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Characteristics of haze change in China since the twenty-first century

Meteorological observation data from 843 national basic/base stations of the China were used to analyze the spatial and temporal distributions of haze in China. The area where there were more than 30 days of haze annually expanded gradually. The average number of haze days across China from 2001 to 2016 was 22.9d. The number of haze days increased gradually from 2001 to 2004 then declined slightly from 2004 to 2012. In 2013, the number of haze days increased to the highest level but then decreased through 2016. The annual number of haze episodes demonstrated a similar trend to the number of haze days. From 2014 to 2016, the cold air was weaker than in the past 30 years. At the same time, the number of breeze days increased and average wind speed decreased. The meteorological conditions were favorable for the formation of haze in China from 2014 to 2016. However, annual haze days, NO2 and SO2 from the OMI (Ozone Monitoring Instrument) data decreased in the three regions from 2014 to 2016, which implies that the emission control program conducted by the Chinese government was effective.


Introduction
The rapid development of the economy and urbanization had led to a high emission of air pollutants in China. The annual haze days in many megacities of China increased [1], while visibility decreased [2]. Many megacities had endured the haze weather for a long time, especially the ones in regions with rapid economic development, such as the Beijing-Tianjin-Hebei, the Yangtze River Delta and the Pearl River Delta regions [3]. The occurrence frequency of a sustained haze episode significantly increased [4]. Haze weather with large coverage played a bad role in the atmospheric environment [5,6], human health [7][8][9] and climate change [10]. Researches had shown that the high concentration of regional fine particles (PM2.5) was the main cause of haze weather [11,12]. The fine particle concentrations were affected by several factors, including the emissions and the meteorological conditions [13][14][15][16]. Changes in the number of haze days were closely related to the increase of air pollutant concentrations, which itself is largely influenced by meteorological conditions. It is of significance to strengthen analysis of temporal and spatial distribution of haze, and discussion of mechanism of haze development. Previous studies on haze in China mostly focused on changing meteorological conditions and concentrations of pollutants in specific processes [17]. Several studies had focused on the long-time trend of haze days in China, and had analyzed the  [18][19][20]. These studies indicated that the annual number of haze days in china had increased significantly science the year of 2000. Mostly, the previous studies represented the meteorological conditions using the meteorological factors from weather stations rather than the synoptic systems. In this paper, the temporal and spatial distribution characteristics of haze are analyzed using meteorological data during 2001-2016. The meteorological conditions, including the cold air masses and breeze days, were also analyzed in this paper.

Data and methods
The meteorological observation data used in this paper were taken from 843 national basic/base stations of the China Meteorological Administration from 2001 to 2016 ( Figure 1). This data includes information on temperature, pressure, visibility, relative humidity, wind speed, wind direction and weather phenomena. Haze days were selected with daily visibility less than 10 km and a relative humidity of 85% [21]. Days with precipitation, blowing snow, blowing sand, floating dust, sandstorms, smoke and other weather with low visibility were excluded. A haze weather process was defined as haze weather lasting for three days, with visibility less than 5 km, and occurring in the majority of regions within three neighboring provinces. Column concentrations of troposphere NO2 and SO2 during 2006-2016 were retrieved by an Ozone Monitoring Instrument (OMI) onboard the NASA EOS/Aura satellite (http://mirador.gsfc.nasa.gov) based on differential optical absorption spectroscopy and a principal component analysis retrieve algorithm, respectively.

Spatial distributions of haze in China
As shown in Figure 2, regions with high mean annual haze days were located in eastern China. Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD) were the three most important regions. The Sichuan Basin was also a haze-prone area. In the past 16 years, the area with more than 30 annual haze days showed an expanding trend. In the central China the annual mean haze days increased rapidly after 2012. In Jiangsu Province, the area with mean annual haze days greater than 100 d was limited to the southwest part of the province from 2001-2004 and then In BTH and its surrounding areas (Shanxi, Shandong, Henan and Inner Mongolia Provinces), the area with more than 60 mean annual haze days showed an increasing trend, covering the entire central and southern regions during 2013-2016. However, there were significant differences across cities in the peak number of haze days and when it occurred. The peak number of annual mean haze days larger than 150 d in southern Hebei and southern Shanxi occurred during 2005-2008, while occurred in 2013-2016 in northern Henan. In the YRD, the area with annual mean haze days larger than 60 d significantly expanded during 2013-2016. The value in Jiangsu and eastern Anhui was more than 100 d. The annual mean haze days in PRD showed a decreasing trend, with most values less than 30 d. These were significantly lower than those in the BTH and YRD regions. In addition, the annual mean haze days in the Sichuan Basin remained high from 2001-2016, showing little fluctuation. In Northeast China, the area with annual mean haze days larger than 11 d also showed an expanding trend during 2013-2016. The occurrence frequency of haze in Northeast China had significant seasonal variations, with a peak in winter, due in part to biomass burning and coal heating. Meanwhile, the large difference between daytime and nighttime temperatures and the high occurrence frequency of inversion in winter prevented the diffusion of pollutants, providing favorable conditions for the occurrence and development of haze.

Temporal variations of haze in China
As shown in Figure 3, visibility in winter (15.1 km) was significantly lower than in summer (18.2 km).The mean visibility in summer and winter was approximately 20 and 17 km, respectively, during    (Figure 6), the more and stronger cold air activity provided favorable meteorological background for the diffusion of atmospheric pollutants in 2016. According to the distribution of cold air activities in different zones, the cold air activities occurred frequently in zone 1 (northeast), zone 2 (the middle east of Northwest China, BTH and surrounding areas) and zone 3 (YRD, Central China and Southwest China), reaching 21, 19 and 14 occurrences, respectively, which were more than the occurrences from 2013 to 2015 and were more conducive to pollutant diffusion. The number of total cold air activities in zone 4 (PRD) was 13 occurrences, but the occurrences of strong cold air were significantly higher than the occurrences from 2013 to 2015. However, in November and December of 2016, the East Asian winter monsoon was weak, affecting fewer occurrences of cold air in Beijing-Tianjin-Hebei, and in the surrounding areas, the overall intensity was generally weak, the activity path was north facing, and the atmospheric diffusion meteorological condition was poor.
Among all kinds of meteorological elements, wind speed had an important influence on haze concentration by affecting the horizontal transport and diffusion capacity of pollutants. The wind speed usually had a significant clearance effect on haze concentration when it was larger than 3m/s. Taking Beijing area as an example, the correlation between wind speed and PM 2.5 concentration was 0.27, and it was delayed for one hour to achieve the greatest impact.    From 2014 to 2016, the diffusion capacity of air pollutants was consistently poor, and the meteorological conditions were favorable for the formation of haze. Meanwhile, the haze days in the Beijing-Tianjin-Hebei, Yangtze River Delta and Pearl River Delta regions decreased significantly, that indicated that the emission reduction measures conducted by the Chinese government were effective. (3)Meteorological conditions affect haze. The number of breeze days increased and the average wind speed decreased. The meteorological conditions were favorable for the formation of haze, but the number of haze days in the three regions decreased. This decrease in haze days reflected the important role that pollution reduction played in China.