In January 2013, a severe fog and haze event (FHE) of strong intensity, long duration, and extensive coverage occurred in eastern China. The present study investigates meteorological conditions for this FHE by diagnosing both its atmospheric background fields and daily evolution in January 2013. The results show that a weak East Asian winter monsoon existed in January 2013. Over eastern China, the anomalous southerly winds in the middle and lower troposphere are favorable for more water vapor transported to eastern China. An anomalous high at 500 hPa suppresses convection. The weakened surface winds are favorable for the fog and haze concentrating in eastern China. The reduction of the vertical shear of horizontal winds weakens the synoptic disturbances and vertical mixing of atmosphere. The anomalous inversion in near-surface increases the stability of surface air. All these meteorological background fields in January 2013 were conducive to the maintenance and development of fog and haze over eastern China. The diagnosis of the daily evolution of the FHE shows that the surface wind velocity and the vertical shear of horizontal winds in the middle and lower troposphere can exert dynamic effects on fog and haze. The larger (smaller) they are, the weaker (stronger) the fog and haze are. The thermodynamic effects include stratification instability in middle and lower troposphere and the inversion and dew-point deficit in near-surface. The larger (smaller) the stratification instability and the inversion are, the stronger (weaker) the fog and haze are. Meanwhile, the smaller (larger) the dewpoint deficit is, the stronger (weaker) the fog and haze are. Based on the meteorological factors, a multi-variate linear regression model is set up. The model results show that the dynamic and thermodynamic effects on the variance of the fog and haze evolution are almost the same. The contribution of the meteorological factors to the variance of the daily fog and haze evolution reaches 0.68, which explains more than 2/3 of the variance.

Meteorological conditions for the persistent severe fog and haze event over eastern China in January 2013

Our analysis of fog and haze observations from the surface weather stations in China in recent 50 years (from 1961 to 2011) shows that the number of fog days has experienced two-stage variations, with an increasing trend before 1980 and a decreasing trend after 1990. Especially, an obvious decreasing trend after 1990 can be clearly seen, which is consistent with the decreasing trend of the surface relative humidity. However, the number of haze days has demonstrated an increasing trend. As such, the role of reduction of atmospheric relative humidity in the transition process from fog into haze has been further investigated. It is estimated that the mean relative humidity of haze days is about 69%, lower than previously estimated, which implies that it is more difficult for the haze particles to transform into fog drops. This is possibly one of the major environmental factors leading to the reduction of number of fog days. The threshold of the relative humidity for transition from fog into haze is about 82%, also lower than previously estimated. Thus, the reduction of the surface relative humidity in China mainly due to the increase of the surface temperature and the saturation specific humidity may exert an obvious impact on the environmental conditions for the formations of fog and haze. In addition, our investigation of the relationship between haze and visibility reveals that with the increase of haze days, the visibility has declined markedly. Since 1961, the mean visibility has dropped from 4-10 to 2-4 km, about a half of the previous horizontal distance of visibility.

Analysis of long-term variations of fog and haze in China in recent 50 years and their relations with atmospheric humidity

. In this paper, the variation and trend of haze pollution in eastern China for winter of 1960–2012 were analyzed. With the overall increasing number of winter haze days in this period, the 5 decades were divided into three sub-periods based on the changes of winter haze days (WHD) in central North China (30–40° N) and eastern South China (south of 30° N) for east of 109° E mainland China. Results show that WHD kept gradually increasing during 1960–1979, remained stable overall during 1980–1999, and increased fast during 2000–2012. The author identified the major climate forcing factors besides total energy consumption. Among all the possible climate factors, variability of the autumn Arctic sea ice extent, local precipitation and surface wind during winter is most influential to the haze pollution change. The joint effect of fast increase of total energy consumption, rapid decline of Arctic sea ice extent and reduced precipitation and surface winds intensified the haze pollution in central North China after 2000. There is a similar conclusion for haze pollution in eastern South China after 2000, with the precipitation effect being smaller and spatially inconsistent.

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Understanding the Recent Trend of Haze Pollution in Eastern China: Roles of Climate Change

The interannual variation of the wintertime fog–haze days across central and eastern China from 1972 to 2014 and its relationship with East Asian winter monsoon (EAWM) are investigated based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and the surface observation data from the weather stations in China. The results show that the wintertime fog–haze days across central and eastern China have close relation with EAWM in the interannual time scale. The stronger (weaker) the EAWM is, the less (more) the wintertime fog–haze days are. In strong (weak) EAWM winters, both near-surface winds and vertical shear of horizontal zonal winds strengthen (weaken). The strengthened (weakened) near-surface winds enhance (reduce) the outward transport of fog and haze and are unfavourable (favourable) for their accumulation over central and eastern China. The enlarged (receded) vertical shear of horizontal zonal winds intensifies (abates) the atmospheric baroclinic instability and vertical diffusion, leading to less (more) fog and haze in near-surface. In addition, a strong (weak) EAWM is also unfavourable (favourable) for the maintenance of the fog and haze in the lower troposphere through the anomalous divergence (convergence) associated with the intense anticyclonic (cyclonic) anomalies in the upper troposphere over southern China.

Interannual variation of the wintertime fog-haze days across central and eastern China and its relation with East Asian winter monsoon

Scientific issues relevant to interactions between aerosols and the Asian monsoon climate were discussed and evaluated at the 33rd “Forum of Science and Technology Frontiers” sponsored by the Department of Earth Sciences at the Chinese Academy of Sciences. Major results are summarized in this paper. The East Asian monsoon directly affects aerosol transport and provides a favorable background circulation for the occurrence and development of persistent fog-haze weather. Spatial features of aerosol transport and distribution are also influenced by the East Asian monsoon on seasonal, inter-annual, and decadal scales. High moisture levels in monsoon regions also affect aerosol optical and radiative properties. Observation analyses indicate that cloud physical properties and precipitation are significantly affected by aerosols in China with aerosols likely suppressing local light and moderate rainfall, and intensifying heavy rainfall in southeast coastal regions. However, the detailed mechanisms behind this pattern still need further exploration. The decadal variation in the East Asian monsoon strongly affects aerosol concentrations and their spatial patterns. The weakening monsoon circulation in recent decades has likely helped to increase regional aerosol concentrations. The substantial increase in Chinese air pollutants has likely decreased the temperature difference between land and sea, which favors intensification of the weakening monsoon circulation. Constructive suggestions regarding future studies on aerosols and monsoons were proposed in this forum and key uncertain issues were also discussed.

Advances in studying interactions between aerosols and monsoon in China

Haze is an important indicator of air quality and,on the other hand,fog has definite meaning indicative of weather With the rapid expansion of economic scale and acceleration of urbanization,hazy weather is getting more and more serious in cities across Chinese mainlandLong-term trends and variation of haze have been obtained by studying meteorological data from 743 surface weather stations in Chinese mainland for the period between 1951 and 2005In terms of geographical distribution, hazy days were relatively few across the nation from 1956 to 1980 with the exception in the Sichuan Basin and the southern Xinjiang where haze exceeds 50 days per yearThe number of haze days increased dramatically since 1980s,and surpass 100 days over most of the eastern part of Chinese mainland and more than 150 days in metropolitan areas at the end of last century,indicating strong linkage between haze days and economic activitiesThe top 10 cities with the most haze days are as follows,Shenyang, Xingtai,Chongqing,Benxi,Xi'an,Chengdu,Suining,Laohekou,Hetian,Qiemo,Minfeng,and NeijiangMost of these cities are located in central Liaoning,the Sichuan Basin,the North China Plain,the Guanzhong Plain and the southern part of Xinjiang where is less affected by the sand stormOn the national scale,haze weather is more frequently observed in December and January than the other months,which account for 30%of the annual hazy daysSeptember is the month with the least number of haze days,only contributes 5%to the annual hazy daysThe weather stations that show an increasing trend of haze days are mainly located in the eastern and southern parts of the China,which are among the most developed zones in economy and industryThe weather stations in which a decreasing trend of haze days is observed are mainly located in the northeastern and northwestern China,due to the relatively laggard economy and industryThe northeastern China is one of the earlier regions to industrialize in China,but the achievements of recent years in industrial structure adjustment and environmental improvement have contributed to the gradual decreasing haze days in this region

WU Xiaojing

Papers

Temporal and spatial variation of haze during 1951-2005 in Chinese mainland

Spatial sampling design optimization of monitoring network for terrestrial ecosystem in China.