path is J I

20 the most typical water vapour transport passage way for the entire NCP region. The Back Close wind convergence zone will hence favour the accumulation and convergence of water vapour, and lead to the formation of fog in this area. Full Screen / Esc

3.4 The impact of RH on low visibility, haze and fog Printer-friendly Version

The ambient RH can influence the visibility by affecting the hygroscopic growth and Interactive Discussion 25 scattering abilities of atmospheric aerosols. Chen et al. (2012) suggest that, under RH < 80 %, visibility is highly dependent on dry aerosol volume concentrations, only 16134 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion under high aerosol loadings does the hygroscopic growth become important for visibility impairment. While for RH greater than 80 %, the hygroscopic growth of aerosols ACPD can greatly affect visibility, even under average aerosol pollution levels. 14, 16123–16149, 2014 The 14:00 LT RH values that were accompanied by haze events (haze event 5 RH) were sorted out and its occurrence frequency in the range of RH < 50 %, 50 % < RH < 60 %, 60 % < RH < 70 % and RH > 70 % was calculated for each station The distribution and in each month during 1981–2010. Figure 7a2–d2 shows the regional average season- trends of fog and decadal variation of the occurrence frequency of haze event RHs. It can be noted that haze haze events mostly occur under RH > 60 % during summer and early autumn, while G. Q. Fu et al. 10 during late autumn, winter and spring, haze events mostly occur under RH < 60 %. This suggests that haze events during the warm season are caused both by high aerosol loadings and their hygroscopic growth, while those in the cold seasons are mostly in- Title Page duced by high aerosol loadings. The occurrence frequency of haze events under low RH (RH < 50 %) in autumn, Abstract Introduction 15 winter and spring has decreased over the past 30 years, with a slight rebound in the Conclusions References winter time data at the end of the last decade. This indicates that aerosol loadings during winter has declined, which could be the result of effective emission control mea- Tables Figures sures during heating seasons. The occurrence frequency of haze events under higher RH (RH > 60 %) in autumn, winter and spring show increasing trends during the past J I 20 30 years. A possible cause could be the fact that atmospheric pollution has become more complicated over the years, leading to a higher fraction of secondary aerosols, J I which are more hygroscopic and are more likely to impair visibility through hygroscopic Back Close growth processes. Full Screen / Esc To further study if this phenomenon exists throughout the entire region, the decadal 25 variation of haze event RH, annual average RH and annual haze days at four repre- Printer-friendly Version sentative stations were analysed as is depicted in Fig. 9. The RH associated with haze events are typically higher than the average RH values. Zanhuang station shows con- Interactive Discussion tinuously increasing number of haze days throughout the past 30 years, while Hengshui displays a continuous decline throughout the past 20 years (Fig. 9b). Shijiazhuang and

16135 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Longyao have both undergone an increase before 2000 and a decrease thereafter. Sig- nificant increasing trends can be found in the ratio of haze event RH and average RH ACPD at all four stations (Fig. 9a). This means that a higher fraction of the haze events are 14, 16123–16149, 2014 now caused by the hygroscopic growth of aerosols. The reduction in primary aerosol 5 emissions further amplifies this effect. The distribution and trends of fog and 4 Summary haze

In this study, the spatial distribution and decadal variation of low visibility, fog and haze G. Q. Fu et al. events in the most polluted southern part of the NCP during the past 30 years were analysed and the impact of wind and RH on those events was investigated. 10 Haze and fog are distinctly distributed, which was determined by the topography of Title Page the NCP and the distribution of wind speed and wind direction. Haze occurs mostly along the south-western edge of the plain region, while fog mostly occurs within the Abstract Introduction central band area parallel to the ridge of the Taihang Mountains. Conclusions References Annual low visibility, haze and fog days have shown increasing trends before 1995, Tables Figures 15 have entered a steady stage during 1995–2003 and have drastically dropped thereafter during the preparation stage for the Beijing Olympic Games in 2008. Low visibility, haze and fog events all occurred most frequently during the heating season in the past 3 J I decades. Wintertime haze and fog both show decreasing trends, benefiting from the J I improvements in central heating and desulfurization techniques. Summertime haze, Back Close 20 however, displayed continuous increasing trends during 1981–2010. Both the distribution of the wind field and the decadal variation of wind speeds have Full Screen / Esc great impacts on the occurrence of low visibility, haze and fog events. South-easterly winds in the southern part of the NCP which are blocked by the weak north-western Printer-friendly Version winds in the Taihang Mountains has resulted in high pollutant concentrations along Interactive Discussion 25 the foot of the mountain, which has led to frequent haze events. The orographic wind convergence zone parallel to the ridge of the Taihang Mountains in the central band area of the southern NCP was responsible for the frequent fog events in this region. 16136 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Wind speed has been decreasing throughout the entire southern NCP, resulting in more stable atmospheric conditions and weaker dispersion abilities, which calls for ACPD even harder efforts to control emissions in order to prevent haze events. 14, 16123–16149, 2014 Haze events were strongly influenced by the ambient RH. Under RH below 60 %, 5 haze events mostly occurred during the heating seasons, while for RH above 60 %, haze events were more likely to happen during summertime. Although annual RH val- The distribution and ues displayed no significant trends within the past 30 years, those RH values associ- trends of fog and ated with haze days were evidently increasing, suggesting that an increasing fraction haze of haze events are caused by the hygroscopic growth of aerosols, rather than simply G. Q. Fu et al. 10 by high aerosol loadings.

Acknowledgements. This work is supported by the National 973 project of China (2011CB403402), the National Natural Science Foundation of China under Grant No. Title Page 41375134, the Beijing Natural Science Foundation (8131003), the Beijing municipal science and technology plan project No. Z131100006113013. Abstract Introduction Conclusions References

15 References Tables Figures

Chang, D., Song, Y., and Liu, B.: Visibility trends in six megacities in China 1973–2007, Atmos. Res., 94, 161–167, doi:10.1016/j.atmosres.2009.05.006, 2009. J I Chen, J., Zhao, C. S., Ma, N., Liu, P. F., Göbel, T., Hallbauer, E., Deng, Z. Z., Ran, L., Xu, W. Y., J I Liang, Z., Liu, H. J., Yan, P., Zhou, X. J., and Wiedensohler, A.: A parameterization of low 20 visibilities for hazy days in the North China Plain, Atmos. Chem. Phys., 12, 4935–4950, Back Close doi:10.5194/acp-12-4935-2012, 2012. Full Screen / Esc China Statistical Yearbook: http://www.stats.gov.cn/tjsj/ndsj/ (last access: 26 March 2014), 1995–2010. Danielson, J. J. and Gesch, D. B.: Global multi-resolution terrain elevation data 2010 Printer-friendly Version 25 (GMTED2010), US Geological Survey Open-File Report 2011-1073, 26, 2011. Interactive Discussion Deng, J., Du, K., Wang, K., Yuan, C.-S., and Zhao, J.: Long-term atmospheric visibility trend in Southeast China, 1973–2010, Atmos. Environ., 59, 11–21, doi:10.1016/j.atmosenv.2012.05.023, 2012. 16137 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Doyle, M. and Dorling, S.: Visibility trends in the UK 1950–1997, Atmos. Environ., 36, 3161– 3172, doi:10.1016/S1352-2310(02)00248-0, 2002. ACPD Fu, C. and Wu, J.: The diﬀerent characteristics of sunny visibility over southwest china in recent 50 years, Procedia Environmental Sciences, 10, Part A, 247–254, 14, 16123–16149, 2014 5 doi:10.1016/j.proenv.2011.09.041, 2011. Liu, H. J., Zhao, C. S., Nekat, B., Ma, N., Wiedensohler, A., van Pinxteren, D., Spindler, G., Müller, K., and Herrmann, H.: Aerosol hygroscopicity derived from size-segregated chemical The distribution and composition and its parameterization in the North China Plain, Atmos. Chem. Phys., 14, trends of fog and 2525–2539, doi:10.5194/acp-14-2525-2014, 2014. haze 10 Ma, N., Zhao, C. S., Nowak, A., Müller, T., Pfeifer, S., Cheng, Y. F., Deng, Z.Z., Liu, P. F., Xu, W. Y., Ran, L., Yan, P., Göbel, T., Hallbauer, E., Mildenberger, K., Henning, S., Yu, J., G. Q. Fu et al. Chen, L. L., Zhou, X. J., Stratmann, F., and Wiedensohler, A.: Aerosol optical properties in the North China Plain during HaChi campaign: an in-situ optical closure study, Atmos. Chem. Phys., 11, 5959–5973, doi:10.5194/acp-11-5959-2011, 2011. Title Page 15 Molnár, A., Mészáros, E., Imre, K., and Rüll, A.: Trends in visibility over Hungary between 1996 and 2002, Atmos. Environ., 42, 2621–2629, doi:10.1016/j.atmosenv.2007.05.012, 2008. Abstract Introduction Ran, L., Zhao, C. S., Xu, W. Y., Lu, X. Q., Han, M., Lin, W. L., Yan, P., Xu, X. B., Deng, Z. Z., Conclusions References Ma, N., Liu, P. F., Yu, J., Liang, W. D., and Chen, L. L.: VOC reactivity and its eﬀect on ozone production during the HaChi summer campaign, Atmos. Chem. Phys., 11, 4657–4667, Tables Figures 20 doi:10.5194/acp-11-4657-2011, 2011. Schichtel, B. A., Husar, R. B., Falke, S. R., and Wilson, W. E.: Haze trends over the United States, 1980–1995, Atmos. Environ., 35, 5205–5210, doi:10.1016/S1352-2310(01)00317-X, J I 2001. J I Vautard, R., Yiou, P., and van Oldenborgh, G. J.: Decline of fog, mist and haze in Europe over 25 the past 30 years, Nat. Geosci., 2, 115–119, doi:10.1038/ngeo414, 2009. Back Close Wu, J., Fu, C., Zhang, L., and Tang, J.: Trends of visibility on sunny days in China in the recent Full Screen / Esc 50 years, Atmos. Environ., 55, 339–346, doi:10.1016/j.atmosenv.2012.03.037, 2012. Xu, W. Y., Zhao, C. S., Ran, L., Deng, Z. Z., Liu, P.F., Ma, N., Lin, W. L., Xu, X. B., Yan, P.,He, X., Yu, J., Liang, W. D., and Chen, L. L.: Characteristics of pollutants and their correlation to Printer-friendly Version 30 meteorological conditions at a suburban site in the North China Plain, Atmos. Chem. Phys., Interactive Discussion 11, 4353–4369, doi:10.5194/acp-11-4353-2011, 2011.

16138 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Zhao, P., Zhang, X., Xu, X., and Zhao, X.: Long-term visibility trends and characteristics in the region of Beijing, Tianjin, and Hebei, China, Atmos. Res., 101, 711–718, ACPD doi:10.1016/j.atmosres.2011.04.019, 2011. 14, 16123–16149, 2014

The distribution and trends of fog and haze

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Table 1. The number (percentage) of stations under various count ranges of low visibility, haze and fog days during 1981–2010. Title Page

Days Abstract Introduction Stations ≥ 3000 2000–2999 1000–1999 500–999 100–499 < 100 Conclusions References 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 %) Tables Figures Fog 0 (0 %) 0 (0 %) 8 (13 %) 47 (73 %) 9 (14 %) 0 (0 %) J I

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Back Close 1 2 Figure 1 The location of the measurement sites (circle), the area of study (red line) and the Full Screen / Esc Figure 1. The3 locationregional topography of the measurementby (Danielson and Gesch, sites 2011) (circle), (color). the area of study (red line) and the regional topography4 by (Danielson and Gesch, 2011) (color). Printer-friendly Version

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17 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion ACPD 14, 16123–16149, 2014 40° N 40° N 2 Langfang a) Zhuozhou b) Gu'an langfang Laiyuan Yixian Xincheng 3200 Baxian Rongcheng 1.8 2900 The distribution and Fuping Baoding Anxin baoding Shunping Wenan 2600 Renqiu Quyang Gaoyang Qingxian trends of fog and Dingzhou 2400 Xingtang Hejian Huanghua 1.6 Xinle Anguo Raoyang 2200 Pingshan Wuji Cangzhou cangzhou Xianxian shijiazhuang haze Jingxing ZhengdingGaocheng Yanshan 1800 ° Shenxian ° 38 N Shijiazhuang 38 N Xinji Dongguang 1.4 1400 Luancheng Zhaoxian Fucheng hengshui Zanhuang Ningjin Hengshui Wuqiao 1100 Lincheng Jixian G. Q. Fu et al. 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 Title Page ° ° 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 Abstract Introduction c) d) langfang langfang 3200 3200 Conclusions References baoding 2900 2900 2600 baoding 2600 2400 2400 Tables Figures cangzhou 2200 shijiazhuang 2200 cangzhou 1800 shijiazhuang 1800 ° ° 38 N hengshui 38 N 1400 1400 hengshui 1100 1100 J I 900 xingtai 900 xingtai 700 700 500 500 J I handan 300 300 handan

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Figure 2. (a) Average OMI SO2 PBL column amount during 2005–2012 with the location of the 2 Figure 2 a) Average OMI SO2 PBL column amount during 2005~2012 with the location of the 64 stations and the count of (b) low visibility, (c) haze and (d) fog days during 1981–2010 in the Printer-friendly Version southern3 64 stations NCP. and the count of b) low visibility, c) haze and d) fog days during 1981~2010 in the Interactive Discussion 4 southern NCP.

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2 Figure 3 a) The annual SO2 emissions in Hebei Province inferred from the China statistical Figure 3. (a) The annual SO2 emissions in Hebei Province inferred from the China statistical Full Screen / Esc yearbooks3 (Nationalyearbooks (National Bureau Bureau of Statistics of Statistics of of China, China, 1995-2010); 1995–2010); b) the(b) annualthe (solid annual lines) (solid and lines) and 4 the 10-year moving average (dashed lines) occurrence count of low visibility, haze and fog days the 10 year moving average (dashed lines) occurrence count of low visibility, haze and fog days Printer-friendly Version among the5 64among stations the 64 stations during during 1981–2010; 1981~2010;(b) b) thethe 10-year 10 year moving moving linear slope linear of the slope annual of the annual average6 occurrence average occurrence count count of low of low visibility, visibility, haze haze and fog fog days days among among the 64 stations the 64 during stations during Interactive Discussion 1981–2010.7 1981~2010. 8 16143

30 15 15 The distribution and a1) b1) c1) Spring Summer trends of fog and 20 10 10 Autumn Winter haze Low visibility at 14h LT 10 5 5 Haze at 14h LT Fog during the day G. Q. Fu et al. 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 Title Page 9 9 9 9 8 8 8 8 Month 7 7 7 7 6 6 6 6 Abstract Introduction 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 Conclusions References Year Year Year Occurence Frequency (%)

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2 FigureFigure 4. 4 (1)1) TheThe trendtrend ofof thethe 1010-year year moving moving average average occurrence occurrence frequency frequency of of(a) a) lowlow visibility visibility J I events at 14:00 LT, (b) haze events at 14:00 LT and (c) fog events during the day among the 64 3 events at 14 LT, b) haze events at 14 LT and c) fog events during the day among the 64 stations J I stations during spring (MAM), summer (JJA), autumn (SON) and winter (DJF); (2) The season- 4 annualduring variation spring (MAM), of the average summer occurrence (JJA), autumn frequency (SON) of and(a) lowwinter visibility (DJF); events 2) The at 14:00season-annual LT, (b) Back Close haze events at 14:00 LT and (c) fog events during the day among the 64 stations and (d) the 5 seasonalvariation variation of the average of the average occurrence occurrence frequency frequency of a) low of low visib visibilityility events events at at 14 14:00 LT, LT, b) haze haze Full Screen / Esc 6 eventsevents at at 14:00 14 LT LT and fogc) fog events events among during the th 64e stationsday among during the 1981–2010.64 stations and d) the seasonal Printer-friendly Version 7 variation of the average occurrence frequency of low visibility events at 14 LT, haze events at Interactive Discussion 8 14 LT and fog events among the 64 stations during 1981-2010. 9

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Figure 5. (a) Averaged 14:00 LT surface wind direction during May–December 2009 given by J I 3 307Figure AWS 5 stations.a) Averaged(b) The14h orographicsurface wind height direction a.s.l. during (shading), May-Dec the average 2009 given 08:00 by LT 307 1000 AWS hPa NCEP ﬁnal analysis wind ﬁeld (black arrows) and the orographic convergence line (red line) Back Close 4 duringstations. a continuous b) The orographic fog event hei duringght above 9–19 sea December level (shading), 2002. the average 8 am 1000 hPa NCEP Full Screen / Esc

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The distribution and trends of fog and haze 1 40 40 4.5 G. Q. Fu et al. a) b) ) c) 39.5 -1 4 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 Title Page 38 38 -0.3 3.9 hengshui hengshui 2 3.7 37.5 -0.4 xingtai 3.5 xingtai 1.5 Abstract Introduction 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 Conclusions References Annual average 14h (LT) wind speed (m s -1 -1

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Figure 6. (a) Distribution of the average 14:00 LT wind speed during 1981–2010 in the southern 3 NCP;Figure(b) 6 a)distribution Distribution of of the the linear average slope 14h of (LT) the wind annual speed wind during speed 1981-2010 during 1981–2010 in the southern in the J I southern NCP; (c) Variation (dashed lines) and linear trend (solid lines) of the regional average J I 4 14:00NCP; LTb) winddistribution speed andof the that linear of Cangzhou slope of andthe Shijiazhuangannual wind duringspeed during 1981–2010. 1981-2010 in the Back Close

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) The distribution and 100 80 80 80 Spring Spring a1) b1) c1) d1) Summer Summer trends of fog and 60 60 60 Autumn Autumn Winter Winter haze 50 40 40 40 RH <50% 50%70% G. Q. Fu et al. 0 0 0 0 Occurence Frequency (% 2 2 2 2 2 1 a2) 1 b2) 1 c2) 1 d2) 1 e) 12 12 12 12 12 11 11 11 11 11 10 10 10 10 10 9 9 9 9 9 8 8 8 8 8 Title Page Month 7 7 7 7 7 6 6 6 6 6 5 5 5 5 5 4 4 4 4 4 3 3 3 3 3 Abstract Introduction 19851990199520002005 19851990199520002005 19851990199520002005 19851990199520002005 0 20 40 60 80 Year Year Year Year Occurence Frequency (%) Conclusions References 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 (%) Tables Figures 2 FigureFigure 7.7 1) (1) TheThe decadal decadal trend trend of the of 10-year the 10 moving year moving average averageoccurrence occurrence frequency frequencyof 14h (LT) of 14:00 LT haze event RH in the range of (a) RH < 50 %, (b) 50 % < RH < 60 %, (c) J I 3 60haze % < eventRH < 70RH % in and the (d)rangeRH of> 70a) %RH during <50% spring, b) 50%70% winter (DJF);during(2) springThe season-decadal(MAM), summer variation (JJA), autumn of the occurrence (SON) and frequency winter (DJF); of 14:00 2) LTThe J I haze event RH in the range of (a) RH < 50 %, (b) 50 % < RH < 60 %, (c) 60 % < RH < 70 % 5 andseason-decadal(d) 70 % < RH variation< 80 % of and the (e)occurrencethe seasonal frequency variation of 14h of (LT) the haze average event occurrence RH in the fre-range Back Close < < < 6 quencyof a) RH of 14:00 <50%, LT b) haze 50% 70 % during 1981–2010. 7 seasonal variation of the average occurrence frequency of 14h (LT) haze event RH in the range Printer-friendly Version 8 of a) RH <50%, b) 50%70% during 1981~2010. Interactive Discussion 9

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36 36 1 114 115 116 117 114 115 116 117 J I 2 Figure 8 The distribution of a) the total average 14h (LT) RH and b) the average 14h (LT) RH Figure 8. The distribution of (a) the total average 14:00 LT RH and (b) the average 14:00 LT RH J I on haze3 on days haze during days during 1981–2010 1981-2010 in in the the southern southern NCP. NCP. Back Close 4 Full Screen / Esc

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Annual count of haze days 0 J I 1985 1990 1995 2000 2005 2010 1 Year Back Close 2 Figure 9 a) The decadal variation of the haze event associated RH divided by the average RH Figure 9. (a) The decadal variation of the haze event associated RH divided by the average RH Full Screen / Esc and (b) the annual3 and count b) the annual of haze count daysof haze days during during 1981–2010 1981~2010 at Shijiazhuang, at Shijiazhuang, Hengshui, Longyao Hengshui, Longyao and Zanhuang.4 and Zanhuang. Printer-friendly Version 5

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