Research and Analysis of a Persistent Heavy Pollution Event in Ningbo, China

From 7 to 12 January 2015, there was a rare persistent severe pollution event in Ningbo. Based on the data from routine weather observation, automatic weather station in Zhejiang Province and urban pollutant monitoring in Ningbo City, by using the particle backward trajectory analysis of NOAA HYSPLIT4 model, the diagnosis and analysis of the pollution development and dissipation process were carried out. The results show that: 1) Pollutants carried by cold air are main reason for pollution in the first stage of the pollution process. The transition from near-surface northwest wind to north-northeast wind is the key to air improvement; 2) Favorable atmospheric circulation is the important reason for the long-term persistence of pollution. The long-term control of high-pressure center and low-level inversion between the two cold air effects make the boundary layer particularly stable, while the low horizontal wind speed is beneficial to polluting particles. With the continuous accumulation, the contaminated particles can not effectively diffuse either vertically or ho-rizontally; 3) In the late period of January 11, the supplement of weak cold air in the East Road made the air quality index (AQI) climb to about 300 again. The two advection conveyances of cold air to pollutant particles and the long-term stable maintenance of weather conditions conducive to the accumulation of pollutant particles are two important reasons for this rare and sustained heavy pollution event.

motor vehicles, fossil fuel combustion, industrial waste gas, etc. have also been rising, and air pollution has occurred frequently in various places (Cao et al., 2018;Li et al., 2018;Yang et al., 2010;Jiao et al., 2016). Ningbo is the economic center of the south wing of the Yangtze River Delta. The economy has maintained a rapid development speed. The continuous increase of energy consumption and the continuous expansion of industrial scale have led to the emergence of a large number of gas pollutants. The atmospheric environment has become increasingly apparent. With the rapid development of the national economy, the living standards of residents have been continuously improved.
The impact of quality on public health and quality of life is receiving increasing attention.
There are many works on air pollution. Numerous studies have shown that PM2.5, the main air pollutant, can be produced not only by local human or natural factors, but also under certain conditions, it can be transported from a distance to the local by atmospheric movement to study pollutants. Transmission characteristics, analysis of potential sources of pollutants, can provide an important basis for the government to carry out air pollution prevention and control Wang et al., 2008).
In addition to local man-made and natural factors, pollutants in the air can also be transported from a long distance to the local or diffused to the local by atmospheric movement. The range of transport and diffusion is as small as several hundred meters, up to a few hundred meters. Regional, intercontinental and even global scales, so pay attention to the quality of a city's atmospheric environment, not only to consider the impact of the local area, but also consider the impact of the surrounding area (Yu et al., 2015;Zhu et al., 2007;Yao et al., 2017).
In the case of little change in the local pollutant discharge season, the air quality in Ningbo has a poor autumn and winter season and good seasonal characteristics in summer, which is not only related to the difficulty in spreading static and stable weather pollutants under the control of continental high pressure in autumn and winter, but also cold in winter. There are many reasons such as the influence of air and the source of external sources of pollution. Exogenous input air pollution caused by cold air carrying pollutants often occurs in autumn and winter, almost accompanied by every cold air intrusion, which has a great impact on air quality, often causing moderate to moderate pollution and living for the people great influence.
Studies have shown that the air quality of all moderate and above pollution levels in Ningbo is 100% in the autumn and winter seasons; the number of lightly polluted air days is 66.7% in the autumn and winter, and the air level is 50% in good days. The number of days in the air accounts for 10%, and autumn and winter are the high incidence of moderate and above air pollution in Ningbo (Jiang et al., 2016).
Other studies have shown that strong cold advection flows intrude to the  (Song et al., 2017).
However, this is exactly the opposite of the experience of environmental forecasting in Ningbo. To understand the relationship between cold air and local pollution, trace the external source of air pollution, study the transmission characteristics of pollutants, and analyze the potential pollution sources of pollutants, which will help the time forecasting and early warning of air pollution. It provides an important basis for government pollutant discharge control and air pollution prevention and control work, and has practical guiding significance for the public to reasonably arrange life production and travel activities. It is also one of the environmental problems that need to be solved urgently in the process of sustainable development of Ningbo economy.
In the middle and early January of 2015, a serious pollution incident occurred in Ningbo City. From January 5 to 12, there were 123 hours of AQI reaching the pollution level. In order to study the cause of the outbreak of this pollution in Ningbo, the boundary layer of Ningbo area Meteorological elements is used for diagnostic analysis. The HYSPLIT4 model is used to analyze the trajectories of aerosol particles to deepen the understanding of the boundary layer circulation characteristics of contaminated particles for a long time, and to establish a conceptual model of particle trajectories to provide technical reference for air quality prediction.

Materials and Methods
The data used include the conventional weather maps from January 6 sources. It has been widely used in the study of atmospheric pollutant transport and diffusion (Weng et al., 2016;Yang et al., 2017;Wang et al., 2015;Wang, 2013;Hua, 2016;Draxler & Hess, 1998).

Circulation Situation
In the process, there are two cold airs with different paths and strengths affecting Ningbo City. Since January 4, 2015, strong cold air has moved eastward and southward from the Hetao area, which belongs to the middle road cold air. The 500 hPa is mainly characterized by westerly wind and 700 hPa high altitude. After the trough transited, it turned to the northwest wind. The ground temperature of the ground station was 10.2 degrees and the lowest was 3.2 degrees, reaching the cold wave standard. Figure 1(a) and Figure 1(b) show the weather conditions at 20 o'clock on the 5 th and 8 o'clock on the 6 th . It can be seen that on the 5 th night, the ground gradually turned from northerly wind to northerly wind, and cold air began to affect Ningbo. At the same time, the average AQI of the city climbed rapidly. At 8 o'clock on the 6 th , it reached the highest value of 297. Since 08:00, weak precipitation began to appear in some areas of the city.
The wind direction also gradually turned from the northwesterly wind at 08:00, and turned northerly at 10 o'clock. The air quality improved rapidly. At 13 o'clock, the AQI has dropped below 70. After that, the wind direction is reversed and the west is slightly swaying. However, the northerly wind is still dominant.
On the morning of the 7 th , the wind direction is northwesterly and the wind is strengthened. The concentration began to rise slowly and reached pollution again at 23:00 on the 7 th . After a favorable circulation situation, the air pollution incident lasted for 107 hours, forming an extremely rare air pollution process in Ningbo.
From the 5 th to the 12 th , 850 hPa has been dominated by high winds in the J. M. Guo et al.  Figure 1. Sea level pressure field (hPa), 700 hPa height field (dagpm) and 850 hPa wind field (m/s) at 20:00 on January 5, 2015 (a) and 08:00 on January 6, 2015 (b) (The red contour is the sea level isobaric line, the black is the 700 hPa contour line, and the feather is the 850 hPa wind direction wind speed).
north. The intensity of cold air is very conducive to the long-distance transport of particulate matter. Before the 8 th , the northerly wind was mainly on the ground, but the wind was not large. The wind speed on the sea was very large. It Journal of Geoscience and Environment Protection gradually turned into high-pressure control from the 8 th . On the 11 th , with the addition of weak cold air, the northerly wind on the ground increased again. The wind direction was mainly. The northeast wind belongs to the cold air of the east road.

Pollution Overview
The monitoring and analysis of six main pollutants (PM10, PM2. This extremely rare pollution process is mainly divided into two stages. The first stage lasts for 16 hours from 21:00 on the 5 th to the 12 th of the 6 th , and the hourly AQI is 297 (8:00 on the 6 th ). After that, the air quality is rapidly improved. The second phase lasted for 107 hours from 23:00 on the 7 th to 9:00 on the 12 th , and the maximum AQI was 286 (0:00 on the 12 th ). The primary pollutants in both stages are PM2.5, and the urban visibility shows a good negative correlation with AQI (mainly PM2.5) (Figure 2). When the AQI reaches 100 or more, the visibility is below 5 KM for a long time. The minimum artificial visibility is only 600 m.

The First Stage Pollution Characteristics and Causes Analysis
Before the first stage of pollution, the surface wind direction is dominated by southerly winds, and the AQI index is basically below 100. Especially on the morning of January 5, as the temperature increases and the southerly wind speed increases, the boundary layer height increases, and the AQI value increases.  The 12 h and 24 h particle back trajectories indicate (Figure 4). The near-surface particles below 100 m are mainly from the southern part of Ningbo, 300 -500 KM in the southwest of Ningbo, and the area of the source of the particles is below 100 m. It is basically consistent with the surface wind direction, and the time when the pollutants soar is also the same as the time when the cold air arrives. It can be seen that this is a typical pollution process caused by cold air carrying pollutants. The advection of contaminated particles by cold air is the most important cause of air pollution. Since Ningbo North (Hangzhou Bay) and East (Donghai) are large areas of water, the sea air is relatively clean. After the wind direction turns from northwest to north and then northeast, the pollution level drops rapidly, and the pollution process is temporarily ended.

The Second Stage Pollution Characteristics and Causes Analysis
On the morning of the 7 th , the wind turned north to northwest. At the same time, the wind strengthened, the concentration of particulate matter began to rise slowly, and reached pollution again at 23 o'clock. After that, it fluctuated until 9:00 on the 12 th , during the evening from 9 th to 10 th . More than ten hours of continuous heavy pollution and reached a stage high of 276 at 21:00 on the 10 th , after which the AQI index remained at 100 -200, and again rose to above 200 at night on the 11 th , up to 286 (0:00 on the 12 th ).
The thickness of the boundary layer is closely related to the atmospheric temperature. When the temperature rises, the boundary layer is well mixed and the thickness is increased, which is conducive to the vertical diffusion of pollutants.  The analysis of the circulation situation found that from 08:00 on the 8 th to 08:00 on the 11 th , the Ningbo area has been near the high-pressure center (not shown), and the weak downdraft flow, according to the analysis of the convective sounding data of the initial field of the EC, from 08:00 to 9. At 08:00, the atmosphere near Ningbo is in a stable state, and the troposphere has an inverse temperature of about 925 hP (about 800 m) (shown in figure), forming a natural pollution cover, which limits the vertical diffusion of pollutants. From noon on the 7 th to the noon on the 12 th of the end of the pollution process, the wind in the urban area has been maintained at 1 -2, the wind direction is irregular, most of the time the wind speed is below 2 m/s (Figure 6), small wind or calm wind. It is not conducive to the diffusion of pollutants, which is another important reason for the long-term pollution in the second stage.
The backward trajectory of the particles indicates ( Figure 7). On the 7 th , there are still pollutant particles in the direction of southern Jiangsu. In the early days of 8 -11, the pollution mainly originated from the accumulation of local polluted particles. From the late 11 th to the noon on the 12 th , along with the East Road With the addition of cold air, AQI has soared to more than 200, up to 286 (0:00 on the 12 th ). The source of pollution is from the eastern coastal areas such as Jiangsu and even Shandong. During the period, the advection of pollutants by cold air is an important reason for AQI's new high.

Conclusion
Through the analysis of the rare persistent heavy pollution process in Ningbo City from January 5 to 12, 2015, it was found that carrying pollutants in cold air is the most important cause of pollution in the first stage of the pollution process.