Ozone affected by a succession of four landfall typhoons in 1 the Yangtze River Delta, China: major processes and health 2 impacts 3

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Research on urban O 3 pollution can be dated back to the early 1950s, beginning with the Los Angeles smog.In China, the photochemical smog, which is characterized by high level of O 3 , was first discovered in Xigu district of Lanzhou in 1970s (Tang et al., 1989).However, with the key atmospheric environmental problem was coal-smoke pollution (such as acid rain) at that time (Wang et al., 2019), little systematic research and coordinated O 3 monitoring were performed in China until the mid-2000s (Wang et al., 2017).
Since the beginning of the 21st century, the complex air pollution, which is dominated by fine particulate matter (PM 2.5 , particles of 2.5 microns or less in aerodynamic diameter) and surface O 3 , has been ingrained in the megacities of China (Chan and Yao, 2008;Jin et al., 2016;Kan et al., 2012).Air pollution has evolved into a political and economic concern in China.Due to the strict air pollution control since 2013, particle pollution has been greatly reduced, appearing a significantly decrease in sulfur dioxide (SO 2 ), NO x and PM 2.5 .However, the concentrations of O 3 and VOCs have increased from 2013 to 2017 (Li et al., 2017), suggesting that more attention should be paid to controlling O 3 and VOCs in the future.Overall, the causes of air pollution in China are remaining challenges to confront, especially in understanding the sources, transport and dispersion processes, and chemical formation mechanisms of O 3 and its precursors (Ding et al., 2016;Guo et al., 2014;Huang et al., 2014).
A typhoon (tropical cyclone, TC) is one of the most severe natural disasters in East Asia.Out of the total provinces in China, 10 coastal and 6 island provinces are affected by typhoon induced disasters, with more than 250 million lives are affected (Liu et al., 2009).The average number of typhoons making landfall in China is 9 each year, and those typhoons usually inflict vast losses in human life and property due to the accompanied strong wind, torrential rains and huge storm surges (Zhang et al., 2009;Zhao et al., 2012).Because of the long lifetime and tremendous energy, typhoons can significantly impact local atmospheric conditions, and thereby can affect surface O 3 concentration through advection, diffusion, deposition and other processes.The impact of typhoons on O 3 has attracted extensive attention in recent years (Deng et al., 2019;Huang et al., 2005;Jiang et al., 2015;Shu et al., 2016;Wang and Kwok, 2002;Wei et al., 2016;Yang et al., 2012).For example, Deng et al. (2019) reported that high O 3 and high aerosol concentrations (double high episodes) are likely to occur when the PRD is under the control of the typhoon periphery and the subtropical high with strong downdrafts.Previous studies were mainly in the southern China (including Hong Kong and Taiwan), where typhoons occur frequently.Still, research on the impact of landfall typhoons on O 3 is rather limited.
The Yangtze River Delta (YRD) region, being one of the most developed and densely populated regions in China, is located on the western coast of the Pacific Ocean.With 3.7% of the area and 16.0% of the population of China, the YRD contributed over 20% of the national total Gross Domestic Product (GDP) in 2019.Due to the rapid economic development and high energy consumption, this region has been suffering from intense air pollution (Ding et al., 2013;Li et al., 2019;Wang et al., 2015;Xie et al., 2016).In 2017, the 90th percentile of the maximum daily 8-hour average (MDA8) O 3 concentration was 170 µg m -3 , and 16 of the 26 cities (Figure 1b) in the YRD failed to meet national standard (http://www.cnemc.cn/jcbg/zghjzkgb/201905/t20190529_704755.html).Therefore, it is urgent to investigate the spatiotemporal characteristic of O 3 as well as its formation mechanisms in the YRD.Influenced by the monsoon weather, the warm and stagnation conditions play an important role in the occurrence of high-level O 3 in summer (Li et al., 2018;Liao et al., 2015;Lu et al., 2018;Zhao et al., 2010).Synoptic weather systems, such as typhoons and cold fronts, can significantly impact O 3 in the YRD (Hu et al., 2013;Shu et al., 2016).This work aims to reveal the main processes of landfall typhoon affecting surface O 3 in the YRD, to fill the knowledge gap and thus provide scientific insight for effective pollution control measures.
In this study, we report a typical case observed in the YRD during the period from 16 July to 25 August, 2018, during which multiday episode of high O 3 occurred and was found to be related to four successive landfall typhoons.Base on the monitoring data and numerical simulation, we explore the impact of landfall typhoons on O 3 in the YRD, including the major processes and health impacts.The following part of this paper is structured as the follows: Section 2 gives a brief description of monitoring data, the analysis methods, and model configurations.The results as well as the discussions are detailed in section 3. Section 4 summarizes the main conclusions.

Air quality data
Surface air pollutants monitored by the China National Environmental Monitoring Center (CNMC) Network are used in this study.The nationwide observation network began operating in 74 major cities in 2013, and it included 1597 nonrural sites covering 454 cities by 2017 (Lu et al., 2018).The monitoring data are strictly in accordance with the national monitoring regulations (http://www.cnemc.cn/jcgf/dqhj/),and can be acquired from the national urban air quality real-time publishing platform (http://106.37.208.233:20035/).Each monitoring site automatically measures hourly air pollutants (PM 2.5 , PM 10 , SO 2 , NO 2 , O 3 and CO), and the urban hourly pollutants are calculated by averaging the pollutants measured at all monitoring sites in that city.The MDA8 O 3 is calculated based on the hourly O 3 with more than 18-h measurements (Liao et al., 2017).Manual inspection, including the identification and handling of invalid and lacking data, is performed following previous studies (Xie et al., 2016;Shu et al., 2017;Zhan et al., 2019).

Surface and sounding meteorological data
With respect to surface observed meteorological data, stations at the three provincial capital cities (Hefei, Nanjing and Hangzhou) and the megacity Shanghai are selected, which are ZSOF (117.23°E,31.87°N),ZSNJ (118.80°E,32.00°N), ZSHC (120.17°E,30.23°N), and ZSPD (121.77°E,31.12°N),respectively.These surface observations, including 2-m temperature, 10-m wind speed and direction and 2-m relative humidity, are recorded hourly and can be obtained from the website of the University of Wyoming (http://weather.uwyo.edu/surface/).The precipitation data is not included in the dataset.
To verify the upper-air fields, the sounding observations at Shanghai (121.46°E,31.40°N) and Nanjing (118.80°E,32.00°N) are used.These sounding observations (pressure, temperature, relative humidity, wind direction and wind speed etc.) are also acquired from the website of the University of Wyoming (http://weather.uwyo.edu/upperair/sounding.html), with a time resolution of 12 h (00:00 and 12:00 UTC).

The best-track TC dataset
To capture the characteristics of landfall typhoons, the best-track TC dataset issued by the China Meteorological Center (CMA) is considered due to its good performance on the landfall typhoons in the mainland China (available at http://tcdata.typhoon.org.cn/zjljsjj_sm.html).The dataset covers seasons from 1949 to the present, the region north of the equator and west of 180°E, and is updated annually (Li and Hong, 2016;Ying et al., 2014).A wealth of information on typhoon is recorded every 6h in the dataset, including location, minimum sea level pressure, etc.For landfall typhoons, 24h before their landing and during their activities in the mainland China, the meteorological data are recorded every 3h.Refer to the national standard for grade of tropical cyclones (GB/T 19201-2006), the intensity category (IC) of tropical cyclones is provided in the dataset, which is based on the near surface maximum 2-min mean wind speed near the tropical cyclone center, ranging from 1 to 6 (Table 1).1a).There are 24 vertical sigma layers from surface to 100 hPa, with about 8 layers located below 1.5 km to resolve the boundary layer processes.Furthermore, the major physical options for the dynamic parameterization in WRF are summarized in Table 2.The anthropogenic emissions are from the Multi-resolution Emission Inventory for China (MEIC) in 2016 with the resolution of 0.25° (http://meicmodel.org/),including anthropogenic emissions from power generation, industry, agriculture, residential and transportation sectors.All emission estimates are spatially allocated to the relevant grid cells based on the meteorological fields obtained from WRF, and are temporally distributed on an hourly basis.The simulation starts from 00:00 UTC on 13 July to 00:00 UTC 27 August, with the first 72 h as spin-up time.

Integrated process rate (IPR) analysis
To quantify the contributions of individual processes to O 3 formation, the IPR analysis provided in the CMAQ is utilized.The IPR analysis can illustrate the contributions to changes in pollutant concentrations from seven different types of processes, including horizontal advection (HADV), vertical advection (ZADV), horizontal diffusion (HDIF), vertical diffusion (VDIF), dry deposition (DDEP), cloud processes with the aqueous chemistry (CLDS) and chemical reaction process (CHEM), with a mass conservation adjustment at each model grid cell.The IPR analysis has been widely applied to investigate regional air pollution (Fan et al., 2015;Li et al., 2012;Wang et al., 2010).In this study, MADV is defined as the sum of HADV and ZADV, and TDIF is defined as the sum of HDIF and VDIF.

Model evaluation
To evaluate the model performance, the simulation results in the innermost domain, including O 3 concentration, air temperature at 2 m (T 2 ), relative humidity (RH), wind speed at 10 m (WS 10 ) and wind direction at 10 m (WD 10 ), are examined against the hourly observations at the representative cities (Table 3).The statistical metrics, including correlation coefficient (R), rootmean-square error (RMSE) and normalized mean bias (NMB), are used.They are defined as follows: where S i and O i are the simulations and observations, respectively.N is the total number of valid data.SandO are the average value of simulations and observations, respectively.In general, the model results are acceptable if the values of R, RMSE and NMB are close to 1, 0 and 0, respectively (Li et al., 2017;Shu et al., 2016;Xie et al., 2016).

Estimate of health impacts
Previous studies showed that surface O 3 pollution can induce a series of adverse health problems from the incidence and mortality of respiratory diseases (Ghude et al., 2016;Jerrett et al., 2009;Lelieveld et al., 2015).To arouse more attention on the issue that O 3 can be significantly affected by typhoons in the YRD, we further estimate the premature mortality attributed to O 3 during the study period.
A standard damage function (Anenverg et al., 2010;Liu et al., 2018;Voorhees et al., 2014; WS/T 666-2019, Technical specifications for health risk assessment of ambient air pollution of China) is employed to quantify premature mortality due to O 3 exposure: where ΔM is the excess mortalities attribute to O 3 exposure, y 0 is the baseline mortality rate, RR is relative risk and (RR-1)/RR is the attributable fraction, and Pop is the exposed population.RR can be calculated using the following relationship: where β is the concentration-response factor, C is the exposure concentration and C 0 represents the theoretical minimum-risk concentration.
In this study, the mortality rate for respiratory disease is obtained from China Health and Family Planning Statistical Yearbook 2018 (https://www.yearbookchina.com/navibooklist-n3018112802-1.html), which is 68.02/100000.The β is generated from Dong et al. (2016), that is 0.461%.The population data are obtained from the Bureau of Statistics of different cities in the YRD.The C 0 is 70 µg m -3 for MDA8 O 3 given by the World Health Organization (WHO).

Characteristic of O 3 episodes
In the midsummer, the warm sea surface (high temperature) is conducive to the generation of typhoons (high O 3 concentration), providing a good opportunity to investigate the mechanism of typhoons affecting O 3 in the YRD. Figure 2 shows the MDA8 O 3 in the typical 26 cities of the YRD in summer of 2018.Actually, it is common for typhoons to affect O 3 in the YRD during summer, and 2018 is special because there were 8 landfall typhoons and many of them landed further north than in the normal years (see Supplement for details).O 3 concentration was relatively high in June, and relatively low in July and August.The relatively low O 3 may be attributed to the maritime air masses transported by the Asian summer monsoon (Ding et al., 2008;Xu et al., 2008).Nevertheless, we notice that there are two regional multiday O 3 pollution episodes from 24 July to 11 August in the YRD, which means that about half of the cities in the YRD exceed the national air quality

Landfall typhoons and their effects
O 3 episodes with regional and long-lasting characteristics may often be associated with slowmoving synoptic weather systems.We find that the O 3 episodes coincided well with activities of  Furthermore, we find that the variations of O 3 was related to the track, duration and landing intensity of the typhoons.For example, during the B1A2 period when the O 3 pollution occurred, the moments that hourly O 3 concentrations first exceed 200 µg m -3 in about half of cities of the categories Ⅰ, Ⅱ, Ⅲ and Ⅳ were 6:00 UTC (14:00 LST) 27 July, 6:00 UTC (14:00 LST) 28 July, 3:00 UTC (11:00 LST) 29 July and 6:00 UTC (14:00 LST) 31 July, respectively.This phenomenon also suggests that O 3 pollution first occurs in cities along the coastline, which may be related to the track of typhoons (Figure 4).Regarding the impact of typhoon duration, the A4B3 period provided a good interpretation.While Typhoon Yagi was still active in the mainland China, Typhoon Rumbia had reached the 24-hour warning line.Hence, the O 3 remained a low level throughout the period (A3B4), which was quite different from B1A2 and B2A3 period.Noted that the landing point and active path of Typhoon Ampil and Typhoon Jongdari were very similar (Figure 4).However, the landing intensity of Typhoon Ampil was severe tropical storm (IC = 3), and that of Typhoon Jongdari was tropical storm (IC = 2), resulting in a difference in O 3 concentrations for Shanghai.Within 24 hours after Typhoon Ampil (Jongdari) reached the 24-hour warning line, the average O 3 concentrations reached 40.9 (80.1) µg m -3 in Shanghai.This is because that the stronger the typhoon landed, the gale (The 10-m wind speed near moment A1 was larger than that near moment A2 in Shanghai, Figure 7a) and precipitation accompanying the typhoon will be more effective in removing O 3 .

Processes of O 3 pollution affecting by typhoons
To reveal the major processes of O 3 pollution episodes affected by landfall typhoons, one municipality and three provincial capital cities with different longitudes, including Shanghai (121.77°E,31.12°N),Hangzhou (120.17°E,30.23°N),Nanjing (118.80°E,32.00°N) and Hefei (117.23°E,31.87°N), are selected for further analysis -based on monitoring data and model results.

Evaluation of model performance
To evaluate the simulation performance, the hourly simulation results are compared with the measurements from 00:00 16 July to 00:00 25 August., 2012;Li et al., 2017).With regards to WD 10 , the simulation error is large based only on these statistical metrics.This is because that near-surface wind fields are deeply influenced by local underlying surface characteristics, and improving the urban canopy parameters might be useful (Liao et al., 2015;Xie et al., 2016).In term of O 3 , the simulated O 3 concentrations behave satisfactorily.R is 0.55, 0.65, 0.66 and 0.54 for the simulations for Shanghai, Hangzhou, Nanjing and Hefei, respectively, while the NMB values are 5.8%, 16.4%, -6.2% and -5.3%, respectively.

Shanghai in category Ⅰ cities
In the study period, Shanghai was usually one of the first cities affected by landfall typhoons.
We can see a multiday episode of O 3 during the period of 24-28 July, with a maximum of hourly O 3 up to 292 µg m -3 at 27 July (Figure 6a).The high O 3 concentrations together with high primary pollutants (CO and NO 2 ) suggest a strong photochemical O 3 production under the condition of high temperature (The daily maximum temperature can reach 35 ℃) during this period, and the weak wind may play a significant role in the accumulation of surface O 3 .The increase of the primary pollutants may be related to a change in wind direction from southeast to southwest causing by Typhoon Ampil (A1 in Figure 6a, -A1 and A1B1 in Figure 7), which originally brought airmass from the ocean, shifted to from inland.Interestingly, PM 2.5 also showed good correlation with O 3 and primary pollutants, especially for NO 2 during this period.This indicates that a high level of oxidizability can promote the formation of secondary particles (Kamens et al., 1999;Khoder, 2002).
From the results of process analysis (Figure 6b), the major contributions to surface O 3 were TDIF, CHEM and DDEP due to the small net contribution of MADV.TDIF had a considerable positive contribution while DDEP did the opposite, suggesting that high surface O 3 may be sourced from the upper layer via TDIF process, and be removed via DDEP process.However, for the whole boundary layer, which is defined as the layer less than 1500 m in this study, the balance was between CHEM and DDEP instead TDIF and DDEP.Thus, TDIF was likely to play the role of "transport" from the upper layer to surface.Figure 6c  As shown in Figure 7, O 3 pollution tends to occur during the period from the end of a typhoon to the arrival of the next typhoon (B1A2 and B2A3) in the YRD.To reveal this phenomenon, we compare these two periods (B1A2 and B2A3) with their previous periods (A1B1 and A2B2) using the skew-T log-P diagram (Figure 6d and 6e).It is found that the atmospheric conditions of B1A2 (B2A3) were hotter and drier than A1B1 (A2B2) below 700 hPa in Shanghai, and wind speed is smaller in B1A2 (B2A3).Those changes in atmospheric conditions after typhoon will be conducive to the generation of high O 3 concentration in Shanghai.Details can be found in Figure 4.

Hangzhou in category Ⅱ cities
Figure 8 presents the case in Hangzhou.It shows that high O 3 concentrations occurred on 27-31 July and 5-7 August, which may also be related to the strong photochemical production of O 3 under the abundance of precursors (Figure 8a) and poor diffusion conditions due to the light wind (B1A2 and B2A3 in Figure 7).Figure 8a further shows that high O 3 was often associated with an increase in CO but the NO 2 concentrations usually remained at the same level.This phenomenon indicates a VOCs-limited regime in this city since CO usually have good correlation with VOCs and can play a similar role as VOCs in the photochemical production of O 3 (Atkinson, 2000;Ding et al., 2013).In fact, O 3 in other representative cities (Shanghai, Nanjing and Hefei) also showed a better correlation with CO than NO 2 .Though Hangzhou is close to Shanghai, there is a significant difference of wind fields over these two cities.Starting from the arrival of Typhoon Ampil (A1).
The wind direction in Hangzhou did not change back to southeast until a few days later after Typhoon Jongdari dissipated (B2).During this period (A1B2), the frequent southwest wind may be the reason for high CO concentrations in Hangzhou.In addition, the chaotic wind field during period B1A2 (B1A2 in Figure 7) may lead to the light wind in Hangzhou.With respect to the simulation results, the model simulated the variation of O 3 but failed to capture the O 3 peaks (e.g., the peak values on 27-31 July), which may be related to the strong upward airflows (Figure 8c) that inhibited the accumulation of O 3 (Figure 8b).This further illustrates that downward airflows may be an important factor for O 3 episodes in this case.

Nanjing in category Ⅲ cities
In Nanjing, the O 3 episode exceeded the national air quality standards was observed on 28 July to 1 August and 7-11 August.These O 3 episodes were characterized by abundant O 3 precursors under the condition of high temperature.Furthermore, light wind (B1A2 and B2A3 in Figure 7) and downward airflows (Figure 9c) also contributed greatly to the occurrence of O 3 pollution, resulting from a mechanism similar to that for Shanghai and Hangzhou.As early as on 22 July, the wind direction in Nanjing changed from southeast to southwest because of the arrival of Typhoon Ampil, and thus the concentrations of the main primary pollutants (CO, NO 2 and SO 2 ) increased (Figure 9a).However, high-level O 3 episodes did not occur until 28 July even though the maximum temperature did not change significantly during 24-31 July.The "obstacle" for enhancing O 3 levels may be the precipitation caused by the strong upward airflows during 23-26 July (Figure 9c).As shown in Figure 9b, high surface O 3 concentration during the pollution episodes is the result of TDIF and CHEM processes, and is lost through DDEP and MADV processes.Regarding vertical structure of atmospheric, B1A2 (B2A3) was also hotter and drier than A1B1 (A2B2) below 700 hPa in Nanjing (Figure 9d and 9e).These consequences, similar to those in Shanghai, further confirm that high O 3 concentrations in a region are more likely to occur during the period from the end of an exciting typhoon to the arrival of the next typhoon (B1A2 and B2A3) than during the period when a typhoon approaches and is active in the region (A1B1 and A2B2).

Hefei in category Ⅳ cities
Hefei is the city farthest from the coast among the four representative cities, and O 3 pollution occurred on 31 July and 8-11 August.We also find the phenomenon that the precursors concentrations had an increase once the wind direction changed from southeast to southwest (Figure 10a).During B1A2 and B2A3, the concentrations of the main precursors of O 3 was high.However, high O 3 concentration was mainly found in B2A3, and not in B1A2.This may be related to the relatively low temperature during B1A2 (Figure 10a), which is not conducive to photochemical production of O 3 (Figure 10b).As shown in Figure 10c, there were distinct upward airflows within the boundary layer, which may be related to urban effect (e.g., urban heat islands).These upward airflows within the boundary layer help mix the air, resulting in a uniform distribution of O 3 in the vertical direction.However, the downward airflows can still inhibit the vertical diffusion of O 3 , and O 3 tends to be trapped within the boundary layer.Typhoon can exert an enormous impact on energy transports and air mass in the troposphere as well as redistribution of pollutants.Though most typhoons generated over the western North Pacific will not land in China, or they are more likely to land in the South China rather than the YRD.In our previous study (Shu et al., 2016), the typhoon did not land in the YRD, but the processes related to high-level O 3 formation may be the same.That is, the processes shown in the open box enclosed by dashed lines in Figure 11, which are unique to landfall typhoons, while the processes inside the box enclosed by solid lines can be found between typhoons.Transport of precursors, downward airflows, high temperature and light wind are crucial factors, and how big roles of those factors play in O 3 episodes depends on behaviors of the typhoons and geographical locations of the cities.Quantify these processes with just a few cases is a large challenge.For example, it is hard to find out whether the downward airflows are modulated by the subtropical high or the periphery circulation of typhoons since they usually occur simultaneously.Furthermore, the behave of particulate matter is intriguing since high-level PM 2.5 often occurs with high-level O 3 after typhoon, which is opposite to the suggestion that high particulate matter concentrations inhibit the formation of O 3 in previous studies (Li et al., 2005;Xing et al., 2017).This may be related to the heterogeneous reactions (Lou et al., 2014) but research on this issue is quite limited to date.

Premature mortalities induced by O 3 exposure
When it comes to typhoons, especially landfall typhoons, the first concern is the huge damage caused by extreme weathers.After the passing of typhoons, people are relieved and go back with their life as usual.However, our research indicates that high O 3 episodes are likely to occur in the short period after a typhoon landing in the YRD, and high O 3 concentrations can do harm to people's health.To arouse attention on this issue, we estimate the premature mortality attributed to O 3 for respiratory disease, we choose two complete cycles, which is the period A1A3 (21 July to 11 August), to do the calculation.In this study, we employ the standard damage function defined by epidemiology studies (Anenverg et al., 2010;Voorhees et al., 2014) to calculate the premature mortalities due to O 3 exposure, the specific formulas and parameters are described in Section 2.7.
Table 4 summarized the premature mortalities in cities in the YRD.The premature mortalities are a function of both the population and O 3 levels, resulting in high premature mortalities in populated and heavily polluted areas.Out of the 26 cities in the YRD, Shanghai showed highest premature mortalities (29.2) due to its high surface O 3 concentrations and huge population.The city with the lowest premature mortalities (0.6) was Zhoushan, which may be related to removing effect of the maritime air masses as Zhoushan is located by the sea (Figure 1b).During this period, the total premature mortalities in the YRD was 194.0, which was larger than the number of casualties caused directly by the typhoons (80 people were killed by landfall typhoons in mainland China in 2018).

Figure 2 .
Figure 2. The MDA8 O 3 in 26 cities of the YRD in June (left panel), July (middle panel), and August (right panel) 2018.The national ambient air quality standard for MDA8 O 3 is 160 µg m -3 in China.These cities are sorted by longitude.

Figure 3
Figure3further shows diurnal variation of O 3 in all 26 cities of the YRD from 00:00 16 July to 00:00 25 August (throughout this paper the time refers to UTC, unless LST is specifically stated).
landfall typhoons, showing in their tracks and intensities in Figure 4. Typhoon Ampil was first observed at 00:00 on 18 July, and landed in Shanghai around 4:30 on 22 July with an intensity of severe tropical storm (IC=3).While Typhoon Ampil remained active, Typhoon Jongdari generated over the western North Pacific at 12:00 on 23 July, and made landfall at the junction of Zhejiang province and Shanghai at 21:00 1 August.After Typhoon Jongdari, Typhoon Yagi generated at 00:00 7 August.At around 15:35 12 August, it landed in Zhejiang province and remained active in the mainland China until 21:00 15 August.Before the end of Typhoon Yagi, Typhoon Rumbia was observed over the western North Pacific at 6:00 14 August.It finally landed in Shanghai at around 20:00 16 August, causing huge economic losses.

Figure 4 .
Figure 4.The track and intensity of Typhoon Ampil, Typhoon Jongdari, Typhoon Yagi, and Typhoon Rumbia.The track is labeled with the date of the month and day (in month.day).The circle, triangle, square and pentagram indicate the intensity category of tropical cyclones is less than 1 (IC < 1), equal to 1 (IC = 1), equal to 2 (IC = 2), and not less than 3 (IC >= 3), respectively.Black solid line and dotted line represent the 24-hour and 48-h warning line for tropical cyclones, respectively.The colored solid points are the locations of cities in the YRD,

Figure 5
Figure 5 further shows hourly variations of O 3 , T 2 , WS 10 and WD 10 for measurements and simulations in four representative cities.The simulations effectively reproduce the diurnal variation of O 3 , T 2 and WS 10 , confirming the reliability of the simulation results.Moreover, the model well captures the shift in wind direction during the study period.Thus, the overall model performance in simulating wind fields is acceptable.In summary, the simulations can capture and reproduce the major meteorological characteristics and O 3 evolution during the O 3 episodes, and thus can provide valuable insights into the formation of the O 3 episodes.
further shows the temporal-vertical distribution of O 3 with vertical wind velocity.The downward airflows prevailed over Shanghai until 23 July, which are induced by the subtropical high.Then, strong upward airflows appeared as Typhoon Ampil arrived, and high level of O 3 disappeared.Around 27 July, the downward airflows gradually resumed and high level of O 3 occurred.The downward airflows are critical because they can not only inhibit the vertical transport of O 3 but also transport high-level O 3 to the surface.The high-level O 3 in the troposphere mainly comes from two sources.One is that O 3 -rich air from the low stratosphere transported by the downdrafts in large-scale typhoon circulation (Jiang et al., 2015).The other is that O 3 produced by photochemical reactions during the day.It is noteworthy that high photochemical production efficiency of O 3 occurred in the middle boundary layer instead of at the surface.Moreover, most of the O 3 remained in the residual layer at night, while surface O 3 concentration was much lower due to NO x titration.By the second day, high O 3 in the residual layer was transported to the surface by the downward airflows as air in the boundary layer is gradually mixed.Combined with the newly generated O 3 , a high concentration of O 3 would eventually appear on the surface.

FigureFigure 7 .
Figure 6.(a) Time series of air pollutants (O 3 , PM 2.5 , SO 2 , CO and NO 2 ) and meteorological factors (T 2 , RH, WS 10 and WD 10 ) in Shanghai.(b) Individual processes contribution to net O 3 density at Shanghai.O 3 is the net increase, MADV is the sum of horizontal advection (HADV) and vertical advection (ZADV), TDIF is the sum of horizontal diffusion (HDIF) and vertical diffusion (VDIF), CHEM is the chemical reaction process, and DDEP is the dry deposition process.The color histograms indicate the results for the layer near the surface, while the solid red lines indicate the average results for all layers below 1500 m.(c) Temporal-vertical distribution of O 3 with vertical wind velocity over Shanghai.The dotted purple line and solid blue line indicate the negative wind speeds (downward airflows) and positive wind speeds

Figure 11 .
Figure 11.A schematic diagram of major processes that summertime O 3 is affected by landfall typhoons in the YRD.The letter A indicates the moment that the typhoon has reached the 24h warning line, and letter B indicates the last moment when typhoon remains active in the mainland China.
photochemical reactions.O 3 is mainly generated in the middle of boundary layer (~ 1000 m) instead of at the surface.The high-level O 3 can remain in the residual layer at night, and can be transported to the surface by downward airflows or turbulent mixing by the second day.The downward airflows also obstruct the vertical diffusion of O 3 .Meanwhile, wind speed is light when the wind readjusts to southeast, which further reduces horizontal diffusion of O 3 .Thus, O 3 can be accumulated and trapped on the ground.The thermal-dynamic effects results in high surface O 3 concentration in the YRD.Those processes will repeat if the next typhoon approaches.The estimated premature mortalities attributed to O 3 exposure for respiratory disease in the YRD during 21 July to 11 August (two complete cycles of typhoons) was 194.0, which is larger