Study on the Pollution Characteristics and Sources of Ozone in Typical Loess Plateau City

atmosphere

Article Study on the Pollution Characteristics and Sources of Ozone in Typical Loess Plateau City

Bin Li 1, Zhuangzhi Zhou 1, Zhigang Xue 2,*, Peng Wei 2, Yanjun Ren 2, Liyuan Cao 3, Xinyu Feng 3, Qingchen Yao 3, Jinghua Ma 2, Peng Xu 2 and Xuan Chen 2,*

1 School of Environment and Safety Engineering, North University of China, Taiyuan 030051, China; [email protected] (B.L.); [email protected] (Z.Z.) 2 State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China; [email protected] (P.W.); [email protected] (Y.R.); [email protected] (J.M.); [email protected] (P.X.) 3 Taiyuan Eco-Environment Monitoring Center of Shanxi Province, Taiyuan 030051, China; [email protected] (L.C.); [email protected] (X.F.); [email protected] (Q.Y.) * Correspondence: [email protected] (Z.X.); [email protected] (X.C.)

Received: 15 April 2020; Accepted: 12 May 2020; Published: 27 May 2020

Abstract: Ground-level ozone is a secondary pollutant produced by photochemical reactions and it adversely affects plant and human health. Taiyuan City, a typical city on the loess plateau, is suffering from severe ozone pollution. We utilized the data from eight national environmental monitoring sites of Taiyuan, including concentrations of O3 and nitric oxide, and meteorological factors, such as air temperature and wind, to study the pollution characteristics and sources of ozone (O3) in Taiyuan in 2018. Results show that during 2018, the maximum value and 90th percentile of the maximum 3 3 8-h running average of O3 concentration were 257 µg/m and 192 µg/m , respectively. There were 72 days where the O3 concentration exceeded the standard in 2018, which were mainly during April to August. The O3 concentration increased from March, reached a high level in April through August, and decreased significantly from September. The O3 concentrations displayed a typical “single peak” diurnal variation, which was high during the day with peak at around 13:00–15:00 and low at night. From April to August, the O3 concentrations at Jinyuan was the highest, followed by Xiaodian and Taoyuan, and the O3 concentrations at Shanglan and Nanzhai were the lowest. When the O3 concentration exceeded the standard value, Jinyuan contributed the most to the O3 pollution of Taiyuan, followed by Taoyuan and Xiaodian. High temperature and pressure, south and southwest winds can lead to an increase in O3 concentration. The O3 pollution in the Taiyuan urban area is caused by local generation, and the transportation of polluted air masses containing oxides of nitrogen (NOx) and volatile organic compounds (VOCs) emitted by industries, such as the coking and steel plants in counties of Jinzhong City in southern Taiyuan, and Qingxu County, and some counties in Lyuliang City to the southwest. In addition, the mountain winds and low nitric oxide concentration are the main reasons for the increase of O3 concentration, often observed in Shanglan at night.

Keywords: ozone concentration; temporal and spatial variation; pollution cause; mountain wind; cluster analysis of backward trajectories; local generation; regional transport

1. Introduction

Ground-level O3 is a major component of photochemical oxidants, which is mainly generated by the photochemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight [1]. Therefore, the formation of O3 is inﬂuenced by meteorological factors, such as solar radiation intensity and air temperature, as well as by the transport of polluted

Atmosphere 2020, 11, 555; doi:10.3390/atmos11060555 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 555 2 of 14

air masses [2–5]. O3 pollution affects not only human health, but also plant health and growth [6]. In recent years, O3 pollution in many areas of China has been more severe year after year with increased urban population and number of motor vehicles. Thus O3 has become another important pollutant next to PM2.5 affecting the air quality [7,8] in China. Taiyuan is a typical loess plateau city with an average elevation of 800 m. The highest and lowest altitudes are 2670 m and 760 m, respectively. It is adjacent to the Taihang Mountains on the east, Lyuliang Mountain on the west, Yunzhong Mountain and Xizhou Mountain on the north, and it has river valleys in the middle and south. Moreover, located in the south of Taiyuan, Xiaodian district together with the areas to its south are characterized by open terrain, and the terrain of the areas to the north of Xiaodian are relatively narrow. However, the northernmost part of Taiyuan, which shares a border with the Yunzhong Mountain, is particularly narrow, making Taiyuan a trumpet-shaped basin shown in Figure1. Only when the north wind blows at a high speed the air pollutants can be dispersed effectively. However, when the south wind blows, the polluted air masses from the south and southwest of Taiyuan can be transported to Taiyuan, affecting the O3 concentration in the urban area of Taiyuan.

Figure 1. Topographic map of Taiyuan. Figure 1. Topographic map of Taiyuan.

In the past two years, although the pollution of PM2.5 has been effectively controlled in Taiyuan, the pollution caused by O3 has been increasing year by year. The first time that ozone concentrations exceeded the standard in Taiyuan in 2017 was 2 May 2017 (173 µg/m3), and the first time that ozone concentrations exceeded the standard in 2018 was 30 March 2018 (174 µg/m3). The time when the ozone concentrations exceeded the standard in 2018 was earlier than the time when the ozone concentrations exceeded the standard in 2017, and the degree of ozone pollution in 2018 was more serious, causing the ambient air quality of Taiyuan City to decline. Therefore, it is particularly important to study the characteristics of O3 pollution in Taiyuan and explore its sources. However, there are still few reports available and this study aims to make clear the temporal variation and spatial distribution characteristics of O3 concentration in Taiyuan, explore its sources and the causes of O3 pollution, which will provide reference for the government to formulate effective measures and improve the air quality of Taiyuan.

2. Data and Methods The data used in this research are collected by the eight national environmental monitoring sites of Taiyuan during 2018, including the concentrations of such pollutant gases as O3 and NOx (in standard condition, 273.15 K and 101.325 Kpa) as well as meteorological factors, such as air temperature

Figure 2. The eight monitoring sites in Taiyuan.

To explore the sources of air masses and their contribution to the O3 pollution in Taiyuan, when the O3 concentration exceeds the standard value, the HYSPLIT model, developed by the United States National Oceanic and Atmospheric Administration, fed by meteorological data provided by the Global Data Assimilation System of the United States National Centers for Environmental Prediction (ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1, resolution: 1° × 1°), has been used to conduct backward trajectory simulations.

3. Results and Discussion

3.1. The O3 Pollution in Taiyuan in 2018

3.1.1. O3 Concentrations Standard and Variation of O3 Concentrations

Atmosphere 2020, 11, 555 3 of 14 and wind direction (Provided by the Taiyuan Ecological Environment Monitoring Center of Shanxi Province). The eight sites from north to south are Shanglan (the background site, not participating in the calculation of the mean value of sampling in Taiyuan), Nanzhai, Jiancaoping, Taoyuan, Wucheng, Jinsheng, Xiaodian, and Jinyuan,Figure as shown 1. Topographic in Figure2 map. of Taiyuan.

Figure 2. The eight monitoring sites in Taiyuan.

To exploreexplore thethe sourcessources ofof airair massesmasses andand theirtheir contributioncontribution toto thethe OO33 pollution in Taiyuan, when thethe O O33 concentrationconcentration exceeds exceeds the the standard standard value, value, the theHYSPLIT HYSPLIT model, model, developed developed by the by United the United States StatesNational National Oceanic Oceanic and Atmospheric and Atmospheric Administration, Administration, fed fedby bymeteorol meteorologicalogical data data provided provided by by the Global Data Assimilation System of the United StatesStates National Centers for Environmental Prediction ((ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1,ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1, resolution: resolution: 1◦ 1°1◦ ),× has1°), been has used been to conductused to backward conduct × trajectorybackward simulations.trajectory simulations.

3. Results and Discussion

3.1. The O3 Pollution in Taiyuan in 2018 3.1. The O3 Pollution in Taiyuan in 2018

3.1.1. O3 Concentrations Standard and Variation of O3 Concentrations 3.1.1. O3 Concentrations Standard and Variation of O3 Concentrations In 2018, the maximum 1-h ozone concentrations in Taiyuan was 316 µg/m3, in this year, a total of 253 h of ozone concentrations exceeded the standard (Grade II Standard in Ambient Air Quality Standard (GB3095-2012) [9], the average 1-h ozone concentrations in this standard is 200 µg/m3). The maximum ozone concentrations of MDA8 for the whole year is 257 µg/m3, and the MDA8 for 72 days in this year exceeds the standard (Grade II Standard in Ambient Air Quality Standard (GB3095-2012), the concentrations of ozone MDA8 in this standard is 160 µg/m3). The ozone concentrations of Taiyuan City is the average ozone concentration of each monitoring site in Taiyuan City (except Shanglan; the Shanglan monitoring site is a background site and does not participate in the calculation of the average value of Taiyuan City). Figure3 shows the temporal variation and monthly average values of MDA8 from January to December, respectively. As can be seen from the two ﬁgures, MDA8 increased from March, reached its maximum in June, and began to decline signiﬁcantly in mid-September. Overall, MDA8 was higher

In 2018, the maximum 1-h ozone concentrations in Taiyuan was 316 μg/m³, in this year, a total of 253 h of ozone concentrations exceeded the standard (Grade II Standard in Ambient Air Quality Standard (GB3095-2012) [9], the average 1-h ozone concentrations in this standard is 200 μg/m³). The maximum ozone concentrations of MDA8 for the whole year is 257 μg/m³, and the MDA8 for 72 days in this year exceeds the standard (Grade II Standard in Ambient Air Quality Standard (GB3095-2012), the concentrationsAtmosphere 2020, 11of, 555 ozone MDA8 in this standard is 160 μg/m³). The ozone concentrations4 of 14 of Taiyuan City is the average ozone concentration of each monitoring site in Taiyuan City (except Shanglan;from the April Shanglan to August, monitoring and lower in January, site is November, a background and December. site Inand addition, does thenot temperature participate in in the calculationJuly of is usuallythe average the highest value in aof year, Taiyuan but due City). to more rainy days and less solar radiation, the average MDA8 in July was signiﬁcantly lower than that in June.

Figure 3. Monthly average of MAD8 in Taiyuan in 2018. Figure 3. Monthly average of MAD8 in Taiyuan in 2018. In 2018, there were 70 days from April to August that the MDA8 exceeded the standard, with Figurethe largest 3 shows number the intemporal June (20 days),variation and theand smallest monthly numbers average in March values and of September MDA8 (Tablefrom 1January). to December,Therefore, respectively. the data fromAs can April be to seen August from was the selected two tofigures, study the MDA8 characteristics increased of O from3 pollution. March, reached its maximumTable in 1.June,The number and ofbegan days that to MDA8 decline exceeded significantly the standard forin eachmid-September. month in 2018. (Jan.: Overall, January, MDA8 was higher fromFeb.: April February, to August, Mar.: March, and Apr: lower April, in Jun.: Janu June,ary, Jul.: November, July, Aug.: August, and Sep.: December. September, In Oct.: addition, the temperatureOctober, in July Nov.: is November,usually Dec.:the December).highest in a year, but due to more rainy days and less solar radiation, theMonth average MDA8 Jan. Feb. in July Mar. was Apr. significantly May Jun. lower Jul. than Aug. that Sep. in June. Oct. Nov. Dec. In 2018,The numberthere ofwere 70 days from April to August that the MDA8 exceeded the standard, with days MDA8 0 0 1 9 14 20 16 11 1 0 0 0 the largest numberexceeded in June (20 days), and the smallest numbers in March and September (Table 1). standard Therefore, the data from April to August was selected to study the characteristics of O3 pollution.

3.1.2. Diurnal Variation of O Concentration Table 1. The number of days that3 MDA8 exceeded the standard for each month in 2018. (Jan.: January, Feb.: February,The O3 concentration Mar.: March, generally Apr: April, shows Jun.: a typical June, “singleJul.: July, peak” Aug.: diurnal August, variation Sep.: on September, sunny days. Oct.: October,It usually Nov.: has November, the lowest concentrations Dec.: December). at 7:00, due to the weak process of precursor conversion to ozone at nighttime and the strong ozone depletion effect caused by NO titration and dry sedimentation [10–12]. It reached its maximum around 13: 00–15: 00, and decreased from 17: 00 or 18: 00 (Figure4), this is Month Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. because O3 is a secondary product of atmospheric photochemical reactions, and high air temperature and high intensity of solar radiation are conducive to its generation. In addition, mixing ozone rich The number of daysair from the free troposphere into the ozone depleted surface layer in the morning, as the nocturnal MDA8 exceededstable layer breaks0 up, 0 it is also 1a significant9 factor14 in the20 shape of16 the diurnal11 cycle [131 ]. On most0 0 0 standard cloudy days, the O3 concentration exhibited the same diurnal variation as on sunny days, except that its maximum was significantly lower than that of sunny days (Figure4), which was due to less solar radiation and lower temperature. 3.1.2. Diurnal Variation of O3 Concentration

The O3 concentration generally shows a typical “single peak” diurnal variation on sunny days. It usually has the lowest concentrations at 7:00, due to the weak process of precursor conversion to ozone at nighttime and the strong ozone depletion effect caused by NO titration and dry sedimentation [10–12]. It reached its maximum around 13: 00–15: 00, and decreased from 17: 00 or 18: 00 (Figure 4), this is because O3 is a secondary product of atmospheric photochemical reactions, and

high air temperature and high intensity of solar radiation are conducive to its generation. In addition, mixing ozone rich air from the free troposphere into the ozone depleted surface layer in the morning, as the nocturnal stable layer breaks up, it is also a significant factor in the shape of the diurnal cycle [13]. On most cloudy days, the O3 concentration exhibited the same diurnal variation as on sunny days, except that its maximum was significantly lower than that of sunny days (Figure 4), which was due to less solarAtmosphere radiation2020, 11, 555 and lower temperature. 5 of 14

Figure 4. Diurnal variations of O3 concentration in Taiyuan on a sunny day (6 June) and cloudy day Figure 4. Diurnal(7 June andvariations 8 June). of O3 concentration in Taiyuan on a sunny day (6 June) and cloudy day

(7 June 3.1.3.and 8 Variation June). of MDA8 at Each Monitoring Site and Their Contributions to the Average of MDA8 in Taiyuan 3.1.3. Variation Figureof MDA85 shows at the Each average Monitoring of MDA8 at each Site monitoring and Their site fromContributions April to August. to The the contribution Average of MDA8 in Taiyuan of each monitoring site to the MDA8 is different under different levels of O3 pollution. When the O3 pollution was slight, Taoyuan had relatively high contribution (Figure6a). When the O 3 pollution Figure was5 shows moderate, the Jinyuan average contributed of MDA8 the most toat the each pollution monitoring (Figure6b). Jinyuansite from to the April southwest to ofAugust. The contributionthe of urban each area mo ofnitoring Taiyuan and site Xiaodian to the to MDA8 the south is are different on the air pollutionunder different transmission levels routes of from O3 pollution. Qingxu County, Lyuliang City, and Jinzhong City to Taiyuan. Due to the impact of transported air When the O3 pollution was slight, Taoyuan had relatively high contribution (Figure 6a). When the O3 masses, the ozone reaches the peak at different times for each site, as evidenced by Figure7, showing pollution wasthe ozonemoderate, diurnal Jinyuan variation atcontributed each monitoring the site most on June to 23.the The pollution Jinyuan and (Figure Jinsheng 6b). sites Jinyuan are to the southwest closer,of the and urban the ozone area concentration of Taiyuan peaks reachedand Xi ataodian 13:00. For to the the Xiaodian, south Jiancaoping, are on Taoyuan,the air pollution transmissionand routes Wucheng from sites, Qingxu the ozone County, concentration Lyuliang peaks were City, reached and at Jinzhong 14:00. Nanzhai City and to Shanglan Taiyuan. are Due to the located in the northernmost part, Nanzhai reached peak at 15:00, and Shanglan reached peak time at impact of transported17:00. There are air many masses, factories, the including ozone coking reache and steels the plants peak in Qingxu at different County and times Lvliang for City, each site, as evidenced byto theFigure southwest 7, showing of Jinyuan, the and ozone in Jinzhong diurnal City, which variation is located at to each the south monitoring of Xiaodian. site Location on June 23. The Jinyuan andof Jinsheng these cities sites are shown are incl Figureoser, 14.and These the plantsozone emit concentration large amounts ofpeaks the O3 reachedprecursors: at NOx 13:00. For the and VOCs. When winds blow from south or southwest, these precursors are transported to Taiyuan’s Xiaodian, Jiancaoping, Taoyuan, and Wucheng sites, the ozone concentration peaks were reached at urban area, leading to O3 formation through chemical reactions. During the transport, or after they 14:00. Nanzhaireach and Taiyuan, Shanglan O3 can be are generated located through in the chemical northernmost reactions, increasing part, Nanzhai O3 concentrations reached in thepeak at 15:00, and Shanglanurban reached area of Taiyuan.peak time Geng. at F17:00. et al. [14There] also are observed many that factories, polluted air including mass transport coking caused and an steel plants in Qingxu Countyincrease of and ozone Lvliang concentrations City, in to Shanghai. the southwest of Jinyuan, and in Jinzhong City, which is located to the south of Xiaodian. Location of these cities are shown in Figure 14. These plants emit large amounts of the O3 precursors: NOx and VOCs. When winds blow from south or southwest, these precursors are transported to Taiyuan’s urban area, leading to O3 formation through chemical reactions. During the transport, or after they reach Taiyuan, O3 can be generated through chemical reactions, increasing O3 concentrations in the urban area of Taiyuan. Geng. F et al. [14] also observed that polluted air mass transport caused an increase of ozone concentrations in Shanghai.

Atmosphere 2020, 11, 555 6 of 14

Figure 5. Average MDA8 at each site in Taiyuan, from April to August 2018. Figure 5. Average MDA8 at each site in Taiyuan, from April to August 2018. FigureFigure 5. Average 5. Average MDA8 MDA8 at at each each site in Taiyuan,Taiyuan, from from April April to Augustto August 2018. 2018.

Figure 6. The difference between the mean value of MDA8 of each site and that of Taiyuan at different O3 pollution levels (a, b) from April to August. FigureFigure 6. 6.TheThe difference diﬀerence between thethe mean mean value value of of MDA8 MDA8 of eachof each site site and and that ofthat Taiyuan of Taiyuan at diﬀerent at different O3 pollution levels (a,b) from April to August. FigureO3 pollution 6. The difference levels (a, betweenb) from April the mean to August value .of MDA8 of each site and that of Taiyuan at different

O3 pollution levels (a, b) from April to August.

Figure 7. Time diﬀerence of peak ozone concentrations at each site on 23 June 2018. Figure 7. Time difference of peak ozone concentrations at each site on 23 June 2018. Figure 7. Time difference of peak ozone concentrations at each site on 23 June 2018. 3.2. Study on the Causes of O3 Pollution in Taiyuan Figure 7. Time difference of peak ozone concentrations at each site on 23 June 2018. 3.2. Study on the Causes of O3 Pollution in Taiyuan

3.2. Study on the Causes of O3 Pollution in Taiyuan

Atmosphere 2020, 11, 555 7 of 14

3.2. Study on the Causes of O3 Pollution in Taiyuan 3.2.1. Temperature 3.2.1. Temperature Figure 8 shows the correlation between MDA8 and the 8-h temperature value (T-8h) in the same Figure8 shows the correlation between MDA8 and the 8-h temperature value (T-8h) in the same period. As can be seen from the scatter plot, there is a strong positive correlation between ozone period. As can be seen from the scatter plot, there is a strong positive correlation between ozone concentration and temperature. Because ozone is the product of a photochemical reaction between concentration and temperature. Because ozone is the product of a photochemical reaction between precursors, such as NOx and VOCs in the atmosphere, an increase in temperature contributes toward precursors, such as NOx and VOCs in the atmosphere, an increase in temperature contributes toward accelerating the production of ozone, as well as promotes the emission of VOCs from plants, both of accelerating the production of ozone, as well as promotes the emission of VOCs from plants, both which contribute to the increase of O3 production. The red-dotted line in the figure indicates the of which contribute to the increase of O production. The red-dotted line in the ﬁgure indicates the concentrations standard value of MDA83 is 160 μg/m³. concentrations standard value of MDA8 is 160 µg/m3.

Figure 8. Correlation between MDA8 and T-8h in the same period. Figure 8. Correlation between MDA8 and T-8h in the same period. 3.2.2. Air Pressure 3.2.2. Air Pressure Normally, the O3 concentration increases with temperature, but sometimes the O3 concentration may beNormally, high when the theO3 concentration temperature isincreases relatively with low te (Figuremperature,9). For but example, sometimes between the O3 concentration 31 May and 2may June be 2018, high the when highest the temperature wasis relatively only 28.5 low◦C, (Figure but the 9). O3 Forconcentration example, between showed 31 a highMay valueand 2 June 2018, 3the highest temperature was only 28.5 °C, but the O3 concentration showed a high value of 273 µg/m . The same phenomenon also appeared during 8–10 May and 14–16 June in 2018. of 273 μg/m³. The same phenomenon also appeared during 8–10 May and 14–16 June in 2018. The Taiyuan area began to be affected by high pressure from 31 May, and on 1 and 2 June the high-pressure air mass gradually moved southward. During this period, the city of Taiyuan was at the center of the high pressure, and the structure of the atmospheric boundary layer was rather stable. This was not conducive to the diffusion of O3; thus, causing gradual accumulation of O3, with the result that the O3 concentration turned out to be the highest on 1 and 2 June (Figure 9). Wang et al. [15] obtained similar results in a study conducted in Fuzhou from 2009 to 2010, and the phenomenon was observed during 8–10 May and 14–16 June in 2018.

Figure 9. Variations of O3 concentration and air temperature in Taiyuan from 31 May to 2 June. Figure 9. Variations of O3 concentration and air temperature in Taiyuan from 31 May to 2 June.

3.2.3. Topography

The daily variation of overall O3 concentration at eight sites was basically consistent. However, it was found that the nighttime O3 concentration was sometimes high at Shanglan when the other 7 sites were low (Figure 10).

Figure 10. Variation of O3 concentration at the national monitoring sites in Taiyuan during 22–24 June.

The Shanglan site (38°00′38.8″ N, 112°26′2.5″ E) is located on a building of Shanglan Village in Taiyuan. Just 150 m north of it is Erlong Mountain (about 960 m above sea level), and Shanglan is at the foot of the mountain. It is known that the weather in the mountains is quite different from that in the plains. The mountain areas are affected by valley winds during the day and by mountain winds at night [16,17]. This is because the air above the mountain slope is more affected by radiation cooling at night than at the bottom of the mountain, and its temperature drops faster. Therefore, the cool air above the mountain slope will flow down to the bottom. The warm air above the valley is lifted,

Atmosphere 2020, 11, 555 8 of 14

The Taiyuan area began to be aﬀected by high pressure from 31 May, and on 1 and 2 June the high-pressure air mass gradually moved southward. During this period, the city of Taiyuan was at the center of the high pressure, and the structure of the atmospheric boundary layer was rather stable. This was not conducive to the diﬀusion of O3; thus, causing gradual accumulation of O3, with the result that the O3 concentration turned out to be the highest on 1 and 2 June (Figure9). Wang et al. [ 15] obtained similar results in a study conducted in Fuzhou from 2009 to 2010, and the phenomenon was Figure 9. Variations of O3 concentration and air temperature in Taiyuan from 31 May to 2 June. observed during 8–10 May and 14–16 June in 2018. 3.2.3. Topography 3.2.3. Topography The daily variation of overall O3 concentration at eight sites was basically consistent. However, The daily variation of overall O3 concentration at eight sites was basically consistent. However, it it was found that the nighttime O3 concentration was sometimes high at Shanglan when the other 7 was found that the nighttime O concentration was sometimes high at Shanglan when the other 7 sites sites were low (Figure 10). 3 were low (Figure 10).

Figure 10. Variation of O3 concentration at the national monitoring sites in Taiyuan during 22–24 June. Figure 10. Variation of O3 concentration at the national monitoring sites in Taiyuan during 22–24 June. The Shanglan site (38◦00038.8” N, 112◦2602.5” E) is located on a building of Shanglan Village in Taiyuan. Just 150 m north of it is Erlong Mountain (about 960 m above sea level), and Shanglan is at the footThe of Shanglan the mountain. site (38°00 It is known′38.8″ N, that 112°26 the weather′2.5″ E) is in located the mountains on a building is quite of di Shanglanfferent from Village that in theTaiyuan. plains. Just The 150 mountain m north areas of it areis Erlong affected Mountain by valley (about winds 960 during m above the day sea and level), by mountainand Shanglan winds is at nightthe foot [16 of,17 the]. Thismountain. is because It is theknown air abovethat the the weather mountain in the slope mountains is more aisff quiteected different by radiation from cooling that in atthe night plains. than The at mountain the bottom areas of theare mountain,affected by and valley its winds temperature during drops the da faster.y and by Therefore, mountain the winds cool airat night above [16,17]. the mountain This is because slope will the flowair above down the to mountain the bottom. slope The is warm more affected air above by the radiation valley iscooling lifted, creatingat night mountainthan at the winds bottom that of blowthe mountain, from the slopesand its to temperature the bottom [drops18]. Generally, faster. Therefore, mountain the winds cool areair formedabove the when mountain the wind slope speed will at flow night down is greater to the than bottom. 1.8 m/ sThe [19 ,20warm]. air above the valley is lifted, The height of the boundary layer at night usually drops to about 900–1000 m [21]. The altitude of Erlong Mountain in the north of Shanglan site is near 1000 m, so its peak is sometimes in the boundary layer at night. In addition, the wind often blows from the north at night at Shanglan, with 55% of the wind speed over 2.0 m/s in April through August (Figure 11). As a result, atmospheric convection is more active at night and mountain winds are likely to occur. Since the O3 concentration increases with the height, the O3 concentration at the top of Erlong Mountain is higher than that at the foot of the mountain. Thus, the mountain wind can transport the high concentration O3 from the top of Erlong Mountain to Shanglan, resulting in an increase in O3 concentration there at night. In addition, the concentration of NO at Shanglan is very low at night, which is close to zero and far lower than that at the other monitoring sites (Figure 12). The low concentration of NO reduces the titration of O3, which leads to increased O3 concentration at Shanglan at night. Wang et al. [21] found that the O3

creating mountain winds that blow from the slopes to the bottom [18]. Generally, mountain winds creatingare formed mountain when the winds wind that speed blow at fromnight the is greater slopes thanto the 1.8 bottom m/s [19,20]. [18]. Generally, mountain winds are formedThe height when of the the wind boundary speed atlayer night at nightis greater usua thanlly drops 1.8 m/s to [19,20].about 900–1000 m [21]. The altitude of ErlongThe height Mountain of the in boundary the north layer of Shanglan at night usua site llyis neardrops 1000 to about m, so 900–1000 its peak mis [21].sometimes The altitude in the ofboundary Erlong Mountain layer at night. in the In northaddition, of Shanglan the wind siteofte nis blows near 1000from m,the so north its peak at night is sometimes at Shanglan, in withthe boundary55% of the layer wind at speednight. overIn addition, 2.0 m/s the in Aprilwind oftethroughn blows August from (Figure the north 11). at As night a result, at Shanglan, atmospheric with 55%convection of the wind is more speed active over at night2.0 m/s and in mountain April through winds August are likely (Figure to occur. 11). SinceAs a theresult, O3 concentrationatmospheric convectionincreases with is more the activeheight, at the night O3 andconcentration mountain windsat the topare likelyof Erlong to occur. Mountain Since isthe higher O3 concentration than that at increasesthe foot of with the themountain. height, Thus,the O 3the concentration mountain wind at the can top transport of Erlong the Mountain high concentration is higher than O3 from that theat thetop foot of Erlong of the mountain.Mountain Thus,to Shanglan the mountain, resulting wind in ancan increase transport in theO3 highconcentration concentration there O at3 fromnight. the In topaddition, of Erlong the Mountainconcentration to Shanglan of NO at, resultingShanglan in is anvery increase low at in night, O3 concentration which is close there to zeroat night. and Infar addition,lower than the that concentration at the other of monitoring NO at Shanglan sites (Figure is very 12). low The at night,low concentration which is close of NOto zero reduces and farthe lowertitration than of thatO3, which at the leadsother to monitoring increased Osites3 concentration (Figure 12). atThe Shanglan low concentration at night. Wang of NO et al. reduces [21] found the Atmosphere 2020, 11, 555 9 of 14 titrationthat the ofO 3O concentration3, which leads atto theincreased top of OTai3 concentration Mao Mounta atin Shanglan (1000 m high)at night. in HongWang Konget al. [21]was found much thathigher the thanO3 concentration that at the foot at the of topthe ofmountain Tai Mao at Mounta night. inThis (1000 was m thought high) in to Hong be caused Kong wasby vertical much concentrationhigherdiffusion than bringing that at theat highthe top foot ofO Tai3 concentrationof Maothe mountain Mountain in theat (1000 night.middle m high)This of boundarywas in Hong thought Kong layer to was tobe thecaused much top higher byof Taivertical thanMao thatdiffusionMountain, at the bringing foot and of the the highweak mountain O titration3 concentration at night.of low This NO in wasconcentrationthe thoughtmiddle of to at beboundary the caused top of bylayer the vertical mountain to the di ﬀtopusion on of O 3 bringingTai [22]. Mao Ren highMountain,et al. O [23]3 concentration observedand the weak that in thetitrationthe middleconcentration of low of boundary NO of concentration pero layerxides to was the at topthehigher oftop Tai atof Mao nightthe mountain Mountain, than during on and O the3 the[22]. day weak Ren on titrationetMount al. [23] Tai. of observed low They NO considered concentrationthat the concentration it to at be the the top result of of pero the ofxides mountainthe reduction was onhigher O of3 [ 22atboundary ].night Ren than et layer al. during [23 depth] observed the combined day that on theMountwith concentration the Tai. mountain They ofconsidered peroxideswind at night. it was to be higher the result at night of than the reduction during the of day boundary on Mount layer Tai. depth They consideredcombined itwith to be the the mountain result of wind the reduction at night. of boundary layer depth combined with the mountain wind at night.

Figure 11. Correlation of wind direction and wind speed at Shanglan at night. Figure 11. Correlation of wind direction and windwind speed at Shanglan at night.

FigureFigure 12.12. TheThe averageaverage concentrationconcentration ofof NONO atat eighteight sitessites betweenbetween 23:0023:00 andand 6:006:00 whenwhenthe the O O3 3 concentrationFigureconcentration 12. The was was average high high at atconcentration Shanglan Shanglan at at night. night.of NO at eight sites between 23:00 and 6:00 when the O3 concentration was high at Shanglan at night. 3.2.4. Wind Direction and Regional Transport

Figure 13 shows the relationship between hourly O3 concentrations and wind direction in Taiyuan 3 from April to August in 2018 when the hourly O3 concentrations exceeded the 1-h standard (200 µg/m ), 3 the hourly O3 concentrations value standard is 200 µg/m (Figure 13). As can be seen from the ﬁgure, when the O3 concentration exceeded the standard, only winds from the southeast to southwest by west were observed. In particular, for the south wind to the south by southwest wind, high concentrations of O3 appeared most frequently. The reason is that there are many coking and steel plants in Qingxu county and Lyuliang City in the southwest of Taiyuan and in Jinzhong City in the south of Taiyuan. Plenty of NOx and VOCs, the precursor of O3 emitted by these plants, can be transferred to Qingxu of Tiayuan (in the southwest of Taiyuan), then to the urban area of Taiyuan when the wind blows from the south to the southwest. These pollutants can generate large quantities of O3 through chemical reactions during the transmission process or after being transferred to Taiyuan, resulting in the increase of O3 concentration in the city.

3.2.4. Wind Direction and Regional Transport

Figure 13 shows the relationship between hourly O3 concentrations and wind direction in Taiyuan from April to August in 2018 when the hourly O3 concentrations exceeded the 1-h standard (200 μ/m³), the hourly O3 concentrations value standard is 200 μg/m³ (Figure 13). As can be seen from the figure, when the O3 concentration exceeded the standard, only winds from the southeast to southwest by west were observed. In particular, for the south wind to the south by southwest wind, high concentrations of O3 appeared most frequently. The reason is that there are many coking and steel plants in Qingxu county and Lyuliang City in the southwest of Taiyuan and in Jinzhong City in the south of Taiyuan. Plenty of NOx and VOCs, the precursor of O3 emitted by these plants, can be transferred to Qingxu of Tiayuan (in the southwest of Taiyuan), then to the urban area of Taiyuan when the wind blows from the south to the southwest. These pollutants can generate large quantities of O3 through chemical reactions during the transmission process or after being transferred to AtmosphereTaiyuan, 2020resulting, 11, 555 in the increase of O3 concentration in the city. 10 of 14

Figure 13. Correlation between O3 concentration and wind direction in Taiyuan from April to August. Figure 13. Correlation between O3 concentration and wind direction in Taiyuan from April to August.In order to investigate the influence of regional transmission on ozone pollution in Taiyuan, the TrajStat software in HYSPLIT model, meteorological data and the hourly concentrations of ozone are In order to investigate the influence of regional transmission on ozone pollution in Taiyuan, the used to analyze the transport path, potential sources and their contribution to O concentrations [24]. TrajStat software in HYSPLIT model, meteorological data and the hourly concentrations3 of ozone are The city of Taiyuan (37◦520 N; 112◦320 E) was taken as the target site to simulate the trajectory of used to analyze the transport path, potential sources and their contribution to O3 concentrations [24]. air mass (500 m above the ground) moving backward for 24 h when the O concentration exceeded the The city of Taiyuan (37°52’ N; 112°32’ E) was taken as the target site to simulate3 the trajectory of air standard from April to August in 2018. A total of 3672 trajectories were simulated, and four types mass (500 m above the ground) moving backward for 24 h when the O3 concentration exceeded the were obtained after cluster analysis, namely Trajectory A, Trajectory B, Trajectory C, and Trajectory standard from April to August in 2018. A total of 3672 trajectories were simulated, and four types D (Figure 14). The length of trajectories in Figure 14 represents the distance of transmission, and the were obtained after cluster analysis, namely Trajectory A, Trajectory B, Trajectory C, and Trajectory proportions represent the ratios of the number of trajectories in this direction to the total number of D (Figure 14). The length of trajectories in Figure 14 represents the distance of transmission, and the trajectories. Among them, Trajectory B with the shortest length starts from Taigu County (Jinzhong proportions represent the ratios of the number of trajectories in this direction to the total number of City), passes through Yuci District and Qingxu County, and then enters the urban area of Taiyuan. trajectories. Among them, Trajectory B with the shortest length starts from Taigu County (Jinzhong It has the largest number of trajectories, accounting for 40%. The longer trajectories of cluster A and City), passes through Yuci District and Qingxu County, and then enters the urban area of Taiyuan. It cluster D indicate that they come from a longer distance of transportation. Among them, Trajectory has the largest number of trajectories, accounting for 40%. The longer trajectories of cluster A and A from the southwest enters the urban area of Taiyuan after passing through Ji County (Linfen City) cluster D indicate that they come from a longer distance of transportation. Among them, Trajectory and Lyuliang City, accounting for 22%. Trajectory D from Jiaozuo City, Henan Province, passes by A from the southwest enters the urban area of Taiyuan after passing through Ji County (Linfen City) Jincheng City and Changzhi City, then goes through Taigu County and Yuci District of Jinzhong City and Lyuliang City, accounting for 22%. Trajectory D from Jiaozuo City, Henan Province, passes by and enters the urban area of Taiyuan, accounting for 23%. Cluster C from the east enters the urban area Jincheng City and Changzhi City, then goes through Taigu County and Yuci District of Jinzhong City of Taiyuan after passing through Zanhuang County, Hebei Province and Pingding County (Yangquan and enters the urban area of Taiyuan, accounting for 23%. Cluster C from the east enters the urban City), accounting for 15%. area of Taiyuan after passing through Zanhuang County, Hebei Province and Pingding County As shown in Figure 15, in order to study the source and transmission route of ozone in Taiyuan (Yangquan City), accounting for 15%. from April to August 2018. It is determined by the potential source contribution function (PSCF) method and concentration weighted trajectory (CWT) method based on backward trajectory analysis. PSCF is a simple and effective method of simulating the source of pollution. The simulated backward trajectory is appended with the corresponding ozone concentration value, and then the trajectory is meshed. The advantage of this method is that it has better resolution [25]. Subsequently, after optimization by Seibert et al. [26], an additional concentration method was obtained, that is, the ozone concentration value of each track in the grid was calculated, and then weighted according to the residence time. Hsu et al. [27] further refined the CWT method. Hao et al. [28] used backward trajectory cluster analysis, potential source contribution function (PSCF), and concentration weighted trajectory (CWT) methods to investigate the transport pathways and potential source regions of PM2.5 on the west coast of Bohai Bay from 2009 to 2018. As shown in Figure 15a, it is the ozone potential source contribution map of Taiyuan City from April to August 2018. The darker the color, the greater the contribution of this area to the ozone concentrations in Taiyuan City, and the greater the possibility of becoming a potential source of pollution. It can be seen from Figure 15a that the local emissions in Taiyuan City and Jinzhong City, Luliang, and Jiaocheng are the most likely sources of ozone pollution Atmosphere 2020, 11, 555 11 of 14 in Taiyuan City. Because the PSCF method can only reflect the contribution rate of the potential pollution source area, but not the pollution degree, the CWT method is used to reflect the pollution degree of the potential pollution source. As shown in Figure 15b, it is the trajectory map of ozone concentration weights during April–August 2018 in Taiyuan City. The darker the color, the greater the ozone pollution in this area. Figure 15b shows that in addition to the local emissions in Taiyuan, the emissions from Qingxu County, Jiaocheng County (Lyuliang city), and Taigu County and Yuci district of Jinzhong City also contribute to the O3 concentration in Taiyuan. This is because winds from the southwest and the south can transport the air masses containing NOx and VOCs emitted by factories, including coking and steel plants in the above counties and cities to Taiyuan. There are also many factories, such as coking and steel plants in some counties south of Jiaocheng County (Lyuliang City) and south of Taigu County (Jinzhong City). Large amounts of NOx and VOCs emitted by these plants will be transported to Qingxu County, Jiaocheng County and Taigu County before being transported to Taiyuan City when the southwest wind or south wind blows; thus, leading to the increase of O 3 concentration in Taiyuan city.

Figure 14. Result of cluster analysis of backward trajectory of airflow when O3 exceeded standard in

FigureTaiyuanFigure 14.14. City ResultResult from ofof Ap clusterclusterril to analysisAugustanalysis 2018. ofof backwardback ward trajectorytrajectory ofof airﬂowairflow whenwhen OO33 exceededexceeded standardstandard inin TaiyuanTaiyuan City City from from April April to to August August 2018. 2018.

Figure 15. The possibility of the backward trajectories as potential sources of ozone (a), and their Figure 15. The possibility of the backward trajectories as potential sources of ozone (a), and their contribution to the concentration of ozone (b) in Taiyuan from April to August, 2018. contributionFigure 15. The to thepossibility concentration of the ofbackward ozone (b )trajecto in Taiyuanries as from potential April tosources August, of 2018.ozone (a), and their contribution to the concentration of ozone (b) in Taiyuan from April to August, 2018. 3.3. Countermeasures and Suggestions for Control of O3 Pollution in Taiyuan

3.3. Countermeasures and Suggestions for Control of O3 Pollution in Taiyuan The O3 pollution in the urban area of Taiyuan is caused by its local generation and the transport of pollutedThe O3 airpollution masses in from the urban the southwest area of Taiyuan and so isuth. caused Therefore, by its local a reduction generation of andNOx the and transport VOCs emissionsof polluted in air the masses city can from reduce the southwestO3 generation and soanduth. increase Therefore, the compliancea reduction rateof NOx of air and quality. VOCs Meanwhile,emissions in it theis necessary city can reduceto strengthen O3 generation the join tand prevention increase and the controlcompliance with rateLyuliang of air City quality. and JinzhongMeanwhile, City it so is asnecessary to reduce to the strengthen transmissi theon joinof pollutedt prevention air masses and control from external with Lyuliang areas. City and Jinzhong City so as to reduce the transmission of polluted air masses from external areas. 4. Conclusions 4. Conclusions The maximum hourly concentration of O3 was 316 μg/m³, with 253 h exceeding the standard valueThe in total. maximum The maximum hourly concentration and 90th percent of ileO3 ofwas the 316 maximum μg/m³, 8-hwith running 253 h exceeding average was the 257 standard μg/m³ andvalue 192 in total.μg/m³, The respectively, maximum andin Taiyuan 90th percent in 2018.ile of The the maximumO3 concentration 8-h running in Taiyuan average increased was 257 μ fromg/m³ March,and 192 reached μg/m³, arespectively, high value during in Taiyuan April inand 2018. Augu Thest, andO3 concentration decreased significantly in Taiyuan from increased September. from DuringMarch, thereached year, a the high O 3value concentration during April of 72 and days Augu exceededst, and thedecreased standard, significantly including from56 days September. of mild pollutionDuring the and year, 16 thedays O 3of concentration moderate pollution, of 72 days which exceeded were themainly standard, from includingApril to August.56 days Theof mild O3 pollution and 16 days of moderate pollution, which were mainly from April to August. The O3

Atmosphere 2020, 11, 555 12 of 14

In summary, the excessive hourly O3 concentration in Taiyuan City from April to August 2018 may be caused by its local generation and the transport of polluted air masses from the southwest (Qingxu County and some counties of Lyuliang City) and south (some counties of Jinzhong City) of Taiyuan.

3.3. Countermeasures and Suggestions for Control of O3 Pollution in Taiyuan

The O3 pollution in the urban area of Taiyuan is caused by its local generation and the transport of polluted air masses from the southwest and south. Therefore, a reduction of NOx and VOCs emissions in the city can reduce O3 generation and increase the compliance rate of air quality. Meanwhile, it is necessary to strengthen the joint prevention and control with Lyuliang City and Jinzhong City so as to reduce the transmission of polluted air masses from external areas.

4. Conclusions

3 The maximum hourly concentration of O3 was 316 µg/m , with 253 h exceeding the standard value in total. The maximum and 90th percentile of the maximum 8-h running average was 257 µg/m3 3 and 192 µg/m , respectively, in Taiyuan in 2018. The O3 concentration in Taiyuan increased from March, reached a high value during April and August, and decreased signiﬁcantly from September. During the year, the O3 concentration of 72 days exceeded the standard, including 56 days of mild pollution and 16 days of moderate pollution, which were mainly from April to August. The O3 concentration in Taiyuan displayed a typical “single peak” diurnal variation, which was high during the day, with peak around 13:00–15:00, and low at night. The peak O3 concentration was far lower on cloudy days than on sunny days. When moderate O3 pollution occurred in Taiyuan, Jinyuan contributed the most to the O3 pollution, followed by Taoyuan. When there was slight O3 pollution, Taoyuan contributed the most, followed by Jinyuan and Xiaodian. High temperature, high pressure, south and southwest winds, can lead to an increase in O3 concentration. The O3 pollution in Taiyuan may be caused by its local generation, and the transport of polluted air mass containing NOx and VOCs emitted by factories, such as coking and steel plants in Jinzhong City to the south and Qingxu County and Lyuliang City to the southwest. In addition, mountain winds and low nitric oxide concentration are the main reasons for the increase of O3 concentration at Shanglan at night.

Author Contributions: Conceptualization, B.L., Z.X. and X.C.; methodology, Z.X. and X.C.; software, P.W.; validation, Z.X. and X.C.; formal analysis, Z.Z. and P.X.; investigation, X.F. and Q.Y.; resources, Z.Z.; data curation, L.C.; writing—original draft preparation, B.L. and Z.Z.; writing—review and editing, B.L. and Z.Z.; visualization, Y.R.; supervision, Z.Z.; project administration, J.M.; funding acquisition, Z.X. and X.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was founded by National Research Program for Key Issues on air pollution control, China (No. DQGG05110). Acknowledgments: We would like to thank the Taiyuan ecological environment bureau of Shanxi Province and the Taiyuan Ecological Environment Monitoring Center of Shanxi Province for their support and assistance in this study. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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