Enhanced Late Spring Ozone in Southern China by Early Onset of the South China Sea Summer Monsoon

The onset of the South China Sea summer monsoon (SCSSM) has profound impacts on meteorological conditions over East Asia. However, whether the interannual variability in monsoon onset date impacts ozone (O3) pollution remains unclear. Here, we investigate the relationship between early onset of SCSSM and late spring O3 in southern China. Our results show notable differences in surface O3 concentrations before and after SCSSM onset during early onset events in southern China. The enhanced O3 of 11.1 μg m−3 is supported by increased air temperature and solar radiation of 1.1 K and 30.9 W m−2 and reduced relative humidity of −5.7%. Both observation and model simulations confirm that O3‐favorable meteorological conditions modulated by early SCSSM onset can be found in May. It increases the boundary layer height and biogenic emissions of volatile organic compounds, enhancing O3 by 10 μg m−3 over southern China. Chemical processes dominate such increases in O3 with enhanced chemical production of 0.27 Tg month−1. Descending motion in southern China vertically transports O3 to surface by 0.10 Tg month−1, whereas horizontal advection reduces O3 concentration by 0.12 Tg month−1. The meteorological responses to colder sea surface temperature (SST) in the central equatorial Pacific are pronounced, leading to higher O3 concentrations over the Yangtze River Delta, while warmer SST in the Philippine Sea contributes O3 over the Pearl River Delta and eastern China. This study highlights the importance of SCSSM onset with respect to O3 in southern China, with promising applications in management of air pollution and agriculture.

rice across China in 2015 (Feng et al., 2019).Understanding the driving factors for O 3 pollution in China is thus crucial to policymakers.
Surface O 3 is primarily formed through photochemical reactions between volatile organic compounds (VOC) and nitrogen oxides (NO x ) in the presence of sunlight.In addition to emissions of precursors, regional weather conditions and large-scale circulation patterns also modulate O 3 concentrations (Liu et al., 2020;Lu et al., 2020;Wang et al., 2017;Yang et al., 2014Yang et al., , 2022;;Yin & Ma, 2020).Generally, higher temperature and enhanced ultraviolet radiation accelerate photochemical reactions and promote emissions of biogenic precursors (Wang et al., 2017;Xiao et al., 2022), which were identified as dominant meteorological drivers for high summer O 3 in China (Wang et al., 2019;Yang et al., 2022;Zhang et al., 2022).The planetary boundary layer height (PBLH) also plays a significant role in vertical mixing (Dong et al., 2020), transport (Gao et al., 2019), and chemical reactions efficiency of O 3 (Zhang et al., 2023), showing a positive correlation with surface O 3 when PBLH is below 1 km (Han et al., 2020).Several studies recognized the significance of large-scale atmospheric circulation in the formation of O 3 pollution with a focus on summer.Higher summer O 3 anomalies (3-6 ppb) were found in central-eastern China due to enhanced transboundary transport during strong East Asian summer monsoon (EASM) years (Yang et al., 2014).Zhao and Wang (2017) argued that there was a negative correlation between O 3 over southern China and the intensity of the western Pacific subtropical high (WPSH), as a strong WPSH with large southwesterly anomalies would lead to more humid, cloudy and cooler conditions.Elevated summer O 3 levels in China were also linked to warm phases of El Niño-Southern Oscillation (ENSO) (Yang et al., 2022), positive phases of Eurasian teleconnection pattern, preceding May Arctic sea ice to the north of Eurasia (Yin et al., 2019), and variability in the joint movements of WPSH and East Asian deep trough (Yin & Ma, 2020).O 3 concentrations in late spring also exhibit relatively high levels in China (Lu et al., 2018;Zhao & Wang, 2017), and fast increasing trends were also identified (Li et al., 2021).However, climate factors that regulate late spring O 3 have not been well understood.
Weather and climate in southern China are largely affected by EASM, the commencement of which is characterized by abrupt shifts of low level zonal wind, precipitation, outgoing longwave radiation, and amount of clouds over the South China Sea (Ding & Chan, 2005;Wang et al., 2004).These changes during late spring indicate the onset of South China Sea summer monsoon (SCSSM hereafter for short) (Ding, 2007;Wang et al., 2009), which exhibits considerable inter-annual variability (Wang et al., 2004;Zhou & Chan, 2005).Previous studies have confirmed that the SCSSM onset plays a critical role in modulating meteorological conditions over East Asia.An early onset of SCSSM, often associated with La Niña, would lead to a northeastward shift of the WPSH (Huang et al., 2005;Zhang, Zhou, & Leung, 2019;Zhou & Chan, 2007), a northward shift of the East Asian trough (Wang & Chen, 2018), and stronger convective activities around the Philippines (Kajikawa & Wang, 2012;Xiang & Wang, 2013).Such climate anomalies are generally accompanied by less East Asian monsoon rainfall over the lower reaches of the Yangtze River valley (He & Zhu, 2015) and increased frequency of tropical cyclones over the western Pacific (Kajikawa & Wang, 2012;Wang & Chen, 2018).These climate anomalies modulated by the onset of SCSSM might also affect O 3 concentrations in southern China, where O 3 is the most concerned pollutant with high values in spring (Gao, Gao, et al., 2020).
The responses of O 3 to monsoon have been well documented, yet whether the interannual variability in monsoon onset date impacts large-scale patterns of O 3 pollution remains unclear.Here we aim to understand how SCSSM onset affects late spring O 3 concentrations in southern China, and the associated implications for management of O 3 pollution.Using reconstructed O 3 data set, meteorological reanalysis and numerical model simulations, we illustrate how meteorological conditions and variations of surface O 3 in southern China are modulated by the onset of SCSSM.Physical and chemical mechanisms for the modulation are also discussed.

Reconstructed Daily Ground-Level O 3 in China
Daily surface maximum 8 hr average (MDA8) O 3 concentration in China from 2005 to 2021 was taken from Zhou et al. (2023) with a spatial resolution of 0.1° × 0.1°.It was reconstructed using the extreme gradient boosting algorithm (XGBoost) with meteorological variables, anthropogenic emissions, land covers, etc, as inputs (Figure S1 in Supporting Information S1).Both cross-validation and independent validation were conducted to confirm the accuracy of this data set.

10.1029/2023JD039029
3 of 14 Considering the significant change in O 3 concentrations over study period due to emission reduction measures in China, we used the empirical mode decomposition (EMD) method to remove the influences of changing emissions in each grid (Gao et al., 2023).EMD can decompose O 3 variation into several physically meaningful components named Intrinsic Mode Functions (IMF) at different resolutions (Huang et al., 1998).We considered the last IMF with the lowest frequency as the signal associated with anthropogenic emission.We then removed this component to partly eliminate the effects of changing emissions.For example, the IMF 7 from EMD decomposition of O 3 concentrations over the North China Plain shows increasing trend during 2013-2017 and a reversal after it (Figure S1 in Supporting Information S1), which agrees with the trend of anthropogenic driver concluded by Li et al. (2020).It is important to note that this detrending approach may not completely eliminate the impact of human emissions, but it helps to focus our investigation on the influence of the SCSSM onset on O 3 concentrations.

Meteorological Reanalysis Data and Definition of SCSSM Onset
Daily meteorological variables with a horizontal resolution of 2.5° × 2.5° over 1979-2021 were obtained from the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) reanalysis data set (Kanamitsu et al., 2002).We included sea level pressure, zonal and meridional winds, near-surface humidity, temperature, and downward solar radiation at the surface in this study, which have been shown in previous studies to play the most important roles in photochemical pollution in China (Zhang et al., 2022).Monthly sea surface temperature (SST) with a spatial resolution of 0.25° × 0.25° was taken from the European Center for Medium-Range Weather Forecasts (ECMWF) ERA-5 reanalysis data set (Hersbach et al., 2020).
The onset of SCSSM was defined as the day after April 1 that satisfies steady easterly to westerly shift of zonal winds averaged over the South China Sea (5°-15°N, 110°-120°E) at 850 hPa (Wang et al., 2004), using the NCEP-DOE reanalysis data set.Such a shift means that the 850 hPa zonal winds in this region become positive and maintain at least two pentads (10 days).The SCSSM onset dates, shown in Table 1, exhibit a distinguished interannual variation.Years with anomalous SCSSM onset date earlier (later) than one standard deviation were selected as early (late) SCSSM onset events in this study (Jiang et al., 2018).

Numerical Model Experiments
The Community Earth System Model (CESM) version 2.1.3was employed to explore the responses of O 3 pollution over China to the onset of SCSSM, with a horizontal resolution of 0.94° × 1.25° and 70 vertical layers (Gent et al., 2011).The atmospheric components were provided by Community Atmosphere Model version 6 (CAM6), while chemical and land processes were simulated by the Whole Atmosphere Community Climate Model version 6 (WACCM6), and the Community Land Model version 5 (CLM5), respectively.Anthropogenic emissions were provided by the Community Emissions Data System (Hoesly et al., 2018).Biogenic emissions were calculated online by the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 2.1.This study conducted a control case with SST data from monthly varying climatology (CESM ctrl ), and a sensitivity simulation by imposing the SST anomaly patterns associated with early SCSSM onset events (CESM early ).Two additional sensitivity simulations were conducted by imposing a warmer SST of 0.6 K over the Philippine Sea (0°N-20°N, 120°E−160°E, CESM PhiSea ) and colder SST of −1.0 K over central equatorial Pacific (5°S-5°N, 160°E−150°W, CESM CenPacif ).All simulations were conducted from January-June 2010.The year 2010 was selected because it did not exhibit an early or late SCSSM onset (Table 1), and had small SST anomalies (Gao et al., 2023;Hu et al., 2022).We evaluated the CESM ctrl based on the mean fractional bias (MFB) and the mean fractional error (MFE) (Boylan & Russell, 2006) (Figure S2 in Supporting Information S1).Given these uncertainties and biases in CESM simulation, simulated results were mainly used to investigate the direction of the response rather than the magnitude of the influence.

Impacts of Early SCSSM Onset on O 3 in Southern China
The onset of SCSSM generally occurs in late spring, preceded by persistent springtime rainfall over southeastern China (Wan & Wu, 2007), and followed by northward movement of the rain belt (Ding & Chan, 2005).To estimate the variation in O 3 concentrations following the onset of SCSSM, the average O 3 concentrations for 15 days after the onset were subtracted from those for 15 days preceding the onset date, as illustrated in Figure 1.Enhanced O 3 concentration of 20 μg m −3 is commonly observed over eastern China within three pentads (15-day average) after onset dates during 2005-2021 (Figure 1a).During early onset events, O 3 concentrations in southern China experience overall enhancements after onsets, with the largest increase of more than 26 μg m −3 in the Yangtze River Delta (YRD) and Pearl River Delta (PRD) (Figure 1b).The frequency of high O 3 days (MDA8 O 3 concentrations exceeded 100 μg m −3 ) is also increased by 4 days within three pentads in southern China after early onset dates (Figure S3b in Supporting Information S1).In contrast, no significant differences in O 3 concentrations for late onset events are found (Figure 1c).
We examined the corresponding differences in meteorological parameters relevant to O 3 formation between postand pre-onset of SCSSM.Following the onset of SCSSM during climatology, the negative anomalous sea level pressure in South China Sea induces not only easterly to westerly shift over South China Sea, but also northeasterly winds over southeastern China, weakening moisture transport (Figure S4a in Supporting Information S1).Downward solar radiation and temperature increase together with reduced humidity over the North China Plain after onset (Figure S4d, S4g and S4j in Supporting Information S1), favoring O 3 production.For early onset events, the stronger cyclonic circulation located in the Philippine Sea results in more O 3 -favorable conditions than those of climatology mean (Figure S4b in Supporting Information S1).Specifically, obvious differences are observed in early onset event that the air temperature and solar radiation are increased by 1.1 K and 30.9 W m −2 , respectively, accompanied by a considerable decrease of 5.7% in relative humidity (Figure 1).Such shifts in meteorological parameters in early onset events contribute to an increase in surface O 3 concentrations of 11.1 μg m −3 over southern China, which are not consistent in late onset events.This reflects the different processes for the early and late onsets of SCSSM.Early SCSSM onsets are generally associated with northwestward-moving tropical convection, whereas late SCSSM onset is primarily affected by the northward migration of the intertropical convergence zone (Kajikawa & Wang, 2012).
As mentioned above, early SCSSM onset accompanied with O 3 -favorable meteorological conditions significantly increases surface O 3 over southern China.We further investigate the impacts of early SCSSM onset on O 3 in May when SCSSM onset dates mainly happened (Table 1).In southern China, there has been an increase of approximately 5 high O 3 days in May due to the early SCSSM onset (Figure S3d in Supporting Information S1).
Based on the detrended O 3 data by EMD method, the composite difference in May O 3 between early SCSSM onset events and climatological mean witnesses a remarkable rise of higher than 5 μg m −3 over southern China (Figure 2a).Statistically significant correlation between surface O 3 in May and the anomalous SCSSM onset dates during 2005-2021 can be found in YRD, PRD and southern China with correlation coefficients of −0.52, −0.66 and −0.67, respectively (Figure 2).However, it is worth noting that despite the occurrence of an early onset event in 2019, the O 3 concentrations were relatively low during this period.This deviation can be attributed to the fact that the early onset event in 2019 was triggered by a cold front, unlike most early onset events that are typically triggered by tropical convection (Hu et al., 2020).On 1 May 2019 (the onset date of SCSSM), a strong cold front extended from Japan to the South China Sea (Hu et al., 2020).Southern China was affected by strong northerly winds after the cold front.The intrusion of cold air from the north significantly inhibited the photochemical reactions responsible for O 3 formation, ultimately leading to the observed lower surface O 3 concentrations.Without considering the early onset event in 2019, we found that early onset events during the period from 2005 to 2021 tend to increase the O 3 concentration in May over YRD, PRD and southern China by 6.1, 8.1 and 9.3 μg m −3 (Figure 2).The high values of O 3 anomaly are overlapped with the composite differences in meteorological conditions (Figure 3).Warmer and drier conditions in May are found over southern China during early onset events with surface temperature increased by 0.8 K and relative humidity reduced by 9% compared to the climatological mean (Figures 3b and 3d).Stronger shortwave radiation fluxes of higher than 30 W m −2 appear in the YRD (Figure 3c).

Mechanism for the Impacts of Early SCSSM Onset on Late Spring O 3 in Southern China
We employed CESM experiments to investigate the influences of early SCSSM onset, which would be helpful to elucidate the underlying mechanisms responsible for the observed changes in surface O 3 associated with early SCSSM onset.SST during February-March-April (FMA) play dominant roles in the variation of the SCSSM onset (Hu et al., 2022;Kajikawa & Wang, 2012).A sensitivity simulation (CESM early ) was conducted by imposing the SST anomaly patterns associated with early SCSSM onset events (Figure 4).The differences between CESM early and CESM ctrl were regarded as the influence of early SCSSM onset. 10.1029/2023JD039029 6 of 14 The steady shift in zonal wind at 850 hPa over the South China Sea from CESM early occurs on 2 May, which is advanced by 20 days compared to CESM ctrl (Figure S5 in Supporting Information S1).The simulated responses are consistent with the results obtained from reanalysis data (Figure S6 in Supporting Information S1), affirming that the impacts of early onset events on meteorological conditions and O 3 enhancements can be successfully reproduced by our simulations.The early onset of SCSSM induces a cyclonic anomaly over the South China Sea, resulting in reduced water vapor entering southern China (Figure S6 in Supporting Information S1).It decreases cloudiness by 20% (Figure 5d), allowing an additional solar radiation of 50 W m −2 to reach the surface.Precipitation is also suppressed by −5 mm per day over the YRD (Figure 5c), accompanied by a 4 K increase in surface temperature.In addition, biogenic VOC emissions from a broad area of forest in southern China are also boosted by higher temperature (Figure 6c), exhibiting enhanced isoprene emission fluxes of 60 kg m −2 h −1 calculated by the MEGAN (Figure 5e).We use the index based on the ratio of the concentration of HCHO and NO y (Sillman, 1995), and find that VOC-limited regime dominates southern China at surface (Figure S7a in Supporting Information S1).With more VOCs available for reactions with NO x , the photochemical reactions are substantially accelerated by 1 kg s −1 (Figure 5f).Consequently, the responses of surface O 3 concentration to early onset of SCSSM are in line with the meteorological and VOC emissions anomalies, presenting the largest increases exceeding 12 μg m −3 over southern China (Figure 7a).
Higher solar radiation and temperature in southern China during early onset event promote the development of boundary layer.Accordingly, PBLH increases by 80 m in southern China but decreases by 100 m in northern China (Figure 5a).These changes in PBLH can affect vertical mixing and transport of O 3 as well as its precursors.O 3 precursors such as total reactive nitrogen oxides (NO y ) and formaldehyde (HCHO) in northern China are trapped near the surface due to lower PBLH, but are more effectively dispersed with higher PBLH in southern China (Figure 6).The O 3 formation sensitivity in the lower troposphere is NO x -limited regime over the south of 29°N, and VOC-limited regime over the north of 29°N (Figure S7b in Supporting Information S1).Positive anomalies of 0.4 ppb for NO y and 0.8 ppb for HCHO are observed below 1 km near 26°N and 30°N, respectively.These anomalies significantly strengthen net chemical production of O 3 within the PBLH (Figure 6a), contributing to O 3 anomalies of 4 μg m −3 at a height of 1 km above the surface over southern China (Figure 7b).
Table 2 summarizes the contributions from physical and chemical processes to surface O 3 in southern China.
The descending motion anomalies of 0.06 Pa s −1 over southern China vertically transport O 3 to surface by 0.10 Tg month −1 (Figure 5b).The O 3 export fluxes of 0.14 Tg month −1 to northern China indicate that the horizontal advection plays a negative role in O 3 concentration in southern China (Table 2).The increases in O 3 over southern China are primarily driven by chemical processes with the enhanced net chemical production of 0.27 Tg month −1 .

Influences of SST Anomalies Associated With Early Onset Events
Previous studies demonstrated the crucial role of SST anomalies during FMA in the equatorial Pacific and the Philippine Sea in modulating the onset date of SCSSM (Hu et   Wang , 2013).In this study, we further investigated the impacts of SST anomalies in these regions on O 3 .As shown in Figure 4, the spatial pattern of FMA-averaged SST anomalies for early onset events resembles the La Niña pattern.Most prominent SST signals are observed in the central equatorial Pacific within 5°S and 5°N, where eminently negative SST anomalies are lower than 1 K within 160°E and 150°W, labeled as the Niño 4 region (Figure 4).The simulated responses of meteorological conditions to such colder SST are similar to those during early onset event, but with greater magnitudes (Figure S6 in Supporting Information S1).The contribution from net O 3 chemical production is 0.41 Tg month −1 , exceeding that from CESMearly (Table 2).This enhancement is  conditions over southern China have been found in CESM PhiSea but much weaker than those in CESM early and CESM CenPacif (Figure S6 in Supporting Information S1), leading to net chemical production of 0.17 Tg month −1 (Table 2).The temperature and solar radiation are also enhanced in north of 30°N by 2K and 20 W m −2 (Figure S6 in Supporting Information S1), slightly promoting an increase of 60m in PBLH (Figure S9a in Supporting Information S1).Positive HCHO anomalies are accordingly found in north of 30°N (Figure 6i), which is VOC-limited regime (Figure S7 in Supporting Information S1).Thus, the surface chemical production anomalies mainly in PRD and eastern China (Figure S9f in Supporting Information S1), with increased O 3 concentrations of 12 and 8 μg m −3 , respectively (Figure 7e).
It is noteworthy that the simulated responses of O 3 in CESM CenPacif are larger than those in CESMearly, and both are consistent with the observed O 3 anomalies during early onset events (Figure 2a).However, the responses in CESM PhiSea align more closely with the changes in O 3 during all onset events (Figure 1a), displaying positive anomalies in PRD and eastern China.The SST in the Philippine Sea can modulate the convective activities to further influence SCSSM onset date, showing a warming trend of 0.02 K year −1 from 1979 to 2021 (Figure S10 in Supporting Information S1).Therefore, the most of SST anomalies were positive during the study period (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021), leading to higher O 3 in PRD and eastern China.These results suggest that SST anomalies in the central equatorial Pacific exhibit significant impacts during early onset events, whereas O 3 variations during most of the SCSSM onsets are linked to SST anomalies in the Philippine Sea in recent decades.

Conclusions
The onset of SCSSM is the most important sub-seasonal phenomenon of the EASM system.Anomalies in circulation patterns and meteorological conditions modulated by the variability of SCSSM onset also affect O 3 pollution in China, which has been less explored.In this study, we illustrate how early SCSSM onset affects late spring O 3 over southern China based on a reconstructed surface O 3 data set and meteorological reanalysis, and further investigate the mechanisms through CESM simulations.It should be noted that the O 3 data set was reconstructed using XGBoost, and detrended by EMD method.
Notable differences in surface O 3 concentrations and associated meteorological conditions before and after SCSSM onset during early onset events are observed over southern China.Following the early onset of SCSSM, increased air temperature and solar radiation by 1.1 K and 30.9 W m −2 , together with the decrease of 5.7% in relative humidity contribute to an increase of 11.1 μg m −3 in surface O 3 concentrations over southern China.The O 3 -favorable meteorological conditions accompanied by early SCSSM onset are also found in May, creating a warmer (0.8 K) and drier (−9%) condition associated with stronger solar radiation (30 W m −2 ) over southern China compared to the climatological mean.Thus, O 3 concentration in May is negatively related to the onset date of SCSSM, with an increase of 9.3 μg m −3 over southern China during early onset events.CESM experiments show that early SCSSM onset increases surface O 3 concentration in May by over 12 μg m −3 over southern China, and the influences are extended to middle troposphere.Chemical processes play dominant roles in the increases in O 3 with enhanced net chemical production of 0.27 Tg month −1 , which is supported by warmer and drier conditions with enhanced solar radiation and less precipitation.Higher temperature associated with early onset events also increases PBLH and boosts biogenic emissions of VOCs from a broad area of forest in southern China.Although descending motion vertically transports O 3 from troposphere to surface layer by 0.10 Tg month −1 , physical processes exhibit negative impacts on O 3 concentrations mainly due to horizontal advection.

Figure 1 .
Figure 1.Differences in surface O 3 and associated weather conditions over southern China between the pre-onset and post-onset of South China Sea summer monsoon (SCSSM) for different phases of SCSSM onset.Three-pentad averaged (15-days average) differences in surface O 3 concentrations (μg m −3 ) between after (during onset to onset date + 15 days) and before (during onset to onset date -15 days) the SCSSM onset for (a) climatological mean, (b) early onset events and (c) late onset events during 2005-2021, respectively (Green box marks southern China).Dotted areas represent statistical significance with 95% confidence according to Student's t-test.(d) Comparisons of three-pentad averaged 2 m air temperature (K) over southern China (the green box shown in Figure 1a) during pre-onset and post-onset of SCSSM for climatological mean, early onset events and late onset events during 1979-2021, respectively.(e) Same as (d) but for surface downward solar radiation flux (W m −2 ).(f) Same as (d) but for near-surface relative humidity (%).

Figure 3 .
Figure 3.The impacts of early South China Sea summer monsoon (SCSSM) onset events on meteorological conditions in May.Composite difference of (a) sea level pressure (Pa, contour) and wind fields (m s −1 , vector) at 850 hPa, (b) 2 m air temperature (K), (c) surface downward solar radiation flux (W m −2 ) and (d) near-surface relative humidity (%) in May between early SCSSM onset events and climatological mean over 1979-2021.

Figure 2 .
Figure 2. Early onset events modulated surface O 3 in May.(a) Composite difference of O 3 (μg m −3 ) in May between early South China Sea summer monsoon (SCSSM) onset events and climatological mean.Dotted areas represent statistical significance with 95% confidence according to Student's t test.Time series of the anomalous SCSSM onset date and O 3 (μg m −3 ) in May over (b) southern China, (c) Yangtze River Delta (YRD), and (d) Pearl River Delta (PRD), respectively.

Figure 4 .
Figure 4. Composite difference of February-March-April averaged sea surface temperature (K) between early South China Sea summer monsoon onset events and climatological mean over 1979-2021 (The green and red box represent the region with sea surface temperature anomaly imposed in Community Earth System Model (CESM PhiSea ) and CESM CenPacif , respectively).

Figure 7 .
Figure 7. Community Earth System Model (CESM) simulated responses of O 3 concentrations.CESM simulated responses of horizontal distribution of near-surface O 3 concentration and pressure-longitude cross sections averaged over 112.5-120°E of O 3 concentration (μg m −3 , contour) and winds (m s −1 , vector) in May to (a, b) early onset event, (c, d) colder sea surface temperature (SST) over central equatorial Pacific, (e, f) warmer SST over Philippine Sea.
SST during FMA featuring early onset of SCSSM event are colder of −1 K in the central equatorial Pacific and warmer of 0.6 K in the Philippine Sea than climatological mean.The colder SST in the central equatorial Pacific causes more O 3 -favorable meteorological conditions and enhances O 3 chemical production by 0.41 Tg month −1 .However, physical processes transport O 3 toward the north boundary, resulting in outgoing CESM early -CESM ctrl CESM CenPacif -CESM ctrl CESM PhiSea -Note.Positive (negative) values indicate incoming (outgoing) O 3 flux.Table 2 Mean O 3 Vertical and Horizontal Fluxes (Unit: Tg Month −1 ) at Different Edges of Southern China (the Green Box Shown in Figure 1a) and Net O 3 Chemical Production (Unit: Tg Month −1 ) at Surface fluxes of 0.36 Tg month −1 .Consequently, the SST anomalies in the central equatorial Pacific mainly increase surface O 3 concentrations in YRD, while higher O 3 concentrations in PRD are attributed by SST anomalies in the Philippine Sea.Colder SST in the central equatorial Pacific has significant impacts on O 3 over southern China during early onset events, whereas variations in O 3 during most of the SCSSM onsets are related to SST anomalies in the Philippine Sea in recent decades.Our results highlight the significant role of SCSSM onset in modulating surface O 3 pollution in late spring in southern China, as summarized in Figure8.Considering the adverse impacts of O 3 on food production and human health, our conclusion suggests promising applications in the management of O 3 pollution and agriculture.

Figure 8 .
Figure 8. Conceptual scheme of modulation of O 3 in Southern China by early SMSSM onset.Meteorological conditions and chemical processes of O 3 during (a) climatological mean and (b) early South China Sea summer monsoon onset event.
. The CESM model can reproduce the general variations of important meteorological factors and O 3 in southern China, with MFB within ±0.18 and MFE lower than 0.30 Dates of South China Sea Summer Monsoon a Denote years that selected as the early onset values.bDenoteyears that selected as the late onset events.Note.SD represents the standard deviation of anomalous SCSSM onset dates.Table1